S. Mattsson^a and J. Börjesson^b
aDepartment of Radiation Physics, Lund University, Malmö University Hospital, SE-205 02, Malmö, Sweden
^bDepartment of Diagnostic Radiology, County Hospital, SE-310 85 Halmstad, Sweden

Introduction

X-ray fluorescence (XRF) is the emission of characteristic secondary (or fluorescent) x-rays from a material that is excited by bombardment of high-energy x-rays or gamma rays. The process in which a photon is absorbed by an atom by transferring all of its energy to an electron is called the photoelectric effect.

Secondary to this process, provided that the photon has a sufficient energy, an electron is ejected primarily from an inner shell, creating a vacancy. This is an unstable condition for the atom and when it returns to its stable condition, an electron from an outer shell is transferred to one of the inner shells. In this process, a characteristic x-ray, whose energy matches the difference between the two binding energies of the corresponding shells, may be created. Because each element has a unique set of energy levels, each element produces a unique set of x-ray energies, allowing us to non-destructively measure the elemental composition of a sample.

The principle of XRF has been known since 1913, when Henry G.J. Moseley (University of Manchester, UK) found the relation between the atomic number of an element (Z) and the wavelength, or energy, of the characteristic x-rays emitted. Moseley measured and plotted the x-ray frequencies for about 40 elements in the periodic table. He showed that the Kα x-rays followed a straight line if Z versus the square root of frequency was plotted, i.e. the Moseley plot. For measurements, Moseley used an x-ray spectrograph with a flat crystal and a photo plate. It had shortly before been constructed by W.H. and W.L. Bragg.

In the early years, the XRF technique was used for element identification, but was not very useful for quantitative analysis and it was not until the development and broader introduction of solid-state detectors for g- and x-ray spectrometry that the technique started to develop. The first applications were for quantitative and specific analysis in metallurgy, the mining industry and metal prospecting. Later it was also used for art identification purposes and for research in geochemistry and archaeology. Similar types of equipment were occasionally used for measurement on human samples, especially to solve forensic problems.

Later on, interest for measurements on human samples of blood, urine, hair, nails, skin, biopsies etc. has grown steadily, benefiting from the intensive development of new x-ray technologies, e.g. µ-XRF, total reflection XRF (TXRF), the advantages of polarised x-ray beams and synchrotron radiation röntgen-fluorescence analysis (SR-RFA). In parallel, there has been a dramatic development of alternative analytical techniques for trace element analysis like atomic absorption spectrometry, inductively coupled plasma atomic emission and mass spectrometry, ICP-AES and ICP-MS, respectively, in the last two decades.

The ultimate use of XRF for medical analysis is in vivo measurements made directly in the living patient or volunteer. It started with quantitative analysis of iodine in the human thyroid. The idea sprang from the pioneering work by Jacobsson, who developed a technique for subtraction radiology of iodine using two x-ray energies, one above and one below the K-absorption edge of iodine. Hoffer et al. realised that if that technology worked, there should be a chance to see the emitted characteristic x-rays from iodine using the semiconductor detectors, which at that time had been developed for nuclear and particle physics. In this way the first in vivo XRF analysis was done, quantifying the iodine concentration in human thyroid, typically around 400 µg g^–1.

The further development of the in vivo XRF technique was related to the analysis of heavy elements, first covering lead and later cadmium and to some extent also mercury in occupationally exposed workers. Platinum was also analysed to investigate uptake and kinetics of the cytostatic agent cis-platinum in tumour patients. The following section describes efforts made to study various toxic elements in vivo in occupationally exposed workers and in patients.

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