Extremity slice matching from one imaging session to the next was accomplished by comparing marrow areas. Back muscle image slices were matched by positioning the center slice on the third lumbar vertebra (L3) and acquiring slices above and below this region. Muscles were grouped as follows: gastrocnemius, soleus, anterior calf (all calf muscles excluding the gastrocnemius and soleus), quadriceps, hamstrings, intrinsic lower back (rotatores, multifidus, spinalis, longissimus, iliocostalis), and psoas. For each muscle region, the outlined area (in pixels) was plotted against position (in millimeters). After comparing identical regions as described above, muscle volume was obtained by adding the number of pixels under each curve. Volumes were expressed as a percent of preflight or pre-bedrest averages.
Using a spine coil, the spinal imaging area was centered within the L3 vertebra, determined from a sagittal scout view. A coronal scout image positioned a 1 cm region through the center of the spinal column. A CPMG sequence obtained images with a Tr=1500 sec and spin echoes at 20, 45, 72, 106 msec (n=5) or 30, 60, 90, 126 msec (n=2). A phantom was imaged during each MRI session to correct for changes in pixel size.
Disc areas for T12-L1, L1-2, L2-3, L3-4 and L4-5 were obtained by calculating the number of pixels in each image in the following manner. Using the echo 1 image, a large region was drawn around the disc to determine the average pixel intensity. All pixels having intensity values greater than one half the mean value were displayed and again the mean and one half the mean were determined. The resultant pixel number was used as the disc area after corrections for pixel size changes. The same procedure was used for the other three echoes using the starting template as echo 1. Echo 1 was selected to represent the entire disc, and echo 3 represented the nucleus.
To examine changes in mobile proton concentrations, the spin-spin relaxation constant (denoted as T2) of discs was measured using the four spin echoes. Each pixel was fit by the weighted least squares method to calculate a T2 value, assuming a single exponential. Signal intensity was used to compute relative weights. T2 was determined by taking an average T2 of all pixels within L2-3 and L3-4. These discs were selected because the field of view was centered on L3 where a high signal to noise ratio would result. T2 was also measured in the lumbar vertebral bodies to determine changes in the fat to water content ratio.
Changes in disc height were determined by calculating the distance from the center of L1 to the center of L5, assuming that spine curvature does not change between measurements.
The statistical analysis was limited to discs L2-3 and L3-4 to reduce the number of comparisons. Preflight measurements were averaged to obtain three time points: preflight, immediate postflight and late postflight (15 days). Data were tested using repeated measures ANOVA and a Greenhouse-Geisser adjustment factor with significance set at p<0.05.
Significant muscle-specific atrophy was seen after only 8 days in weightlessness. Muscle volume changes found in the four SL-J crewmembers were: -3.9% from the anterior calf, -6.3% from the soleus and gastrocnemius, -6.0% from the quadriceps, -8.0% from the hamstrings, -10.3% from the intrinsic lower back and -3.1% from the psoas. Statistical analysis of R+1 data demonstrated that the soleus, gastrocnemius, anterior calf, hamstrings and intrinsic back muscle volumes were significantly decreased compared to preflight values. The quadriceps (p=0.06) and the psoas (p=0.13), while lower immediately following recovery, were not significantly decreased. All muscle regions increased in volume by R+15 relative to R+1. However, the hamstrings and intrinsic lower back muscles were still significantly lower in volume than preflight. These changes explain in part the large losses in muscle strength reported after Shuttle missions. The muscle volume changes observed after flight are similar to those observed after bed rest of similar duration.
Results indicate that the disc area of bed rest patients expanded, reaching an equilibrium value of 22%. Disc area of SL-J crewmembers had returned to preflight values within the first 24 hours after landing. As expected from the disc area results, no changes were observed in the lumbar spine lengths (L1-L5), intervertebral discs, lumbar vertebral bodies, or marrow transverse relaxation times. This work has demonstrated that significant adaptive changes in intervertebral discs can be expected during weightlessness. Knowledge of these changes is important in understanding spine mechanics and in selection of exercise countermeasures employed during flight.
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