In the last decade, magnetic resonance imaging (MRI) has
revolutionized the diagnosis of stroke. Prior to the use of MRI, physicians
based their stroke diagnoses largely on physical symptoms together with negative
findings on computed tomography (CT), explains Jeffry R. Alger, Ph.D., medical
biophysicist and a researcher in the UCLA Ahmanson-Lovelace Brain Mapping
Center. While CT was used to rule out the less common form of stroke caused by
bleeding, it provided no positive sign of blood vessel blockage, the more common
cause.
Two hurdles hindered the use of MRI for diagnosing
stroke. First, it was difficult to get patients just hours out from a stroke
into imaging studies to prove the value of MRI because they were being evaluated
and treated. Second, the imaging procedure was lengthy, limiting the amount of
information obtained. That is changing, particularly at UCLA. MRI can be vital
in the assessment and care of stroke patients, says Dr. Alger, whose research
focuses on the development of novel in vivo imaging techniques that use MRI and
magnetic resonance spectroscopy for study of the brain and the nervous system.
“At UCLA, our MRI protocols are very specific in defining
whether a patient has had a stroke. We can differentiate hemorrhagic stroke from
ischemic stroke, and these procedures sometimes even allow us to see the blood
clot itself,” Dr. Alger notes. “By using certain MRI procedures that measure
functional parameters, we can tell how severe the stroke is, which usually is
related to how long the patient has been suffering. Stroke onset is not always
clear from clinical measurements, particularly if a patient wakes up with
symptoms of stroke. We can’t know when during the night the stroke began. There
are now MRI procedures that show indications of how old a stroke is. That has a
lot to do with how the patient is then managed.”
Using MRI in stroke diagnosis and assessment came about
by scientific accident. About 17 years ago, Michael Moseley, Ph.D., a researcher
at the University of California, San Francisco, decided to try a new type of MRI
called diffusion-weighted MRI in an animal model of stroke, just to see what he
could see. Dr. Alger says the procedure delineated the tissue involved in the
brain attack much better than any tool then in existence. This marked the
beginning of the development of a battery of MRI procedures that helps in the
diagnosis of stroke. “We now have a number of MRI stroke assessment protocols at
UCLA that can be completed in 25 minutes,” Dr. Alger says. “These procedures
clearly define whether a patient has had a stroke, which vessels are involved
and how severe the stroke is. That information is quite useful in defining what
the patient is suffering from, and very helpful in deciding how to manage
individual patients. These procedures provide a tremendous amount of
information.”
UCLA’s Clinical Image Processing Service, under the
direction of diagnostic neuroradiologist Pablo Villablanca, M.D., exclusively
processes three-dimensional image data. “Within five to 10 seconds, we can
create a complete data set of three-dimensional renderings from MRIs or CT
angiograms. That information can be used for diagnosis, triage and for treatment
planning,” he explains. “We can also fuse images—a CT image to an MR image, for
example—to superimpose two different data sets to gain new perspectives about
the disease.” Today, four MRI procedures are used at UCLA to evaluate stroke
patients:
Diffusion MRI measures the rate at which water molecules move around in brain tissue. Where blood flow is interrupted by stroke, the molecules slow down, allowing the physician to see how large the stroke is and what part of the brain is affected. This procedure is being tested to see if it can provide an early sign that the patient might improve when given new drugs or treatment procedures.
Gradient echo MRI identifies blood in brain tissue. Recent UCLA studies have shown that it can identify strokes caused by bleeding. The use of gradient echo MRI to identify bleeding in the brain and blood clot within an artery that is blocking flow provides information that a conventional MRI cannot capture.
MR angiography or MRA allows clinicians and researchers to visualize blood vessels and the blood flowing through them to identify narrowed or blocked segments.
Perfusion MRI measures the amount of blood flowing through individual parts of the
brain in the small blood vessels that feed the cells. Researchers use this
procedure to identify whether various areas of the brain are being properly
supplied with oxygen-rich blood.
Because MRI shows how severe a stroke is and when it
happened, researchers may be able to determine, for example, that patients with
less severe or newer strokes may benefit from a particular therapy, while those
with more severe or older strokes do not. In this way, MRI could help patients
avoid undergoing costly treatments that in the end would not help them. “That is
the future—doing therapeutic trials to improve understanding of individual
patients so we can personalize their therapies,” Dr. Alger says. “Cost is
important as well. Some therapies are expensive. If we can be certain a therapy
won’t work because of the individual characteristics of a patient, we can lower
costs as well as save patients from unnecessary treatments.”
For example, one new stroke treatment, clot-busting
drugs, appears to work only within a very narrow time window— three hours after
initiation of the stroke. There’s interest in using MRI to define a group of
patients who are outside that narrow time window, but perhaps could still
benefit from clot-busting therapy. The biggest complication of clot-busting
drugs is bleeding, but researchers can now monitor patients for that using MRI.
“Maybe some patients outside that narrow window after stroke could benefit,” Dr.
Alger says. “We don’t know this now.”
In another UCLA-based clinical trial funded by the
National Institutes of Health, researchers are using MRI to characterize
patients who might benefit from a new corkscrew-like device used to pull out the
clot. For patients who are unable to undergo MRI due to severe claustrophobia or
implanted devices that are MR incompatible, CT-based stroke protocols can be
employed. “Three complementary studies— a noncontrast CT scan of the brain, a CT
angiogram of the neck and brain and CT perfusion of the brain—provide similar
information asrevealed through multimodal MRI,” Dr. Villablanca notes. “The high
special resolution of these techniques provides extremely detailed images of the
brain vessels and may give information about vessels and aneurysms that is not
available by MRI. CT perfusion provides information about blood flow and volume
in cubic millimeter per volume of brain tissue, thereby allowing the stroke
neurologist to gain a deeper understanding of tissue ischemia.”
The knowledge base derived from imaging is expanding
exponentially, and is driving clinical research. “To me, this has happened
phenomenally fast,” Dr. Alger says. “The work we are doing here is going to
improve the way we treat patients and, hopefully, enhance their
recovery.”
