Motion capture (Mocap) is a technique used in the film industry to digitally track a human actor's movements and precisely transfer those motions to an animated figure. But it has other applications as well. Digitally tracking a person's motions can be used to analyze medical problems such as degenerative disc disease, chronic lower back pain, and scoliosis in adults and children.

A Vicon Motion Systems (Oxford, UK) Mocap system is being used in a research setting to allow physicians to capture information about a person's spine. People can be observed while they walk freely in a manner that mimics real-world movement conditions, enabling research on human motion and the spine, beyond focusing primarily on gait as measured on a treadmill. In the laboratory, it is used to accurately capture a person's motions and transfer the collected data to computer software where it is analyzed for presentation to a physician. The software generates reports that enable the physician to identify gait cycles and calculate joint angles to explore where the spine is affected.

Surgery is traditionally based on static imaging, but as soon as patients start to move, things change. The motion capture system allows physicians to see precisely how patients enter their gait cycle and it enables analysis of joint angles and movements. When the spine is engaged, the lumbar, neck, thorax, and head can all be affected.

Figure 1. A researcher applies retro-reflective markers to the subject's body. (Photo courtesy of Vicon Motion Systems)

Patients requiring surgical procedures have their walking motion captured prior to their operation (baseline analysis), short-term follow-up, and year post-surgery follow-up. The system allows for objective data to be collected to enable comparisons not only between pre-and-post surgery data, but also to healthy controls.

Mocap

The system consists of ten Vicon Vantage V16 mocap cameras, set up throughout a 900-square foot lab. Each camera has an 18 mm lens with 16 megapixel resolution custom greyscale sensors. The frame rate at 16 mega-pixels is 120 Hz (120 frames per second), and each camera can go up to 2000 Hz.

The setup features five Vantage cameras in the back and five in the front, alongside two Bonita video cameras. Each mocap camera captures information from forty-one retroreflective markers applied to the full body to capture patients’ walking, balance, and lifting tests. The markers are typically made from plastic with a retroreflective coating. This means the light emitted from a strobe on the camera is reflected directly back to the camera — the camera uses this reflection to determine the position of the marker in two-dimensions. Since all of the cameras are calibrated together, the position of the marker can be calculated in three-dimensions (3D). The cameras feature OLED display and status lights, which provide camera identification and system feedback, for example, that the camera is calibrating.

A built-in accelerometer enables several functionalities, such as a tab to select prior to system calibration and a realtime camera position monitor (bump detection). A temperature sensor monitors the thermodynamics of the camera body, which is important because the optical properties of the sensor and lens change with temperature. The user therefore needs to be informed if the temperature of the camera changes between calibration and capture.

The cameras capture grayscale imagery to precisely track reflective data points from markers and ignore everything else, which provides more relevant information and increases motion measurement accuracy. They perform some of the front-end data processing, generating grayscale blobs from the retroreflective markers and using centroid-fitting algorithms to determine accurate centers. The camera data is then sent to Vicon Nexus software for further processing.

The software uses the data to produce submillimeter accurate representations of a patient's movement for future analysis by the medical team. Three-dimensional gait analysis is captured at 300 Hz (300 frames per second) to measure joint kinematics (joint angle ranges e.g. hip flexion and extension) and spatial-temporal parameters (e.g. walking speed and cadence). A spine biome-chanical model that includes nine additional markers is applied to the dataset to calculate the kinematic joint angles. For example, for the balance and lifting test, lumbar swing range is measured.

Psychology

Figure 2. Balance test. (Photo courtesy of Vicon Motion Systems)

The Bonita video camera is Vicon's HD color reference camera, which has HD resolution of 1280 x 720 (720p) at a frame rate of 120 Hz. The video cameras provide full-color reference video of the trial, which allows for further context of the optical data. Functionality within the software allows for three-dimensional optical data overlay onto video captured data. One Bonita Video Camera (sagittal view) is used to capture the gait profiles of the patient walking along the gait walkway. The other (coronal view) is used for psychological testing to access pain scales that help identify the interaction between pain and motion of the patient. This procedure is critical for the assessment and treatment process. For instance, the camera records facial movements such as eyebrow or lip twitch, which are typical traits for patients experiencing pain. This procedure is also objectively quantified for pain by utilizing surface electromyography (EMG) data, which is integrated into the Nexus software. The EMG data provides physicians with a full picture, including how much muscle energy a patient is expending in the gait cycle, the degree of swing in lumbar balance tests, lifting, and balance details. The pain measurement score can be compared to how the patient is walking in terms of range of motion and walking speed. Since pain inhibits motion, before treatment or surgery, patients tend to take shorter steps, and they adopt a wider stance to accommodate loss of balance.

Figure 3. Computerized gait display is matched to video. (Photo courtesy of Vicon Motion Systems)

Surface Electromyography (EMG)

EMG assesses muscle energy expenditure during the gait. Electrodes are applied to various muscle sites (e.g. gastrocnemius) to measure electric muscle activity. The EMG system connects directly to a Vicon System PC via USB. With a digital third-party EMG plug-in file placed in the software, Nexus can synchronize the channels of the third-party device to the Vicon motion capture System. EMG integration allows the center of mass to be calculated to determine the center of mass displacement during a one-minute test. Dr. Haddas explains that physicians “track the extent of displacement of the center of mass of a scoliosis patient, which averages almost a full meter, while a non-affected person moves only 20 – 25 centimeters.” “While there is almost unlimited data from Vicon, it needs to be distilled. We generally provide five to ten highlights for the doctors.”

Data Presentation

Figure 4. Using a wand to calibrate the system. (Photo courtesy of Vicon Motion Systems)

Nexus reconstructs the 2D marker data into 3D, which can be presented in a number of ways specific to the user's requirements or preferences. This could be as an ASCII file showing the data for each individual marker for each frame, or it could be a 3D representation of the data, showing the movement of the patient throughout the trial. Vicon also has a reporting tool that enables the user to create clinical reports showing the captured 3D data and the video data.

Calibration

The provided Vicon Active Wand is used to calibrate the system. It uses LEDs, which are able to be manufactured to tighter tolerances than passive markers, for simultaneous calibration of both the optical (Vantage) and video reference (Bonita) cameras. The integrated Photodiode automatically synchronizes the wand to the optical and video reference cameras. A dynamic calibration is performed by waving the wand within the capture volume in such a manner that it can be seen by all cameras. Since the wand markers are at known distances, details of the two-dimensional position of each camera in three-dimensional space can be obtained. A static calibration is then performed to set the camera origin to determine the global coordinate system of the capture volume. This process allows for accurate three-dimensional data, not only of the optical data, but also for the video cameras, thereby enabling accurate overlay of the 3D optical data to the video footage.

One application for the calibration system, is to perform a correction if the accelerometer determines that the camera has been bumped — moved from its calibrated position. The user is notified by status LEDs and a display on the camera as well as a notification in the software. Selective camera calibration could then be performed by waving the wand to recalibrate the cameras affected by the bump.

Looking to the Future

The use of Mocap for medical research is an excellent example of synergy between technologies developed for completely different purposes. Precise 3D measurement of human movement in order to impart realism to animated figures in movies and video games is being used as a way of analyzing spinal malfunctions. That kind of cross-pollination is what moves technology forward.

This article was written by Ed Brown, Associate Editor of Photonics & Imaging Technology. For more information, visit here.