13.1 Multiphase Liver, Pancreas & Adrenal CT

Why This Topic Matters

On the ARRT CT exam, "multiphase" organ protocols are one of the most heavily tested procedural themes because they combine two domains at once: Image Production timing knowledge and Procedures anatomy/pathology recognition. The official CT Content Specifications (implementation date September 1, 2026) list multiphase liver, multiphase pancreas, and multiphase adrenals as three of the eight leaf items under Procedures → Abdomen and Pelvis → Abdomen, and vignette-style items routinely test whether you know which phase answers which clinical question — not just that "contrast was given." Expect scenarios that give you a specific Hounsfield unit (HU) value, a specific phase name, or a specific clinical indication (cirrhosis, jaundice, an incidental nodule) and ask you to pick the correct interpretation or the correct next step.

Core Terms and Phase Timing

Every multiphase organ protocol is built around the same three questions: what does an unenhanced (noncontrast) phase show, what does an arterial-dominant phase show, and what does a venous or delayed phase show. The three organs below share the same underlying physiology — differential arterial versus portal venous blood supply — but use different timing windows and different diagnostic thresholds.

Liver. A dedicated liver CT for lesion characterization typically acquires up to four phases: unenhanced (baseline HU values), a late hepatic arterial phase (roughly 25-35 seconds post-injection), a portal venous phase (roughly 60-70 seconds), and, for hepatocellular carcinoma (HCC) surveillance in cirrhotic patients, a delayed/equilibrium phase (roughly 3-5 minutes). The arterial phase exists because the liver receives about 75-80% of its blood supply from the portal vein and only 20-25% from the hepatic artery — but most liver tumors (HCC, hypervascular metastases) are supplied almost entirely by the hepatic artery, so they enhance early and fade fast relative to background liver. This produces the classic LI-RADS (Liver Imaging Reporting and Data System) pattern used for HCC: nonrim arterial phase hyperenhancement (APHE) followed by nonperipheral "washout" on the portal venous or delayed phase, where the lesion becomes hypodense relative to the now-enhancing background parenchyma. A benign hemangioma shows the opposite temporal signature: peripheral, nodular, discontinuous enhancement on the arterial phase with slow centripetal fill-in on delayed images. Recognizing this pattern is what keeps a benign hemangioma from being sent for an unnecessary biopsy. Typical liver-protocol contrast delivery is 3-5 mL/second at a total volume of roughly 100-150 mL of iodinated contrast (concentration around 300-370 mgI/mL) — injection rate and volume are what make phase timing reproducible from patient to patient.

Pancreas. A "pancreas protocol" CT is a dual-phase study built around one fact: pancreatic adenocarcinoma is hypovascular relative to normal pancreatic parenchyma, so the tumor is most conspicuous exactly when the normal gland is enhancing maximally. That moment is the pancreatic parenchymal (late arterial) phase, roughly 35-40 seconds post-injection, where the tumor appears as a relatively hypodense mass against a brightly enhancing gland. A second portal venous phase (roughly 65-70 seconds) is acquired to evaluate the liver for metastases and to assess the peripancreatic venous structures. Thin collimation (0.5-1 mm) with multiplanar reformation is standard practice, because staging depends on whether tumor contacts a vessel — the superior mesenteric artery (SMA), superior mesenteric vein (SMV), celiac axis, or portal vein — over less than 180° or 180° or more of its circumference. Neutral (low-density, water-based) oral contrast is often preferred over positive oral contrast for pancreas protocol exams specifically so it does not obscure the enhancing peripancreatic vasculature on reformatted images.

Adrenal glands. The adrenal washout protocol exists to answer one question about an incidentally discovered adrenal nodule: is this a benign, lipid-rich adenoma, or does it need further workup for something else (metastasis, pheochromocytoma, adrenocortical carcinoma)? The protocol is unenhanced CT, then a contrast-enhanced phase at 60-70 seconds, then a delayed phase at 15 minutes. Two calculations decide the answer:

Worked example: An adrenal nodule measures 12 HU unenhanced, 92 HU on the 60-second enhanced phase, and 32 HU on the 15-minute delayed phase. Absolute washout = (92 − 32) ÷ (92 − 12) × 100 = 60 ÷ 80 × 100 = 75%. Because 75% clears the 60% threshold, this pattern is consistent with a benign adenoma rather than a metastasis or pheochromocytoma. Historically, technologists were taught to avoid IV contrast in suspected pheochromocytoma without premedication because of concern for a hypertensive crisis; current evidence with modern low- or iso-osmolar nonionic contrast shows this risk is much lower than once believed, but the exam may still test that a known or strongly suspected, unmedicated pheochromocytoma warrants extra caution before injection.

Realistic Exam Scenario

A 64-year-old with known cirrhosis is referred for HCC surveillance. The technologist performs unenhanced, arterial, and portal venous phases. A 2.3-cm lesion shows arterial hyperenhancement and becomes hypodense relative to background liver on the portal venous phase. The exam expects you to identify this as classic washout — a LI-RADS major feature for HCC — and to know that a single-phase (portal-venous-only) acquisition would have missed the diagnostic arterial hyperenhancement entirely, potentially delaying diagnosis. A second scenario: a patient with vague epigastric pain and painless jaundice is referred for a pancreas protocol; the technologist times the acquisition to the late arterial (pancreatic parenchymal) phase specifically because a portal-venous-only scan would show the hypovascular tumor with far less contrast against the normal gland, reducing detection sensitivity for a small lesion.