In Vivo Measurement and Visualization of Pelvic Position and Orientation and Changes in Soft Tissue Shape and Thickness with Respect to Changes in Seating Surface Shape

Abstract

Pressure ulcers are volumes of tissue necrosis that evolve after prolonged exposure of skin and soft tissue to the forces of reclining or sitting. They are serious problems for people confined to beds and wheelchairs. Treating them is expensive, and if untreated fatal infections can evolve, so substantial clinical effort is focussed on prevention. However the precise conditions for and mechanisms of ulcer development and evolution are poorly understood, so prevention is largely ad hoc. Current models of pressure ulcer formation and evolution postulate that soft tissue distortion is a more important issue than pressure per se. This thesis reports the development and evaluation of a system that uses reclining MRI, non-invasive measurement of pelvic bone position and orientation, external pressure sensing, and ultrasonic soft tissue thickness sensing to measure the distortions and distortion changes of the various soft tissue layers of the buttocks when a subject sits on a surface of dynamically variable shape. This multi-modality system is required because current volumetric imaging scanners cannot accomodate seated subjects, and they certainly cannot accommodate subjects seated on a dynamically shaped surface. Pressure and thickness data are fused with pelvic bone position and orientation data to yield shapes and thicknesses of skin, fat, and muscle layers as a function of seating contour. Results are rendered on an interactive 3D visual display for qualitative intuitionbuilding by human researchers, and they are analyzed quantitatively to generate functional relationships between subjects and seating contours. The approach is validated by a small pilot study of two healthy subjects and one spinal-cord injured subject. The study concretely explores and evaluates the kinds of comparisons and analyses that can be done. If this pilot study is successfully validated by a large enough future study drawn from a broad enough population, it is apparent that the currently accepted models of soft tissue distortion for seated subjects will need to be revised substantially.