New MRI technique
pinpoints oxygen-starved areas in stroke patients' brains
BY KRISTA CONGER
Magnetic resonance imaging
(MRI) allows physicians to see inside the body, looking
through bone to diagnose abnormalities in the soft
tissues including the brain. Now a study by Stanford
physicians indicates that a technique that uses the same
equipment and technology is even more sensitive than
standard MRI in pinpointing oxygen-starved areas in the
brains of stroke patients. Almost half of the new
extra-sensitive scans picked up lesions that were
undetectable by conventional methods, and in some cases
the findings radically changed the course of therapy for
the patient.
"We looked at three
categories of findings that could be clinically useful:
location, number and age of the lesions," said
Gregory Albers, MD, professor of neurology and lead
author of the study. "Almost 50 percent of the time
we found something potentially important that we couldn't
see on the conventional MRI," he said. The results
are published in the April 25 issue of the journal
Neurology.
It's important for
physicians to accurately determine the location and
number of stroke-damaged brain areas before they begin
treatment. A misdiagnosis might steer a surgeon to the
incorrect blood vessel, or obscure the true cause of the
symptoms. Many small, scattered lesions can indicate that
blood clots are originating from a source outside the
brain, often due to cardiac problems. Treatment to
prevent these traveling clots is different from that used
to prevent clots originating in the blood vessels of the
neck or the brain.
To make an accurate
diagnosis and begin the appropriate treatment for a
suspected stroke victim, Stanford Stroke Center
physicians routinely use MRI to generate a series of
"snapshots" of different slices of the
patient's brain. The quick, noninvasive scan can help
pinpoint the location and nature of the oxygen-starved
tissue.
But the conventional MRI,
which works by applying a magnetic field to the body and
detecting the release of energy absorbed by hydrogen
nuclei in bones and tissues, isn't perfect. Often it is
difficult to detect a very recent stroke, or to identify
particularly small or deep lesions. A standard MRI is
also not very good at differentiating between recent
damage and necrotic tissue left over from a previous
stroke (that may not be responsible for the patient's
current symptoms).
The new method, diffusion
weighted imaging (DWI), uses the same equipment and
scanning procedure as the MRI. However, DWI specifically
hones in on the hydrogen-rich water molecules in the
brain and measures how free they are to move about, or
diffuse. Water in the extracellular space can squeeze
around individual neurons, but water trapped inside a
cell has much less room to maneuver.
"It's like going to a
party where there are only a few people and it's easy to
mingle with each other," said Albers, director of
the Stanford Stroke Center. "In contrast, when the
party gets really crowded it's much more difficult to
circulate," he said.
In a DWI, the restricted
diffusion of water molecules inside a cell serves as a
flag for cells in trouble. When a dying cell is no longer
able to maintain proper internal chemical concentrations,
water crowding into the cell to compensate for the
imbalance shows up on the DWI as very bright areas
against a dark background. This "light bulb"
effect using DWI makes it easier for physicians to
identify the size and location of stroke-damaged tissue
compared with conventional MRI.
Previous studies have
indicated that DWI is a useful tool for identifying dead
or dying cells very soon after the onset of symptoms, but
it was unclear how often the technique would be useful
for patients who are imaged beyond six hours after stroke
symptom onset. Most people don't appear in the emergency
room until several hours after their symptoms begin.
In the new study, the
patients were first examined by a physician who made a
preliminary diagnosis of the suspected location and type
of lesion based on their symptoms. Each patient underwent
first a conventional MRI, and then a DWI. The time from
onset of symptoms until scans varied from six hours to 48
hours, with an average time of 27 hours.
Of the 40 patients who
participated in the study, the results of the DWI found
that the true symptomatic lesion was located in a
different region of the brain than that identified by
conventional MRI in seven people, or about 18 percent of
the total. Five people, about 13 percent of the total,
were found to have multiple new lesions when the
conventional MRI had only identified one.
Monitoring the rate of
water diffusion is also a good way to determine the age
of the lesion. Dead cells disintegrate in about 20 to 30
days, leaving their trapped water free to slosh about in
the empty space remaining. The freely diffusing water is
easily detectable, and indicates that the lesion is at
least three weeks old. In Albers' study, 8 patients, or
20 percent of the total, were found to have only old
lesions and no new ones, essentially ruling out the
possibility that they had suffered a recent stroke.
All together, the DWI
findings changed the diagnosis of 19 of the 40 patients
participating in the study.
In addition to Albers'
study, another study from the Stanford Stroke Center in
which Michael Marks, MD, associate professor of radiology
is the senior author, appears in the same issue of
Neurology. His study confirms the usefulness of DWI in
helping to determine the appropriate treatment for stroke
patients within seven hours of symptom onset. Marks
compared DWI to computed tomography (CT), another common
imaging technique that uses X-rays to visualize the
brain.
In this study the
researchers compared DWI and CT scans taken within seven
hours of symptom onset in 19 patients. DWI was able to
correctly pinpoint the location of the lesion in all of
the cases, but CT scans were correct in only 42 percent
to 63 percent of the patients.
Although neither study
analyzed whether the clinical outcomes were different
when DWI changed a patient's diagnosis, both urge that
further study is warranted to address the issue.
In addition to Albers and
Marks, Stanford physicians participating in the two
studies include Martin Lansberg, MD, formerly a research
assistant at the Stroke Center and now an intern in
Baltimore; David Tong, MD, assistant professor of
neurology and neurological sciences; Michael O'Brien,
formerly a stroke fellow and now in private practice in
San Jose; Michael Moseley, PhD, associate professor of
radiology; and Christopher Beaulieu, MD, PhD, assistant
professor of radiology. SR
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