![Visualization of ultrasound image acquisition and analysis. (A) The fascicle probe angle (FPA) is subject to variation in both the pennation angle (α, middle) and the gel pad angle (GA, right). Only GA can be controlled by the operator, while α stays constant in a resting muscle. The ultrasound gel pad enables angulation affording minimal ultrasound attenuation prior to coupling into the tissue. At a muscle/IMCT interface, reflection, transmission, absorption, and scattering (not indicated) occur and change with the FPA. A change in FPA causes a change in the angle of reflection x =|FPA|, and hence causes the reflected ultrasound ray to be displaced by ∆x ∝ sin(2 ·\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\cdot$$\end{document}|FPA|) in the probe. Thus, a change in FPA will cause all reflected rays within a detector area of fascicle-transverse length ∆x to be reflected away from the probe, and thus the MGV decreases by an amount ∝ ∆x. The image in the middle exemplifies the trigonometric relationship between the probe angle and the amount of ultrasound waves being reflected back to the probe. The echo intensity detected is determined by the function y = β0 – (β1/2) ∙ sin(2|FPA|) where β0 is the mean gray value at 0° FPA (MGV_00) and β1 is the tilt echo gain (TEG). (B) Set-up for ultrasound image acquisition: The linear probe is held in place by a probe holder that is attached to a robotic axis translating the probe in the shown direction. (C) Representative ultrasound images of an infraspinatus muscle scanned with a 0° (left), + 12° (middle) and − 2° (right) gel pad. α is the pennation angle, between fascicle (F) and epimysium (E). ω is the fascicle probe angle (FPA), between fascicle and probe (P). The mean gray value is visibly higher at a smaller FPA (middle < top < bottom). (D) Segmentation of macroscopic photo of muscle cross section (left) with ilastik (right).](https://www.researchgate.net/publication/378239397/figure/fig1/AS:11431281224058778@1708061008680/Visualization-of-ultrasound-image-acquisition-and-analysis-A-The-fascicle-probe-angle_Q320.jpg)
February 2024
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This study aimed to validate the concept of spatial gain sonography for quantifying texture-related echo intensity in B-mode ultrasound of skeletal muscle. Fifty-one bovine muscles were scanned postmortem using B-mode ultrasonography at varying fascicle probe angles (FPA). The relationship between mean gray values (MGV) and FPA was fitted with a sinusoidal and a linear function, the slope of which was defined as tilt echo gain (TEG). Macroscopic muscle cross sections were optically analyzed for intramuscular connective tissue (IMCT) content which was plotted against MGV at 0° FPA (MGV_00). MGV peaked at FPA 0°. Sine fits were superior to linear fits (adjusted r²-values 0.647 vs. 0.613), especially for larger FPAs. In mixed models, the pennation angle was related to TEG (P < 0.001) and MGV_00 (P = 0.035). Age was relevant for MGV_00 (P < 0.001), but not TEG (P > 0.10). The correlation between the IMCT percentage and MGV_00 was significant but weak (P = 0.026; adjusted r² = 0.103). The relationship between fascicle probe angle and echo intensity in B-mode ultrasound can be modeled more accurately with a sinusoidal but more practically for clinical use with a linear fit. The peak mean gray value MGV_00 can be used to compare echo intensity across muscles without the bias of pennation angle.