Apparent Intramyocellular Lipid Index Using Spiral Magnetic Resonance Spectroscopic Imaging at 3 T

Abstract Purpose: This work introduces a rapid, high-resolution method based on spiral Magnetic Resonance Spectroscopy Imaging (MRSI) to map the apparent content of intramyocellular (IMCL) and extramyocellular (EMCL) lipids in human skeletal muscle.

Methods

Unsuppressed water spiral MRSI data were acquired from healthy volunteers using a dual-tuned 1H/31P transmit/receive coil positioned under the right calf. Frequency registration of spectra was ensured using the Free Induction Decay (FID) modulus approach. By computing the cumulative sum of the normalized spectral amplitude over the 1.09–1.71 ppm range, we derived the Apparent Intramyocellular Lipid Index (AIMLI), which provides an estimate of the relative IMCL contribution within the total lipid signal (IMCL + EMCL) in each voxel. Signal simulations were conducted to optimize the AIMLI’s method to minimize its sensitivity to frequency shifts of the EMCL peak induced by magnetic susceptibility effects. In vivo, AIMLI performance was compared to classical LCModel quantification in muscles of interest (Gastrocnemius Medialis (GM) and Soleus Medialis (SM)) and discussed regarding fibers’ orientation (assessed from SE-EPI diffusion-weighted), and high-resolution water, fat, and fat fraction images (derived from 3D Chemical-Shift-Encoded (CSE) gradient multiecho T1 VIBE Dixon data). Repeatability and reproducibility were systematically evaluated. Additionally, AIMLI was applied in a longitudinal study to monitor metabolic changes during long-term fasting and compared with conventional Single Voxel Spectroscopy (SVS)-LCModel quantification.