Biomedical Engineering Reference
In-Depth Information
Figure 3.6: Two-dimensional TOF MRA pulse sequence (left) and 3D TOF pulse
sequence (right) are shown. Several gradient waveforms are combined to reduce
TE.
enhancement to distinguish two types of spins i.e., moving spins and stationary
spins. The advantage of this flow-related enhancement is mainly fast scanning
time. It uses minimized FRE because each slice represents an entry slice.
3.2.1.1 2D TOF MRA
A typical pulse sequence is represented for TOF MRA. A presaturation pulse is
applied above or below each slice to eliminate signal from overlapping venous
or arterial structures. Usually a short TR (about 50 msec), a moderate flip angle
(45
−
50
◦
), and a short TE (a few msec) are used. This method has the following
advantages. It generates high SNR as signal is generated from a large volume. It
improves spatial resolution. During time of flight, blood flowing into the imag-
ing slice is fully magnetized and appears brighter than the partially saturated
stationary tissues. Optimized scan parameters to acquire angiographic images
are described.
3.2.1.1.1 Image Contrast.
To get better image contrast, repetition times
(TR) must be kept short with respect to the stationary tissues' T1 times. It helps
to suppress the signal from stationary tissue and to maximize the vessel contrast
due to flow-related enhancement (see Fig. 3.7). TR of 45-50 msec are adequate
for suppressing the signal from stationary spins within the slice. During this
time, the fully relaxed blood moving into the slice remains unsaturated. As a
result, blood appears bright (high signal intensity) compared to the low-signal-
intensity stationary tissues. Other flow and imaging parameters influencing the
