Rectus Femoris Ultrasound Echo Intensity Is a Valid Estimate of Percent Intramuscular Fat in Patients Following Anterior Cruciate Ligament Reconstruction

Abstract

Objective: The aim of the work described here was to evaluate quadriceps muscle ultrasound metrics and common demographic variables to create a conversion equation that validly predicts magnetic resonance imaging (MRI) percent intramuscular fat after anterior cruciate ligament reconstruction (ACLR).

Methods: We recruited 15 participants between the ages of 18 and 35 y who were 1-5 y post-ACLR. For the MRI assessment, we used an iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) sequence to assess the mid-thigh. A single reader manually segmented the rectus femoris on two consecutive MRI slices using ITK-Snap to estimate the percent intramuscular fat. For the ultrasound assessment, a single investigator captured transverse panoramic ultrasound images of the mid-thigh with the participant positioned supine and the knee flexed to 30°. A separate single reader used ImageJ to manually segment the rectus femoris ultrasound images. Ultrasound metrics included muscle cross-sectional area, echo intensity and subcutaneous fat thickness. A stepwise linear multiple regression was used to develop an equation to predict MRI percent intramuscular fat using the ultrasound metrics and common demographics (i.e., age, sex, height, mass). Additionally, intraclass correlation coefficients (ICC_2,k) and Bland-Altman plots were used to assess the agreement between true and estimated percent intramuscular fat.

Results: Echo intensity and age significantly predicted MRI intramuscular fat percent (p = 0.003, r² = 0.62). When using the conversion equation, there was high agreement (ICC_2,k = 0.87, 95% confidence interval: 0.62-0.96) between the estimated and true percent intramuscular fat.

Conclusion: Our patient population-specific conversion equation that uses quadriceps muscle ultrasound echo intensity and age is a valid estimate of MRI percent intramuscular fat.

Keywords: Anterior cruciate ligament reconstruction; Conversion equation; Magnetic resonance imaging; Muscle composition; Muscle echo intensity.