Magnetic resonance imaging (MRI) provides a complete evaluation of solid organs of the gastrointestinal system, including the liver, pancreas, gallbladder, and spleen, and is the most sensitive noninvasive imaging modality for evaluating the biliary system. Applications of MRI for imaging the gastrointestinal tract have expanded rapidly due to development of faster imaging techniques, which have helped overcome effects of motion and susceptibility, enabling examination of structures and organs that were previously not reliably imaged.

Magnetic resonance imaging has several advantages over CT. All image contrast with CT relies on attenuation or density, and vascularity of structures can be inferred by the changes in density imparted by intravenous contrast agents. Attenuation differences that can be distinguished include air, calcification, bone, soft tissue, fat, and changes in soft tissue imparted by intravenous contrast agents. MRI utilizes multiple imaging parameters to definitively characterize tissues including T1, T2, fat‐saturated sequences, diffusion, and postcontrast imaging. The generation of MR images involves the spatial localization of radiofrequency signals elicited from water‐ or fat‐containing tissue in the body. The differences in gray scale on the image (white vs black) represents the strength of these signals, and is known as the signal intensity. Tissues and structures that are brighter on the image are described as being of high signal intensity, and tissues and structures that are darker on the image are low signal intensity.

When the specific technical parameters are changed on the MR magnet system, images can be generated that evaluate different tissue properties. Two tissue properties, T1 and T2, are examined on T1‐ and T2‐weighted sequences respectively. The presence of lipid within tissue is identified using chemical shift images or with fat saturation techniques. Other properties, such as vascularity, biliary secretion, or macrophage activity, are imaged using a wide array of contrast agents which are excreted in the vessels or bile ducts, causing T1 shortening.

Blood flow in vessels is selectively visualized via a family of pulse sequences collectively known as magnetic resonance angiography (MRA). These include time‐of‐flight (TOF), which shows flowing blood as bright (high signal intensity) on certain gradient echo (GRE) sequences, phase contrast angiography, bolus tracking, black blood techniques, and three‐dimensional gadolinium‐enhanced MRA.

Magnetic resonance imaging of the abdomen is often used for definitive characterization of a liver lesion. The most common benign liver lesions are hepatic cysts (Figure 90.1), biliary hamartomas, and hepatic hemangiomas (Figures 90.2 and 90.3), which have very high signal on T2‐weighted sequences. Hemangiomas demonstrate peripheral discontinuous nodular enhancement with progressive fill‐in of the lesion while following blood pool. Hepatic cysts do not enhance unless there is superimposed infection/inflammation. Arterially enhancing lesions that are benign and commonly seen in women are focal nodular hyperplasia (see Figure 90.2) and hepatic adenomas (Figure 90.4). Hepatic lesions that are of moderate signal intensity on T2‐weighted sequences similar to spleen and restrict diffusion are typically malignant, including hepatocellular carcinoma (Figures 90.5 and 90.6) and hepatic metastases (Figures 90.7–90.9).

Magnetic resonance imaging can be used to diagnose diffuse liver disease such as fibrosis, cirrhosis, hepatic steatosis, and iron deposition. Fibrosis/cirrhosis can be evaluated using MR elastography, which analyzes shear wave propagation to look at liver stiffness (Figure 90.10). Hepatic steatosis is diagnosed using chemical shift imaging (Figure 90.11), looking for drop of signal in the liver parenchyma on the out‐of‐phase sequences relative to the in‐phase sequences. Iron deposition is characterized by marked low signal on T2 and GRE sequences (Figure 90.12), with drop in signal of the liver parenchyma on the in‐phase sequences relative to the out‐of‐phase sequences. Cirrhosis is accurately diagnosed on MRI by visualization of regenerating nodules which are separated from each other by the fibrovascular septae, nodular liver contour, interlobar/right lobe atrophy, and lateral segment left hepatic lobe hypertrophy (Figure 90.13). MRI can also assess for portal hypertension, including varices, ascites, and splenomegaly (Figure 90.14).

Magnetic resonance cholangiopancreatography (MRCP) is used for evaluating the biliary tract and pancreatic duct. By utilizing an extremely high echo time (TE), these sequences are very heavily T2 weighted in the abdomen, with fluid structures including bile and secretions in the biliary tract and pancreatic duct being the brightest on these sequences. Displaying these images in a format similar to conventional endoscopic retrograde cholangiopancreatography or cholangiography, causes of obstruction of the biliary duct and pancreatic duct can be identified. Filling defects such as calculi appear as hypointense signal voids within the duct (Figure 90.15). Characterization of obstructing lesions is aided by dynamic gadolinium‐enhanced images. Cholangiocarcinoma typically demonstrates a heterogeneous mass with progressive delayed enhancement (Figures 90.16 and Figure 90.17) and biliary obstruction. In cirrhotic patients, cholangiocarcinoma can present as a diffuse infiltrative tumor involving an entire lobe (Figure 90.18).

The normal pancreatic parenchyma is high signal on T1‐weighted images (Figure 90.19). Chronic pancreatitis is characterized by loss of this high T1 signal as well as irregular dilation of the pancreatic duct (Figure 90.20). Autoimmune pancreatitis is a special form of chronic pancreatitis typically caused by IgG4‐associated lymphocytic infiltration of the pancreatic parenchyma with male predilection. Typical MRI characteristics include focal or diffuse pancreatic enlargement, absence of parenchymal atrophy, mild pancreatic duct dilation proximal to the site of stenosis, and a T2‐hypointense capsule‐like rim (Figure 90.21). Pancreatic adenocarcinoma typically is an infiltrating hypoenhancing mass relative to the normal pancreas, and often encases surrounding vasculature with secondary pancreatic ductal dilation (Figures 90.22 and Figure 90.23).

The potential of MRI for evaluating the bowel has been explored using MR enterography. Bowel motion can be reduced by the intravenous injection of antiperistaltic agents such as glucagon. These techniques can differentiate bowel from pathological processes. Intrinsic intestinal wall abnormalities such as inflammatory bowel disease can be characterized (Figures 90.24 and Figure 90.25). Additional abnormalities including perianal fistulas can also be accurately depicted using high spatial resolution T2 and T1 postcontrast techniques (Figure 90.26).