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Abdominal CE-MRI protocol helps detect very small lesions

Best Practice
Low, Rusell Sharp and Children's MRI Center • USA


CE-MRI's excellent soft tissue contrast resolution versus helical CT helps us detect peritoneal lesion implants far earlier in their growth and spread, to improve prognosis and guide interventional decisions, according to Russell Low, M.D., Medical Director of Sharp and Children's MRI Center (San Diego, Calif.). The peritoneum, the lining of the abdominal cavity, is a critical imaging target among patients with primary abdominal cancers, which may shed cells that migrate and implant throughout the peritoneum via ascitic flow. Implantation in contiguous and non-contiguous organs also is possible, because the peritoneum's many inter-organ connections can serve as pathways of direct tumor spread. Dr. Low has used Intera 1.5T and a protocol featuring 3D THRIVE and high resolution T1-WATS since April 2003 to obtain exceptional visualization of even miniscule implants.

 Russell Low, M.D. (second from right, standing) and his team of technologists. The Sharp Outpatient Pavilion houses the Philips Intera MR scanner.
Russell Low, M.D. (second from right, standing) and his team of technologists.
The Sharp Outpatient Pavilion houses the Philips Intera MR scanner.

Helical CT provides outstanding image quality for many applications, but is at a distinct disadvantage when exploring the abdominal cavity for peritoneal and many small primary tumors.


"Contrast-enhanced MRI's soft tissue contrast is considerably greater than that of CT with iodinated contrast," Dr. Low says. "CT's sensitivity for tumor detection has been reported to be as low as 51%, regardless of tumor size. With small tumors, the numbers drop off to perhaps 30%. CE-MRI allows me to see very small, subtly enhancing tumors. A 10 mm peritoneal tumor is huge by our standards."


Sharp and Children's MRI Center uses three 1.5T systems, one of which is Intera 1.5T. Dr. Low had been impressed with Philips MR image quality in early 2003 when his local representative showed him some images.


"The images were spectacular," he recalls. "Homogeneity and sharpness were excellent, and the single-shot TSE images were considerably sharper, more detailed and had better signal homogeneity than those obtained with single-shot Fast Spin Echo. We also thought the 3D THRIVE images were spectacular - they were very thin and the SPIR, soon-to-be SPAIR, fat suppression was extraordinarily homogeneous. Our imaging needs demand very fine detail and very homogenous images. Any bright or dark areas are a big problem."

Peritoneal tumors a common target at Sharp and Children's

At Sharp and Children's, abdominal imaging needs to be exceptional, as nearly one-third, or about 500 cases, of the monthly MRI volume are body examinations. A major percentage of these are patients with primary abdominal malignancies, such as ovarian, pancreatic, gastrointestinal or endometrial tumors.


"All of our oncology patients have a potential risk of metastatic disease to the peritoneum, so we always scan this structure," Dr. Low observes.


From a single cell shed from a primary tumor, peritoneal implants can grow to massive, space-occupying sizes (e.g. pseudomyxoma) if left untreated. Ascitic (peritoneal fluid) flow and the peritoneum's reflections - ligamentous connections to and between internal organs - govern a tumor cell's migration through the peritoneal cavity and eventual deposition. Likely areas for implantation are where ascitic flow is static, such as the subphrenic area, Morrison's pouch, near the sigmoid colon and terminal illeum and any tiny recess in the peritoneal cavity. Implants can be nodular, but typically form a confluent sheet that spreads over peritoneal surfaces. In end-stage carcinomatosis, tumor implants cover the entire peritoneum.


Patient with pseudomyxoma peritonei.

Resp. triggered TSE Sagittal contr.enh. THRIVE
Resp. triggered TSE
Sagittal contr.enh. THRIVE

Patient with pseudomyxoma peritonei. Respiratory-triggered TSE (left) shows mucinous tumor (white arrows) encasing the liver. The tumor extends into the gastrohepatic ligament (black arrow) and perihepatic fissures. The sagittal contrast-enhanced THRIVE image (right) shows an enhancing peritoneal tumor (arrow) anterior to the stomach.


