Correcting radiofrequency inhomogeneity effects in skeletal muscle magnetisation transfer maps

The potential of MRI to provide quantitative measures of neuromuscular pathology for use in therapeutic trials is being increasingly recognised. Magnetisation transfer (MT) imaging shows particular promise in this context, being sensitive to pathological changes, particularly in skeletal muscle, where measurements correlate with clinically measured muscle strength. Radiofrequency (RF) transmit field (B1) inhomogeneities can be particularly problematic in measurements of the MT ratio (MTR) and may obscure genuine muscle MTR changes caused by disease. In this work, we evaluate, for muscle imaging applications, a scheme previously proposed for the correction of RF inhomogeneity artefacts in cerebral MTR maps using B1 information acquired in the same session. We demonstrate the theoretical applicability of this scheme to skeletal muscle using a two‐pool model of pulsed quantitative MT. The correction scheme is evaluated practically in MTR imaging of the lower limbs of 28 healthy individuals and in two groups of patients with representative neuromuscular diseases: Charcot–Marie–Tooth disease type 1A and inclusion body myositis. The correction scheme was observed to reduce both the within‐subject and between‐subject variability in the calf and thigh muscles of healthy subjects and patient groups in histogram‐ and region‐of‐interest‐based approaches. This method of correcting for RF inhomogeneity effects in MTR maps using B1 data may markedly improve the sensitivity of MTR mapping indices as measures of pathology in skeletal muscle. Copyright © 2011 John Wiley & Sons, Ltd. We evaluate a scheme for the correction of radiofrequency inhomogeneities in skeletal muscle magnetisation transfer ratio (MTR) maps using B1 mapping data. A quantitative model of pulsed MT in muscle is used for theoretical validation. We demonstrate experimentally that the method reduces the within‐ and between‐subject variation of muscle MTR measures in 28 healthy volunteers and 23 patients with neuromuscular conditions, increasing the potential value of MTR as a quantitative marker of disease.