Multiphase Liver, Pancreas & Renal CT Protocols

Acquisition settings and contrast timings below are illustrative adult protocol examples. Select the authorized protocol for the indication, scanner, body size, access device, and clinical condition. Treatment decisions belong to the responsible clinical team.

Hemodynamic Principles of Abdominal Multiphase CT

Computed tomography of the abdomen and pelvis requires precise synchronization between intravenous contrast media administration and image acquisition. Routine single-phase abdominal CT, commonly acquired in the portal venous phase, often answers a general question. Dedicated phases may be needed for characterizing hypervascular neoplasms, occult parenchymal lesions, vascular encasement, or collecting system pathologies. Multiphase CT exploits the distinct microvascular hemodynamics and differential perfusion rates of abdominal parenchymal organs, tumors, and vascular networks. Technologists preparing for the NMTCB(CT) specialty examination must master the physiological rationale, timing windows, contrast kinetics, and diagnostic interpretation criteria governing multiphase hepatic, pancreatic, and renal examinations.

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The Dual Blood Supply & Hemodynamics of the Liver

The liver is unique among visceral organs due to its dual vascular inflow:

1. Portal Vein ($75\%\text{ to }80\%$ of Total Hepatic Inflow): Transports deoxygenated, nutrient-rich blood from the gastrointestinal tract, spleen, and pancreas to the hepatic sinusoids at a relatively low pressure (approx. $7\text{ to }10\text{ mmHg}$). The portal venous transit is delayed because contrast must first traverse systemic arteries, the splanchnic capillary beds, and mesenteric veins before entering the liver.
2. Hepatic Artery ($20\%\text{ to }25\%$ of Total Hepatic Inflow): Delivers oxygen-saturated arterial blood directly from the celiac axis via the common and proper hepatic arteries at systemic systolic pressures.

Renal enhancement is assessed by comparing consistent unenhanced and enhanced measurements while considering phase, lesion size, noise and pseudoenhancement. Measurable enhancement warrants characterization; it does not independently establish malignancy. Liver and pancreatic phases should be timed for the prescribed task and patient circulation rather than inferred from a universal percentage of tumor blood supply.

Conversely, normal hepatic parenchyma receives the vast majority of its contrast-laden blood via the portal vein. Consequently, when contrast is injected intravenously:

• During the arterial phase, the arterial-fed tumor enhances intensely while the surrounding parenchyma has relatively less enhancement.
• During the portal venous phase, the normal liver parenchyma reaches maximal contrast enhancement as portal blood enters the sinusoids. Meanwhile, contrast rapidly clears from the tumor's dysplastic arteriovenous channels, making the tumor appear conspicuously hypoattenuating ("washout appearance").
• In contrast, hypovascular metastases (e.g., colon adenocarcinoma, breast, lung, and gastric carcinomas) receive minimal arterial supply. They appear most conspicuous as dark, non-enhancing filling defects against the brightly enhancing normal liver during the portal venous phase.

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Multiphase Liver Protocol Timing & Findings

A dedicated liver protocol consists of three or four sequential helical acquisitions through the liver:

1. Pre-Contrast (Unenhanced) Phase

• Technical Objectives: Scan through the entire liver prior to contrast injection. Slice thickness is typically $2.5\text{ to }5.0\text{ mm}$.
• Baseline Attenuation: Normal, unenhanced liver parenchyma demonstrates homogeneous soft-tissue attenuation ranging between $+50\text{ and }+65\text{ HU}$, which is typically $8\text{ to }10\text{ HU}$ higher than the normal unenhanced spleen ($+40\text{ to }+55\text{ HU}$).
• Diagnostic Assessment of Hepatic Steatosis (Fatty Liver): Because lipid has negative attenuation (approx. $-100\text{ to }-50\text{ HU}$), intracellular triglyceride accumulation reduces parenchymal attenuation. Objective CT criteria for hepatic steatosis on unenhanced scans include:
  • Absolute hepatic parenchymal attenuation of $<40\text{ HU}$; OR
  • Hepatic attenuation that is $>10\text{ HU}$ lower than splenic attenuation (spleen minus liver $>10\text{ HU}$).
• Additional Utility: Identifies hyperdense acute hemorrhage, baseline calcifications (granulomas, echinococcal cysts, mucinous metastases), radiopaque gallstones, and provides the baseline $HU$ necessary to calculate true contrast enhancement ($\Delta\text{HU}$). Unenhanced scans also distinguish radiopaque arterial embolization beads from active tumor recurrence in treated HCC patients.

