Lead in Bone

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Measuring Lead in Bone; the Physics of Bone Lead X-Ray Fluorescence

The presence of most toxic heavy metals in the body can be measured in the laboratory by the analysis of blood, urine or biopsy samples. However, the sensitivity of such measurements varies from element to element and, more importantly, the measurements do not always directly indicate the amount of the element in the organ of accumulation or interaction. For example, lead in blood does not measure cumulative exposure because it has a biological half life of approximately 36 days. In vivo elemental analysis provides a direct, non-invasive measurement of the element of interest in the organ of accumulation. The principle of X-Ray Fluorescence (XRF) is the use of photons to fluoresce atoms of the element of interest and measurement of the subsequent characteristic X-rays with a radiation detector, indicating the amount of the element present in the sample.

Photons from the 'fluorescing source' remove an inner shell electron from a lead atom, leaving the lead atom in an excited state. De-excitation can occur via the emission of one of a series of X-rays whose energy is specific to lead. The X-rays are recorded by a radiation detector and the number of X-rays is directly proportional to the amount of lead present in the bone. Appropriate calibration of the system against lead-doped samples allows the number of emitted lead X-rays to be quantified as a measure of lead in the sample. Bone-lead measurements are non-invasive; the subject is required to sit in a chair and have the measurement system moved into place. 109Cd K XRF measurements are typically performed for approximately 30 minutes.

Advantages of the technique we use for Bone Lead XRF

The 109Cd source is mounted co-axially with and on the front of an intrinsic germanium detector, the detector thus measures the radiation spectrum backscattered into it and is referred to as being in a backscatter geometry. The energy of the incident photons is 88 keV, and such scattering ensures that the Compton peak lies at 65 keV, below the energy of the lead K X-ray peaks (74-87 keV). This lowers the background under the lead peaks and improves the detection limit. A significant feature of the spectrum is the elastic scatter peak from 109Cd photons which interact with elements in the sample such that their energy is essentially unchanged; the peak thus occurs at 88 keV. The probability of a photon being elastically scattered by an element of atomic number Z depends on Z5 or Z6, and in a bone-lead measurement 98-99% of elastically scattered photons arise from the bone mineral. Normalizing the lead X-ray peak counts to the elastic scatter peak counts results in a bone-lead content in units of mg Pb per gram of bone mineral. The advantage of the normalization is that a measurement result is thus independent of bone shape, size or mass, of tissue overlay thickness, source to subject distance, orientation of detector assembly with respect to the bone, the orientation of the bone within the leg and of subject movement. No correction factors need therefore be applied to take these into account and subjects need not be restrained or sedated in any way. The normalization assumes that the lead and elastic scatter signal arise out of the same incident photon fluence and are subject to the same interactions with the surrounding media. The measurement precision (the uncertainty on an individual measurement) does vary somewhat, depending upon the source to bone distance and the thickness of overlying tissue.

The considerable robustness of the K XRF technique and the consequent ease with which different bone sites can be measured are probably the reasons for the widespread acceptance of this technique among the research community.