Robust Assessment of Macromolecular Fraction (MMF) in Muscle with Differing Fat Fraction Using Ultrashort Echo Time (UTE) Magnetization Transfer Modeling with Measured T1

Magnetic resonance imaging (MRI) is widely regarded as the most comprehensive imaging modality to assess skeletal muscle quality and quantity. Magnetization transfer (MT) imaging can be used to estimate the fraction of water and macromolecular proton pools, with the latter including the myofibrillar proteins and collagen, which are related to the muscle quality and its ability to generate force. MT modeling combined with ultrashort echo time (UTE-MT modeling) may improve the evaluation of the myotendinous junction and regions with fibrotic tissues in the skeletal muscles, which possess short T2 values and higher bound-water concentration. The fat present in muscle has always been a source of concern in macromolecular fraction (MMF) calculation. This study aimed to investigate the impact of fat fraction (FF) on the estimated MMF in bovine skeletal muscle phantoms embedded in pure fat. MMF was calculated for several regions of interest (ROIs) with differing FFs using UTE-MT modeling with and without T1 measurement and B1 correction. Calculated MMF using measured T1 showed a robust trend, particularly with a negligible error (<3%) for FF < 20%. Around 5% MMF reduction occurred for FF > 30%. However, MMF estimation using a constant T1 was robust only for regions with FF < 10%. The MTR and T1 values were also robust for only FF < 10%. This study highlights the potential of the UTE-MT modeling with accurate T1 measurement for robust muscle assessment while remaining insensitive to fat infiltration up to moderate levels.

Automated summary

Magnetic resonance imaging (MRI) is considered the most comprehensive imaging technique for assessing skeletal muscle quality and quantity. Magnetization transfer (MT) imaging can estimate the water and macromolecular proton pools, including myofibrillar proteins and collagen, which are related to muscle quality and force generation. The combination of MT modeling and ultrashort echo time (UTE-MT modeling) can improve the evaluation of myotendinous junction and fibrotic tissue regions in skeletal muscles with shorter T2 values and higher bound-water concentration. This study investigated the impact of fat fraction (FF) on macromolecular fraction (MMF) estimation in bovine skeletal muscle phantoms embedded in pure fat using UTE-MT modeling with and without T1 measurement and B1 correction. The results demonstrate the potential of UTE-MT modeling with accurate T1 measurement for robust muscle assessment while being insensitive to fat infiltration up to moderate levels.