Gamma-ray scattering for mandibular bone density measurement.

There has been considerable interest in the measurement of bone density in the mandible, either to study bone resorption following tooth loss or to determine the relationship between mandibular and skeletal bone mineral density. Measurements mostly have been made on dental radiographs but more significant correlations between mandibular and skeletal bone density have been obtained when more sophisticated techniques have been used, such as quantitative CT and dual energy X-ray absorptiometry. The present work investigates the feasibility of using gamma-ray scattering measurements to estimate mandibular bone density. Using a phantom to simulate the jaw, an 241Am source and a hyperpure germanium detector it is shown that bone density may be measured with a precision of about 1%.

Gamma-ray backscatter for body composition measurement.

The purpose of this study was to examine the potential of using backscatter information to assess regional body composition at selected sites. Two measurement techniques are examined: the measurement of the ratio of coherent to Compton scatter, and the measurement of the Compton scatter profile. Two possible applications are considered: the measurement of trabecular bone mineral density, and the measurement of the average fat/muscle ratio in a tissue volume. The results presented indicate that the analysis of coherent and Compton backscattered gamma-ray spectra from an 241Am source has the potential for measuring both trabecular bone mineral density and average fat/muscle ratio in a tissue volume, with a low absorbed dose to the subject.

Gamma-ray scattering for fat fraction measurement.

The work reported examines the potential of using gamma-ray photon backscatter information to measure in vivo the percentage of fat in specific tissue volumes. 241Am gamma rays are used as the source and the backscatter detected with a hyperpure germanium detector, with ethanol (approximately 80% fat, 20% muscle) and water (muscle) being used as tissue substitutes. Two measurement techniques are examined; the measurement of the ratio of coherent scatter to Compton scatter and the measurement of the Compton scatter profile. Both are shown to be sensitive to the composition difference between ethanol and water. For the coherent-Compton scatter ratio, the measured difference between water and ethanol is 1.85:1, close to the value calculated (about 2:1). A similar difference in the coherent-Compton ratios between muscle and fat is calculated (2.2:1). The FWHM of the Compton profile has also been shown to vary with tissue composition with a difference of 0.10 keV (5%) between the ethanol and water profile widths.

Dose rates and energy spectra in the maze of a linear accelerator treatment room.

When designing an entrance maze for a linear accelerator facility, it is useful to be able to estimate both the dose rate at the entrance and, in case further shielding is necessary, the energy spectrum of the scattered photons reaching the maze entrance. This study examines the applicability of the commonly used empirical formula contained in the National Council on Radiation Protection and Measurements (NCRP) Report No. 51 for estimating the dose rates at the maze entrance in a typical treatment room, and proposes an improvement allowing for the fact that the initial scattering event occurs most often in the patient at the isocentre. The X-ray energy spectrum at the maze entrance is also measured and shown to have a lower average energy than is assumed in NCRP Report No. 51.

Technical note: an X-ray fluorescence system for the determination of gold in vivo following chrysotherapy.

This paper describes a low cost mobile measurement system for the determination of gold in vivo base around a 153Gd radiation source and a hyper pure germanium detector. Early clinical results are also presented to demonstrate the efficacy of the system.

BIR presidential address: exploring the niches.

In his 1990 Presidential Address Professor C R Hill posed the question of whether, given the ethos of the late 20th century, there was a future for radiological science. To answer his question he reviewed the scientific basis of current major branches of radiology, demonstrating progress resulting from initial physical discovery, combined with engineering and medical inventiveness. In the process of the development of radiological science, small-scale speculative research, outside mainstream areas, has proved to be an important element. This address complements Professor Hill's by examining the role of a small-scale speculative research (the niches of the field). Examples are taken from the early pioneers of radiological science and the first British Institute of Radiology Congress. Finally some lesser known research activity in this category, being presented at the Golden Jubilee Congress, is reviewed.

Coherent-Compton scattering for the assessment of bone mineral content using heavily filtered x-ray beams.

