Magnetic resonance imaging of the liver, biliary tract, and pancreas

CHAPTER 19  Magnetic resonance imaging of the liver, biliary tract, and pancreas Scott R. Gerst and Richard Kinh Gian Do

Magnetic resonance imaging (MRI) is a cross-sectional multiplanar imaging technique (Fig. 19.1). MRI uses magnetic fields and radiofrequency pulses to generate images with outstanding tissue contrast and excellent spatial resolution. The principles of nuclear magnetic resonance were first described in the 1940s by Bloch and colleagues (1946) and Purcell and colleagues (1946) as a method for in vitro chemical analysis. Several decades later, Damadian (1971) and Lauterbur (1973) applied some of these basic principles to design MRI to allow in vivo imaging. Today, MRI is used extensively as a medical imaging tool throughout the body to visualize and distinguish normal and pathologic tissue.

PRINCIPLES OF MAGNETIC RESONANCE IMAGING MRI harnesses the signal available from the magnetic moment or spin present in certain nuclei, including hydrogen atoms. When placed in a large static magnetic field, the nuclei align themselves in a parallel or antiparallel direction to the field and also rotate or spin at a precise frequency termed the Larmor frequency. This frequency depends on the specific type of nuclei imaged and the strength of the magnetic field. The most commonly imaged nucleus in clinical practice is hydrogen (H1) because of its great abundance in the human body, mostly in the form of water. Other nuclei that may be imaged by MRI include phosphorus (P31), sodium (Na23), and carbon (C13). When unperturbed in a static magnetic field, nuclei with magnetic spins are in equilibrium: An almost an equal number of nuclei are in an “up” (parallel) or “down” (antiparallel) alignment. The slight difference between the two states creates a net magnetic moment. A measurable signal is generated from the magnetic moment after excitation by a radiofrequency (RF) pulse at the resonance Larmor frequency. This signal is generated as the excited nuclei in the body return to equilibrium, releasing energy in the form of an electromagnetic field that is captured by a receiver coil. The strength of this emitted signal determines the signal intensity (SI) of a tissue. The precise tissue SI depends on several factors, including longitudinal relaxation (T1), transverse relaxation (T2), proton density (essentially the number of nuclei present), and flow. Some of these factors can be manipulated (e.g., through T1 and T2 weighing) to create images highlighting various soft tissue properties. All tissues have an intrinsic T1 value for relaxation along the longitudinal magnetic axis, which varies between 200 and 800 milliseconds. T2 time, or transverse relaxation, is a measure of signal loss perpendicular to the long axis of the magnetic field and typically varies between 20 and 200 millisecconds for most 358

tissues. Differences in T1 and T2 times intrinsic to various soft tissues can be exploited to improve image contrast and diagnostic accuracy. For example, free water, which has a long T1 relaxation, is low signal on standard T1-weighted imaging and markedly high signal on T2-weighted imaging. Thus so-called heavily T2-weighted imaging sequences, such as those used in MR cholangiopancreatography, highlight high water content structures, such as bile and pancreatic ducts, while reducing the signals of other organs. Diffusion-weighted MRI (DWI) is an alternative tissue contrast mechanism that produces images dependent on the local magnitude of water diffusion. DWI of the liver may be used to help detect focal hepatic lesions, especially metastases, and may also be used to assess changes in the liver parenchyma, such as liver fibrosis. Tremendous interest in this sequence first arose due to its increased sensitivity to early changes in the brain after a cerebrovascular accident compared with more traditional T1or T2-weighted imaging. Applications for DWI in body imaging, initially hampered by hardware and software limitations, were addressed in the mid-2000s. With new advances in receiver coil design, improved gradient coils used to acquire images, and motion correction through respiratory or navigator techniques, DWI has flourished as a functional imaging sequence in hepatopancreatobiliary imaging (Taouli et al, 2010).

MAGNETIC RESONANCE IMAGING SAFETY MRI is safe for most patients. However, physicians and patients should be aware of some important considerations, such as MR contrast safety, and the risk of interactions between the patient (including their implants) with the strong static magnetic field of the scanner as well as the RF field used to generate images. The magnetic field interactions are potentially dangerous by causing the motion of ferromagnetic aneurysm clips or causing electronic malfunctioning of cardiac pacing devices and defibrillators. Many MRI facilities have screening programs in place to address potentially contraindicated devices, under new MRI safety guidelines proposed by the American College of Radiology (Expert Panel on MR Safety, 2013). However, referring physicians should be aware of their local imaging centers’ approach to MRI safety, such as their ability to scan patients with MR-conditional or MR-unsafe pacemakers, for example. MRI has a role in patients with minimal or mild renal insufficiency. Iodinated contrast used in CT is associated with a risk of contrast-induced nephropathy; however, administration of intravenous gadolinium contrast agents in patients with severe renal insufficiency carries a risk of nephrogenic systemic fibrosis (NSF). NSF is a serious and potentially fatal complication

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C FIGURE 19.1.  Multiplanar T2-weighted images through the liver in a patient with multiple hepatic metastases. A, Axial image. B, Sagittal image. C, Coronal image. A variety of techniques can be used to obtain these images. Note that fluid-containing structures, including small bowel (curved arrow, A), gallbladder, biliary tree, and pancreatic duct (arrow, C), are bright.

related to free gadolinium deposition within soft tissues and organs, with resulting scleroderma-like fibrosis. The exact causal mechanism of NSF remains unknown, but the risk of NSF increases in patients with a glomerular filtration rate (GFR) less than 30 mL/min, and particularly in patients with severe, end-stage disease (GFR < 15 mL/min). In addition, linear agents with lower thermodynamic stability carry a greater risk than others. If available, more stable, lower-risk macrocyclic agents can be used in patients with a GFR less than 30 mL/ min. Gadolinium contrast usage in patients with a GFR less than 15 mL/min should only be performed if essential, after careful evaluation of risks versus benefits, and with consultation with a nephrologist, if available. Since NSF was initially described, the rapid international investigation, dissemination of information, and swift adjustment to policy have led to a precipitous drop of reported NSF cases. In multiple countries, NSF has essentially disappeared since 2009 (Bennett et al, 2012; Grobner, 2006; Idee et al, 2014).

MAGNETIC RESONANCE IMAGING CHOLANGIOPANCREATOGRAPHY MR cholangiopancreatography (MRCP) is an imaging technique used to evaluate the bile and pancreatic ducts and plays

an important role in imaging benign disorders, as well as comprehensive evaluation of malignancies of the biliary system (Mandelia et al, 2013; Singh et al, 2014; Vaishali et al, 2004). Heavily T2-weighted images are used to provide an overview of biliary and pancreatic ductal anatomy, removing the signal of surrounding structures. Excellent diagnostic-quality images are obtainable, with high sensitivity and specificity for evaluation of ductal dilatation, strictures, and intraductal abnormalities (Hekimoglu et al, 2008; Palmucci et al, 2010a; Palmucci et al, 2010b; Sandrasegaran et al, 2010). Cross-sectional images and maximum intensity projection images (Fig. 19.2) are produced with current MRCP techniques, and projection images are similar to direct contrast-enhanced cholangiograms obtained with either endoscopic retrograde cholangiopancreatography (ERCP) or percutaneous transhepatic cholangiography (PTC; see Chapter 18). MRCP is noninvasive, eliminating the potential morbidity associated with ERCP or PTC (Zhong et al, 2004). The basic principle of MRCP is to use T2-weighted imaging to highlight stationary or slowly moving fluid, including bile, as high in signal intensity; surrounding tissues, including retroperitoneal fat and the solid visceral organs, with shorter T2 values, are markedly reduced in signal. In addition to heavilyT2-weighted sequences, MRCP protocols also include routine intermediate

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FIGURE 19.2.  Mixed main- and branch-duct intraductal papillary mucinous neoplasm (IPMN) of the pancreatic tail. A, Coronal T2 image. A lesion with high T2 signal indicating cystic contents involves the pancreatic tail and shows multiple thin septations (arrow). B, Coronal three-dimensional maximum intensity projection T2-weighted magnetic resonance cholangiopancreatography image. The multiseptated cystic pancreatic tail mass is apparent (arrow). C, Axial T2 image. Multiple septations are apparent with dilated main pancreatic duct (star). Incidental note is made of a right renal cyst with thin septations (open circle). D, Axial T1-weighted fat saturation image postintravenous gadolinium contrast. The numerous septations enhance within the lesion (arrow). Some components appeared nodular, and the lesion was resected. Histopathology revealed an IPMN with a focus of noninvasive mucinous cancer.

