Joanne M. O’Meara

A study of MDL improvement for the in vivo measurement of lead in bone

Αbstract 1 0 9 Cd induced K XRF has been used for in vivo lead measurement for about two decades. Recently, the need for improvement of this system has been emphasized due to an increased understanding of low-level lead exposure. The conventional XRF bone-lead measurement system includes a 25 mm radius HPGe detector and one set of electronics. In this work, a cloverleaf detector system is investigated. This system consists of four 8 mm radius detectors and four sets of electronics. We measured bare plaster of Paris phantoms and phantoms in a soft tissue equivalent leg phantom to represent in vivo measurement. A Monte Carlo simulation for XRF measurement was also used to simulate this system. We compared both the experimental results and the simulation and found that the minimum detectable limit (MDL) is greatly improved by using the cloverleaf system and a stronger source. The effect of geometry is also discussed. An overall MDL ratio of about 0.3 (experimental value 0.278 ′ 0.016 and simulation value 0.273) is obtained by using the cloverleaf system compared to the conventional system for in vivo measurement, which means a decrease of MDL from about 6-10 to about 2-3 μg/(g bone mineral).

Feasibility of measuring arsenic and selenium in human skin using in vivo x-ray fluorescence (XRF)--a comparison of methods.

In recent years, in vivo measurement systems of arsenic in skin by K-shell x-ray fluorescence (XRF) have been developed, including one which was applied in a pilot study of human subjects. Improved tube-based approaches suggest the method can be further exploited for in vivo studies. Recently, it has been suggested that selenium deficiency is correlated with arsenic toxicity. A non-invasive measurement of both elements could therefore be of potential interest. The main aim of this current study was to evaluate and compare the performance of an upgraded portable XRF system and an advanced version of the benchtop XRF system for both selenium and arsenic. This evaluation was performed in terms of arsenic and selenium Kα detection limits for a 4W gold anode Olympus InnovX Delta portable analyzer (40 kVp) in polyester resin skin-mimicking phantoms. Unlike the polychromatic source earlier reported in the literature, the benchtop tube-based technique involves monochromatic excitation (25 W silver anode, manufactured by x-ray optics, XOS) and a higher throughput detector type. Use of a single exciting energy allows for a lower in vivo dose delivered and superior signal-noise ratio. For the portable XRF method, arsenic and selenium minimum detection limits (MDLs) of 0.59  ±  0.03 ppm and 0.75  ±  0.02 ppm respectively were found for 1 min measurement times. The MDLs for arsenic and selenium using the benchtop system were found to be 0.35  ±  0.01 ppm and 0.670  ±  0.004 ppm respectively for 30 min measurement times. In terms of a figure of merit (FOM), allowing for dose as well as MDL, the benchtop system was found to be superior for arsenic and the two systems were equivalent, within error, for selenium. We shall discuss the performance and possible improvements of each system, their ease of use and potential for field application.

Optimization of an in vivo X-ray fluorescence mercury measurement system

Abstract A non-invasive in vivo X-ray fluorescence (XRF) method of measuring renal mercury concentrations has previously been reported, as a potential occupational monitoring tool for those who work with this toxic element [Phys. Med. Biol. 40 (1995) 413]. However, the detection limits remain high compared to the typical values anticipated in these populations. Our approach for further enhancing the XRF renal mercury detection limit has been threefold: investigations of the ideal filtration and tube voltage with a conventional tungsten anode X-ray tube, and the replacement of the existing tungsten X-ray tube with a Fluorex tube [Phys. Med. Biol. 36 (1991) 1573]. In all cases the systems were compared by Monte Carlo simulation to that reported by Borjesson et al. [Phys. Med. Biol. 40 (1995) 413]. The optimal filtration was found to be a 0.035 cm uranium filter, positioned after the polarizer. Modest improvement was achieved by increasing the tungsten tube voltage from 160 [Phys. Med. Biol. 40 (1995) 413] to 200 kV, decreasing the system detection limit by 27% for the same subject dose. It was found that the Fluorex tube did not improve the system sensitivity for a given dose rate, either when the tube was used for direct excitation or in a polarized configuration. Despite the improved performance reported here at 200 kV, detection limits remain high compared to typical levels in occupationally exposed individuals.

Performance comparison of two Olympus InnovX handheld x-ray analyzers for feasibility of measuring arsenic in skin in vivo – Alpha and Delta models

The Figure-Of-Merit (FOM) performance, a combination of detection limit and dose, is compared between two generations of handheld X-Ray Fluorescence (XRF) spectrometers for the feasibility of in vivo XRF measurement of arsenic (As) in skin. The Olympus InnovX Delta model analyzer (40 kVp using either 37 or 17μA) was found to show improvements in Minimum Detection Limit (MDL) using arsenic As-doped skin calibration phantoms with bulk tissue backing, when compared to the first generation InnovX Alpha model (40kVp, 20μA) in 120s measurements. Differences between two different definitions of MDL are discussed. On the Delta system, an MDL of (0.462±0.002) μg/g As was found in phantoms, with a nylon backing behind to mimic bulk tissue behind skin. The equivalent and effective doses were found to be (10±2) mSv and ~7×10-3μSv respectively for the Alpha and (15±4) mSv and ~8×10-3μSv respectively for the Delta system in 120s exposures. Combining MDL and effective dose, a lower (better) FOM was found for the Delta, (1.7±0.4) ppm mSv1/2, compared to (4.4±0.5) ppm mSv1/2 for the Alpha model system. The Delta analyzer demonstrates improved overall system performance for a rapid 2-min measurement in As skin phantoms, such that it can be considered for use in populations exposed to arsenic.