Computed Tomography Versus Magnetic Resonance Imaging for Hepatic Lesion Characterization/Diagnosis

Abstract

Computed tomography (CT) and magnetic resonance imaging (MRI) are both extremely helpful in the diagnosis of liver lesions. Each has strengths and weaknesses for particular indications, and having an appropriate history is important in choosing the best modality for the patient. In CT, there are several protocols to choose from when imaging the abdomen and, in particular, when imaging the liver. In addition to the unenhanced scan, a dedicated liver CT has three phases after intravenous contrast agent administration: the late arterial (25-35 seconds), portal venous (60-75 seconds), and delayed (3-5 minutes) phases. Liver parenchyma is mainly supplied by the portal vein, and thus predominantly enhances during the portal venous phase. Liver lesions, however, are supplied only by the hepatic artery.1 The multiphasic CT and magnetic resonance protocols take advantage of the various enhancement patterns of each type of lesion to noninvasively characterize them. For instance, hypervascular lesions appear brighter than the background liver on the arterial phase, whereas hypovascular ones are darker on the portal venous phase when the background liver enhances maximally. Moreover, peculiar enhancement patterns are typically seen in some liver lesions, for example, early peripheral nodular enhancement and delayed fill-in is characteristic of liver hemangioma. Newer hepatobiliary-specific agents used in MRI are specifically taken up by hepatocytes and excreted in the bile. The enhancement of normal liver parenchyma and bile ducts peaks at the hepatobiliary phase (20-60 minutes), helping to detect nonhepatocellular origin lesions, such as metastases, which have no functioning hepatocytes to take up the contrast and will be hypoenhancing during the hepatobiliary phase, while functioning hepatocytes of lesions of hepatocellular origin, such as focal nodular hyperplasia (FNH), take up the contrast and are enhancing during the hepatobiliary phase.3 Liver MRI protocol varies according to the provided clinical history. In patients who have diffuse liver disease and a suspected liver lesion, a standard liver MRI is performed with multiphasic postcontrast imaging. If the patient presents with a cholestatic liver profile, magnetic resonance cholangiopancreatography (MRCP) should be added to assess the degree and level of biliary obstruction and to determine whether the obstructive lesion is periductal or endoluminal. In asymptomatic patients who have incidentally detected liver lesions, hepatobiliary-specific agents may be used instead of extracellular fluid contrast agents to differentiate FNH from hepatocellular adenoma (HCA). Hepatobiliary contrast agent-enhanced MRI is also recommended in oncology patients who need accurate detection and mapping of hepatic metastases. Each modality has advantages and specific limitations (Table 1). CT attenuation or MRI signal characteristics are exploited to characterize liver lesions. Awareness of the key imaging features would help to decide the modality of choice for each diagnostic category (Table 2). In this review, we discuss the spectrum of imaging features of the most common focal hepatic pathologies (Table 3). Cysts are incidentally discovered on cross-sectional imaging of the liver and show CT fluid attenuation or MRI fluid SI without enhancement. The hyperintense T2 signal on MRI similar to cerebrospinal fluid is characteristic of benign cysts. CT is usually sufficient for detecting cysts, but in cases of lesions <1 cm, MRI may be more useful in differentiating cysts from subcentimeter metastases.4, 5 The imaging features of liver abscess vary according to its evolution. At an early stage, it can appear as a heterogeneous solid mass on CT, whereas a mature abscess has a necrotic hypoattenuating center and enhancing rim.3 The clinical and laboratory findings, including fever, right upper quadrant abdominal pain, neutrophilic leukocytosis, and elevated alkaline phosphatase, can be very useful for the diagnosis of abscess.6 In some cases, however, it may be quite challenging to differentiate an abscess from a necrotic tumor, and biopsy with tissue diagnosis may be required.6 Some features on MRI may help to make this distinction. The “double target sign” refers to a layered-wall enhancement surrounding the abscess cavity where the outer layer shows a delayed enhancement relative to the inner one.6 In one study, this lesion was more frequently found in abscesses. However, delayed washout of the outer layer was more noted in malignant tumors.7 The typical features of hemangioma of early peripheral interrupted enhancement and delayed progressive enhancement (fill-in) are sufficient to make the diagnosis on CT and MRI.8 However, MRI may be required in some atypical hemangiomas, particularly in patients with a known malignancy or at risk for hepatocellular carcinoma (HCC).9 Capillary hemangiomas appear as small hyperenhancing lesions and may be difficult