3D Ultrasound Imaging: Fast and Cost-effective Morphometry of Musculoskeletal Tissue

The overall goal of this methodology is to perform a 3D in vivo morphological assessment based on an ultrasound analysis of human muscle tissue. The method is a promising technique for answering key questions in healthcare and sports about diagnosis and followup of muscle function after treatment and training. The advantages of this technique is that measuring the muscles by 3D ultrasound is a fast and cost-effective alternative to magnetic resonance imaging.

This 3D ultrasound technique provides a non-invasive method for determination of muscle volume, fascicle length in a physiological cross-sectional area of skeletal muscles in vivo. Since 1980, we've been active studying the form function relation of muscles, first in human cadaver muscles and making mathematical models of those muscles, and then in animal experiments. With the improved ultrasound technique, we will now be able to study human muscles in vivo.

To assess the gastrocnemius medialis muscle, or GM, first have the subject lie prone on an examination table with both feet over the edge of the table. Place a support under the tibia to align the lower leg horizontally and fix the thigh to the examination table with padded lashing strips to prevent knee extension during the experiment. Fit the foot of the leg to be scanned into a custom-made foot plate and connect a custom-made torque wrench with a goniometer attached to the foot plate.

Find the foot plate angle corresponding to the appropriate externally-applied torque and use an extendable rod connected to the table to fix the foot plate in the corresponding appropriate orientation. Then, identify the most prominent dorsal aspects of the medial and lateral femur epicondyles and the centers of the malleoli of the tibia and fibula by palpation and mark their locations with a surgical skin marker. Identify the most superficial points of the medial and lateral femur condyles using ultrasound and mark using a surgical skin marker.

Instructing the subject not to move during the examination, apply ample ultrasound gel on the region of interest and open the frame grabber software on the measurement computer. It is essential for the patient to remain still during the measurements to prevent movement artifacts in the 3D ultrasound image. Click Capture to begin the ultrasound image acquisition and subsequently press the button on the synchronization device to initiate and activate the motion capture data acquisition.

The synchronization device, located close to the ultrasound probe, will be activated creating a distinct artifact in the ultrasound image upon motion capture initiation. Exerting minimal pressure while maintaining the image quality, place the ultrasound probe proximal to the femur condyles on the medial aspect of the thigh and sweep the probe at a constant speed in the proximal distal direction along the medial border of the GM.Using the gel from the previous sweep, add additional 0.5-centimeter overlapping sweeps until the entire region of interest has been scanned and the medial border of the muscle is completely imaged. It is important to apply ample ultrasound gel to prevent tissue deformation as a result of excessively applied probe pressure while obtaining ultrasound images.

When all of the images have been acquired, use a custom script to reconstruct a single 3D ultrasound voxel array from an individual sweep over a specific region of interest by bin filling and in-painting of the array. Then, use the same voxel array coordinate system to reconstruct all of the individual sweeps covering one larger region of interest to merge multiple sweeps. To measure the muscle volume, first load the 3D ultrasound image of interest in the medical interaction tool kit.

Set the slicing to Coupled crosshair rotation and align the axis with muscle or bony structures to precisely retrieve the coordinates of the origin, insertion and distal end of the muscle belly. Open the segmentation tool and create a new segmentation outlining the muscle boundaries identified in a cross-section halfway along the muscle belly. Press A on the keyboard to add a manual segmentation clicking the left mouse button and moving the cursor along the muscle boundaries.

To segment a new cross-section, press the A or S button according to the last selected mode to move the crosshair to other cross-sections along the muscle belly and trace the next muscle boundary as just demonstrated. When all of the segments have been selected, set interpolation to enable and review the proposed segmentations of the muscle boundaries in all of the cross-sections along the muscle belly length. When all muscle boundaries have been correctly segmented, press Confirm for all slices and select the plane in which the segmentations were made.

Save the binary volume as a Nearly Raw Raster Data file and use a custom script to calculate the labeled volume size. Then, use a custom script to locate the orientation of the mid-longitudinal fascicle plane of the muscle belly containing the full length of fascicles. From the mid-longitudinal plane, measure the fascicle length at a predefined standardized position between the origin and the distal end of the muscle belly and segment the muscle boundaries.

Place a line halfway through the region of interest and rotate the line until it matches the direction of the underlying fascicles. The intersection of the line with the muscle boundaries will represent the estimate of the fascicle length. This image of a male human cadaver illustrates an atrophied state of the quadriceps muscle halfway along the thigh at death making identification of the boundaries of individual heads of the quadriceps muscle difficult compared those identified in the quadriceps muscle of a sedentary 30-year-old male or a 30-year-old male athlete.

The validities of the fascicle length as well as the muscle volume were confirmed with significant and high correlations by the 3D ultrasound method compared to the physical dissection data for the same tissues with a high interrater reliability for the 3D-ultrasound-method-derived measurements. Confirming that the 3D ultrasound approach is a valid and reliable tool for the volume and fascicle length assessment of human vastus lateralis and gastrocnemius medialis muscles. Following this procedure, muscle morphology can be quantified and related to joint range of motion and physical performance.

This technique is now ready to be implemented as a routine evaluation of muscle function in patients with neuromuscular diseases or those who are suffering from sarcopenia. This 3D ultrasound imaging technique could also potentially be used to measure the morphology of other tissues such as bones, tendons and blood vessels. Development of this technique has paved the way for researchers in the neuromuscular field to investigate how muscle morphology limits mobility.

And in exercise physiology, to assess morphological determinants of sprint and endurance performance of athletes.