The results that triggered our concern are shown in Fig. 2, where changes in total-body BMD and BMC using Enhanced Analysis are plotted against change in weight. The results were very similar whether they were plotted as absolute changes or percentages. There are highly significant linear regressions, positive for BMC, but negative for BMD. There were some substantial changes of weight and it may appear that the regressions are dominated, or even determined, by outliers. However, when all results with a weight change greater than one standard deviation (SD; 5 kg or 9%) away from the mean were removed, the revised regressions were still highly significant, with slope and intercept not significantly different from those with the full population. The regression equation indicates that a 10% change of weight was associated with a BMC change of approximately 2.5%. During the period of the study, 39% of the subjects lost and 40% gained more than 1% in weight.
Figure FIG. 2.. (Upper) Correlation between changes in total BMC and weight. (Lower) Correlation between changes in total BMD and weight.
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Further inspection revealed that the negative dependence of BMD on weight was due to an apparent positive correlation between changes in bone area and weight. The analysis provides results for regions of the body as well as the total, so regression equations for the correlations between changes in BMC (ΔBMC) and weight (ΔW) and ΔAREA and ΔW are presented in Table 1A. With the exception of the pelvis and the head, all the regressions were highly significant. The slopes differed between body regions, but the slope for ΔAREA was always similar to that for ΔBMC. As a consequence, BMD changed little with weight; there was a negative correlation for the total and for the legs, but no other significant correlations.
Table Table 1.. LINEAR REGRESSION EQUATIONS OF PERCENTAGE ΔBMC AND ΔAREA AGAINST PERCENTAGE ΔWEIGHT IN VIVO
The most significant correlation was that between changes in AREA and BMC, Fig. 3. There were similar correlations for each of the regions. The linear regressions are given in Table 2A. The slope of the regression equation for the pelvis was significantly lower than that for the total body, but there were no significant differences between the other regression coefficients.
Figure FIG. 3.. (Upper) Correlation between changes in total bone area and BMC. (Lower) Correlation between changes in bone area and BMC of lumbar spine.
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Table Table 2.. LINEAR REGRESSION EQUATIONS OF PERCENTAGE ΔAREA AGAINST PERCENTAGE ΔBMC
The foregoing results are for the whole group studied. Separate calculations were also made for groups comprising patients with continuing eating disorders, recovered patients, and normal controls. The corresponding regression equations for total-body bone mineral were not significantly different from each other or from those for the whole group.
It can be noted in Fig. 2 that there are intercepts for the regressions, so that at zero weight change there was a 1.1% increase in BMC, a 1.6% increase in BMD, and a reduction of 0.5% in AREA. No evidence of a change in the calibration of the scanner was evident in the quality control records. These are based on daily measurements of the Hologic spine phantom, so there was a possibility that they might not be representative of the stability of total-body scanning. Further evidence was obtained from repeated measurements of a semianthropomorphic total-body phantom11 that had been scanned for other purposes at intervals over a period of 4 years, including the duration of the study reported here. There were no significant changes of total body AREA or BMC. Over the 4-year period, there was a significant increase of BMD (p = 0.02), but during 1 year this amounted to only 0.4%.
The data from the subject measurements were examined for evidence of calibration change. A group of 45 subjects in whom weight change during 1 year was less than 2% was selected and the percentage change in total BMC was plotted against the initial scan date; these dates covered a period of 10 months. There was no significant correlation, supporting the existence of a stable calibration. Given the variance of the BMC and AREA changes (SEE of the regressions 2.8%, range about twice this), it is not too surprising that there were apparently mean changes of 1% and 0.5% in a year, particularly as they may be related to the anomaly under investigation.
In addition to the results based on the usual regions of interest (ROIs), some individual patient scans were examined to investigate the contributions made by areas of low BMD, because the phantom measurements suggested that these might have a disproportionate effect. Manually selected ROIs were placed over the hands and ribs. Typical measured BMDs in these areas were found to be 0.3–0.4 g/cm2. However, the initial contributions to the total BMC were less than 5%, so that any influences of a BMD threshold would have a negligible effect on the total-body bone changes.
