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High-res images pinpoint small peritoneal tumors
Gadolinium-enhanced MRI has become the exam of choice for patients with known or suspected peritoneal tumor
By Russell Low, M.D.

Accurate depiction of tumor involvement of the peritoneum is critical to the diagnosis and management of cancer patients. Unfortunately, the small size of peritoneal tumor implants renders them difficult to image by helical CT and nonenhanced MR.1-5

We have shown that gadolinium-enhanced MRI with fat-suppressed, spoiled gradient-recalled-echo (SGE) sequences is exquisitely sensitive for depicting peritoneal tumors.6-9 At our institution, gadolinium-enhanced MR imaging has become the exam of choice for all patients with known or suspected peritoneal tumor.

The parietal and visceral peritoneal lining covers the free peritoneal surfaces of the abdominal cavity and envelops the abdominal organs and bowel. The peritoneum thus provides an extensive and continuous surface area, which may become involved in malignant or inflammatory diseases of the abdomen and pelvis.10

Malignant involvement of the peritoneum may occur as a result of intraperitoneal seeding of tumor cells from primary malignancies of the ovary and gastrointestinal tract. These tumor cells will then spread according to the pathways of ascitic flow, as shown by Myers.11 Eventually, dissemination of intraperitoneal tumor may involve all of the peritoneal surfaces of the abdomen and pelvis, including the free peritoneal surfaces, the bowel serosa, perihepatic and perisplenic ligaments, mesentery, and omentum.

Alternatively, these same peritoneal structures may serve as pathways of direct tumor spread from contiguous or noncontiguous tumors within the abdomen and pelvis.12-17 Primary tumors of the pancreas, liver, gallbladder, stomach, or spleen, for example, often spread to contiguous organs via the peritoneal reflections of the upper abdomen. A thorough understanding of these potential anatomic pathways of tumor spread will assist the radiologist in interpreting cross-sectional exams in patients with abdominal malignancy.

Inflammatory diseases involving the peritoneum result in acute or chronic peritonitis.10 Peritonitis may be due to an infectious etiology, such as bacterial or tuberculous peritonitis, or it may occur in the setting of bowel perforation, appendicitis, diverticulitis, or intra-abdominal abscess. Bacterial peritonitis can also result as a complication of intraperitoneal dialysis, surgery, or abdominal trauma. Spontaneous bacterial peritonitis can occur in the setting of alcoholic cirrhosis and other forms of chronic liver disease.

Noninfectious cases of peritonitis may occur with severe chemical reactions following leakage of bile or pancreatic enzymes into the peritoneal cavity. Peritonitis can occasionally be the result of inflammation related to a systemic process such as systemic lupus erythematosus (SLE) or familial Mediterranean fever. In patients with SLE, peritonitis may be accompanied by development of ascites, pleural effusions, and pericardial effusions.

MRI Protocol

Our MR imaging exam for patients with peritoneal disease includes breath-hold, fat-suppressed T2-weighted imaging and fat-suppressed, gadolinium-enhanced SGE imaging of the abdomen and pelvis in the axial and coronal planes.6-9

  • T2-weighted MR images. For the fat-suppressed T2-weighted imaging, the examiner may use a single-shot rapid acquisition relaxation-enhanced (RARE) technique, such as single-shot fast spin-echo (SSFSE) or half-Fourier acquired single-shot turbo spin-echo (HASTE), to rapidly acquire T2-weighted images. Since each image is acquired independently, there is minimal or no motion artifact. Both SSFSE and HASTE acquisitions use half-Fourier techniques, which reduce signal-to-noise ratio. The addition of fat suppression brings a further reduction in signal, making these images less useful for body coil imaging. For body coil imaging of the abdomen and pelvis, we prefer a fat-suppressed, breath-hold fast spin-echo acquisition. The time of breath-holding is 25 seconds for each of the 12 slices. These images have better SNR than the fat-suppressed SSFSE images but exhibit more artifact from bowel peristalsis.

