Biomedical Engineering Reference
In-Depth Information
The larger slice-select gradient amplitudes require larger flow compensation
gradients. These gradients in turn limit the minimum TE to 8-9 msec.
3.2.1.1.5 Longitudinal Relaxation Times (T1).
T1 values of blood may
have a significant effect on intravascular signal intensity, particularly in slow-
flow conditions. Suppose the velocity is not sufficient to completely refresh spins
within the imaged slice; in this case the blood will begin to become saturated. In
the 2D TOF procedure described for the carotid bifurcation, saturation would
occur at flow velocities of approximately 3 cm/sec. It can be represented as 1.5
mm divided by 50 msec. The ability of TOF angiography to detect slow flow may
be improved by shortening the T1 of blood through the use of MR contrast agents.
3.2.1.1.6 Flip Angle.
Flip angle affects the slice-saturation rate. It is selected
to saturate the stationary tissues without compromising intravascular signal
intensity. Normally, flip angle of 45-60
◦
is selected for 2D TOF angiography.
3.2.1.1.7 Asymmetric Echo Acquisitions.
These acquisitions are ob-
tained using the fractional echo or partial echo times. Shortest echo times are
obtained for optimizing MR angiographic images. These short echo times are ob-
tained by the use of asymmetric echo acquisition. In this approach, the gradient
echo may offset in the acquisition window by shortening the duration and size of
the dephasing and flow-compensation gradient pulses. This permits shortening
of the sequence and reduced echo times. Asymmetric echo acquisition has the
advantage of reducing the size of the refocusing gradients on the read-out axis.
It results in decreased signal loss and reduces artifacts from accelerations or
higher order moments.
3.2.1.2 Presaturation Pulses
Blood may flow into the imaging plane from any direction. The blood may pro-
duce flow-related enhancement i.e. 2D TOF images of carotid artery show blood
flowing to the imaging plane from above or below. As a result, the carotid arter-
ies appear bright. This brightness is due to overlapping of the carotid arteries
and jugular arteries. To eliminate the signal from overlapping vascular struc-
tures in the projection images, a presaturation pulse is applied asa3cmwide
saturation band at the level of 0.5 cm superior to the slice. The saturation pulse