The absorptiometer also measures the fat and lean components of the soft tissue and, in view of the influence these may have on the bone results, the results are presented in Table 3. It will be noted that the fat in the limbs comprises more than 60% of the total and contributes more than 50% of the changes. Changes in the fat component made up 67% of the weight change on average, but 80% in the legs.
Table Table 3.. CONTRIBUTION OF BODY REGIONS TO THE TOTAL BONE AND SOFT TISSUE MASSES AND TO CHANGES, EXPRESSED AS A PERCENTAGE
The results of the reanalysis of scans using the Standard software are shown in Table 4. There were no significant differences between the mean values of ΔBMC, ΔAREA, or ΔBMD for the selected group and the whole population. The slope of the ΔBMC/ΔW plot was little different from the whole population for Enhanced analysis, but steeper for Standard. Because there was no significant difference between the ΔAREA/ΔW plots for the two protocols, the ΔBMD/ΔW relationship was different. The negative slope observed with Enhanced was reversed with Standard. The ΔAREA/ΔBMC plot was steeper for Enhanced. There were significant differences between the mean initial bone measurements, with Enhanced producing a higher BMC and BMD and a lower AREA, included in Table 4. However, it was found that the ratios of Enhanced/Standard results were dependent on the percentage fat of the body as determined by the absortiometer. The parameters of linear regressions of this correlation are also included in Table 4. The two protocols give the same result for all three variables at about 30% fat, but at lower fat proportions. Enhanced Analysis leads to values that are up to 20% higher.
Table Table 4.. LINEAR REGRESSION EQUATIONS OF TOTAL-BODY RESULTS FROM 20 SUBJECTS ANALYZED WITH ENHANCED WHOLE BODY SOFTWARE AND STANDARD SOFTWARE
All subjects also had separate lumbar spine scans on the same visits. There was no dependence of ΔBMC or ΔAREA on ΔW. However, there was a strong correlation between ΔBMC and ΔAREA, although with a much lower slope than that found for the total body, Fig. 4. The linear regression, using percentages, was ΔAREA = 0.312 × ΔBMC + 0.25, r = 0.61, p < 10−10, SEE = 1.6.
Figure FIG. 4.. Plots of AREA and BMC against uniform fat proportion in phantom limb with no variation of bone or soft tissue thickness. The regression equations are:
where ΔF is the change in fat%.
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Software is available to make use of the attenuation data to measure the body thickness at any point in terms of the equivalence of epoxy resin plastic, used as one of the calibration standards. This facility was used to measure the thickness over the abdomen and midthigh of a number of subjects who had changed weight appreciably. Changes of thickness were then correlated with weight change. There was no significant difference between the slopes for the two regions, and a combined slope of 2.4 mm/kg was found.
To ascertain whether anomalies occurred in different populations, the database of total-body scans performed for clinical purposes was examined, and 62 who had had two scans at an interval of at least 1 year were identified. They were not part of a weight change study, but there had been some such changes, in both directions. The SD of ΔW was 4.4 kg, and 25 subjects had changed by more than 2 kg. When the whole group was considered, there was no correlation between total-body ΔBMD and ΔW, but both ΔBMC and ΔAREA rose with ΔW. The parameters of the regression equations are included in Table 1B. The intercepts were not statistically significant. The correlations ceased to be significant when the analysis was confined to the 37 subjects whose weight changed by less than 2 kg. However, for each of the categories, there was a highly significant correlation between ΔAREA and ΔBMC, as presented in Table 2B. The results for the 12 men included in the group were analyzed separately. There were no significant differences of any of the regressions from those of the whole population.
The records of repeated total-body scans undertaken to establish short-term reproducibility were examined. Two measurements had been made without an interval on 17 subjects. There were no systematic differences between them, but there was a significant correlation between the differences of AREA and BMC, Table 2C.