T2-weighted sequences and T1-weighted sequences to evaluate surrounding structures. MR-specific techniques for obtaining cholangiographic images include breath-hold, as well as respiratory and navigator gated techniques, which can generate images in a patient during free breathing.

MAGNETIC RESONANCE IMAGING CONTRAST AGENTS MRI contrast agents work primarily by altering the intrinsic T1 relaxation times of various soft tissues where contrast accumulates. In the hepatobiliary system, MRI contrast agents can improve the detection of liver lesions and improve the characterization of focal liver abnormalities. MRI contrast agents for hepatobiliary imaging are divided into two basic categories: extracellular fluid contrast (ECF) agents, and those with additional hepatobiliary excretion. In the past, the most commonly used contrast agents were the ECF agents, such as gadopentetate dimeglumine (gadolinium diethylenetriaminepentaacetic acid [Gd-DTPA]), which is distributed within the intravascular compartment initially and rapidly diffuses through the extravascular space, similar to the action of iodinated contrast agents in computed tomography (CT; see Chapter 18). Differences in

enhancement patterns between benign and malignant liver lesions have been well reported and are discussed in individual entities later. Several new hepatobiliary-specific contrast agents have been developed to add additional functional information compared with traditional ECF contrast agents. These hepatobiliary specific contrast agents are taken up to varying degrees by functioning hepatocellular tissue and are excreted in bile as well as through the kidneys. Hepatobiliary specific agents include mangafodipir trisodium, gadobenate dimeglumine, and gadoxetic acid (Gd-EOB-DTPA). Both gadobenate dimeglumine (MultiHance; Bracco Imaging, Cranbury Township, NJ) and gadolinium-ethoxybenzyl-diethylenetriamine pentaacetic acid (Eovist/Primovist; Bayer Healthcare, Wayne, NJ) have been approved in the United States. These contrast agents are all administered intravenously, although the dose and pharmacokinetics are different. These two agents are sometimes referred to as combination agents because of their dual capacity for imaging both in the dynamic phase (as done routinely with ECF agents) and in the delayed, hepatocyte-specific phase. These agents provide comprehensive information about the hepatic parenchyma, bile ducts, and liver vasculature (Burke et al, 2013; Seale et al, 2009).

Chapter 19  Magnetic resonance imaging of the liver, biliary tract, and pancreas



NORMAL HEPATIC APPEARANCE ON MAGNETIC RESONANCE IMAGING MRI is routinely used to evaluate diffuse and focal liver abnormalities. With liver MRI, a combination of precontrast T1, T2, and DWI sequences are used. Normal hepatic parenchyma is brighter (hyperintense) than the spleen on T1-weighted images, whereas on T2-weighted images, the spleen is relatively brighter than the liver (Fig. 19.3). Although most hepatic lesions are low in signal intensity on T1-weighted images, they have more variable intensity on T2-weighted images, with cysts and hemangiomas having the highest T2 signal intensity in general. Precontrast and postcontrast T1-weighted imaging is also performed to assess enhancement patterns of the liver parenchyma and liver lesions. If a hepatobiliary contrast agent is used, additional delayed hepatobiliary phase images are acquired.

DIFFUSE HEPATIC DISEASE Fatty Liver Fat may accumulate within hepatocytes for many reasons, including alcohol intake, diabetes, medications, and obesity (see Chapters 71 and 100). Hepatic steatosis may be diffuse, patchy,

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or focal. The pattern of fatty liver is related to regional differences in perfusion. It may be difficult to distinguish focal fat from focal hepatic lesions on CT because both appear low in attenuation. Additionally, focal fatty sparing may appear similar to an enhancing neoplasm on contrast-enhanced CT. Although geographic margins and location may aid in the diagnosis, this is more readily distinguished on MRI due to different signal characteristics. T1-weighted in-phase and out-of-phase imaging allows differentiation of signals from fat and water when they are present in the same voxel, as it exploits minute differences in resonance frequencies between fat and water protons (Fig. 19.4). Using fast gradient-echo techniques, fat and water protons that share the same voxel will be imaged both “in phase” or 180 degrees “out of phase.” Tissues that have relatively equal quantities of fat and water will appear dark on the out-of-phase sequence because the signals from fat and water are “opposed in phase” and can cancel each other out (Boll et al, 2009; Springer et al, 2010). Areas of fatty sparing will remain hyperintense relative to the surrounding fatty liver on out-of-phase imaging. On standard T1-weighted imaging, areas of focal fat can show increased signal. The appearance on T2-weighted images depends on the type of sequence acquired and whether fat saturation is used. The ability of MRI to exploit those signal

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C FIGURE 19.3.  Normal hepatic magnetic resonance image. A, T1-weighted spin-echo image. The liver is brighter (hyperintense) relative to the signal of the spleen. There are normal flow voids that appear as dark areas in the visualized vessels (straight arrow, intrahepatic portion of the inferior vena cava; curved arrow, aorta). B, T2-weighted fast spin-echo image. Because of reversal of the liver/spleen contrast, the normal spleen is brighter than the liver. Note the flow voids within the vessels (straight arrow, intrahepatic portion of the inferior vena cava; curved arrow, aorta). C, Postcontrast T1-weighted gradient-echo image. Normal enhancement in the liver and spleen is evident, and the parenchyma of both is about equal in this phase of the injection. All the vessels are bright as a result of the T1 shortening effect of gadolinium (straight arrow, intrahepatic portion of the inferior vena cava; curved arrow, aorta).

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E FIGURE 19.4.  Focal fat within the liver. A, Non–contrast-enhanced computed tomography image in a young adult with history of testicular cancer shows heterogeneous regions of low attenuation in the right hepatic lobe (arrows). B, T1-weighted in-phase image at the same level now shows signal abnormality in this region. C, T1-weighted out-of-phase image shows geographic areas (arrow) that have lost signal. On the out-of-phase images, a black line “India ink effect” surrounds tissue interfaces. D, T1-weighted fat saturation precontrast image also shows mild signal loss in this area (arrow). E, T1-weighted fat saturation postcontrast image. The area in question has normal vascularity coursing through it; no mass was present—the typical appearance of fatty infiltration.



Chapter 19  Magnetic resonance imaging of the liver, biliary tract, and pancreas

differences, as well as to apply diffusion-weighted and postcontrast imaging, offers an advantage over CT when evaluating for focal lesions on a background of hepatic steatosis. Newer, breath-hold three-dimensional (3D) T1 techniques with interpolation allow fast single breath-hold acquisition with added fat and water separation (Bashir et al, 2012). Enhancement characteristics also aid in the diagnosis, as focal fatty liver enhancement should parallel surrounding normal hepatic parenchyma. This is important because other lesions, including hepatocellular carcinoma (HCC) and adenoma, may contain small quantities of fat within them (see Chapters 90A and 91).

