ARRT Computed Tomography (CT) Exam — Questions and Answers

Question 1: Which technique allows for reduced radiation dose in CT imaging while maintaining image quality?

Conventional
High-dose CT
Low-dose CT (Correct answer)
Ultrafast CT

Correct answer: Low-dose CT

Low-dose CT (LDCT) refers to specific protocols and techniques designed to reduce the amount of radiation exposure to the patient while still maintaining diagnostic image quality. This is achieved through various methods, including adjusting X-ray tube current, voltage, and using iterative reconstruction algorithms. It's particularly important for screening applications like lung cancer screening, where repeated scans might be necessary.

Question 2: In CT, what does the Hounsfield unit (HU) value of approximately -100 indicate?

Fat (Correct answer)
Soft tissue
Water
Air

Correct answer: Fat

Fat typically measures between -50 and -150 HU, with approximately -100 HU being a classic fat attenuation value, while air is near -1000 HU.

Question 3: A CT brain performed for trauma shows a lens-shaped (biconvex) hyperdense collection between the inner table of the skull and the brain. The most likely source of bleeding is:

Anterior cerebral artery
Middle meningeal artery (Correct answer)
Vertebral artery
Bridging cortical veins

Correct answer: Middle meningeal artery

Epidural hematomas are most commonly caused by tearing of the middle meningeal artery, producing the classic biconvex hyperdense collection.

Question 4: Which reconstruction algorithm produces sharper edges but increased image noise in CT?

Bone/sharp kernel (Correct answer)
Smooth/soft-tissue kernel
Ultra-low-dose kernel
Standard kernel

Correct answer: Bone/sharp kernel

Sharp (bone) kernels apply high-frequency enhancement to improve edge definition, which amplifies noise compared to smooth kernels.

Question 5: What is the primary safety concern when scanning a patient with a pacemaker using CT?

Radiofrequency interference causing pacemaker malfunction
Gadolinium contrast nephrotoxicity
Potential image artifacts affecting diagnostic quality (Correct answer)
Excessive radiation dose to the cardiac device

Correct answer: Potential image artifacts affecting diagnostic quality

While CT does not emit RF waves that affect pacemaker function, metallic pacemaker components cause streak artifacts that can limit evaluation of surrounding cardiac and mediastinal structures.

Question 6: What is the primary purpose of a CT gantry tilt?

To align the scan plane with anatomical structures while avoiding radiation-sensitive organs (Correct answer)
To increase patient comfort
To improve contrast resolution
To reduce scan time

Correct answer: To align the scan plane with anatomical structures while avoiding radiation-sensitive organs

Gantry tilt allows the scan plane to be angled relative to the patient's body axis to align with anatomy or minimize dose to sensitive structures like the lens of the eye.

Question 7: In CT imaging, what does the term 'image noise' primarily refer to?

Blurring caused by insufficient spatial resolution
Distortion caused by metallic implants in the patient
Random variation in CT numbers (HU) across a uniform object (Correct answer)
Artifacts caused by patient motion during scanning

Correct answer: Random variation in CT numbers (HU) across a uniform object

CT image noise is the random fluctuation in Hounsfield units across a uniform region, primarily caused by quantum mottle from insufficient photon statistics.

Question 8: Which detector configuration in multi-detector CT (MDCT) acquires data simultaneously from multiple rows, allowing faster volume coverage?

Photomultiplier tube array
Single-row detector array
Xenon gas detector
Multi-row detector array (Correct answer)

Correct answer: Multi-row detector array

Multi-row detector arrays allow simultaneous acquisition of multiple slices per gantry rotation, dramatically increasing volume coverage speed.

Question 9: Which CT finding is characteristic of renal cell carcinoma (RCC)?

Non-enhancing cyst with thin walls
Calcified lesion with no enhancement
Fat-containing lesion with negative HU values
Hypervascular solid renal mass with arterial phase enhancement ≥20 HU above baseline (Correct answer)

Correct answer: Hypervascular solid renal mass with arterial phase enhancement ≥20 HU above baseline

RCC is highly vascular and shows significant enhancement (typically >20 HU increase) on arterial phase CT, distinguishing it from cysts and benign lesions.

Question 10: Which medication must be withheld 48 hours before and after iodinated contrast administration in patients with renal impairment?

Aspirin
Metformin (Correct answer)
Atorvastatin
Lisinopril

Correct answer: Metformin

Metformin must be withheld because contrast-induced nephropathy can impair its renal clearance, leading to toxic accumulation and risk of lactic acidosis.

Question 11: What is the primary purpose of using automatic exposure control (AEC) in CT?

Improve spatial resolution
Adapt tube current to patient size and anatomy (Correct answer)
Reduce contrast agent volume
Increase scan speed

Correct answer: Adapt tube current to patient size and anatomy

AEC (also called tube current modulation) automatically adjusts mA based on patient attenuation to optimize image quality while minimizing dose.

Question 12: What is the most common type of iodinated contrast media used for CT examinations?

Ionic high-osmolar contrast
Barium sulfate suspension
Gadolinium-based contrast
Non-ionic, water-soluble iodinated contrast (Correct answer)

Correct answer: Non-ionic, water-soluble iodinated contrast

Non-ionic, water-soluble iodinated contrast agents are preferred for CT because they have lower osmolality and fewer adverse reactions compared to ionic agents.

Question 13: The 'reconstruction kernel' (convolution filter) in CT primarily affects:

Contrast injection timing
Gantry rotation time
Slice thickness
Spatial resolution and noise (Correct answer)

Correct answer: Spatial resolution and noise

Sharp kernels enhance spatial resolution but increase noise, while smooth kernels reduce noise at the cost of spatial resolution.

Question 14: Which oral contrast agent is preferred in CT when bowel perforation is suspected?

Negative contrast (water only)
Barium sulfate
Water-soluble iodinated contrast (e.g., Gastrografin) (Correct answer)
No oral contrast is needed

Correct answer: Water-soluble iodinated contrast (e.g., Gastrografin)

Water-soluble iodinated agents are used when perforation is suspected because barium leaking into the peritoneum causes severe peritonitis.

Question 15: What is the primary goal of automatic tube current modulation (ATCM) in CT?

To speed up gantry rotation during scanning
To reduce scan time by increasing tube voltage
To maintain consistent image noise while minimizing radiation dose (Correct answer)
To increase image sharpness uniformly across the scan

Correct answer: To maintain consistent image noise while minimizing radiation dose

ATCM automatically adjusts tube current based on real-time patient attenuation, delivering consistent noise levels while reducing dose in less-attenuating regions.

Question 16: What is the 'gantry' in a CT scanner?

The high-voltage power supply
The patient support table
The computer processing unit
The rotating frame housing the X-ray tube and detector array (Correct answer)

Correct answer: The rotating frame housing the X-ray tube and detector array

The gantry is the large donut-shaped housing that contains the rotating X-ray tube, detector array, and associated electronics.

Question 17: The term 'Z-axis dose profile' in CT refers to dose distribution along which direction?

Anterior-to-posterior axis
Patient cranio-caudal (head-to-foot) axis (Correct answer)
Rotational axis of the gantry
Left-to-right axis

Correct answer: Patient cranio-caudal (head-to-foot) axis

The Z-axis is the cranio-caudal axis along which the patient moves through the gantry, and the dose profile describes radiation distribution along this direction.

Question 18: What is the primary advantage of using saline flush after contrast injection in CT?

Prevents nephrotoxicity
Pushes residual contrast into central circulation, improving vascular opacification and reducing dose (Correct answer)
Increases image noise
Reduces scan time

Correct answer: Pushes residual contrast into central circulation, improving vascular opacification and reducing dose

A 30–50 mL saline chaser propels contrast remaining in the IV tubing and peripheral vein into the bloodstream, maximizing the usable contrast bolus.

Question 19: A digital radiograph was produced while the patient bed was translated (moved) through the gantry, maintaining the X-ray tube and detector array in place.

What is a Scout Image in CT ? (Correct answer)
What is the purpose of an scout image in CT ?
What Does a SSCT AND MSCT require?
What is the purpose of Flying Focal Spot Technology ?

Correct answer: What is a Scout Image in CT ?

The description accurately defines the acquisition of a scout image in CT, also known as a topogram or surview. A scout image is a preliminary digital radiograph produced by translating the patient bed through the gantry while the X-ray tube and detector array remain stationary. This image is essential for planning the subsequent axial or helical CT scans, allowing the technologist to define the precise scan range and angle.

Question 20: BEAM COLLIMATION and DETECTOR COLLIMATION settings determine the section width in the MSCT System.

SSCT Beam Collimation Consists of:
MSCT Beam Collimation Consists of:
Collimation (msct)-detector (section) collimation
MSCT Beam Collimation Consists of: Post-Patient Collimation (Correct answer)

Correct answer: MSCT Beam Collimation Consists of: Post-Patient Collimation

In Multi-Slice Computed Tomography (MSCT), both pre-patient and post-patient collimation are critical for defining the section width and managing radiation dose. Post-patient collimation, located just before the detectors, further refines the X-ray beam after it has passed through the patient. This ensures that only the desired X-rays reach the detectors, contributing to precise slice thickness and optimal image quality.

