X-ray Fluorescence Measurements Of Lead Research Articles

Improvements and reproducibility of an optimal grazing-incidence position method to L-shell x-ray fluorescence measurements of lead in bone and soft tissue phantoms

L-shell x-ray fluorescence (LXRF) is a non-invasive approach to lead (Pb) concentration measurements in human bone. The method is based on the detection of the characteristic x-ray photons of Pb at 10.5 and 12.6 keV and experimental studies were designed to perform in vivo human bone Pb measurements. In later studies, however, the initial LXRF methodology was shown to have poor accuracy and precision. In a recent publication, we investigated an optimal grazing-incidence position (OGIP) approach using a sub-millimeter x-ray beam from an integrated x-ray tube and polycapillary x-ray lens table-top system. The OGIP method effectively reduced the x-ray scatter and produced a Pb detection limit of ∼5 μg/g for a 2 mm soft tissue phantom thickness. In this study, the OGIP methodology was improved by using 10 s x-ray spectra acquisitions at sequential positions 0.5 mm apart. The measured Sr Kα peak height versus position data was used to spectroscopically identify the bone phantom and the OGIP. The data was fitted with the analytical convolution between a Gaussian and an exponential decay. The position corresponding to the maximum of the fitted convolution function was then selected as the OGIP. Four phantom sets were used. A cylindrical plaster-of-Paris bone phantom doped with Pb in a concentration of 74 μg/g was used as a bare bone phantom or with one of the three overlying polyoxymethylene cylindrical shell soft tissue phantoms of 1, 2, and 3 mm thickness. The reproducibility of the OGIP method was assessed in five independent trials using each of the four phantom sets. The coefficient of variation (COV) percentage values of the Sr Kα peak height measurements were below 5%. The new procedure decreased by more than threefold the duration and radiation dose of the earlier approach.

THE INFLUENCE OF BONE-DENSITY ON IN VIVO K X-RAY FLUORESCENCE BONE-LEAD MEASUREMENTS

Mathematical simulations and benchmark measurements were performed to assess the impact that normal variations in human calcium content have on in vivo K x-ray fluorescence measurements of lead in bone. Four sets of cortical bone tissue simulants were fabricated containing from 20.8% to 23.8% calcium (by weight) for measurement in a surrogate (phantom) of the human leg. The net counts detected in the coherent backscatter peak at 88.034 keV using a Cd source indicate a positive trend, with a variability of up to 17% over the range of assessed calcium content. Mathematical simulations confirm this trend and also demonstrate that the contribution of 87.3 keV Pb Kβ2 counts, which are unresolved in measurements, do not contribute significantly to the coherent peak at low levels of bone-lead content. Both measurements and simulations confirm that calcium is a statistically significant parameter in predicting the K-XRF response and suggest that lead levels may be over-predicted for individuals having low bone density compared to the calibration matrix. Simulations identify a 4.5% negative bias in measured lead values for each 1% increase in calcium weight percent in the bone matrix as compared to the calibration matrix. It is therefore important to accommodate this uncertainty when performing epidemiological studies of populations having a wide range of bone densities.

The effect of filters and collimators on Compton scatter and Pb K-series peaks in XRF bone lead analysis

The utility of Al, Cu and In filters and collimators of different diameters in the X-ray fluorescence measurement of lead in the bone was investigated. The changes in Compton scattering and Pb K-series peaks were analyzed with different filters and collimators. Compton scattering and fractional dead time were significantly reduced with the use of filters. The FWHM values of Pb Kα1, Kβ1,3 peaks decrease in a nearly linear relation with the ratio of length of collimator to the diameter as the small diameter collimator with long length reduces divergence of spectra, thus improves resolution. The measurement precisions of Kα1 and Kβ1,3 peaks were improved by applying an indium filter. The ratio of signal-to-noise slightly increases when an indium filter was used.

