MR Imaging of Bowel and Peritoneum

Introduction

The outstanding contrast conspicuity of MR imaging makes it a powerful and robust technique for imaging the extrahepatic abdomen. New advances in MR imaging hardware and software have eliminated motion artifact so that one can now routinely perform rapid breath-hold abdominal imaging. By combining intravenous gadolinium chelates with readily available and inexpensive intraluminal contrast agents used to distend the bowel one can depict subtle diseases of the bowel wall and adjacent peritoneum. MR imaging of the bowel and peritoneum is a vitally important part of our clinical practice. In this article I will review clinical applications, MR protocols, and techniques for MR imaging inflammatory, infectious and ischemic intestinal diseases as well as benign and malignant diseases of the peritoneum.

Gastrointestinal MR Imaging

The unique ability of MR imaging to directly visualize the bowel wall is the foundation of its power for evaluating inflammatory, infectious, ischemic, and malignant gastrointestinal tract diseases (1-12). Mural involvement of the GI tract is the common denominator for all of these varied diseases. By combining breath-hold MR imaging with intravenous gadolinium, and readily available intraluminal agents, MR imaging can consistently depict subtle mural diseases of the GI tract as areas of bowel wall thickening and enhancement (1-10).

Techniques and Protocols – GI Tract MR Imaging
Intraluminal Contrast Agents

Adequate distension of the stomach, small intestine, and colon is essential for MR imaging of the GI tract (Fig 1). Incomplete intestinal distention may mask important findings or may mimic an inflammatory or neoplastic mass. Our experience has shown the benefits of a biphasic intraluminal agent that is bright on T2-weighted images and dark on T1-weighted images. A dark or low signal intraluminal agent on the gadolinium-enhanced gradient-echo T1 images is very useful as it helps to accentuate enhancement of the adjacent diseased bowel wall. A high signal intraluminal agent on T1 images could obscure subtle mural enhancement.

Water mixed with psyllium fiber (0.8 mg/kg body weight mixed in 1 – 1.5 liters of water is effective in distending the small bowel and is well tolerated by patients (13,14). Metamucil, (The Proctor & Gamble Co. Cincinnati OH), is composed of or psyllium fiber with orange flavoring. It is a dietary fiber supplement that can be purchased off the shelf. Its active ingredient is psyllium husk, a natural plant fiber with a high percentage of soluble fiber. Since the Metamucil is orange flavored it is typically well tolerated by patients. We mix ½ scoop of Metamucil per 8 ounce glass of water and have patients ingest 4-5 glasses over 1 hr prior to the examination. To improve filling of the distal small bowel we have patients drink the first two glasses of Metamucil at home before leaving for their appointment. As water soluble contrast material Metamucil produces a biphasic appearance on MR images.

Dilute barium sulfate (ReadiCat2, EZ-EM, Lake Success, NY), which is 98% water can also be used as a biphasic intraluminal agent for MR imaging (1-3). It is well tolerated and readily available. Dilute barium sulfate is iso-osmolar so it will stay in the GI tract and not be reabsorbed. We have patients ingest 1400 cc of oral contrast material beginning 30-45 min before exam. Rectal water (500-1000) can be administered through a balloon-tipped barium enema catheter. The balloon should be filled with water to avoid the susceptibility artifact generated by the air in the balloon.

Oral contrast may also be administered via a nasojejunal tube for more controlled distension of the small bowel. This MR enteroclysis technique has been described for evaluation of inflammatory bowel disease (8,9). Alternative intraluminal agents include 2% polyethylene glycol and 2.5% Mannitol mixed with 2% locust bean gum.

Intravenous Contrast Agents

Intravenous gadolinium (0.1 mmol/kg) is administered for the gadolinium-enhanced GE images. Although not essential to the technique, we use a power injector with a bolus injection at a rate of 2 cc/sec. The non-specific extra cellular gadolinium chelates will enhance inflammatory, infectious, and malignant disease of the GI tract and peritoneum, markedly increasing their conspicuity. One mg IV glucagon is preloaded into the IV tubing and is administered at the time of gadolinium injection to decrease bowel peristalsis. An alternative antiperistaltic agent is 0.25 mg Levsin (Hyoscyamine sulfate) administered IV 5-10 min prior to the gadolinium-enhanced imaging).

