MRI Measurement of Heart Wall Motion - Advance Image Processing in Magnetic Resonance Imaging - page 352

90 90 90

rf

ab c de r

g

δ ∆ δ

FIGURE 10.11 Stimulated echo version of the pulsed gradient spin echo. Displacement
during the interval ∆ is encoded in the phase of the signal measured at time point f.

technique (Subsection 10.3.5), which applied an encoding sequence at a fixed point
in time and imaged at multiple subsequent times, this novel technique encoded
velocity at multiple points in time but imaged at a single fixed time. This results in
a series of functional images of a single plane of myocardium which describe veloc-
ities and strain rates throughout the cardiac cycle, referenced to the imaged frame.

10.6 DENSE

10.6.1 T HEORY

In order to avoid the tracking errors arising from integrating velocity measure-
ments over time, displacement information over long time periods (potentially
up to the period of the cardiac cycle) can be encoded in the phase of the MR
signal using stimulated echoes [32,35]. The 180

° pulse of the pulse gradient spin

echo (PGSE) experiment ( Figure 10.9 ) is split into two 90 ° pulses separated by

a time interval which is typically substantially longer than T2 but shorter than
T1 (Figure 10.11). The velocity-encoding gradients of the PGSE now encode
displacement over the longer “mixing time.” Thus, the phase of the image is
proportional to the displacement occurring over

∆ ; however, the signal-to-noise

ratio is halved [32,35].
Due to the longer mixing time, the displacement-encoded signal fades over time,
and a nonencoded signal gains in strength due to T1 relaxation. The relaxed, non-
encoded signal corrupts the displacement map. Traditionally, the size of the encode
and decode gradients in the slice direction were set to a large value in order to crush
the relaxed component signal. The decode gradient was thus used to both refocus
the encoded signal and defocus the relaxed component. However, a common problem
with stimulated echo imaging techniques in the heart is that myocardial strain or
rotation leads to nonhomogeneous displacement over a voxel. In this case, the phase
of the transverse magnetization within the voxel will not completely refocus, leading
to signal loss. The size of the signal loss depends on the magnitude of the decode
gradient and the amount of strain or rotation occurring in the myocardium. Fischer
et al. [35] examined this problem and concluded that stimulated echo techniques
had limited application in the heart if the displacement decode gradient is also used
as a crusher gradient to destroy signals arising due to T1 relaxation.