Iron Deposition Disease Hepatic iron deposition may occur from primary genetic hemochromatosis or from secondary nongenetic systemic overload (see Chapter 76). Primary hemochromatosis is an autosomal recessive genetic disorder characterized by abnormal intestinal iron absorption, with accumulation of iron in the nonreticuloendothelial organ system. This predominantly occurs in hepatocytes later in the disease when other viscera, including the pancreas and heart, are more likely to be involved. The liver shows abnormally low signal intensity compared with spleen on T1-weighted in-phase sequences. T1-weighted gradient-echo sequences are very sensitive for detecting the presence of iron within the hepatic parenchyma due to the local field inhomogeneities caused by the presence of iron. Primary, or genetic, hemochromatosis is important to diagnose because this entity may be unnoticed until late in the disease process, and its longterm sequelae include fibrosis, cirrhosis, and HCC. Nongenetic systemic iron overload from secondary causes can sometimes be distinguished from primary hemochromatosis based on the pattern of organ involvement. Nongenetic systemic iron overload is typically associated with multiple blood transfusions, myelodysplastic syndrome (MDS), or abnormal iron absorption in patients with cirrhosis (QueirozAndrade et al, 2009). In secondary iron overload, there is predominant accumulation of iron in the reticuloendothelial system (RES), with splenic and bone marrow iron deposition early and hepatic deposition later. If the iron deposition is limited to the RES only, it is termed hemosiderosis. If non-RES organ involvement is seen, it is termed secondary hemochromatosis. These distinguishing characteristics and the patient’s history often allow correct differentiation. MRI is excellent for identifying the abnormal signal intensity due to iron deposition, and gradient-echo imaging has been shown to reliably quantify hepatic iron load (Fig. 19.5) (Mavrogeni et al, 2013; QueirozAndrade et al, 2009). Iron quantification may have indirect added uses; for example, limited data suggest it may help predict nonrelapse mortality prior to allogeneic stem cell transplant in patients with MDS and acute myeloid leukemia (Wermke et al, 2012).

FOCAL HEPATIC LESIONS The advantage of MRI over CT is in the improved characterization of liver lesions by improved soft tissue contrast combined with novel tissue contrast mechanism. We will discuss common benign and malignant liver lesions and emphasize distinguishing imaging characteristics. Familiarity with typical lesion T1 and T2 signal, as well as enhancement characteristics, will aid the clinician in developing an approach to assess the most commonly encountered hepatic lesions.

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Cysts Cysts are common hepatic lesions that may be sporadic (see Chapters 75 and 90B) or associated with polycystic disease of the kidney (Fig. 19.6). Small cysts (<1 cm) may be difficult to characterize on CT because of volume averaging but are easily diagnosed by MRI. The MRI criteria for a simple cyst is a nonenhancing lesion that is homogeneously low in signal intensity on T1-weighted images and homogeneously very bright on T2-weighted images, similar to cerebrospinal fluid. Cysts maintain their markedly high signal on heavily T2-weighted images, similar to fluid in the cerebrospinal fluid, bile ducts, and pancreatic duct.

Hemangioma Hepatic hemangiomas (see Chapter 90A) are benign tumors of the liver with an estimated incidence of 20% (Albiin, 2012). Most hemangiomas are thus commonly found incidentally on other imaging studies, such as CT or ultrasound. MRI is ideal for characterizing hemangiomas and is the preferred modality for imaging. On T1-weighted images, hemangiomas are hypointense compared with the surrounding hepatic parenchyma, have smooth, well-marginated borders, and are frequently lobulated. Hemangiomas are markedly hyperintense compared with normal liver on T2-weighted images. Hemangiomas also tend to retain a high signal on more heavily T2-weighted sequences, almost to the same degree as cysts, and become more conspicuous compared with surrounding liver parenchyma. After the administration of contrast, the presence of peripheral nodular discontinuous enhancement, with either partial or complete filling in on successive postcontrast phases, has been shown to be highly accurate for the diagnosis of hemangioma (Fig. 19.7; Silva et al, 2009). Dynamic contrast-enhanced MRI after administration of intravenous gadolinium is useful for increasing the specificity of diagnosis of hepatic hemangioma and for differentiating this lesion from others, including metastases and HCCs (Albiin, 2012). Larger hemangiomas tend to follow these criteria more frequently, although giant hemangiomas also may have variable signal intensity and a central scar (Prasanna et al, 2010). Problems arise with smaller hemangiomas, however, which may “flash fill” and appear as hypervascular, homogeneous lesions. These smaller lesions (usually <1 cm) may be distinguished by both their T2 signal characteristics and enhancement pattern paralleling enhancing vessels (Albiin, 2012). In addition, hemangiomas have higher apparent diffusion coefficient values obtained from DWI than metastases (Miller et al, 2010). Caution must be used when assessing enhancement using the hepatobiliary contrast agent gadoxetate disodium, however, as the overlapping extracellular phase and hepatobiliary excretion of this contrast may confound the typical hemangioma enhancement and lead to “pseudowashout” in late dynamic phase, making characterization more difficult (Goshima et al, 2010). For this reason, we prefer not to use gadoxetate as the contrast agent for initial liver lesion characterization and use a traditional ECF agent instead. Sclerosed hemangiomas represent a diagnostic challenge, often mimicking malignancies in the liver; limited data suggest that increasing sclerosis shows corresponding decreased T2 signal and less avid enhancement, probably corresponding to areas of fibrosis. Adjacent capsular retraction and adjacent wedge-shaped transient perfusional changes, as well as size

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G FIGURE 19.5.  Iron deposition. A, T1-weighted in-phase gradient-echo image (echo time [TE] 4.8) shows abnormal liver/spleen contrast. The liver is homogeneously darker than the spleen from preferential deposition of iron within the hepatic parenchyma. This is a pediatric patient who had undergone numerous transfusions while undergoing therapy for cancer. Note the difference from Figure 19.X. B, T1-weighted out-of-phase gradientecho image (TE 2.2) shows the liver/spleen contrast reverses, with the liver being brighter than spleen. This is opposite from Figure 19.X and shows iron deposition disease within the liver. C, Axial T2-weighted fast spin-echo sequence shows marked low T2 signal within both liver and spleen. D-G, Using T2-weighted multigradient-echo technique, there is progressive signal loss in the liver. This type of multiecho imaging allows estimation of iron quantification using computer software.

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FIGURE 19.6.  Hepatic cysts associated with polycystic kidney disease. A, Axial T2-weighted image at the level of the kidneys shows bilaterally enlarged kidneys with multiple hyperintense cysts. Little normal renal parenchyma is present at this level, and multiple small hepatic cysts (arrow) are seen. B, Coronal T2-weighted images through the kidneys show similar findings of enlarged kidneys containing multiple cysts (black arrows), and small hepatic cysts are identified (white arrow).

decrease over time, may all offer clues to suggest the diagnosis, but biopsy may be necessary for confirmation, depending on the clinical picture (Ridge et al, 2014).

