Triple photon absorptiometry cannot correct for fat inhomogeneities in lumbar spine bone mineral measurements

Phantom Studies of Triple Photon Absorptiometry and Bone Mineral Measurement at a Hip Prosthesis

The feasibility of using triple photon absorptiometry (TPA) for the measurement of bone mineral mass about a hip prosthesis was examined. A theoretical expression describing the variance of TPA measurements was verified using a triple photon source and phantom materials which simulate the soft tissue-bone mineral-metal prosthesis system. The expression for the variance was used to determine an optimized set of photon energies. It was shown that a precision of 3% could be obtained for reasonable measurement times using this optimized set of energies, and that TPA should be a feasible approach for measurement of bone mineral about a hip prosthesis.

The effect of bone strontium on BMD is different for different manufacturers' DXA Systems

Osteoporotic patients treated with strontium ranelate show relatively large increases in bone mineral density (BMD) measured by dual-energy X-ray absorptiometry (DXA) due to the replacement of some of the calcium atoms in bone by strontium. A study published by Pors Nielsen and colleagues reported that replacement of 1% of calcium atoms by strontium causes a 10% increase in BMD. We refer to the ratio of the percentage increase in BMD to the molar percentage of strontium in bone as the strontium ratio. Theoretically it is expected that the strontium ratio should vary between different manufacturers' DXA equipment depending on the effective photon energy of the device, an effect that arises because of the proximity of the X-ray energies produced by lower energy devices to the strontium K-edge at 16 keV. In this study we report theoretical estimates of the strontium ratio for two axial DXA systems and two peripheral DXA devices based on their broad spectrum X-ray emission. The theoretical figures were verified in an experimental study in which the strontium ratio for each device was measured using phantoms containing mixtures of hydroxyapatite and strontium hydrogen-phosphate. The theoretical values of the strontium ratio were 11.0 for the Hologic Discovery, 9.9 for the GE-Lunar Prodigy, 9.1 for the Demetech Calscan, and 8.5 for the Osteometer Dexacare G4. Experimental results were 11.2 for the Discovery, 9.9 for the Prodigy, 8.6 for the Calscan and 6.3 for the Dexacare G4. The results confirm both theoretically and experimentally that the effect of bone strontium on BMD measurements is different for different DXA systems. In the future it might be possible to exploit this effect to make a non-invasive estimate of average bone strontium content in groups of patients receiving strontium medication for osteoporosis.

Bone densitometry using x-ray spectra

In contrast to the two distinct energy regions that are involved in dual-energy x-ray absorptiometry for bone densitometry, the complete spectrum of a beam transmitted through two layers of different materials is utilized in this study to calculate the areal density of each material. Test objects constructed from aluminum and Plexiglas were used to simulate cortical bone and soft tissue, respectively. Solid-state HPGe (high-purity germanium) detectors provided high-resolution x-ray spectra over an energy range of approximately 20-80 keV. Areal densities were obtained from spectra using two methods: a system of equations for two spectral regions and a nonlinear fit of the entire spectrum. Good agreement with the known areal densities of aluminum was obtained over a wide range of PMMA thicknesses. The spectral method presented here can be used to decrease beam hardening at a small number of bodily points selected for examination.

Photon absorptiometry, bone densitometry and the challenge of osteoporosis

During the lifetime of Physics in Medicine and Biology, osteoporosis has been recognized as the cause of a major health burden for societies, particularly within developed countries. The health detriment is associated with the consequences of bone fractures and the subsequent increases in morbidity and mortality. Much of the credit for the current availability of means for identifying groups of subjects at risk of fracture and the provision of means for the effective treatment of excessive bone loss can be attributed to the technique of dual photon absorptiometry. In this review, the history of the development of techniques based on the interactions of x- and gamma-rays with bone is considered and the ultimate dominance of x-ray based absorptiometry is described. The advantages and disadvantages of current absorptiometric techniques are presented and the likely future path for bone measurement is outlined.

