Keywords

Brief Introduction of MRI

MRI is acquired using the property of water molecules, specifically hydrogen protons, which behave as small magnets and align within the strong magnetic field , therefore requiring no radiation. Depending on pulse sequences, different soft tissue contrast can be achieved. For example, T1 sequences can highlight intrinsic blood, protein, melanin, and administered gadolinium-based contrast, while T2 can be used to identify edema, ascites, CSF, bile, and other fluids. Magnetic resonance cholangiopancreatography (MRCP ) utilizes the T2 characteristics of fluid in bile to assess the biliary system [1]. More recently, diffusion-weighted imaging (DWI ) has been incorporated into routine abdominal imaging. DWI captures the free motion of the water molecules and can magnify tissues that restrict that movement such as tumor, ischemic tissue, abscess, and lymph nodes. For pancreatic adenocarcinoma, DWI is more sensitive than computed tomography (CT) for accentuating the tumor and also detecting lymph nodes and metastases, making it useful for staging [2].

Clinical MRI is available in various magnetic field strengths; however, the current standard of care for most imaging applications, and most commonly available scanners, has a field strength of 1.5 T (1.5 Tesla). 3 T strength MRI systems are becoming more widely available; however, while 3 T theoretically allows for higher signal to noise, in practice there is no appreciable imaging quality difference between 1.5 T and 3 T [3,4,5,6]. In fact, there are some limitations with 3 T MRI of the abdomen, which are beyond the scope of this chapter, and 1.5 T remains the standard of care for most abdominal MRI applications. As technology evolves, however, 3 T may ultimately replace 1.5 T systems.

Generally, patients fast for at least 4 hours to distend the gallbladder and to minimize artifacts related to motion from the stomach and regional bowel. As a complete evaluation protocol, many institutions use the following sequences: T2-weighted sequences ; T1 in and opposed phases; T1-weighted sequences before and after intravenous administration of gadolinium-based contrast material at arterial, portal venous, and equilibrium phases (20–40 s, 45–65 s, and 3–5 min, respectively); 2D and 3D MRCP; and DWI [Table 2.1] [15, 16].

Table 2.1 Pancreatic mass MR protocol
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Additionally, while most images are routinely obtained in the axial plane, MRI allows for acquisition in any orientation, and coronal, sagittal, or other non-traditional planes of imaging can be acquired for many of the imaging sequences.

Patient Factors

While widely used, not all patients are suitable to undergo MRI. One major consideration is the presence of metallic foreign bodies such as cardiac or other electronic implants, embolization coils, endoscopic hemostatic clips, aneurysm clips, and shrapnel/bullet fragments. The most important concern with metallic objects is patient safety; ferromagnetic objects can be deflected by the magnetic field with potential for damage to adjacent tissues and organs. Magnetic fields can also induce electrical currents within wires or circular metallic objects leading to heating and potential thermal injury. However, many metallic implants are considered MRI compatible and do not necessarily preclude patients from undergoing imaging. By carefully assessing MRI compatibility of the device and risk, MRI can be safely performed [7]. All MRI facilities utilize extensive checklists to ensure that a patient scheduled for an exam is safe to undergo the procedure. The third issue with metallic objects is not related to safety but to image quality. Some metallic objects, although safe for imaging, can cause local alterations to the magnetic field and lead to significant image artifacts limiting diagnostic quality. For example, embolization coils in the gastroduodenal artery or endoscopic hemostatic clips in the duodenum may preclude evaluation of the adjacent pancreas due to associated susceptibility artifact [Fig. 2.1].

Fig. 2.1
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(a) A metallic surgical clip in the left upper quadrant (black arrow) as noted in the abdominal radiograph can create (b, c) MRI susceptibility artifact and limit the evaluation of the surrounding structures

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Obesity is a factor to consider, as MRI scanners typically have lower table weight limits and smaller bore size than CT scanners, and some patients may not be physically able to undergo MRI. However, newer MRI machines have larger bore sizes equivalent to CT (70 cm). So-called “open MRI” scanners can accommodate some of these patients, but usually at the expense of image quality given lower magnetic field strengths (typically 0.2–1.2 T). Even with a larger bore size, the MRI bore length is significantly greater than that of a CT scanner, frequently leading to patient anxiety and claustrophobia which may lead to early termination of the scan or outright refusal. Estimates are that 1–15% of patients are unable to complete an MRI scan secondary to claustrophobia [8, 9].

