Several studies have explored how MR images compare with other imaging modalities in determining structural parameters (Table I). Hipp et al. and Hopper et al. used small-bore MRI with resolutions of 92x92x92 |im3 and 23x23x39 |im3 respectively. All other studies were performed on 1.5 or 3T scanners with in-plane resolution of 100-150 mm and a slice thickness of 300 |im on in vitro bone cubes. Weber et al. compared MR in vivo and in vitro trabecular bone images from mice with histological sections. They found parameters derived from in vivo images correlated better with histological parameters than did in vitro images and attributed the difference to the better MR signal from bone marrow than formalin. These studies indicate that MR derived architectural parameters correlate well with measures taken at much higher resolutions. In general, MR tended to overestimate BV/TV and Tb.Th and underestimate Tb.Sp due to partial volume effects.
Architechtural parameters have also been compared to bone mineral density (BMD) and mechanical strength in the radius, lumbar vertebrae, femur, calcaneus and among various sights. In these studies correlations coefficients for BV/TV, Tb.Th, and Tb.N with BMD or mechanical strength were between 0.5 and 0.8. All studies found that Tb.Sp had a correlation coefficient with BMD or mechanical strength of -0.5 to -0.6, indicating that the spacing between the trabeculae increases as BMD and mechanical strength decrease. Studies also found that combining BMD and trabecular structural parameters improved correlations with mechanical strength.
Figure 6 – Axial MR images of the distal radius (156x156x500 |im). A. Non-osteoporotic 37 year old. B. Osteoporotic 76 year old with radial fracture. Note the thinning of the trabeculae and loss of trabecular bone volume. The figure on the right illustrates pre and post-menopausal decrease in bone fraction in the radius as one moves from the joint line into the shaft.
In vivo imaging in humans
DXA is the gold standard for diagnosing osteoporotic bone, however only provides an areal measure of bone mineral density. Multi-slice CT can be used for volumetric bone mineral density and structural measurements. Though MR cannot provide measures of BMD, it can provide trabecular bone structural measures and does not require radiation. Trabecular bone structure also varies considerably depending on the skeletal site, as well as within a given skeletal site (Figure 6). Studies have examined the trabecular structure in the calcaneus of normal and osteoporotic women and found that structural parameters (especially BV/TV, Tb.Sp, Tb.N, and connectivity measures) were significantly different between normals and osteoporotic trabecular bone. The same was found to be true in the calcaneus of normal and osteoporotic men and in the radius of premenopausal, postmenopausal normals, and postmenopausal patients with hip fractures. Tb.Sp demonstrated that largest change with age, increasing significantly in postmenopausal women with hip fractures. Benito et al. detected bone loss in hypogonadal men using MR. They found that the ratio of plates to rod (surface voxels to curve voxels in their analysis) and bone volume fraction decreased in hypogo- nadal men. Correspondingly, the erosion index, a combination of topological parameters that increases as bone architecture deteriorates, was higher in men with hypogonadism. MR has been used to measure structural bone changes in steroid induced osteoporosis in patients after renal and cardiac transplantation. Structural parameters were significantly lower (except for Tb.Sp, which was higher) after cardiac transplantation due to the altered bone metabolism caused by immunosuppres- sive drugs. Large pre- and post-transplant differences in structural parameters were not seen in renal patients probably because renal failure can alter bone metabolism and trabecular structure before transplantation occurs. Chesnut et. al. have published the first longitudinal study showing that nasal spray calcitonin preserves trabecular bone micro-architecture in the distal radius. Source your medication needs online. generic viagra online pharmacy home delivered
Table I – Correlation of MR derived trabecular parameters with those derived from other imaging modalities. All values are statistically significant with p<0.05. n.s. denotes correlations that were not statistically significant.
Correlation coefficients Tb.Sp Tb.N
X-ray tomographic microscopy (18 |im)
Optical images (23 |im)
Optical images (20 |im)
Scanning electron microscopy (20x)
Macro section radiograph (5 |im)
Macro section radiograph (5 |im)
CT (247x247x1000 |im3)
MicroCT (22 |im)