Pulse Sequences

MR imaging of the gastrointestinal tract requires rapid imaging to minimize the effects of respiratory motion and peristalsis. GI tract disease can be subtle so that normal physiologic motion can obscure important findings. Rapid, breath-hold MR imaging is clearly essential for effective GI tract imaging. The entire abdomen and pelvis is imaged in the axial plane and coronal planes using Single shot rapid acquisition with relaxation enhancement (RARE and fat suppressed, gadolinium-enhanced gradient-echo MR imaging (Table 1).

Single shot RARE (single shot turbo spin-echo (SSTSE), single shot fast spin-echo (SSFSE), and half-Fourier acquisition single shot turbo spin-echo (HASTE) ) imaging is performed in the axial and coronal planes providing breathing independent T2-weighted images in a breath-hold (11, 12The examination also includes axial dual echo T1 SGE images, fat suppressed T2-weighted FSE imaging, and breath hold diffusion weighted imaging (DWI) using an intermediate b-value of 400-500 s/mm2.

Fat suppressed gadolinium-enhanced images are critical for depicting mural enhancement (1-3). They are performed following the intravenous injection of 0.2mmol/kg gadolinium chelate. An immediate and delayed set of axial images are obtained through the abdomen and pelvis combined with coronal and sagittal imaging. Gadolinium-enhanced images can be obtained with either a 2D or 3D gradient-echo pulse sequence. The 3D gradient-echo images typically use a thinner slice thickness and provide efficient anatomic coverage.

The 3D gradient-echo images typically use a slice thickness of 4mm with overlapping slices. The 3D acquisitions are known by different acronyms including THRIVE (T1W high resolution isotropic volume exam), VIBE (Volumetric interpolated breath-hold examination, and LAVA (Liver Acquisition with Volume Acceleration) (7-10). The 3D sequences are more sensitive to breathing artifact, which may degrade image quality in the uncooperative patient or one with limited respiratory reserve. The 2D gradient-echo images including FFE (fast field echo), SGE (spoiled gradient-echo), and FLASH (fast low angle shot) use a slice thickness of 8-10 mm but are characteristically sharper and show greater contrast conspicuity. In practice we often combine a 3D gradient-echo sequence for the first dynamic acquisition followed by a 2D gradient-echo sequence for the delayed set of gadolinium-enhanced images. The coronal and sagittal gadolinium-enhanced images may be obtained with either the 2D or 3D acquisitions. Specific imaging parameters will depend upon the particular MR imager and software one has available. On our current scanner we use the imaging parameters shown in Table 1.

Coil Selection

GI tract and peritoneal MR imaging is performed with phased array surface coils. Newer phased array surface coils providing coverage of the abdomen and pelvis combined with improved surface intensity correction algorithms are now available. The excellent image homogeneity and extended coverage new surface coils are important for GI tract and peritoneal imaging. Phased array surface coils improve image signal-to-noise ratio and overall image quality.

Clinical Applications for Gastrointestinal MRI
Crohn’s Disease

Crohn’s disease is a chronic inflammatory disease of the gastrointestinal tract characterized by apthous ulceration, cobblestoning, strictures, and fistula formation. Changes in the bowel wall are typically discontinuous and asymmetrical and can be depicted on cross sectional imaging studies. Helical CT and MR imaging can be used to assess the mural changes of Crohn’s disease and to depict extra intestinal complications, including fistula formation, abscesses, and phlegmons .

The MR depiction of mural changes of Crohn’s disease and other inflammatory intestinal diseases requires adequate distension of the bowel. Collapsed bowel may enhance and mimic abnormal bowel. In addition, subtle bowel wall changes may be hidden by inadequately distended bowel. Optimal intestinal distension can be achieved by combining oral and rectally administered water-soluble contrast material.