Question 21: What is the primary advantage of iterative reconstruction over filtered back projection (FBP) in CT?

Higher spatial resolution
Shorter reconstruction time
Fewer beam hardening artifacts
Reduced image noise at equivalent or lower radiation dose (Correct answer)

Correct answer: Reduced image noise at equivalent or lower radiation dose

Iterative reconstruction algorithms model the imaging process and iteratively reduce noise, allowing diagnostic quality images at lower mAs and reduced patient dose compared to FBP.

Question 22: Increasing the mAs in CT by a factor of 4 will have what effect on image noise?

Double the noise
Reduce noise by 50% (Correct answer)
Quadruple the noise
Eliminate noise entirely

Correct answer: Reduce noise by 50%

Since noise is proportional to 1/√mAs, quadrupling mAs reduces noise by half (√4 = 2).

Question 23: What is the CTDI (Computed Tomography Dose Index) primarily used to measure?

Scanner radiation output for a single axial scan (Correct answer)
Contrast-to-noise ratio of an image
Patient effective dose in millisieverts
Image noise level across a reconstructed slice

Correct answer: Scanner radiation output for a single axial scan

CTDI measures the average absorbed dose within the scan volume from a single rotation and is used as a standardized descriptor of scanner output, not true patient effective dose.

Question 24: Which condition is best diagnosed using CT pulmonary angiography (CTPA)?

Pulmonary embolism (Correct answer)
Pulmonary fibrosis
Community-acquired pneumonia
Pleural effusion

Correct answer: Pulmonary embolism

CTPA is the gold standard for diagnosing pulmonary embolism, demonstrating filling defects within pulmonary arteries during the pulmonary arterial phase.

Question 25: The term 'isotropic voxel' in CT means the voxel dimensions are:

Equal in all three dimensions (Correct answer)
Larger in-plane than in the z-axis
Larger in the z-axis than in-plane
Variable depending on window settings

Correct answer: Equal in all three dimensions

Isotropic voxels have equal x, y, and z dimensions, enabling high-quality multiplanar reformations in any orientation.

Question 26: What is the primary function of the pre-patient collimator in a CT scanner?

To focus the X-ray beam onto the detector
To reduce the energy of the X-ray beam
To filter scattered radiation from the patient
To define the width of the X-ray beam along the z-axis (Correct answer)

Correct answer: To define the width of the X-ray beam along the z-axis

The pre-patient collimator controls beam width along the z-axis, defining the nominal slice thickness for CT acquisition.

Question 27: What defines a 'dual-source' CT scanner?

A system with two separate patient tables
A scanner capable of imaging two patients simultaneously
A system using two different X-ray energies from a single tube
A system with two X-ray tube and detector array pairs offset at approximately 90° (Correct answer)

Correct answer: A system with two X-ray tube and detector array pairs offset at approximately 90°

Dual-source CT uses two X-ray tube/detector systems mounted approximately 90° apart, enabling ultra-fast acquisition and simultaneous dual-energy imaging.

Question 28: Which term describes the ability of a CT system to distinguish between two objects with very similar attenuation values?

Low-contrast resolution (Correct answer)
Spatial resolution
Noise index
High-contrast resolution

Correct answer: Low-contrast resolution

Low-contrast resolution (contrast resolution) measures the system's ability to differentiate objects with nearly identical attenuation coefficients.

Question 29: What technique is used to time the CT acquisition to arterial contrast peak in CT angiography?

ECG gating
Bolus tracking (SmartPrep) or test bolus technique (Correct answer)
Breathing command protocol
Fixed-delay scanning

Correct answer: Bolus tracking (SmartPrep) or test bolus technique

Bolus tracking uses real-time low-dose monitoring of a target vessel; when attenuation reaches a threshold, the diagnostic scan is automatically triggered.

Question 30: In CT, what does 'z-axis resolution' refer to?

Contrast resolution between adjacent soft tissue structures
Temporal resolution of the rotating gantry
In-plane pixel size in the axial plane
Spatial resolution along the longitudinal (head-to-foot) axis of the patient (Correct answer)

Correct answer: Spatial resolution along the longitudinal (head-to-foot) axis of the patient

Z-axis (longitudinal) resolution determines the ability to distinguish structures stacked along the patient's long axis and improves with thinner collimation.

Question 31: Which window setting is most appropriate for evaluating brain parenchyma on CT?

Window width 2000 HU, level 600 HU
Window width 1500 HU, level -600 HU
Window width 400 HU, level 40 HU
Window width 80 HU, level 40 HU (Correct answer)

Correct answer: Window width 80 HU, level 40 HU

A narrow window (80 HU width) centered at 40 HU maximizes contrast between gray matter, white matter, and CSF for brain evaluation.

Question 32: Which brain imaging technique is most sensitive for detecting acute ischemic strokes?

Magnetic Resonance Imaging (MRI) (Correct answer)
Single-Photon Emission Computed Tomography (SPECT)
Computed Tomography (CT)
Positron Emission Tomography (PET)

Correct answer: Magnetic Resonance Imaging (MRI)

Magnetic Resonance Imaging (MRI), particularly with diffusion-weighted imaging (DWI) sequences, is the most sensitive technique for detecting acute ischemic strokes. DWI can detect changes in water movement within brain tissue that occur minutes after the onset of ischemia, often before any changes are visible on a standard CT scan. This early detection is crucial for timely intervention and treatment to minimize brain damage.

Question 33: Which type of artifact is caused by high-density materials like metal in the CT scan's field of view?

Ring artifact
Partial volume artifact
Beam-hardening artifact (Correct answer)
Motion artifact

Correct answer: Beam-hardening artifact

Beam-hardening artifact occurs when the X-ray beam passes through dense materials, such as metal implants or thick bone. The lower-energy photons in the beam are absorbed more readily, leaving a 'harder' (higher-energy) beam. This causes streaks and dark bands in the image, often appearing between two dense objects or at the edges of very dense objects.

Question 34: When performing a CT brain for evaluation of acute stroke, the primary reason to acquire a non-contrast series BEFORE contrast-enhanced sequences is:

To assess the degree of atherosclerotic calcification
To reduce radiation dose from overlapping sequences
To measure baseline Hounsfield units of the tumor
To detect acute hemorrhage, which is a contraindication to thrombolytic therapy (Correct answer)

Correct answer: To detect acute hemorrhage, which is a contraindication to thrombolytic therapy

Identifying acute hemorrhage on non-contrast CT is critical because thrombolytics (tPA) are absolutely contraindicated if hemorrhage is present.

Question 35: Which parameter most directly controls CT image noise?

kVp
FOV
Reconstruction interval
mAs (Correct answer)

Correct answer: mAs

mAs (milliampere-seconds) determines the quantity of X-ray photons produced; higher mAs means more photons, less quantum noise, and better image quality.

Question 36: A CT of the head shows enlargement of all four ventricles with periventricular hypodensity. The most likely diagnosis is:

Communicating hydrocephalus with transependymal edema (Correct answer)
Diffuse axonal injury
Cerebral atrophy
Bilateral subdural hematomas

Correct answer: Communicating hydrocephalus with transependymal edema

Communicating hydrocephalus causes symmetric dilation of all ventricles and periventricular hypodensity from transependymal CSF migration.

Question 37: What is the primary advantage of using contrast enhancement in brain CT imaging?

Faster image acquisition
Reduced radiation dose
Improved spatial resolution
Enhanced visualization of blood vessels (Correct answer)

Correct answer: Enhanced visualization of blood vessels

Contrast agents, typically iodine-based, are injected intravenously and absorb X-rays more effectively than surrounding tissues. This differential absorption makes structures like blood vessels, tumors, and areas of inflammation appear brighter on the CT scan. This enhanced contrast allows for improved detection and characterization of vascular abnormalities, such as aneurysms or arteriovenous malformations, and helps differentiate pathological tissues from normal brain parenchyma.

Question 38: What does the term CTDI stand for in CT radiation dosimetry?

Computed Tomography Density Index
Computed Tomography Dose Index (Correct answer)
CT Dose Interval
CT Diagnostic Imaging

Correct answer: Computed Tomography Dose Index

CTDI (Computed Tomography Dose Index) is a standardized measure of radiation dose output from a CT scanner for a single rotation.

Question 39: What type of CT artifact appears as dark streaks radiating from dense metallic objects such as hip prostheses?

Partial volume artifact
Motion artifact
Ring artifact
Beam hardening and metal streak artifact (Correct answer)

Correct answer: Beam hardening and metal streak artifact

Metal objects severely attenuate and preferentially absorb low-energy photons (beam hardening), causing dark streaks that obscure surrounding anatomy.

Question 40: On a CT scan of the abdomen, what Hounsfield unit threshold is typically used to identify an acute hemorrhage?

20–40 HU
40–60 HU
0–20 HU
55–80 HU (Correct answer)

Correct answer: 55–80 HU

Acute hemorrhage typically measures 55–80 HU on non-contrast CT due to the high protein content of clotted blood, making it hyperdense relative to surrounding soft tissues.

Question 41: What effect does increasing CT tube voltage (kVp) have on patient dose?