2. Late Arterial Phase ($35\text{--}40\text{ s}$ Delay)

• Timing Differentiation: An "early arterial phase" ($15\text{--}25\text{ seconds}$) shows dense contrast in the celiac axis and hepatic arteries without parenchymal capillary filling (useful only for hepatic arterial CT angiography or surgical roadmapping). The late arterial phase (also known as the hepatic arterial phase, acquired at $35\text{--}40\text{ seconds}$ post-injection or $15\text{--}20\text{ seconds}$ after bolus tracking triggers at $100\text{ HU}$ in the abdominal aorta) captures peak capillary perfusion of hypervascular lesions.
• Pathological Findings:
  • Arterial Phase Hyperenhancement (APHE): Hallmark LI-RADS feature of HCC. The lesion enhances intensely, appearing significantly brighter than the adjacent background liver.
  • Focal Nodular Hyperplasia (FNH): Intense, homogeneous arterial enhancement with a non-enhancing central fibrous scar.
  • Hepatic Adenoma: Robust, early arterial enhancement, frequently homogeneous or with areas of internal hemorrhage.
  • Cavernous Hemangioma: Peripheral, nodular, discontinuous globular enhancement matching aortic blood pool attenuation.

3. Portal Venous Phase ($65\text{--}75\text{ s}$ Delay)

• Timing & Enhancement: Acquired at $65\text{ to }75\text{ seconds}$ post-injection (or $40\text{ to }50\text{ seconds}$ post-trigger). At this stage, contrast has transited the splanchnic bed and floods the portal vein, resulting in peak parenchymal enhancement ($100\text{ to }150\text{ HU}$).
• Pathological Findings:
  • Hypovascular Liver Metastases: Colorectal and other adenocarcinoma metastases appear maximally conspicuous as dark, hypoattenuating lesions surrounded by brightly enhancing liver parenchyma.
  • HCC Washout Appearance: Contrast rapidly clears from the HCC into hepatic venous branches. As the surrounding liver parenchyma reaches peak enhancement, the tumor becomes distinctly hypoattenuating compared to background tissue.
  • HCC Pseudocapsule: A smooth, uniform enhancing rim develops around the lesion in the portal venous or delayed phase, reflecting a compressed fibrous capsule or surrounding sinusoids.
  • Portal Vein Thrombosis: Differentiates bland (non-enhancing) thrombus from malignant tumor thrombus (enhancing vascular invasion with APHE, expanding the vein caliber).

4. Equilibrium / Delayed Phase ($3\text{--}5\text{ min}$ Delay)

• Timing & Hemodynamics: Scanned at $180\text{ to }300\text{ seconds}$ post-injection. Systemic arterial and venous contrast concentrations achieve equilibrium, and contrast redistributes into the extracellular interstitial space.
• Pathological Findings:
  • Definitive HCC Washout & Capsule: Confirms true washout appearance (a major LI-RADS criterion) and delineates the enhancing pseudocapsule against faded parenchyma.
  • Cavernous Hemangioma Centripetal Fill-In: Peripheral globular puddling seen on arterial phases progresses inward (centripetally), resulting in complete or partial uniform fill-in that remains iso-attenuating or hyperattenuating to the blood pool on delayed images.
  • Intrahepatic Cholangiocarcinoma (ICC): Peripheral, irregular arterial enhancement followed by progressive, prolonged delayed enhancement on 3- to 5-minute scans, caused by contrast retention within the dense, desmoplastic fibrous stroma characteristic of bile duct tumors.

