The dynamics of water diffusion apply as much in the body as they do in the brain; restricted diffusion arising from pathological processes causes tissues to "light up" on diffusion-weighted images - sometimes dramatically so compared to normal surrounding anatomy. Dr. Russell Low at Sharp and Children's MRI Center (San Diego, Calif.) is exploiting this variability in the Brownian motion of water in all abdominal imaging studies. For certain body MRI applications, whole-body DWI aids in the detection of pathology that may be virtually invisible on T1-,T2- and contrast-enhanced images. Technical challenges, such as susceptibility artifacts and physiologic motion, have been addressed by implementing an optimized rapid diffusion-weighted version of EPI that enables short breath holds and reduced artifacts when combined with SENSE.
Russell Low, M.D.
An artist conception of the new Sharp Memorial Hospital, scheduled for opening in 2008.
"Radiologists aren't using diffusion-weighted imaging as extensively as they should due to a lack of awareness and - to a certain extent - DWI's technical limitations in body imaging," says Dr. Low, Medical Director of Sharp and Children's MRI Center. "Whole-body diffusion-weighted imaging is an entirely new way to look not only at tumors but also inflammatory diseases in the abdomen and pelvis. Number one, abdominal DWI is feasible and number two, the images are clinically very useful."
Sharp and Children's radiologists became familiar with whole-body DWI's potential in abdominal and pelvic imaging in March 2004, during a visit by Philips clinical scientist Alun Jones. Dr. Low used DWI on the Center's Achieva 1.5T to scan a patient who later was diagnosed with a hepatoma.
"We began seeing things on the diffusion-weighted images, but we were initially unsure what they represented," he recalls. "Finally, we realized we were looking at enlarged lymph nodes that were so conspicuous they stood out like light bulbs. We had never seen anything like this before. The contrast of the images and the conspicuity of the nodes in this case really were quite striking. On conventional images we were used to seeing lymph nodes that blended in with surrounding structures."
It soon became apparent to Dr. Low that definitive whole-body DWI images might facilitate the task of reviewing the 1,000 or more standard T1- and T2-weighted, dynamic post-contrast and multiplanar images in an effort to detect very subtle contrast differences. Subsequently, Sharp and Children's added a DWI sequence to its abdominal and pelvic protocol and validated this theory.
"The diffusion-weighted images really direct my focus to the area of the abnormality," he observes. "Many times I'll look at the diffusion images first to see what appears bright - which areas have restricted diffusion - and then go back and simultaneously review the conventional images. This helps me see the abnormalities more quickly - particularly in the lymph nodes. Unequivocally, DWI is the sequence to use to look for lymphadenopathy."
DWI's value in helping to visualize possible disease states in small lymph nodes is most apparent because the anatomy can easily blend in with adjacent soft tissues, Dr. Low adds, but occasionally DWI can help reveal subtle larger nodal masses that are inconspicuous on conventional MR images.
While lymphadenopathy is clearly the signature application for DWI, he continues, the technique also is quite valuable in helping clinicians detect all types of primary and metastatic tumors in the abdomen and pelvis, in addition to other pathologies associated with restricted diffusion - such as Crohn's disease, inflammation, osteomyelitis, abscesses and cysts.
A further benefit of the DWI sequence is to guard against potential false-positive interpretations, Dr. Low adds. "Sometimes you see a suspicious finding on, say, a T2-weighted series, and then you realize the diffusion-weighted image is entirely normal," he says.
To cope with motion, Dr. Low opted for a breath hold strategy using a rapid DWI pulse sequence on the Achieva 1.5T system. Other investigators favor high-NSA (i.e. 6 to 8 per station) imaging as a way to average out motion, but the cost-benefit of breath hold imaging seemed ubstantial, he says.
"The diffusion-weighted version of single shot EPI is extremely fast and can be accomplished in a 20-second breath hold for the abdomen and an additional 20-second breath hold for the pelvis," he explains. "Versus high-NSA imaging, our approach is to get the sequence done quickly and simply as an additional sequence added to our conventional imaging. For the short breath holds involved, we gain a tremendous amount of additional information."
Preceding the DWI sequence are a T1-weighted dual echo FFE sequence followed by axial T2-weighted and T1-weighted TSE sequences with SPIR fat suppression. After these, the breath hold DWI for abdomen or pelvis is performed. Subsequently, clinicians implement a dynamic contrastenhanced THRIVE (T1-weighted high-resolution isotropic volume examination), followed by a time-delayed T1-weighted FFE with WATS. SENSE factor 2 is used in all sequences.
"The beauty of this Philips body protocol is that they're all breath hold sequences now," he notes. "We can easily perform an abdominal MR exam in 15 minutes and we have some technologists who can scan an abdomen and pelvis in 20 minutes. "We're exploiting technological advances in scanning speed in body imaging much more so than in neuro and musculoskeletal imaging," he remarks.
