Imaging and classifying HCC with CT and MR

Hepatocellular carcinoma is currently the only tumor entity that can be diagnosed solely using non-invasive CT and MR imaging. Reliable diagnosis requires multiphasic imaging with precise timing. LI-RADS allows for the standardized estimation of the relative risk for HCC.

According to Guiseppe Brancatelli (University of Palermo, Italy) the hepatocellular carcinoma (HCC) is a “peculiar tumor”, as it is the only tumor for which non-invasive imaging can be used for a definitive diagnosis. In his talk Brancatelli highlighted the cornerstones of liver CT and MRI imaging, presented differences between CT and MRI imaging for HCC detection and explained the value of the LI-RADS classification system.

Multiphasic imaging is key for liver CT and MRI

Liver CT and MR imaging for the detection of HCC relies on differences in enhancement between the liver and HCC. “The greatest differences in enhancement occur during the hepatic arterial phase (HAP) and in the portal venous phase (PVP)”, explained Brancatelli. One crucial parameter for reliable diagnostic imaging of HCC with CT and MRI is the timing of HAP. Brancatelli made clear that maximal enhancement occurs in the late HAP and “contrast also needs to be present in the portal vein”.

Brancatelli illustrated his point with CT images with inadequate timing resulting in no enhancement in the PVP. Brancatelli stressed that “timing is crucial to detect HCC”. To enable the detection of hypovascular lesions a delayed phase, with a three-minute delay after contrast injection (= transitional phase), should be used. “This is important to demonstrate the wash-out”, Brancatelli said.

Hypervascular HCC can be detected using hepatobiliary MRI contrast agents and analyzing the hepatobiliary phase. The increased sensitivity, however, comes at the cost of decreased specificity. “The cornerstone of liver CT and MRI is multiphasic imaging“, Brancatelli concluded.

Differences between CT and MRI

Compared to CT, MRI has a higher technical complexity and is prone to less consistent image quality, mainly due to longer acquisitions times which require longer breath holds and patients holding still. In addition, ascites may also negatively influence image quality.
On the other hand, advantages of MRI over CT include the possibility of recording additional sequences (e.g. T2, DWI) and a higher contrast resolution, which is particularly beneficially for identifying the tumor capsule. Overall, “MRI is slightly more sensitive than CT, especially for lesions <2 cm”, Brancatteli said.
LI-RADS helps to standardize reporting
According to Brancatelli “many guidelines paint a black and white picture of HCC diagnosis with CT and MRI”, focusing mainly on enhancement and wash-out. The LI-RADS system on the other hand comprises 5 categories that denote the likelihood for the presence of HCC. Categories range from LI-RADS 5 (definitely HCC) to LI-RADS 1 (definitely benign).

Brancatelli briefly introduced the main components of the LI-RADS classification system: arterial phase enhancement, nodule size and the presence of additional signs such as washout, tumor capsule and threshold growth. Based on these factors the LI-RADS category can be determined. Using the LI-RADS system the likelihood of HCC can then be reported to the hepatologist in a standardized fashion.

Conclusion

The cornerstone of liver CT and MRI is multiphasic imaging. For diagnosis of HCC using CT and MRT timing of image acquisition is crucial. MRI is slightly more sensitive than CT, especially for lesions <2 cm. Standardized reporting of the likelihood for HCC is possible with the LI-RADS system.