Inflammatory diseases of the GI tract such as Crohn's disease or ulcerative colitis are exquisitely depicted with MR imaging (1-7). By combining a negative oral contrast agent with fat suppressed, gadolinium-enhanced MR imaging one may show bowel wall thickening and enhancement (Fig. 2). In our experience, compared to helical CT, MR imaging is much more sensitive to earlier or milder forms of inflammatory bowel disease. In a study of 26 patients with Crohn's disease (2), depiction of mural thickening and / or enhancement was superior on the MR images which showed 55 (85%) and 52 (80%) of 65 abnormal bowel segments for the two observers, compared to helical CT which showed 39 (60%) and 42 (65%) (P <. 001, P < .05) of bowel segments affected by Crohn's disease.

The activity of Crohn’s disease can be assessed by a number of different MR imaging parameters including degree of bowel wall enhancement, the pattern of enhancement, the thickness and length of the involved diseased segment, and the presence of edema in the bowel wall on T2-weightd images (1, 5-7). Perienteric changes including enhancing lymph nodes, infiltration of mesenteric fat and increased mesenteric vascularity can also reflect active Crohn’s disease.

Gadolinium enhancement of the inflamed bowel segments facilitates detection of diseased bowel. The degree of bowel wall enhancement correlates with the activity of the inflammatory process. The enhancement of the normal bowel wall is equal to or less than that of the liver parenchyma. Bowel wall enhancement that is more than the liver is mild and enhancement that is equal to intravascular gadolinium is marked in intensity. Bowel that is actively inflamed from Crohn’s disease will show gadolinium enhancement more than that of the liver parenchyma (1). A layered pattern of bowel wall enhancement also indicates active Crohn’s disease (Fig. 3). With this layered pattern, the mucosa shows marked enhancement while the submucosa and muscularis layers show diminished enhancement to due to edema (5), 7). On the other hand, thickened bowel that does not enhance correlates with non-acute disease.

On DWI active segments of bowel with active inflammation demonstrate restricted diffusion. In our experience chronic fibrotic strictures and inactive Crohn’s disease does not show restricted diffusion. Most bowel contents are suppressed on DWI improving the depiction of bowel wall inflammation.

It is equally important to accurately depict complications of Crohn’s disease. In our experience MR imaging and helical CT are equivalent for depicting a fistula, abscess, or phlegmon. Gadolinium enhancement of an extra intestinal abscess or phlegmon facilitates their detection on MR imaging. Fistulas will be depicted directly as fluid or air-filled tracts between adjacent bowel loops, viscera, and/or the abdominal wall. More commonly, one may see distortion and tethering of bowel loops at the site of fistulous connection. Perirectal fistulas are often best shown on surface coil thin section STIR images angled perpendicular and parallel to the rectum.

Mesenteric Ischemia

Diagnosing intestinal ischemia can be a clinical and imaging challenge (15-24). The presenting symptoms of cramping abdominal pain, leukocytosis, diarrhea, and hematochezia are not specific and can be seen with inflammatory or infectious intestinal diseases. Mesenteric ischemia is caused by inadequate arterial supply or insufficient venous drainage of the involved segment of bowel. Arterial insufficiency is much more common and may be due to occlusive or non-occlusive causes. The much less common non-occlusive causes are related to low flow states and hypo perfusion. Occlusive mesenteric ischemia may be due to atheromatous plagues, embolic disease, and abdominal aortic aneurysms with a dissection involving the SMA. Less common mechanical causes of intestinal ischemia include adhesions, small bowel herniation, intusception, or a volvulus. Hypercoaguable states such as antiphospholipid syndrome can also predispose patients to thrombotic occlusion of mesenteric vessels.

