Imaging bone structure and osteoporosis using MRI

In addition to Bone Mineral Density (BMD) bone quality plays an important role in defining bone strength. Trabecular bone quality can potentially be defined by several factors, for example trabecular micro-architecture, matrix composition of trabeculae and trabecular bone damage-repair. Considerable effort is being expended in developing techniques to assess trabecular bone micro-architecture non-invasively. Site-specific bone structure information would significantly contribute to understanding the results of different therapeutic interventions, and potentially assist in optimizing the course of treatment. Three dimensional techniques that reveal trabecular bone structure are emerging as important contenders for defining bone quality, at least partially. Techniques such as micro-computed tomography have recently been developed and provide high resolution images of the trabecular architecture. A more recent development in the assessment of trabecular bone structure is the use of magnetic resonance imaging techniques that make it possible to obtain non-invasive bone biopsies at multiple anatomic sites. Cortical and trabecular bone have a low water content and short T2 and are not detectable using routine MR imaging methods. However, the marrow surrounding the trabecular bone network, if imaged at high resolution, reveals the trabecular network. Using such images, multiple different image processing and image analysis algorithms have been developed. The goal of all of these is to quantify the trabecular bone structure in 2 or 3 dimensions. The measures that have been derived so far are many, some of them synonymous with the histomorphometric measures such as trabecular bone volume fraction (BV/TV), trabecular thickness (TbTh), trabecular spacing (TbSp), trabecular number (TbN), others include connectivity or Euler number, fractal dimension, tubularity, maximal entropy, etc. A number of calibration and validation studies (in vitro and in vivo) have been undertaken in which MR-derived measures of structure are compared with measures derived from other modalities, such as histology, micro-CT, BMD, and with biomechanics. With recent advances in phased array coils and higher strength magnets, the potential of MR imaging of bone structure is ever increasing. At the present time, the skeletal sites most commonly imaged are the radius and calcaneus. Studies currently underway are exploring the possibility of obtaining micro-architectural features of trabecular bone and the understanding whether bone turnover and micro-architecture are related, and the underlying relationship between turnover, bone mineral density and architecture, is the first step towards untraveling the therapeutic efficacy of different treatment regimen