The latest advances in MRI
Excerpts from the Luminary Programme at
Bombay and Delhi, September 5-6, 1997
Dr. Lawrence N Tanenbaum, MD, Section Chief,
MRI & Neuroradiology, New Jersey Neuroscience
Institute, JFK Medical Centre, Edison, NJ
|
Dr. Lawrence Neil Tanenbaum
delivered lectures at Bombay Hospital and
Hinduja Hospital in Mumbai and at GMRI
in New Delhi
during his recent visit.
In his lectures on "Latest
advances in MRI"
which focussed on
Functional Imaging,
Dr. Tanenbaum explained
the following techniques.
|
|
1.BOLD Imaging( Blood Oxygen Level Dependent Imaging )
In BOLD imaging, image contrast depends on the
amount of oxygen in blood. Activated centres in the
brain will have higher oxygen content in their local
blood volume. In a typical task activation imaging,
the patient is asked to perform some simple task
(like finger movement) for a short period of time
while multiple Echo Planar images of the motor
cortex are acquired. The task is then discontinued
for a short period while the imaging continues.
This "task-on, task-off " procedure is then repeated for
a number of cycles. The resulting time series images
are post processed to generate a map of those regions,
that exhibit cyclic intensity changes which correlate
with the "task-on, task-off " exercise.
Clinical Advantages
The FMRI images can be used to identify, avoid
and preserve areas of functional brain tissue during
treatment. Brain maps of the motor and/or sensory
regions are generated and overlaid directly on to
corresponding spin echo images of the pathology.
This yields a convenient visual correlation of the vital
regions relative to the pathology so that these regions
can be circumvented during surgery or other
treatments.
|
BOLD Imaging: Fundamental Principle
This patient was subjected to a repeated auditory signal during
the FMRI experiment. Activation signal appears posterior and superior
to the vacinity of AVM
|
2. Diffusion Weighted Imaging
Diffusion Weighted Echo Planar Imaging, DW-EPI,
is one of a battery of imaging sequences that enhance
the ability of magnetic resonance to detect and
characterise diseases of the brain.
Clinical Advantages
Diffusion gradients sensitize MR images to the
motion of water molecules between
cells in tissues.
A derangement of tissue integrity can restrict the
motion of water molecules producing increased signal
on the diffusion weighted MR image. Attuning to this
unique tissue parameter provides the greatest sensitivity
of any imaging modality to acute cerebral ischemia.
Since the diffusion weighted images will detect
ischemia almost immediately after onset, they are
invaluable in the diagnosis of stroke and in
combination with Dynamic Susceptibility Contrast EPI,
essential if therapeutic intervention is contemplated.
DW-EPI is useful in detecting areas of active
demyelination against the background of chronic lesions in case of multiple sclerosis (MS), once again
avoiding the need for injection of an MR contrast agent. Diffusion weighted images, obtained with single-shot
EPI techniques, are acquired rapidly - thin section, whole brain studies can be acquired in as little as 10
seconds for each direction sampled.
Acute infarction. Acute left thalamic infarction very
conspicuous on DW-EPI image
|
