Analysis of Dynamic Magnetic Resonance Elastography Data - Advance Image Processing in Magnetic Resonance Imaging - page 445

Synchronous Trigger Pulses

Wave
Generator

θ

Pivot Actuator

Coil

Synchronous Motion-Sensitizing Gradient

Motion

RF

Agar Gel Phantom

Direction of Motion-
Sensitizing Gradient

FIGURE 14.1 Schematic diagram of the MRE system. Conventional MRI gradients and

RF pulses that encode spatial positions are shown at the bottom left. The electromechanical
driver applies transverse acoustic waves to the object to be imaged via a surface plate
(right). The cyclic motion-sensitizing gradients and the acoustic drive are synchronized
using trigger pulses provided by the imager. The phase offset ( ) between the two can be

varied. As shown by the shaded regions, the motion-sensitizing gradients can be superim-
posed along any desired axis to detect cyclic motion.

θ

of the
mechanical and magnetic oscillations and the dot product of the displacement
amplitude vector and the motion-sensitizing magnetic gradient vector . Par-
ticles whose components of motion along the gradient vector are exactly in phase
or out of phase with the magnetic oscillation have maximum phase shifts of
opposing polarities. Particles whose components of motion along the gradient
vector is 90

This accumulated phase shift is proportional to the relative phase θ

ξ 0

G

out of phase with the magnetic oscillation have no net phase shift.
Because the response is also proportional to the number of gradient cycles (N )
and the period of the gradient waveform (T ), high sensitivity to small-amplitude
synchronous motion can be achieved by accumulating phase shifts over multiple
cycles of mechanical excitation and the motion-sensitizing gradient waveform. The
quantity

°

r

is the gyromagnetic ratio, and is the spin position vector.
Generally, two acquisitions are made for each repetition in an interleaved
fashion reversing the polarity of the motion-sensitizing gradients. This reduces
systematic phase errors and doubles sensitivity to small displacements. Typical
data-acquisition parameter ranges for 2-D sequences are: TR, 10 to 300 msec; TE,
10 to 60 msec; acquisition time, 20 to 120 sec; and flip angle, 10 to 60

γ

. The number
of gradient pulses (N) varies from 2 to 30 cycles, and the frequency of mechanical
excitation ranges from 50 to 1000 Hz. Acquiring and processing 2-D slices captures
only two of the three components of the wave propagation vector and may yield
misleading results unless the shear wave is propagating in a plane, but in some
cases, time considerations or other factors mandate their use. 3-D MRE pulse

°