High resolution MR and micro MR have received considerable attention both as research tools and potential clinical tools for assessment of trabecular bone architecture. Magnetic resonance is a complex technology based o nthe application of high magnetic fields, transmission of radiofrequency (RF) waves and detection of RF signals from excited hydrogen protons. As a noninvasive, nonionizing radiation technique, MR can provide three dimensional images in arbitrary orientations and can depict trabecular boen as a negative iamge by virtue of the strong signal generated by the abundant fat and water protons in the surroundign marrow tissue. In MR the appearance of the image is affected by many factors beyond spatial resolution, including the field strength and specific pulse sequence used, the echo time and the signal-to-noise acheived. MR microscopy holds promise as an additional valuable technology for quantitative assessment of trabecular structure both in vivo and in vitro. Investigators have obtained 78 um isotropic resolution of human and bovine bone cubos using three dimensional imaging at 9.4 Tesla and found changes in accordance with histomorphometry measures. Studies have extended the in vitro techniques to obtain images in an ovariectomized rat model and have shown the ability to measure changes in trabecular structure following ovariectomy. We examined human cadaver specimens using a standard clinical MR scanner at 1.5 Tesla and a spatial resolution of 117~*117~*300 um, and observed resolution dependence for traditional stereological parameters, some of which could be modulated by appropriate processign techniques. In an early study the feabibility of using such images to quantify trabecular structure, MR images of the distal radius were botained using a modified gradient echo seuqnece, a 1.5 Tesla imager, and a spatial resolution of 156 um and slice thickness of 0.7 mm was established. In axial sections from normal and osteoporotic MR iamges clearly depicted the loss of the integrity of the trabecular network with the development of osteoporosis. Similar images of the calcaneus of normal subjects showed that the orientation of hte trabeculae is significantly different in various anatomic regions. Clinical images at 1.5 Tesla with special TF soil designs and have Been used in the phalanges at resolutions of 78-150 um and slice thickness of 300 um. The capability for spine and/or hip fracture discrimination usign trabecular structure or textural parameters fromin vivo MR images of the radius or calcaneus have also been demonstrated.
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