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Multiphase Pancreatic CT Protocol

Pancreatic adenocarcinoma carries a dismal prognosis, and computed tomography is the gold standard imaging modality for determining surgical resectability. Resectability hinges on defining tumor relationship to adjacent vascular structures: the celiac axis, common hepatic artery (CHA), superior mesenteric artery (SMA), superior mesenteric vein (SMV), and portal vein (PV).

Protocol Parameters

• Oral Contrast: Neutral contrast (plain water or negative enteral contrast, $500\text{--}750\text{ mL}$ ingested immediately before scanning). Water distends the stomach and duodenal C-loop without causing beam hardening artifacts that would obscure the pancreatic head, ampulla, or peripancreatic arterial calcifications.
• Intravenous Contrast Delivery: High delivery rate of $4.0\text{ to }5.0\text{ mL/s}$ via an 18- or 20-gauge antecubital peripheral catheter. Standard volume is $100\text{ to }150\text{ mL}$ of high-concentration iodinated contrast ($350\text{ to }370\text{ mg I/mL}$).

Assess enhancement, lesion margins, ducts and vascular relationships together. Arterial enhancement and washout can support hepatocellular carcinoma in the appropriate at-risk population using the applicable diagnostic system; they are not universally diagnostic in every patient. Pancreatic tumor contact with arteries and veins must be described for multidisciplinary assessment, because one contact-angle cutoff alone does not establish resectability for all vessels and circumstances.

1. Pancreatic parenchymal phase

A dedicated pancreatic phase, often around 35–50 seconds in an illustrative adult protocol, emphasizes pancreatic enhancement and relevant arterial anatomy. Thin multiplanar images show the pancreatic head within the duodenal C-loop, the body anterior to the splenic vein and the tail toward the splenic hilum. Compare a lesion with background pancreas, look for ductal dilation and describe vessel contact. A venous phase complements this by assessing veins and the metastatic survey. Timing and selected phases follow the supervising protocol.

2. Portal Venous Phase ($65\text{--}70\text{ s}$ Delay)

• Diagnostic Objective: Maximally opacifies the mesenteric-portal venous axis: the superior mesenteric vein, splenic vein, and portal vein confluence.
• Venous Invasion Criteria: Assesses tumor impingement, caliber narrowing, venous contour irregularity, intraluminal tumor thrombus, or occlusion of the SMV and PV. Evaluates whether venous reconstruction is surgically feasible.
• Metastatic Survey: Complete coverage of the liver and peritoneum to detect occult hepatic metastases or peritoneal carcinomatosis.

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Multiphase Renal CT Protocol & Bosniak Classification

Multiphase renal CT is indicated for evaluating gross or microscopic hematuria, staging known renal cell carcinoma (RCC), characterizing indeterminate cystic or solid masses, and conducting CT urography.

2. Corticomedullary Phase ($30\text{--}40\text{ s}$ Delay)

• Hemodynamics: High-rate arterial inflow selectively opacifies the renal cortex and columns of Bertin ($>150\text{ HU}$), while the tubular fluid has not yet reached the renal medullary pyramids, leaving the medulla unenhanced and dark.
• Diagnostic Role: Delineates renal arterial anatomy (accessory or aberrant renal arteries for pre-surgical partial nephrectomy planning) and identifies renal arteriovenous malformations.
• Renal Vein Thrombosis: Optimal phase for detecting tumor thrombus extending from an RCC into the main renal vein or inferior vena cava (IVC).
• Limitation: Inadequate for characterizing renal masses located within or abutting the renal medulla, as unenhanced pyramids mimic hypoenhancing tumors.

