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A recent advance in the field of medical technology is the adaptation of x-ray fluorescence (XRF) to measure the amount of lead sequestered in bones. The physiological basis for this measurement relies on the fact that the skeleton serves as the major reservoir for ingested lead incorporating it into the bone matrix during calcification where it remains until bone is remodeled or resorbed. Depending on the type of bone (trabecular or cortical) turnover of lead is two to 30 years, whereas in blood it is about 35 days. The pharmacokinetics of lead toxicity are such that beginning at age two, lead from the blood starts to move into bone cells, although it is not until after age four that the amount sequestered in bone exceeds the amount that moves back into blood. After that age, the measureable levels of blook-lead indicate recent exposure while teeth and bone-lead levels indicate long-term exposure. By adolescence, about 75% of the body burden of lead has been stored in the bones. By adulthood this figure rises to 90-95% in the non-occupationally exposed.
There are two circumstances in which lead sequestered in bones may be resorbed into the blood and become available to damage soft tissue (including fetuses): in pregnant and lactating females and in the aging, especially in females. A noninvasive method to measure lead in bone would be a useful biomarker of long-term exposure and provide a mechanism for answering research questions that could not previously be examined.
Several XRF devices have been developed that use a radioactive source to fluoresce atoms of the element of interest and measure the characteristic x-rays emitted. The sensitivity of the XRF measurement systems is critically dependent upon the choice of fluorescing source energy and the source-target-detector geometry. Three techniques have been developed to measure lead in bone in vivo. Two use high energy (K) either from a radioactive cadmium or cobalt source and one uses low energy (L) x-rays from an x-ray generator. The geometry is also different in the three techniques. Whatever the method, x-rays are shined on bone at a point where there is very little skin and tissue above it. The x-rays cause the ejection of electrons from lead atoms sequestered in the bone matrix in direct proportion to the concentration of lead in that bone.
Initially safety from radiation exposure was an issue. K-XRF can penetrate 10 mm to the bone marrow. L-XRF, while it does not penetrate more than 1 mm, has more x-ray scatter to the surrounding tissue. The National Academy of Sciences sponsored a meeting in 1993. Dosimetry data from each method were presented to a panel of experts. The conclusion was that both methods are safe and researchers could concentrate on establishing the efficacy of the methods.
Ideally, XRF measurements should give reproducible results that correlate well with lead body burden. This requires that the system hardware (light source, collimator, detector for the emitting rays, etc.) be matched with a computer software package that measures and interprets the x-ray energy released. NIEHS supported making a series of plaster of Paris phantom limbs to simulate bones impregnated with varying concentrations of lead. These phantoms were calibrated and analyzed by the National Institute of Standards and Technology (NIST). The phantoms are circulating so that researchers can make sure they are using the appropriate software package to analyze the signal and that measurements done on one machine are comparable with another.
Two workshops sponsored by the NIEHS were held in 1993 and 1994 to determine the value of the XRF in measuring lead in bone and what the problems were both in the instrumentation as well as bone-lead biology. The consensus of the attendees was that K-XRF could be used successfully in epidemiological studies because the higher energy (k) x-ray penetrates to the marrow, making the technique less sensitive to whether lead is laid down uniformly in bone or whether trace amounts of arsenic in the blood will interfere with the L-line lead spectra. Current state-of-the-art K-XRF instruments can measure lead in bone lead at levels as low as 5 parts per million (ppm). The workshops highlighted research areas that should be pursued to refine both techniques. The goal of one project NIEHS supports is to improve the minimum detectable concentration of bone-lead using both K and L instruments . Another is to compare the precision and accuracy of K and L on bone samples impregnated with a known amount of lead.
Several studies supported by NIEHS are using K-XRF instruments developed with NIEHS funds (either through the small business innovative research (SBIR) program or through the regular grant (R01) mechanism) to examine the long-term effects of lead exposure on a number of health outcomes. One study evaluated the association between body lead burden and social adjustment (Needleman; R01ES05015) in a sample of 301 male Pittsburgh primary school students. Lead burden was measured by K line X-ray fluorescence spectroscopy at age 12 years, while at earlier ages the subjects were evaluated for antisocial behavior and given a battery of neurobehavioral tests. It was concluded from the study that cumulative lead exposure is associated with increased risk for antisocial behavior and delinquency, and that the effect follows a developmental course.
The main objective of another project was to clarify the inter- relationships between accumulated lead burden, blood lead levels, dietary calcium, and blood pressure (Hu) using a cohort from the Normative Aging Study (NAS), a group of men who have been followed longitudinally since 1962 and whose lead exposures should be similar to those in the general population. K-XRF was used to measure bone-lead burden in NAS subjects. Archived frozen red cell samples (available on each NAS subject from multiple visits since the 1960s) were analyzed as well as fresh blood leads. Their findings suggest that chronic lead accumulation, as reflected by bone lead levels, may be an independent risk factor for developing hypertension in men of the general population.
A prospective epidemiologic study using K-XRF is examining the relationship between relatively low-level environmental exposure to lead in adults and chronic neurotoxicity (Todd; R01ES05697). Neurologic function is being evaluated in lead battery workers at the beginning of their occupational exposure to elevated lead levels and again 1.0 to 3.5 years later, the hypothesis being that increased exposure to lead can have neurological consequences to adults. This study is unique in its prospective examination of neurologic outcomes in relation to a cumulative biologic indicator of chronic lead exposure, lead content in bone.
Recognizing the importance of elucidating the factors that affect the accumulation and release of lead in bone, scientists at the Harvard School of Public Health and Mt Sinai Medical Center have been conducting two on-going epidemiological studies that address questions surrounding the uptake and release of lead from bone into blood (Hu; P42ES05947, Berkowitz; P42ES07384) when it may become available to damage soft tissue (including fetuses): during pregnancy and lactation and during aging. The studies are employing K-XRF to make safe, accurate and non-invasive measurements of bone lead levels in the subjects. By reviewing the histories of the subjects and comparing them with measured bone lead levels, the factors that contribute the most to accumulated lead burden are being identified. Also, by comparing bone lead levels with health outcomes, it is being determined whether bone lead, and therefore accumulated lead exposure, serves as a risk factor for adverse health effects.