Clearly, early tumor detection is critical, and CE-MRI's sensitivity makes it preferable for imaging peritoneal tumors when they are small (= 5 mm) and prognosis is favorable. CE-MRI also now plays a useful role in detection of residual tumor post-treatment. For example, in the past, patients who had surgery and chemotherapy for ovarian cancer and secondary peritoneal tumors often underwent second-look surgery to detect residual tumor. Fifty percent of these women went on to have recurrent tumor, Dr. Low notes.


"Second look surgery is very rare in the United States today, because it's ineffective in predicting recurrence," he observes. "This simply means surgeons couldn't see very small tumor implants. Similarly, blood tests for the marker CA 125 also are poor in predicting recurrence.


" 'See it earlier, see it smaller' has always been our goal," he says. "With CE-MRI, we can see implants as small as 5 mm and very subtle enhancement of peritoneal surfaces. You want to see these small tumors because these patients have the best chance for cure or long-term disease-free survival. If you give them more chemotherapy at that point, they're more likely to do better. If you wait until the tumor is big enough to detect it with CT or another test, these patients will be much further down the course of disease."


Patient with treated ovarian cancer

Single-shot TSE Contrast-enhanced THRIVE
Single-shot TSE
Contrast-enhanced THRIVE

The single-shot TSE image (left) shows ascites and thickened bowel loops. The contrast-enhanced THRIVE image (right) shows abnormal enhancement of the thickened bowel, indicating suspicious lesions and diffuse serosal peritoneal tumor.


Peritoneal disease evaluation with CE-MRI

Because peritoneal reflections are invisible on MR, Dr. Low maintains that a thorough understanding of these ligamentous connections will help clinicians interpret cross-sectional examinations in patients with abdominal malignancy.


"If you're looking at the liver, which has many peritoneal reflections, and you see enhancing soft tissue that is in the expected course of these reflections, that's a peritoneal tumor," Dr. Low says. "Similarly, if I see enhancing soft tissue interposed between the stomach and the liver's left lobe, that's a peritoneal tumor involving the lesser omentum, the gastrohepatic ligament. In a pancreatic cancer case, if you see tumor extending up toward the liver, you know that's a tumor extending from the pancreas along reflections up into the liver."


Naturally, getting peritoneal tumors to enhance in the first place is the goal. In CE-MRI, both peritoneum and ascites have low MR signal intensity, making contrast-enhanced peritoneal tumors stand out. Conversely, CT's limited soft tissue contrast limits visualization of peritoneal enhancement, and furthermore, distinguishing tumor from ascites is difficult.


In CE-MRI, peritoneal tumor is typically easily distinguished from peritonitis's similar MR appearance by patient presentation, Dr. Low adds.

Intera 1.5T abdominal protocol emphasizes 3D THRIVE

For all abdominal and pelvic imaging, Sharp and Children's uses an all-purpose protocol that combines both the abdomen and pelvis in a single scan. "This imaging of the abdomen and pelvis at once is something we've done for the last 12 years, because oncologists have said they need to see everything - so we use MR in much the same way as CT," Dr. Low notes. "Philips' MobiTrak method conforms to that strategy, enabling us to move smoothly from abdomen to pelvis, obtaining a single stack of images that starts with the diaphragm and ends with the pelvis. When I look at these images on the PACS, I just line them up and run from top to bottom. Without MobiTrak, you get discrete packages."



Pre-scan bowel preparation consists of an oral Metamucil solution before the patient arrives at the hospital, followed by another oral dose shortly before scanning. Patients are also asked to take a Fleets Enema at home to cleanse the rectum and sigmoid colon. To prepare the sigmoid colon, 500-100 cc's of water are introduced via a balloon-tipped enema catheter. Ovarian cancer patients receive IV glucagon during contrast administration to decrease bowel peristalsis.


Following a single-shot TSE localizer, clinicians perform a respiratory triggered TSE (A & P). Because of slow contrast uptake within peritoneal tumor, two sets of axial 3D THRIVE images are obtained at 0 minutes and 3-5 minutes after contrast injection, yielding a total of 150 x 6 mm slices (3 mm overlap) covering the top of the liver to the symphysis pubis. For these images, Philips' MobiTrak technique is used to automatically step through three 50-slice stations during three 17-second breath holds. During the contrast uptake period, coronal and sagittal 3D THRIVE image sets are acquired.