Fracture threshold in the femur and tibia of people with spinal cord injury as determined by peripheral quantitative computed tomography

OBJECTIVE

To determine bone traits of the femur and tibia with peripheral quantitative computed tomography (pQCT) that best distinguish between spinal cord injury (SCI) subjects with and without fractures.

DESIGN

Cross-sectional study.

SETTING

In- and outpatient paraplegic center in Switzerland.

PARTICIPANTS

Ninety-nine motor complete SCI subjects (duration of paralysis, 2 mo-49 y), 21 of whom had sustained fractures of the femur or tibia.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Subjects with SCI were questioned about the occurrence, location, and approximate date of fractures to their lower extremities. Trabecular and cortical bone mineral density (BMD), as well as bone geometric properties of distal epiphyses and midshafts of the femur and tibia, were measured by pQCT.

RESULTS

Trabecular BMD of the femur and tibia distal epiphyses was found to distinguish best subjects with fractures from those without. Fractures occurred in subjects with trabecular BMD of less than 114 mg/cm 3 and less than 72 mg/cm 3 for the femoral and tibial distal epiphysis, respectively (corresponding to 46% and 29% of mean values of an able-bodied reference group). Approximately 50% of the subjects with chronic SCI (defined as time postinjury >5 y for femur data and >7 y for tibia data) had trabecular BMD values above the fracture threshold in the femur and about one third above the fracture threshold in the tibia.

CONCLUSIONS

By using pQCT, it may be possible to identify subjects with SCI who are at risk of sustaining fractures of the femur and tibia through minor trauma.

Assessment of anthropometric, systemic, and lifestyle factors influencing bone status in the legs of spinal cord injured individuals

The aim of the present study was to assess the influence of muscle spasms, systemic or lifestyle factors on bone mass and geometry of the femur and the tibia in people with long-standing spinal cord injury (SCI). Fifty-four motor complete SCI people with paralysis duration of between 5 and 50 years were included in the study. Spasticity was measured by means of the Ashworth scale. Distal epiphyses and mid shafts of the femur, tibia, and radius were measured by peripheral quantitative computed tomography. From the epiphyseal scans, trabecular and total bone mineral density (BMDtrab and BMDtot) were calculated, and from the shaft scans, cortical BMD (BMDcort), total and cortical cross-sectional area (CSAtot and CSAcort), and muscle cross-sectional areas (CSAmus) were determined. Personal characteristics, anthropometric, as well as life-style factors, were assessed by means of a questionnaire. A Spearman correlation matrix was produced with measured data. Correlation coefficients exceeding 0.3 were tested for significance by performing linear regression for parametric data and ANOVA for non-parametric data. Subjects with higher spasticity scores had significantly larger CSAmus in the upper and lower leg. Both spasticity and CSAmus were found to be significantly related to BMDtrab and BMDtot of the distal epiphysis of the femur and to CSAcort of the femoral shaft. In the lower leg, bone parameters of the tibia were found to be strongly related to corresponding bone parameters of the radius, which suggests a systemic origin. No significant relationships were found between bone parameters and any of the life-style factors. The extent of bone loss caused by disuse of the lower extremities in people with long-standing SCI is influenced by systemic factors. Additionally, spasticity has a positive effect on bone parameters of the femur.

Relationship between the duration of paralysis and bone structure: a pQCT study of spinal cord injured individuals

