FAQ-049: Why is the late arterial phase strongly preferred? And what is the optimal scan delay?
Late arterial phase is strongly preferred, because HCC enhancement is usually higher in the late than in the early arterial phase and some HCCs show enhancement only in the late arterial phase. The optimal delay depends on the modality (CT/MRI); type, volume and concentration of contrast agent; rate of contrast injection; timing method (fixed/bolus tracking/timing run); and number and time to K-space center of arterial phases (for MRI).
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FAQ-050: How do I gauge if liver enhangement is adequate during the hepatobiliary phase (HBP)?
Liver enhancement during the HBP is adequate if the parenchyma is unequivocally hyperintense relative to hepatic blood vessels. It is suboptimal otherwise. The mechanism for suboptimal HBP enhancement is not well understood but probably reflects reduced number of functional hepatocytes or dysfunctional cellular transport mechanisms. Pitfall: visible excretion of gadoxetate into bile duct does not indicate adequate liver enhancement.
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FAQ-051: If HBP is suboptimal, should I delay the acquisition or increase the flip angle?
Delaying the HBP acquisition may improve image quality in cirrhotic livers with diminished function, but has unknown impact on diagnostic accuracy. Increasing the flip angle improves lesion-to-liver contrast-to-noise ratio for metastases in normal livers, but has unknown impact on image quality and diagnostic accuracy in cirrhotic livers with diminished function.
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FAQ-052: If liver enhancement during the HBP is suboptimal, how do I characterize observations that are hypointense, isointense, or hyperintense relative to liver?
See Imaging Features
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FAQ-053: What modality and contrast agent type does LI-RADS recommend for diagnosis or staging?
LI-RADS provides guidance on proper imaging technique for each modality (CT, MRI) and agent (ECA, HBA) but does not recommend any particular modality or agent. The choice of modality and agent depends on patient preference, tolerance, and safety; numerous factors that may affect image quality or exam feasibility; prior imaging modality and agent; and institutional and radiologist expertise. Radiologists are encouraged to tailor the modality and agent to the individual patient.
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FAQ-054: What about for monitoring treatment response?
Although treatment response may be monitored with CT or MRI and with any contrast agent:
- MRI may be preferable to CT after iodized oil-TACE because high-density oil within an embolized tumor may obscure residual or recurrent tumoral enhancement
- MRI with an extracellular agent may be preferable to MRI with gadoxetate, which is prone to arterial phase motion artifacts. In patients in which detection of new lesions is considered more important than assessing response of treated lesions, gadoxetate-MRI may be appropriate.
Radiologists should tailor the modality and agent to the individual patient and applied treatment.
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FAQ-055: Can I use LI-RADS for interpretation and reporting if recommended images are omitted?
Yes. A specific LI-RADS category often can be assigned even if recommended images are omitted. For example, an exam with only arterial and delayed phase images would allow confident LR-5 categorization if those images depicted a mass with APHE, “washout”, and “capsule”.
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FAQ-056: Are LI-RADS technical recommendations for CT and MRI consistent with OPTN?
Yes.
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FAQ-057: Why does LI-RADS use “transitional phase” rather than “delayed phase” for gadoxetate?
For gadoxetate-enhanced MRI, the period from 2-5 minutes postinjection represents a transition from extracellular-dominant (i.e., portal venous phase) to intracellular-dominant (i.e., hepatobiliary phase) enhancement, and so is termed the “transitional phase”. During this period, both the intracellular and the extracellular pools of gadoxetate contribute substantially to parenchymal enhancement. This is fundamentally different from the conventional delayed phase using other agents, where enhancement reflects extracellular distribution of contrast material.