The effect of covariance between the Kα and the Kβ lead peak concentrations on the uncertainty in the result of in vivo 109Cd KXRF bone lead measurement

The effect of covariance between the Kα and Kβ lead peak concentrations on the uncertainty in the 109Cd K x-ray fluorescence measurement of lead in bone is addressed here. It is commonly believed that this covariance arises as a result of the mutual dependence of the ratios of x-ray to coherent amplitudes on the same coherent peak amplitude. Previous work assumes statistical independence between spectral quantities of interest, crudely estimates the uncertainties in the lead peak concentrations, and suggests that the effect of covariance on the measurement uncertainty is small and can be ignored. Consequently, the current method followed by most laboratories reports the measurement uncertainty as if the fluctuations in the measured peak concentrations were independent. The robustness of such assumption, however, is undermined by existing epidemiological data. This paper assesses the magnitude of the covariance effect, using a method based on the observed significant correlations between the ratios of x-ray to coherent peak amplitudes in series of repeat phantom measurements. The revised uncertainties following this approach can exceed the uncertainties estimated by the accepted method by as much as 40%, which suggests a much stronger effect of covariance on the measurement uncertainty than previously reported.

Curve fitting using a genetic algorithm for the X-ray fluorescence measurement of lead in bone

We investigated the potential application of the genetic algorithm in the analysis of X-ray fluorescence spectra from measurement of lead in bone. Candidate solutions are first designed based on the field knowledge and the whole operation, evaluation, selection, crossover and mutation, is then repeated until a given convergence criterion is met. An average-parameters based genetic algorithm is suggested to improve the fitting precision and accuracy. Relative standard deviation (RSD%) of fitting amplitude, peak position and width is 1.3-7.1, 0.009-0.14 and 1.4-3.3, separately. The genetic algorithm was shown to make a good resolution and fitting of K lines of Pb and g elastic peaks.

Effects of measurement distance and source activity on the precision of X-ray fluorescence measurements of lead in a bone phantom

The effects of measurement distance and source activity on the measurement of lead in a plaster-of-Paris bone phantom were investigated for a 109Cd source K-shell X-ray fluorescence system. Measurement precision was assessed from analyses of average measurement uncertainty and reproducibility. For a source activity of 1000 MBq, a source to sample distance of 9 mm and sample to detector distance of 25 mm were identified as yielding superior measurement precision. Source activities of 710 and 320 MBq produced superior precision results with a source to sample distance of 4 mm and sample to detector distance of 20 mm. Significant improvements of 18% in average measurement uncertainty and 39% in reproducibility were observed when compared with the previously standard distance protocol. Individual estimates of measurement uncertainty tended to underestimate measurement reproducibility. An effect on measured phantom lead concentration as a function of measurement distance was observed, with closer range trials producing slightly lower concentrations of lead when using the same bone phantom. Reductions in 109Cd source activity affected measurement precision adversely, but not to the degree expected from a consideration of radiation intensity alone.

L-shell x-ray fluorescence measurements of lead in bone: accuracy and precision

This study aimed to quantify the accuracy and precision of a method for in vivo measurements of lead in bone using L-shell x-ray fluorescence (LXRF), the former via comparison with independent measurements of lead in bone obtained using electrothermal atomic absorption spectrometry (AAS) following acid digestion. Using LXRF, the lead content of adult human cadaver tibiae was measured, both as intact legs and as dissected tibiae with overlying tissue removed, the latter at several proximal–distal locations. After LXRF, each tibia was divided into nine cross-sectional segments, which were further separated into tibia core and surface samples for AAS measurement. The proximal–distal variability of AAS-measured core and surface tibia lead concentrations has been described elsewhere (the lead concentration was found to decrease towards both ends of the tibia). The subjects of this paper are the proximal–distal variability of the LXRF-measured lead concentrations, the measurement uncertainty and the statistical agreement between LXRF and AAS. There was no clear proximal–distal variability in the LXRF-measured concentrations; the degree of variability in actual tibia lead concentrations is far less than the LXRF measurement uncertainty. Measurement uncertainty was dominated by counting statistics and exceeded the estimate of lead concentration in most cases. The agreement between LXRF and AAS was reasonably good for bare bone measurements but poor for intact leg measurements. The variability of the LXRF measurements was large enough, for both bare bone and intact leg measurements, to yield grave concerns about the analytical use of the technique in vivo.

L-shell x-ray fluorescence measurements of lead in bone: theoretical considerations

This paper reviews several theoretical considerations pertinent to the use of lead L-shell x-rays for the in vivo measurement of lead in bone: the method of correcting for attenuation, the contributions to the measurement uncertainty, interferences, the depth of bone sampled and the signal strength. Both the predicted bone lead concentration and the measurement uncertainty therein are influenced by the choice of linear attenuation coefficient with which to correct for overlying tissue. Measurement uncertainty is also influenced by inter-individual variability in body composition, methodological uncertainty in the ultrasound measurement of overlying tissue thickness and discrepancy between the site of LXRF and the site of ultrasound measurement. Interference with the Pb Lα x-rays by As Kα has been overstated and is probably negligible; interference from lead in non-bone tissues may not be. The depth of bone from which the signal is obtained and a crude estimate of signal strength are calculated for different bone compositions for both K and L x-ray fluorescence.

