TH‐D‐201C‐10: Improved Quantification of the CH2/CH3 Ratio of Lipids: Illustration in Vivo on Tibial Bone Marrow at 3 T

Purpose : To improve the accuracy of lipid CH 2 /CH 3 ratios determined by proton magnetic resonance spectroscopy (MRS) by minimizing J‐coupling modulations of the CH 3 lipid peak. Method and Materials : Experiments were conducted in vivo on the tibial bone marrow lipids of four volunteers at 3 T. A regular PRESS (Point RESolved Spectroscopy) sequence was used to estimate the T 2 (transverse relaxation) of the CH 2 protons by acquiring spectra at five echo times (TEs) and plotting the areas as a function of TE. The curve was fitted to the function M o exp(−TE/T 2 ) where M 0 is proportional to the CH 2 proton concentration. The CH 3 response to a regular PRESS sequence is modulated due to J‐coupling interactions and does not decay monotonically. To determine M o of the CH 3 protons a narrow‐bandwidth PRESS sequence was designed that rewinds the J‐coupling evolution of the CH 3 protons in the voxel of interest and spectra were acquired with the same five TEs. The CH 2 /CH 3 ratio was calculated by dividing the CH 2 M o by that determined for CH 3 and the result was multiplied by 1.5 to compensate for the different proton multiplicities. Results : The mean T 2 of the CH 2 protons was estimated to be ≈ 88 ms. Applying the narrow‐bandwidth PRESS sequence minimized CH 3 signal variations due to J‐coupling and resulted in a decay curve that could be described by a monoexponential T 2 decay function. The mean T 2 for the CH 3 protons was ≈ 133 ms. A mean ratio of 12:1 was calculated for the CH 2 :CH 3 ratio of the tibial bone marrow lipids of all volunteers. Conclusion : The presented PRESS sequence enables the T 2 of the CH 3 lipid protons to be measured with more accuracy than would be determined by using short‐TEs thereby allowing a more accurate measure of the CH 2 /CH 3 lipid composition ratio to be determined.