### Key Points > [!tip] > MRI provides superior soft tissue contrast compared with CT and is often the preferred modality for characterizing indeterminate liver lesions, evaluating biliary pathology, and assessing pancreatic cystic lesions. - MRI is frequently complementary to CT rather than a replacement. - Multiphasic contrast-enhanced MRI remains the cornerstone of liver lesion characterization. - Hepatobiliary contrast agents can improve detection and characterization of focal liver lesions. - MRI is often the most sensitive modality for detecting small liver lesions. --- ## Common MRI Sequences ### T1-Weighted Imaging Useful for: - Fat-containing lesions - Hemorrhage - Proteinaceous material - Baseline lesion characterization Many liver lesions appear hypointense on T1-weighted imaging. ### T2-Weighted Imaging Useful for: - Fluid-containing lesions - Biliary structures - Cystic lesions Common findings: - Simple cysts are markedly T2 hyperintense - Hemangiomas are typically very bright on T2 imaging - Bile ducts are well visualized on heavily T2-weighted sequences ### Diffusion-Weighted Imaging (DWI) Assesses restriction of water movement within tissues. Useful for: - Detecting small metastases - Characterizing liver lesions - Treatment response assessment > [!important] > Restricted diffusion often reflects hypercellularity and may improve detection of small malignant lesions that are difficult to identify on conventional sequences. --- ### Contrast Agents |Clinical Question|Preferred Contrast| |---|---| |Suspected CRLM|Eovist| |Indeterminate liver lesion|Eovist| |HCC evaluation|Eovist| |Bile leak|Eovist| |Pancreatic mass|Extracellular agent| |Pancreatic cyst|Often no contrast or extracellular agent| #### Extracellular Contrast Agents Examples: - Gadobutrol (Gadavist) - Gadoterate (Dotarem) Characteristics: - Similar to iodinated CT contrast - Remain within vascular and extracellular spaces - Most commonly used MRI contrast agents Common uses: - Pancreatic lesions - General abdominal MRI - Vascular assessment #### Hepatobiliary Contrast Agents ##### Gadoxetate Disodium (Eovist) Characteristics: - Initial vascular phase similar to extracellular contrast agents - Approximately 50% excreted through the biliary system - Taken up by functioning hepatocytes - Allows hepatobiliary phase imaging Common uses: - HCC - FNH - Liver metastases - Cholangiocarcinoma - Indeterminate liver lesions > [!clinical] > Eovist is particularly valuable for detecting small liver metastases and characterizing indeterminate hepatic lesions. > [!warning] > Eovist performance may be reduced in patients with significant cholestasis, biliary obstruction, or advanced hepatic dysfunction because hepatocyte uptake and biliary excretion are impaired. #### Why Eovist Is Useful Normal hepatocytes take up Eovist and become hyperintense during the hepatobiliary phase. Lesions can then be detected because they either: - Fail to take up contrast and appear dark (_negative contrast_) - Retain hepatocyte function and appear bright (_positive contrast_) This often improves lesion conspicuity compared with conventional extracellular contrast agents. > [!important] > Rather than highlighting the lesion itself, Eovist often highlights the surrounding liver, making lesions easier to detect against a bright hepatic background. --- ## MRI Phases ### Arterial Phase Best for: - HCC - Hypervascular lesions - Neuroendocrine tumors - Arterial anatomy Typical findings: - Arterial phase hyperenhancement - Hypervascular metastases - Arterial variants ### Portal Venous Phase Best for: - Liver metastases - Portal venous anatomy - General liver assessment ### Delayed Phase Best for: - Fibrotic lesions - Cholangiocarcinoma - Washout assessment - Post-treatment evaluation > [!clinical] > Delayed enhancement is often seen in fibrotic tumors such as cholangiocarcinoma because contrast gradually accumulates within the desmoplastic stroma. ### Hepatobiliary Phase Obtained approximately 20 minutes after Eovist administration. Best for: - Small liver metastases - HCC characterization - FNH diagnosis - Biliary evaluation |Structure / Lesion|Hepatobiliary Phase Appearance| |---|---| |Normal liver parenchyma|Bright| |FNH|Bright or isointense| |Most HCC|Dark| |Colorectal liver metastases|Dark| |Cholangiocarcinoma|Dark| > [!tip] > The hepatobiliary phase often provides the greatest lesion-to-liver contrast. Small metastases that are difficult to identify on CT may become conspicuous as dark lesions against a bright liver background. --- ## MRCP MRCP uses heavily T2-weighted sequences to highlight fluid-filled structures and is the primary MRI technique for evaluating biliary and pancreatic ductal anatomy. --- ## Liver MRI ### Common Indications - Characterization of indeterminate liver lesions - HCC evaluation - Colorectal liver metastases - Cholangiocarcinoma - Transplant evaluation ### Advantages Over CT - Superior lesion characterization - Better small lesion detection - Diffusion-weighted imaging - Hepatobiliary phase imaging > [!important] > Liver MRI is generally the most sensitive cross-sectional imaging modality for detecting small hepatic lesions. --- ## Pancreas MRI ### Common Indications - Pancreatic cystic lesions - IPMN surveillance - Indeterminate pancreatic masses - Neuroendocrine tumors - Chronic pancreatitis ### Advantages Over CT - Superior ductal evaluation - Better cyst characterization - MRCP capability --- ## Eovist Leak Protocol Delayed hepatobiliary phase imaging following Eovist administration can identify active biliary leaks. ### Technique - Eovist administration - Standard dynamic imaging - Hepatobiliary phase imaging - Additional delayed imaging when necessary Delayed images may be obtained: - 20 minutes - 60 minutes - 1–3 hours depending on the clinical question and hepatic function. ### Findings - Extravasation of hepatobiliary contrast outside the biliary tree - Contrast accumulation within bilomas - Contrast accumulation within surgical drains - Communication between bile ducts and fluid collections ### Common Indications - Suspected bile leak after hepatectomy - Suspected bile leak after cholecystectomy - Evaluation of postoperative biloma - Biliary-enteric anastomotic complications - Liver transplant complications > [!clinical] > Delayed Eovist-enhanced MRI can identify bile leaks that may be occult on CT and often provides a noninvasive alternative to diagnostic cholangiography. > [!warning] > Significant cholestasis, biliary obstruction, or impaired hepatic function may reduce Eovist uptake and biliary excretion, decreasing study sensitivity. > [!note] > Delayed excretion of Eovist may provide functional information regarding hepatocyte function and biliary drainage.