By combining gadolinium-enhanced MR angiography and anatomic MR imaging of the abdomen and pelvis, one can perform a comprehensive exam in the patient with suspected intestinal ischemia (20-24). The MRA will evaluate possible occlusive disease of the SMA and IMA, while the anatomic images will show mural thickening of the involved bowel segments. Our current approach involves administering the intraluminal contrast agents described above, followed by a gadolinium-enhanced MR angiograms of the abdominal aorta and mesenteric vessels. This is followed the breath hold SSFSE and gadolinium-enhanced fat suppressed SGE images of the abdomen and pelvis.

Findings of mesenteric ischemia include occlusion of the SMA, IMA, or rarely of the celiac artery branches. Due to collateral circulation, typically two of the three mesenteric vessels must be involved to produce symptoms of mesenteric ischemia. It is important to realize that distal small vessel occlusion may produce focal ischemia. Such distal occlusions are beyond the resolution of current MRA techniques. For this reason we always assess the bowel wall for secondary changes of mural thickening.

On the anatomic SSFSE images and the gadolinium-enhanced gradient-echo images one will see focal mural thickening in a vascular distribution. In patients with acute arterial insufficiency, there will be diminished or absent enhancement within the thickened segments of ischemic bowel. A target appearance with enhancement of the mucosal and serosal surrounding a nonenhancing muscularis layer can be seen (Fig. 4). This assessment of lack of enhancement should be made on the initial gadolinium-enhanced images. On delayed images, bowel wall enhancement may be evident from leakage of the contrast from the capillaries into the damaged bowel wall. In the non-acute setting the pattern of enhancement will be variable depending upon the degree of revascularization, fibrosis, or tissue necrosis. In our experience the findings on MR imaging can resolve very rapidly following spontaneous revascularization of the ischemic segment of bowel.

Gastrointestinal Tract Tumor

Primary tumors of the GI tract may involve the esophagus, stomach, small bowel, colon or rectum. MR imaging with intraluminal contrast material can be used to effectively stage primary gastrointestinal cancers (25-35). Combining unenhanced T1-, T2-weighted, single shot RARE, and fat suppressed gadolinium-enhanced MR imaging provides a comprehensive evaluation of the primary tumor and distant metastases (34). In our experience thin section T2-weighted and fat suppressed gadolinium-enhanced imaging is most effective for determining the transmural extent of the tumor or T-staging (29-33). Thin section surface coil images angled orthogonal to the tumor will depict the layers of the bowel wall and the relationship of the tumor to the bowel wall and surrounding fat.

T1 and T2 tumors are confined to the bowel wall and show a smooth outer contour to the intestinal wall (Fig. 5). T3 tumors with transmural tumor extension show nodular tumor extending beyond the muscularis propria into the adjacent fat and T4 tumors will show invasion of adjacent structures (Fig. 6). This distinction is particularly important for rectal cancers T3 and T4 cancers will undergo preoperative radiation therapy to reduce the tumor bulk and to downstage the tumor prior to surgical resection. Accurate preoperative T staging of rectal cancers has become the accepted practice at many institutions. The same principles for T-staging also apply to cancers of the colon, small bowl, and stomach. Local regional nodal metastases can also be assessed with MR imaging. Using size criteria alone MR imaging and helical CT are insensitive to microscopic metastases to normal sized lymph nodes. The sensitivity of MR imaging for nodal metastases may improve with the use of iron oxide contrast agents that are taken up by normal nodes with a loss of signal, but not take up by nodes infiltrated with tumor. Diffusion-weighted MR imaging is particularly useful for depicting lymphadenopathy.

Peritoneal MR Imaging

The depiction of small peritoneal implants and carcinomatosis is a challenge for cross-sectional imaging studies, including CT scanning and unenhanced MR imaging. Coakley et al noted that the sensitivity of helical CT for peritoneal tumors <1cm was only 25-50% compared to 85-93% for all tumors (36). However, with gadolinium-enhanced MR imaging small peritoneal tumors are routinely depicted with a level of conspicuity which is unmatched by other imaging studies (Fig. 7). Marked enhancement of small peritoneal implants with IV gadolinium on MR images facilitates detection of metastases to free peritoneal surfaces and bowel serosa (37-40). Following the injection of gadolinium, peritoneal implants slowly accumulate the contrast material and are; therefore, most conspicuous on images obtained 5-10 min following the IV injection of the gadolinium chelate. The addition of fat suppression reduces the competing high signal of the adjacent fat and is an important element in this technique.

