Abstract
Purpose: Panoramic ultrasound imaging (pUSI) is an extended field of view (FOV) imaging technique that enables visualization of larger muscles; however, it is not available in all ultrasound systems. Using an acoustic standoff pad that is compatible with any ultrasound system may be an alternative method to increase FOV, but it has not been used to evaluate limb muscles. The purpose of this study was to evaluate the reliability and feasibility of using pUSI and an acoustic standoff pad to measure the limb muscle cross-sectional area (mCSA). Method: A cross-sectional study was conducted. Using pUSI and an acoustic standoff pad, we obtained B-mode ultrasound images of the rectus femoris, biceps brachii, and lateral gastrocnemius muscles of 26 healthy participants on two occasions 7–10 days apart. The agreement between the two methods was determined using intra-class correlation coefficients (ICCs) and Bland–Altman plots. Test–retest reliability was assessed using ICCs and standard error of measurement (SEM). The feasibility of acquiring and analyzing the images was evaluated using a Likert scale. Results: The acoustic standoff pad and pUSI demonstrated strong agreement (ICC[3,3] > 0.85); however, the mCSAs were different (p < 0.05). Test–retest reliability for each technique was high for all muscles (ICC[3,3] > 0.85; SEM = 0.6–1.5 cm2). Image acquisition was highly feasible, but there were some limitations in analyzing the images. Conclusions: pUSI and an acoustic standoff pad are two reliable techniques for measuring mCSA, but the measurements are not directly comparable. Future studies should evaluate the accuracy of the acoustic standoff pad compared with gold-standard methods.
Key Words: quadriceps muscle, reproducibility of results, skeletal muscle, ultrasonography
Abstract
Objectif : l’imagerie panoramique (IP) est une technique à champ étendu (CÉ) qui permet de visualiser les grands muscles, mais qui n’est pas offerte dans tous les systèmes d’échographie. Un panneau acoustique autonome compatible avec les autres systémes d’échographie peut être utilisé pour accroître l’étendue du champ, mais il n’a pas été utilisé pour évaluer les muscles des membres. La présente étude visait à évaluer la fiabilité de l’IP et la faisabilité de l’utiliser comme panneau acoustique autonome pour mesurer la surface transversale du muscle (STM) du membre. Méthodologie : les chercheurs ont réalisé une étude transversale. Ils ont obtenu des images d’échographie en mode B du muscle droit antérieur de la cuisse, du biceps brachial et des muscles gastrocnémiens latéraux à l’aide de l’IP et d’un panneau acoustique autonome chez 26 sujets en santé à deux reprises (à sept à dix jours d’écart). Ils ont établi la concordance entre les deux méthodes à l’aide des coefficients de corrélation intraclasse (CCI) et des courbes de Bland-Altman. Ils ont évalué la fiabilité test-retest à l’aide des CCI et de l’écart-type de mesure standard (ÉTM). Ils ont évalué la faisabilité d’acquérir et d’analyser les images à l’aide de l’échelle de Likert. Résultats : Le panneau acoustique autonome et l’IP présentaient une forte concordance (CCI[3,3] > 0,85), mais les STM étaient différentes (p < 0,05). La fiabilité test-retest de chaque technique était élevée pour tous les muscles (CCI[3,3] > 0,85; ÉTM = 0,6 à 1,5 cm2). Il était hautement faisable d’acquérir les images, mais il y avait certaines limites à les analyser. Conclusions : Un IP et un panneau acoustique autonome sont deux techniques fiables pour mesurer la STM, mais les mesures ne sont pas directement comparables. De futures études devront évaluer la précision du panneau acoustique autonome par rapport aux méthodes de référence.
Mots-clés : : échographie, muscle squelettique, quadriceps, reproductibilité
Clinicians and researchers use musculoskeletal ultrasound imaging (USI) to evaluate muscle size (cross-sectional area, layer thickness), muscle quality (echogenicity), and architecture (e.g., fiber length and pennation angle), as well as to detect changes in muscle size in response to training, disuse, or disease.1–6 The muscle cross-sectional area (mCSA) is of particular interest to both clinicians and researchers because it can be used as an objective indicator of a muscle’s force-generating capacity.7
USI is a valid and reliable measure of mCSA in multiple muscle groups, such as rectus femoris (RF), vastus lateralis, and biceps brachii (BB).1,2,8 Compared with MRI and computed tomography (CT), which are considered gold-standard tools for evaluating muscle size, USI is a cost-effective and accessible tool for clinicians and researchers alike to obtain accurate and reliable measures of mCSA.2 However, USI has a relatively small field of view (FOV) compared with MRI and CT, which limits its application in measuring larger muscle groups.5,9 Thus, several techniques have been introduced to increase the FOV for USI.
