Table of contents

A variable Jacobian and weighted back projection algorithm, used for medical CT, was adapted to perform CT reconstructions on data obtained with a dental panoramic X-ray unit. A detector array, fitted to the unit for the purpose of acquiring digital panoramic radiographs, was used to collect the data. Compensations were made for the incomplete (230') rotation of the panoramic X-ray unit, the non-fixed centre of rotation, the irregular rotation of the X-ray target and detector, and the resulting variances in magnification. The algorithm was tested on mathematically simulated phantoms and on acquired data. Reconstruction of simulated data proved the success of the algorithm. Real data reconstructions showed some defects as a result of inaccuracies in quantifying the experimental panoramic device.

The accuracy of the analogue to digital conversion (ADC) process is a critical element in the production of high quality SPECT images. Sub-optimal ADC performance can lead to the production of non-uniformities in the projection data which are amplified by the reconstruction process to produce circular artefacts in the transaxial sections. A simple method for the objective, quantitative assessment of ADC differential linearity has been developed. The analytical approach uses the fact that a differential non-linearity in the ADC will produce a non-uniformity in the digitized image which has a well defined periodicity. This can be most clearly demonstrated in the frequency space domain by determining the Fourier transform of a thick profile which is taken through the centre of a flood field image. The accuracy of the method permits deteriorations in the performance of ADCs to be detected well before significant reduction in SPECT image quality are produced.

The parallelism before existing air kerma formalisms based on the 'absorbed dose to air' chamber factor, N_D (or N_gas), and the more simple approach based on calibrations in terms of absorbed dose to water, N_w, is described. The importance of avoiding steps performed by the user that introduce avoidable uncertainties in the dosimetric procedure, is emphasized. Radiation beam quality factors normalized to ⁶⁰ Co gamma -rays have been calculated from a comparison between the two formalisms, and sets of tables produced for the different ionization chambers included in the 1987 IAEA Code of Practice. The calculated set of data is compared with existing experimental determinations at Primary Standard Dosimetry Laboratories, showing agreement within estimated uncertainties.

The Prony method in connection with Burg, Householder and singular value decomposition algorithms for the computation of the autoregressive coefficients, has been applied to simulated signals with different signal to noise ratios. The lines in the frequency domain are characterized by large linewidths and small differences in chemical shifts. The analysis performed by comparing FFT and the above mentioned methods shows that the best results, as far as both stability and performances of the solutions are concerned, are reached by the Prony-Householder method. Furthermore, the authors describe and apply the criterion of the minimum of the residual norm as a new procedure to find the order of autoregression. A validation procedure based on analysis of the residuals is also described.

For pt.I see ibid., vol.37, no.12, p. 2213-30 (1992). Conjugates of 9- beta -D-arabinofuranosyladenine 5'-monophosphate (ara-AMP) with lactosaminated albumin (L-SA), obtain by using two different coupling procedures, produce antibodies in rats and mice which display only a small cross-reactivity, ³¹P NMR signals from the two conjugates have been examined to clarify whether different lysine and histidine residues are involved in the two reaction pathways. The occurrence of different chemical shifts and linewidths between the two conjugates, as evidenced by processing the signals with the Prony-Householder method, indicates the formation of two different complexes.

The diffusion of ferric ions produced by irradiation in a dosimeter gel, consisting of a ferrous sulphate solution and agarose gel, has been studied. The diffusion coefficient of ferric ions in the gel was found to be 1.91*10^-2 cm² h^-1+or-5%. It was shown that the dose image obtained with an MR scanner deteriorates due to diffusion. This deterioration can be predicted with the aid of the measured diffusion coefficient. It was concluded that if the MR measurements (1/T1 image) of a typical depth-dose distribution are carried out within 2 hours of irradiation the diffusion will not have a significant effect on the results.

A dual-energy X-ray tomodensimeter adapted for the determination of bone mineral density (BMD) of the lumbar vertebrae has been directly developed from a typical dual-energy X-ray absorptiometer. This apparatus consists of two fundamental parts: a dual energy X-ray tube, and a multidetector made by an array of 24 NaI(Tl) crystals. It provides both tomographic and non-tomographic (anteroposterior, lateral etc) measurement. The detection area is limited to 132 mm. In this condition, the choice of the best reconstruction algorithm in order to give a direct BMD is considered. Preliminary studies based on numerical simulated projections and hydroxyapatite phantoms demonstrated the superiority of algebraic reconstruction algorithms, such as conjugated gradient.

