Computed Tomography (CT)

Computed tomography is an imaging technique that can be used for the examination of the liver. The helical-CT mode (rotation of image acquisition apparatus around the patient) and multislice technique can be considered as the standard examination technique in today’s clinical practice using iodinated contrast agents.

Contrast-enhanced CT of the liver (HCC).

Multi-slice helical-CT (CT)

In comparison to sequential mode CT, helical-CT has three major advantages:

The continuity between the different slices prevents the risk of non-detection of small focal lesions.
The ‘time window’ for the performance of one helical is short enough to perform multiple acquisitions in different phases of contrast enhancement (i.e. arterial, portal-venous, venous).
The acquisition of data in a helical mode offers the possibility of secondary reconstruction (i.e. multiplanar, 3D).

Additionally to the above mentioned advantages CT allows reduction of the amount of contrast agent needed and suppression of motion artefacts. CT has had great impact in the evaluation of the liver (Hamer et al., 2007). The early arterial phase acquired after injection of iodinated contrast agent has been demonstrated to be particularly useful for the depiction of hepatic arteries in a mapping of vascular architecture of the liver (Kim et al., 2007).
Evaluation of multiphasic images (early arterial, late arterial, and portal-venous phase)¹ have been shown to increase accuracy in detection of HCC (Kim et al., 2007).

Benefits	Drawbacks
Broad availability Short examination time and a wide clinical acceptance MS-CT, entire image acquisition in a short period of time Multiplanar reconstruction of helical CT datasets are possible	Lower sensitivity and specificity for lesion detection and characterization compared to MRI Iodinated contrast agents can be contraindicated Higher false-positive rate

¹Different phases of contrast-enhanced (CE) liver imaging

The liver has a different blood supply as compared to all other organs. The arterial blood is supplied by the hepatic artery (approximately 20% of total blood supply), a branch of the celiac axis (Truncus coeliacus) and venous blood from the mesenteric and splenic veins is supplied by the portal vein (approximately 80%). Blood drains into the hepatic veins. Contemplation of this dual blood supply is mandatory for the understanding of the different phases of CE dynamic liver imaging.

Arterial phase: When a contrast agent is injected intravenously, the hepatic arterial system is enhanced during the first pass, whereas only minimal enhancement is registered in the parenchyma. These arterial or presinusoidal phase images (acquired approximately 15-25 sec after injection) are especially important for the detection of hypervascular malignancies of the liver, i.e. lesions which are predominantly supplied by the hepatic artery (e.g. HCC, FNH), and provide important information on the character of the lesion.

Portal-venous phase: Images of this phase of CE liver imaging are acquired approximately 40-70 sec after administration of the contrast agent, when the portal vein system and the hepatic veins are enhanced. These images show maximal parenchymal enhancement and maximal contrast differentiation between liver and hypovascular lesions.

Equilibrium phase: Images of this phase are acquired 80-120 sec after injection of contrast agent, when it has already diffused into the interstitium. Contrast enhancement in the equilibrium phase is particularly prominent in edematous tissues, such as neoplasms and areas of inflammation.