Gemini 5: L-10, L-4, L-2, L-0, R+0, R+1, R+10, and R+58
Gemini 7: L-10, L-3, L-0, R+0, R+1, R+11, and R+47
Apollo 7 and Apollo 8: unknown except for L-0 and R+0
The equipment used in this experiment included standard x-ray machines, standard x-ray films, aluminum alloy calibration wedges, and a bone densitometer assembly.
The os calcis was selected as an anatomical site for evaluation because it was a large bone with a minimum of over- and underlying soft tissue and it contained a substantial amount of trabecular bone tissue. (The interchange of minerals between bone and extracellular tissue occurs more readily in newly formed trabecular bone than in older compact bone.) The phalanges were selected as anatomical sites for evaluation because they represented bone that contained a substantial amount of compact skeletal tissue. Both the os calcis and the phalanges were known to be little affected by soft tissue x-ray scatter, and they were easily accessible for x-raying. All of the sites selected were considered non-hazardous from the irradiation aspect since blood-forming marrow would not be exposed to radiation. The x-ray exposure field was strictly limited to these small areas.
The x-ray machines were standardized by three methods. The first method used an aluminum alloy wedge exposed on the film adjacent to the bone. The x-ray films were calibrated by placing an aluminum alloy reference wedge adjacent to the bone to be x-rayed. The wedge served as a means of correcting any bone scan that was traced by first correcting the trace of the wedge for deviations resulting from slight differences in film characteristics or development techniques. The alloy in the wedge was selected because it exhibited an x-ray absorption coefficient similar to that of bone. Separate calibration wedges were used for the heel bone and the little finger.
The second method used a roentgen meter immediately before making a x-ray to determine the calibrated kilovoltage that would produce identical beam qualities in each of the x-ray machines. The third method used a specially prepared phantom that was shaped like an os calcis and made of calcium hydroxyapatite in an organic matrix. The wedges were individually calibrated in terms of calcium hydroxyapatite, the major mineral component of bone. Each wedge was x-rayed on the same film with a series of known quantities of calcium hydroxyapatite encased in thin-walled Lucite containers. The values obtained from scanning certain sections of bone could also be equated in terms of the mass of calcium hydroxyapatite by means of a conversion factor. The phantom was x-rayed before and after each series of films made at any one time to detect possible technique variations. For the os calcis, milliamperes, kilovolts, and time were set to give an exposure level of 167 ± 2 milliroentgens for each x-ray made.
The x-ray film development procedures were closely controlled regarding chemicals, temperature, and timing. Densitometer control readings of all films agreed within 2 percent.
Bone mass was determined from exposed x-rays by means of a bone densitometer computer assembly. The densitometer system evaluated the x-rays in two ways. First, a single path approach known as a "conventional scan" was used. The densitometer was programmed to trace one small strip (1.3 millimeters in width) across the x-ray image between two conspicuous landmarks on the bone. Each x-ray was made with extreme care so that the image of the bone on each film could be superimposed exactly over that of the initial film. With a small steel needle, the initial film was punctured at each end of the bone image; this defined the limits of the conventional trace. The two needle picks could be identified by the densitometer, which made possible exact positioning of the film prior to scanning. The same technique was applied to the image of the calibration wedge on the same film. Before tracing the same segment on each successive film, each film was superimposed over the first film with the needle pricks made in exactly the same positions. By superimposing successive x-rays, accurate serial films could be produced for the same individual showing the density changes. The os calcis and the talus were evaluated this way. These were revealing sites for measuring bone mineral changes, since the path penetrated a highly trabecular area of bone, surrounded by a periphery of cortical bone. The conventional scan gave bone mass, in terms of wedge mass equivalency in grams.
The "parallel path" system was used to evaluate a larger section of the same bone. Without removing the x-ray film from the densitometer after making a conventional scan, a series of parallel strips 1.0 millimeter (mm) apart were made, beginning 1.0 mm above the conventional path and continuing to the lowest portion of the bone. The center of each scan was 1.0 mm from the center of the next scan, with the edges of the successive scans slightly overlapping. The total number of paths scanned was proportional to the size of the bone with its individual variations. The os calcis and phalanges were measured in this fashion.
Initial results from the three missions indicated that bone mineral losses in the Gemini 7 crewmembers were significantly less than those of the Gemini 4 and Gemini 5 crewmembers. The most significant differences occurred in the os calcis between the Gemini 5 and Gemini 7 crewmembers. Bone density changes in the os calcis ranged from -15.10 and -8.90 % in the Gemini 5 subjects to -2.91 and -2.84 % in the Gemini 7 subjects. The bone mineral density changes in the Gemini 4 astronauts were -7.80 and -10.30 %. Losses in the phalanges were also less in the Gemini 7 crewmembers than those found on Gemini 4 and Gemini 5, although the differences were not as great as those changes in the os calcis.
Rapid gains in bone mass were found during the first 12 hours after recovery in all subjects. Before this study was closed, each of the six astronauts had met or exceeded his preflight bone density level in all anatomical sites that were tested. The backup crewmembers who acted as control subjects experienced only those changes in bone density found in normal healthy persons.
Several years after the close of the Gemini program, the experiment data were reevaluated using photon absorptiometry. Researchers concluded that certain factors may have been conducive to misinterpretation of bone mineral changes in the Gemini 4 and 5 astronauts. X-ray evaluations of a standard bone phantom made at the same times and on the same x-ray machines as the original films indicated that the reported mass losses in the Gemini 4 and 5 astronauts were 7 % too high. This error was attributed to a 4 % inherent absorption difference between the two calibration wedges used prior to launch and after recovery. Three percent of the error was attributable to variation in x-ray beam qualities among the x-ray machines used in the experiment.
The corrected density changes for the Gemini 4 crewmembers were -2.9 and -3.8 % instead of the -7.8 and -10.3 % values originally reported. Similarly, the corrected density changes for the Gemini 5 crewmembers -9.2 and -2.5 % instead of the originally reported values of -15.1 and -8.9 %. The modified data indicated that the observed changes in bone density pre- and postflight were so close to the overall experimental error that it was questionable whether any changes occurred at all.