Measuring Lead in Bone; A Technical Description

Lead K-shell X-rays are fluoresced with cyclotron-produced ¹⁰⁹Cd in a solid encapsulated form. The source is usually 1.295 GBq (35 m Ci) activity at delivery. The ¹⁰⁹Cd source is a sealed source. It has an Amersham design assignment number of CUC.D4 and is registered as a sealed source with the US Nuclear Regulatory Commission (NRC), license number IL-136-S-342-S.

Source holder with copper filter mounted onto a detector end cap and measuring lead in the leg on the right of the photograph. This photograph also shows a section of the leg brace (far left) which is attached around the aluminum detector housing.

The ¹⁰⁹Cd is placed in a cylindrical source holder/collimator (6 mm diameter, 6 mm length) of 90% tungsten, 6% nickel and 4% copper. The source fits snugly within, but is removable from, the source holder. The source is not removed from the source holder/collimator except for the mandated biannual wipe tests.

At all times the source has 0.25 mm of copper over the source window that absorbs 98.25% of the silver X-rays that accompany the decay of 109Cd. When the source is being used for measurements, the source in its holder/collimator is removed from the lead storage container and mounted onto the middle of the end cap of a 50 mm diameter intrinsic germanium detector; the Cd source can thereby be mounted co-axially with and on front of the detector.

Fluorescent X-ray emissions are detected and quantified with the detector, which has a built-in resistive feedback preamplifier connected to a standard spectroscopy system of electronics, i.e., amplifier, analog-to-digital converter and multi-channel pulse height analyzer.

In addition to in vivo spectra, calibration spectra are obtained each day using 10 lead-doped plaster-of-Paris phantoms containing between 0 and 150 parts-per-million of lead.