Several studies have shown that gadolinium-enhanced, fat suppressed MR imaging is superior to CT scanning in depicting peritoneal tumor. The superior performance of enhanced MR imagining compared to CT scanning is most noticeable in the depiction of small (<1cm) tumor implants and carcinomatosis. In one study gadolinium-enhanced MR imaging detected 75% - 80% of small tumor implants (<1cm) compared to 22% - 33% for CT scans (39).

Techniques and Protocols - Peritoneal MR Imaging

The MR protocol and techniques we use for evaluating peritoneal disease are very similar to those that we use for MRI of the GI tract. We again use breath-hold acquisitions to reduce motion artifact. The abdomen and pelvis are scanned in the axial and coronal planes with SSRARE and breath-hold, fat suppressed T2-weighted fast spin-echo (FSE) acquisition or a respiratory-triggered T2-weighted acquisition followed by fat suppressed, gadolinium-enhanced MR imaging. Two sets of axial fat suppressed gadolinium-enhanced images are obtained through the abdomen and pelvis with additional coronal and sagittal imaging.

DW imaging using an intermediate b-value of 400-500 s/m2 is also routinely performed. Diffusion-weighted MR images are also very useful for depicting peritoneal diseases. Single shot EPI DW images using an intermediate b-value of 500 s/mm2 show restricted diffusion with peritoneal tumors and inflammation. On DW images ascites and bowel contents are suppressed while peritoneal and serosal tumors show restricted diffusion and are depicted as areas of high signal intensity. Suppression of ascites and bowel contents improves the conspicuity of peritoneal and serosal tumors on DW images. We have found that the most accurate examination for detecting peritoneal tumors is the combination of DWI and delayed gadolinium-enhanced MRI.

Intraluminal Contrast

We use the same intraluminal contrast agents to distend the stomach, small intestine, and colon for patients with suspected peritoneal tumor. As with GI tract disease, inadequate bowel distension can significantly limit image interpretation. Distension of pelvic bowel is especially important in patients with treated gynecologic malignancy, in whom subtle tumor recurrence often occurs near the rectosigmoid colon.

Intravenous Contrast Agents

A single dose (0.1 mmol/kg) of gadolinium chelate administered IV as a bolus at 2 cc/sec. Antiperistaltic agents are also administered intravenously to decrease bowel peristalsis.

Clinical Applications for MRI of Peritoneal Disease
Ovarian Cancer

Gadolinium-enhanced MR imaging is useful in women with ovarian cancer to monitor response to therapy by depicting residual peritoneal tumor and to detect recurrence in patients with a rising serum CA-125 level (37-45) (Fig. 8). Detecting clinically occult tumor is critical in determining appropriate patient management. Following chemotherapy declining serum CA-125 values indicate tumor response to treatment. Unfortunately, a normal CA-125 valued does not exclude residual tumor, as up to 50% of women with a normal CA-125 value following chemotherapy still have residual tumor (46-50).

As fewer second-look surgeries are being performed, our oncologists now use the results of gadolinium-enhanced MR imaging to determine response to chemotherapy. In a recent study we compared MR imaging and laparotomy reassessment in 76 women with treated ovarian cancer using clinical outcome at 1 year as the gold standard. MR imaging and surgical reassessment were equally accurate for depicting residual tumor following treatment (37). MR imaging had 90% sensitivity, 88% specificity, and 89% accuracy compared to laparotomy (88%, 100%, 89%). The positive predictive values for MR imaging and laparotomy were 98% and 100% while the corresponding negative predictive values were 50% for both tests. In our current practice surgical reassessment in women with treated ovarian cancer is rarely performed. The results of serial MR examinations are combined with those from serial CA-125 values to make treatment decisions regarding the need for additional chemotherapy or surveillance.

