13.2 Renal CT — Multiphase Kidney, Urogram & Stone Protocol

Why This Topic Matters

Renal complaints — flank pain and hematuria — are among the most common indications for abdominal CT in everyday practice, which is exactly why ARRT tests three near-neighbor renal protocols against each other: a multiphase renal mass protocol used to characterize an incidental kidney lesion, a CT urogram used to evaluate hematuria for urothelial pathology, and a noncontrast stone protocol used for acute flank pain. Confusing these is a classic real-world and exam trap — giving IV contrast for a stone-protocol indication, or skipping the delayed excretory phase on a hematuria workup, defeats the entire purpose of the study. The official content outline lists multiphase kidneys, urogram/IVU, and renal stone as three separate leaf items under Procedures → Abdomen precisely because each has a distinct contrast strategy and a distinct diagnostic target.

Core Terms and Protocol Comparison

Multiphase renal CT (renal mass protocol). Built around the kidney's two vascular compartments — the rapidly perfused cortex and the more slowly perfused medulla. The key phases are:

• Unenhanced — baseline HU values; detects calcifications, stones, and hemorrhage before any contrast is given
• Corticomedullary phase (roughly 25-70 seconds, peak around 40 seconds) — the cortex enhances intensely before the medulla does, which makes this phase ideal for vascular mapping (renal artery and vein anatomy) before partial nephrectomy or living-donor evaluation, but a poor phase for detecting small renal masses because normal parenchymal enhancement is itself heterogeneous at this moment
• Nephrographic phase (roughly 85-120 seconds) — cortex and medulla enhance homogeneously; this is the single best phase for detecting and characterizing a renal mass, because any focal region that fails to enhance uniformly with the rest of the parenchyma stands out clearly
• Excretory/delayed phase (roughly 3-5 minutes, extended to 10-15 minutes when urogram-quality collecting system detail is needed) — evaluates the collecting system, ureters, and bladder

An renal lesion is considered truly "enhancing" — and therefore concerning for renal cell carcinoma (RCC) rather than a simple cyst — when its attenuation increases by 20 HU or more compared to its unenhanced value. This threshold underlies the Bosniak classification (categories I, II, IIF, III, IV) used to risk-stratify cystic renal masses for malignancy potential, ranging from a simple benign cyst (Bosniak I) to a lesion that is surgical by definition (Bosniak IV).

CT urogram (CTU). Ordered primarily for hematuria — especially painless gross hematuria in a patient over 40, which is worrisome for urothelial carcinoma until proven otherwise. Rather than optimizing for renal parenchyma, CTU is built to opacify the urothelium: the mucosal lining of the collecting system, ureters, and bladder, so that a filling defect (tumor) can be seen against a contrast-filled lumen. A standard three-phase CTU includes an unenhanced phase, a nephrographic phase (roughly 100 seconds), and an excretory (delayed) phase at 10-15 minutes, sometimes assisted by IV furosemide to promote diuresis, distend the collecting system, and dilute what would otherwise be very dense contrast that can obscure a small filling defect. A split-bolus technique — a first, smaller bolus followed after a delay by a second, larger bolus, timed so the nephrographic and excretory phases are captured on a single acquisition — is a common dose-reduction strategy that turns three separate acquisitions into two.

Renal stone (renal colic) protocol. Performed without IV contrast — the single fact tested most often about this protocol, and for a specific reason: contrast in the collecting system can obscure a small ureteral stone, and dense contrast within the ureter can itself mimic a stone. The scan covers kidneys to bladder (CT KUB) at low dose, since stone formers are frequently young and undergo repeat imaging many times over a lifetime. Beyond simple detection, measuring the stone's HU density helps predict its composition and fragility: uric acid stones tend to measure lower (roughly 200-450 HU) and may respond to medical dissolution therapy, while calcium oxalate or calcium phosphate stones measure higher (often above 600-900 HU) and are more resistant to dissolution, informing whether lithotripsy or another intervention is more likely to be needed.

Realistic Exam Scenario

A 55-year-old presents with painless gross hematuria and no flank pain. The correct study is a CT urogram with delayed excretory-phase imaging, because the clinical concern is urothelial malignancy, not a stone. Ordering only a noncontrast stone protocol here would miss a bladder or ureteral tumor entirely. Contrast this with a 38-year-old who presents with sudden, colicky left flank pain radiating to the groin: the correct study is a noncontrast stone protocol, and giving IV contrast in that scenario is a protocol error that can obscure the very stone the study is meant to find. A third variant: a 3-cm cystic renal lesion is found incidentally on an unrelated scan. To classify it, the correct next step is a dedicated multiphase renal mass protocol with unenhanced and nephrographic phases so its HU enhancement can be measured and it can be assigned a Bosniak category — not simply repeating the original single-phase study.