Genitourinary Imaging
Core framework
GU questions concentrate on renal mass characterization, stone disease, and prostate/uterine imaging. The Bosniak classification (2019 update) is essential: I and II are benign (II includes homogeneous high-attenuation cysts <3 cm and mildly thick septa); IIF requires follow-up; III and IV require surgery or ablation. Key 2019 changes: IIF now requires measurable enhancement or thicker septa/wall, and the "calcified nodule" criterion was removed.
Renal masses
Angiomyolipoma with macroscopic fat is diagnosed by negative HU (-20 or lower) on unenhanced CT; a fat-poor AML is a mimic. Renal cell carcinoma subtypes: clear cell is hypervascular and enhances avidly; papillary is hypovascular and homogeneous; chromophobe is intermediate with a spoke-wheel pattern. The RENAL nephrometry score quantifies surgical complexity. Transitional cell carcinoma arises in the collecting system and shows a filling defect on excretory phase.
Stones and prostate
Noncontrast CT is the test of choice for urolithiasis; know secondary signs (hydronephrosis, ureteral stranding, tissue rim sign) and the location of stone impaction at three narrowing points. For prostate MRI, use PI-RADS v2.1: score 4–5 lesions in the peripheral zone are diffusion-restricted with low ADC; transition zone lesions rely more on T2. The dominant sequence drives the score.
Traps and rule of thumb
Traps: assigning Bosniak III to a nonenhancing high-attenuation cyst; confusing a parapelvic cyst with hydronephrosis; calling a fat-poor AML an RCC without biopsy; forgetting that DWI drives peripheral-zone PI-RADS.
Rule of thumb: enhancement is the discriminating variable in Bosniak - if a cyst enhances measurably, it is at least IIF; if it has solid enhancing components, it is IV.