Biomedical Engineering Reference
In-Depth Information
Using the parallel re-slicing, a good segmentation in the slices close to the
middle of the prostate is possible. But when the slice is nearly tangential
to the boundary of prostate, i.e., at the ends of the prostate, only a small
portion of the prostate appears in the image and the segmentation is more
difficult (see Figure 12). Considering that the prostate is approximately
ellipsoidal shaped, an alternative re-slicing technique—called rotationally
re-slicing— is superior. In this method, the 3DUS image of the prostate is
re-sliced rotationally about an axis approximately through the center of the
prostate into
N
rotational slices separated by
π/N
(Figure 11b). Hence,
the prostate shapes and sizes in the re-sliced 2D images are similar (see
Figure 13).
Step 2: Initialization.
The contour initialization in the initial slice requires the
operator to select four points on the prostate boundary as described in
Section 2. The 4-point-based model generates useful prostate initialization
models for simple prostate shapes but not for complex abnormally shaped
prostates. In order to represent an abnormal prostate more accurately, an
alternative model based on cardinal-spline interpolation can be used [42].
Using this model, a number of initial points (usually 3-8 depending on
the complexity and contrast of the prostate) are chosen on the prostate
boundary to obtain an initial contour.
Step 3: Refinement in 2D image.
Refinement of the prostate contour is performed
using the DDC method described in Section 2. If the refined contour in
the initial slice is far from the “true” boundary of the prostate, it may be
edited by adding or moving its vertices. Then, the edited contour is refined
again as the initial contour. The procedure is repeated until the final refined
contour matches the prostate boundary.
Step 4: Propagation to three dimensions.
The refined contour in the initial slice
can be propagated to its adjacent slice and used as a good initial approx-
imation of the prostate contour in the distance between the two slices is
small. For the rotational re-slicing method, the distance between slices
increases with the distance to the rotational axis. In order to decrease the
spatial interval located at the boundary of the prostate, a small angular
step has to be used in the rotationally re-slicing, which can be typically 2
r
[42]. Thus, for a large prostate with a 40-mm diameter, the largest spatial
interval at the prostate boundary in rotational re-slicing is about 0.7 mm,
which is the approximate resolution of the US images and is less than the
parallel re-slicing spacing of 1.0 mm used in [41].
Step 5: 3D reconstruction.
Steps 3 and 4 are repeated until all the prostate bound-
aries in the 2D slices have been segmented. The 3D prostate boundary sur-
face can then be reconstructed by interpolation of the contours obtained in
the 2D segmentations of all the 2D slices.
