Taking a New Look at MRI

Due to its expense and limited availability, magnetic resonance imaging (MRI) is rarely considered a first-line diagnostic tool, taking a back seat to less costly and more mobile imaging modalities, such as ultrasound and computed tomography (CT). But thanks to advancements in speed and imaging — and the absence of ionizing radiation — MR has managed to gain ground where CT has previously dominated, and in the process, raise the bar on clinical diagnostics.
However, Steven Wolff, M.D., Ph.D, Director of Advanced Cardiovascular Imaging, New York City and assistant professor at Columbia University, is quick to point out that MR and CT are not competing technologies. The cardiovascular radiologist spends 99 percent of his time performing cardiac and vascular MR and CT, currently with GE Healthcare’s TwinSpeed 1.5T MR unit and the new Discovery VCT XT scanner with low-dose algorithm.
“If a question about a patient’s condition can’t be definitively answered with a CT, then MR is a nice complementary test that provides better functional, viability and valvular information,” Dr. Wolff explained. “It’s difficult to think of a disease that could not be diagnosed after performing both CT and MR tests.”
“People assume that one technology can do it all and that’s simply not the case,” Timothy Albert, M.D., Director, Salinas Valley Memorial Hospital Cardiovascular Imaging Center and Assistant Consulting Professor of Medicine at Duke University, added. “They don’t understand the strengths and weaknesses of each technology.”
In recent years, advancements in CT technology have allowed cardiologists to visualize the coronary arteries much less invasively. Because of this, Dr. Wolff believes CT is superior for coronary imaging, but he believes MR is an excellent imaging tool for every other aspect of studying the heart.
How well does the heart pump? How well are the valves functioning? How big is the heart? Is the heart wall muscle alive or dead? For answering these questions and more, MR can be an invaluable tool. In fact, it is the test of choice for assessing viability or evaluating damage to the heart muscle, according to Dr. Wolff.
“What you see on a CT scanner, or even an angiogram, is not always obvious. What do you do with a patient who has, let’s say, 50 percent stenosis? Do you fix it? Well, you shouldn’t fix it if it isn’t causing a problem. Perfusion imaging with MR allows you to determine whether the narrowing is severe enough to prevent enough blood from getting to the heart muscle,” he explained.
“An exciting thing about MRI that CT does not provide is dynamic, or functional, capabilities,” said Dr. Albert.
MRI has always been a good quantifying tool, but one of the technology’s biggest enemies has been time. Thanks to advances in parallel imaging and multi-element arrays, MRI’s speed of acquisition is improving. For example, MRI is now an excellent tool for quantifying the amount and the velocity of blood flowing through the vessels — using flow velocity imaging techniques — says Dr. Albert.
Salinas Valley Memorial Hospital has partnered with Toshiba to further develop MRI applications as it anticipates its Cardiovascular Imaging Center’s official opening in Spring 2008. They will be installing Toshiba’s newest MRI magnet, the Vantage Atlas. Dr. Albert is also enthusiastic about Toshiba’s unique noncontrast imaging techniques, which he believes has the potential to open up the field of vascular imaging.
“In the short run, the biggest potential for MRI to displace CT is in the area of vascular imaging,” said Dr. Albert. Issues that have hampered the technology in the past, such as speed of acquisition and ease of application, are improving, and along with noncontrast applications, will drive the transition.
“If we can make MRI as push-button, as simple, as CT and get high-quality, reproducible images without radiation and without contrast, then it’s going to be hard to make the argument to do CT over MR,” said Dr. Albert.
Chasing CT
MR’s gain on CT is evidenced outside the cardiology realm, as well. This summer at the American Association of Physicists in Medicine (AAPM) meeting in Minneapolis, MN, B. Gino Fallone, Ph.D. presented information
on a new radiation therapy-MRI system he co-invented with Marco Calone, Ph.D. and Brad Murray, Ph.D. The system, known as Advanced Real-Time Adaptive Radiotherapy (ART), combines a linear accelerator with a MRI, which the inventors believe will allow for a significant reduction of treatment margins during radiotherapy. The system’s prototype is being built at the University of Alberta Cross Cancer Institute, Canada.
Radiation therapy currently relies on X-ray or CT for image guidance — most commonly performed during treatment planning — but these imaging modalities are not as proficient as MR in imaging organs or tumors. As a result, clinicians end up applying larger treatment margins to ensure the entire tumor is treated. The ART system, however, fixes the linear accelerator and open-bore MRI to each other — in such a way to reduce interference between the two — and rotates around the patient, delivering treatment from all angles. Because MR can track tumor motion in real time, radiation beam adjustments can be made as necessary during treatment — a significant innovation, the inventors say, in how radiation treatments are currently performed.
MRI’s new spin
John A. Detre, M.D., Associate Professor and Director, Center for Functional Neuroimaging, University of Pennsylvania, is involved in studies of a MRI pulse sequence known as Arterial Spin Labeling (ASL) that measures — without contrast — perfusion in the brain and other organs.
ASL is the MRI-equivalent of O15 PET that instead of using an injected radioactive tracer, magnetically labels arterial blood water noninvasively. Within three to five seconds the magnetic tracer is gone, which is enough time in most instances, for the tracer to perfuse the tissue of interest.
“With this endogenous tracer, ASL measures classical tissue perfusion because the tracer that is used — magnetically labeled water — is a diffusable tracer that can exchange with the tissue water,” explained Dr. Detre.
To date, most of ASL’s studies have concentrated on blood flow disorders in the brain, such as chronic cerebral vascular disease or acute stroke, and as a marker of neural function in degenerative and other diseases.
“It still remains to be seen how useful ASL will be in hyper acute stroke,” said Dr. Detre. “Probably the best role for the technique in these cases will be when the patient is outside the diagnostic window for thrombolytic therapy,” he said. “But it is quite useful for guiding treatment in patients with chronic cerebral vascular disease.”
According to Dr. Detre, there are other potential advantages to the noncontrast, noninvasive ASL technique, not the least of which is the ability to perform an unlimited number of repeated measurements. However, he admits, ASL still needs improvement in quantifying very low blood flow.
In order for ASL to be readily available and adopted, Dr. Detre believes two things must happen. First, vendors must develop robust ASL imaging sequences. Up until now, ASL has only been available at about a dozen major MRI research centers around the world with the physics expertise to implement it. Siemens Medical Solutions has licensed ASL technology from the University of Pennsylvania and is developing a product that will allow users to access the technology at the “push of a button.”
Second, clinicians must be willing to use the technology.
“In general,” added Dr. Detre, “radiologists and clinicians are not used to having ready access to a measurement of blood flow. Once they realize what they can measure with ASL, they will begin to demand it.“