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Trabecular bone volume fraction (TBVF) is highly associated with the mechanical competence of trabecular bone. TBVF is ordinarily measured by histomorphometry from bone biopsies or, noninvasively, by means of high-resolution microcomputed tomography and, more recently, by micro-MRI. The latter methods require spatial resolution sufficient to resolve trabeculae, along with segmentation techniques that allow unambiguous assignment of the signal to bone or bone marrow. In this article it is shown that TBVF can be measured under low-resolution conditions by exploiting the attenuation of the MR signal resulting from fractional occupancy of the imaging voxel by bone and bone marrow, provided that a reference signal is available from a marrow volume devoid of trabeculation. The method requires accurate measurement of apparent proton density, which entails correction for various sources of error. Key among these are the spatial nonuniformity in the RF field amplitude and effects of the slice profile, which are determined by B(1) field mapping and numerical integration of the Bloch equations, respectively. By contrast, errors from variations in bone marrow composition (hematopoietic vs. fatty) between trabecular and reference site are predicted to be small and usually negligible. The method was evaluated in phantoms and in vivo in the distal radius and found to be accurate to 1% in marrow volume fraction. Finally, in a group of 12 patients of varying skeletal status, TBVF in the calcaneus was found to strongly correlate with integral bone mineral density of the lumbar vertebrae (r(2) = 0.83, p < 0.0001). The method may fail in large imaging objects such as the human trunk at high magnetic field where standing wave and RF penetration effects cause intensity variations that cannot be corrected. Magn Reson Med 46:103-113, 2001., (Copyright 2001 Wiley-Liss, Inc.)

In micro-magnetic resonance imaging of cancellous bone architecture, involuntary subject motion even on a sub-millimeter scale is detrimental and generally precludes accurate quantification of architectural parameters. In this work a navigator-assisted three-dimensional spin-echo technique is described and evaluated for imaging at 137 microm resolution in humans. The method is based on gradient navigator echoes following the spin-echo readout for sensing translational displacements alternately in x- and y-directions with a spatial resolution of 273 microm and a temporal resolution of 0.2 sec. The technique is shown to improve micro-images of the distal forearm significantly and to enhance accuracy and reproducibility of bone volume fraction, transverse contiguity, and tubularity, parameters introduced in prior work to characterize the trabecular network. It is further shown that a fourfold reduction in navigator sampling time, along with zero-filling, improves the accuracy of the navigator correction while reducing the minimum pulse repetition time or gradient heating. The data indicate that navigator-assisted micro-imaging is capable of effectively correcting sub-millimeter displacements in micro-imaging.

This article reviews recent progress in magnetic resonance microimaging of cancellous bone in vitro and in vivo from the perspective of the authors' laboratory. It is shown that in particular in vivo the key technical prerequisites to satisfy are: (i) achieving sufficient signal-to-noise ratio (SNR) to allow for adequate spatial resolution; (ii) the image processing algorithms have to be robust enough to provide accurate structural information in the limited spatial resolution regime, i.e., in the presence of inevitable partial volume blurring and noise. The practical lower limit of voxel size in vivo was found to be about 6 x 10(-3) mm3 in the radius, and about 10(-4) mm3 for small specimens in vitro with state-of-the-art equipment and scan times of 10 and 30 minutes, resp., and SNR approximately 10. Finally, data are presented highlighting the potential of these methods for predicting the bone's elastic modulus in vitro and fracture risk in vivo.

A method is described for measuring R'2, the RF reversible contribution to the effective transverse relaxation rate in yellow trabecular marrow, as a means to evaluate trabecular bone structure and density. The method exploits the similarity in spectral composition of the marrow and fat in subcutaneous tissue. Under these conditions the gradient echo envelope of the marrow signal can be regarded as a convolution of a function describing the bone marrow intravoxel line broadening (R'2) with a function expressing chemical shift modulation, which is obtained from the echo envelope of the subcutaneous fat signal in a reference region. Simple division of each of a series of echoes by the reference signal is shown to afford a smooth decay which can be fitted to a model to extract R'2. The method has been evaluated in the upper femur of test subjects and a strong correlation of the thus derived R'2 values with those obtained by the GESFIDE technique is demonstrated. The close correspondence in spectral composition of proximal femur marrow and subcutaneous fat is further illustrated by means of localized spectroscopy. The major potential error source is global inhomogeneity in the reference region which can lead to an underestimation of the demodulation-derived R'2., (Copyright 1998 Academic Press.)