Biomedical Engineering Reference
In-Depth Information
technique may be called “minimum TE.” It produces the shortest possible TE
with the PC sequence, and is selected by not choosing the flow compensation.
3.2.6 MRA Image Reformation
The MR data from MRA images is reformatted and this reformatting plays a major
role in vascular anatomy observed in the MRA imaging. The common method
for reformatting TOF-MRA uses the technique known as MIP. This technique
also generates 3D images of blood vessels with blood motion. The other method
for reformatting MRA images is shaded surface display. This method reformats
image data in such a way that it appears as if a light is thrown onto structures
to generate 3D appearance of vasculature.
3.2.6.1 Maximum Intensity Projection
The method of reformatting based on 'maximum intensity projection' is known
as 'mipping.' The mipping of blood can be done based upon the blood flow char-
acteristics. Flowing blood in MRA techniques has a high intensity. The intensity
of a pixel in a slice is compared with that of the corresponding pixels in all the
other slices (as in a channel), and the one with maximum intensity is selected.
For example, pixel (1, 1) in slice 1 is compared with other pixels (1, 1) of all
other slices. For this, an internal threshold is used, below which no pixel in
the channel falls. This threshold process is repeated for all the pixels in the
slice to connect high intensity dots in space in order to generate an MRA image.
Thus MRA image represents the highest intensities (caused by flowing blood)
in the imaging volume. A major drawback of this method is that bright struc-
tures other than blood may be included in the mipped image i.e. fat, posterior
pituitary glands and subacute hemorrhage. This problem is observed only with
TOF MRA and not with PC MRA. PC MRA is a subtraction technique based on
velocity-induced phase shifts rather than on tissue T1 and T2 relaxation times.
3.2.6.2 Saturation Effects
The saturation effects can minimize the loss in signal intensity if small (15-20 )
flip angles are used. The 3D phase contrast technique can image large volumes,
such as the entire head, without serious signal loss due to saturation effects. As
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