Other patient factors do not necessarily preclude patients from undergoing MRI but may adversely affect image quality leading to a suboptimal study. MRI of the abdomen requires a much longer scan time than CT, approximately 30–40 minutes for a complete study. Some of the imaging sequences require minutes and are obtained with free-breathing; others require 10–20 seconds and are obtained with breath-holds. Suboptimal breath-holding as well as irregular breathing can lead to artifacts degrading image quality; this is much less of an issue with modern CT scanners. Also, as MRI utilizes the characteristics of water molecules to create images, the presence of large fluid collections, ascites, and soft tissue edema/third spacing can deteriorate imaging quality, particularly MRCP [Fig. 2.2].

Fig. 2.2
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(a) Biliary ducts on MRCP coronal images are not well visualized due to (b) artifacts from the ascites (white arrow) and anasarca (arrow head) as seen on the axial T2 sequence

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Probably the most commonly encountered patient condition regarding suitability for MRI is renal dysfunction. Gadolinium contrast agents are not nephrotoxic and do not impart a risk for impairing renal function even in patients with end-stage renal disease. However, since the FDA initially reported an association between nephrogenic systemic fibrosis (NSF) and gadolinium-based contrast agent administration, it has been found that patients were at increased risk with severe renal dysfunction (GFR <30) and specific types of contrast agents (termed group 1 agents). With continued research and experience, however, it has been realized that there is very low or negligible risk of NSF development, regardless of renal function status, with what are termed group 2 agents. In conjunction with improved screening of patients for renal dysfunction and change in practice to using group 2 agents, the incidence of NSF has essentially been eliminated. The current American College of Radiology Contrast Manual recommends usage of group 2 gadolinium agents for renally impaired patients requiring contrast-enhanced MRI [10]. Many institutions and imaging centers have switched to using exclusively group 2 contrast agents, in which case screening for renal dysfunction may no longer be necessary. In short, even patients with renal dysfunction can safely undergo contrast-enhanced MRI with current screening standards and modern contrast agents.

Role of MRI

With MRI’s higher sensitivity compared to CT in determining resectability of tumor (93% vs. 87%), vascular infiltration (80% vs. 50%), and liver involvement (90–100% vs. 70–76%), it is a useful tool in tumor staging [10,11,12,13]. As such, the American Society of Clinical Oncology (ASCO) recommends MRI as a part of the initial assessment after histopathologic confirmation of pancreatic adenocarcinoma diagnosis and before initiating therapy [14]. The National Comprehensive Cancer Network guidelines also suggest MRI as a problem-solving tool for initial workup and treatment response surveillance in addition to the multi-detector CT [15]. Especially for small, isoattenuating, or CT indeterminant masses, high soft tissue contrast of MRI is superb for diagnosis. MRCP is also a noninvasive way to evaluate for small mural nodules or differentiate stones from masses. With a combination of these characteristics, MRI and MRCP aid in early detection, staging, and surveillance of pancreatic cancer.

Imaging Characteristics of Pancreatic Cancer

Normal Versus Pancreatic Cancer

The normal pancreas typically has high signal on non-contrast-enhanced T1 fat-suppressed sequences. Following contrast administration, there is homogeneous intense enhancement during the early arterial phase which becomes isointense to the liver on later phases [Fig. 2.3]. Pancreatic adenocarcinoma generally appears hypointense compared to the surrounding parenchyma on T1 fat-suppressed images. It is also hypoenhancing and is best delineated during the late arterial phase as hypoenhancing tissue relative to the surrounding pancreatic parenchyma, with progressive enhancement on later post-contrast phases [Fig. 2.4] [16, 17]. These imaging characteristics may be due to fibrotic characteristic of pancreatic cancers [17, 18].

Fig. 2.3
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Normal pancreas enhancement pattern. (a) In non-contrast T1 fat-suppressed sequence, the normal pancreas is brighter than the liver and spleen. (b) During the early arterial phase, the pancreas enhances homogeneously and (c) becomes isointense to the liver during delayed phases. (d) On equilibrium post-contrast sequence, the pancreas appears darker than the liver

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Fig. 2.4
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Pancreatic adenocarcinoma (white arrow) at the head of the pancreas shows (a) hypoenhancement during the early arterial phase and (b) progressive enhancement on the delayed phase. (c) It also appears bright on DWI as it restricts diffusion. (d) On T2, the mass can be seen with intermediate signal and vague margins

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Secondary signs related to obstruction of the pancreatic duct, such as peripheral pancreatic ductal dilation and atrophy of the pancreatic tissue peripheral to the tumor, also aid in the detection of the tumor on MRI. Ductal dilation in particular is best visualized on MRCP, and a hallmark feature of pancreatic adenocarcinoma is dilation of both the common bile duct and the pancreatic duct, or “double duct sign ” [Fig. 2.5]. Although not entirely specific to malignancy, it is commonly seen with pancreatic cancer [17].

