Image Processing Reference
In-Depth Information
signal difference between two acquisitions with different through-plane tag-
encoding gradients to estimate strain in the direction normal to the slice. Relative
through-plane displacement was determined from the phase of the DENSE image.
However, the effects of motion parallel to the image plane (e.g., shears) were
ignored. In theory, strain-weighted imaging could be used to estimate all com-
ponents of the strain tensor, using at least six different displacement- encoding
directions, in a manner similar to MR diffusion tensor imaging.
10.8
FUTURE DIRECTIONS
The field of MRI cardiac motion estimation is developing rapidly. True fast imaging
with steady-state precision (FISP) imaging sequences allow faster image acquisition
of tagged images [44]. With phase-sensitive quadrature acquisitions, the phase of
both peaks in a 1-1 SPAMM-encoded image can be combined to reduce artifacts
due to field inhomogeneity, susceptibility, etc. [45]. Parallel imaging techniques using
8 or more element cardiac coils allow 3-D cardiac acquisitions in a single breath
hold, or in real time [46]. Eulerian HARP or DENSE strain analysis can also be
performed in real time, allowing interactive display as the images are acquired.
An alternative technique for removing the relaxed component of SPAMM-
tagged images, proposed by Aletras et al. [47], involves inversion RF pulses
regularly spaced in the imaging sequence (alternating inversion recovery [AIR]
SPAMM). This effectively keeps the relaxed component alternating about zero,
while the tagged component alternates in sign at each inversion.
Another method for removing the complex conjugate signal (the third term
at ~2
k
e
in Equation 22) was recently proposed by Epstein and Gilson [48]. Instead
of increasing the size of
k
e
so that the peak at 2
k
e
is pushed out of the readout
window, two additional CSPAMM acquisitions are performed with sin(
k
e
⋅
X
) and
−
sin(
k
e
⋅
X
) encoding. These can be combined with the standard cos(
k
e
⋅
X
) and
−
cos(
k
e
⋅
X
) acquisitions in order to cancel both the second and third terms in
Equation 22, leaving just the displacement-encoded signal. This technique can
be used to reduce the size of
k
e
(reducing the amount of SNR loss due to strain)
or to encode displacement perpendicular to the slice.
This issue of which type of strain should be reported requires further study.
Lagrangian strains have traditionally been quoted for SPAMM-tagged studies
[25] and studies using radiopaque markers [24]. However, Eulerian strains are
easier to calculate in HARP, DENSE and tissue Doppler ultrasound studies.
Statistical procedures to compare one patient group with another need to be
devised. Preliminary work in this area, using a principal component analysis of
finite element models, is reported in Reference 9,
Chapter 3
.
10.9
CONCLUSIONS
It can be seen that the field is in a stage of rapid development, and any review
of current techniques is almost immediately outdated. The techniques of SPAMM,
HARP, and DENSE are closely related and can be thought of as variants on the
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