New MR technique quickly builds 3-D images of knees
A faster magnetic resonance imaging (MRI) data-acquisition technique will cut the time many patients spend in a cramped MR scanner, yet deliver more precise 3-D images of their bodies.
Developed at the University of Wisconsin-Madison, the faster technique will enable clinics to image more patients—particularly the burgeoning group of older adults with osteoarthritis-related knee problems—and can help researchers more rapidly assess new treatments for such conditions.
MR has long been touted as the ideal method for capturing 3-D images of the human body. “But unfortunately, it’s kind of a slow technique,” says Walter Block, an associate professor of biomedical engineering and medical physics. “You can only sample a few pieces of information needed to build the image at a time.”
Consequently, most MR technicians acquire images as a series of 2-D slices, which yield high resolution in a single plane and poor resolution in the remaining direction, he says.
To capture an image, an MR scanner commonly conducts hundreds to thousands of little “experiments,” or encodings, that help to make up the big picture. Block’s data-acquisition technique capitalizes on recent MR hardware advances that, coupled with a clever way of maintaining a high-level MR signal throughout the scan, will speed an MRI session. “But to maintain the high-level signal,” he says,” you need to be able to complete each of these smaller encodings within a couple of milliseconds.”
Rather than using the conventional Cartesian raster method, which sweeps horizontally to gather MR data, Block’s technique acquires the body’s signals radially, in a way that looks somewhat like a toy Koosh ball. “The Koosh ball is much more efficient in terms of doing these encodings very rapidly,” he says. “We can essentially acquire data during the whole experiment, where in the Cartesian case, a lot of time was spent either prepping for the experiment or returning it to the steady state so that you could do the next experiment. What we’re doing now is capable of a study that you can visualize in any plane in about the same time as people are doing one plane.”
For example, when imaging a joint like the knee — Block’s inspiration for developing the new technique—supressing the fat signal in bone provides image contrast between bone and the cartilage surface. The conventional data-acquisition method would spend half its scan time suppressing the signal from fat, instead of imaging cartilage. However, Block’s technique exploits the difference in resonant frequencies between fat and water. During the scan time, then, the technique maximizes each component of the image, so that a technician can view any aspect.
High-resolution 3-D images are important not only from diagnostic and clinical standpoints, but also to help patients better understand their health conditions, says Block. “If you could actually look at a 3-D model from different perspectives, you’d have a much better chance to make sense of the pain you’re feeling, your doctor’s diagnosis, and your treatment options,” he says.
The technique, which Block patented through the Wisconsin Alumni Research Foundation, also will make it easier to image parts of the body, such as the heart or abdomen, in which motion is a factor.
In related research, Block also has developed an algorithm that, within less than a second, can calibrate an MR system to use non-conventional methods of data acquisition, yet produce clearer images.
Grants from the National Institutes of Health, the Coulter Foundation, and the UW-Madison Graduate School fund Block’s research.