Relaxation time measurements of bone marrow protons in the calcaneus using a compact MRI system at 0.2 Tesla field strength

Abstract

Relaxation times (T 1 and T 2 ) of the bone marrow protons and trabecular bone volume fraction (TBVF) in the calcaneus were measured for 100 female volunteers using a compact MRI system at 0.2 T field strength. The speed of sound (SOS) through the calcaneus was measured also for the same subjects using a quantitative ultrasound system. Both relaxation times were found to have positive correlations with age (R ‫؍‬ 0.40; P < 0.0001 and R ‫؍‬ 0.31; P < 0.002, respectively) and negative correlations with SOS (R ‫؍‬ ؊0.38; P < 0.0001 and R ‫؍‬ ؊0.38; P < 0.0001, respectively). Although TBVF had a fairly high positive correlation with the SOS (R ‫؍‬ 0.67), neither T 1 nor T 2 were correlated with TBVF (R ‫؍‬ ؊0.062 and ؊0.024, respectively). These results suggest that the age dependence of both T 1 and T 2 is caused by the microdynamic properties of the lipid molecules in bone marrow observed using acoustic or elastic modalities. Bone density measurements using dual-energy X-ray absorptiometry (DXA) and quantitative ultrasound (QUS) are widely used for diagnosis and the assessment of drug therapy for osteoporosis. DXA is a transmission method for measuring areal bone mineral density (BMD) by correcting soft tissue absorption using two different energy X-ray beams. QUS is also a transmission method through a bulk tissue (mostly the heel) in which speed of sound (SOS) and other acoustic parameters such as the frequency dependence of sound attenuation are measured.Bone density measurements using MRI have several advantages over the above conventional methods. MRI can measure the "volume" density of trabecular bone (TB), which is much more sensitive to bone metabolism than cortical bone. Furthermore, unlike DXA, MRI does not use ionizing radiation and provides a much clearer physical interpretation of the measured results than QUS.Three different MRI approaches to bone density measurement have been proposed (1-9). The first is based on high resolution MRI, which can spatially resolve TB from bone marrow. This also provides a measure of TB microstructure, which gives useful information on TB strength (1-4). The second approach is based on measuring the relaxation rate R 2 * of transverse nuclear magnetization arising from the difference in susceptibility between TB and bone marrow (5-7). The third approach is based on quantification of bone marrow protons in a voxel, which is a complementary quantity of TB volume fraction (TBVF) in a voxel (8,9).Because the first and second approaches need a high magnetic field strength (Ͼ 1.0 Tesla [T]), we used the third method to construct a compact MRI system for measuring TBVF in the calcaneus (10). A particular advantage of the system is that it can measure TBVF in the calcaneus in a few minutes, thereby allowing the rapid scanning of many subjects. We measured the relaxation times of bone marrow protons, TBVF, and SOS for the calcaneus of 100 female volunteers to clarify the NMR properties of bone marrow protons. MATERIALS AND METHODSOne male (age 51 years) and 100 female volunteers...

Mobile and Compact NMR

“…8c) [30,71,166,167]. Another major field of interest of mobile and compact MRI concerns plants as another type of biological tissue [168,169]. Mobile instruments with center-field magnets have been developed to study trees ( Fig.…”