Panoramic ultrasound imaging (pUSI) is a technique that extends the FOV by using built-in software to merge successive images taken as the ultrasound transducer traverses the surface of the body.5 Accordingly, pUSI can be used to image larger anatomical structures than can be measured using conventional USI.5 This technique has been found to have good to excellent test–retest reliability for mCSA (intra-class coefficient [ICC] > 0.95), and it has been validated against MRI for appendicular muscles such as quadriceps, gastrocnemius, and BB.10–12 However, pUSI is not readily available or compatible with all ultrasound systems, a fact that limits its wider application in research and clinical settings.9
An alternative to pUSI that can be used with any ultrasound system is an acoustic standoff pad: a gel-filled pad placed under the ultrasound transducer to increase the distance between the transducer and the skin, thereby increasing its FOV.9 The acoustic standoff pad has been used to image superficial structures such as facial musculature and peripheral nerves.13,14 In a previous study conducted in our laboratory, we used an acoustic standoff pad to measure inter-rectus distance and thereby evaluate diastasis recti in postpartum women.9 We found that the acoustic standoff pad was valid and reliable, with a small measurement error compared with pUSI and conventional USI.9 The reliability of using an acoustic standoff pad to image limb muscles has not been investigated.
Therefore, the objectives of this study were to (1) compare the mCSA measurements of the RF, BB, and lateral gastrocnemius (LG) obtained by pUSI and acoustic standoff pad imaging; (2) investigate the test–retest reliability of pUSI and the acoustic standoff pad in measuring the mCSA of these muscles; and (3) determine the feasibility of acquiring and analyzing images for each imaging technique.
Methods
Study design
A cross-sectional, observational, single-group study design was used. Participants were recruited from the university campus using convenience sampling: we put up posters, sent out messages on email lists, and put out information by word of mouth. Healthy men and women aged 18–45 years were included to avoid age-related muscle changes that may influence image quality.15 Exclusion criteria were the presence of connective tissue disorders, neuromuscular conditions, cardiorespiratory or metabolic disease, current cancer or chemotherapy, statins or oral corticosteroid medication use, current musculoskeletal injuries of the muscles of interest, and inability to lie in supine for 30 minutes or prone for 15 minutes.
This study was approved by the University of Toronto Health Science Research Ethics Board (REB No. 32300). All participants provided written and informed consent before the study began, and the rights of the participants were protected.
Sample size
Sample size estimates were based on those of a similar study.9 For test–retest reliability (Objective 2), a sample size of 21 participants was determined using the Streiner and Norman equation.16 Our underlying assumptions included an expected ICC of 0.80; 95% CI: 0.6, 1.0; and Type I and II error rates of 0.05 and 0.20, respectively.5 To compare the two imaging techniques (Objective 1), a sample size of 17 participants was required, with underlying assumptions of an expected correlation of r ≥ 0.60 and Type I and II error rates of 0.05 and 0.20, respectively.9,10 For this, we used G*Power, Version 3.1.9.2, a free tool for calculating statistical power.17 The sample size was inflated to 26 participants to address both objectives and account for participant attrition.
Study procedures
Participants attended two sessions held 7–10 days apart. They were instructed to avoid any vigorous exercise for 24 hours beforehand to minimize the amount of water accumulating in the muscle. All measurements were performed on the dominant limb – reported by the participant as the limb most commonly used for writing (hand) and kicking (foot). We measured the participants’ height and weight on a standing scale before performing the USI. Physical activity level was self-reported using the Rapid Assessment of Physical Activity questionnaire.18
Ultrasound imaging
The USI of the RF, BB, and LG was conducted using a portable USI system (General Electric Logiq e; GE Medical Systems, Milwaukee, WI) with a two-dimensional linear transducer (8–13 MHz; footprint 12.7 mm × 47.1 mm) in brightness mode (B-mode). The system had built-in software for performing pUSI. The acoustic standoff pad imaging was performed using a gel-filled pad, 5 centimetres long × 10 centimetres wide × 4 centimetres deep (ATS Laboratories, Bridgeport, CT) placed between the skin and the ultrasound transducer. Ultrasound gel was smoothed on the top and bottom of the pad. The ultrasound settings for pUSI were frequency = 8–13 megahertz, depth = 3.5–13 centimetres, and gain = 46–98 decibels; for the acoustic standoff pad, frequency = 8–13 megahertz; depth = 4–13 centimetres, and gain = 46–98 decibels. Representative images for each muscle and technique are shown in Figure 1.
