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Measuring Lead in Bone; A Practical Description
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To undergo a
bone-lead measurement, a subject sits in a lead-free, plastic chair for
35-45 minutes. This time includes time for preparation of the subject and
the time for the bone-lead measurement itself. The variation in time
(30-40) minutes results from the decay of the 109Cd source over
time. As the source grows older and, consequently, weaker, the measurement time is increased
until, at its least active, the measurement is conducted for 40 minutes.
The increase in time ensures constant measurement precision and does not,
of course, mean a larger dose. Preparation of the subject is simply
rolling up of the left trouser leg and wiping of the leg with diluted
glacial acetic acid (3%) to remove any lead contamination of the skin.
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Cleaning the leg |
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The subject's left
leg is then gently restrained to maintain 3 cm between the skin and the
source. Restraint is performed with broad nylon straps fitted with
non-metallic fasteners and is not uncomfortable. When the subject is
positioned, the source/detector is moved into place. The source/detector
combination is mounted on a platform that moves in all three spatial
dimensions (XYZ). Measurements are made at the midshaft of the left tibia,
the calcaneus and the patella. When positioning the system the subject is
shielded from the source photons by a source shield of tungsten alloy that
is retracted when the measurement begins.
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The XRF system in place |
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| Pediatric
subjects are "immobilized" in the same ways as adults are; only
by restraint of their left tibia. Only the leg needs to be restrained;
sedation of pediatric subjects is not necessary because the XRF
measurement result is independent of non-gross subject movement. Pediatric
subjects are not held by another person. There is no need to exclude the
custodian of the child from the measurement room. During the measurement,
subject boredom is often alleviated by administering a questionnaire of
work, occupational and medical histories. For children. we have found a
choice of videos helpful (and entertaining). |
A measurement in progress in Taiwan |
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