    Figure 1
    Figure 1. Fat-suppressed, gadolinium-enhanced, SGE image helps characterize right subphrenic peritoneal tumor (arrows) in patient with ovarian cancer. Delayed images obtained three to five minutes after contrast injection are especially useful for depicting peritoneal cancer. Fat suppression further aids subtle peritoneal enhancement.
    Figure 2
    Figure 2. Rim of thick, nodular-enhancing right subphrenic tumor (white arrows) and confluent bulky tumor (black arrows) in the left upper quadrant encasing the stomach and spleen appear in this fat-suppressed, gadolinium-enhanced SGE image of a patient with pseudomyxoma peritonei. The stomach has been distended with water to facilitate depiction of adjacent peritoneal tumor. (TR 165/TE 2, 512 x 192, 1 nex, 10-mm slice thickness, 0-mm gap, ±20-kHz receiver bandwidth, FA 70°, fat sat, 3/4 FOV, 25 sec each for 12 slices.)
    Figure 3
    Figure 3. Diffuse carcinomatosis involving bowel serosa, mesentery, and peritoneum can be appreciated from this fat-suppressed, gadolinium-enhanced SGE image of a woman with ovarian cancer. Note the abnormal bowel wall thickening and enhancement (arrows) from serosal tumor implants. Distention of the bowel with water-soluble contrast is essential to serosal tumor depiction.

  • Gadolinium-enhanced SGE MR images. The key images for evaluating peritoneal disease are the fat-suppressed, gadolinium-enhanced SGE MR images (Figure 1). Due to the slow accumulation of gadolinium within the peritoneal tumor, a delayed set of images obtained five minutes after injection of gadolinium is most sensitive in depicting peritoneal disease.6-9 We obtain two sets of axial SGE MR images at zero and approximately five minutes after injection of 0.2 mmol/kg gadolinium. A set of coronal SGE MR images is also obtained. Fat suppression is a critical element in this protocol as it accentuates subtle peritoneal enhancement by suppressing the competing high signal of subcutaneous, retroperitoneal, and mesenteric fat. Since the SGE MR images are sensitive to bowel peristalsis, we administer 1 mg IV glucagon at the time of the gadolinium injection. In our experience, this improves depiction of peritoneal tumor involving bowel serosa and mesentery.

    The specific parameters for SGE MR images will vary with the vendor. On our LX GE 1.5-tesla MR imager, we use the parameters described in Figure 2 for the gadolinium-enhanced SGE MR images. Selecting minimum TE will improve fat suppression when combined with chemical fat suppression, as well as shortening the time of breath-hold. The receiver bandwidth should be kept as low as possible to maintain image quality.

  • Bowel preparation. Depiction of peritoneal disease in the middle abdomen requires distention and separation of bowel loops (Figure 3). This can be easily accomplished by having the patient drink two to three bottles of ReadiCat 2, starting 30 minutes prior to the MR examination.6 From experience we have found that drinking the barium more rapidly and then scanning immediately provides better distention of small bowel. The dilute barium serves as a positive oral contrast agent on the T2-weighted images and as a negative oral contrast agent on the T1-weighted SGE images.

    We have begun to use rectal water for distention of the colon. We administer 500 to 1000 cc of rectal water via an enema tip just prior to the gadolinium injection. Distending the rectosigmoid colon is particularly important in patients with ovarian cancer who may have primary or recurrent pelvic tumor involving the rectum, sigmoid colon, or bladder.

  • Gadolinium injection. We inject a double dose of gadolinium 0.2 mmol/kg via either hand or power injection. Since the delayed gadolinium-enhanced images are most important, the rate and timing of injection are not critical. At the time of gadolinium injection, 1 mg IV glucagon is also administered to reduce bowel peristalsis.

    Ovarian Cancer

    Gadolinium-enhanced, fat suppressed MR imaging is highly effective in depicting small-volume peritoneal tumor and carcinomatosis.7,8,18 As fewer second look surgeries are being performed, our oncologists have come to rely on the results of MR imaging to help determine tumor response to chemotherapy. Information from the MR exam combined with serial serum cancer antigen (CA) 125 values is often the basis for either confirming complete clinical response or determining the need for additional consolidative chemotherapy.

    In a study comparing MR imaging, CT scanning, and immunoscintigraphy in 16 patients with ovarian cancer, we found that MR imaging was superior.7 For detection of individual sites of tumor, the sensitivity of enhanced fast multiplanar spoiled GRASS MR images was highest (81%) compared with CT (51%, P<0.001) and immunoscintigraphy (50%, P<0.01)

    We subsequently performed a longitudinal study of 69 women with treated ovarian cancer whom we followed over a five-year time period.8 We compared the results of MR imaging, with serial CA 125 values and physical examination for detecting residual tumor following chemotherapy. Thirty-nine of the 69 patients were in clinical remission with a normal CA 125 level and physical examination. Twenty-three of these patients had subclinical tumor proved by laparotomy or clinical follow-up.

