Docteur de l'université Automatic Segmentation and Shape Analysis ...

38 Chapter 2 Literature Review
2.3.1 Volume measurement
Usual methods to assess hippocampal atrophy are based on the manual or auto-
matic segmentation (e.g. Hsu et al., 2002; Chupin et al., 2009) of the hippocampal
volume. The volume of hippocampus can be measured by the count of voxels di-
rectly on the segmentation. Due to the difference in the head size which correlates
with the hippocampal volume (Free et al., 1995), the hippocampal volumes are
often normalized by the total intracranial volume (TIV, e.g. Yavuz et al., 2007) or
the intracranial coronal area (ICA, e.g. Järvenpää et al., 2004). Decreased volume
in hippocampus and asymmetry in the volume reduction has been reported (for
review, see Barnes et al., 2009b; Shi et al., 2009).
Figure 2.5: Tracing hippocampal volume. The region of interest (ROI, green)
includes the hippocampal proper, subiculum, dentate gyrus and part of the
alveus. Image source: Barkhof et al. (2011).
In longitudinal studies, serial scans are acquired at baseline and follow-up time
points. The rate of atrophy indicating the change in volume can be computed
if the measurements on the scans over time are available. Since the atrophy rate
compares the follow-up measurements with baseline of the same subject, the inter-
subject difference in the head size is less important. Images are usually registered
to the first scan (e.g. Jack et al., 2004), or a common atlas space (e.g. Barnes
et al., 2007). Greater rate of hippocampal atrophy in AD than the control group
has consistently been reported in the literature, which have been reviewed in the
meta-analysis by Barnes et al. (2009a).