Figure 1.
Ultrasound images of the limb muscles using panoramic imaging and an acoustic standoff pad to obtain the muscle cross-sectional area: (a) RF, (b) BB, and (c) LG.
RF = rectus femoris; BB = biceps brachii; LG = lateral gastrocnemius.
The positioning of the participants and the landmarks for acquiring the ultrasound images are described in Table 1. The participants rested supine or prone for 10 minutes before the USI began to allow any fluid shifts to stabilize.21 For both imaging techniques, the investigator maintained a perpendicular alignment of the transducer to the skin surface and maintained minimal pressure to prevent tissue deformation. Imaging using the acoustic standoff pad and pUSI was performed in a randomized order, and three images of each muscle were obtained using each technique.
Table 1.
Positioning of Participants and Location of Ultrasound Images Acquisition for Each Muscle of Interest
| Muscle | Position | Anatomical location of image acquisition (study) |
|---|---|---|
| Rectus femoris | Supine, with hip and knee extended and leg neutrally rotated | 40% of the distance from the center point of the patella to the medial aspect of the anterior superior iliac spine10 |
| Biceps brachii | Supine, with shoulder at 45° of abduction, elbow extended, and upper limb supinated | 40% of the distance from the lateral epicondyle to the lateral aspect of the acromion process and midway between the humeral epicondyles19 |
| Lateral gastrocnemius | Prone, with ankles freely resting off the plinth | 40% of the distance from the articular cleft between the femoral and tibial condyles to the distal aspect of the lateral malleolus and 25% of the distance from the lateral femoral epicondyle to medial femoral epicondyle20 |
Before starting the data collection phase of the study, two investigators (CD, AH) were trained by a physical therapist with 5 years of experience in musculoskeletal USI (NK), after which they practised for 10 hours using both pUSI and an acoustic standoff pad. Each investigator acquired the images for half of the study’s participants.
Image analysis
Images were exported in DICOM format and analyzed using the open-source image analysis programme OsiriX, version 10.0 (Pixmeo SARL, Bernex, Switzerland) by two investigators (AC, KN). Each analyst measured only the images captured by one imager (e.g., AC analyzed all the images acquired by AH, and KN analyzed all the images acquired by CD). The muscles were manually outlined on the fascial border. We used the average mCSA from the three images obtained for each muscle using each imaging technique for statistical analysis.
Feasibility of acquiring and analyzing the images
The feasibility of acquiring and analyzing the images was rated using a 5-point Likert scale. The investigators who captured the ultrasound images rated the ease of image acquisition on a scale ranging from 1 = unable to capture images/see borders clearly to 5 = very easy to obtain images/all muscle borders were easily visible. The time (in minutes) required to image each muscle using each technique was recorded. The image analysts rated the ease of outlining the muscle on a scale ranging from 1 = unable to trace any border easily to 5 = able to trace 100% of borders easily. The analysts also recorded the time (in minutes) required to outline three images for each muscle using each imaging technique. Subjective comments on acquiring and analyzing the images were also recorded.
The feasibility criteria were set a priori. Acquiring the images was considered feasible if the following criteria were met 80% of the time: (1) acquisition was completed in 10 minutes or less for each muscle, (2) visualization of the muscle borders was rated 3 or more out of 5, and (3) ease of obtaining the images was rated 3 or more out of 5. Analyzing the images was considered feasible if the following criteria were met 80% of the time: (1) it took 5 minutes or less to outline all three images of one muscle and (2) ease of tracing the muscle borders was rated 3 or more out of 5.
Statistical analysis
Statistical analysis was performed using IBM SPSS Statistics, version 22 (IBM Corporation, Armonk, NY). The demographic variables and mCSAs were described using means (SDs). The variables were tested for normality using the Shapiro–Wilk test, and significance was set at α = 0.05. The mCSAs from the two imaging techniques were compared using paired t-tests (RF and LG muscles) or the Wilcoxon rank-sum test (BB), depending on normality, using values from the first testing session. ICCs with 95% CIs and the standard error of measurement (SEM) were calculated to assess the agreement between pUSI and the acoustic standoff pad; an ICC > 0.90 was considered excellent agreement.22,23 Bland–Altman plots were used to visualize the agreement between the two imaging techniques.24 The test–retest reliability for each imaging technique was determined using ICCs (Model 3) with fixed effects (image acquisition technique) and 95% CIs.22 The error associated within each imaging technique between days was also evaluated using the coefficient of variation (% CV) and SEM.23,25 The feasibility data were described using the mean (SD) of the time to acquire and analyze an image and the median and range of the Likert scale responses.
