What is the protocol for a triphasic Computed Tomography (CT) scan?

Triphasic CT Scan Protocol

A triphasic CT scan involves three distinct imaging phases: non-contrast, arterial, and portal venous phases, allowing comprehensive evaluation of tissue perfusion characteristics for optimal detection and characterization of lesions, particularly in the liver. 1

Protocol Components

Timing of Phases

• Non-contrast phase: Initial scan without contrast enhancement 1, 2
• Arterial phase: Acquired 15-25 seconds after contrast injection (typically 22-27 seconds) to capture arterial enhancement 1, 3
• Portal venous phase: Acquired 49-73 seconds after contrast injection to capture portal venous enhancement 1, 3
• Some protocols may include a delayed/equilibrium phase at 8-10 minutes post-injection for further characterization 3

Contrast Administration

• Contrast medium: Non-ionic iodinated contrast agents are typically used 4
• Injection protocol options:
  • Standard bolus: Single injection followed by saline flush 2
  • Split-bolus technique: Two separate boluses of contrast medium 5
    • First bolus (75-90 mL at 2.0 mL/sec): For portal venous phase enhancement
    • Second bolus (60 mL at 3.5 mL/sec): For arterial phase enhancement
    • Each bolus followed by 20 mL saline flush at same flow rate 5
  • For left-sided cardiac valve assessment: Triphasic protocol consisting of contrast bolus, followed by contrast-saline mixture, then pure saline 6

Technical Parameters

• Scanner type: Multidetector-row CT is preferred for optimal resolution 5
• Slice thickness: Thin slices (3-5 mm) are essential for adequate staging accuracy 7
• ECG gating: Recommended for cardiac applications to eliminate motion artifacts 8, 6
• Radiation dose considerations: Split-bolus technique can reduce radiation dose by 35-40% compared to traditional triphasic scanning 5

Clinical Applications

Liver Imaging

• Triphasic CT enables characterization of a wide range of focal liver lesions 3
• Different lesions show characteristic enhancement patterns across the three phases:
  • Hypervascular lesions (e.g., hepatocellular carcinoma) are better detected in arterial phase 9, 10
  • Hypovascular lesions are better visualized in portal venous phase 3, 10
  • Well-differentiated HCCs may remain hyperattenuated in portal venous phase 10

Other Applications

• Thoracic aortic aneurysm/dissection: Triple-phase imaging helps detect endoleaks and evaluate stent placement 2
• Colorectal cancer staging: Multiphase imaging improves detection of liver metastases 7
• Pyelonephritis: Nephrographic phase is most important, but triphasic protocol may be used for comprehensive evaluation 11
• Cardiac valve assessment: Triphasic protocol optimizes visualization of prosthetic heart valves 6

Protocol Variations and Considerations

• Dual-energy CT option: Can potentially reduce radiation dose by generating virtual non-contrast images from contrast-enhanced acquisitions 2, 12
• Patient-specific adjustments:
  • Contrast volume and flow rate may need adjustment based on patient weight and cardiac output 6, 4
  • For renal insufficiency patients, consider alternative imaging modalities or modified protocols 12
• Specialized protocols:
  • For pulmonary valve assessment: Use protocols similar to CT pulmonary embolism studies 6
  • For tricuspid valve: Consider tailored triphasic contrast protocol with decreased flow rate during final phase 6

Common Pitfalls to Avoid

• Inadequate timing of phases can lead to suboptimal enhancement and missed lesions 3, 9
• Failure to use appropriate contrast concentration may result in poor tissue differentiation 4
• Insufficient slice thickness can reduce detection of small lesions 7
• Not accounting for patient-specific factors (weight, cardiac output, renal function) may lead to suboptimal imaging or complications 6, 12