    Gadolinium-enhanced MR imaging correctly showed residual tumor in 20 of 23 patients, with one false-positive interpretation. For all 69 patients, MR images had an 91% sensitivity, 87% specificity, 90% accuracy, and 72% negative predictive value. MR was superior to serum CA 125 level (53%, 94%, 63%, and 38%) (P<0.0001) and physical examination (26%, 94%, 43%, and 29%) (P<0.0001) for depicting residual or recurrent tumor. Based upon this experience we are confident in our ability to detect residual ovarian cancer following chemotherapy.

    Pseudomyxoma Peritonei

    Pseudomyxoma peritonei is a rare condition characterized by the accumulation of copious gelatinous masses throughout the peritoneal cavity. It may be associated with appendiceal mucin-producing tumors or may occur as a complication of ovarian mucinous cystadenoma. Pseudomyxoma peritonei 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 bloodstream, it is a progressive condition, which if untreated eventually leads to death through replacement of the peritoneal cavity by mucinous tumor.19

    The primary tumor of the appendix or ovary 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 are most common. Deposition of tumor cells on bowel surfaces is uncommon except at the ileocecal region, the rectosigmoid regions, and the gastric antrum.

    On gadolinium-enhanced MR imaging, pseudomyxoma peritonei is depicted as thick and heterogeneously enhancing peritoneal tumor masses. The peritoneal implants are typically less homogeneous in appearance than in ovarian cancer. This more heterogeneous appearance may reflect varying amounts of nonenhancing mucinous material versus enhancing cellular tumor in the pseudomyxoma peritoneal implants. As in ovarian cancer, all of the peritoneal surfaces eventually become encased in tumor.

    Other Abdominal Malignancies

    Gadolinium-enhanced, fat-suppressed SGE imaging is equally effective for evaluating peritoneal metastases from other primary tumors of the gastrointestinal tract. The ability of gadolinium-enhanced MRI to depict subtle peritoneal tumor and carcinomatosis makes it a valuable study in the oncology patient.9

    Primary tumors of the stomach, pancreas, colon, and appendix often spread by intraperitoneal tumor shedding and subsequent peritoneal carcinomatosis. Accurate depiction of sometimes subtle peritoneal tumor can completely alter patient management. In patients with pancreatic cancer, for instance, 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. With concurrent peritoneal metastases, however, hepatic tumor resection is obviously contraindicated. In the patient with gastric cancer, although we are all familiar with the drop metastases to the pelvis producing large complex Krukenberg tumors, it is more common to find subtle peritoneal metastases elsewhere in abdomen on MR images.

    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 Tumors

    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 the spread of abdominal malignancies.13-17

    The gastrohepatic ligament is also known as the lesser omentum. It extends from the lesser curvature of the stomach to the left lobe of the liver.17 On the liver surface, the gastrohepatic ligament extends into the fissure for the ligamentum venosum, which 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, which 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. A pancreatic cancer, for example, 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, and splenorenal ligaments, greater omentum, and transverse mesocolon are additional peritoneal reflections that also serve as potential pathways for tumors to spread to the liver, stomach, spleen, kidneys, and colon. A thorough understanding of these interconnected peritoneal reflections improves our interpretation of imaging studies in patients with metastatic abdominal tumor.

    Peritonitis

    Inflammation of the peritoneum produces peritoneal enhancement with IV gadolinium, which is indistinguishable from peritoneal metastases. The distinction between peritonitis and peritoneal tumor can generally be based on clinical presentation and laboratory values. MR confirmation of findings of peritonitis can at times be clinically important in patients with fever, leukocytosis, and abdominal pain of unclear etiology.

    Both infectious and noninfectious forms of peritonitis will show similar peritoneal enhancement with gadolinium. Patients who have undergone recent transhepatic cholangiopancreatography, for example, uniformly show abnormal peritoneal enhancement due to bile peritonitis. Patients with pancreatitis will commonly show upper abdominal and pericholecysticperitoneal enhancement due to the chemical irritation of the peritoneum by pancreatic enzymes.

    Dr. Low is medical director of Sharp and Children's MRI Center in La Jolla, CA.

    References

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