Assessment of trabecular structure using high resolution magnetic resonance imaging

Although bone mineral density is one of the most important contributing factors to bone strength and risk of fracture, studies have shown that changes in bone quality and structure independent of bone mineral density, influence both bone strength and individual risk of fracture. The influence of these other factors is thought to explain at least partially the observed overlap in bone mineral measurements between patients with and without osteoporotic fractures, irrespective of measurement site or technique. Thus, several new emerging techniques have been aimed at quantifying trabecular bone structure in addition to bone density. With recent hardware and software advances magnetic resonance (MR) images with spatial resolutions of 80-150 microns and slice thickness of 300-700 microns which resolve the trabecular structure have been obtained both in vitro and in vivo. Both modified spin-echo and gradient echo based imaging sequences have been used to obtain these images, and although the technical parameters and the sequence specific mechanisms affect the depiction of trabecular bone. In conjunction with three dimensional image processing and an understanding of the mechanisms of image formation, these high resolution images may be used to quantify trabecular bone architecture. In addition to obtaining standard stereological measures such as trabecular bone volume, mean trabecular width, mean trabecular spacing, mean intercept length as a function of angle, parameters such as three dimensional connectivity as measured by the Euler number, fabric tensor in three dimensions and texture related parameters such as fractal dimension may be derived from such images. Quantitative measures of trabecular architecture derived from such images have been compared with those obtained from higher resolution 18 microns images, and with biomechanical properties. In vivo studies in the radius and calcaneus have been performed and differences between osteoporotic and normal subjects are distinguishable. Thus, MR imaging techniques coupled with computerized image analysis may potentially be very useful for studying osteoporosis and quantifying trabecular bone architecture and may provide information in addition to bone density.