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

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

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. $$ 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.

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 $$ ) nor ³¹ P density (P = 0.55) were found to correlate with subject age. Average coefficients of variation for the repeat study were 1.5%, 2.6%, and 4.4% for bone ³¹ P T₁ , $$ , and density, respectively.

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