The improved sensitivity of gadolinium-enhanced MR imaging in depicting small volume tumor, compared to CA 125 levels alone, provides our oncologists with information critical to patient management; providing a more accurate means of monitoring response to adjuvant chemotherapy and detecting recurrence after initial response. In our experience, gadolinium-enhanced MR imaging often shows residual tumor in patients following adjuvant chemotherapy indicating a need for additional treatment.

Peritoneal Metastases from Other Abdominal Tumors

Gadolinium-enhanced, fat suppressed, SGE MR imaging is equally effective for evaluating peritoneal metastases from other primary tumors of the pancreas and gastrointestinal tract (34, 38, 51) (Fig 9). Primary tumors of the stomach, pancreas, colon, and appendix often spread by intraperitoneal tumor cell exfoliation and subsequent peritoneal carcinomatosis. At our institution the information from MR imaging is used to determine the appropriate course of therapy. Accurate depiction of subtle peritoneal tumor can completely alter patient management. For instance, in a patient with pancreatic cancer surgical resection is not indicated if metastatic peritoneal tumor is confirmed on preoperative MR imaging. Similarly, in the patient with colon cancer metastatic to the liver, possible hepatic resection of isolated liver metastases may prolong survival. However, with concurrent peritoneal metastases, hepatic tumor resection is obviously contraindicated. In the patient with gastric cancer, we are all familiar with the drop metastases to the pelvis producing large complex Krukenberg tumors. However, it is more common to find subtle peritoneal metastases elsewhere in abdomen on MR images. The ability of gadolinium-enhanced MR imaging to depict subtle peritoneal tumor and carcinomatosis makes it a valuable study in the oncology patient. At our institution we often use we use MR imaging as the primary imaging study in these patients. This approach becomes especially important when peritoneal tumor is of immediate clinical concern.

Spread of Abdominal Tumors Along Peritoneal Reflections

The peritoneal reflections also form the ligaments that connect the abdominal organs and viscera to one another and to the retroperitoneum and abdominal wall. In the upper abdomen a complex network of peritoneal reflections surround and connect the liver, stomach, spleen, kidneys, and duodenum. These peritoneal reflections thus serve as important potential pathways for spread of abdominal malignancies (52, 53).

The gastrohepatic ligament is also known as the lesser omentum and extends from the lesser curvature of the stomach to the left lobe of the liver. On the liver surface the gastrohepatic ligament extends into the fissure for the ligamentum venousum that separates the caudate lobe from the left hepatic lobe. The hepatoduodenal ligament is located along the free margin of the lesser omentum or gastrohepatic ligament. It extends from the porta hepatis to the duodenal sweep and contains the components of the portal triad; the portal vein, hepatic artery, and bile ducts. The hepatoduodenal ligament is an important pathway of spread of inflammation or tumor from the retroperitoneum to the liver or from the liver to the retroperitoneum.

Once tumor gains access to the liver it can spread along the periportal space that then communicates with the left intersegmental fissure and the falciform ligament. The falciform ligament connects the liver with the anterior abdominal wall. Using these peritoneal reflections a continuous pathway is thus established from the retroperitoneum through the liver to the abdominal wall. For example, a pancreatic cancer can spread along the hepatoduodenal ligament from the retroperitoneum to the liver, and then along the periportal tissues to the falciform ligament and finally to the anterior abdominal wall. The gastrohepatic, gastrosplenic, splenorenal ligaments, greater omentum, and transverse mesocolon are additional peritoneal reflections that serve as potential pathways of spread for tumors of the liver, stomach, spleen, kidneys, and colon. A thorough understanding of the interconnected peritoneal reflections will improve our interpretation of imaging studies in patients with metastatic abdominal tumor.