to differentiate from small hypervascular tumors. On MRI, marked hyperintensity on heavily T2-weighted imaging (T2WI) is helpful to diagnose a hemangioma.8 Sclerosed hemangiomas have atypical imaging appearance on CT and MRI. A comparison with the previous examination is advisable to determine whether typical features of hemangioma were present in this location previously. A “bright dot sign” is a helpful feature to diagnose this type of hemangioma and refers to a peripheral enhancing dot without complete delayed enhancement. However, a biopsy may still be needed to confirm the diagnosis because of overlapping appearances between sclerosed hemangiomas, cholangiocarcinoma (CC), and metastases.8, 9 On CT/MRI, FNH shows marked arterial phase hyperenhancement and becomes isoenhancing to the liver parenchyma during the portal venous and delayed phases.3 Hepatobiliary contrast agent-enhanced MRI has the advantage over CT by showing contrast taken up by the functioning hepatocytes within the FNH, which enhances during the hepatobiliary phase. This allows confident diagnosis of FNH and differentiating it from HCAs, HCC, and metastasis.10 In a systematic review, gadoxetic acid (hepatobiliary contrast agent)-enhanced MRI was able to discriminate between HCA and FNH on the hepatobiliary phase with 91% to 100% sensitivity and 87% to 100% specificity.11 HCAs are difficult to accurately diagnose on CT due to a similar appearance to benign lesions (e.g., FNH) and malignant tumors (e.g., HCC).9 However, MRI can be useful in diagnosing HCA and in recognizing HCA subtypes.12 Inflammatory HCAs usually demonstrate high SI on T2WI and marked hyperenhancement on the arterial phase, which persists on the portal and delayed phases. These MRI features have 85% to 88% sensitivity and 88% to 100% specificity for the diagnosis of inflammatory HCA.13, 14 In HCA with hepatocyte nuclear factor-1 alpha (HNF-1A) mutation, diffuse intratumoral microscopic fat is shown as diffuse and homogeneous signal dropout on T1 opposed-phase relative to in-phase images (Fig. 1), with a sensitivity of 87% to 91% and specificity of 89% to 100%.13, 14 β-Catenin-activated HCA and unclassified HCA have no specific imaging features on MRI. Thus, if a suspected HCA cannot be confidently classified as either an inflammatory subtype or an HNF-1A mutated subtype, a biopsy may be considered for definitive diagnosis.10 On CT/MRI, CC may appear as a biliary stricture in periductal infiltrative (perihilar) CC, hypovascular tumor with peripheral enhancement in mass-forming CC, or intraductal nodular mass.15 CT provides an accurate assessment of vascular invasion but tends to underestimate the biliary extent of perihilar CC.16 MRCP is an accurate, noninvasive method for evaluation of the biliary extension of the tumor and the entire biliary tree, with an accuracy rate of 95%. However, MRCP may suffer motion artifact.17 Although vascular invasion also can be identified on MRI and magnetic resonance angiography, it may be overestimated because of misinterpretation of the peritumoral inflammation and fibrosis as tumor infiltration, especially in patients after biliary stenting. Therefore, unless immediately required, stenting should be delayed after staging MRI.18 In mass-forming CC, hepatobiliary-specific agent MRI, including DWI, is more accurate in detecting intrahepatic metastasis. On the contrary, it may mask perihilar CC because of the simultaneous enhancement of liver parenchyma during the hepatobiliary phase.17 CT is the preferred imaging modality for the initial assessment and posttreatment surveillance because it allows an excellent overview of the primary tumor and other potential sites for metastases.19 Calcified metastases, such as colorectal metastases, are better assessed with CT20 (Fig. 2). MRI has better sensitivity in the detection of small liver metastasis that can be missed on CT21 (Fig. 3). MRI is also useful in presurgical planning for resection of metastasis or when hepatic metastasis precludes resection of the primary tumor (e.g., pancreatic cancer).19, 22 In one study, gadoxetic acid (hepatobiliary contrast agent)-enhanced MRI had a sensitivity of 86% for the detection of colorectal hepatic metastases ≤1 cm compared with 50% with contrast-enhanced CT. Gadoxetic acid-enhanced MRI combined with DWI had the highest sensitivity (95%).23 Moreover, liver steatosis is a known limitation for the assessment of small hypoattenuating metastases on CT; therefore, MRI is a problem-solving tool in this condition.19, 22 CT and MRI have been established cross-sectional imaging modalities useful for the evaluation and characterization of liver lesions. CT is usually more easily available, and scan time is short. MRI has a better contrast resolution and is an excellent problem-solving tool. Choosing which modality to start with is dependent on many factors and requires good communication between requesting physicians and the radiologist to optimize the imaging modality and technique.

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