Morphometric MRI study of hippocampal shape in MCI using spherical harmonics

Abstract

Background: Mild cognitive impairment (MCI) is characterized by memory complaints and impairment in the absence of dementia and confers a high risk for AD. Identifying medial temporal morphological abnormalities, in circuits required for learning and memory, may be critical for early diagnosis and treatment of MCI and AD. Objective: Volumetric analysis can identify hippocampal atrophy in MCI, but does not localize the structural changes. Shape analysis has the potential to provide important information beyond volume and may localize regionally specific structural changes in the absence of volume differences. This study performed hippocampal shape analysis aiming at a global and local quantitative representation of shape changes in MCI. Methods: Participants were 40 adults with amnestic MCI (age 72.5 3.3), 40 adults with cognitive complaints (CC) but no impairment (72.6 2.6), and 42 normal controls (CN) (70.8 2.6). MRI data were a T1-weighted SPGR coronal series acquired on a GE 1.5T LX magnet. The hippocampi were segmented using BRAINS software. The left and right hippocampi were treated as a single shape configuration. The spherical harmonics (SPHARM) description was used for surface modeling, with the parameter space being aligned according to the first order ellipsoid for surface correspondence. After normalizing for the total volume, landmarks were created by uniform surface sampling (Figure 1(a,b)) and aligned by a quaternion-based algorithm. For each landmark, the local shape change was defined as the distance between an individual and the mean along the normal direction of the mean surface. Surface signals were modeled as Gaussian random fields. Heat kernel smoothing was employed to increase SNR on the hippocampal surface (Figure 1(c)) and statistical inference was performed via random fields theory. Conclusions: The results of group analyses (t-maps in Figure 2) show that statistically significant regions of shape changes only appear between CN and MCI. The CC group showed a more intermediate pattern. The structural changes in MCI were primarily located in the anterior right hippocampus and posterior left hippocampus (Figure 3). Shape analysis has the potential to inform early detection and is likely to be useful for longitudinal monitoring of response to therapeutic agents. P-121 RESTING FDG-PET IN NEUROLOGICALLY NORMAL INDIVIDUALS REPORTING DREAM ENACTMENT BEHAVIOR: PRECLINICAL DEMENTIA WITH LEWY BODIES?