The aim of the present study was to describe bone loss of the separate compartments of trabecular and cortical bone, as well as changes in bone geometry of a large number of spinal cord injured (SCI) individuals. Eighty-nine motor complete spinal cord injured men (24 tetraplegics and 65 paraplegics) with a duration of paralysis of between 2 months and 50 years were included in the study. Distal epiphyses and midshafts of the femur, tibia, and radius were measured by peripheral quantitative computed tomography. The same measurements were performed in a reference group of 21 healthy able-bodied men of the same age range. In the femur and tibia, bone mass, total and trabecular bone mineral density (BMDtot and BMDtrab, respectively) of the epiphyses, as well as bone mass and cortical cross-sectional area of the diaphyses, showed an exponential decrease with time after injury in the spinal cord injured subjects. The decreasing bone parameters reached new steady states after 3-8 years, depending on the parameter. Bone mass loss in the epiphyses was approximately 50% in the femur and 60% in the tibia, while the shafts lost only approximately 35% in the femur and 25% in the tibia. In the epiphyses, bone mass was lost by reducing BMD, while in the shaft bone mass was lost by reducing cortical wall thickness, a process achieved by endosteal resorption advancing at a rate of about 0.25 mm/year within the first 5-7 years after injury. Except for a slight transient decrease in cortical BMD of the femoral and tibial shaft during the first 5 years after the spinal cord lesion, cortical BMD of the spinal cord injured subjects was found to be at reference values. Bone parameters of the radial epiphysis in paraplegic subjects showed no deficits compared to the reference group. Furthermore, a trend for an increased radial shaft diameter suggests periosteal apposition as a consequence of increased loading of the arms.

Inaccuracies inherent in dual-energy X-ray absorptiometry in vivo bone mineral densitometry may flaw osteopenic/osteoporotic interpretations and mislead assessment of antiresorptive therapy effectiveness

New, anatomically realistic simulation studies based on a cadaveric lumbar vertebra and a broad range of soft tissue anthropometric representations have quantitatively delineated inaccuracies inherent in dual-energy X-ray absorptiometry (DXA) in vivo bone mineral density (BMD) methodology. It is found that systematic inaccuracies in DXA BMD measurements may readily exceed +/-20% at typical in vivo lumbar vertebral sites, especially for osteopenic/osteoporotic, postmenopausal, and elderly patients. These findings are quantitatively compared with extensive clinical evidence of strong, positive correlations between soft tissue anthropometrics and DXA in vivo BMD upon which prior significant bone biology interpretations and implications have been based. The agreement is found to be both qualitatively and quantitatively excellent. Moreover, recent extensive multicenter clinical studies have also exposed new facets of strong linkages between body mass/percent body fat/body mass index (BMI) and DXA-measured BMD that are particularly relevant to osteopenia/osteoporosis and remedial effectiveness of antiresorptive drug therapy. These seemingly disparate and unrelated diagnostic and prognostic aspects of clinically observed associations between soft tissue anthropometrics and measured vertebral BMD are, in this study, self-consistently shown to share the common origin of being manifestations of systematic inherent inaccuracies in DXA in vivo BMD methodology, without the need to invoke any underlying biologically causal mechanism(s). These inaccuracies arise principally from absorptiometric disparities between the intra- and extraosseous soft tissues within the DXA scan region of interest. The present evaluative comparisons are based exclusively on an incisive and diverse body of clinical data that appears difficult to dismiss or discount. Previous invocations of biologically causal mechanisms responsible for this broad range of observations linking body mass, percent body fat, and/or BMI to measured BMD now appear questionable. This doubtful status has also been extended in the present work to previously reported relationships between antiresorptive therapies and observed changes in DXA-derived BMD. These findings strongly indicate that critical and insightful reassessments of diagnostic/prognostic imputations underpinned by DXA in vivo BMD measurements are warranted. It is suggested that a good deal of what is known of bone fragility, bone densitometry, antiresorptive drug efficacy, and/or other therapeutic regimens, if based on patient-specific in vivo DXA methodology, may prove to be equivocal and tenuous.