Results
A total of 26 healthy adults were screened for eligibility consecutively between January and May 2016, and all participants were deemed eligible and completed both study sessions. The participants’ characteristics are given in Table 2, and their level of physical activity is given in Table 3. The mCSAs obtained using each imaging technique are shown in Table 4. One participant’s LG data from Session 2 was removed from analysis as a result of an error in capturing the image.
Table 2.
Characteristics of Study Participants (N = 26)
| Variable | Mean (SD)* |
|---|---|
| Age, y | 25.0 (3.0) |
| Sex, no. (%) | |
| Male | 13 (50.0) |
| Female | 13 (50.0) |
| Height, cm | 170.2 (8.7) |
| Weight, kg | 70.7 (13.9) |
| BMI, kg/m2 | 24.5 (5.1) |
Unless otherwise indicated.
Table 3.
Physical Activity Level of Study Participants Assessed with the RAPA (N = 26)
| Type of activity | No. (%) |
|---|---|
| Aerobic activity | |
| Sedentary | 1 (3.8) |
| Underactive regular – light activities | 3 (11.5) |
| Underactive regular | 9 (34.6) |
| Active | 13 (50.0) |
| Strength and flexibility activities | |
| None | 5 (19.2) |
| Strength exercises only | 10 (38.5) |
| Flexibility exercises only | 2 (7.7) |
| Both strength and flexibility exercises | 9 (34.6) |
RAPA = rapid assessment of physical activity.
Table 4.
Muscle Cross-Sectional Areas (in cm2) Obtained by Each Imaging Technique for Each Session (N = 26)
| Muscle | Session 1, mean (SD) |
Session 2, mean (SD), p-value |
||
|---|---|---|---|---|
| pUSI | Acoustic standoff pad | pUSI | Acoustic standoff pad | |
| Rectus femoris | 5.7 (1.9) | 6.3 (2.1) | 6.1 (2.3), 0.10* | 6.5 (2.3), 0.05 |
| Biceps brachii | 8.2 (3.0) | 9.0 (3.3) | 8.2 (3.0), 0.42* | 8.8 (3.3), 0.20 |
| Lateral gastrocnemius† | 4.0 (2.2) | 3.5 (1.8) | 3.8 (2.1), 0.33* | 3.8 (2.2), 0.16 |
p-values represent a comparison between Day 1 and Day 2 with the same imaging method using paired t-tests for the rectus femoris and lateral gastrocnemius and the Wilcoxon sign-rank test for the biceps brachii.
N = 25 (1 participant excluded because of an error in capturing the image in Session 2). pUSI = panoramic ultrasound imaging.
Comparing the mCSAs obtained using pUSI and the acoustic standoff pad
High agreement was found between the mCSAs obtained using the two measurement techniques: for RF, ICC(3,3) = 0.960, 95% CI: 0.913, 0.982; for BB, ICC(3,3) = 0.951, 95% CI: 0.893, 0.978; and for LG, ICC(3,3) = 0.882, 95% CI: 0.750, 0.946. The SEM between the techniques was 0.6 cm2 for RF, 0.6 cm2 for BB, and 1.1 cm2 for LG.
However, the mCSAs of RF and BB obtained using pUSI were smaller than those obtained using the acoustic standoff pad: the mean differences were −0.6 cm (95% CI: −0.8, −0.3, p < 0.001) and −0.8 cm (95% CI: −1.2, −0.4, p = 0.001), respectively. The mean mCSA of LG was larger using pUSI than using the acoustic standoff pad: the mean difference was 0.5 cm (95% CI: 0.1, 0.9, p = 0.010). The bias between the two techniques is depicted in the Bland–Altman plots (see Figure 2).
Figure 2.
Bland–Altman plots for pUSI and the acoustic standoff pad for measuring the mCSA of (a) rectus femoris, (b) biceps brachii, and (c) lateral gastrocnemius.
Note: In each plot, the solid line represents the mean difference between the two techniques, and the dotted lines represent the mean limits of agreement (SD 1.96).
pUSI = panoramic ultrasound imaging; mCSA = muscle cross-sectional area.
