Magnetic resonance imaging measurements of bone density and cross-sectional geometry

Magnetic resonance imaging (MRI) is commonly used in the assessment of the musculoskeletal system and associated pathology. The ability of MRI to meas ure the signals from water and lipid protons enables quantitative measureme nts of bone porosity. The goal of this investigation was to demonstrate tha t the density and cross-sectional geometry of whole bones can be noninvasiv ely measured using MRI. Ten trabecular specimens cored from whale vertebrae were used to compare apparent bone density measured directly, and using a quantitative MRI algorithm. Bone density and several cross-sectional geomet ric properties were also measured using MRI in the distal tibia of 14 volun teers. The MRI measurements were compared with measurements made using quan titative-computed tomography (QCT). A proton density sequence was used for all MRI studies. A porosity phantom was included in the MRI examinations an d used to convect the MRI signal intensity to bone volume fraction. Bone de nsity and cross-sectional bone geometry were calculated from the bone volum e fractions by assuming constant tissue properties. The apparent density of trabecular bone cores measured directly and using quantitative MRI were Li nearly related (r(2) = 0.959; P < 0.01). A strong linear relation also exis ted between MRI. and QCT measurements of ash density (r(2) = 0.923; P < 0.0 1) and cross-sectional geometric properties (r(2) = 0.976-0.992; P < 0.01). MRI data can be used to measure bone density and cross-sectional geometry of whole bones if a proton density sequence is used to homogenize differenc es in marrow composition and a porosity phatom is used for slice-specific v olume fraction calibration.