Has no effect on patient dose
Reduces dose by hardening the beam
Decreases dose by reducing scan time
Increases dose due to higher photon energy and greater beam penetration (Correct answer)

Correct answer: Increases dose due to higher photon energy and greater beam penetration

Higher kVp increases photon energy and quantity (via space charge effect), generally increasing patient dose, though it may allow mAs reduction in large patients.

Question 42: The 'hyperdense MCA sign' on non-contrast CT brain indicates:

Contrast material within the MCA from prior study
Calcification of the MCA wall
Arteriovenous malformation involving the MCA
Acute thrombus within the middle cerebral artery (Correct answer)

Correct answer: Acute thrombus within the middle cerebral artery

The hyperdense MCA sign reflects acute thrombus in the proximal MCA lumen, an early sign of large-vessel occlusion.

Question 43: What is the purpose of the bow-tie (wedge) filter in CT?

To reduce motion artifacts
To remove high-energy photons from the beam
To increase radiation dose to the center of the patient
To compensate for the varying thickness of the patient's body cross-section (Correct answer)

Correct answer: To compensate for the varying thickness of the patient's body cross-section

The bow-tie filter attenuates the beam more at the periphery (thinner patient edges) than at the center, equalizing detector signal and reducing skin dose.

Question 44: Which unit is used to express the effective radiation dose received by a patient during a CT scan?

Roentgen (R)
Milligray (mGy)
Rad
Millisievert (mSv) (Correct answer)

Correct answer: Millisievert (mSv)

Millisievert (mSv) is the SI unit used to express effective dose, which accounts for the biological effect of radiation on different tissues.

Question 45: What does the term 'partial volume averaging' refer to in CT imaging?

Use of partial rotation for faster data acquisition
Blending of attenuation values from different tissues within a single voxel (Correct answer)
Reduction in FOV to improve spatial resolution
Averaging of multiple scan acquisitions to reduce noise

Correct answer: Blending of attenuation values from different tissues within a single voxel

Partial volume averaging occurs when a voxel contains more than one tissue type, resulting in an averaged attenuation value that misrepresents both tissues.

Question 46: What is the purpose of using a higher kVp setting when scanning a bariatric patient?

To reduce scan time
To increase penetrating power of the X-ray beam through greater tissue thickness (Correct answer)
To improve temporal resolution for cardiac imaging
To decrease patient radiation dose

Correct answer: To increase penetrating power of the X-ray beam through greater tissue thickness

Higher kVp produces more energetic photons with greater penetrating ability, reducing photon starvation and noise when imaging larger patients with greater tissue attenuation.

Question 47: Which material is used in CT phantoms to simulate human tissue?

Water (Correct answer)
Plastic
Metal
Air

Correct answer: Water

Water is widely used in CT phantoms because its X-ray attenuation properties are very similar to those of soft tissue in the human body. Phantoms filled with water or water-equivalent materials are essential for quality assurance and calibration of CT scanners. They allow for consistent measurements of image quality parameters like noise, uniformity, and CT number accuracy.

Question 48: What is the primary radiation protection principle that requires justifying each CT examination?

ALARA
Dose constraint
Justification (Correct answer)
Optimization

Correct answer: Justification

Justification is the ICRP principle requiring that no examination be performed unless the expected benefit exceeds the radiation detriment to the individual.

Question 49: Which of the following brain CT findings is most consistent with diffuse axonal injury (DAI) following high-speed trauma?

Hyperdense basilar cisterns
Multiple small hemorrhages at the gray-white junction and corpus callosum (Correct answer)
Large epidural hematoma with midline shift
Symmetric hypodensity in both MCA territories

Correct answer: Multiple small hemorrhages at the gray-white junction and corpus callosum

DAI results in petechial hemorrhages at gray-white junctions, corpus callosum, and brainstem due to rotational shear forces.

Question 50: Which artifact appears as a dark band between two high-density objects in CT images?

Partial volume artifact
Motion artifact
Beam hardening artifact (Correct answer)
Ring artifact

Correct answer: Beam hardening artifact

Beam hardening causes dark streaks or bands between dense structures because lower-energy photons are preferentially absorbed, shifting the beam spectrum.

Question 51: CT colonography (virtual colonoscopy) is performed with the patient in which position to evaluate both the colon positions?

Left lateral decubitus only
Trendelenburg position
Both supine and prone positions (Correct answer)
Upright sitting position

Correct answer: Both supine and prone positions

Both supine and prone acquisitions are standard for CT colonography; repositioning redistributes residual fluid and stool, allowing evaluation of dependent and non-dependent colonic segments.

Question 52: Which factor most significantly increases a patient's risk for a severe contrast reaction?

Age over 65
Asthma controlled with inhalers
History of a prior severe contrast reaction (Correct answer)
Mild shellfish allergy

Correct answer: History of a prior severe contrast reaction

A prior severe contrast reaction is the single strongest risk factor for a repeat severe reaction, increasing risk approximately 5-fold.

Question 53: What is the purpose of contrast bolus tracking in CT angiography?

To improve image resolution
To minimize patient motion
To monitor contrast agent injection (Correct answer)
To enhance the patient's comfort

Correct answer: To monitor contrast agent injection

Contrast bolus tracking is a technique used in CT angiography to optimize the timing of image acquisition relative to contrast agent injection. It involves monitoring the arrival of the contrast bolus in a target vessel or organ using real-time low-dose scans. This ensures that the diagnostic scan is performed precisely when the contrast enhancement is at its peak, maximizing vessel opacification and image quality.

Question 54: What is the purpose of 'prospective triggering' in cardiac CT?

To scan continuously throughout the cardiac cycle
To reconstruct multiple cardiac phases from one acquisition
To automatically adjust kVp based on heart rate
To acquire data only during a specific, predetermined phase of the cardiac cycle (Correct answer)

Correct answer: To acquire data only during a specific, predetermined phase of the cardiac cycle

Prospective triggering uses the ECG signal to trigger X-ray exposure only during a specific quiescent phase, significantly reducing radiation dose compared to retrospective gating.

Question 55: What effect does increasing the pitch in helical CT have on patient dose?

Decreases dose by reducing overlap (Correct answer)
Increases dose proportionally
Has no effect on dose
Doubles the dose

Correct answer: Decreases dose by reducing overlap

Increasing pitch means the table moves faster per rotation, resulting in less beam overlap and a reduced effective dose to the patient.

Question 56: A thyroid cancer patient requires radioiodine ablation therapy. Why must iodinated CT contrast be avoided before this treatment?

Iodinated contrast saturates the sodium-iodide symporter, blocking therapeutic radioiodine uptake (Correct answer)
It permanently destroys the thyroid gland
It interferes with TSH levels only
It causes thyroid storm

Correct answer: Iodinated contrast saturates the sodium-iodide symporter, blocking therapeutic radioiodine uptake

Excess stable iodine from contrast saturates thyroid transporters and competes with therapeutic I-131, significantly reducing radioiodine ablation efficacy for 2–8 weeks.

Question 57: During a dual-energy CT scan, what primary advantage does dual-energy provide over single-energy CT?

Faster image acquisition
Material differentiation and virtual non-contrast imaging (Correct answer)
Lower radiation dose in all cases
Improved temporal resolution

Correct answer: Material differentiation and virtual non-contrast imaging

Dual-energy CT acquires data at two different kVp settings, enabling material decomposition to distinguish iodine, calcium, and uric acid, and to create virtual non-contrast images.

Question 58: X-rays of the body are captured from various angles during computed tomography (CT), which produces images that resemble 2- and 3-dimensional slices (cross-sections). All jewelry and metal objects, such as zippers and belt buckles close to the body portion being scanned, should be taken off by anyone getting a CT scan. Why are patients asked to remove metal before a CT scan?

The items can damage the CT machine
The items may block the x-rays and blur the images (Correct answer)
The goods could burn the individual receiving the scan since they heat up in the machine.
The machine's rays could cause metallic items to release poisonous fumes.

Correct answer: The items may block the x-rays and blur the images

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Question 59: Which ionizing radiation unit is used to express the effective dose that accounts for tissue radiosensitivity?

Sievert (Sv) (Correct answer)
Rad
Roentgen (R)
Gray (Gy)

Correct answer: Sievert (Sv)

The Sievert (Sv) is used to express effective dose, which weights absorbed dose by tissue-specific radiation sensitivity and radiation type weighting factors.

Question 60: A patient's CT scan demonstrates a hyperdense lesion in the basal ganglia on non-contrast CT. What is the most common cause?

Calcified granuloma
Acute hemorrhage (Correct answer)
Ischemic infarction
Air embolism

Correct answer: Acute hemorrhage

Fresh blood is hyperdense on non-contrast CT (55–80 HU) due to the protein content of clotted hemoglobin, making acute hemorrhage the most common cause of basal ganglia hyperdensity.

Question 61: What is the significance of a 'split-bolus' technique in contrast-enhanced CT?

It pre-mixes contrast with saline to reduce osmolality
It administers oral and IV contrast simultaneously
It doubles the contrast volume to improve enhancement
It combines two contrast injection phases into a single acquisition to reduce radiation dose (Correct answer)

Correct answer: It combines two contrast injection phases into a single acquisition to reduce radiation dose

The split-bolus technique injects contrast in two separated boluses so that a single scan acquisition captures both nephrographic and excretory phase opacification, reducing the number of scan passes and radiation dose.

