Assessment of peripheral muscle thickness and architecture in healthy volunteers using hand-held ultrasound devices; a comparison study with standard ultrasound

Pocket-sized ultrasound devices are becoming increasingly common in the clinical setting. The devices are being used in the qualitative assessment of patients, allowing rapid assessment and diagnosis of abdominal [1], aortic [2] and gynaecological [3] pathologies. They have also been shown to be valuable in the assessment of intra-abdominal free fluid in patients with traumatic injuries [4]. Quantitatively, pocket-sized devices have been validated for obstetric measurements [5] and in the estimation of optic nerve sheath diameter [6]. These studies have used the pocket sized Vscan device (GE Healthcare, United States), although other pocket sized devices are described in the literature, such as the Acuson P10 system (Siemens, United States) for use in the assessment of ascites [7].

The use of ultrasound to assess muscle thickness and architecture is well documented across a number of patient populations. For example, ultrasound is used in the critically ill to assess changes in muscle thickness as a result of atrophy [8], and loss of quadriceps muscle has been of interest in patients with respiratory [9], endocrine [10] and renal [11] diseases. In healthy volunteer studies, assessment of muscle architecture with ultrasound has been used to link muscle structure with function [12]. Furthermore, ultrasound systems have been used in sports science to detect fat content within muscle [13], and to estimate depth of subcutaneous fat layers [14].

A recent systematic review concluded that pocket-sized devices produce images that can be used to answer distinct clinical questions, and provide good agreement with high-end devices [15]. The ability to use a pocket-sized device to track changes in muscle size in many patient populations could make muscle assessment easier and cheaper compared to a larger, more expensive device. Similarly, accurate measurement with a pocket-sized device may be of benefit in studies of healthy volunteers. We aim to assess whether a pocket-sized device will accurately measure muscle thickness and architecture in the anterior and lateral thigh, in healthy male volunteers, compared to a standard ultrasound machine.

Ethical approval for this study was granted from the University of Liverpool’s central ethics committee. The study was conducted in the gait laboratory within the University’s Institute of Aging and Chronic Disease over a 4 month period, from September to December 2018. This study was performed when participants attended to take part in other studies that involved ultrasound measurements of the thigh muscles. Participants were recruited for these studies through e-mail communications and advertising posters positioned across university notice boards. All recruited participants signed a consent form, having read a participant information sheet and following a verbal briefing from the authors.

This study demonstrates that a pocket-sized ultrasound system can be used in healthy volunteers to give accurate assessment of muscle thickness in both the vastus lateralis and the anterior compartment of the quadriceps, compared to a standard ultrasound system. Further, the pocket-sized system can image muscle fascicles to a satisfactory resolution in comparison to a standard system. When fascicle length is determined using trigonometric methods, the resulting length is also comparable to the estimations of fascicle length obtained from the standard device. The lack of proportional bias indicates that this accuracy is maintained across the range of obtained measurements. To our knowledge, this is the first time a pocket-sized device has been used to assess limb muscle architecture, although a different smartphone based pocket-sized ultrasound device has been used successfully in the assessment of the hyomental muscle [24]. The Vscan device used in our study has also been assessed as suitable in the diagnosis of musculoskeletal pathology of the shoulder [25].

The Vscan has been has used in a previous study to assess its accuracy in the measurement of diaphragm thickness at both tidal and maximal end points of respiration [26]. The authors found that measurements of muscle thickness at both inspiration and expiration gave ICC values of greater than 0.9, at both tidal and maximal volumes. They did however find that the system could not accurately assess maximal diaphragm movement, although this is ordinarily measured in M-mode [27], which the Vscan does not possess.

The techniques for assessment of peripheral muscle size and architecture using ultrasound are well established in both healthy volunteers [28] and certain patient populations, with muscle parameters relating to functional outcomes [29]. Muscle architecture specifically describes the arrangement of fascicles within a muscle, with the angle that a fascicle inserts into the deep aponeurosis being known as the pennation angle; this angle is important as it is positively related to force generation, with larger angles being able to pack more muscle into a particular volume [30]. The length of the fascicle spanning between the two aponeuroses indicates the maximal shortening velocity of a muscle [31]. It can be measured directly [32] or estimated using trigonometry if the muscle thickness and pennation angle are known [33]. Rectus femoris cross-sectional area and thickness, vastus intermedius thickness and vastus lateralis thickness have all been shown to significantly correlate with functional measures in patients with critical illness [34], and in healthy volunteers [35], in particular the thickness of vastus intermedius [36].

Our study is limited by the Vscan probe being unable to image the entire cross sectional area of the rectus femoris, possibly due the size of the probe. Although measurement of muscle thickness of the anterior compartment of the thigh is a well-established technique, at present we cannot extrapolate these results to patient populations. For example, in critically ill patients, comparison between thickness and cross sectional area has shown that thickness underestimates loss of muscle by around 8% [37]. Another study has shown that both quadriceps thickness and rectus femoris cross-sectional area decrease significantly in sepsis, but thickness decreases to a lesser degree and does not correlate with volitional measures of strength [38]. Furthermore, the effect of tissue oedema and changes in muscle echogenicity could not be examined in this study of volunteers, and is another limitation in trying to apply these results to any patient population.

Although the absolute agreement across the 6 variables was high, there were reduced ICC values for the measurement of the rectus femoris and vastus intermedius muscles. A possible explanation could be the position and appearance of the hyperechoic interface between the two muscles: themuscular fascia of the rectus femoris is superficial and easily identifiable, as is the echobright tip of the femur, making measurement of the overall anterior compartment of the thigh straightforward [23]. In comparison, the hyperechoic interface between the two muscles that make up the anterior compartment may be harder to clearly delineate using the pre-programmed resolution settings of the Vscan.

Further work in this field should concentrate on the potential benefits of using a pocket-sized devices over a standard ultrasound device. For example, whether a pocket-sized device confers an ergonomic benefit to users, or if the length of time spent acquiring images is similar between the two methods; a simulated study of central venous cannulation found that the time taken to achieve image acquisition and puncture of the vessel was similar between pocket-sized and standard devices [39]. In order to establish a pocket-sized device as a bedside measuring tool, measurements using the device’s own caliper function need to be compared against those obtained using formal computer software, and the use of other commercially available pocket-sized devices could be similarly evaluated for use in assessment of muscle thickness.

Finally, as this was a study performed in healthy volunteers, the device needs to be compared to a standard ultrasound device in patients at risk of muscle wasting, such as those with chronic or critical illness, to assess the effect of oedema and pre-existing sarcopenia on the accuracy of image acquisition.

In healthy male volunteers, a pocket-sized ultrasound device provides measurements that are comparable to a standard device in the assessment of muscle layer thickness and fascicle architecture. Further work is required to determine if such devices perform as well in patient populations.