Quantification of the PCF volume and the degree of PCF crowdedness were shown to be beneficial for differential diagnosis of tonsillar herniation7,11
and for prediction of surgical outcome.10
The lack of a reliable automated method for PCF volumetry by use of MR imaging, however, limits the clinical use of these PCF markers. Advanced automated methods for brain parcellation have matured in recent years and are becoming more widely used.16,17
This work represents adaptation of established brain segmentation techniques tailored toward PCF volumetry in CMI. The proposed atlas-guided PCF segmentation method is enhanced by the creation of a CMI-specific reference atlas that captures the altered PCF morphology associated with CMI. An excellent agreement between the proposed automated method and manual segmentation by an expert observer is evident by the small relative percentage difference of −0.3 ± 1.9% and the very high mean Dice coefficient of 0.96. The delineation of the PCF obtained by use of the proposed automated method highly agrees with the manual delineation in terms of accuracy and spatial overlap in patients with CMI. Furthermore, a high degree of repeatability is evident from the small absolute percentage difference of 0.6 ± 0.2% found by use of quantification of the repeated scans in 3 healthy subjects. The automated volume measurement of PCF is minimally affected by the normal variability in patient positioning in the MR imaging scanner.
The mean PCF volume measurement obtained in our small cohort of adult patients with CMI (196 ± 8.7 mL) tends to be larger than previously reported CT and MR-based measurements of 186 mL by Nishikawa et al,11
174 ± 25 mL by Noudel et al,10
and 166 ± 8 mL by Milhorat et al.7
The bias in the mean volume measurements may be attributed to the differences in the modalities and the possible differences in the segmentation protocols, particularly how the PCF boundaries were defined. Another contributing factor may be related to the difference in the sampling resolution of the volumetric data. In contrast to isotropic 1-mm 3D imaging used in this work, previous reports used 2D-based imaging with thicker sections for the volumetric measurements that can lead to measurement errors caused by large partial volume effect. In addition, the limited number of subjects used in this study to validate the proposed automated method against manual segmentation may not be representative of a CMI population in terms of PCF volume.
The tonsillar herniation in CMI has been attributed to overcrowding of the PCF as a result of a small PCF and normally developed brain tissue volume.7,10,11
Therefore, in addition to PCF volume measurement, accurate quantification of brain tissue volume is also critical. Our measurement of mean PCF tissue volume of 162.1 ± 8.2 also tends to be slightly larger than previously reported values of 156 mL by Nishikawa et al11
and 151.8 ± 3.1 mL by Milhorat et al.7
However, the measurement of crowdedness, the ratio of PCF tissue volume to PCF volume of 0.826 ± 0.012, is in good agreement with the mean value of 0.833 reported by Nishikawa et al.11
The comparison of the hindbrain tissue volume measurements between the proposed method and FreeSurfer revealed a statistically significant mean difference of 3.6 ± 1.1% (P = .005). The tissue volumes found through the use of FreeSurfer were consistently larger than volumes obtained by using the proposed method. As demonstrated in , this difference is the result of the exclusion of the tonsillar tissue volume that descends below the foramen magnum. This tissue is excluded because it is outside the PCF and thus does not contribute to the PCF overcrowding.
Assessment of the associations between PCF volumes and the linear PCF markers revealed that none of the 6 measures were significantly associated with the PCF volume. The 5 linear landmarks of the PCF were all modestly positively correlated with the PCF volume. The lack of significance can be explained in part by the small sample. The length of herniation negatively correlated with the PCF volume, which is expected when a normal size cerebellum is compressed inside an increasingly smaller PCF.
The reference atlas used to guide the segmentation was prepared by use of MR images from patients with CMI, all of whom had tonsillar herniation >5 mm. Therefore, this atlas may not be optimal for segmentation of healthy subjects because of morphologic differences. However, for the purpose of reproducibility estimate, data from healthy subjects were used because repeated scans from patients with CMI were not available. Even with this limitation, an excellent reproducibility with an average difference of 0.6 ± 0.2% is obtained, reflecting the robustness of the proposed method.