Biomedical Engineering Reference
In-Depth Information
to its original orientation and in doing so echo's its own radio signal that the scanner
detects and deciphers into images. The time taken for the disturbed protons to return
to their original orientation is called the relaxation time. There are two relaxation
times of interest: longitudinal relaxation (T1) is the time for approximately 63 % of
protons to realign with the magnetic field along the longitudinal axis; and transverse
relaxation (T2), is the time for approximately 63 % of protons to process out of
phase. Different tissues exhibit different T1 and T2 relaxation times, which results
in high-contrast images. The contrast is provided by saturating the imaging plane
with radio-frequency energy. Air that moves into the plane provides fresh spins and
appears bright in the image, in contrast to the saturated tissues surrounding the airway.
The general MRI procedure may include the following steps:
•
A contrast agent called “gadolinium” may be injected in order to help obtain a
clearer picture of the area being examined
•
All metal objects need to be removed
•
The patient should lie as still as possible although between sequences some minor
movement may be allowed
•
Depending on the medical investigation, the MRI scan may take around 15-45 min
although some may take as long as 60-minutes or longer. This is obviously longer
than the limit of a single breath hold, and imaging of the respiratory airway is
susceptible to the natural cycle of the lung movement due to breathing. Optimal
breathing techniques such as shallow, free breathing instead of maximum breath
hold may be suggested to the patient in conjunction with of a higher magnetic field
using dynamic contrast-enhanced MRI to minimize lung parenchyma movement
(Kino et al. 2007).
3.2.3
CT vs MRI
Both MRI and CT are non-invasive techniques that have ability to see inside the
body by providing 3D volumetric data sets from a series of stacked 2D pixels which
have an associated depth, or slice thickness. CT scans require the use of x-rays and
therefore ionizing radiation exposure is a problem. On the other hand MRI scans
poses almost no risk to the average patient, although the strong magnetic field will
affect implanted medical devices that contain metals such as pacemakers, cochlear
implants, or metallic pins. Therefore MRI has the advantage that a patient can undergo
multiple scans successively in the short term, without the radiation exposure that
would be encountered with CT. Depending on the anatomy for imaging, either MRI
or CT will produce better quality images over the other. For example MRI has the
ability to change the contrast of the images by small changes in the radio waves and
magnetic field. The different contrast settings produce much higher detail in soft
tissues and can highlight different types of tissue. MRI also has the ability to display
the anatomy in multiple planes of projection while CT scans are limited to the plane
of the gantry that is usually axial or coronal. CT images produce better detection of
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