Focal Nodular Hyperplasia Focal nodular hyperplasia (FNH; see Chapter 90A) is the second most common benign tumor of the liver after hemangioma. Pathologically, FNH contains all the elements of normal liver and may have a central fibrous scar, which shows delayed enhancement, and is surrounded by hepatocytes and small bile ducts (see Chapter 89). FNH has been associated with oral contraceptive use (Scalori, 2002) as well as chemotherapy in the pediatric age group (Do et al, 2011; Sudour et al, 2009). Most FNH are isointense to normal liver parenchyma on T1and T2-weighted images, and a few are mildly T1 hypointense or minimally hyperintense on T2-weighted images (Fig. 19.8A and B). When discussing relative T1 and T2 signal, a normal background liver parenchyma is assumed. In a liver with iron deposition, which diffusely lowers the signal intensity of the liver parenchyma, T1 and T2 relative hyperintensity can be expected. This may be seen, for example, in pediatric oncologic patients who have concomitant iron deposition (Do et al, 2011). Frequently, FNH is visualized by displacement of the normal hepatic vasculature, and classic FNH shows strong arterial peak enhancement following administration of an intravenous contrast agent, with lack of washout in portal venous or late dynamic phase imaging. A common feature of FNH is the presence of a central scar. This distinguishing feature is hypointense on T1-weighted images and hyperintense on T2-weighted images with enhancement on delayed imaging beginning at approximately 3 minutes by using ECF contrast agents (Fig. 19.8C) (Karam et al, 2010). The presence of enhancement, similar to or higherthan-normal parenchyma, during the delayed hepatobiliary phase with a hepatocyte-specific contrast agent is expected for the majority of FNHs. Of note, with the hepatobiliary agent gadoxetate disodium, a central scar, if present, does not usually enhance relative to the hepatocytes present in this lesion.

Hepatic Adenoma Hepatocellular adenomas (HCAs) (see Chapter 90A) also are benign liver lesions, but unlike FNH, are associated with complications, including abdominal pain, bleeding, and rarely, malignant degeneration. HCAs are strongly associated with oral contraceptive or steroid use and are more common in women of childbearing age. Newer low-dose oral contraceptives have a less strong association with HCAs. Although HCA may be large at presentation, they are increasingly incidentally discovered and often less than 4 cm in diameter (Grazioli et al, 2012). Hemorrhage may be life threatening if it extends into the peritoneum. Although imaging characteristics vary, a combination of features, including enhancement characteristics, intralesional lipid, and hypointensity on delayed hepatobiliary phase imaging, may help increase diagnostic confidence (Mohajer et al, 2012). Nonetheless, small adenomas without hemorrhage may be difficult to differentiate from FNH without a central scar and from well-differentiated HCC. Recently, distinct genetic subtypes of adenomas have been described and show differing behaviors, histopathology, and imaging features. These include inflammatory, hepatocyte nuclear factor-1α (HNF-1α–mutated), and β-catenin–mutated HCAs (see Chapter 89). Of note, some adenomas may be both β-catenin mutated and inflammatory. A group remains unclassified and encompasses other HCAs without any genetic abnormalities (Grazioli et al, 2013). Inflammatory HCA, the most common subtype (30% to 50% of HCAs) typically seen in women with a history of oral contraceptive use, is generally hypointense on T1 images, hyperintense on T2, and frequently heterogeneous. They usually show moderate arterial enhancement, persistent dynamic phase enhancement, and variable uptake in delayed hepatobiliary phase (Grazioli et al, 2013). The enhancement pattern may reflect a mix of poorly functioning hepatocytes, arterioles without accompanying veins, inflamed bile ducts, and dilated sinusoids. On MRI, small amounts of intralesional lipid may be seen. A small percentage of these HCA show contrast washout in late dynamic phases, whereas others retain contrast similar to hemangiomas. Delayed

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FIGURE 19.7.  Hepatic hemangioma undergoing sclerosis. A, Heavily T2-weighted image (echo time [TE] 150) shows a mass (m) that is hyperintense to hepatic parenchyma with well-defined margins. B, Precontrast T1-weighted fat saturation image at the same level shows the mass to be low signal compared with background parenchyma. C, T1-weighted postcontrast image in early portal venous phase shows peripheral nodular enhancement within this mass (arrows). Portions of the lesion fail to show enhancement during this phase. D, Equilibrium phase T1-weighted image postcontrast shows the lesion has partially filled in toward the central portion. This constellation of findings is typical for hepatic hemangioma.  E-F, The same lesion 5 years later on T1-weighted postcontrast scans in portal venous and equilibrium phase imaging shows less avid enhancement and has shown slight size decrease, consistent with developing sclerosis. Sclerosing hemangiomas may show size decrease with less avid enhancement, associated capsular retraction, and lower T2 signal than nonsclerosed hemangiomas.

peripheral rim enhancement in hepatobiliary phase may also be seen (Grazioli et al, 2013). Hepatocyte nuclear factor-1α mutated HCAs (30% to 35% of HCAs) are almost exclusively seen in women and often show large amounts of intralesional lipid. They enhance less avidly in arterial phase than inflammatory HCA and typically are homogeneously low in signal on delayed hepatobiliary phase. They may also remain low signal in portal venous or equilibrium dynamic phases, but that may be due to the opposedphase property of postcontrast T1-weighted imaging (Grazioli et al, 2013). β-Catenin–mutated HCAs (10% to 15% of HCAs) are associated with glycogen storage disease, androgen use, and have a male predilection; they also have a higher chance of undergoing

malignant degeneration. They show moderate, often heterogeneous arterial phase enhancement, which may persist but is variable in late dynamic and delayed phases. Although T1 hypointensity and T2 hyperintensity are common, it is often heterogeneous. These lesions show diffuse glutamine synthetase expression from upregulation, although it may be heterogeneous. This is in contradistinction to FNH, which shows maplike glutamine synthetase expression distribution adjacent to hepatic veins (Agarwal et al, 2014; Balabaud et al, 2013; Grazioli et al, 2013). Unclassified HCA (10% of HCAs) have no specific genetic or histopathologic abnormalities. No specific imaging features have been identified for these lesions, although experience remains limited due to their infrequent diagnosis.

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FIGURE 19.8.  Focal nodular hyperplasia (FNH). A, T2-weighted image shows a mass that is isointense to hepatic parenchyma (arrows). Centrally, within this mass is a linear, hyperintense focus (arrowhead). B, A precontrast, T1-weighted gradient-echo image shows the mass (arrows) to be mildly hypointense to liver. C, Arterial dominant phase T1-weighted gradient-echo sequence shows that the mass enhances intensely with respect to hepatic parenchyma (arrows). The central focus, which was bright on the T2-weighted images, does not show enhancement on this phase of the injection (arrowhead). D, Postcontrast equilibrium phase image shows the mass to be isointense to background hepatic parenchyma. The central portion of scar shows delayed enhancement (arrowhead) characteristic of FNH.

Hepatic Abscess/Infection CT is generally the modality of choice in recent postoperative patients in whom there is a clinical suspicion of a hepatic abscess (see Chapter 72). Patients with more atypical symptoms may present a diagnostic dilemma. Although some patients show symptoms suggesting abscesses, others do not, but patients suspicious for abscesses or bile leaks may be imaged with MRI. Abscesses are usually hyperintense on T2-weighted images with an irregular rim of intermediate signal intensity surrounding a hyperintense outer rim. Abscesses are generally hypointense on T1-weighted images, unless they have hemorrhage or proteinaceous debris within them. After administration of contrast material, the rim enhances, whereas the central portion does not. The imaging findings may overlap with malignancies, including gallbladder cancer. Infected cystic hepatic lesions also may occur, such as parasitic echinococcal infections. Echinococcal cysts are typically multiloculated with internal thin-walled daughter cysts, and typically show no internal enhancement. A peripheral low T1 and T2 signal rim, as well as correlation with a history of possible exposure, may provide added clues to the diagnosis (see Chapter 74) (Qian et al, 2013).