The feasibility of using heavily filtered x-ray beams to assess trabecular bone mineral content has been investigated by measuring the ratio of coherent to incoherent scattered x-rays with a high purity Ge detector. The technique uses the strong dependence of coherent scattering on the effective atomic number of the scattering medium. With an x-ray beam filtered with a high-atomic-number filter, a spectrum characterised by a sharp discontinuity at the K-absorption edge is produced. Analysis of the spectral shape after scattering allows the coherent to Compton scattering ratio to be obtained. Theoretical and experimental results from phantom studies are presented and a comparison made between the results obtained with x-ray beams and radionuclide sources respectively. The influence of overlying tissue thickness on the sensitivity of the measurements is demonstrated.

Tungsten anode tubes with K-edge filters for mammography.

A number of authors have calculated the optimum X-ray energies for mammography using, as a criterion, the maximum signal to noise ratio (SNR) per unit dose to the breast, or conversely the minimum exposure for constant SNR. Filters having absorption edges at appropriate energy positions have been used to modify the shape of tungsten anode spectra to bring them close to the calculated optimum. The suitability of such spectra for practical use has been assessed by comparing both the film image quality and the incident breast dose obtained using a K-edge filtered tungsten anode tube with that obtained using a molybdenum anode tube. Image quality has been assessed both by using a "random" phantom and by comparing mammograms of women attending a screening clinic where one breast was radiographed using a filtered tungsten anode tube and the other using a standard molybdenum anode unit. Relative breast doses were estimated from both ionisation chamber measurements with a phantom and thermoluminescent dosimetry measurements on the breast. The film image quality assessment indicated that the filtered tungsten anode tube gave results which were not significantly different from those obtained with a molybdenum anode tube for a tissue thickness of about 4 cm and which were better for larger breast thicknesses. The dose could be reduced to between one-half and one-third with the filtered tungsten anode tube.

Optimum x-ray spectra for mammography.

A number of authors have calculated x-ray energies for mammography using, as a criterion, the maximum signal-to-noise ratio (SNR) obtainable per unit dose to the breast or conversely the minimum exposure for constant SNR. The predicted optimum energy increases with increasing breast thickness. Tungsten anode x-ray spectra have been measured with and without various added filter materials to determine how close the resultant spectra can be brought to the predicted optimum energies without reducing the x-ray output to unacceptable levels. The proportion of the total number of x-rays in a measured spectrum lying within a narrow energy band centred on the predicted optimum has been used as an optimum energy index. The effect of various filter materials on the measured x-ray spectra has been investigated both experimentally and theoretically. The resulting spectra have been compared with molybdenum anode, molybdenum filtered x-ray spectra normally used for mammography. It is shown that filters with K-absorption edges close to the predicted optimum energies are the most effective at producing the desired spectral shape. The choices of filter thickness and Vp are also explored in relationship to their effect on the resultant x-ray spectral shape and intensity.

The recovery of iron uptake in erythropoietic bone marrow following large field radiotherapy.

Large X-ray fields, such as those used to irradiate major lymph-node areas in the treatment of lymphomas, often irradiate large areas of haemopoietic bone marrow. The absorbed doses received by the marrow are close to the tolerance level. Quantitative scanning of the bone marrow, using 52Fe and a digital whole-body scanner, has been carried out in a series of 22 patients treated by radiotherapy up to nine years previously in order to assess the extent and recovery of their erythropoietic marrow. The results showed large variations from patient to patient; in some patients the iron uptake in the marrow returned to near normal levels in two to three years, while in others the uptake remained suppressed after seven to eight years. The tolerance dose for the bone marrow appeared to be about 1100 ret. There was some evidence of a more rapid recovery of erythropoietic function in males than females, but no evidence to show that age of patient, type of disease or whether chemotherapy had been used influenced the results. Patients who showed a decreased iron uptake in an irradiated region of marrow generally showed a compensating increased uptake in unirradiated regions, which in some cases was associated with anaemia.