Role of Advanced MRI Techniques in Characterization of Hepatic Nodules in Liver Cirrhosis

Abstract

Liver cirrhosis is characterized by advanced fibrosis and formation of hepatocellular nodules which are classified to regenerative nodules, dysplastic nodules, or neoplastic lesions (HCC). However, their accurate characterization may be difficult even at histopathological analysis. Differential diagnosis may be facilitated by comparing the clinical and pathological findings with radiological imaging features. The detection and characterization of hepatic focal lesions in liver cirrhosis is important to decide the treatment options available for the patient because surgical resection and/or transplantation offer the most effective treatment in patients with only limited disease so it is crucial not to miss early small HCCs. Triphasic CT was believed to be the standard in evaluating the hepatic focal lesions together with alpha fetoprotein but unfortunately, not all cases with HCC having high alpha fetoprotein & not all cases having typical imaging criteria of HCC by Triphasic CT scan & also not all lesions are detected by US or are seen in the dynamic CT study Biopsy of all lesions detected with MRI in the setting of liver cirrhosis would be impractical, extremely difficult, if not impossible, and likely to be of low yield also distinguishing between HCC and benign hyper-vascular lesions such as flash filling hemangiomas & pseudo lesions remain a major challenge in management of patients at risk for developing hepatocellular carcinoma. MRI is the most sensitive modality in detection and characterization of hepatic focal lesions including small lesions (≤ 2 cm) also in follow up post locoregional therapy as TACE or RF ablation. Hyperintensity on both T2- and diffusion-weighted images is helpful in the diagnosis of HCC even those smaller than 2 cm. Fat-containing hyper-vascular liver lesions that are hypo-intense on in-phase with further signal drop out in the out-of-phase sequence (chemical shift artifact) are strongly associated with the diagnosis of HCC. In follow up post treatment moderate hyper intensity of the lesion in T2-weighted images suggests residual/recurrent lesions while marked hyper intensity which appears in heavy T2-weighted images should suggest liquifactive necrosis. Most of HCC appear hypointense in T1-weighted images but they may appear hyperintense if it contains glucogen, fat or cupper. In follow up post intervention T1 hyperintensity denotes coagulative necrosis & hemorrhagic products. Diffusion restriction with measuring the ADC value could be used in detection & characterization of hepatic focal lesions but unfortunately in liver cirrhosis not all malignant lesions have low ADC values due to the decrease in the ADC values of the liver itself secondary to the fibrotic changes in the liver. DWI could be considered complementary to conventional MRI in the diagnosis of hepatocellular carcinoma; particularly for new subcentimetric lesions in hepatocellular carcinoma patients following interventional therapy or diagnosis of intrahepatic metastases of HCCs. Moreover, because of the potential risk of nephrotoxicity in cirrhotic patients with hepatorenal syndrome, DWI as well as T2 seems to be used as a substitute for dynamic imaging in the assessment of a variety of focal liver lesions in cirrhotic patients with poor renal function. Arterial phase enhancement (especially late arterial phase) with washout of the contrast in the venous & delayed phases and delayed capsular enhancement are diagnostic for HCC in liver cirrhosis and also denotes residual/recurrent lesions in follow up post intervention. In follow up post intervention necrotic tissue shows no contrast enhancement in the arterial phase which should be supported by subtraction images to remove the intrinsic hyperintense signal of the coagulative necrosis and hemorrhagic products. Semi-quantitative perfusion images in our study showed preliminary good results as an objective method in characterization of hepatic lesions and in follow up post locoregional therapy. As being quantitative study it makes the diagnosis much easier and with more confidence level than other qualitative and subjective parameters.