Question 62: In CT image quality assessment, the modulation transfer function (MTF) is used to evaluate:

Image noise as a function of dose
Temporal resolution at different heart rates
Contrast sensitivity at low kVp
Spatial resolution as a function of spatial frequency (Correct answer)

Correct answer: Spatial resolution as a function of spatial frequency

The MTF describes how faithfully the CT system reproduces contrast at various spatial frequencies, characterizing the system's spatial resolution capability.

Question 63: Which factor influences the spatial resolution of a CT scan?

Detector size
Patient weight
X-ray tube current
Slice thickness (Correct answer)

Correct answer: Slice thickness

Spatial resolution refers to a CT scan's ability to distinguish between two closely spaced objects. Slice thickness directly influences the resolution along the z-axis (the patient's long axis). Thinner slices provide finer detail and better separation of structures in the axial plane, thereby improving the overall spatial resolution. This allows for clearer visualization of small anatomical features and subtle abnormalities.

Question 64: What is the purpose of the CT table's indexing capability?

To increase the gantry rotation speed
To adjust the X-ray tube voltage
To synchronize with the patient's breathing
To move the patient through the gantry during scanning (Correct answer)

Correct answer: To move the patient through the gantry during scanning

The CT table's indexing capability refers to its ability to precisely move the patient incrementally through the gantry opening. This controlled movement allows for the acquisition of sequential slices or a continuous helical scan, ensuring that the entire anatomical region of interest is covered systematically. This precise positioning is critical for accurate imaging and reconstruction.

Question 65: In CT imaging, what is a "Hounsfield Unit" (HU) a measure of?

Radiation dose
Tissue density (Correct answer)
Pixel size
Image brightness

Correct answer: Tissue density

The gantry is the large, donut-shaped component of a CT scanner that houses the X-ray tube and the detector array. During image acquisition, the gantry rotates rapidly around the patient, allowing the X-ray tube to emit radiation from multiple angles and the detectors to capture the transmitted X-rays. This rotation is essential for collecting the data needed to reconstruct cross-sectional images.

Question 66: What is the normal attenuation range (HU) for the liver parenchyma on a non-contrast CT?

50 to 70 HU (Correct answer)
0 to 20 HU
20 to 40 HU
80 to 100 HU

Correct answer: 50 to 70 HU

Normal liver parenchyma measures approximately 50–70 HU on non-contrast CT, slightly higher than the spleen under normal conditions.

Question 67: Which population group requires the most careful dose optimization in CT due to higher radiation sensitivity?

Athletic adults
Elderly patients
Male patients
Pediatric patients (Correct answer)

Correct answer: Pediatric patients

Children are more radiosensitive than adults and have more years of life remaining in which radiation-induced cancers could develop, requiring strict dose minimization.

Question 68: What does multiplanar reformation (MPR) allow clinicians to do with CT data?

Remove contrast enhancement from images
View reconstructed images in coronal, sagittal, and oblique planes from axial data (Correct answer)
Increase spatial resolution after acquisition
Generate 3D models for printing

Correct answer: View reconstructed images in coronal, sagittal, and oblique planes from axial data

MPR uses isotropic or near-isotropic volumetric CT data to generate reconstructions in any desired imaging plane without additional scanning.

Question 69: What is the primary advantage of using a higher tube voltage (kV) in CT imaging?

Reduced patient dose
Faster scan times
Enhanced image contrast
Improved spatial resolution (Correct answer)

Correct answer: Improved spatial resolution

Higher tube voltage (kV) increases the energy and penetrating power of the X-ray beam. This reduces beam hardening artifacts, especially in dense tissues, and improves the signal-to-noise ratio. Consequently, it allows for better differentiation of small structures and fine details, leading to improved spatial resolution in the CT image.

Question 70: Contrast extravasation during CT injection most commonly occurs at which site?

The renal pelvis
The pulmonary veins
The aorta
The IV catheter insertion site in the antecubital fossa (Correct answer)

Correct answer: The IV catheter insertion site in the antecubital fossa

Extravasation occurs when contrast leaks from a dislodged or infiltrated IV catheter into surrounding soft tissue, most often at the antecubital injection site.

Question 71: What is the significance of the 'z-flying focal spot' technology in multidetector CT?

It allows dual-energy acquisitions
It doubles the number of effective slices by alternating focal spot positions along the z-axis (Correct answer)
It eliminates beam hardening artifacts
It reduces gantry rotation time

Correct answer: It doubles the number of effective slices by alternating focal spot positions along the z-axis

Z-flying focal spot alternates the focal spot between two positions along the z-axis within each rotation, doubling the number of independently acquired slices and improving z-axis resolution.

Question 72: Which principle of radiation protection states that doses should be kept as low as reasonably achievable?

NCRP
BERT
LNT
ALARA (Correct answer)

Correct answer: ALARA

ALARA (As Low As Reasonably Achievable) is the guiding principle for minimizing radiation exposure to patients and staff.

Question 73: Which serum creatinine-based calculation is most commonly used to screen for contrast-induced nephropathy risk before CT?

BUN/creatinine ratio
Creatinine clearance via timed urine collection
eGFR (estimated Glomerular Filtration Rate) (Correct answer)
Urine specific gravity

Correct answer: eGFR (estimated Glomerular Filtration Rate)

eGFR calculated from serum creatinine is the standard screening tool; an eGFR <30 mL/min/1.73m² generally indicates high risk for contrast nephrotoxicity.

Question 74: What is the primary advantage of helical (spiral) CT scans over conventional CT scans?

Reduced radiation dose (Correct answer)
Slower image acquisition
Higher spatial resolution
Enhanced motion artifacts

Correct answer: Reduced radiation dose

Helical (spiral) CT scans offer several advantages over conventional CT, including faster image acquisition and reduced motion artifacts. A significant benefit is the potential for reduced radiation dose. By continuously rotating the X-ray tube and moving the patient through the gantry, helical CT acquires a volume of data in a single breath-hold, minimizing the need for repeated scans or overlapping exposures, leading to more efficient radiation use.

Question 75: What is the primary purpose of a bow-tie filter in a CT scanner?

To increase contrast resolution throughout the image
To reduce image noise at the center of the scan field
To correct for detector calibration errors
To shape the X-ray beam and reduce dose to peripheral tissues (Correct answer)

Correct answer: To shape the X-ray beam and reduce dose to peripheral tissues

A bow-tie (wedge) filter is thicker at the edges to attenuate X-rays where the patient cross-section is thinner, reducing peripheral skin dose and detector saturation.

Question 76: Which CT image artifact results from inconsistent attenuation readings caused by rapid patient breathing during thoracic imaging?

Beam hardening streak
Ring artifact
Respiratory motion artifact (Correct answer)
Windmill artifact

Correct answer: Respiratory motion artifact

Respiratory motion during thoracic CT causes misregistration of lung boundaries and blurring of structures, classified as a motion artifact.

Question 77: On CT of the spine, which finding is most indicative of an acute compression fracture?

Normal cortical outline with disk space narrowing only
Lytic lesion with pathological fracture
Posterior element involvement with kyphotic deformity
Loss of vertebral body height with band-like edema pattern on CT (marrow hyperdensity) (Correct answer)

Correct answer: Loss of vertebral body height with band-like edema pattern on CT (marrow hyperdensity)

Acute vertebral compression fractures show loss of vertebral body height with preserved posterior elements and often show band-like hyperdensity representing compressed trabecular bone.

Question 78: For what group have weight-based CT protocols with color-coding been established to help lower patient doses?

Pediatric (Correct answer)
Geriatric
Low-income
Adult

Correct answer: Pediatric

Filtration in CT imaging involves placing thin sheets of metal (like aluminum or copper) in the X-ray beam path. This process selectively absorbs lower-energy, longer-wavelength photons, which contribute more to patient dose without significantly improving image quality. By removing these 'soft' X-rays, filtration results in a more monochromatic and higher-energy beam, reducing patient radiation exposure and improving image quality through a process known as beam hardening.

Question 79: What is the purpose of the CT couch indexing feature?

To adjust image contrast
To increase the tube current
To move the patient through the gantry during scanning (Correct answer)
To synchronize with the patient's breathing

Correct answer: To move the patient through the gantry during scanning

The CT couch indexing feature refers to the precise, controlled movement of the patient table (couch) through the gantry during a CT scan. In helical (spiral) CT, the couch moves continuously, while in axial (sequential) CT, it moves in discrete steps between slice acquisitions. This indexing ensures that the entire anatomical region of interest is systematically scanned.

Question 80: A CT scan shows an adrenal lesion with an unenhanced attenuation of 8 HU. What is the most likely diagnosis?

Pheochromocytoma
Adrenal metastasis
Adrenal adenoma (Correct answer)
Adrenal carcinoma

Correct answer: Adrenal adenoma

Adrenal lesions with unenhanced HU ≤10 are consistent with lipid-rich adenomas due to intracellular fat content.

Question 81: Which parameter directly controls the amount of radiation delivered per CT rotation?