3. Nephrographic Phase ($80\text{--}100\text{ s}$ Delay)

• Hemodynamics: Contrast is filtered across glomeruli into the loops of Henle and collecting tubules, producing uniform, homogeneous parenchymal enhancement across both the cortex and medulla ($100\text{ to }120\text{ HU}$).
• The Gold Standard Phase: This is the single most sensitive phase for detecting, delineating, and characterizing renal parenchymal neoplasms. Normal renal tissue enhances homogeneously, maximizing contrast against RCCs.
• Enhancement measurement: compare matched unenhanced and enhanced ROIs. An increase around 20 HU supports measurable enhancement; less than 10 HU generally supports nonenhancement, with intermediate change requiring assessment. Noise, partial volume and pseudoenhancement complicate small-lesion measurements. Enhancement does not prove cancer or automatically require surgery.

4. Excretory / Pyelographic Phase ($5\text{--}15\text{ min}$ Delay)

• Hemodynamics: Contrast is concentrated and excreted into the calyces, renal pelvis, ureters, and urinary bladder.
• Diagnostic Role: Evaluates the urothelium in CT Urography (CTU). Detects filling defects caused by urothelial carcinoma (transitional cell carcinoma), fibroepithelial polyps, ureteral strictures, and papillary necrosis.

Cystic renal masses: Bosniak version 2019

Apply the Bosniak system to an appropriately characterized cystic renal mass, not to every enhancing solid mass. Version 2019 uses morphology and enhancement: a thin wall or septum is at most 2 mm, minimal thickening is 3 mm, and thickening of at least 4 mm or irregular enhancing wall/septa supports category III. Category IIF includes specified minimally thickened or many thin enhancing septa. Enhancing nodules support category IV, with the defined angle and size criteria. The old 3 cm cutoff for an otherwise homogeneous hyperattenuating nonenhancing cyst is not retained as an automatic IIF rule.

An enhancement difference around 20 HU can support measurable CT enhancement when properly measured, but small lesions can show pseudoenhancement and noisy measurements. Enhancement does not by itself prove renal cell carcinoma. Compare unenhanced and enhanced data using consistent ROIs and appropriate phase selection. Categories guide risk assessment; management depends on patient factors and the treating team rather than an automatic surgical instruction in a technical guide.

Renal, adrenal and urinary questions

A noncontrast stone examination depicts calculi in the kidneys and ureters and their secondary obstruction signs without needing an IV bolus. Trace the ureter from the renal pelvis to the bladder and distinguish a ureteric calculus from adjacent vascular calcification or a phlebolith. CT urography adds appropriately selected phases to evaluate renal tissue and the collecting system; an excretory phase emphasizes ureters and urothelial lesions rather than serving as a substitute for retrograde traumatic cystography.

Adrenal CT may characterize a lesion using unenhanced attenuation and, when specifically ordered, delayed enhancement behavior. Washout calculations require the correct acquisitions and cannot classify every lesion in isolation. An adrenal mass should not be biopsied without the clinical team addressing the possibility of pheochromocytoma. Multiphase imaging carries added dose, so use the phases needed for the actual question.

Reference: Bosniak version 2019 proposal.

Routine abdominal assessment and trauma

Routine abdominal CT commonly covers the diaphragmatic domes through the prescribed pelvic endpoint, using a phase chosen for the question. Follow the liver and spleen, pancreas, kidneys, adrenals, bowel, mesentery, aorta and retroperitoneum through consecutive slices. Coronal and sagittal reformations help relate a lesion to organ boundaries and vessels. Do not add every multiphase protocol to a routine examination.

In trauma, maintain resuscitation and immobilization and follow the ordered range and phase plan. Contrast-enhanced data can show organ laceration, devascularization, hematoma and active bleeding. Arterial or delayed phases may be selected for a vascular or urinary-leak question, with added dose weighed against the clinical need. Compare a focus across phases to help distinguish contained vascular injury from spreading extravasation. A noncontrast scan cannot exclude active bleeding merely because no dense focus is visible.

The spleen lies beneath the left diaphragm beside the stomach and left kidney; renal and splenic injuries can share nearby fluid. Trace organ margins and collecting systems rather than attributing all left upper abdominal fluid to one source. For adrenal assessment, identify the right gland near the IVC and the left gland anterior-medial to the kidney. Unenhanced attenuation and selected washout measurements can help characterize a mass, but neither establishes every histologic diagnosis.