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Advancing Microelectronics • Volume 29, No. 6 • November/December, 2002

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Electronic Instrument Development for Applications in Space Biomedicine and Astronaut Health

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Harry K. Charles, Jr., Ph.D.

ABSTRACT

The space environment can cause significant alterations in the human physiology that could prove dangerous for astronauts on long-duration space missions. The NASA program to develop countermeasures for these deleterious human health effects is being carried out by the National Space Biomedical Research Institute (NSBRI). The NSBRI has 11 research teams, nine of which are primarily physiology based, one addresses on-board medical care, and the eleventh focuses on technology development in support of the other research teams. This Technology Development team is creating novel electronic instruments that should have a major impact on the health of astronauts and the human population on Earth.

INTRODUCTION

As evidenced by astronauts returning from the Russian space station MIR and other long-duration space missions, the environment of space causes significant alterations in human physiology that can ultimately affect astronaut health and, in certain circumstances, be life threatening. Just as the prior generations of Earth-bound men and women have adapted to difficult conditions (climbing mountains, crossing oceans, etc.) to explore and learn more about their surroundings, the current and future generations of mankind must adapt to the space environment in order to explore and develop space beyond Earth orbit.

The National Aeronautics and Space Administration (NASA) effort to develop countermeasures or solutions to the physiological and psychological problems faced by astronauts on long-duration space flights is being carried out by the National Space Biomedical Research Institute (NSBRI) [1]. Currently, envisioned space missions will last from two to three years and allow astronauts to travel to planets such as Mars as well as studying long-term space health-related issues aboard the International Space Station (ISS).

The weightless and reduced-gravity environments encountered during space exploration, in addition to the altered natural radiation fields that exist within these environments, will seriously affect multiple body parts and systems. The human body has been uniquely designed to live with the Earth’s gravitational force and thus must adapt to the new and foreign conditions of space. The changes experienced by the human body in space, if not mitigated, can seriously affect astronauts’ health and performance on extended missions as well as pose a significant risk upon their subsequent return to Earth. While most humans will never explore outer space, countermeasures or solutions to space-related health issues can have a significant impact on medical care and treatment on Earth.

For example, a solution or countermeasure to the rapid bone loss in astronauts (typically 1 to 1.6% per month [2]) experiencing microgravity may prove beneficial to the treatment of osteoporosis on Earth. Osteoporosis and related conditions are on the rise within our aging population and may ultimately affect 60% of the women and 40% of the men. Solutions to other space-related health conditions such as muscle wasting, sleep disturbances (shift related), balance disorders, cardiovascular, and immune system problems have direct applications in the treatment of many sufferers of these conditions in the Earth-bound population.

NSBRI AND TECHNOLOGY DEVELOPMENT

The NSBRI consortium was established in April, 1997, through a competitive solicitation by NASA. The NSBRI unites scientific and medical experts from leading institutions. The NSBRI consortium has 12 institutional members as shown in Table 1. Each consortium member is widely known for its excellence in scientific and medical research, education, and patient care. The research program carried out by the NSBRI directly supports NASA’s Strategic Program for the Human Exploration and Development of Space by seeking to prevent or solve human health problems associated with long-duration space travel and the continued exposure to microgravity and high radiation environments. The NSBRI also investigates ways to deliver medical care on these missions through the application of new technologies and remote treatment advances. While the focus of the NSBRI research is on space-related issues, the NSBRI also has a strong technology transfer activity that will ensure that NSBRI research discoveries are rapidly and effectively introduced into human health treatment on Earth.

The NSBRI’s current research program involves 100 peer review projects involving almost 300 investigators at over 80 institutions and government laboratories. The research program is team-based, with 11 teams addressing a wide gamut of space-related human health issues, including bone loss, muscle alteration and atrophy, radiation effects, neurovestibular adaptation, cardiovascular alterations, immunology, nutrition, physical, fitness, etc. Of the 11 research teams, the Technology Development Team [3] has the primary responsibility for electronic instrument development. Within the Technology Development Team suite of projects, there are two projects that address NASA’s top priority health risks: bone loss and radiation effects. The effective measurement of bone loss is being addressed by the development of an Advanced Multiple Projection Dual Energy X-ray Absorptiometry (AMPDXA) system capable of being flown in space. An advanced Neutron Energy Spectrometer (NES) is being developed to provide highly accurate radiation measurements over a wide range of energy. The remainder of this article will focus on the development of these instruments.

