Positions and velocities of hundreds of implanted targets could be measured simultaneously.
An x-ray photogrammetric technique for minimally invasive measurement of displacements in muscles, limbs, and organs is undergoing development. This technique could be alternative or complementary to the use of strain sensors. Measurements obtained by this technique could also be compared directly with both absolute and differential measurements of muscle and skeleton kinematics obtained by exoskeletal devices or from video images of external motion.
The technique is based on x-ray pointsource/ shadow imaging of small implanted biocompatible metal targets (see figure). The dimensions of the targets would be much smaller than those of the body parts into which they would be inserted; e.g., a target for implantation in a 50-µm-diameter muscle fiber would have a diameter of about 5 µm. The targets would be implanted by use of a hypodermic needle. The targets would show up as dark spots in an x-ray image, which would be projected onto a fluorescent screen monitored by a video camera. A complete imaging system would include at least two (and preferably three) pointsource/ fluorescent-screen/video-camera assemblies oriented along different coordinate axes, plus frame grabbers, video image digitizers, and an image-data-processing computer.
Individual targets would be identified and their three-dimensional positions computed from the positions of their shadow spots in the x-ray images. With sufficiently fast frame grabbers and sufficient computing capacity, it would be possible to compute the coordinates of hundreds of targets at video frame rates. Velocities and accelerations of targets could readily be computed from changes in their positions in successive frames. Combining the results of these computations with knowledge of which particle is embedded in which tissue, one could establish the kinematics of the entire organism or body part in which the ensemble of targets was embedded. Inasmuch as gray-scale x-ray images generally show such tissues as bones and dense muscles, the information thus generated could be processed further to obtain enhanced animated images of body parts in motion. The original intended application for this technique is monitoring of fibrillation or operation of heart valves. Other potential applications include monitoring fatigue or cramps in muscles or acquiring data for calibration of mathematical models of muscle and limb movements and of muscle forces and displacements.
This work was done by Frank Hartley of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Bio- Medical category.
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X-Ray Measurement of Kinematics in Muscles and Limbs (reference NPO-20467) is currently available for download from the TSP library.
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