The effect of body thickness on the bone measurements was investigated with the hardboard phantom. When the thickness was reduced by the same proportion in each body compartment, the total-body BMC was unchanged, but the AREA increased significantly, by 2.2% for a 15% thickness reduction and 2.5% for a 25% thickness reduction. There was no further reduction when thickness was reduced to 50% of the original. The consequence of these changes was therefore an increase of BMD with increasing phantom thickness and weight. This is in the opposite direction to the findings in vivo, so the thickness effect does not provide an explanation for the anomaly; rather it dilutes it.
The effect of fat proportion on bone measurements when thickness and weight were almost constant was examined using the Bush phantom thigh section. In the analysis of the results, a fixed ROI was chosen to exclude the polyethylene end plates of the cylinder. It was found that the results were the same with the cylinder in the upper or lower leg position. Mean results from at least six measurements at each point are plotted in Fig. 4 against the fat proportion as recorded by the QDR 1000W. One of the points at 18% fat relates to the cylinder with water alone and the other to a combination of a semicylinder of wax and a solution of 200 g sodium chloride in 2.5 l of water. The close agreement of these points provides a justification of the means used to vary the soft tissue composition.
Both the AREA and BMC appear to change with the fat proportion. There was significant linear regression with percent fat, with similar negative slopes for both bone parameters. There was no significant change of BMD, a correct result because there had been no alteration of the aluminium skeleton. By contrast, the BMC measurements gave a false impression. The similarity of the regression coefficients for AREA and BMC leads to a correlation between ΔAREA and ΔBMC, just like that observed in vivo. However, AREA and BMC in the phantom decrease with increasing fat proportion, whereas they increase in vivo. Once again the results in vitro dilute, rather than explain, the measurements on subjects.
When 4 kg of lard was laid flat on the trunk and thighs of the hardboard phantom, there was a significant reduction of 2% in the total-body BMC but an insignificant reduction of AREA. When the lard thickness was doubled, using 8 kg, the BMC reduction increased to 3% and the ΔAREA became significant at 1%. For both added fat masses, the total-body BMD reduced significantly by 1%. These findings are similar to those observed when fat proportion or thickness were increased separately.
However, when lard was added as an annulus wrapped around the limbs, different results appeared. They are presented in Tables 5 and 6. Table 5 shows the results of measurements of the complete hardboard phantom with and without 1 kg of lard around each arm and 2 kg around each leg. Using the Enhanced protocol, results for the arms and legs were similar, with the addition of the lard leading to increases in both AREA and BMC of 5–7%. There was no significant change of BMD in the limbs, but there was in the total body. Interestingly, there was a highly significant change in the trunk AREA, although no lard was added here. The increase in the BMC of the trunk was not significant and the BMD decreased. The nominal AREA for each arm was 150 cm2, so it was overestimated in all the measurements. The arm BMC derived from the calibration against calcium hydroxyapatite was 139 g, so that was also overestimated. With a nominal arm BMD of 0.925 g/cm2, a correct value was obtained. The nominal leg AREA was 360 cm2. The measured values are below this, but a small proportion of the femur is excluded by the ROIs used. The nominal leg BMC of 416 g and BMD of 1.156 g/cm2 are exceeded.
Table Table 5.. RESULTS OF MEASUREMENTS OF THE COMPLETE HARDBOARD PHANTOM WITH AND WITHOUT THE ADDITION OF 6 KG LARD AROUND THE ARMS AND LEGS
Table Table 6.. RESULTS OF BONE MEASUREMENTS OF HARDBOARD PHANTOM, WITH AND WITHOUT 1 KG LARD WRAPPED AROUND EACH LOWER LEG SECTION, PLACED IN THE ARM, LOWER AND UPPER LEG POSITIONS
With the Standard software there was no significant effect of added lard on total AREA, BMC, or BMD, but BMC and BMD were reduced in the legs. All values of BMC recorded using the Standard protocol were lower than those measured using Enhanced, markedly so in the trunk.