Quantitative estimation of the increased risk of non-melanoma skin cancer (NMSC) in British people that may result from depletion of the stratospheric ozone layer is given for the present generation of British people. For adults alive today continuing ozone depletion at current rates is predicted to result in a relatively small additional lifetime risk ((5%) of NMSC, assuming no changes in climate, time spent outdoors, behaviour or clothing habits. The lifetime risk incurred by today's children, however, is 10%-15% greater than expected in the absence of ozone depletion. However, if the production and use of substances which deplete ozone are reduced, as expected under the current provisions of the Montreal Protocol, the increased lifetime risk of skin cancer is likely to be less than this estimate. These predicted increases in risk, resulting from greater solar ultraviolet exposure, can be offset by adopting changes to behaviour during the summer months.

The authors report a novel application of a neural network to perform curve fitting to simulated experimental data, obviating the need for lengthy non-linear least-squares fitting procedures. This application arose in the non-invasive determination of tissue optical properties, namely the transport scattering coefficient, mu '_s and the absorption coefficient, mu _a. This problem is relevant in clinical applications such as photodynamic therapy of cancer where the optical properties are used for the calculation of light fluence distributions in tissue, and for measuring the concentration of endogenous or exogenous chromophores. One approach to determine this optical coefficients noninvasively is to use a pencil beam of light to irradiate the tissue and to measure the multiply scattered light remitted from the tissue at different radial distances, rho , from the pencil beam on the tissue surface.

The feasibility study was of the use of an accelerator based neutron source, using the 3 MV Dynamitron accelerator in the School of Physics and Space Research, The University of Birmingham. Neutrons were produced via the ³H(¹H,n)³He reaction and the incident proton energy was varied to obtain neutron spectra with differing energies. The sensitivity for aluminium measurements (counts per unit mass per unit dose) was determined for the different neutron energies used; the possibility of using high resolution germanium, as opposed to NaI(Tl), detectors was examined, as were interferences and projected detection limits.

The magnitude of the observer variations intrinsic to contrast-detail measurements were found to be independent of the type of image used in their determination. The two sources of variation identified, within-observer and between-observer variation, were deduced to be 0.115 and 0.111, respectively (variations expressed as a relative standard deviation). When using the Leeds fluoroscopy test objects, an uncertainty on the measured threshold contrasts of 16% for one observer and 11% for two observers may be anticipated when each observer makes one independent reading. This then implies that a change of one perceived disc on the T010 test object (in which the contrast step between discs is approximately 40%) for one independent measurement made by one observer will be significant within two standard deviations. A further implication is that a more sensitive contrast-detail test object could be developed (with approximately 20% contrast steps between discs), provided that the test object is viewed by two or more observers.

A-150 tissue-equivalent (TE) plastic is widely used as a wall material for tissue-equivalent proportional counters (TEPCs) used in experimental microdosimetry. The choice of wall thickness allows provision of adequate equilibrium for secondary charged particles originating in the wall. An important feature in the development of a suitable counter is the construction of thin self-supporting TE films of good electrical conductivity for use as a TE dividing wall. The authors give a technical account of how A-150 TE plastic film can be fabricated in the laboratory from commercially available A-150 TE plastic.

The tissue air ratios far ⁶⁰Co gamma radiation tabulated in the British Journal of Radiology, supplement 17 are shown to be low by about 2%. A theoretical argument implies that this is due to an error in the measurement of peak scatter factors. Monte Carlo calculations also indicate that peak scatter factors are too low by about 2%.

The ionization yield of charged particles in gases represented as the average energy, W, required to produce an ion pair if the particles are completely stopped in the medium, together with the corresponding differential quantity, w, are essential interaction quantities for radiation research and dosimetric work. As a result of a continuous demand for precise W data protons in dry air, preliminary data of the authors presented graphically at the 8th Symposium on Microdosimetry (Willems et al. 1982) have been re-evaluated together with an evaluation of numerous subsequent measurements for proton energies between 0.7 and 100 keV.