Biomedical Engineering Reference
In-Depth Information
more meaningful approach that consists of finding the average displacement of
voxels in a region-of-interest (ROI) [20]. The 3D distances between transformed
voxels are calculated in millimeters and averaged over a cubic ROI just covering
the prostate.
3.2.3.4
Other Evaluation Methods Including Displacement
of Prostate Centroids
A variety of other methods are used to evaluate the registration of the pelvis and
prostate. First, we measure potential displacements of the 3D centroid of man-
ually segmented prostates. Second, we use multiple visualization and analysis
methods found in RegViz such as contour overlap and color overlay. Third, we
calculate the intensity difference between the reference and registered volumes
on a voxel-by-voxel basis and compute statistics. Fourth, we calculate the cor-
relation coefficient (CC) between corresponding voxels to measure the quality
of registration of two MR volumes acquired with identical parameters.
3.2.4
Examples and Results
3.2.4.1
Data Acquisition
We acquired MRI volumes from a 1.5 T Siemens MRI system (Magnetom Sym-
phony, Siemens Medical Systems, Erlangen, Germany). An 8-element phased
array body coil was used to ensure coverage of the prostate with a uniform sensi-
tivity. Typically two anterior and two posterior elements were enabled for signal
acquisition. We used two different MR sequences. First, we used a 3D RF spoiled
gradient echo steady state pulse sequence (FLASH) with TR/TE/flip parameters
of 12/5.0/60 which give 256
×
256
×
128 voxels over a 330
×
330
×
256-mm field
of view (FOV) to yield 1
.
29
×
1
.
29
×
2
.
0-mm voxels oriented to give the highest
resolution for transverse slices. The acquisition time was 5.63 min. This sequence
was good for pelvic imaging but was not ideal for the prostate. Second, we used
a 3D rapid gradient echo sequence (PSIF) designed to acquire the spin-echo
component of the steady state response, rather than the free induction decay.
The spin echo component formed immediately prior to the RF pulse, and it was
shifted toward the prior RF pulse through appropriate gradient waveform de-
sign. The sequence with 9.4/5.0/60 (TR/TE/flip) yielded 160
×
256
×
128 voxels
