Imaging bone structure and osteoporosis using MRI: Imaging in animal studies

23 Dec
2011

MR also has been used to measure structural parameters in animal models of osteoporosis. Jiang et al. treated an ovariec- tomized sheep model of osteoporosis with salmon calcitonin, an osteoclast inhibitor, to determine if structural parameters in the neck of the femur could be maintained. It was found that BV/TV and Tb.N decreased and Tb.Sp increased in ovariectomized sheep. Structural parameters of sheep treated with salmon calcitonin were equivalent to sham operated sheep. Small-bore micro-MRI has been used to study osteo- porotic bone structure in ovariectomized rats. Analysis of MR images revealed differences in osteoporotic trabecular structure that DXA could not detect.

Takahashi et al. have investigated the effects of corticosteroid on bone structure in rabbit femurs using magnetic resonance microimaging (|iMRI). They found that short term, high doses of corticosteroids resulted in a decrease in trabecular bone volume through trabecular thinning with little change in trabecular network, trabecular number or trabecular spacing. Using MR spectroscopy they also determined that hematopoietic bone marrow was converted to fatty marrow in rabbits treated with corticosteroid.

Future

Recent advances in micro-CT imaging in vivo make it possible to obtain radius and tibia images using this methodolo­gy. However, comparative studies, in-vivo case-control studies, longitudinal studies using micro-CT in vivo in humans have not been undertaken and are clearly warranted. MR imaging has proved to be a valid method for analyzing trabecular structure and offers distinct advantages over other imaging modalities. Be­sides being non-ionizing, and providing the ability to image skele­tal sites such as the calcaneus, hip, tibia, femur, it offers the ad­vantages of characterizing bone and the adjoining soft tissues. In particular, MR images soft tissue, such as cartilage, muscle, marrow, and meniscus, which is not possible with x-ray based imaging modalities. Understanding the relationship between bone and cartilage is critical, particularly in cases of arthritis or injury. It has been found that degradation of cartilage on one compartment of the knee corresponds with a loss of trabecular structure in the other compartment, which is probably linked to mechanical load between the compartments. Most MR images display proton signals from water or fat. It is possible, however, to detect signals from other molecules in a technique called MR spectroscopy. This technique has been used to a limited amount in bone imaging, in particular to im­age phosphorus in cortical and trabecular bone and lipids in the red bone marrow in hematological diseases. It has also been suggested that MRI can be used to detect the in­crease in lipid-to-water ratio in the vertebral bodies in patients with osteoporosis.

The combination of MR imaging and finite element (FE) analy­sis has been used to determine mechanical properties of trabecular bone. This allows the in vivo estimation of me­chanical properties, which are usually determined by in vitro compression testing. In FE models derived from MR images it is possible to incorporate soft tissue structures in the model. This would be useful not only in mechanobiological models of tissue differentiation and bone remodeling, but also in models of fracture healing where cartilage formation is critical to the process. Medication you can afford 

Bone quality has been an emerging concept in the area of os­teoporosis. Trabecular bone micro-architecture, bone geometry and associated marrow changes in osteoporosis can all be probed using MRI. Thus, MR techniques have the potential for providing a complete whole-organ assessment of skeletal sta­tus in osteoporosis, and further developments in this imaging modality and research studies are clearly warranted.

 

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