Overview
Compact, High Precision, Multiple Projection DXA Scanner
Principal Investigator:
Harry K. Charles, Jr., Ph.D.
Organization:
Johns Hopkins University Applied Physics Laboratory
Technical Summary
The development of the AMPDXA is being carried out in a multistage process beginning with an initial Laboratory Test Bed to prove basic principles and develop initial hardware and software. Following the Laboratory Test Bed was a Clinical Test System, which is now operational, which will allow the testing of AMPDXA principles on human subjects as well as on special phantoms and other test objects. The third stage is the design and development of a protoflight system that incorporates all the design and performance features of the previous units into a form, fit and functional configuration that could evolve into the low mass flight unit. Preliminary designs for this protoflight system incorporate advanced electronic fabrication technologies (chip-on-board, multichip modules) coupled with commercial (off-the-shelf) parts to produce a reliable, integrated system which not only minimizes size and weight, but because of its relative simplicity, is also cost effective to build and maintain. Additionally, the protoflight system is being designed to minimize power consumption. Methods of heat dissipation and mechanical stowage (for the unit when not in use) are being optimized for the space environment.
The AMPDXA Project is a joint effort between the NSBRI's Technology Development Team and both the Bone Demineralization/Calcium Metabolism Team and the Muscle Alterations and Atrophy Team. Its goal is to provide the high precision monitoring system necessary to fully assess both the deleterious effects of weightlessness on the bones and muscles and the effectiveness of any countermeasures. We believe that any pharmacological or exercise-related countermeasures used by astronauts to mitigate microgravity effects will require efficient and timely monitoring. Moreover, the monitoring device must be capable of being used by astronauts during spaceflight so that feedback can be dynamically employed to regulate countermeasure doses. The system design will be such that intelligent but not necessarily medically trained personnel will be able to create scans that will provide all of the accuracy and precision necessary. Readouts and displays for the AMPDXA instrumentation will be specifically designed to provide useful (real-time) feedback information to both the astronauts and the ground-based physician monitoring team (as permitted by the mission dynamics).
We believe the key to understanding the mechanism of bone (and muscle) loss in space (microgravity) lies in the bone's structural details and the changes in the structure due to prolonged weightlessness. Our hypothesis is that throughout most of adult life, aging bones become more structurally efficient and retain their strength even though BMD declines. The homeostatic mechanism for strength maintenance depends on skeletal loading. Thus, to maintain bone strength, normal loading on the skeletal system must be maintained. Absence of loading during prolonged spaceflight (or disuse) can cause uncompensated loss of bone strength. Even reduced loading (caused by muscle wasting and inactivity in the elderly) can cause a disruption in the bone strength maintenance mechanism.
Current bone and muscle mass measurements (via conventional DXA or ultrasound) are regional averages that obscure structural details. Since the mechanical consequences of lost bone and muscle are reflected in the structure, an absolute determination of skeletal mechanical competence is needed to supplement the loss measurements. Engineering properties of the bones can be derived from DXA-generated BMD data. Our method derives geometrical measurements from the BMD images. From such images, we extract BMD profiles at important skeletal locations (e.g., proximal shaft and femoral neck). Key properties measured and derived from these profiles include the BMD, the subperiosteal width, the section modulus (related to strength), and the cortical dimensions.
During the course of the AMPDXA Project, significant progress has been made in several key areas: (1) instrument development, (2) algorithm development for BMD image extraction and structural analysis, and (3) bone reconstruction and modeling techniques. As mentioned above, a full size Laboratory Test Bed (1 meter source to detector distance) was constructed to verify principles and theoretical predictions. Scanning is provided by high-precision rotation and translating stages. This Laboratory Test Bed, in conjunction with a high-resolution detector and our analysis software, has produced some exciting preliminary results. The improvement in spatial and contrast resolution with our scanner is quite evident.
The next important instrumentation step has been the development of a Clinical Test System. The Clinical Test System incorporates a high precision rotation and translation stage to provide the scanning capability to carry out qualification tests on human subjects. Since the Clinical Test System is designed to operate only on Earth, the table and gantry were not built to the size and mass requirements of a protoflight AMPDXA. In fact, the unit was built on a used CT Scanner structure. Employing used equipment for some of the structural elements and the rotating parts and machinery has allowed critical resources to be focused on the information extraction and analysis issues leading to human testing.
The image extraction capability of the AMPDXA is excellent. In a test using a human femur immersed in water, the BMD image is of higher resolution and the mass distribution in a projected thickness of a femur slice contains much more structural detail than conventional DXA's. The high frequency content of the BMD spatial projections are reproducible and provide information on the bone's microstructure. Work on multiple image analysis has progressed quite well. Using multiple projections about the bone axis allows structural properties (e.g., bending strength) to be obtained independent of patient position. To do this at least three arbitrary projections over 90 degrees (two of which are orthogonal) must be obtained. Such analysis can provide maximum and minimum moments of inertia for bending or torsion in any plane. Our experiments to date with different sets of three projections show that the principal moments of inertia can be determined within 3 to 4 percent. Additional projections (above 3) reduce this number further. Our experimental systems also have some known non-linearities which when removed will drop the error in the three projection estimation of moments to less than 1 percent.
Initial multiple projection work also shows that three projections are not sufficient for total image reconstruction; however, it appears that a cone beam type reconstruction from as few as three to seven projections is sufficient to produce a pseudo three-dimensional geometry that is mechanically equivalent to the measured hip.
The AMPDXA, as described above, has direct application to risk reduction in NASA's Critical Research Path. The AMPDXA is capable of real-time monitoring of bone and muscle loss at extremely high precision. Since the results are patient-specific and not tied to volumetric averages and statistical norms, the AMPDXA is a very useful tool for monitoring the effectiveness of countermeasures as well as determining the risk of fracture under deployment scenarios. The AMPDXA also appears to be a natural adjunct to Earth-bound research on the effects of aging and disuse on bone integrity. It could also be used as a routine screening tool for osteoporosis and as a monitoring tool for osteoporosis drug therapy.