Mucinous Appendiceal Neoplasm with Peritoneal Spread

A mucinous appendiceal neoplasm with peritoneal spread is commonly referred to as pseudomyxoma peritonei syndrome. It is a rare condition characterized by the accumulation of copious gelatinous masses throughout the peritoneal cavity. This is typically a slowly progressive disease in which patients present with increasing abdominal girth, an inguinal hernia, or a palpable ovarian mass. While pseudomyxoma does not metastasize via the lymphatics or blood stream, it is a progressive disease which if untreated eventually leads to death by replacement of the peritoneal cavity by mucinous tumor.

Mucinous appendiceal neoplasms with peritoneal spread are divided into three clinical and pathologic categories. Disseminated peritoneal adeonmucinosis (DPAM) is a benign condition arising from appendiceal adenomas while peritoneal mucinous carcinomatosis (PMCA) is characterized architectural and cytological features of adenocarinoma. PMCA arises from appendiceal or intestinal mucinous adenocarcinomas. An intermediate category occurs with features are in between those of DPAM and PMCA. The classification of pseudomyxoma peritonei determines the clinical course and long term survival. The age adjusted 5-year survival for DPAM is 84% compared to 37.5% for patients with intermediate features, and 6.7% for those with PMCA (54).

The primary tumor of the appendix is typically inconspicuous at the time of diagnosis. Mucin producing tumor cells escape from the appendix or ovary and distribute throughout the peritoneal cavity. The eventual deposition of the tumor cells is determined by pathways of flow of peritoneal fluid and by gravity. Bulky tumor deposits in the omentum and right and left subphrenic spaces is most common. Deposition of tumor cells on bowel surfaces is uncommon except at the ileocecal region, the rectosigmoid regions, and the gastric antrum.

Helical CT and MR imaging are often utilized in patients with mucinous appendiceal neoplasms (55-57). However, on helical CT scans it is often difficult to distinguish ascites, mucin, and tumor in patients with pseudomyxoma peritonei syndrome. In our experience delayed gadolinium-enhanced MR imaging with fat suppression is useful to stage and characterize the peritoneal tumors in pseudomyxoma peritonei (Fig. 10). The excellent contrast resolution of MR imaging allows one to distinguish non enhancing ascites and mucin from the enhancing cellular component of pseudomyxoma peritonei. Pools of ascites or mucin will not demonstrate any enhancement with gadolinium. Solid adenocarcinoma will show marked enhancement while adenocarcinoma mixed with mucin will demonstrate mild to moderate enhancement.

Conclusions

MR imaging of the gastrointestinal tract and peritoneum is a powerful clinical and imaging tool that can provide essential information to your patients and clinicians. By combining breath-hold MR imaging with intravenous and intraluminal contrast agents, one can generate images that capitalize on the superior contrast conspicuity of MRI to depict subtle diseases of the GI tract and peritoneum.

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Learning Objectives
After completing the material in this chapter, the learner should be able to describe clinical applications and MR imaging techniques for evaluation of GI tract and peritoneal disease.

Questions and Answers

1. Important elements in the MRI technique for evaluation of the GI tract include:
A. Oral contrast material
B. Breath-hold imaging
C. Intravenous gadolinium
D. All of the above
Answer: D

2. Compared to helical CT, MR imaging of the GI tract is characterized by:
A. Superior in plane resolution
B. Faster scan times
C. Superior soft tissue contrast
D. More radiation exposure
Answer: C

3. For MR imaging of peritoneal tumor
A. Fat suppression is optional
B. Single dose gadolinium is administered
C. Delayed gadolinium-enhanced imaging is most sensitive
D. Oral contrast material is not useful
Answer: C

4. Cancers that spread via intraperitoneal dissemination may arise in the:
A. Stomach
B. Colon
C. Ovary
D. Pancreas
E. All of the above
Answer: E

5. Peritoneal tumor
A. Rarely involves the terminal ileum
B. Dose not spread according to patterns of ascitic flow
C. Preferentially involves the left paracolic gutter
D. Localizes in the “hot spots” Pouch of Douglas, sigmoid colon, terminal ileum, right paracolic gutter, Morrison’s Pouch, and the right subphrenic space.
E. Is better depicted on helical CT than on gadolinium-enhanced MRI
Answer: D