Potential role of nuclear magnetic resonance for the evaluation of trabecular bone quality

Abstract

This paper discusses two novel applications of nuclear magnetic resonance (NMR) as an investigational tool for the assessment of cancellous bone microarchitecture. It further outlines extensions of the method for in vivo clinical evaluation of bone strength in patients with skeletal disorders such as osteoporosis. The first method relies on the hypothesis that the presence of two phases of different magnetic permeability, i.e., bone and bone marrow, causes a spatial nonuniformity of the magnetic field across the measurement volume. The resulting spread in resonance frequency shortens the decay time constant (T2*) of the time domain proton signal in bone marrow or its substitute (water). Increased trabecular spacing, such as it occurs in osteoporosis, reduces the spatial field inhomogeneity and thus prolongs T2*, which has been shown both in vitro and in vivo. Subjects with osteoporosis, characterized by either low bone mineral density and/or spine compression fractures, have T2* values that are significantly prolonged. The second method focuses on a direct measurement of micromorphometric parameters of cancellous bone, using the principles of proton NMR microscopy in conjunction with computer processing of the resulting digital images. Image contrast between the trabeculae and the intertrabecular space is based on the marrow protons providing a signal, as opposed to bone, which appears with background intensity. Once tissues have been classified (into bone and marrow), for example, by means of a histogram-based segmentation algorithm, bone area fraction, mean trabecular plate density (MTPD), and mean trabecular plate thickness (MTPT) can be computed without the need for further operator intervention. The most critical parameter for successful implementation is image slice thickness which determines the extent of partial volume blurring. At 400 MHz spectrometer frequency (9.4 T field strength), images of appropriate resolution can be obtained from a 1 cm3 specimen of vertebral cancellous bone in 1 hour or less. It is shown that for relatively isotropic cancellous bone such as the one found in the vertebrae, a slice thickness on the order of 200 μm is adequate, with an inplane resolution on the order of 50 × 50 μm2 As an illustration of the technique, the relationship among the different stereologic parameters in cadaver specimens of human lumbar vertebrae is reported, showing a strong association between the area fraction and both MTPD and MTPT. The chief benefit of the new technique is its nondestructive nature and its ability to provide histomorphometric images from multiple physical locations and in multiple planes, which is desirable because of the large spatial variations in the morphologic parameters within the bone. Finally, the technique is demonstrated to be potentially also noninvasive, as illustrated with images from the human finger, acquired on a modified 1.5 Tesla clinical magnetic resonance imaging system at a pixel size of 95 × 95 μm2 © 1993 Springer-Verlag New York Inc.

Publication Title

Calcified Tissue International

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