CECT Abdomen-Pelvis: Triple Phase CT

Why Triple Phase?

• Hepatic artery supplies ~20-25% of liver blood flow
• Portal vein supplies ~75-80% of liver blood flow

The Four Phases (Non-contrast + Triple)

Phase 0: Non-Contrast (Unenhanced) Phase

• Calcifications in the pancreas, kidneys, gallbladder, aortic wall
• Hemorrhagic lesions appear hyperdense
• Fat-containing lesions (angiomyolipoma, fatty liver) are identified
• Baseline for measuring enhancement (enhancement = contrast HU - non-contrast HU)

Phase 1: Arterial Phase (Late Arterial / Hepatic Arterial Phase)

• Hypervascular lesions - these enhance brightly because they derive blood supply from the hepatic artery: HCC (hepatocellular carcinoma), hypervascular metastases (RCC, neuroendocrine tumors, melanoma, thyroid, breast), focal nodular hyperplasia (FNH), hepatic adenoma, hemangioma (peripheral nodular enhancement)
• Arterial anatomy - pre-operative vascular mapping, anomalous vessels, aneurysms
• Active arterial bleeding (trauma)
• Pancreatic ductal adenocarcinoma (hypovascular against enhancing parenchyma)
• Endocrine tumors of the pancreas (hypervascular)

Key teaching point: On the arterial phase, the portal vein is NOT yet opacified. This distinguishes it from the portal venous phase. The hepatic veins are also not visible.

Phase 2: Portal Venous Phase (Hepatic Venous Phase)

• Hypovascular lesions - these appear hypodense (dark) against a now-brightly enhancing liver background: hypovascular metastases (colorectal, lung, gastric), liver cysts, abscesses
• Portal vein thrombosis
• Solid organ injuries (liver, spleen, kidney lacerations)
• Bowel wall enhancement and thickening (mesenteric ischemia, inflammatory bowel disease)
• Peritoneal and mesenteric pathology
• HCC "washout" begins to appear (lesion appears relatively hypodense compared to peak parenchymal enhancement)
• Best phase for staging most abdominal malignancies

Phase 3: Delayed Phase (Equilibrium Phase / Excretory Phase)

• HCC washout appearance - HCC lesion appears hypodense with a "capsule" enhancement (very characteristic)
• Cholangiocarcinoma - fibrous tumors with delayed progressive enhancement (hallmark)
• Hemangioma - complete centripetal fill-in (becomes isodense or hyperdense to liver)
• Fibrous tissue / scars - enhance progressively on delayed phase (FNH central scar)
• Urographic phase (subset) - when done at 7-10 min, collecting system, ureters and bladder are opacified - used for urothelial tumors, transitional cell carcinoma
• Post-ablation or post-embolization residual tumor
• Lymph node characterization
• Peritoneal deposits

Comparison Table

Clinical Indications for Triple Phase

Contrast Technique

• Contrast volume: 80-120 mL iodinated contrast (~300-370 mg I/mL)
• Injection rate: 3-5 mL/sec (faster for arterial phase quality)
• Timing method: Bolus tracking (most reliable for arterial phase - trigger at aortic HU ~100 HU above baseline) or fixed delay
• Radiation: Triple phase triples radiation exposure compared to single-phase; always use minimum phases clinically required

Organ-Specific Phase Logic: Kidneys

1. Corticomedullary phase (~35-40 sec) - cortex enhances brightly, medulla less so
2. Nephrographic phase (~80-120 sec) - uniform enhancement of entire renal parenchyma - best for detecting renal masses
3. Excretory/pyelographic phase (~3-7 min) - contrast in collecting system, ureters, bladder

• Sabiston Textbook of Surgery (Sabiston, p.1826)
• Bailey & Love's Short Practice of Surgery, 28th Edition
• CT Techniques & Protocols in Liver Disease - PMC
• Abdominal CT Phases - LITFL Radiology Library

CECT Abdomen-Pelvis Triple Phase - Technologist's Protocol Guide

Pre-Scan Checklist (Your Job Before You Start)

• IV access: 18G or 20G cannula - antecubital fossa preferred. Test with 10 mL saline flush at fast rate manually before injecting contrast. 18G minimum for 4-5 mL/sec injection rate.
• Creatinine / eGFR: Verify before contrast. If eGFR <30, flag to radiologist. Hold metformin 48 hrs post-contrast if eGFR <45.
• Allergy history: Previous contrast reactions? Pre-medication protocol if needed.
• Fasting: 4-6 hours for elective studies (reduces motion artifact from peristalsis, reduces aspiration risk if reaction occurs).
• Hydration: Oral or IV hydration especially in diabetics and renal impairment patients.
• Oral contrast: Water (neutral) 1000 mL, 20-30 min before scan for bowel opacification. Positive contrast (dilute barium/iodine) for abscess/fistula cases.
• Patient position: Supine, arms raised overhead (reduces beam-hardening artifact through the liver). Feet-first or head-first per department preference.
• Breath-hold instruction: Coach the patient - all phases are in inspiration breath-hold. Practice before scanning. A bad breath-hold ruins the arterial phase.

