SAN FRANCISCO — Advances in imaging techniques are providing new insights into trabecular and cortical bone structure and may help assess bone quality, a key component of bone strength identified by a 2001 National Institutes of Health consensus panel.
Recent studies suggest that high-resolution MRI (hrMRI), multidetector CT, and high-resolution peripheral quantitative CT (hr-pQCT) each may be useful in assessing bone quality. But each brings different advantages and disadvantages, and it's unclear which imaging modality will be best for identifying osteoporotic fractures and monitoring treatment-related changes in bone structure.
The three imaging modalities can produce significantly different absolute numbers compared with each other when assessing trabecular or cortical bone structure, yet all correlate reasonably well with micro-CT as a standard of reference, Dr. Thomas M. Link said at a conference sponsored by the International Society for Magnetic Resonance in Medicine. Because of differences in acquisition and analysis of the images, bone structure data from the three imaging modalities are not directly comparable.
Trabecular and cortical bone structure are key components of bone quality, which was deemed to be an important component of bone strength, according to NIH (JAMA 2001;285:785–95).
In one randomized, double-blind study, for example, 51 postmenopausal women with osteopenia were treated with alendronate or placebo and followed over a 2-year period by 3T MRI of the radius, tibia and femur, hr-pQCT of the radius and tibia, and dual x-ray absorptiometry measures of bone mineral density. Both 3T MRI and hr-pQCT results for trabecular bone showed moderate but significant correlation with bone density as a reference, even though there was a twofold to fourfold difference between 3T MRI and hr-pQCT in parameter values such as trabecular number, thickness, or separation (J. Bone Miner. Res. 2008;23:463–74).
In earlier studies, hrMRI showed that salmon-calcitonin nasal spray helped maintain trabecular microarchitecture, compared with placebo (J. Bone Miner. Res. 2005;20:1548–61) and that testosterone replacement may improve trabecular architecture in hypogonadal men (J. Clin. Endocrinol. Metab. 2003;88:1497–502).
Another study of 106 postmenopausal women found no difference in conventional bone mineral density measurements between the 35 women with a history of fractures and the fracture-free women in the rest of the cohort, but hr-pQCT imaging showed significant differences in trabecular structure (J. Clin. Endocrinol. Metab. 2005;90:6508–15).
Multidetector CT was used in a separate study showing significant increases in trabecular microstructure in 65 postmenopausal women who were treated for 12 months with teriparatide for osteoporosis (J. Bone Miner. Res. 2007;22:1426–33).
For corticol bone imaging, a newer area of research, two 2008 studies using hr-pQCT showed substantial differences between postmenopausal women with hip or wrist fractures, compared with fracture-free women, said Dr. Link, professor of radiology at the University of California, San Francisco.
Both hrMRI and hr-pQCT are being used experimentally to assess cortical bone porosity, which affects bone stability. One recent study using hr-pQCT found significant differences between normal premenopausal women, normal postmenopausal women, and postmenopausal women with renal osteodystrophy. “This is quite exciting to see these changes in cortical bone porosity. We don't really know what they mean yet, but they're clearly associated with fracture risk,” said Dr. Link, who has has received research funding and support from Merck, which markets medication to treat osteoporosis.
MRI or hr-pQCT provide high spatial resolution and produce no or relatively little radiation, compared with high-radiation exposure from multidetector CT. Multidector CT has the advantage of allowing imaging of more central skeletal sites such as the spine or proximal femur, he said. The hr-pQCT scanners image only peripheral sites such as small areas of the radius and tibia and possibly the calcaneus, while hrMRI covers larger areas of the radius, tibia, and possibly the femur.
The CT techniques provide measures of bone densitometry. Although hrMRI gives no densitometric data, some studies suggest it may be used to analyze bone marrow composition through spectroscopy in order to assess bone stability. The three techniques appear to have similar rates of reproducibility.
MRI is expensive, and the time needed for imaging results in motion artifacts. In comparison, hr-pQCT requires a dedicated scanner. Although the exam time is shorter with hr-pQCT, motion artifacts remain a problem. Multidetector CT is widely available and requires less time for a scan. Postimage processing requires very sophisticated techniques with MRI, but also is technically challenging with hr-pQCT.
Trabecular and cortical bone architecture of the distal tibia is seen here with hr-pQCT.
High-resolution 3T MRI of the distal tibia shows trabecular and cortical bone architecture. IMAGES COURTESY DR. THOMAS M. LINK