What is the recommended imaging phase for a patient with ascites?

Recommended Imaging Phases for Patients with Ascites

For patients with ascites undergoing CT or MRI evaluation for hepatocellular carcinoma (HCC), multiphase imaging including late hepatic arterial phase, portal venous phase, and delayed phase is essential, though MRI is preferred over CT due to superior lesion detection and the absence of radiation exposure, particularly important given that moderate to severe ascites can cause artifacts on MRI. 1

Multiphase Imaging Protocol

Required Phases for HCC Detection

Both late hepatic arterial and portal venous phases are absolutely necessary for accurate HCC diagnosis on multiphase imaging. 1

The delayed phase is considered essential by most experts and is recommended by UNOS because it:

• Increases conspicuity of HCC's washout and capsular appearance 1
• Helps distinguish HCC from cholangiocarcinoma 1
• Improves sensitivity compared to using only arterial and portal venous phases 2

Specific Timing for CT Phases

• Arterial phase: 30 seconds after contrast injection 2
• Portal venous phase: 68-70 seconds after contrast injection 2
• Delayed phase: 5 minutes after contrast injection 2

Noncontrast Phase Considerations

A noncontrast phase is unnecessary if the patient has not received previous liver treatment. 1

MRI vs CT in Patients with Ascites

MRI is Preferred Despite Ascites-Related Challenges

MRI has superior sensitivity for HCC detection compared to CT:

• For all HCC sizes: MRI sensitivity 59-95% vs CT 43-63% 1
• For HCC >2 cm: MRI sensitivity 100% vs CT 98% 1
• For HCC <2 cm: MRI sensitivity 58-100% vs CT 53-68% 1

Critical Caveat: Ascites and MRI Artifacts

Moderate to severe ascites more frequently causes artifacts on MRI, which is a significant disadvantage. 1 However, this does not negate MRI's overall superiority in lesion detection and characterization, particularly given the absence of ionizing radiation—crucial for patients requiring repeated surveillance imaging. 1

MRI Protocol Components

MRI for HCC diagnosis should include:

• Pre- and post-contrast T1-weighted sequences 1
• T2-weighted sequences 1
• Diffusion-weighted imaging (DWI) for increased lesion conspicuity 1
• Gadolinium-based contrast (extracellular or hepatobiliary agents) 1

Diagnostic Criteria Using Multiphase Imaging

HCC diagnosis requires the following appearance on post-contrast imaging:

• Late hepatic arterial-phase hyperenhancement 1
• Venous- or delayed-phase washout appearance 1
• Venous- or delayed-phase capsule appearance 1

This imaging pattern has nearly 100% specificity and positive predictive value for HCC. 1

Practical Algorithm for Imaging Selection

1. First-line choice: Multiphase MRI with arterial, portal venous, and delayed phases 1
2. If MRI unavailable or contraindicated: Multiphase CT with the same three phases 1
3. If severe ascites causes significant MRI artifacts: Consider CT, but recognize the trade-off of lower sensitivity and radiation exposure 1
4. For repeated surveillance: Strongly favor MRI to avoid cumulative radiation exposure from multiple CT scans 1

Common Pitfalls to Avoid

• Do not omit the delayed phase—it significantly improves diagnostic accuracy for small HCCs and helps differentiate HCC from other lesions. 1, 2
• Do not use CT routinely for repeated HCC screening in patients with chronic liver disease due to cumulative radiation exposure concerns. 1
• Do not assume ascites precludes MRI—while artifacts may occur with moderate to severe ascites, MRI remains superior for lesion detection overall. 1
• Do not rely on two-phase imaging alone—the combination of all three phases (arterial, portal venous, delayed) achieves significantly higher diagnostic performance than two-phase protocols. 2