Triple-Phase CT Scan for Focal Hepatic Lesions
A triple-phase CT scan is a specialized, multi-phasic imaging technique used to evaluate focal liver lesions. It takes advantage of the liver's dual blood supply from the hepatic artery (approximately 25%) and the portal vein (approximately 75%) to characterize the vascularity of lesions and differentiate them from normal liver parenchyma. Most primary and secondary liver tumors receive their blood supply predominantly from the hepatic artery.
The protocol typically includes three distinct phases of imaging after the administration of intravenous contrast: a non-contrast phase may also be included, making it a "four-phase" protocol in some centers.
Scan Protocol and Technique
The patient is positioned supine with arms above their head. A bolus injection of non-ionic contrast material is administered at a high flow rate (3-5 mL/s), often followed by a saline flush to optimize contrast enhancement. Bolus tracking, which monitors the contrast density in a target vessel (e.g., the aorta), is used to time the acquisitions accurately.
The three main phases are:
- Late Arterial Phase: This phase is acquired approximately 35-45 seconds after contrast injection. During this time, the hepatic artery is fully opacified, and hypervascular lesions, which are mainly supplied by this artery, will "light up" brightly. The portal vein may begin to enhance, but the hepatic veins should not be enhanced yet.
- Portal Venous Phase: This is the routine phase for most abdominal CT scans, typically acquired at 60-75 seconds after contrast injection. The liver parenchyma, which receives most of its blood supply from the portal vein, shows maximum, homogeneous enhancement. In this phase, hypovascular lesions are best evaluated as they appear hypoattenuating (darker) compared to the surrounding enhanced liver tissue.
- Delayed Phase (or Equilibrium Phase): Images are acquired 2-5 minutes after contrast injection. This phase shows how the contrast is distributed and retained within the lesion and the normal liver tissue. Some lesions, such as those with a fibrous component, may show persistent or delayed enhancement, while others may demonstrate "washout" of contrast.
Triple-Phase CT Findings in Hepatocellular Carcinoma (HCC)
Hepatocellular carcinoma (HCC) is a primary liver malignancy that often arises in the setting of chronic liver disease, particularly cirrhosis. The characteristic imaging features of HCC on a triple-phase CT scan are based on its unique blood supply and a process called "washout".
- Non-contrast Phase: The lesion may appear as a hypodense nodule. This phase is useful for detecting features like fat or calcification within the tumor.
- Late Arterial Phase: The hallmark feature of HCC is arterial phase hyperenhancement (APHE). The tumor, being primarily supplied by the hepatic artery, shows strong, often heterogeneous enhancement, making it appear brighter than the surrounding liver parenchyma.
- Portal Venous Phase: HCC typically demonstrates "washout" in the portal venous phase. This means the lesion becomes iso- or hypoattenuating (fades to a similar or lower density) compared to the now-enhanced liver parenchyma. This is a key diagnostic feature of HCC.
- Delayed Phase: The washout effect typically persists or becomes even more pronounced in the delayed phase, with the HCC lesion appearing distinctly hypoattenuating relative to the liver. Some HCCs may also show an enhancing "capsule" or rim in this phase.
The ability of multiphasic CT to reliably detect and diagnose HCC in cirrhotic livers is based on these characteristic enhancement patterns.