Definition:

• Change from the usual blood supply

LI-RADS Categorization

• Observations thought to definitely represent perfusion alteration should be categorized LR-1.
• Observations thought to probably represent perfusion alteration should be categorized LR-2.
• Observations that are indeterminate for perfusion alteration versus HCC should be categorized LR-3 or higher.

LI-RADS Reporting

• Observations that are easily recognized as definite perfusion alteration (LR-1) or probable perfusion alteration (LR-2), that cause no diagnostic confusion, and that are considered to have little or no clinical relevance do not necessarily need to be reported.
• Radiologists at their discretion may report perfusion alterations. If they are reported, they may be reported in aggregate.
• Exception: Definite (LR-1) or probable (LR-2) perfusion alterations that on the previous examination were reported as LR-3, LR-4, or LR-5 usually should be reported. If they are reported, it may be more appropriate to report them individually rather than in aggregate.
• Rationale: the interval downgrade in category may alter management or prognosis.

Synonyms

• Synonyms: Transient hepatic enhancement difference (THED), Transient hepatic intensity difference (THID), Transient hepatic attenuation difference (THAD)
• Preferred terms:  Perfusion alteration, Transient hepatic enhancement difference (THED)
• Rationale for preferred terms:
  • THED and perfusional alteration are applicable for both CT and MR imaging
  • Perfusional alteration is preferred as it describes the entity, while THED describes the resulting imaging finding

Characteristic imaging features

• Perfusion alterations/THEDs typically show, relative to liver:
• Hyperenhancement in the arterial phase
• Isoenhancement in the portal venous phase and delayed phase
• Isoattenuation at unenhanced CT and isointensity at T2w, DW, and unenhanced T1w MRI
• Perfusion alterations/THEDs may have variable morphologies (wedge-shaped, rounded) and distributions (diffuse, lobar, segmental, peri-tumoral, subcapsular, patchy).
• Perfusion alterations/THEDs are not masses. Hence they exert no mass effect and they preserve the underlying hepatic parenchyma. Undistorted vessels traverse them.
• Multiplanar images (source or reformatted) may help correctly characterize observations as perfusion alterations by showing undistorted vessels, preserved hepatic architecture, wedge shape.

Background:

• Perfusion alterations may be caused by several mechanisms:
  • Regional arterial hyperemia induced by hyper-vascular tumor.
  • Arterio-portal shunting due to cirrhosis, benign or malignant tumor, or arterio-portal fistula. The shunting causes increase in arterial flow to the territory supplied by the portal vein/venule.
    • Shunting due to a macroscopic fistula usually causes a wedge-shaped perfusion alteration.
    • Many arterioportal shunts in cirrhosis are due to tiny arterio-portal communications in the microcirculation. These microcirculatory shunts may cause small perfusion alterations, often nodule-like in configuration.
  • Portal hypo-perfusion due to portal vein obstruction, portal vein invasion, or regional elevation in sinusoidal pressure. Portal hypo-perfusion causes compensatory increase in arterial flow (hepatic arterial buffer response).
  • Anomalous (non-portal) venous inflow. Compared to portal veins, these anomalous veins have a shorter circulatory path from aorta to liver and are fully enhanced in the hepatic arterial phase.
• While perfusion alterations/THEDs are benign, they may be caused by HCC via various mechanisms (regional hyperemia, trans-tumoral arterio-portal shunting, portal vein obstruction/invasion). Hence, perfusion alterations/THEDs should be scrutinized for presence of underlying HCC.
  • In the setting of a geographic or triangular perfusion alteration, look carefully at the apex of the perfusion alteration for evidence of a small mass or portal vein obstruction.

Potential pitfalls and challenges

• While perfusion alterations/THEDs characteristically are hypo-attenuating at unenhanced CT and isointense at T1w and T2w MRI, perfusion alterations/THEDs occasionally show abnormalities on unenhanced imaging and delayed imaging and may not be truly “transient.”  These abnormalities include:
  • Mild hypo-attenuation at CT or mild T1 hypointensity and T2 hyperintensity at MRI (attributed to parenchymal edema)
  • Focal changes in hepatic parenchymal fat content (attributed to altered oxygen and nutrient supply)
• Imaging features that, if present, favor perfusion alterations/THEDs over HCCs include:
  • Isoenhancement to liver in portal venous phase and, if acquired, delayed phase
  • Undistorted vessels traversing the observation
  • Preserved hepatic architecture
  • Absence of mass effect
  • Elongated shape (e.g., along orientation of shunt vessel)
  • Isoattenuation at unenhanced CT and isointensity at T2w, DW, and unenhanced T1w MRI
• <20mm nodule-like areas of hyperenhancement visible only in the arterial phase are known as NAPHs.
  • NAPHs are thought to usually represent either perfusion alterations or small non-malignant masses (e.g., FNH-like lesion, hemangioma, dysplastic nodule), but occasionally small HCC