The results of scanning with the lower leg section in the three limb positions are presented in Table 6. Fixed, manually chosen, ROIs were used in the analysis, within the ends of the bones, but wider than the soft tissue boundaries. There were four sets of conditions: with and without aluminium added to the hardboard to increase leanness and with and without added superficial lard, and they need to be considered in pairs. The differences between results from the arm and leg positions also need to be considered. There were no differences between upper and lower leg positions.
The position and soft tissue composition clearly have a big influence on the observed bone mineral parameters. The nominal AREA defined by the ROI and the width of the bone was 146 cm2 and values between 118 and 161 cm2 were observed. The BMC derived from the calibration against calcium hydroxyapatite was nominally 169 g, and the measured values ranged from 159 to 225 g. Section A of Table 6 shows that the degree of initial leanness had no significant effect on AREA or BMC, but altered the BMD by 1–2%. When lard had been added, the basic leanness did affect the bone measurements to a small extent, an increase of fat proportion leading to a 3% reduction of AREA and a 6–10% reduction in BMC. The points for the leg are included in Fig. 5 for comparison with the comparable results from the Bush phantom. The data are rearranged in Table 6B to show the effect of the addition of lard, which is seen to be important only for the arm position, increasing the AREA and BMC by 20–40%, with no change in BMD.
Figure FIG. 5.. Variation of measured BMD with aluminium thickness expressed as nominal BMD in the arms, legs, and spine of the phantom.
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There were some differences between the results from the same lower leg section scanned in the arm and leg positions (Table 6C). The positional effect was itself dependent on the soft tissue composition. With no added lard, the AREA and BMC in the leg position were 15–29% higher than in the arm position.
The effect of an annulus of lard around the limbs in vivo is illustrated in Table 7. Using the Enhanced protocol, the results are very similar to the measurements in vitro presented in Table 5. The linked increases in AREA and BMC are greater in the arms and slightly lower in the legs, but the total body changes are the same, and in each case there is a small but significant reduction of BMD. The use of Standard software gives very similar results in the limbs but introduces increases of AREA and BMC and reductions of BMD in the trunk. The increase in total AREA is greater with Standard software, so that the negative slope of BMD is increased.
Table Table 7.. MEAN RESULTS OF THREE MEASUREMENTS OF VOLUNTEER WITH AND WITHOUT 1 KG LARD WRAPPED AROUND EACH ARM AND 2 KG AROUND EACH LEG
The variations of measured BMD with the thickness of aluminium in the skeleton, translated into nominal BMD from the calibration against calcium hydroxyapatite, are plotted in Fig. 5. The pattern varies with the position in the body. There are thresholds of about 0.4 g/cm2 for the legs and 0.2 g/cm2 for the arms, below which no bone is recorded. Exchange of the arms and legs of the phantom demonstrated that the differences were mainly due to the position in the body, but soft tissue thickness also had an effect. The spine registered some bone when none was present, as did the ribs.
The total-body results included in Fig. 5 were obtained by varying the aluminium thickness in the arms, legs, and spine by the same amount. The lowest total-body BMD plotted, 0.5 g/cm2, was achieved with BMDs of 0.23 g/cm2 in the arms, legs, and spine. The highest total-body BMD of 0.87 g/cm2 corresponds to 0.92 g/cm2 in the variable components. The apparent minimum in the total-body BMD is an anomaly due to AREA rising faster than BMC as aluminium thickness is increased at low BMDs, but it is not of practical significance because the proportion of the skeleton with BMDs below 0.5 g/cm2 is small.
At nominal BMDs above 0.5 g/cm2 for the arms, legs, and spine, the variations of AREA and BMC with aluminium thickness are such that the increases of measured BMD are close to linear. However, Fig. 5 does not reveal clearly how near the slopes are to unity. The measurements were further analyzed to determine the slopes at three BMD levels, and the results are presented as the measured increase in BMD for a true increase of 10% in Table 8. These results show that, at BMD levels similar to those found in humans, real changes of BMD are underestimated by 10–30%. Above a BMD of 0.6 g/cm2, the BMC was closely proportional to the nominal values, so a 10% change of leg, arm, or spine BMC resulted in a deviation from that expected by less than 2%.
