Obtaining Quality Extended Field-of-View Ultrasound Images of Skeletal Muscle to Measure Muscle Fascicle Length

Being able to obtain quality extended field of view ultrasound images can increase the number of muscles for which we have in vivo fascicle length data. The main advantage of extended field of view ultrasound is that it can enable direct measurement of longer curved muscle fascicles which cannot be captured with traditional ultrasound. This method may be applied to the over 90%of upper limb and 85%of lower limb muscles with optimal fascicle lengths longer than the field of view of common ultrasound probes.

Successful implementation of this method requires a good understanding of musculoskeletal anatomy. Practice obtaining images and practice assessing the quality of images. Begin by taking time to become familiar with the systems method for obtaining EFOV-US and with the anatomy of the muscles of interest.

After explaining the study protocol, seat the participant in an adjustable chair that can be locked in place, and adjust the chair so that the participant is as comfortable as possible while still providing access to the muscle of interest. Place the joint that the muscle of interest spans in a posture that can be controlled and repeated and use clinical guidance to locate anatomical landmarks. Using the ISB standards for defining the joint coordinate system, mark the anatomical landmarks with a skin-safe marker.

Align the center of a handheld goniometer with the axis of rotation of the joint and align the arms of the goniometer with the joint's segments, then, measure the joint angle. Then use cloth straps to secure the limb to minimize movement during the imaging protocol. For image acquisition, plug in and turn on the ultrasound system and ensure that the exam is set to musculoskeletal, the transducer is set to the transducer in use, and the transmit frequency is set between five to 17 megahertz.

In the system's software, set the foot switch to start-stop the imaging. If the foot switch in use has multiple pedals, set additional pedals to freeze or pause and to print or store the image. Next, apply a generous amount of ultrasound gel to the head of the transducer and place the transducer onto the participant's skin in the approximate region of interest.

Move the transducer across the shorter axis plane of the muscle keeping the indicator pointed laterally. Identify the muscle of interest in the short axis plane perpendicular to the muscle fiber direction and move the transducer distally and proximally to acquire a full visualization of the muscle path using the skin-safe markers to mark important anatomical landmarks as they are identified. Once the location of the muscle has been identified and marked, move the ultrasound transducer in the long axis plane with the indicator pointed distally.

When either the distal or proximal end of the muscle has been identified, rotate and tilt the transducer to identify the fascicle plane at that point and mark the skin when the correct transducer position has been established. Once the approximate fascicle plane has been established along the entire desired length to be scanned, practice following the muscle path and set the ultrasound system to the extended field of view mode. Starting at one end of the muscle, click the foot switch to start the image acquisition and slowly and continuously move the ultrasound transducer along the long axis.

When the end of the muscle has been reached, click the foot switch to end image acquisition. To optimize image visibility and clarity, if the image acquisition ends before the desired length of the muscle can be captured, increase the depth of the image. To adjust the focus, place the focus arrow in the lower 1/2 of the image just below the muscle of interest.

Ensure that the gain is balanced through the depth of the image and use the indicator to determine the optimal speed for moving the transducer. Once qualitatively good images have been collected, export the images as uncompressed DICOM images. Adjust the chair so that the participant is comfortable and the tibialis anterior can be accessed with ultrasound.

Then, locate and mark anatomical landmarks and make joint angle measures. After finding the location of the fascicle plane and practicing capturing images, capture an EFOV-US image of the tibialis anterior. When all of the images have been acquired, open a long axis image of interest in an appropriate image analysis software program.

Identify the connective tissues surrounding the muscle as bright hyperechoic lines indicating the muscle boundaries around a dark hypoechoic shape and check that the image does not have excessive non-anatomical bending, gaps in the image, or a jagged, flexible ruler line over the image. If the image is missing one or more of these tissue structures, deem the image qualitatively poor. To quantify the muscle fascicle length, first, open the image of interest in ImageJ and use the straight line tool to draw a 10 millimeter straight line on the ruler on the side of the ultrasound image.

Select analyze and measure to measure the line. If the image properties have been preserved, the length of the straight line should be approximately one centimeter. To measure the fascicle lengths in the image, right click on the straight line tool to select segmented line.

Click one end of the fascicle to be measured and continuously click along the muscle path to ensure that the curvature in the fascicle has been captured. Then, double-click at the end of the fascicle and select analyze and measure to measure the length of the line. In these representative analyses, extended field of view ultrasound was used to obtain images from the long head of the biceps brachii and the tibialis anterior in four healthy volunteers.

Extended field of view ultrasound imaging of these muscles was used to observe important aspects of the tissues such as central tendon and fascicle path. After imaging, qualitatively good images were then analyzed for each muscle in each individual. ImageJ was implemented to measure four fascicles with paths that could be convincingly visualized from their origin to insertion and that were located in different portions of the muscle in each image.

The average fascicle lengths obtained in this study for the biceps brachii and the tibialis anterior were within the range of previously reported fascicle lengths. As it can be difficult to determine which images are appropriate for fascicle evaluation, here, representative, qualitatively bad images are shown with the portions of the images that are specifically bad highlighted. In these images, variations in the muscle quality and landmarks for different individuals can be observed.

When attempting this protocol, take time to understand the validity of the algorithm used by the ultrasound system. Review the anatomy of the muscle of interest and obtain sufficient practice imaging. This protocol is intended to provide pointers and describe the necessary standards.

So extended field of view ultrasound may be applied to muscles beyond the two examples we provide. We expect that this method will be applied to steady muscle in both healthy and impaired populations to better understand healthy muscle function and muscle adaptation following injury or disease.