The final sequence is a T1-WATS, a high resolution T1-weighted scan with ProSet fat suppression, in which MobiTrak is again employed to automatically obtain 48 slices in two 24-slice stations covering the abdomen and pelvis, each station acquired during three breath holds, eight x 10 mm slices per breath hold. Total scan time is about 30 minutes.


Sharp and Children's' variety of complementary sequences combines thin slices, high resolution, contrast enhancement and different orientations to make it extremely difficult for even minute peritoneal tumors to hide, Dr. Low says. Philips application consultant Alun Jones and clinical scientist Jan De Becker have further refined select sequences to make the protocol even more robust.


The respiratory triggered TSE sequence is useful for detecting lymph node disease - as 25-30% of ovarian cancers metastasize to these sites - and produces T2-weighted images that make the ascites appear very bright, he says. These images also provide a helpful comparison with post-contrast THRIVE images, increasing diagnostic confidence.


The post-contrast 3D THRIVE images feature thin slices with excellent resolution. "The THRIVE sequence we use has higher resolution than what is specified on delivery," Dr. Low notes. "We pushed the resolution up to 512 x 192, to obtain ultrahigh resolution for each individual slice. We could have used thinner slices, but I'd rather have a thicker slice and sharper images when looking for these small, subtle implants. The slices are still very thin and very homogenous."


The extra coronal and sagittal 3D THRIVE images provide an added perspective on tiny implants, he notes.


Initially a somewhat less useful sequence, Dr. Low says the T1-WATS has proved of great value after Philips' Jones and De Becker boosted resolution substantially to 512 x 228. "These 2D images are very sharp and crisp," he says. "At 10 mm, they tend to be thicker, but sharp detail is more important when imaging peritoneal tumors."


Despite CT's poorer sensitivity in detecting peritoneal tumors, its use in this application nationwide is still more widespread due to greater clinical availability, Dr. Low observes. Sharp and Children's MRI Center is the exception, as referring oncologists have come to appreciate CE-MRI's superb sensitivity in detecting peritoneal tumors earlier and smaller - before they can impact on their patients' prognosis and quality of life.


Example: Patient with ovarian cancer

Coronal single-shot TSE Respiratory-triggered TSE
Coronal single-shot TSE
Respiratory-triggered TSE

The coronal single-shot TSE image shows a bulky omental tumor (arrows). The respiratory-triggered TSE image through the middle abdomen shows a bulky omental tumor (arrows) and ascites (A).



Contrast-enhanced THRIVE Contrast-enhanced THRIVE
Contrast-enhanced THRIVE
Contrast-enhanced THRIVE

Contrast-enhanced THRIVE. The left image, from the first set of axial images, shows an omental tumor (arrows). The right image, through the pelvis, shows a confluent tumor (arrows) encasing the sigmoid colon and extending to the pelvic sidewalls.



Delayed CE T1-WATS (FFE) Delayed CE T1-WATS (FFE)
Delayed CE T1-WATS (FFE)
Delayed CE T1-WATS (FFE)

Delayed contrast-enhanced high-resolution T1-WATS (FFE). The left image shows a subtle enhancing right subphrenic peritoneal tumor (white arrow) and a peritoneal tumor on the surface of the liver (black arrows). The right image shows a subtle enhancing peritoneal tumor (short arrows) involving the parietal peritoneum. A bulky confluent tumor (arrows) encases the sigmoid colon.

Example: Patient with treated overian cancer

Delayed CE T1-WATS Delayed CE T1-WATS
Delayed CE T1-WATS
Delayed CE T1-WATS
Delayed contrast-enhanced high-resolution T1-WATS (FFE) images of a patient with treated ovarian cancer. The left image depicts subtle enhancing left subphrenic tumor (short white arrows) adjacent to the spleen. Peritoneal tumor is also present extending along the medial aspect of the spleen (dashed arrow). Note the irregular enhancing peritoneal tumor in the right hepatorenal fossa (long white arrow). The right image shows enhancing peritoneal and serosal tumor (arrows).

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Nov 26, 2004

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