Test–retest reliability of the mCSA using pUSI and the acoustic standoff pad
We found no significant differences between the mCSAs obtained using the same technique in the two testing sessions (all ps > 0.05; see Table 4).
The ICCs(3,3), 95% CI, and corresponding SEMs for the mCSA using pUSI were as follows: for RF, ICC(3,3) = 0.885, 95% CI: 0.760, 0.947, % CV = 6.9%, SEM = 1.0 cm2; for BB, ICC(3,3) = 0.981, 95% CI: 0.959, 0.992, % CV = 4.2%, SEM = 0.6 cm2; and for LG, ICC(3,3) = 0.868, 95% CI: 0.723, 0.940, % CV = 18.3%, SEM = 1.1 cm2. Slightly higher reliability was found for the RF muscle using the acoustic standoff pad: ICC(3,3) = 0.960, 95% CI: 0.914, 0.982, % CV = 6.5%, SEM = 0.6 cm2. The reliability was slightly lower for BB using the acoustic standoff pad: ICC(3,3) = 0.970, 95% CI: 0.933, 0.986, % CV = 4.6%, SEM = 0.8 cm2; and LG: ICC(3,3) = 0.858, 95% CI: 0.705, 0.935, % CV = 17.6%, SEM = 1.5 cm2.
Feasibility of acquiring and analyzing the images
Both ultrasound techniques met the predetermined feasibility criteria for acquiring the images (see Table 5), but not all criteria for analyzing them were met consistently. The time required to analyze the images for RF and LG was less than 5 minutes; however, for BB only 52% of the images obtained using pUSI and 58% of those obtained using the acoustic standoff pad, were outlined in less than 5 minutes. For ease of tracing muscle borders, only 77% of the LG images acquired with the acoustic standoff pad were rated 3 or more out of 5. The ratings for the images of other muscles were 3 or more in 80% of the images or higher, therefore meeting the a priori criteria for feasibility (Table 5).
Table 5.
Feasibility of Using pUSI and the Acoustic Standoff Pad to Measure the mCSA (N = 26)
| Feasibility | pUSI |
Acoustic standoff pad |
||||
|---|---|---|---|---|---|---|
| RF | BB | LG* | RF | BB | LG* | |
| Image acquisition | ||||||
| Time, min, mean (SD) | 3 (2) | 5 (3) | 3 (2) | 3 (2) | 4 (2) | 4 (3) |
| Ease of visualizing muscle borders | 5 (2–5) | 4 (2–5) | 5 (3–5) | 4 (2–5) | 4 (2–5) | 4 (2–5) |
| Ease of obtaining images | 5 (3–5) | 4 (2–5) | 5 (3–5) | 5 (3–5) | 4 (2–5) | 4 (2–5) |
| Image analysis | ||||||
| Time, min, mean (SD) | 4 (2) | 5 (2) | 4 (2) | 4 (2) | 5 (2) | 4 (2) |
| Ease of tracing muscle borders | 4 (3–5) | 4 (2–5) | 4 (2–5) | 4 (2–5) | 3 (2–5) | 4 (1–5) |
Note: Unless otherwise indicated, results are presented as median (range, min–max).
n = 25.
pUSI = panoramic ultrasound imaging; mCSA = muscle cross-sectional area; RF = rectus femoris; BB = biceps brachii; LG = lateral gastrocnemius.
Discussion
This is the first study to report on the use of an acoustic standoff pad in measuring the mCSA of limb muscles. We found high test–retest reliability for both pUSI and the acoustic standoff pad in measuring the mCSA of the upper and lower limb muscles. Despite demonstrating good agreement between the image acquisition methods, the mCSAs obtained using each technique were different: the acoustic standoff pad had higher values for BB and RF, and pUSI had higher values for LG. We also examined the feasibility of the two methods and found that acquiring the images was feasible with both. However, there were challenges in analyzing the images (increased time and the ease of tracing muscle borders), particularly for images obtained with the acoustic standoff pad. Future studies may use either of these methods to capture a larger FOV, which is needed to visualize the cross-sectional area (CSA) of limb muscles; however, the measurements obtained from the two techniques should not be directly compared or used interchangeably.
