Master's Thesis - Studierstube Augmented Reality Project - Graz ...

3.5 Cine Cardiac Fast Low-Angle Shot with Parallel-Imaging Technique
of the gradient pulse pair which is incorporated into the sequence. Figure 3.5.1 and
Figure 3.13, show the remaining phase components of moving spins for the case of a
certain bipolar gradient field in one direction.
(a) Measuring a flow compensated data requires
to compensate an arbitrary bipolar pulse G bp1 so
that out-of-phase spins are brought back to the
same phase of the stationary spins. The result is a
phase image with flow-sensitivity zero.
(b) Applying a self-compensating bipolar
gradient in one direction leads to a
phase-related flow sensitivity in that direction.
Computing the complex difference
of this signal with the complex reference
signal from (a) produces an intensity
value proportional to the quantitative
flow value of this voxel in the
gradient’s direction.
Figure 3.12: Comparing a flow-compensated complex reference signal measurement (a)
with a flow-sensitive one (b). Both use a bipolar gradient but in different fashion.
The complex subtraction of two of these data points produces an image with signal
intensities depending on local flow velocities. A complex differentiation ∆S performed
for one direction is shown in Figure 3.14. The length of the difference ∆S depends on
the phase shift Φ V for each pixel. The amount of phase shifts,
Φ V = γ · A · τ · v (3.33)
depends on the area of each gradient pulse and distance between the pulses, with
γ [Hz/T ] ... gyro magnetic ratio
A [m 2 ] ... area of each gradient pulse
τ [s] ... time between the pulses
56