Contrast Injection Parameters

Why injection rate matters so much: The arterial phase window is only about 10-15 seconds wide. If you inject too slowly, the contrast bolus is spread out and you miss the peak arterial enhancement. Faster injection (4-5 mL/sec) = tighter, denser bolus = better arterial phase. The portal venous phase is more forgiving - even 3 mL/sec works.

Bolus Tracking - How You Trigger the Scan

Setup at the Console:

1. Place your ROI (Region of Interest) on the descending aorta at the level of the diaphragm / T12-L1 level
2. Set your trigger threshold at 100-150 HU above baseline (most departments use 150 HU)
3. Start the monitoring low-dose scan (single slice, repeated every 1-2 seconds, very low dose)
4. Inject contrast → watch the HU value rising on your screen
5. When aorta hits your threshold HU → scanner auto-triggers OR you manually trigger

Scan Delays After Trigger:

Practical note: The portal venous phase timing is calculated independently from the arterial phase. Most scanners let you set it as a fixed delay from the injection start (70-80 sec) regardless of when the arterial phase triggered. The delayed phase is always a fixed clock delay from injection start.

Scan Parameters at the Console

Dose tip: Some departments reduce the noise index (increase dose) on the unenhanced phase to 3x higher noise (lower dose) since it is primarily used only as a baseline comparator. The arterial and portal venous phases need the best image quality.

What You Will See on Each Phase - Console/Viewer Checklist

Phase 0: Non-Contrast

• All organs are grey/uniform, no vessel-organ differentiation
• No bright vessels
• What to check: Any hyperdense lesion here (hemorrhage, dense stone, calcification) - if you see a bright white structure, it's either calcium or blood, not contrast
• PACS window: soft tissue window (W:400 / L:40)

Phase 1: Arterial Phase (~35-40 sec)

• Aorta is bright white (should be >200-250 HU)
• Portal vein is NOT yet opacified (this is your quality check - if the portal vein is already brightly enhancing, you're too late; you missed the arterial window)
• Hepatic veins NOT visible
• Liver parenchyma is minimally enhanced (only 20% arterial supply)
• Celiac axis, SMA, renal arteries well-defined for vascular mapping
• Pancreatic parenchyma enhances brightly (pancreas is highly arterially supplied - great window for pancreatic ductal carcinoma which appears hypodense)
• Kidneys: cortex enhances brightly, medulla is still dark = corticomedullary differentiation

Tip: If the portal vein is already full on your so-called "arterial" phase, your bolus trigger threshold was too low or your patient had fast circulation. You've essentially got an early portal phase. Inform the radiologist.

Phase 2: Portal Venous Phase (~70-80 sec)

• Portal vein is bright (your quality check)
• Hepatic veins now visible and filling
• Liver parenchyma is at maximum brightness - this is when the liver looks "full" of contrast
• Aorta is less dense than arterial phase (this confirms you're not still in arterial phase)
• Bowel wall enhancement visible
• Spleen shows maximum enhancement
• Kidneys: entire parenchyma (cortex + medulla) uniformly enhanced = nephrographic phase - best for detecting renal masses

This is the most used single phase in abdominal CT. If your department only does single-phase abdomen, it is always the portal venous phase.

Phase 3: Delayed Phase (3-5 minutes)

• All vessels become faint - contrast has redistributed into interstitium
• Liver parenchyma less enhanced than portal phase
• Collecting systems opacified - if you wait 7-10 minutes this becomes the urographic phase (ureters, bladder visible)
• Fibrotic tissue appears relatively brighter than surrounding liver (because fibrous tissue retains contrast while normal liver washes it out) - this is how you spot cholangiocarcinoma
• HCC: lesion that was bright on arterial phase should now appear dark (washout) - confirms HCC diagnosis
• Hemangioma: contrast has filled in completely, lesion now isodense or slightly hyperdense to liver (centripetal fill-in)

Scan Coverage Per Phase

Common Technologist Errors and How to Avoid Them

Organ-Specific Protocol Variants You'll Encounter

Radiation Dose Awareness

• Use auto-mA / tube current modulation (ATCM) on every phase
• Use iterative reconstruction (ASIR, ADMIRE, AIDR) - allows lower mAs with acceptable noise
• Reduce dose on non-contrast phase - use a higher noise index (e.g., NI 59 vs. NI 22) since it's only a baseline comparator, not the primary diagnostic phase
• Justify triple phase: it is ordered for a reason. If a routine staging case has been ordered as triple phase without clear indication, check with the radiologist before scanning