The feasibility of a sensitive low-dose method for the in vivo evaluation of Fe in skin using K-shell x-ray fluorescence (XRF)

An x-ray fluorescence (XRF) system designed for monitoring of skin Fe concentrations has been performance tested for use on patients treated for beta-thalassaemia. The essentials of the system are: a collimated x-ray tube operated at 20 kV and 20 mA; energy selection of the x-ray beam by means of a Cu K-edge filter; use of skin phantoms containing concentrations of Fe in the range 10 to 100 parts per million (ppm); and a high-purity germanium detector placed at 90 degrees to the incident beam. For a Cu K-edge filter of 0.15 mm thickness a quasi-monoenergetic beam of approximately 8.4 keV is obtained which is close to the absorption edge of Fe (7.11 keV). For a real-time counting period of 400s the systemis capable of detecting Fe concentrations of 15+/-2 ppm at a skin dose of the order of 5 mSv. This level of Fe is at the higher end of the normal range found in the skin. In using the same system and operating parameters, measurements on a sample of ferritin obtained from a rat's liver yield an Fe concentration of 36+/-3 ppm for a measurement time of 500 s; this can be compared with suppliers' data indicating an Fe level of 36 ppm.