Slice thickness
mAs (Correct answer)
kVp
Pitch

Correct answer: mAs

Milliampere-seconds (mAs) is the product of tube current and rotation time, directly determining the number of X-ray photons produced per rotation.

Question 82: Which type of CT detector converts X-ray photons directly into electrical charge using amorphous selenium?

Scintillator-photodiode detector
Direct-conversion flat-panel detector (Correct answer)
Xenon ionization detector
Indirect-conversion flat-panel detector

Correct answer: Direct-conversion flat-panel detector

Direct-conversion detectors (e.g., amorphous selenium) convert X-rays directly to electrical charge without an intermediate light step, potentially improving spatial resolution.

Question 83: A non-contrast CT brain in a neonate with seizures reveals periventricular calcifications. The most likely causative infection is:

Rubella virus
Toxoplasma gondii
Herpes simplex virus
Cytomegalovirus (CMV) (Correct answer)

Correct answer: Cytomegalovirus (CMV)

Congenital CMV classically produces periventricular calcifications on CT, along with microcephaly and cortical malformations.

Question 84: The quantity of detector rows X Size of each detector is Section Width; for example, 64 x 0.625 is 40mm. <br> <br> Beam Collimation/Number of Detector Rows = Detector Collimation, where 0.625 = 40 mm/64.

MSCT Beam Collimation Consists of: Beam
MSCT Beam Collimation Consists of: Post-Patient Collimation
MSCT Beam Collimation Consists of: Beam (Formula) (Correct answer)
Desired Quality in a CT Detector: High Dynamic Range

Correct answer: MSCT Beam Collimation Consists of: Beam (Formula)

This question describes the fundamental formulas used in Multi-Slice Computed Tomography (MSCT) to determine beam and detector collimation. The provided equations illustrate how the total section width is calculated from the number of detector rows and the size of each detector, and how detector collimation is derived from beam collimation. This directly relates to the technical specifications of the X-ray beam in MSCT systems.

Question 85: In CT, the concept of 'noise power spectrum' (NPS) provides information about:

The frequency-dependent distribution of image noise (Correct answer)
The dynamic range of the display monitor
The maximum achievable mA for a given kVp
Patient dose as a function of scan length

Correct answer: The frequency-dependent distribution of image noise

The NPS (or Wiener spectrum) describes how noise energy is distributed across spatial frequencies, capturing the texture and magnitude of noise in CT images.

Question 86: In CT perfusion imaging of the brain, which parameter best reflects the integrity of cerebral autoregulation?

Cerebral blood flow (CBF)
Cerebral blood volume (CBV) (Correct answer)
Time to peak (TTP)
Mean transit time (MTT)

Correct answer: Cerebral blood volume (CBV)

Cerebral blood volume (CBV) reflects the total volume of blood in a given brain region and is maintained by autoregulation; preserved CBV with reduced CBF indicates penumbral tissue.

Question 87: What technology allows modern helical CT scanners to rotate continuously without stopping?

Electromagnetic braking
Gear reduction transmission
Slip ring technology (Correct answer)
Cable drive system

Correct answer: Slip ring technology

Slip ring technology enables continuous electrical contact between the rotating gantry and stationary components, allowing uninterrupted 360° rotation.

Question 88: Using CT and a radiopaque contrast agent, CT angiography creates 2- and 3-dimensional pictures of blood vessels to look for anomalies. Which of the following statements most accurately describes why CT angiography is frequently employed instead of traditional angiography?

Is more accurate
Takes more time
Detects blood clots in arteries
Is less invasive (Correct answer)

Correct answer: Is less invasive

Iodine-based contrast agents are the most frequently used with CT scans due to their high atomic number, which allows them to effectively absorb X-rays and enhance the visibility of blood vessels and soft tissues. They are typically administered intravenously to highlight specific structures and differentiate them from surrounding tissues. Other agents like barium are used for gastrointestinal imaging, while gadolinium is primarily for MRI.

Question 89: What is the primary advantage of using a higher pitch value in helical CT scans?

Reduced radiation dose
Increased spatial resolution
Improved image contrast
Faster scan speed (Correct answer)

Correct answer: Faster scan speed

In helical CT, pitch is the ratio of table movement per 360-degree rotation of the X-ray tube to the total beam collimation. A higher pitch value means the patient table moves faster through the gantry during each rotation. This results in a quicker overall scan time, which is beneficial for reducing motion artifacts and scanning large anatomical regions efficiently, though it can sometimes compromise Z-axis resolution.

Question 90: Which of the following is an advantage of CT over conventional X-ray imaging?

Better soft tissue contrast (Correct answer)
Faster image acquisition
Lower radiation exposure
Portable equipment

Correct answer: Better soft tissue contrast

The fundamental principle behind Computed Tomography (CT) imaging is X-ray absorption. A CT scanner uses an X-ray tube to generate X-rays that pass through the patient's body, and detectors measure the amount of X-rays absorbed by different tissues. The varying absorption rates, based on tissue density, allow a computer to reconstruct detailed cross-sectional images.

Question 91: On non-contrast CT of the head, acute intraparenchymal hemorrhage typically appears as:

Isodense to brain
Hyperdense (bright white) relative to adjacent brain parenchyma (Correct answer)
Hypodense (dark) relative to brain
Ring-enhancing

Correct answer: Hyperdense (bright white) relative to adjacent brain parenchyma

Acute blood is hyperdense on NCCT due to the high protein content of clotted hemoglobin (55–75 HU), making it easily distinguishable from normal brain (~35 HU).

Question 92: In CT, spatial resolution is most commonly measured using which parameter?

Signal-to-noise ratio (SNR)
Modulation Transfer Function (MTF) or line pairs per centimeter (lp/cm) (Correct answer)
Hounsfield Unit (HU) accuracy
Contrast-to-noise ratio (CNR)

Correct answer: Modulation Transfer Function (MTF) or line pairs per centimeter (lp/cm)

Spatial resolution in CT is quantified by the MTF or high-contrast resolution measured in line pairs per centimeter, describing the system's ability to resolve fine detail.

Question 93: What is the main clinical application of CT fluoroscopy?

Real-time guidance for percutaneous biopsies and drainage procedures (Correct answer)
Dynamic assessment of swallowing disorders
Whole-body oncologic staging
Cardiac stress perfusion imaging

Correct answer: Real-time guidance for percutaneous biopsies and drainage procedures

CT fluoroscopy provides continuous or near-continuous CT imaging to guide needle placement during interventional procedures such as lung biopsies or abscess drainage.

Question 94: What is the purpose of a CT scout image (localizer) in patient positioning?

To assess motion artifacts
To determine optimal contrast dose
To provide a preliminary diagnosis
To outline the scanning region (Correct answer)

Correct answer: To outline the scanning region

The CT scout image, also known as a localizer or topogram, is a preliminary low-dose X-ray image taken before the diagnostic scan. It serves as a reference image for the technologist to accurately define the anatomical boundaries and precise start and end points for the subsequent diagnostic CT scan. This ensures that the correct area of interest is included in the scan and minimizes unnecessary radiation exposure.

Question 95: Which pre-medication regimen is typically used for patients with prior moderate contrast reactions undergoing non-emergent contrast CT?

Single dose of epinephrine before the scan
No premedication is effective
Oral prednisone 13 hours, 7 hours, and 1 hour before, plus diphenhydramine 1 hour before (Correct answer)
IV methylprednisolone 30 minutes before only

Correct answer: Oral prednisone 13 hours, 7 hours, and 1 hour before, plus diphenhydramine 1 hour before

The standard ACR-recommended steroid premedication protocol uses oral prednisone at three intervals before the scan combined with antihistamine to reduce breakthrough reaction risk.

Question 96: The Nyquist theorem in CT states that to accurately represent a spatial frequency, the sampling rate must be at least:

Equal to the frequency of interest
Half the frequency of interest
Twice the frequency of interest (Correct answer)
Four times the frequency of interest

Correct answer: Twice the frequency of interest

The Nyquist criterion requires a sampling frequency at least twice the highest spatial frequency to avoid aliasing artifacts in the reconstructed image.

Question 97: In CT angiography, what is the purpose of a test bolus or bolus tracking technique?

To determine patient weight-based contrast volume
To calibrate the CT detector gain before contrast injection
To optimize scan timing relative to contrast arrival in the target vessel (Correct answer)
To assess patient allergy risk before full injection

Correct answer: To optimize scan timing relative to contrast arrival in the target vessel

Bolus tracking and test bolus methods synchronize scan initiation with peak contrast enhancement in the target vessel to ensure optimal opacification.

Question 98: What is the typical oral contrast agent used to opacify the GI tract during abdominal CT?

Gadolinium
Dilute barium or water-soluble iodinated contrast (Correct answer)
Iohexol injection
Normal saline

Correct answer: Dilute barium or water-soluble iodinated contrast

Dilute barium sulfate or water-soluble iodinated contrast is given orally to distend and opacify the bowel, distinguishing it from adjacent pathology.

Question 99: What does the term 'partial volume effect' describe in CT imaging?

Loss of signal at scan edges
Increased noise at the image periphery
Artifact from metal implants
Averaging of attenuation from different tissues within a single voxel (Correct answer)

Correct answer: Averaging of attenuation from different tissues within a single voxel

Partial volume averaging occurs when a voxel contains multiple tissue types, and the assigned HU is an average of all tissues present, potentially obscuring small lesions.

