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 3.0T. Theory and Methods SIR is a quantitative method for measuring magnetization transfer (qMT) that uses 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 min/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 SNRs 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 Δ B 0 , , and T 1 ) when optimized t I ‐ t D combinations are selected.