"We're imaging probably four times faster in the abdomen than we did five to eight years ago, while a lot of musculoskeletal and brain MR imaging is still being scanned at the same pace. For example, when I began performing body MR 16 years ago, conventional spin echo T2-weighted images with fat sat took 12 minutes. Now it takes just a breath hold of 20 seconds to acquire
T2-weighted images with better resolution and detail."
To further increase SNR, Dr. Low also uses the new 16-channel SENSE Torso coil, which he has been using for several months. "It's a phenomenal coil," he says. "It appears to be a relatively small coil, yet the coverage is excellent. If we position it carefully, we can cover the abdomen and pelvis very easily, up to 48 to 50 cm.
Although some investigators have sought to use ADC values to characterize tumors as benign or malignant, at Sharp and Children's DWI is intended to serve as a guidepost mainly, indicating where potential problems may be. The standard sequences in the protocol are designed to further help characterize what is seen on diffusion and pinpoint the pathology's anatomical coordinates.
"One of the limitations of diffusion is that a lot of the background features fade out," he explains. "You end up with a big bright spot or black [i.e. inverted-contrast] spot, so correlating the diffusion with the anatomic images is necessary. We roll through all the series simultaneously on PACS to view DWI and the other images concurrently."
Instead of post-processing the DWI data to create inverted-contrast MIP images, Dr. Low prefers to view the source images. "These inverted images are quite attractive and they promote awareness of the technique, but source images - whether they be MRA images or DWI images - represent the unadulterated data set," he says. "The MIP doesn't 'know' the brightest signal intensity pixel is a tumor, so if it happens to be T2 shine-through from bowel, for example, it will still be projected as bright signal."
Some specific pathologies that DWI has helped Dr. Low and his colleagues visualize - in addition to lymphadenopathy - include tumors of the peritoneum, pancreas, liver and gastrointestinal tract.
"Contrast-enhanced imaging is very good at helping us to detect peritoneal tumors, but DWI sometimes helps us sort out complex relationships - such as separating bowel from peritoneal tumors and bowel from mesenteric tumors," he observes. "In imaging rectal tumors, we would try to make high resolution images, but if the bowel is collapsed it's hard to distinguish collapsed bowel from tumor. With DWI, these tumors are exceptionally hyperintense. And you can see the lymphadenopathy around the tumor."
To improve hepatic imaging, Dr. Low credits Dr. Shadid Hussain of Erasmus University of Rotterdam, The Netherlands, for a modification of the standard DWI sequence. The concept is to use a b-value of 20 and replace the T2-weighted sequence, yielding nominal diffusion weighting and substantial T2 weighting - creating a black blood sequence, essentially.
"All the vessels become dark, but the rest of the image will basically be a combination of some diffusion weighting and a lot of T2 weighting," Dr. Low explains. "I like this sequence - it works well to show small perivascular tumors."
For all other imaging, Dr. Low prefers a moderate diffusion sensitivity to limit artifact, therefore uses a b-value of 500 (versus a b-value of 1000 commonly used in the brain). "There still is some T2 shine-through, which may be seen as some bright signal in the bowel; a lot of the bowel does suppress on the DWI, but typically there still are some remaining areas of high intraluminal signal. However, if you compare the diffusion image to the other images, it's always easier to tell what's going on."
Post contrast THRIVE
Patient with increasing abdominal girth. The gadolinium-enhanced THRIVE image shows peritoneal enhancement and infiltration of the small bowel mesentery (long arrow). Omental tumor (short arrow) is also present. The DW image shows hyperintense masses in the small bowel mesentery and omentum (arrows).
Patient with primary ovarian cancer. The T2-weighted image shows pelvic ascites (A).The DW image shows markedly hyperintense omental (long arrows) and peritoneal tumor (short arrow).The ascites suppresses on the DW image increasing the conspicuity of the tumor.
Post-contrast keyhole THRIVE
Patient with metastatic prostate cancer. The T1-weighted and gadolinium-enhanced keyhole THRIVE images show liver metastases and osseous metastases.The DW image best shows the innumerable hyperintense liver metastases and osseous metastases in spine (arrows).
Given the obvious benefits that Sharp and Children's MRI Center has experienced by adding whole-body DWI to their abdominal and pelvic protocols, DWI's use in the body should begin to increase rapidly through increased awareness, Dr. Low predicts.
"Clinicians will follow as they learn about a technique's advantages," he says. "Then, it's a simple matter of plugging it in an running it. Of course, after that they need to figure out what they're looking at, since DWI's dichotomous bright-dark contrast scheme is different from what clinicians are accustomed to.
Clearly, diffusion-weighted imaging is one of the things that Philips MR does really well," Dr. Low adds. "It's a beautiful sequence that Philips has optimized for body DWI. It works very nicely and is an important part of our abdominal and pelvic MR protocol."