Question 100: Which modality provides ionizing radiation-free cross-sectional imaging and is preferred for repeated abdominal follow-up in young patients?

Magnetic resonance imaging (MRI) (Correct answer)
Fluoroscopy
Positron emission tomography (PET)
CT with low-dose protocol

Correct answer: Magnetic resonance imaging (MRI)

MRI uses magnetic fields and radiofrequency pulses with no ionizing radiation, making it safer for serial follow-up imaging in young or pediatric populations.

Question 101: On CT of the abdomen, the normal appendix should have a maximum diameter of:

Less than 6 mm (Correct answer)
Less than 1 cm
Size is not diagnostically relevant
Less than 2 cm

Correct answer: Less than 6 mm

A normal appendix is typically less than 6 mm in outer wall diameter; a distended appendix ≥6 mm with periappendiceal inflammation is diagnostic criteria for acute appendicitis.

Question 102: What is the purpose of "windowing" in CT image display?

To add color to the image
To enhance motion artifacts
To reduce radiation exposure
To adjust image brightness and contrast (Correct answer)

Correct answer: To adjust image brightness and contrast

Windowing is a crucial post-processing technique in CT image display that allows for the adjustment of image brightness (window level) and contrast (window width). By manipulating these parameters, specific tissue types or pathologies can be optimally visualized. This enables radiologists to highlight structures with similar densities, improving diagnostic accuracy for various anatomical regions like bone, soft tissue, or lung.

Question 103: The concept of isotropic voxels in modern multi-detector CT (MDCT) means that voxels are:

Equal in all three dimensions (x, y, z) (Correct answer)
Larger in the z-axis than x-y plane
Round rather than square
Variable depending on reconstruction kernel

Correct answer: Equal in all three dimensions (x, y, z)

Isotropic voxels (e.g., 0.5 × 0.5 × 0.5 mm) allow high-quality MPR and 3D reconstruction in any plane without staircase artifacts.

Question 104: In CT, which body region is most sensitive to radiation-induced cancer risk?

Extremities
Skin
Bone marrow and gonads (Correct answer)
Brain

Correct answer: Bone marrow and gonads

Bone marrow and gonads have the highest tissue weighting factors (wT), making them most sensitive to stochastic radiation-induced cancer risk.

Question 105: What part of the CT gantry measures photons that are transmitted and pass through the patient?

Filters
Collimators
Detectors (Correct answer)
Slip rings

Correct answer: Detectors

Solid state crystalline detectors, such as those made from cadmium tungstate or ceramic scintillators, are widely used in modern CT scanners. They are highly efficient at converting X-ray photons into light, which is then converted into electrical signals. Their high efficiency and fast response times contribute to improved image quality and reduced patient radiation dose.

Question 106: Which CT technique reduces dose by acquiring fewer projections and reconstructing with iterative algorithms?

Low-dose CT with iterative reconstruction (Correct answer)
High-pitch spiral CT
Dual-energy CT
Sequential CT

Correct answer: Low-dose CT with iterative reconstruction

Low-dose CT combined with iterative reconstruction algorithms can reduce dose by up to 80% compared to traditional filtered back projection while maintaining diagnostic quality.

Question 107: A technologist notes that a CT image has a mottled, grainy appearance. Which adjustment would most directly improve this?

Widen the window width
Increase mAs (Correct answer)
Decrease pitch
Increase kVp

Correct answer: Increase mAs

Increasing mAs increases X-ray photon flux, reducing quantum (photon) noise and improving the signal-to-noise ratio of the image.

Question 108: On CT brain, 'sulcal effacement' in one hemisphere most likely indicates:

Calcification of the falx cerebri
Bilateral ventricular enlargement
Normal aging-related brain atrophy
Cerebral edema causing compression of sulci (Correct answer)

Correct answer: Cerebral edema causing compression of sulci

Sulcal effacement occurs when edema or a mass expands the brain hemisphere, compressing and obliterating normal cortical sulci.

Question 109: What is the term for the phenomenon where a single tissue appears with different densities in different CT images due to varying X-ray attenuation angles?

Cupping artifact
Beam-hardening artifact
Streak artifact
Partial volume effect (Correct answer)

Correct answer: Partial volume effect

The partial volume effect occurs when a single voxel (the smallest 3D unit of a CT image) contains more than one type of tissue. The CT number assigned to that voxel will be an average of the attenuation values of all the tissues within it. This can lead to misrepresentation of tissue densities and blurring of small structures, especially when the slice thickness is large relative to the size of the object.

Question 110: What is the recommended lead apron shielding thickness to protect gonads from scattered CT radiation?

0.5 mm lead equivalent (Correct answer)
2.0 mm lead equivalent
No shielding is needed
0.1 mm lead equivalent

Correct answer: 0.5 mm lead equivalent

A 0.5 mm lead equivalent apron is standard for gonadal shielding, though its effectiveness in CT is debated due to scatter from all directions.

Question 111: What type of CT artifact is caused by patient movement during data acquisition?

Cupping artifact
Motion artifact (Correct answer)
Star artifact
Ring artifact

Correct answer: Motion artifact

Motion artifacts appear as blurring, streaks, or ghosting on images when the patient moves during the scan, causing inconsistent projection data.

Question 112: Which intravenous catheter gauge is recommended for power injection during contrast-enhanced CT?

14 gauge or larger
Any gauge is acceptable
24–26 gauge
18–20 gauge (Correct answer)

Correct answer: 18–20 gauge

An 18–20 gauge IV catheter is recommended for power injection; smaller gauges risk rupture at high flow rates, while larger gauges are unnecessary.

Question 113: The DLP (Dose Length Product) in CT is calculated by multiplying CTDI_vol by which parameter?

Scan length (Correct answer)
Rotation time
Slice thickness
Pitch factor

Correct answer: Scan length

DLP = CTDIvol × scan length (in cm), providing an estimate of total energy deposited during the entire CT examination.

Question 114: Which window setting is most appropriate for evaluating pulmonary nodules?

Brain window (WW 80, WL 40)
Bone window (WW 2000, WL 500)
Lung window (WW 1500, WL -600) (Correct answer)
Soft tissue window (WW 400, WL 40)

Correct answer: Lung window (WW 1500, WL -600)

The lung window uses a wide window width and low window level to optimize contrast between air-filled lung parenchyma and soft tissue nodules.

Question 115: Which part of a CT scanner rotates around the patient during the scan?

Gantry (Correct answer)
Console
Detector array
X-ray tube

Correct answer: Gantry

The gantry is the large, donut-shaped component of a CT scanner that houses the X-ray tube and the detector array. During a CT scan, the X-ray tube and detectors rotate rapidly within the gantry around the patient. This continuous rotation allows for the acquisition of X-ray projections from multiple angles, which are then used to reconstruct detailed cross-sectional images.

Question 116: What is the function of the analog-to-digital converter (ADC) in a CT system?

To modulate the tube current based on patient size
To control the rotation speed of the gantry
To amplify the X-ray signal from the tube
To convert the electrical detector signal into digital data for processing (Correct answer)

Correct answer: To convert the electrical detector signal into digital data for processing

The ADC converts the analog electrical signal from the detector into a digital format that can be processed by the reconstruction computer.

Question 117: A patient experiences mild urticaria after CT contrast injection. What is the most appropriate immediate action?

Continue the scan without intervention
Immediately inject epinephrine
Administer diphenhydramine (Benadryl) and observe the patient (Correct answer)
Administer IV corticosteroids and discharge

Correct answer: Administer diphenhydramine (Benadryl) and observe the patient

Mild urticaria (hives only, no systemic symptoms) is treated with antihistamines and close observation while monitoring for progression to a more severe reaction.

Question 118: Which organ is typically used as a reference when setting window level for abdominal CT?

Spleen
Pancreas
Kidney
Liver (Correct answer)

Correct answer: Liver

The liver is commonly used as the window level reference in abdominal CT because it has a consistent, predictable attenuation value of approximately 50-70 HU.

Question 119: What is the primary shielding material used in CT scanning room walls?

Aluminum
Lead or lead-equivalent concrete (Correct answer)
Steel
Fiberglass

Correct answer: Lead or lead-equivalent concrete

CT room walls are constructed with lead or high-density concrete to attenuate primary and scatter radiation to acceptable levels for adjacent areas.

Question 120: At what injection rate (mL/sec) is intravenous contrast typically administered for a CT pulmonary angiography (CTPA)?

Less than 0.5 mL/sec
10–15 mL/sec
3–5 mL/sec (Correct answer)
0.5–1 mL/sec

Correct answer: 3–5 mL/sec

CTPA requires a brisk injection rate of 3–5 mL/sec to achieve adequate pulmonary artery opacification during the brief arterial phase.

Question 121: What is the purpose of CT image post-processing?

To improve the patient's comfort
To reduce the number of images acquired
To increase the radiation dose
To enhance image quality and visualization (Correct answer)

Correct answer: To enhance image quality and visualization

CT image post-processing involves applying various computational techniques to the reconstructed images to improve their diagnostic utility. This includes multiplanar reconstructions (MPR), 3D volume rendering, maximum intensity projections (MIP), and windowing adjustments. These techniques allow radiologists to better visualize anatomical structures, identify pathologies, and tailor the images for specific clinical questions.

