As in chapter 1, we have stated the problems and given the research questions and objectives. Now we examine whether we have achieved the objectives in our experiments and implementations. Firstly, we made the anthropomorphic prostate phantom with soft PVC, which could give the phantom sufficient elastic deformations during the intervention. With different concentrations, the phantom has different bio-mechanical properties in different parts. The prostate model and pubis model were made based on real patient MR segmentations and CT scans and the relative positions were determined under the doctor’s suggestions. By examining the MR images, we concluded that the T2-weight scans showed the good
visibility and MR compatibility. In both the BEAT IRTTT sequence and full 3D sequence the deformations were obvious and the displacements were minor. The motion under intervention was clear in the real time sequence. The data resolution and acquisition time were acceptable. The collected MR images can be easily implemented for 3D image processing. By doing all of these, the objective ‘Make an anthropomorphic deformable prostate phantom to simulate prostate motion in needle intervention and collect MR image data with respect to prostate deformations.’ is accomplished.
Secondly, in terms of the other objective, we built the statistic data set using multi-direction needle insertions. For each image, shape and texture informa- tion was collected by a manual shape segmentation. By means of a spherical parameterization based MDL group-wise correspondence optimization, we built the shape data variance statistic model using PCA. With a 3D image warping method the texture of different scans can be associated and modeled statistically. The active appearance model can be established by combining the shape model and the texture model. We improved the standard model search approach which conventionally uses 3D image for shape and texture segmentation, with respect to three orthogonal slices that contain more information of shape deformations. By means of searching the intensities on these 2D slices, the parameters for adjusting the shape, transformation, and modeled intensities were updated, until the differ- ence between modeled intensity and the searched intensities were minimized. We used the leave-all-in method for validation and it proved the 2D slices based AAM fitting is convergent and the expected shape and texture can be generated with a small acceptable error distance. Though in the leave-one-out verification, the performance was not as good as that in leave-all-in scheme. Therefore, the objective ‘build active appearance models using the collected data and segment the prostate by means of three orthogonal 3D slices based model search’ is not completely achieved. However, the main problem is that the data set is too small for the AAM system to learn most of the shape and texture variations. The verification can be developed using more data set such as building a new phantom and choosing more practical insertion points to collect the data as many as possible. We can anticipate the segmentation results would be better with more training images presence.
Now we will see with these mostly achieved objectives, whether the research questions can be answered.
1 Is it feasible to learn the prostate deformation and motion knowledge in a 3D space?
We planned to use the prostate phantom to simulate the possible motions in the MR guided intervention and indeed we have seen a satisfying dynamic property in the MRI based intervention. This observation stimulated us to aim at learning the rules of motion and deformation of a general prostate. For the sake of making the shape more representative, we designed the prostate shape from a real case. The multi-direction needle insertions were designed for including most of the cases. To learn the prostate deformation and the motion in the
biopsy, we want to generalize the situation from the phantom to the real case. However, what we have is the AAM trained from only a phantom and the limited experimental insertions. In consequence, the obtained statistical models worked well with the phantom, but could not be applicable in the real case. There are several reasons for this. First, PCA extracts the principal axes that represents the most shape point variance. But the shape, though, is almost the same (because all the shapes are from the same phantom), the correspondence cannot be perfect since the image registration approach could be problematic and the segmentation for each image was done manually which always has a huge amount of deviated labeling. When the shape points difference in the deformed part has an equivalent magnitude with the variation in other points caused by false manual segmentation, the principal components for the deformation would be sinked in the false shape variations. In this case, if we apply the shape principal components to the real case, the generated shape would be problematic. Another problem is that the texture pattern of the phantom is way too much different with the MRI scan of male pelvis. In conclusion, the motion and deformation were simulated, and the shape variance of phantom could be learnt if the noise can be neglected.
2 Is it feasible to segment the prostate based on three orthogonal slices in a 3D MR environment?
As we discussed above, we have implemented the 2D slices based AAM search for segmenting the shape and texture in 3D space. The leave-all-in validation has proven the feasibility of prostate segmentation depending on the 3D AAM training and 2D fitting. However, the reliability of this approach has not been verified for the leave-one-out scheme. As a result, we need to collect sufficient training data set that has a precise manual segmentation and well establish the shape points correspondence. Another problem is the slices for testing were orthogonal and parallel to the three MR planes (axial, sagittal and coronal). While in the biopsy implementation, it could not be absolute orthogonal and also not parallel to the three planes. Theoretically this problem can be solved as long as the slices include most of the deformable part and the AAM system is precisely employed, which requires the phantom to be simulated the deformation correctly. Moreover, when we compare the 3D T2-weighted MR images with the image slices from BEAT IRTTT sequence, we can see a lot of differences mainly in intensities. Because the real time sequence is very fast, more artifacts could be observed which can significantly influence the phantom image quality. This would lead to a bad model search with problematic intensities. In fact, in human tissue there would only be the texture difference between the intensities. But for the phantom, since it’s hard to get rid off all the air bubbles so the artifact could be a problem in a faster sequence. In summary, prostate segmentation depending on three orthogonal slices is feasible, but for the practical verifications it requires more work.
In short, we have used the phantom to mimic the motion of prostate in biopsy, and tested the deformation by means of using AAM modeling. We successfully
reconstructed the 3D representation with the aid of three orthogonal slices. For a generalization from the phantom to the real case, the current work cannot guarantee a good performance.