Table Table 8.. MEASURED PERCENTAGE INCREASE IN BMD OF PHANTOM FOR A 10% TRUE INCREASE IN THE ARMS, LEGS, AND SPINE, AT THREE INITIAL BMD LEVELS
There was a possibility that the use of aluminium rectangles to simulate the variable parts of the skeleton might place different demands on the bone recognition algorithms from those found in vivo. This point was investigated by substituting aluminium cylinders with the same outer diameter, but different wall thicknesses, in the arms. The resultant pattern of variation of AREA, BMC, and BMD with wall thickness was very similar to that found with rectangles. Because the bone mineral-equivalent volume density of aluminium is somewhat greater than that of the mineral dispersed in cortical bone itself, an alternative model was also used. Thin aluminium sheets of different widths were stacked to create an edge thickness gradient matching that of a thick-walled tube simulating a bone shaft. Again the results were very similar to those obtained with rectangles.
The results presented in Fig. 5 were obtained using the Enhanced Whole Body software. The scans were reanalyzed using Standard software. The results were little different. The BMD threshold was reduced for the legs to 0.2 g/cm2, but increased for the arms to 0.25 g/cm2. A threshold of 0.2 g/cm2 was introduced for the spine, and the spine BMD and BMC were underestimated at higher BMD levels. The slopes of the BMD plots at BMDs greater than 0.6 g/cm2 were similar to those with Enhanced software.
The relevance of the phantom measurements to the results in vivo can be assessed by considering the changes that would follow a weight change of 9 kg or a fat change of 6 kg. The results are summarized and illustrated in Table 9. The soft tissue thickness measurements showed that such a weight change would be associated with a thickness change of 2.2 cm, or 15% of the mean of 15 cm. The hardboard experiments with a constant fat proportion revealed no changes of BMC or AREA with thickness at this initial thickness. At greater thicknesses there was a reduction of AREA and an increase of BMD with increasing thickness. The changes were small and in the wrong direction to explain the observed anomaly.
Table Table 9.. MEASURED PERCENTAGE CHANGES OF TOTAL BMC, AREA, AND BMD FOR AN INCREASE IN BODY WEIGHT OF 9 KG OR FAT MASS OF 6 KG
Changes of fat proportion without a change in thickness could also not provide an explanation. If all of a 9 kg increase in weight were attributed to the fat compartment it would result in an increase in fat proportion from the mean initial 24% to 40%. Figure 4 shows that such an increase would result in AREA reducing by 7.6% and BMC by 8.3%. These are again in the opposite direction to that observed in vivo, although they are compatible with there being no change in BMD.
A change of weight of 9 kg in vivo implies a change of fat of approximately 6 kg, the amount used in the lard annulus studies, the results of which are shown in Tables 5 and 7. The addition of 6 kg of lard led to increases of AREA of 5.5% and BMC of 3.4%, in both phantom and in vivo studies using Enhanced Analysis. In the clinical measurements, a corresponding 9 kg weight increase led to an AREA increase of 5.4% and a BMC increase of 5.4%, taking into account the intercepts, 4.3% and 5.9% increases if the slopes alone are considered. While the similarity of these results may be somewhat fortuitous, given the fact that we did not attempt to simulate fat distribution changes in the trunk, they do demonstrate that the results in vivo could be explained by the inappropriate effect of a realistic change of superficial fat in the limbs, without there being any real change in bone mass. The associated apparent change of BMD is in the opposite direction, but the magnitude of the error is less than that of ΔBMC and ΔAREA.
With the Standard Analysis, the annular lard had a similar effect on the total-body BMC and AREA of the volunteer to that with the Enhanced Analysis but no significant effect on the phantom. Weight change in vivo produced greater increases of BMC and AREA with the Standard Analysis.