We found good to excellent test–retest reliability for both imaging techniques; the two muscles with larger mCSAs (BB and RF) were associated with higher reliability metrics. Our results suggest that the size of a muscle may influence the ability to obtain consistent mCSA measurements. Noorkoiv and colleagues10 reported similar results; the reliability of the mCSA measurements they obtained using pUSI from the quadriceps muscle was poorest at the most distal anatomical site compared with proximal sites where the muscle was larger. We took images of the LG muscle closer to the musculotendinous junction, where the mCSA is smaller, and measurements of this muscle had the lowest reliability. This location was chosen so that the mCSA would fit within the boundaries of the acoustic standoff pad. On the basis of these findings, the location of a muscle’s maximum CSA should be considered when selecting the imaging landmarks to obtain reliable results, but the acoustic standoff pad may still have some limitations because of its size.
The reliability of the mCSA measurements acquired using either imaging technique may also be influenced by the curvature of the limb, which affects the contact between the acoustic standoff pad and the surface of the limb, as well as the ability of the imager to maintain perpendicular alignment of the transducer during pUSI. In our previous study of the abdominal muscles using pUSI and the acoustic standoff pad, we had greater difficulty obtaining images in participants with a curved abdomen.9 To address this issue, Noorkoiv and colleagues10 used a custom-built rig, which was placed over the thigh to help maintain perpendicular alignment of the transducer during quadriceps imaging. The use of a transducer guide may improve the reliability of the mCSA measurements obtained using pUSI.
The reliability of the mCSA measurements obtained using the acoustic standoff pad may also be affected by the decrease in image quality resulting from signal attenuation from the gel-filled pad itself.9 We noted that it was less feasible to analyze images obtained using the acoustic standoff pad because of its poorer visualization of the muscle borders. Thus, it is important for investigators to use visual inspection while acquiring images to ensure that they are distorted as little as possible; this will optimize the subsequent measurements of the mCSA.
Our study demonstrated that the measurement error between days ranged from 0.6 to 1.5 cm2; this appears to be a reasonable degree of error to still detect increases in the mCSA that occur with strength training or decreases that occur with disuse. A study investigating the effects of strength training in young men (N = 21) reported an increase of approximately 4 cm2 in the vastus lateralis mCSA using both USI and MRI.5 Moreover, a study investigating muscle atrophy in critically ill patients (N = 62) found that the RF mCSA measured using conventional USI decreased by approximately 0.9 cm2 over a 10-day stay in an intensive care unit.6
Our results suggest that the measurement error associated with pUSI and the acoustic standoff pad using our protocol would be sensitive enough to detect these clinically important changes in muscle size. For pre–post comparisons, however, it is important to note that the values obtained from pUSI and the acoustic standoff pad cannot be directly compared or used interchangeably because the measurements obtained from these methods differ. We found that acoustic standoff pad measurements were larger than those obtained from pUSI for the RF and BB mCSA but smaller for the LG mCSA. The results of the LG mCSA may also have been affected by its lower reliability and larger measurement error, as previously discussed.
Limitations
This study had potential limitations. First, we had novice ultrasound imagers (physical therapy students) conduct the measurements, which may be considered a limitation. However, the high reliability we found for both imaging techniques suggests that with some hands-on training, these techniques are straightforward to learn and may be feasible for a wide range of researchers and clinicians to conduct. Second, we did not have a true gold standard such as CT or MRI with which to compare the results obtained from the two extended FOV techniques; therefore, we cannot conclude that one technique is more accurate than the other for measuring mCSA.
Conclusion
The results of this study demonstrate that both the acoustic standoff pad and pUSI are reliable imaging techniques for extending the FOV when using USI to measure limb mCSA in healthy individuals. However, these techniques do not provide comparable results and should not be used interchangeably. The acoustic standoff pad is advantageous because it can be used with any ultrasound system, but further research comparing it with gold-standard tools for measuring mCSA such as MRI and CT is needed to reliably determine its accuracy.
Key Messages
What is already known on this topic
The acoustic standoff pad is a gel-filled pad that is placed under the ultrasound transducer to increase the distance between the transducer and the skin, thereby increasing its field of view. This method has been shown to be reliable in imaging the rectus abdominus muscle to measure inter-rectus distance and evaluate diastasis recti in postpartum women.
What this study adds
The results of this study show that the acoustic standoff pad is a feasible method for imaging the limb muscles of the upper and lower extremities to obtain the muscle cross-sectional area (mCSA) in healthy people. The imaging and analysis protocol for obtaining the mCSA using the acoustic standoff pad also demonstrated high reliability with novice imagers. The measurements obtained from the acoustic standoff pad demonstrate good agreement with panoramic ultrasound imaging, but discrepancies between the measures obtained from these methods prevent the use of these techniques interchangeably or direct comparisons of the measurements obtained using different imaging methods.
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