Question 122: In CT, the term 'z-axis resolution' refers to resolution along which dimension?

Anterior-to-posterior patient axis
Superior-to-inferior patient axis (Correct answer)
Left-to-right patient axis
Diagonal plane of the gantry

Correct answer: Superior-to-inferior patient axis

The z-axis is the longitudinal axis running head-to-foot along the patient, and z-axis resolution determines the ability to distinguish structures in that direction.

Question 123: What is the significance of the CT dose index (CTDI) metric?

It measures the effective dose to the patient in millisieverts
It quantifies the radiation output of a CT scanner for a standardized scan protocol (Correct answer)
It measures the absorbed dose to the skin surface only
It calculates the contrast-to-noise ratio of an image

Correct answer: It quantifies the radiation output of a CT scanner for a standardized scan protocol

CTDI measures the scanner's radiation output in a standardized phantom and is used to characterize and compare dose output across protocols and scanners.

Question 124: What is the name of the CT artifact that appears as concentric rings centered on the isocenter?

Partial volume artifact
Aliasing artifact
Beam hardening artifact
Ring artifact (Correct answer)

Correct answer: Ring artifact

Ring artifacts are caused by defective or miscalibrated detector elements and manifest as circular bands centered on the gantry rotation axis.

Question 125: Which measure quantifies the overall radiation output of a CT scanner independent of scan length?

Effective dose
SSDE (Size-Specific Dose Estimate)
CTDI (CT Dose Index) (Correct answer)
DLP (Dose-Length Product)

Correct answer: CTDI (CT Dose Index)

CTDI quantifies the radiation output of the CT scanner per rotation, independent of scan length, and is measured in mGy.

Question 126: What is the typical scan direction for most helical CT acquisitions?

Inferior to superior
Alternating with each rotation
Determined by patient positioning only
Superior to inferior (Correct answer)

Correct answer: Superior to inferior

Most helical CT acquisitions proceed from superior to inferior (craniocaudal), though specific protocols may vary for certain applications like runoff studies.

Question 127: What CT finding is pathognomonic (diagnostic) for acute epidural hematoma?

Crescent-shaped hyperdense collection spanning the brain surface
Ring-enhancing lesion with surrounding edema
Biconvex (lens-shaped) hyperdense collection adjacent to the inner table of the skull (Correct answer)
Diffuse brain swelling with effacement of sulci

Correct answer: Biconvex (lens-shaped) hyperdense collection adjacent to the inner table of the skull

An acute epidural hematoma appears biconvex because it is limited by dural attachments at suture lines, and it is hyperdense (bright) due to fresh clotted blood.

Question 128: On CT brain, the normal Hounsfield unit (HU) range for white matter is approximately:

0–10 HU
20–30 HU (Correct answer)
80–100 HU
50–70 HU

Correct answer: 20–30 HU

White matter typically measures 20–30 HU, slightly lower than gray matter (30–40 HU) due to its higher myelin lipid content.

Question 129: What is the primary advantage of dual-energy CT (DECT) over single-energy CT?

Ability to characterize tissue composition based on material decomposition (Correct answer)
Faster scan acquisition time
Lower radiation dose per examination
Superior soft tissue spatial resolution

Correct answer: Ability to characterize tissue composition based on material decomposition

DECT acquires data at two energy levels, enabling material decomposition to distinguish substances like iodine, uric acid, and calcium.

Question 130: In CT fluoroscopy-guided procedures, which group is at highest risk for radiation injury?

The interventionalist's hands if placed in the beam (Correct answer)
Technologists monitoring from the control room
The patient's eyes
Other patients in adjacent rooms

Correct answer: The interventionalist's hands if placed in the beam

CT fluoroscopy uses continuous X-ray exposure, and the interventionalist's hands can receive very high doses if placed near or in the primary beam.

Question 131: What does the size-specific dose estimate (SSDE) correct for compared to CTDIvol?

Patient body size and diameter (Correct answer)
Scan protocol complexity
Contrast agent type
Scanner manufacturer differences

Correct answer: Patient body size and diameter

SSDE adjusts CTDIvol based on the patient's effective diameter to provide a more accurate estimate of actual patient dose.

Question 132: Which contrast injection phase best visualizes hepatocellular carcinoma (HCC)?

Non-contrast
Delayed phase
Arterial phase (Correct answer)
Portal venous phase

Correct answer: Arterial phase

HCC is hypervascular and enhances avidly during the arterial phase (approximately 25-35 seconds after injection) when hepatic artery flow predominates.

Question 133: Colon cancer patient after brain contrast MRI Which diagnosis is more likely?

Two primary brain tumors
Two benign meningiomas
Subarachnoid haemorrhage
Two enhancing metastases (Correct answer)
Two areas of normal calcification

Correct answer: Two enhancing metastases

In a patient with a known history of colon cancer, the presence of new brain lesions that enhance with contrast on an MRI is highly suspicious for metastatic disease. Colon cancer frequently metastasizes to the brain. Enhancing lesions typically indicate a breakdown of the blood-brain barrier, which is common in aggressive tumors like metastases, making this the most likely diagnosis.

Question 134: Which reconstruction algorithm uses a statistical model of the imaging process to minimize a cost function iteratively, achieving better noise properties than FBP?

Maximum-likelihood expectation maximization (MLEM) (Correct answer)
Simple back-projection (SBP)
Filtered back-projection (FBP)
Algebraic reconstruction without penalty

Correct answer: Maximum-likelihood expectation maximization (MLEM)

MLEM iteratively updates the image estimate to maximize the likelihood that the projection data would be observed given the current image, reducing noise and artifacts.

Question 135: What does 'z-axis coverage' refer to in multi-detector CT?

The spatial resolution in the x-y image plane
The total anatomical length imaged per gantry rotation (Correct answer)
The maximum tube voltage achievable
The number of projection views acquired per rotation

Correct answer: The total anatomical length imaged per gantry rotation

Z-axis coverage is the total length along the patient's long axis covered per rotation, determined by the number of detector rows multiplied by individual detector width.

Question 136: Which type of contrast medium is often used in CT imaging to enhance visualization of blood vessels and other structures?

Barium sulfate
Gadolinium
Iron oxide
Iodine-based (Correct answer)

Correct answer: Iodine-based

A Hounsfield Unit (HU) is a quantitative scale used in CT imaging to measure the radiodensity of tissues. It is a standardized unit that relates the attenuation coefficient of a tissue to that of water, which is assigned a value of 0 HU. Different tissues have characteristic HU values, allowing radiologists to differentiate between structures like bone, soft tissue, fat, and air.

Question 137: During a CT pulmonary angiography (CTPA), the technologist observes the pulmonary arteries are not well opacified. What is the most likely cause?

Excessive kVp causing beam hardening
The patient moved during acquisition
Scan initiated too early before contrast reaches the pulmonary arteries (Correct answer)
The reconstruction kernel was set incorrectly

Correct answer: Scan initiated too early before contrast reaches the pulmonary arteries

CTPA timing is critical; scanning before contrast bolus reaches the pulmonary circulation results in non-diagnostic images — bolus tracking or test bolus timing is used to prevent this.

Question 138: Which regulatory body in the United States sets occupational radiation dose limits for CT technologists?

Nuclear Regulatory Commission (NRC) (Correct answer)
Joint Commission
FDA
ACR

Correct answer: Nuclear Regulatory Commission (NRC)

The NRC sets the annual occupational whole-body dose limit of 50 mSv (5 rem) for radiation workers in the US, including CT technologists.

Question 139: In CT scanner design, what is the 'focal spot' of the X-ray tube?

The area of the detector that receives the highest X-ray intensity
The small area on the anode where electrons strike and X-rays are produced (Correct answer)
The center point of the reconstruction matrix
The region of patient tissue receiving the highest dose

Correct answer: The small area on the anode where electrons strike and X-rays are produced

The focal spot is the small area on the rotating anode target where the electron beam strikes, causing X-ray emission; its size directly affects spatial resolution.

Question 140: What is the appropriate action when a patient reports a previous moderate contrast reaction and requires contrast-enhanced CT today?

Premedicate with corticosteroids and antihistamines per protocol and notify the radiologist (Correct answer)
Switch to a higher osmolality contrast agent
Administer contrast immediately with double the dose to speed the exam
Refuse the exam and refer back to the ordering physician without further action

Correct answer: Premedicate with corticosteroids and antihistamines per protocol and notify the radiologist

Patients with prior moderate reactions are at increased risk and should receive premedication (typically oral prednisone plus diphenhydramine) with radiologist awareness before contrast administration.

Question 141: Which post-processing technique in CT creates a three-dimensional display by showing only the highest attenuation voxel along each line of sight?

Volume rendering (VR)
Maximum intensity projection (MIP) (Correct answer)
Multiplanar reformation (MPR)
Minimum intensity projection (MinIP)

Correct answer: Maximum intensity projection (MIP)

Maximum intensity projection (MIP) projects the highest HU value along each ray through the data, making it ideal for visualizing contrast-enhanced vessels and calcifications.

