Relationship between NMR transverse relaxation, trabecular bone architecture, and strength.

Abstract

Structure, biomechanical competence, and incremental NMR line broadening (R') of water in the intertrabecular spaces of cancellous bone were examined on 22 cylindrical specimens from the lumbar vertebral bodies of 16 human subjects 24-86 years old (mean, 60 years old). A strong association (r = 0.91; P < 0.0001) was found between Young's modulus of elasticity and R' for a wide range of values corresponding to cancellous bone of very different morphologic composition. NMR line broadening is caused by the inhomogeneity of the magnetic field induced as a consequence of the coexistence of two adjacent phases of different diamagnetic susceptibility-i.e., mineralized bone and water in the marrow spaces. Structural analyses performed by means of NMR microscopy and digital image processing indicated that the variation in R' is closely related to the trabecular microstructure. Mean trabecular plate density measured along the direction of the magnetic field was found to play a major role in predicting Rj (r = 0.74; P < 0.0001). This behavior was confirmed when the plate density was varied in individual specimens, which was achieved by rotating the specimen, making use of the bone's structural anisotropy. It is concluded that the NMR transverse relaxation rate in human cancellous bone of the spine is significantly determined by trabecular structural parameters relevant to biomechanical strength. The results further underscore the important role played by the transverse trabeculae in contributing to cancellous bone strength. The work has implications on possible in vivo use of quantitative magnetic resonance for the assessment of fracture risk in osteoporotic patients.It is widely accepted that the mechanical strength of the skeleton in vertebrates is largely determined by the material density of trabecular bone, generally referred to as bone mineral density (BMD). Osteopenia, characterized by a loss in bone mass, leads to impaired bone strength, which, in turn, has been associated with atraumatic vertebral fractures suffered by patients with osteoporosis (1). The association between bone strength and BMD has been suggested by a large number of studies, with bone strength statistically showing a quadratic dependence on density (2). However, in vivo BMD measurements by either quantitative computed tomography or dual-energy x-ray absorptiometry do not very well predict fracture risk in osteoporotic patients (3).More recently, the search for a better predictor of bone strength, and thus fracture risk, has shifted toward trabecular bone morphology (4-8). Kleerekoper (5) found in a study in which postmenopausal women with vertebral fractures were compared to age-matched normal women of equal mean BMD that osteoporotics had significantly lower trabecular plate density and, as a corollary, higher trabecular plate thickness. The data were interpreted as suggesting that anThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accorda...

Quantitative MRI for the assessment of bone structure and function

“…Surfaces were modeled with triangular elements of constant magnetic surface charge density, which allows the induced field to be computed from eqn (5). The data indicated the width of the induced field distributions to be narrowest for the polarizing field applied parallel to the preferred orientation of the trabeculae, confirming previous experimental findings (40). Figure 2 shows a histogram of the induced magnetic field, derived from 3D MR images of bovine tibia with the magnetic field oriented along each of three orthogonal directions.…”

“…The relationship between the locally induced fields and architecture (density and orientation of the trabeculae), begs the question whether R Ã 2 or R 0 2 can serve as a surrogate for the bone's mechanical competence. Several studies have investigated this aspect by comparing the elastic modulus or ultimate strength measured by mechanical testing, with R 0 2 or R Ã 2 (40,47,48). As pointed out previously, the TB structure is highly anisotropic and, as expressed by eqn (6), R 0 2 has a strong angular dependence.…”

“…those orthogonal to the field) are primarily polarized and therefore are expected to be the principal cause of the susceptibility-induced line broadening. In cadaver studies of the human vertebrae, Chung et al (40) provided compelling evidence for the critical role of the horizontal trabeculae in conferring strength from measurements of R 0 2 when the polarizing field was oriented parallel to the body axis. Specimens representing subjects of widely varying age were, after marrow removal, immersed in water and R 0 2 measured at 1.5 T. They found R 0 2 to be highly correlated with Young's modulus of elasticity for compressive loading (r ¼ 0.91; P < 0.0001).…”

Measurements of mobile and bound water by nuclear magnetic resonance correlate with mechanical properties of bone

“…While both NMR and magnetic resonance imaging (MRI) have been used to characterize bone in a number of studies [40,45,[48][49][50][51][52][53][54][55][56], the present work is the first to quantify water distribution in bone for two age groups. A previous study had actually characterized water distribution in dentin samples from a 20 year, 35 year, and 50 year old donor [50].…”

In vivo NMR microscopy allows short-term serial assessment of multiple skeletal implications of corticosteroid exposure

“…However, it is well known that BMD is not the sole determinant of bone strength and that architecture plays a significant role in conferring strength to the trabecular network (6)(7)(8). Unlike bone mineral densitometry, which affords a measure of global or regional bone density, we and others have shown that magnetic resonance microimaging ( -MRI) can quantify trabecular bone microarchitecture in vitro in specimens and biopsies (9)(10)(11)(12)(13) and in vivo in laboratory animals and humans (14)(15)(16). Whereas microcomputed tomography ( -CT) can generate high-resolution images of trabecular bone architecture (17), magnetic resonance (MR) is unique in that it is also able to provide detailed information on structure and function of soft tissues and bone marrow, which are also affected by steroid treatment.…”