Multivariate tensor-based morphometry on surfaces: application to mapping ventricular abnormalities in HIV/AIDS.

Here we developed a new method, called multivariate tensor-based surface morphometry (TBM), and applied it to study lateral ventricular surface differences associated with HIV/AIDS. Using concepts from differential geometry and the theory of differential forms, we created mathematical structures known as holomorphic one-forms, to obtain an efficient and accurate conformal parameterization of the lateral ventricular surfaces in the brain. The new meshing approach also provides a natural way to register anatomical surfaces across subjects, and improves on prior methods as it handles surfaces that branch and join at complex 3D junctions. To analyze anatomical differences, we computed new statistics from the Riemannian surface metrics-these retain multivariate information on local surface geometry. We applied this framework to analyze lateral ventricular surface morphometry in 3D MRI data from 11 subjects with HIV/AIDS and 8 healthy controls. Our method detected a 3D profile of surface abnormalities even in this small sample. Multivariate statistics on the local tensors gave better effect sizes for detecting group differences, relative to other TBM-based methods including analysis of the Jacobian determinant, the largest and smallest eigenvalues of the surface metric, and the pair of eigenvalues of the Jacobian matrix. The resulting analysis pipeline may improve the power of surface-based morphometry studies of the brain.

Amygdalae morphometry in late-life depression.

OBJECTIVE: The amygdalae have been a focus of mood disorder research due to their key role in processing emotional information. It has been long known that depressed individuals demonstrate impaired functional performance while engaged in emotional tasks. The structural basis for these functional differences has been investigated via volumetric analysis with mixed findings. In this study, we examined the morphometric basis for these functional changes in late-life depression (LLD) by analyzing both the size and shape of the amygdalae with the hypothesis that shape differences may be apparent even when overall volume differences are inconsistent. METHODS: Magnetic resonance imaging data were acquired from 11 healthy, elderly individuals and 14 depressed, elderly individuals. Amygdalar size was quantified by computing total volume and amygdalar shape was quantified with a shape analysis method that we have developed. RESULTS: No significant volumetric differences were found for either amygdala. Nevertheless, localized regions of significant shape variation were detected for the left and right amygdalae. The most significant difference was contraction (LLD subjects as compared to control subjects) in a region typically associated with the basolateral nucleus, which plays a key role in emotion recognition in neurobiologic models of depression. CONCLUSIONS: In this LLD study, we have shown that, despite insignificant amygdalar volumetric findings, variations of amygdalar shape can be detected and localized. With further investigation, morphometric analysis of various brain structures may help elucidate the neurobiology associated with LLD and other mood disorders.

Guidance of retrobulbar injection with real-time tomographic reflection.

OBJECTIVE: Retrobulbar and peribulbar injections are common ophthalmologic procedures used to deliver anesthetics and other medications for ophthalmic therapy and surgery. These injections, typically performed without any type of guidance, can lead to complications that are rare but visually devastating. The needle may penetrate the optic nerve, perforate the globe, or disperse toxic quantities of drugs intraocularly, causing major visual loss. Sonographic guidance may increase the accuracy of the needle placement, thereby decreasing the incidence of complications. However, difficulties arise in coordinating the relative location of the image, the needle, and the patient. Real-time tomographic reflection is a new method for in situ visualization of sonographic images, permitting direct hand-eye coordination to guide invasive instruments beneath the surface of the skin. METHOD: In this preliminary study, real-time tomographic reflection was used to visualize the eye and surrounding anatomic structures in a cadaver during a simulated retrobulbar injection. RESULT: The needle tip was easily followed as it was advanced into the retrobulbar space. CONCLUSIONS: The images presented in this preliminary study show the use of real-time tomographic reflection to visualize insertion of an invasive instrument into the human body.