Question 142: Which of the following best describes the dose profile of a helical CT scan with a pitch greater than 1?

Overlapping dose profiles occur
Dose is uniformly distributed
Gaps exist between dose profiles from successive rotations (Correct answer)
Patient dose is eliminated

Correct answer: Gaps exist between dose profiles from successive rotations

A pitch >1 means the table moves faster than one detector width per rotation, creating gaps in the dose profile and generally reducing patient dose.

Question 143: On a CT abdomen, a lesion measures -120 HU. What does this attenuation value suggest?

Simple cyst
Calcified granuloma
Fat-containing lesion (lipoma or angiomyolipoma) (Correct answer)
Acute hemorrhage

Correct answer: Fat-containing lesion (lipoma or angiomyolipoma)

Fat has a characteristic negative attenuation of approximately -50 to -150 HU, so a value of -120 HU strongly indicates a fat-containing lesion.

Question 144: What is the recommended fasting time before IV contrast-enhanced CT in non-emergency patients?

No fasting required
12-hour fast
4 hours NPO for solids (Correct answer)
24-hour fast

Correct answer: 4 hours NPO for solids

A 4-hour fast is recommended before contrast CT to reduce aspiration risk in the event of nausea or vomiting, while preventing dehydration.

Question 145: Which CT parameter, when increased, directly increases patient radiation dose?

Detector row width
Pitch (above 1.0)
FOV
mAs (Correct answer)

Correct answer: mAs

Increasing mAs increases the number of X-ray photons produced, directly increasing the radiation dose delivered to the patient.

Question 146: Which phase of contrast enhancement is best for evaluating hypervascular liver tumors such as hepatocellular carcinoma (HCC)?

Non-contrast phase
Delayed phase (3–5 minutes)
Portal venous phase (60–70 seconds)
Arterial phase (25–35 seconds post-injection) (Correct answer)

Correct answer: Arterial phase (25–35 seconds post-injection)

HCC derives its blood supply primarily from the hepatic artery and enhances dramatically during the arterial phase, a hallmark feature used for diagnosis.

Question 147: Which artifact occurs when an object is positioned partially in and partially out of the scan field of view?

Motion artifact
Metallic artifact
Beam-hardening artifact
Partial volume artifact (Correct answer)

Correct answer: Partial volume artifact

A partial volume artifact occurs when a voxel contains tissues or structures with significantly different attenuation values, such as bone and soft tissue, or when an object is only partially included in the slice thickness. The CT number assigned to that voxel represents an average of these different attenuations, leading to a misrepresentation of the true density and potentially blurring or obscuring small structures.

Question 148: Poor historian, his perplexity has grown over the past few weeks. What does the picture depict?

Brain atrophy without evidence of haemorrhage
Subarachnoid haemorrhage
Left subdural collection
Bilateral subdural collections (Correct answer)
Intraventricular haemorrhage

Correct answer: Bilateral subdural collections

A 'poor historian' and 'perplexity growing over weeks' suggests a chronic or subacute process, which is characteristic of chronic subdural hematomas. These collections of blood or fluid accumulate slowly in the subdural space, often appearing isodense or hypodense to brain tissue on CT, and can cause gradual neurological symptoms. Bilateral collections are common, especially in elderly patients or those with brain atrophy.

Question 149: Which structure is best evaluated using a 'brain window' setting (W:80, L:40)?

Liver lesions
Pulmonary nodules
Gray-white matter differentiation in the brain (Correct answer)
Cortical bone fractures

Correct answer: Gray-white matter differentiation in the brain

A narrow window width centered around 40 HU maximizes contrast between gray matter (~40 HU) and white matter (~30 HU), essential for evaluating brain parenchyma.

Question 150: Which of the following signs may a patient in shock display?

Cyanosis- discolored skin {bluish}
Rapid, shallow breathing
All of the above (Correct answer)
Tachycardia - rapid heart rate

Correct answer: All of the above

Shock is a critical condition characterized by inadequate tissue perfusion, leading to a systemic response. Tachycardia (rapid heart rate) is a compensatory mechanism to maintain cardiac output, while rapid, shallow breathing attempts to increase oxygen intake. Cyanosis (bluish discoloration of the skin) indicates poor oxygenation due to insufficient blood flow. Therefore, all these signs are commonly displayed by a patient in shock.

Question 151: For an adult, the usual range of respiration is

12-20 breaths per minute (Correct answer)
5-10 breaths per minute
35-50 breaths per minute
20-30 breaths per minute

Correct answer: 12-20 breaths per minute

A CT scout image, also known as a topogram or pilot scan, is a preliminary low-dose X-ray image taken before the main CT scan. While primarily used to plan the scan range and ensure proper patient positioning, it can also be utilized for specific measurements, including bone density in certain clinical contexts. It helps the technologist define the exact area to be scanned and optimize parameters.

Question 152: Which term refers to the ability of a CT scanner to acquire images in a very short time interval?

Spatial resolution
Axial resolution
Temporal resolution (Correct answer)
Contrast resolution

Correct answer: Temporal resolution

Temporal resolution in CT refers to the ability of the scanner to capture images of moving structures with minimal motion blurring, essentially how quickly the data for a single image can be acquired. High temporal resolution is critical for imaging rapidly moving organs like the heart, reducing artifacts caused by cardiac motion or patient breathing. It is often achieved with faster gantry rotation times and multi-detector arrays.

Question 153: In CT angiography, what timing method uses real-time monitoring of contrast arrival in a target vessel?

Fixed delay scanning
Bolus tracking (SmartPrep/Care Bolus) (Correct answer)
Saline flush technique
Test bolus technique

Correct answer: Bolus tracking (SmartPrep/Care Bolus)

Bolus tracking monitors a region of interest in real time and automatically triggers scanning when contrast attenuation reaches a preset threshold.

Question 154: In CT of the chest, the 'ground-glass opacity' (GGO) pattern is best described as:

Hazy increased attenuation that does not obscure underlying vessels (Correct answer)
Thickening of bronchial walls only
Complete white-out of the lung with no visible vessels
Air-filled cysts throughout the lung

Correct answer: Hazy increased attenuation that does not obscure underlying vessels

GGO is a non-specific finding of partial air space filling or interstitial thickening where the lung appears hazy but underlying vascular structures remain visible.

Question 155: On CT brain, the normal attenuation of CSF within the ventricles approximates:

20–30 HU
35–45 HU
0–10 HU (Correct answer)
50–60 HU

Correct answer: 0–10 HU

CSF has nearly the same density as water and measures 0–10 HU on CT, appearing dark (hypodense) relative to brain parenchyma.

Question 156: What is the primary advantage of dual-energy CT (DECT) over single-energy CT?

Faster scan acquisition
Lower radiation dose always
Better spatial resolution
Material decomposition allowing tissue characterization (e.g., uric acid vs. calcium) (Correct answer)

Correct answer: Material decomposition allowing tissue characterization (e.g., uric acid vs. calcium)

DECT acquires data at two kVp levels and uses material decomposition algorithms to distinguish substances by their different attenuation characteristics at different energies.

Question 157: What procedure results in a more monochromatic beam and lowers patient radiation exposure by reducing the longest wavelength photons?

Filtration (Correct answer)
Slip rings
Detector collimation
Bean hardening

Correct answer: Filtration

Detectors are the critical components within the CT gantry that measure the X-ray photons that have passed through the patient's body. These detectors convert the transmitted X-ray energy into electrical signals, which are then processed by a computer to reconstruct the cross-sectional images. The number and arrangement of detectors directly impact image quality and scan speed.

Question 158: Dual-energy CT (DECT) can be used to create virtual non-contrast (VNC) images. What does this eliminate for the patient?

Post-processing time
The need for a separate pre-contrast CT acquisition (Correct answer)
The need for IV access
The use of a contrast injector

Correct answer: The need for a separate pre-contrast CT acquisition

VNC images derived from DECT data simulate non-contrast images by mathematically subtracting iodine signal, eliminating a dedicated unenhanced acquisition and reducing dose.

Question 159: What type of adverse reaction to iodinated contrast media involves an immune-mediated hypersensitivity response?

Contrast extravasation
Vasovagal syncope
Contrast nephropathy
Anaphylactoid (allergy-like) reaction (Correct answer)

Correct answer: Anaphylactoid (allergy-like) reaction

Anaphylactoid reactions mimic true anaphylaxis (urticaria, bronchospasm, hypotension) but do not require prior sensitization and are not IgE-mediated.

Question 160: What does ALARA stand for in radiation protection?

Allowable Limit for Any Radiation Application
Automatic Level Adjustment for Radiation Areas
Adjusted Limit of Acceptable Radiation Activity
As Low As Reasonably Achievable (Correct answer)

Correct answer: As Low As Reasonably Achievable

ALARA is the guiding principle in radiation protection meaning doses should be kept as low as reasonably achievable while maintaining diagnostic image quality.

ARRT Computed Tomography (CT) Exam

This exam certifies technologists in the specialized imaging modality of Computed Tomography, demonstrating competency in patient care, safety, and image acquisition.

160

Questions

210 min

Time Limit

75%

To Pass

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