Hepatic Metastases The liver is the most common site for the hematogenous spread of malignant neoplasms (see Chapters 92 to 94). Other hepatic

lesions, including hemangiomas or focal fat, may appear similar to metastases when imaged with CT or ultrasound. In general, metastases are mildly hypointense on T1-weighted images and are mildly hyperintense on T2-weighted images. Unless they are necrotic, mucinous, or cystic, metastases are not as bright on T2-weighted images as hemangiomas or cysts and often become less conspicuous with greater (longer echo time [TE]) T2 weighting. A number of metastases are hypervascular and best seen on arterial phase imaging, such as those arising from a primary neuroendocrine tumor, renal cell carcinoma, or intrahepatic metastases from hepatocellular carcinoma. Most metastases, however, are hypovascular and tend to have less well-defined borders after contrast enhancement than other benign lesions (Fig. 19.9). During contrast administration, metastases often show early peripheral marginal enhancement. These peripheral rims may wash out and appear hypointense on delayed images (Fig. 19.10). Larger metastases tend to have a thick irregular rim of enhancement representing viable tumor with areas of central necrosis. Multiple studies have demonstrated superior accuracy of hepatobiliary agent contrastenhanced MRI over contrast-enhanced CT (Lafaro et al, 2013). Both ECF and hepatobiliary agent contrast-enhanced MRI has shown higher sensitivity and specificity than CT–positron-emission tomography (PET) imaging as well, particularly for small (<1 cm) lesions, and hepatobiliary contrast-enhanced MRI combined with PET imaging may offer added benefit (Donati

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FIGURE 19.9.  Hepatic metastasis from colorectal carcinoma. A, T1-weighted image shows a low signal intensity metastasis (arrow) within segment IVB of the liver. B, T2-weighted image shows the mass is mildly brighter than background hepatic parenchyma. C, Post–contrast-enhanced, T1-weighted fat saturation image shows the lesion as centrally hypointense, in contrast to the normal surrounding hepatic parenchyma, with minimal areas of marginal enhancement. D, Fused computed tomography/fluorodeoxyglucose–positron-emission tomography image confirms avid tracer uptake within the lesion, consistent with active tumor. Note the differences in appearance from hemangioma (see Fig. 19.7).

et al, 2010; Maegerlein et al, 2015; Seo et al, 2011). Arterial phase imaging may also provide anatomic detail preoperatively; however, CT angiography offers improved spatial resolution over MRI.

Hepatocellular Carcinoma The diagnosis of HCC (see Chapter 91) on CT and ultrasound can be challenging. It is often associated with cirrhosis and parenchymal heterogeneity (Fig. 19.11), increasing the difficulty in distinguishing focal hepatic lesions. Cirrhotic livers show both nodular hepatic contour and parenchymal multinodular change. These nodules represent a spectrum, from regenerative or dysplastic nodules to HCC. These nodular lesions are generally thought to constitute a multistep spectrum

of disease, progressing from benign to malignant, which occurs in response to hepatic parenchymal damage and scarring (Lee et al, 2012). The advantage of MRI in the evaluation of HCC is that areas of carcinoma show differences in signal intensity, diffusion, blood pool, and functional hepatocyte enhancement and growth compared with regenerative or dysplastic nodules, or background cirrhotic parenchyma. MRI also has an advantage over other modalities in assessing vascular invasion. Although HCC generally is hypointense on T1-weighted and hyperintense on T2-weighted images, and shows arterial phase hyperenhancement with delayed dynamic phase washout, signal intensity may be variable (Silva et al, 2009), particularly for early HCC measuring less than 2 cm (Rhee et al, 2012). On T2-weighted images, larger, more poorly differentiated

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FIGURE 19.10.  Hepatic metastasis from breast carcinoma. A, T2-weighted image shows a small lesion within the liver (straight arrow) that is mildly hyperintense. Portal lymphadenopathy also is present (curved arrow). B, Pre-contrast-enhanced, T1-weighted gradient-echo image shows a welldefined mass (arrow) that is hypointense to liver. C, T1-weighted gradient-echo image after the administration of contrast shows the peripheral portion that enhanced irregularly during the arterial dominant phase (not shown) beginning to wash out, suggesting an early target appearance (arrow). D, Delayed postcontrast gradient-echo image shows continued filling of the central portion of this lesion; the periphery has washed out (arrow).

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FIGURE 19.11.  Liver with cirrhotic configuration. A, T1-weighted spin-echo image at the level of the portal vein shows hypertrophy of the left lobe and caudate with atrophy of the right lobe of the liver. B, T2-weighted image of the liver at the same level shows mildly heterogeneous signal in the atrophied right lobe, making exclusion of an underlying mass difficult. Note also the focal hepatic scarring (arrow).

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FIGURE 19.12.  Large hepatocellular carcinoma showing the mosaic pattern. A, T1-weighted image shows a large heterogeneous mass (m) in the liver. Note the hypertrophied left hepatic lobe and caudate. B, T2-weighted image also shows the mass (m) to be heterogeneous, with islands of tissue that are hyperintense with respect to other portions of the same mass. This pattern is known as the mosaic pattern, and it can be seen in hepatocellular carcinoma, especially when the lesion is large.

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FIGURE 19.13.  Hepatocellular carcinoma containing intracellular lipid. A, T1-weighted in-phase gradient-echo image shows a mass in the liver that is darker than normal parenchyma, with a small internal area of brighter signal (arrow). B, T1-weighted out-of-phase gradient-echo image shows the same area (arrow) has lost signal, which is consistent with an admixture of fat and water-containing tissue.

HCCs may be heterogeneous due to areas of necrosis (Fig. 19.12). Areas of heterogeneity may indicate moderate to poorly differentiated tumors (Witjes et al, 2012). HCC may show high T1 signal (Fig. 19.13) (Basaran et al, 2005), and lesions with intracellular lipid show signal loss on out-of-phase T1-weighted imaging. Intracellular lipid may be an important clue to small HCCs without typical enhancement patterns, as early HCC are sometimes hypovascular in arterial phase (Rhee et al, 2012). Contrast washout in portal venous or later dynamic phase MRI, as well as heterogeneous enhancement, has been associated with moderate to poorly differentiated tumors; a higher percentage of well-differentiated tumors may not demonstrate washout (Okamoto et al, 2012; Tan et al, 2011; Witjes et al, 2012). In addition to washout, peritumoral capsules, which are low in signal intensity on the arterial dominant phase and enhance later, are also associated with microvascular invasion, an important feature with clinical significance (Witjes et al, 2012). These are distinguishing features in contrast to benign or dysplastic nodules. Although dysplastic nodules may also

hyperenhance, they tend to be more homogeneous and isointense to background liver parenchyma during the equilibrium phase, often show low T2 signal, and are variable on delayed hepatobiliary phase imaging (Cruite et al, 2010). In HCC, hepatocyte contrast agents generally show hypointensity in delayed hepatobiliary phase imaging compared with surrounding liver in the vast majority of cases, with a small percentage showing variable internal uptake, which may reflect either the grade of tumor differentiation or specific enzyme production and tumor receptors. (Cruite et al, 2010) Rarely, welldifferentiated HCC may show tracer concentration in delayed hepatobiliary phase with areas of increased signal intensity. Due to the complexity of imaging features and overlap, as well as multimodality availability, there have been organized efforts to improve report standardization and communication regarding imaging findings, as well as recommendations regarding surveillance imaging and screening. The American Association for the Study of Liver Diseases (AASLD) issued revised recommendations in 2010, recommending HCC screening for



