Definition

• Intensity in the transitional phase that unequivocally is less than that of the surrounding liver

Usage

• An observation that is not definitely or probably benign but is hypointense in the transitional phase and isointense on all other sequences should be categorized LR ≥ 3. The presence of transitional phase hypointensity should prompt a careful review of all other pre- and postcontrast sequences to identify subtle imaging features that might have been missed initially.

• Ancillary feature that favors malignancy and may be used to upgrade an observation (up to but not beyond LR-4)

If unsure whether an observation in the transitional phase is hypo-intense:

• Do not characterize as transitional phase hypointensity

Synonyms:

• Synonyms: transitional phase hypoenhancement

• Preferred terms: transitional phase hypointensity

• Rationale for preferred terms: Feature is descriptive of intensity relative to background liver parenchyma and may not truly represent the ability of the lesion to take up or retain hepatobiliary contrast agents.

Background:

• Transitional phase hypointensity is an ancillary feature that favors malignancy and may be used to upgrade an observation (up to but not beyond LR-4).

◦ Rationale: After the portal venous phase, the hepatic parenchyma continues to enhance progressively due to take up of gadoxetate disodium by hepatocytes. The relative hypointensity of an observation in the transitional phase may be due to rapid transit of contrast, lack of functional hepatocytes, or a combination of the two. Given this uncertainty, transitional phase hypointensity does not have the same diagnostic implication as washout appearance and does not constitute a major feature. Transitional phase hypointensity is not specific for HCC and can be seen with hemangiomas, non-HCC malignancies, some dysplastic nodules, siderotic nodules, nodular or confluent fibrosis, some cases of focal fat deposition, and some perfusion alterations.

• Transitional phase hypointensity usually occurs in conjunction with hepatobiliary phase hypointensity. That is, most observations with transitional phase hypointensity also have hepatobiliary phase hypointensity.

Potential pitfalls and challenges:

• Transitional phase hypointensity is not equivalent to washout appearance

◦ Washout appearance is a major feature used to categorize LR-3, LR-4, and LR-5 observations and should be assessed only during the extracellular phase of contrast (portal venous or delayed phase) prior to significant uptake of hepatobiliary agents within the hepatocytes. The phases appropriate for assessing washout appearance depend on the gadolinium-based contrast agent as described below:

o For gadoxetate disodium, the phase appropriate for assessing washout appearance includes only the portal venous phase; it does not include the transitional phase or hepatobiliary phase

o For gadobenate dimeglumine, the phases appropriate for assessing washout appearance include the portal venous and delayed phases; it does not include the hepatobiliary phase.

Rationale: Gadobenate dimeglumine has only mild hepatocellular uptake. Although this mild hepatocellular uptake permits hepatobiliary imaging at a delay of about 1-3 hours, it has negligible impact on enhancement of liver and liver observations during the dynamic phases after administration of gadobenate dimeglumine.

o For extracellular gadolinium-based contrast agents, the phases appropriate for assessing washout appearance include the portal venous and delayed phases.

Rationale: These agents have negligible hepatocellular uptake.