Fig. 2.5
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(a) Normal MRCP maximal intensity projection (MIP) shows nondilated common bile duct (white arrow) and pancreatic duct (white arrow). (b) Corresponding T2 sequence shows a normal pancreatic duct (white arrow). In contrast, (c) “double duct sign” can be seen in the abnormal MRCP MIP with dilation of both the common bile duct (+) and the pancreatic duct (*) due to (d) the pancreatic head mass (black arrow head) in this T1 sequence in early arterial phase. Dilated common bile (+) and pancreatic ducts (*) are again seen adjacent to the mass

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Diffusion-weighted imaging has particular sensitivity for detecting pancreatic adenocarcinoma. Lesions that may not be detectable on contrast-enhanced CT, or subtle on other MRI sequences, can be readily demonstrated [Fig. 2.6].

Fig. 2.6
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(a) Subtle hypoenhancing pancreatic body lesion (white arrow) on CT also appears subtle on (b) the arterial phase of T1 sequence MRI as a hypoenhancing mass with (c) progressive enhancement on the delayed phase. (d) On DWI, however, the mass restricts diffusion and is readily seen

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Metastatic Disease

Liver

Identifying and characterizing liver lesions is critical in determining the resectability of pancreatic adenocarcinoma. Small lesions that may be indeterminate on CT imaging can be more definitively characterized with MRI. Relative to the surrounding liver parenchyma, metastases are minimally hypointense on T1-weighted sequences and isointense to moderately hyperintense on T2-weighted sequences . Contrast enhancement pattern varies, from peripheral ring enhancement to wedge-shaped with differences in vascularity of the lesion [17]. DWI also can aid in detecting small metastases that can be overlooked in other sequences [Fig. 2.7].

Fig. 2.7
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(a) Small hypoenhancing liver metastatic lesion (black arrow) on arterial phase of T1 sequence is readily visible on (b) the DWI sequence (white arrow)

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Peritoneum and Lymph Nodes

Peritoneal involvement is common, however, in small volume, making detection difficult. MRI is found to be more sensitive than CT in detecting subcentimeter enhancing peritoneal implants [17, 19]. Peripancreatic and porta hepatis lymph node involvement is also common and can be seen as enlarged (> 1 cm in short axis) and bright (diffusion restricting) lesions on DWI sequence [Fig. 2.8]. MRI is more sensitive than CT for the detection of lymph nodes, although still limited in characterizing benign versus malignant nodes as size remains the primary factor.

Fig. 2.8
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(a) A lymph node (black arrow) by the porta hepatis is noted in a contrast-enhanced CT in arterial phase. (b) The same lymph node (white arrow) is seen restricting diffusion and bright on MR DWI sequence

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Vascular

Vascular invasion by tumor, especially the celiac artery, superior mesenteric artery, superior mesenteric vein, and portal vein, is an important factor in determining tumor resectability. T1 fat-suppressed images and gadolinium contrast-enhanced images can aid in differentiating abutment versus encasement of vasculature [Fig. 2.9] [17].

Fig. 2.9
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Venous phase of the T1 sequence clearly delineates the splenic vein (*) with gradual narrowing (arrow head) due to encasement from the surrounding pancreatic head mass (arrow)

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Future Trends in Pancreatic Adenocarcinoma Imaging

Positron emission tomography (PET) combined with CT is a useful tool in diagnosis, initial staging, and treatment response for pancreatic adenocarcinoma. With the recent advent of concurrent PET/MRI scanners, many clinical studies are underway for assessing different cancer imaging [20,21,22,23,24,25]. Currently available concurrent scanners combine PET detectors with co-located 3 T MRI detectors for better co-registration of images. Generally, protocols include whole-body MRI survey component with organ-specific focused MRI sequences for better characterization of liver, peritoneal, lymph node, or vascular involvement [20]. With the high sensitivity of PET imaging and superior soft tissue contrast resolution of MRI, combined PET/MRI has the potential to increase accuracy in early detection, staging, and recurrence.