Chapter 19  Magnetic resonance imaging of the liver, biliary tract, and pancreas

patients at risk with ultrasound (US) every 6 months combined with serum α-fetoprotein (AFP). MRI imaging has an emphasis on arterial phase hyperenhancement and washout, with biopsy improving sensitivity for indeterminate lesions (Tan et al, 2011). The Liver Imaging Reporting and Data System (LIRADS), an initiative supported by the American College of Radiology, as well as the Liver and Intestinal Organ Transplant Committee (OPTN) have also issued guidelines regarding HCC classification. LI-RADS was revised in 2014 to better incorporate OPTN and AASLD guidelines regarding growth and lesion detection on screening ultrasound. Ancillary features, including nodule-in-nodule or mosaic appearance, intralesional fat, diffusion restriction, and perilesional or coronal enhancement, for example, are also incorporated into the LI-RADS imaging interpretation algorithm (http://nrdr.acr.org/ lirads/). Although continued revisions are inevitable, consensus guidelines will encourage report standardization, improve communication, and ultimately improve decision making. Agreement among readers using newer algorithms tends to be moderate to substantial for expert readers but lower among novices, suggesting implementation of criteria may require a learning curve (Davenport et al, 2014).

Fibrolamellar Carcinoma Fibrolamellar HCC (FLHCC), a rare variant, occurs in young adults, and is distinct from conventional HCC on multiple levels, including molecular and clinical differences (see Chapter 91). The average age at presentation is 25 years, and the presence of nodal metastases is common (Ganeshan et al, 2014). FLHCC may have a central scar, and these scars tend to be low in signal intensity on T1-weighted and T2-weighted images due to fibrous changes. FNH also may have a central scar, although FNH scars generally show increased T2 signal; however, it is not always a reliable differentiating feature (Ganeshan et al, 2014; Kim et al, 2009). FLHCC may show early heterogeneous enhancement with variable washout and may show partial hepatocyte agent concentration in delayed hepatocyte phase imaging (Meyers et al, 2011). Metastatic disease is also common at presentation, both in the abdomen and chest, with adenopathy being prominent (>2 cm) in a majority of cases (Do et al, 2014).

LESS COMMON HEPATIC TUMORS Lymphoma Non-Hodgkin lymphoma accounts for most hepatic lymphomas. MRI shows masses of varying size, which are generally low in signal intensity on T1-weighted images and hyperintense on T2-weighted images (Coenegrachts et al, 2005; Rizzi et al, 2001), but not as great as with other benign hepatic lesions, such as hemangiomas or cysts. This difference in signal intensity makes it relatively easy to determine the malignant nature of the lesion but not the specific tumor type. Lymphoma may be relatively hypoenhancing in early dynamic phases, with isointensity in later phases (Coenegrachts et al, 2005). The imaging characteristics of lymphoma overlap with other malignant hepatic lesions.

Angiomyolipoma Hepatic angiomyolipoma (HAML) is an uncommon tumor, more frequent in females, which currently presents a diagnostic challenge for imaging diagnosis (see Chapters 89 and 90).

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Although extracellular lipid is typical in HAMLs, they often show brisk, avidly enhancing soft tissue components with washout, which overlaps with other liver malignancies, such as hepatocellular carcinoma. HAML is generally considered a benign, solitary tumor comprising three elements: smooth muscle, thick-walled blood vessels, and mature adipose tissue (Cai et al, 2013), although they can also present without fat (Butte et al, 2011). Limited data suggest a well demarcated, lipid-containing tumor with a thin peripheral enhancing rim, lack of peritumoral capsule, and an early draining vein that may offer clues to the diagnosis, particularly in patients without cirrhosis or elevated serum AFP (Fig. 19.14) (Cai et al, 2013; Du et al, 2012). Surgical excision remains preferable, as there have been reports of HAML hemorrhage and capsular rupture, as well as venous obstruction or Budd-Chiari syndrome from mass effect. Furthermore, pathologic analysis remains essential for diagnostic confirmation; immunohistochemical stains for markers may offer increased accuracy (Du et al, 2012).

Mesenchymal Tumors Rarely, tumors of mesenchymal origin, including hepatic angiosarcoma (HAS), leiomyosarcoma, and fibrous histiocytoma, may be present within the liver (Anderson et al, 2009). The MRI appearance of these tumors is nonspecific; they are generally of low intensity on T1-weighted images and hyperintense on T2-weighted images, with heterogeneous enhancement. HAS typically shows hyperenhancement, often peripheral, but sometimes central and irregular, with progressive fill-in on more delayed phases following blood pool. Although similar to cavernous hemangiomas, HAS may also show centrifugal or “reverse” enhancement patterns and show greater degrees of variance and heterogeneity, with more disordered peripheral enhancement than nodular, discontinuous, clump-like enhancement typically seen in benign hemangiomas. HAS is typically multiple, of varying size, involving both hepatic lobes, and rapidly growing, as median survival has been reported as less than 6 months (Huang et al, 2014; Pickhardt et al, 2015).

Epithelioid Hemangioendothelioma Epithelioid hemangioendothelioma (EH) is a rare tumor of adults that should be distinguished from infantile hemangioendothelioma, which occurs in infants and children. The prognosis is better for EH than for other parenchymal tumors, such as sarcoma, although extrahepatic metastases do occur. EH is generally low in signal intensity on T1-weighted images and hyperintense on T2-weighted images but not as hyperintense as hemangiomas. They also may show a distinct target sign on T1- and T2-weighted images (Economopoulos et al, 2008). After Gd-DTPA administration, an irregular nodular and concentric enhancement may be seen. Capsular retraction also may be seen with EH, helping to elucidate the diagnosis (Lin et al, 2010) (see Chapter 89).

Biliary Cystadenoma and Biliary Cystadenocarcinomas Biliary cystadenomas and biliary cystadenocarcinomas are rare tumors of the liver that present as a predominantly cystic mass containing enhancing septations (Fig. 19.15; see Chapters 89 and 90B). These lesions may be of low, intermediate, or increased signal on T1-weighted images, depending on the protein or hemorrhagic content within the mass, and they are usually hyperintense on T2-weighted sequences. Enhancing mural or internal irregular soft tissue nodularity, as well as

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FIGURE 19.14.  Angiomyolipoma. A, T1-weighted in-phase image shows a high-signal right hepatic mass (arrow). B, T1-weighted out-of-phase image shows low signal at its margin due to chemical shift artifact between the mass and surrounding liver, confirming the presence of bulk fat.  C, Axial T2-weighted image with fat saturation shows the mass is dark. D, Axial T1-wieghted fat saturation image shows the mass is dark as well.

internal hemorrhage, has been shown in to be more often associated with cystadenocarcinomas; however, differentiation may not be possible (Arnaoutakis et al, 2015; Qian et al, 2013; Wang et al, 2012). Communication with the biliary ductal system on delayed hepatobiliary phase imaging with hepatocyte contrast agents has been reported, helping to differentiate these tumors from nonneoplastic simple hepatic cysts. (Billington et al, 2012; Marrone et al, 2011). Infectious cysts may also communicate with the biliary tree, however, such as with complex cysts from echinococcal disease.

