Definition:

• The transitional phase is a post-gadoxetate disodium injection time range after the portal venous phase and before the hepatobiliary phase in which liver vessels and hepatic parenchyma are of similar signal intensity and in which both the intracellular and extracellular pools of gadoxetate disodium contribute substantially to parenchymal enhancement.

Usage:

• The term transitional phase applies only to contrast enhancement with gadoxetate disodium.  Hepatocyte uptake with gadobenate dimeglumine is sufficiently delayed that there is no overlap between the extracellular delayed phase and the hepatobiliary phase.
• Refers to a phase of imaging occurring between the arterial/portal venous and hepatobiliary phases

Synonyms:

• Synonyms: transitional phase, late dynamic phase
• Non-preferred term: Equilibrium phase
  • Rationale: gadoxetate disodium is not in equilibrium between the vascular, interstitial, and intracellular compartments during the transitional phase. 

Background: 

• Unlike other agents, gadoxetate disodium does not provide a conventional delayed phase. The reason is that hepatocellular uptake of the agent begins during its first pass through the hepatic sinusoids. Thus by the end of the portal venous phase, considerable hepatocellular uptake has occurred, and from about two to five minutes post-injection, both the intracellular and extracellular pools of gadoxetate disodium contribute substantially to parenchymal enhancement. This two- to five-minute phase, which represents a transition from extracellular-dominant to intracellular-dominant enhancement, is called the transitional phase.
• The transitional phase is routinely obtained in dynamic phase liver imaging using gadoxetate disodium.  Defining this phase of imaging alerts the interpreting radiologist to draw distinction between hypo-intensity on the transitional phase (pseudo-washout) and washout appearance seen on the portal venous phase.

 Potential pitfalls and challenges:

• The interpretation of hypoenhancement in the transitional phase is not yet fully understood.

• Hypoenhancement of an observation in the transitional phase but not in the portal venous phase should not be characterized as washout appearance.
• Rationale:  hypoenhancement of an observation during the transitional phase may be due to relative hyperenhancement of the surrounding liver parenchyma rather than “washout” of the observation. This appearance is considered pseudo-washout appearance. The rationale for this distinction is that many observations (metastases, hemangiomas, cholangiocarcinomas) may be hypointense during the hepatobiliary and transitional phases due to enhancement of the liver parenchyma and lack of transporter expression.