Optimization of selective inversion recovery magnetization transfer imaging for macromolecular content mapping in the human brain

PURPOSE: To optimize a selective inversion recovery (SIR) sequence for macromolecular content mapping in the human brain at 3.0 T. METHODS: SIR is a quantitative method for measuring magnetization transfer (qMT) that employs a low-power, on-resonance inversion pulse. This results in a biexponential recovery of free water signal that can be sampled at various inversion/predelay times (t(I)/t(D)) to estimate a subset of qMT parameters, including the macromolecular-to-free pool-size-ratio (PSR), the R(1) of free water (R(1f)), and the rate of MT exchange (k(mf)). The adoption of SIR has been limited by long acquisition times (≈4 mins/slice). Here, we use Cramér-Rao Lower Bound theory and data reduction strategies to select optimal t(I)/t(D) combinations to reduce imaging times. The schemes were experimentally validated in phantoms, and tested in healthy volunteers (N=4) and a multiple sclerosis patient. RESULTS: Two optimal sampling schemes were determined: i) a 5-point scheme (k(mf) estimated) and ii) a 4-point scheme (k(mf) assumed). In phantoms, the 5/4-point schemes yielded parameter estimates with similar signal-to-noise ratios as our previous 16-point scheme, but with 4.1/6.1-fold shorter scan times. Pair-wise comparisons between schemes did not detect significant differences for any scheme/parameter. In humans, parameter values were consistent with published values, and similar levels of precision were obtained from all schemes. Furthermore, fixing k(mf) reduced the sensitivity of PSR to partial-volume averaging, yielding more consistent estimates throughout the brain. CONCLUSIONS: qMT parameters can be robustly estimated in ≤1 min/slice (without independent measures of ΔB(0), B(1)(+), and T(1)) when optimized t(I)−t(D) combinations are selected.