BILIARY TUMORS Gallbladder Carcinoma MRI offers improved characterization of gallbladder cancer compared with other modalities. However, its differentiation from inflammatory conditions, which may occur concurrently, may be difficult (see Chapters 33 and 49). As opposed to the inflammatory associated wall thickening with maintained mucosal and submucosal layers, with differential enhancement, gallbladder cancer typically shows irregular intermediate to

high T2 signal thickening of the gallbladder wall, with early and prolonged heterogeneous enhancement, often in patients with multiple gallstones (Tan et al, 2013). Cholelithiasis is a predisposing condition. Evidence of liver invasion and spread to regional lymph nodes also may be identified with MRI, which is optimal for evaluation of these lesions because of its outstanding tissue contrast, ability to image directly in multiple planes (Dai et al, 2009; Sikora et al, 2006), with DWI offering added sensitivity and specificity for the primary tumor as well as nodal or hepatic metastatic disease (Tan et al, 2013). Fluorodeoxyglucose (FDG) PET imaging may also provide improved sensitivity and specificity for nodal or distant metastatic disease (Lee et al, 2010).

Bile Duct Cancer Bile duct tumors (see Chapters 50, 51, and 59) are well depicted on MRI and may be intrahepatic, hilar, or extrahepatic, with a mass-forming appearance or as periductal infiltrating or papillary intraductal morphologies (Chung et al, 2009). Tumors may show focal nodular thickening along ductal walls or be discrete intrahepatic masses. Central hilar

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C FIGURE 19.15.  Biliary cystadenoma. A, Coronal T2-weighted image shows a lobulated mass (m) in close proximity to the left hepatic duct (arrowhead). B, Projection image in the coronal oblique plane shows the well-defined hyperintense mass to be in continuity with a mildly dilated left-sided biliary radicle (straight arrow). The normal right-sided biliary radicle (curved arrow) is easily seen with this technique. C, Postcontrast, T1-weighted gradient-echo image shows no enhancement within this mass (m).

cholangiocarcinomas are the most common and are generally associated with biliary ductal dilation. Bile duct tumors may be seen as intermediate, mildly increased T2 signal (Fig. 19.16), however, less so than the surrounding biliary dilatation. The level of obstruction and continuity of the tumor with the vasculature are well evaluated with MRI (Peporte et al, 2013), although CT angiography with multiphasic imaging offers improved spatial resolution and similar accuracy to MRI to assess vascular involvement, and exceeds MRI accuracy when assessing for nodal and distant metastases (Aljiffry et al, 2009). Focused assessment of ductal, portal venous, and hepatic arterial involvement is performed in staging and preoperative imaging. Peripheral, mass-forming intrahepatic cholangiocarcinomas occur in approximately 10% of cases and generally have high T2 signals, lobulated heterogeneous masses with peripheral enhancement, and gradual centripetal fill-in (Fig. 19.17) (Hennedige et al, 2014, Kang et al, 2012) (see Chapter 50). Satellite lesions are common within the liver, and these lesions tend to show low signal intensity on delayed hepatocyte imaging phase (Chung et al, 2009; Kim et al, 2011; Peporte et al, 2013). Intrahepatic cholangiocarcinomas may show adjacent

capsular retraction as well as vascular encasement, typically without tumor thrombus (Chung et al, 2009). Smaller tumors may show early, more uniform arterial enhancement, particularly when less than 4 cm in size (Kim et al, 2011). Intraductal papillary neoplasm of the bile duct is an uncommon subtype of bile duct cancer with characteristics similar to pancreatic intraductal mucinous neoplasms. The majority of tumors occur near the hilum or in extrahepatic ducts, and the clinical outcomes are better than conventional bile duct cancers (Rocha et al, 2012).

BENIGN DISEASES OF THE BILIARY TRACT Cholelithiasis and Choledocholithiasis Gallstones are well identified on T2-weighted images and on MRCP sequences (Fig. 19.18; see Chapters 32, 33, and 36). They usually appear as low signal intensity structures in a fluid-filled gallbladder. Ultrasound is usually the modality of choice in the evaluation of uncomplicated cholelithiasis or cholecystitis due to its high sensitivity and lower cost, but choledocholithiasis is more effectively imaged by MRI (Johnson et al, 2010; Samaraee et al, 2009). Coronal T2-weighted imaging,

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C FIGURE 19.16.  Hilar mass. A, Axial T2-weighted image shows dilatation of the left hepatic duct and a low to intermediate–intensity mass expanding the right hepatic duct (arrow). B, T1-weighted late arterial phase postcontrast image shows early enhancement of the tumor (arrow). C, Portal venous phase postcontrast T1-weighted imaging shows the mass (arrow) is hypoenhancing to liver. Biopsy of a more superior intrahepatic mass in this patient with cirrhosis showed poorly differentiated hepatocellular carcinoma. Extension into the biliary ducts with ductal expansion and obstruction is an uncommon finding in hepatocellular carcinoma (HCC). Although HCC and cholangiocarcinoma may exist simultaneously as two distinct tumors, a distinct subtype of mixed HCC-cholangiocarcinoma is increasingly recognized.

performed routinely with MRI, readily identifies common duct stones (Fig. 19.19). MRCP can also be obtained to evaluate whether retained stones are present after cholecystectomy, although clips may limit examinations in the perioperative time period. If no stones are present on MRCP, this may obviate the need for ERCP.

Choledochal Cysts Choledochal cysts (see Chapter 46) represent dilatation of the extrahepatic bile ducts with possible associated intrahepatic biliary duct dilatation. This entity is a relatively uncommon congenital anomaly that usually presents before 10 years of age. The classic triad includes a palpable mass, abdominal pain, and jaundice. Cysts may be associated with chronic inflammation and increase the risk for cholangiocarcinoma. Five types of cysts

have been described (Lee et al, 2009; Todani et al, 1977): type I has been subdivided into A, B, and C, in addition to types II, III, IVa, IVb, and V. Recently, there have been advocates for dropping the numeric classification system, instead using a more descriptive, clinically meaningful nomenclature (Visser et al, 2004). MRI is well suited not only to diagnose these cysts, but also to classify the type of cyst. Not only can 3D MR cholangiograms depict the normal and abnormal anatomy, but direct coronal imaging and delayed scans post–hepatocytespecific gadolinium-based contrast agents can be obtained for further evaluation (Gupta et al, 2010).

Postoperative Biliary Complications Complications from surgical procedures include bile leaks, abscess formation, and biliary strictures (see Chapters 27 and

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FIGURE 19.17.  Mass forming intrahepatic cholangiocarcinoma. A, T1-weighted in-phase gradient-echo image shows a peripheral hypointense mass (m) with capsular retraction. B, A T2-weighted fat saturation image shows increased signal intensity within the mass. C, Diffusion-weighted image shows restricted diffusion within the mass, which is brighter than liver. D, T1-weighted fat saturation post–contrast-enhanced image in late arterial phase shows peripheral enhancement. E, T1-weighted post–contrast-enhanced image in late portal venous phase shows only partial filling of the mass. F, T1-weighted post–contrast-enhanced image in late portal venous phase at a separate level shows an additional satellite lesion (arrow), smaller in size with marginal enhancement, typical of intrahepatic metastasis.

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FIGURE 19.18.  Choledocholithiasis. A, Coronal T2-weighted single-shot fast spin-echo image through the common duct in a patient after cholecystectomy shows multiple stones within the common bile duct. Note the distal stone impacted at the level of the ampulla (arrow). B, Axial T2-weighted image with the same technique also shows a stone, surrounded by bile, in the distal common bile duct (arrow).

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biliary phase scans by opacifying the biliary tree, which may improve diagnostic confidence to evaluate for active biliary leak, anatomic variants, and choledocholithiasis (Boraschi & Donati, 2014; Gupta et al, 2010).