AMPDXA

Project Description

The purpose of the AMPDXA project is to design, build, and test a precision scanner system for monitoring the deleterious effects of weightlessness on the human musculoskeletal system during prolonged spaceflight. The instrument uses dual energy x-ray absorptiometry (DXA) principles and is designed to measure bone mineral density (BMD), decompose soft tissue into fat and muscle, and derive structural properties (cross-sections, moments of inertia). Such data permits assessment of microgravity effects on bone and muscle and the associated fracture risk upon returning to planetary gravity levels. Multiple projections, coupled with axial translation, provide three-dimensional geometric properties suitable for accurate structural analysis. This structural analysis, coupled with bone models and estimated loads, defines the fracture risk. The scanner will be designed to minimize volume and mass (46kg goal), while maintaining the required mechanical stability for high-precision measurement. The AMPDXA will be able to detect changes of less than 1% in bone mass and geometry and changes of less than 5% in muscle mass.

Bone Loss Mechanisms

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 [4]. 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.

Structural Observations

Initial structural studies on adult population (ages 20 to over 90) have produced the following information. On average, there was little decline in the section modulus at the femoral neck and proximal shaft through the seventh decade of life for females and the eighth decade for males. Over the same age range, the BMD in female decreased by approximately 25% (through the seventh decade) and in males by approximately 20% (through the eighth decade). Even though the BMD decreased significantly, the maintenance of section modulus suggests that the bones became mechanically more efficient with age — due to subperiosteal expansions. Analysis of subperiosteal widths produced an almost linear subperiosteal width increase with increasing age. The rage of increase was about 2% per decade relative to nominal young adult values. Aging causes an increase in endocortical diameter and with appropriate activity (loading), bone is added to the subperiosteal surface. Hence, the bones get larger with thinner cortical walls, thus maintaining strength despite BMD loss. For example, a 10% loss in BMD can be compensated by less than a 1 mm increase in subperiosteal diameter.

Russian cosmonaut data (20 individuals with six months average space time) produced supporting results. Since loading stimulates subperiosteal expansion, it was expected that BMD loss would not be accompanied by subperiosteal expansion and, thus, both BMD and bone strength (section modulus) would be reduced. This is exactly what the cosmonaut data shows. BMD decreased by 4-8% and section modulus decreased by 4-8%, while the subperiosteal width remained relatively constant.

In a study of postmenopausal women (age 65 plus), we have compared BMD data, section modulus, and cortical dimensions for women who maintained or gained weight with those that had significant weight loss. Women with static (increasing) weight had reduced BMD (nominally 1 to 2% over a 4-year period) and some decline in cortical thickness, but there was an offsetting increase in subperiosteal diameter, thus preserving bone strength or section modulus. In women with significant weight loss, the BMD reduced 2 to 4% (over four years) and the decline in cortical thickness was greater than the subperiosteal diameter increase, thus causing a decline in bone strength, which, again, is consistent with our hypothesis.

Instrument Progress

The current focus of our activities has been on (1) instrument development, (2) algorithm enhancement for BMD image extraction and structural analysis, and (3) bone reconstruction and modeling techniques. In the instrument development phase, we have constructed two instruments: (1) a Laboratory Test Bed (LTB) and (2) a Clinical Test Unit (CTS). The LTB was utilized to verify principles and theoretical predictions and demonstrate that the AMPDXA techniques worked and produced results with the expected precision. Such results are shown in Fig. 1. Fig. 1(a) is a BMD of a human femur immersed in a cylinder of water (to simulate fatty tissue). The same bone was imaged on a new commercial DXA scanner located at The Johns Hopkins Hospital, as shown in Fig. 1(b).

The CTS incorporates high-precision, rotational and translational stages to provide the scanning capability to carry out qualification tests on human subjects. Since the CTS is designed to operate only on Earth, the table, gantry, and associated equipment were not built to the size and mass requirements of an AMPDXA unit for spaceflight (see Cover illustrations). In fact, the unit was built on a used computed tomography scanner.

