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Cardiac MRI Finds Small Areas of Heart Cell Death Missed By Nuclear Imaging Techniques

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Duke Health News 919-660-1306

DURHAM, N.C. -- Researchers from Duke University Medical
Center and Northwestern University Medical School have
demonstrated that cardiac magnetic resonance imaging (MRI)
technology can detect small areas of heart muscle death that
cannot be detected by commonly used imaging techniques.

Their finding is important, the researchers said, because
these small areas of muscle cell death, known as infarcts, can
be early indicators of future, more severe heart problems.

In their study involving 91 patients with known or suspected
coronary artery disease, the researchers found that the
traditional nuclear imaging technique, "single photon emission
computed tomography" (SPECT) detected only 53 percent of these
microinfarcts that were detected by cardiac MRI. Additionally,
13 percent of study patients with microinfarcts were shown to
have none when SPECT alone was used.

The findings of the study were reported today (Feb. 1, 2003)
in the journal Lancet.

"While both cardiac MRI and SPECT are extremely accurate in
detecting large infarcts, our study shows that only cardiac MRI
systemically detected smaller infarcts that are missed by
SPECT," said lead researcher Robert Judd, Ph.D., co-director of
the Duke Cardiovascular Magnetic Resonance Center (DCMRC). "The
smaller the infarct, the more likely that SPECT will miss
it."

Heart attacks occur when blood flow to an area of the heart
is cut off or blocked, depriving those muscle cells of needed
oxygen and nutrients. When these cells die, they tend to die
from the inside of the heart's pumping chamber and move
outward. In a large heart attack, the area of cell death can
cover the entire thickness of the chamber's wall.

"However, in these smaller microinfarcts, the cell death may
only travel a short distance," Judd explained. "Since the
spatial resolution of SPECT is roughly equivalent to the
thickness of the heart chamber wall, it can only detect those
infarcts that have traversed a good portion of the chamber
wall."

The spatial resolution of cardiac MRI is 60 times greater
than SPECT, Judd said, allowing it to pick up these
microinfarcts.

During a cardiac MRI examination, which is non-invasive and
radiation-free, a patient is guided through the cavity of a
large doughnut-shaped magnet. The magnet causes hydrogen nuclei
in cells to align, and when perturbed by radio waves, they give
off characteristic signals, which are then converted by
computers into three-dimensional images of the heart and its
structures. While MRI technology itself is 20 years old, only
in the past few years has technology improved to the point
where accurate images of moving tissues can be taken.

SPECT technology creates a series of "slices" of the area to
be studied, with a computer assembling the slices to create an
image. Patients are usually given a radioisotope, which
provides information about blood flow and metabolism of tissues
being studied. In the study, both groups of patients were given
a contrast-enhancement agent.

To verify the findings on human subjects, the researchers
then performed the same tests on a series of animals, some
without heart disease and others with known disease; and then
analyzed the heart tissue after testing. The scientists
performed the confirming studies in the animals to ensure that
the smaller areas detected by cardiac MRI were actual areas of
cell death, and not something else.

"In the animals, cardiac MRI and SPECT detected infarcts
involving greater than 75 percent of the chamber wall," Judd
said. "However, for infarcts involving less than 50 percent of
the chamber wall, cardiac MRI detected 92 percent, while SPECT
detected only 28 percent."

Judd said that further studies are needed to establish the
role of cardiac MRI in the diagnosis of heart disease.

"If we see these spots on cardiac MRI, the patient likely
has coronary artery disease," Judd said. "However, if they
don't have spots, we can't say for certain they don't have
coronary artery disease."

Duke cardiologists estimate that about 30 percent of
patients with heart disease find that conventional methods for
imaging the heart fall short in providing accurate information
by which to guide treatment. They said that MRI provides crisp
3-D views of cardiac anatomy with no interference from adjacent
bone or air.

The image quality of cardiac MRI also surpasses that of
echocardiography -- another common imaging technique, Judd
said. Cardiac MRI can better show physicians how well the heart
muscle is contracting, as well as precisely reveal areas of
damaged tissue.

"It wasn't until a few years ago that engineers developed
scanners fast enough to clearly capture a beating heart," Judd
said. "The discipline is still defining itself. We want to
advance the field by improving existing cardiovascular imaging
techniques and also by creating entirely novel ways to look at
the heart and its vessels."

The experiments reported in this study were conducted at
Northwestern University. Some team members have since come to
Duke to establish the DCMRC earlier this year, including Judd,
co-director Raymond Kim, M.D., Anja Wagner, M.D., and Heiko
Mahrholdt, M.D. Team members still at Northwestern are Thomas
Holly, M.D., Michael Elliott, M.D., Matthias Regenfus, M.D.,
Michele Parker, Francis Klocke, M.D., and Robert Bonow,
M.D.

The study was funded by grants from National Institutes of
Health, the Deutsche Forschungsgemeinschaft and the Robert
Bosch Foundation.

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