L-shell x-ray fluorescence measurements of lead in bone: system development

This paper reports on the development of an L-shell x-ray fluorescence (XRF) bone lead measurement system. A secondary target gave greater lead x-ray peak signal-to-background ratios than partially plane polarized XRF. Filtration did not improve the lead x-ray peak signal-to-background ratio: the gains in spectrum quality were outweighed by the losses caused by attenuation. There was a substantial matrix effect: the signal from a calcium-rich matrix was far lower than that from a calcium-free matrix. The effect of attenuation was, as expected, profound for the lead L x-rays: detection limits ranged from 18 to 217 μg Pb/g plaster with attenuation equivalent to 0–2.1 mm of skin or 0–3.7 mm of adipose tissue for the Pb Lα x-ray group (10.5 keV), and from 16 to 184 μg Pb/g plaster with attenuation equivalent to 0–1.3 mm of skin or 0–2.3 mm of adipose tissue for the Pb Lβ x-ray group (12.6 keV).

The Monte Carlo modelling of in vivo X-ray fluorescence measurement of lead in tissue

A Monte Carlo model has been developed, using the EGS4 code, to model the in vivo X-ray fluorescence (XRF) measurement of Pb in non-superficial bone/tissue. Unlike previous work in this field the current model incorporates a correction for Doppler broadening of the Compton scatter peak due to the electron momentum distribution of the medium (tissue/water) in which the photons are Compton scattered by convolving the Compton peak of the Monte Carlo generated spectrum with a modified Compton profile for water. This correction improves the agreement between the measured spectral shape obtained using an experimental in vivo X-ray fluorescence Pb analyser with a 109Cd/180 degrees source/geometry combination, measuring a bone phantom at depth in water and the generated spectral shape obtained from the equivalent Monte Carlo model. The model enables improved estimates to be made of the spectral background beneath the Pb K alpha 1 and K alpha 2 X-ray peaks compared with estimates based on simpler models that assume that Compton interactions are with 'free' electrons and hence permits better optimization of in vivo analyser system design.

In Vivo X-Ray Fluorescence of Lead and Other Toxic Trace Elements

Abstract The first in vivo x-ray fluorescence measurements of lead in bone used y-rays from a 57Co source to excite Pb K x-rays. Later systems used γ-rays from 109Cd to excite Pb K x-rays or polarized x-rays to excite Pb L x-rays. All three approaches involve an extremely low effective dose to the subject. Of the two K x-ray techniques, 109Cd is more precise and more flexible in choice of measurement site. Pb L x-ray fluorescence (L-XRF) effectively samples lead at bone surfaces, whereas Pb K x-ray fluorescence (K-XRF) samples through the bulk of a bone. Both the polarized L-XRF and 109Cd K-XRF achieve similar precision. Renal mercury has recently been determined using a polarized x-ray source. Both renal and hepatic cadmium can be measured using polarized x-rays in conjunction with a Si(Li) detector. Platinum and gold have been measured both by radioisotopic source excitation and by using polarized x-rays, but the latter is to be preferred. Applications of Pb K-XRF have shown that measured bone lead relates strongly to cumulative lead exposure. Secondly, biological half lives of lead in different bone types have been estimated from limited longitudinal data sets and from some cross sectional surveys. Thirdly, the effect of hone lead as an endogenous source of lead has been demonstrated and it has been shown that a majority of circulating blood lead can be mobilized from bone, rather than deriving from new exposure, in some retired lead workers.

The factors affecting in vivo x-ray fluorescence measurements of lead in bone

The factors affecting the sensitivity of X-ray fluorescence determination of lead in bone phantoms using 57Co are investigated with the object of applying the technique to in vivo measurements in tibia. An analysis of the components of the measured pulse height spectrum in a hyperpure Ge detector is presented and the implications for the optimisation of the source-detector configuration discussed. Finally, some preliminary investigations using a 109Cd source are outlined.