Biomedical Engineering Reference
In-Depth Information
the electron. To induce electron precession, a temporal magnetic field is applied perpen-
dicular to the
z
φ
-axis. This causes the net electron magnetic field to precess at angle
from
z
the
-axis. By measuring the change in the direction of the magnetic field and the time
for the magnetic field to return back to an aligned state, information about the tissue com-
position can be obtained.
There are a few major components to a MRI system, which include a super-conducting
magnet, surface coils that can induce a transient magnetic field (to investigate precession),
and data collection/analysis components. The super-conducting magnet is used to gener-
ate a static magnetic field within the interior of the MRI machine. This static magnetic field
is used to align all of the atoms within a certain axis. This static field is normally around
3T. The surface coils, use transient magnetic fields, typically within the range of 5% of the
static magnetic field to localize the spin, precession, and time constants for the atoms as
they are pulled away from their alignment and return back to the aligned configuration.
The data collection software can detect the magnetic fields generated by the electrons
themselves. The data analysis software takes the collected frequency data and deter-
mines the average tissue homogeneity/composition. Image processing techniques can then
be used to generate a two-dimensional image of the tissue composition. It is typical that
the magnetic field alignment is changed during the image processing time (e.g., from the
z
-axis) to allow for the data acquisition at different slices/orientations
throughout the body. Images can be acquired on the order of every 50 ms with the current
technology.
MRI technology has been applied to many different functional applications to detect
changes within the tissue composition. Changes in brain activity can be quantified by
changes in cerebral metabolism, blood flow, or oxygenation, in response to various stimuli.
Changes in these parameters are all linked, because increased metabolism generally
decreases tissue oxygenation, while increased blood flow increases tissue oxygenation
levels. This is typically quantified as differences in the alignment and time constants of
hemoglobin. Phase contrast MRI has been used to obtain pictures of arteries and entire
vascular networks. This has been used to diagnose the likelihood of cardiovascular disease
onset (such as stenosis and/or aneurysm). There are two common techniques that
have been used to image the vasculature with phase contrast MRI. The first is through
the administration of a paramagnetic contrast agent, most commonly gadolinium, into the
bloodstream. That contrast agent (similar to standard angiography) is responsive to
changes in the magnetic field and emits a strong signal that can be recorded by the MRI
data acquisition equipment. A second technique, termed FLASH MRI, saturates the tissue
that neighbors a vascular region of interest. As new blood enters the vascular bed, the
blood itself will not be saturated; therefore, the MRI signal from the blood is much higher
than the MRI signal from the surrounding tissue. This provides a direct measurement of
velocity changes, as well as the vascular architecture.
-axis to the
y
15.4 REVIEW OF OTHER TECHNIQUES
. Usually,
a catheter will be inserted through the femoral artery of a patient, with a contrast dye
Angiography or arteriography is a technique to visualize blood vessels
in vivo
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