Monte Carlo modelling of an extended DXA technique

The precision achieved in measuring bone mineral density (BMD) by commercial dual-energy x-ray absorptiometry (DXA) machines is typically better than 1%, but accuracy is considerably worse. Errors, due to inhomogeneous distributions of fat, of up to 10% have been reported. These errors arise because the DXA technique assumes a two-component model for the human body, i.e. bone mineral and soft tissue. This paper describes an extended DXA technique that uses a three-component model of human tissue and significantly reduces errors due to inhomogeneous fat distribution. In addition to two x-ray transmission measurements, a measurement of the path length of the x-ray beam within the patient is required. This provides a third equation, i.e. T = ts + tb + tf where T, ts, tb and tf are the total, lean soft tissue, bone mineral and fatty tissue thicknesses respectively. Monte Carlo modelling was undertaken to make a comparison of the standard and extended DXA techniques in the presence of inhomogeneous fat distribution. Two geometries of varying complexity were simulated. In each case the extended DXA technique produced BMD measurements that were independent of soft tissue composition whereas the standard technique produced BMD measurements that were strongly dependent on soft tissue composition. For example, in one case, the gradients of the plots of BMD versus fractional fat content were for standard DXA (-0.183+/-0.037) g cm(-2) and for extended DXA (0.027+/-0.044) g cm(-2). In all cases the extended DXA method produced more accurate but less precise results than the standard DXA technique.

The feasibility of triple-energy absorptiometry for the determination of bone mineral, Ca and P in vivo

The theoretical feasibility of triple-energy absorptiometry in general and the experimental conditions when using triple-energy absorptiometry for the determination of bone mineral, elemental calcium and phosphorus content in vivo have been investigated. A theoretical analysis of the decomposition of the mass attenuation coefficients is presented and discussed. The main obstacle to the effective use of triple-energy absorptiometry in vivo is the large number of pulses which must be detected to reduce the statistical fluctuations.

Measurement of bone mineral using multiple-energy x-ray absorptiometry

Our laboratory has previously reported a method of determining the amount of bone mineral using triple-energy absorptiometry with a continuous x-ray spectrum. In the present study, the experimental properties of the technique were examined. The accuracy, the influence of fat content and body thickness and the in vitro and in vivo precision were analysed. The results found in this investigation showed that despite the complexity of the technique, the amount of bone mineral can be accurately determined. The in vivo precision was determined to be 3.4%, expressed as the coefficient of variation (CV), for different skeletal parts. The in vitro precision was found to be 2.1% (CV). Neither the fat content nor the body thickness had any effect on the measured bone mineral values. Excellent linearity and a close correlation were found between the true and the measured bone mineral values.

Precision and accuracy of measurements of whole-body bone mineral: comparisons between Hologic, Lunar and Norland dual-energy X-ray absorptiometers

Measurements of whole-body bone mineral made by Hologic, Lunar and Norland dual-energy X-ray absorptiometers have been compared. It was found that in each case the results were changed by new software protocols introduced by the manufacturers during the course of the study. With a moderately anthropomorphic model, the later software corrected some anomalies of regional bone mineral content (BMC) observed earlier. There was some slight dependence of total BMC on thickness and fat proportion and up to 15% difference between instruments. Measurements on volunteers showed good precision, but there were differences between instruments made by different manufacturers. There were high correlations, but the slopes of regression lines suggested differences of calibration of up to 8%; the standard errors of the estimates were 110 to 190 g. with maximum deviations from regression of 17%. There were regional disparities in BMC, particularly in the trunk, which arise (in part at least) from the imposition of a higher bone threshold by Hologic. From the pattern of results it was concluded that different assumptions were made by the manufacturers, particularly concerning the fat distribution model, which preclude the interchangeability of results from different instruments.

Bone mineral and fat measurement with a novel dual photon absorptiometer

The authors have produced an original dual-photon absorptiometer, with specific algorithms, permitting real-time automatic bone recognition, to reduce examination time (to 5 min). It permits the simultaneous measurement of the mineral density of an area of bone (BMD), the fat percentage of the soft tissues (PFST) that surround it, and the variation in this percentage. It visually represents, on one and the same picture, freely defined in terms of dimension and definition, bone mineral distribution and fat mass distribution. The performances are analysed, using phantoms and taking ten control subjects, in the region of the femoral neck. The variations in PFST in the control subjects are compared with the measurements carried out in MRI in the same region. In all these control subjects, there is a regular decrease in PFST in this region along an upper external/lower internal axis. This decrease has been likened to a regression line, the slope of which, representing between -0.16 per cent of fat per cm and -1.7 per cent per cm, has a mean value of -0.81 per cent per cm. The BMD is thus reduced by 0-2.5 per cent, depending on the subjects.