In vivo bone 31 P relaxation times and their implications on mineral quantification.

PURPOSE: The intersubject variations in bone phosphorus-31 (31 P) T1 and T2*, as well as the implications on in vivo 31 P MRI-based bone mineral quantification, were investigated at 3T field strength.

METHODS: A technique that isolates the bone signal from the composite in vivo 31 P spectrum was first evaluated via simulation and experiments ex vivo and subsequently applied to measure the T1 of bone 31 P collectively with a spectroscopic saturation recovery sequence in a group of healthy subjects aged 26 to 76 years. T2* was derived from the bone signal linewidth. The density of bone 31 P was derived for all subjects from 31 P zero TE images acquired in the same scan session using the measured relaxation times. Test-retest experiments were also performed to evaluate repeatability of this in vivo MRI-based bone mineral quantification protocol.

RESULTS: The T1 obtained in vivo using the proposed spectral separation method combined with saturation recovery sequence is 38.4 ± 1.5 s for the subjects studied. Average 31 P density found was 6.40 ± 0.58 mol/L (corresponding to 1072 ± 98 mg/cm3 mineral density), in good agreement with an earlier study in specimens from donors of similar age range. Neither the relaxation times (P = 0.18 for T1 , P = 0.99 for T2*) nor 31 P density (P = 0.55) were found to correlate with subject age. Average coefficients of variation for the repeat study were 1.5%, 2.6%, and 4.4% for bone 31 P T1 , T2*, and density, respectively.

CONCLUSION: Neither 31 P T1 nor T2* varies significantly in healthy adults across a 50-year age range, therefore obviating the need for subject-specific measurements.