PANCREAS Solid Tumors of the Pancreas

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Pancreatic solid tumors include both neoplastic and nonneoplastic origins (see Chapter 59). Although MRI characteristics alone may help predict etiologies, there may be imaging-feature overlap between entities. Imaging characteristics combined with clinical presentation and demographic data often guide therapy and predict the diagnosis. Neoplastic etiologies include tumors arising in the pancreas, such as adenocarcinoma, solid pseudopapillary tumor, neuroendocrine tumors, or pancreatic lymphoma. Metastases, lymphomas, and other rare tumors may involve the pancreas (Low et al, 2011). In addition to enhancement characteristics, such as early arterial enhancement in neuroendocrine tumors, precontrast signal characteristics, such as hemorrhage with increased T1 signal in solid pseudopapillary tumors, may provide clues to the diagnosis. Nonneoplastic etiologies include focal pancreatitis, accessory spleen, focal fat, congenital anomalies, and other rare etiologies. Intrapancreatic splenic tissue shows signal intensity and enhancement characteristics paralleling that of spleen on all MRI sequences. Focal pancreatitis may mimic pancreatic adenocarcinoma; however, restricted diffusion on DWI is typically seen in adenocarcinoma (Fattahi et al, 2009; Kartalis et al, 2009), compared with focal pancreatitis. Focal pancreatitis may also cause irregular narrowing of the main pancreatic duct, as opposed to the abrupt cutoff and upstream dilatation with associated parenchymal atrophy noted in adenocarcinoma (Low et al, 2011; Siddiqi et al, 2007).

Pancreatic Cancer B FIGURE 19.19.  Afferent loop syndrome. A, Coronal T2-weighted sequence in a patient after pancreaticoduodenectomy for pancreatic carcinoma. The patient has a hepaticojejunostomy with a patent anastomosis (white arrow). The afferent loop is markedly dilated (black arrow) compared with other loops of bowel. No mechanical obstruction was noted. B, Axial T2-weighted image through the level of the anastomosis (arrow) shows the site to be patent without evidence of a stricture. The afferent loop is dilated.

30). Although using other imaging modalities in the immediate postoperative setting to assess for bile leaks and abscesses may be prudent, MRI is uniquely suited for the assessment of the postoperative biliary tract. Bile duct injuries after surgery can be caused by a number of things, but these may all lead to the same result—a stricture at the anastomotic site. MRCP is uniquely suited for addressing this problem. Surgical clips near the anastomosis may create streak artifacts during CT scanning, but currently used clips are less problematic for MRI. MRI has multiplanar capabilities and superior tissue contrast, and MRCP sequences can minimize susceptibility to artifacts to allow improved visualization of the region of the anastomosis and improved diagnostic confidence (see Fig. 19.19). More recently, hepatocyte contrast agents also allow delayed hepato-

Pancreatic adenocarcinoma (see Chapters 59 and 62) is well evaluated on MRI and MRCP. T2, diffusion-weighted, and dynamic contrast-enhanced T1-weighted MRI sequences are particularly helpful to characterize pancreatic cancer and stage patients (Tamm et al, 2012). MRI is generally thought to be highly accurate in assessing for vascular invasion or encasement (tumor surrounds >50% of the vessel circumference), localregional tumor extension, and liver metastases (Tamm et al, 2012), although recent data have suggested MRI may underestimate tumor size (Hall et al, 2013; Legrand et al, 2015). Similar to other modalities, accuracy is limited for assessment of lymph node metastases with MRI techniques. Pancreatic cancer subtypes may be predicted based on imaging features, patient demographics, and clinical presentation, particularly neuroendocrine tumors; however, there is overlap in the imaging appearance and location of solid and cystic neoplasms. Preliminary data suggest grade of tumor enhancement may correlate with tumor grade in adenocarcinomas (Lauenstein et al, 2010).

Cystic Lesions of the Pancreas Detection of pancreatic cystic lesions (see Chapter 60) has increased in recent years with increased imaging and improved spatial resolution. They are increasingly found incidentally. Benign lesions may also progress to malignant lesions over time. Diagnostic possibilities include benign lesions, such as pseudocysts, serous cystadenomas, and true epithelial cysts; to



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benign lesions with potential for malignant degeneration, such as intraductal mucinous or parenchymal mucinous cystic neoplasms; to malignant lesions, such as rare cystic or necrotic adenocarcinomas or cystic neuroendocrine tumors (Kucera et al, 2012). Multiple imaging modalities may be used to identify and characterize pancreatic cystic lesions, including ultrasound, CT, and MRI.

to have a higher association with malignancy; their presence may warrant surgical excision (Kim et al, 2014). Evaluation after contrast administration helps assess cyst wall irregularity or enhancement of soft tissue components. Molecular analysis of cyst contents to assess for genetic mutations or gene silencing associated with IPMN may offer improved accuracy in diagnostic workup (Freeny et al, 2014).

Intraductal Papillary Mucinous Neoplasms

Autoimmune Pancreatitis

Cystic lesions arising from intraductal papillary mucinous neoplasms (IPMNs) of either branch-duct, main-duct, or mixed type have been increasingly discovered (Campbell et al, 2015), and the International Association of Pancreatology has published consensus guidelines for their management, which were last revised in 2012 (see Chapter 60). The International Association of Pancreatology stratifies patients based on “high risk” or “worrisome” features, with guidelines regarding resection, endoscopic ultrasound, and imaging follow-up (Tanaka et al, 2012). A recent revision includes main duct dilatation greater than 5 mm (without obvious cause) as a worrisome feature, but also downgraded size greater than or equal to 3 cm from high risk to worrisome (Tanaka et al, 2012). Soft tissue contrast is improved with MRI imaging, which permits optimal depiction of features deemed worrisome or high risk, such as septations, thick enhancing walls, nodularity, or soft tissue components. The single-shot fast spin-echo T2-weighted sequence is particularly useful, as cystic lesions are high in T2 signal and bright on these sequences, and communication with the main duct may be more evident (Acar et al, 2011; Campbell et al, 2015). Although branch-duct IPMNs have a lower incidence of malignancy compared with main- or mixed-duct subtypes, recent meta-analysis has shown mural nodules in branch-duct IPMNs

An increasing incidence of autoimmune pancreatitis is most likely due to increased awareness and thereby increased detection (Vlachou et al, 2011) (see Chapter 59). Imaging features may suggest the diagnosis and are correlated with clinical features, including elevated serum immunoglobulin G4, relatively indolent clinical presentation, and response to corticosteroid therapy. Autoimmune pancreatitis may be diffuse, focal, or multifocal, and often shows heterogeneous T1 hypointensity, slight T2 hyperintensity, and delayed enhancement due to associated fibrosis. Fibrosis may also show diffuse T2 hypointensity (Vlachou et al, 2011). Diffuse main pancreatic ductal narrowing or irregularity, and upstream common bile duct dilatation or wall thickening, may be present. There may also be a peripancreatic halo, which appears hypointense on T1- and T2-weighted imaging postcontrast (Heyn et al, 2012). Focal disease, particularly when involving the head with associated upstream pancreatic and main bile duct dilatation, may mimic ductal adenocarcinoma. Extrapancreatic manifestations may also be present within the spectrum of immunoglobulin G G4related sclerosing disease, including bile duct, renal, and retroperitoneal involvement (Kim et al, 2013). References are available at expertconsult.com.



Chapter 19  Magnetic resonance imaging of the liver, biliary tract, and pancreas 377.e1

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