The image extraction capability of the AMPDXA is illustrated in Fig. 1(a) and (b), where not only is the BMD image higher resolution, but also the mass distribution in a projected thickness of a femur slice contains much more structural detail than conventional DXAs [see Fig. 1(c)]. The high frequency content of the BMD spatial projections as shown in Fig. 1(c) are reproducible and provide information on the bone’s microstructure. 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. Initial experimental measurements with different sets of three projections showed that the principal moments of inertia could be determined within 3 to 4%. Additional projections (above 3) reduce this number further. Our original experimental system also had some known non-linearities, which have since been removed, and our error in the three-projection estimation of moments has been reduced to less than 1%.

The CTS scanner is now operational with an advanced antiscattering grid [Fig. 2(a)], which has produced exciting initial results [Fig. 2(b)]. The comparison of Fig. 2(b) with Fig. 1(a) shows the improvement in resolution in a BMD image over the LTB. Initial human scans have been taken on the CTS with similar results. Comparison of these results with a commercial DXA is now underway.

NEUTRON ENERGY SPECTROMETER

The NES is being developed to monitor the flight radiation environment on board the ISS. The NES is designed to measure the neutron energy spectrum from 20 keV to 500 MeV, with a goal of 10% energy resolution and the ability to count neutrons whose energies are less than 20 keV. Radiation and bone loss are considered the most threatening risks for astronauts on long-duration space missions. Furthermore, neutrons are estimated to contribute 30 to 60% of the radiation dose equivalent inside space structures such as the ISS and below remote planetary atmospheres. Due to their uncharged nature, neutrons cause little damage to cells; however, the human body contains large quantities of hydrogen, a constituent of the water molecules that occupy 70% of the human body. When energetic neutrons impact a hydrogen atom, they knock out a proton that causes ionization in the body. As a result, neutrons can cause more severe damage to the body, at equivalent absorbed doses, than do gamma rays. On Earth, neutrons have very low energies due to the multiple collision effects within the Earth’s atmosphere, while inside the Shuttle or the ISS at shield depths of 10 to 50 g cm2, the secondary neutrons will be the progeny of only one or two collisions and will have much higher energies. Thus, the neutron dose can be deposited within or near critical organs such as the liver or spleen, thereby increasing the risk of carcinogenesis at these locations. There is also the possibility of a DNA double strand break from a single neutron strike. Deep deposition of neutron dose also threatens the central nervous system.

The NES project is in the last stages of developing an engineering prototype that will be directly adaptable to the radiation monitoring needs of the ISS and Shuttle, and could be used on future planetary missions. To cover the required neutron energy range (20 keV to 500 MeV), the NES uses two detectors: (1) a 3He gas tube for the low energy neutrons, and (2) a 5 mm thick, lithium-drifted silicon detector for the high energies.

The current unit weighs 27 kg with an ultimate goal of 10 kg. The NES prototype as shown in Fig. 3, has flown on several F15 aircraft flights at altitudes in excess of 40,000 feet. The high voltage power supply had to be especially designed to allow the unit to function at or near the corona region (pressure less than 1 psi) that exists not only in the unpressurized F15 instrument pod, but will also exist on the surface of Mars. Currently, the unit is being tested in a balloon flight to 90,000 feet.

SUMMARY

The major health and risk factors induced by long-duration exposures to the environment of space must be addressed. Effective solutions or countermeasures to the deleterious effects of weightlessness and increased radiation exposure must be found. The NSBRI research program is a major driver in the race to find effective countermeasures and solutions. Opportunities exist for the electronics community to have a significant impact on the outcome of this research which will not only benefit astronauts engaged in long-duration space travel, but also will address major health needs on Earth. Instruments that fly in space must be relatively small, lightweight, low power, highly reliable, and easy to use. These drivers are the same as the electronic industry has in the commercial marketplace. The AMPDXA and NES are but two examples of how electronic instrument development is playing a key role in NSBRI research and the monitoring and treatment of human health problems.

REFERENCES

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