Mapping to Talairach Coordinate System 67

tween individual brains occurs in cerebellum resion in both the female template and the bisexual template. However, the bisexual template results more normalization distortion in cerebellum and occipital region while normalizing female subjects to it. The female tem-plate causes less variance of nonlinear deformation for female subjects.

5.2.2 Correlation Ratio between Subjects and Templates

We calculate the correlation ration between warped images and the template to evalu-ate the similarity of them. In other words, we study the performance of registration when different templates participate. The results of CR between warped images in both gender group and the bisexual template are shown in Table 4.11. The CR between warped male images and the male template is 0.8776 and between warped male images and the bisex-ual template is 0.8647. Otherwise, the CR between warped female images and the female template is 0.8868 and between warped female images and the bisexual template is 0.8845.

The result reveals the gender templates for the subjects in their group participated improves the registration accuracy.

5.3 Mapping to Talairach Coordinate System

In order to verify the mapping coordinate in Talairach coordinate system from our tem-plate space, we manually identified five tissue landmarks and transform them to Talairach space. The labeled position of these five landmarks, including AC, PC, ACC, GU and CB, are listed in Table 4.12. We located these landmarks in Talairach atlas to acquire the ground truth. Fig. 4.16 shows the coordinates of the landmarks in Talairach atlas. Fig. 4.17 demon-strates the position of the five landmarks in our unbiased prime template.

We used the proposed method, which used MNI template as the bridge, to convert the coordinates from our template to Talairach space. Table 4.13 shows the original coordi-nates in our template space, the derived mapping coordicoordi-nates and the real coordicoordi-nates in Talairach space (the ground truth). However, due to the artifact of manual identification of landmarks and transformation error from MNI to Talairach space, the derived mapping coordinates are not identical to the real coordinates in Talairach coordinates. Even though, the results are still promising when the transformation from MNI space to Talairach space advances [5].

Chapter 6

Conclusions

In this study, we developed an automatic procedure of constructing MRI brain templates and provided the transformation from our template space to Talairach space. The construc-tion procedure contained selecconstruc-tion of representative brain and determinaconstruc-tion of unbiased stereotaxic space. First, because we need a reference space to derive template space, we chose a brain volume, which is one subject of the image set and has the minimum variation of deformation magnitude to the other subjects, as the representative brain. Second, we computed the unbiased space according to the representative brain and all other brain im-ages. In this step, we developed an interpolation algorithm to calculate the corresponding position from the template space to the space of the representative brain. Besides, in order to obtain the structural and functional labels, we also provided the transformation from our template to the Talairach coordinate system.

It is important to have a brain template for functional and structure researches. Brain comparison, based on normalizing to a standard brain template, needs a suitable template which diminishes the distortion of normalization and improves the registration accuracy.

In this study, we compared the study specific unbiased template, which was constructed from 191 Taiwanese brains, and ICBM152 template. The results revealed that the unbiased template could diminish the nonlinear deformation and improved the registration accuracy evidently.

To evaluate the performance of using the study specific template, we applied our proce-dure to construct the templates for both gender groups. We compared the gender templates with the bisexual template. The observation from this study showed that the study spe-cific templates improved the registration accuracy. Thus, we may conclude that the study specific unbiased template is more suitable for the subjects in this study group.

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