SPEX Radiology and Imaging Flashcards

A 58-year-old patient with chronic kidney disease (eGFR 28 mL/min/1.73m²) requires urgent CT angiography to evaluate suspected mesenteric ischemia. Which contrast administration strategy best balances diagnostic yield against nephrotoxicity risk?

Answer: Use iso-osmolar iodinated contrast (iodixanol) with pre-hydration using isotonic saline at 1 mL/kg/hr for 3 hours before and after the procedure

Iso-osmolar iodinated contrast (iodixanol) carries the lowest nephrotoxic risk among iodinated agents. Pre- and post-procedure isotonic saline hydration (1 mL/kg/hr) is the only intervention with strong evidence for reducing contrast-induced nephropathy. Gadolinium-based agents are contraindicated below eGFR 30 due to the risk of nephrogenic systemic fibrosis. Unenhanced CT cannot diagnose mesenteric ischemia reliably. N-acetylcysteine alone without hydration is insufficient and its benefit remains controversial in current evidence.

On a T2-weighted MRI of the knee, a meniscal signal abnormality extending to the inferior articular surface is classified as Grade 3. Which additional MRI characteristic most strongly predicts that this finding represents a TRUE tear rather than a pseudotear from magic-angle artifact?

Answer: The signal abnormality persists on proton-density sequences with echo time (TE) greater than 20 ms

Magic-angle artifact occurs when collagen fibers are oriented at approximately 55 degrees to the main magnetic field (B0), producing spurious T2 shortening that can mimic grade 1–2 signal. Critically, magic-angle effect diminishes on sequences with TE > 20 ms (longer echo times), because the T2* relaxation of the artifact is very short. If the abnormal signal persists on proton-density sequences with TE >20 ms, it is far more likely to represent true pathology. The location in the posterior medial meniscus increases clinical suspicion but is not a discriminating MRI characteristic for artifact vs. tear.

During fluoroscopic-guided lumbar facet joint injection, the physician notes unexpected contrast spread into an elongated paravertebral structure rather than pooling within the joint capsule. Which pattern of inadvertent injection has occurred, and what is the primary management concern?

Answer: Intravascular injection into a segmental artery; risk of spinal cord infarction if particulate steroid is injected

Contrast outlining an elongated, tubular, flowing structure rather than a focal articular pool during facet injection indicates intravascular uptake, most likely a segmental artery or vein. The critical danger is injection of particulate corticosteroids (e.g., triamcinolone, methylprednisolone acetate) which can embolize via anastomotic connections to the radiculomedullary artery (artery of Adamkiewicz) causing spinal cord infarction and permanent paralysis. Non-particulate steroids (dexamethasone) or aborting the procedure are the appropriate responses. Capsular tears cause epidural spread (not elongated tubular pattern), and discitis/perineural patterns have distinct appearances.

A radiologist reviews a Tc-99m MAA (macroaggregated albumin) lung perfusion scan for a patient with suspected pulmonary embolism. Perfusion defects are present bilaterally in a segmental distribution. However, the ventilation scan (Xe-133) shows matched ventilation-perfusion abnormalities in the same regions. Which additional scintigraphic finding would UPGRADE this study from INTERMEDIATE to HIGH probability for PE according to PIOPED II criteria?

Answer: Two or more large segmental perfusion defects with normal ventilation in the corresponding lung zones

PIOPED II high-probability criteria require ≥2 large segmental perfusion defects WITHOUT matched ventilation defects (V/Q mismatch). If ventilation is normal in regions with perfusion defects, this is a classic mismatch pattern consistent with high-probability PE. A single large matched defect is intermediate. Subsegmental matched defects on a normal CXR remain intermediate. The stripe sign indicates that perfusion is partially preserved peripherally adjacent to a defect, which actually DECREASES the probability of PE by suggesting alternative pathology like atelectasis, not increasing it.

A 45-year-old woman undergoes breast MRI for high-risk screening. A 12 mm enhancing lesion demonstrates rapid initial enhancement followed by a washout kinetic curve on DCE-MRI. Its T2 signal is markedly hypointense. Based on these combined morphologic and kinetic features, what is the most likely pathologic diagnosis, and why does the T2 signal narrow the differential?

Answer: Invasive lobular carcinoma; T2 hypointensity reflects dense fibrotic stroma reducing water content and correlates with poor tumor conspicuity on conventional imaging

Invasive lobular carcinoma (ILC) classically presents on DCE-MRI with rapid initial enhancement and washout kinetics. Critically, ILC has characteristically low T2 signal due to its desmoplastic, fibrotic stroma with densely packed malignant cells and minimal extracellular free water. This T2 hypointensity distinguishes ILC from most benign lesions (fibroadenomas, lymph nodes, and cysts are typically T2 hyperintense) and from mucinous or medullary carcinomas (which are T2 bright). Fibroadenomas with washout kinetics are possible but are typically T2 hyperintense, not hypointense. Fat necrosis has variable T2 signal and characteristic T1 fat signal.

During interpretation of a Tc-99m sestamibi myocardial perfusion SPECT study with pharmacologic stress (adenosine), a patient demonstrates a fixed perfusion defect in the inferior wall. The post-processing attenuation correction (AC) images show near-normalization of this defect. Which statement BEST characterizes the appropriate next step and the imaging pitfall encountered?

Answer: Prone imaging should be performed to confirm that the inferior defect on non-AC images is attenuation artifact before concluding the study is normal

A fixed inferior wall defect that normalizes on attenuation-corrected (AC) images is a classic presentation of diaphragmatic attenuation artifact, but AC algorithms themselves can introduce artifacts if registration between CT (used for AC) and SPECT is imperfect. The recommended confirmatory step is prone imaging: repositioning the patient prone shifts the diaphragm, and if the inferior defect disappears or markedly diminishes on prone images, attenuation artifact is confirmed. This avoids false-positive diagnoses of inferior MI and unnecessary catheterization. Accepting AC normalization alone without confirmation is insufficient. Tl-201 redistribution is used for viability assessment but is not the first-line response to this specific diagnostic ambiguity.