Glass beads and Ge-doped optical fibres as thermoluminescence dosimeters for small field photon dosimetry.

An investigation has been made of glass beads and optical fibres as novel dosimeters for small-field photon radiation therapy dosimetry. Commercially available glass beads of largest dimension 1.5 mm and GeO2-doped SiO2 optical fibres of 5 mm length and 120 µm diameter were characterized as thermoluminescence dosimeters. Results were compared against Monte-Carlo simulations with BEAMnrc/DOSXYZnrc, EBT3 Gafchromic film, and a high-resolution 2D-array of liquid-filled ionization chambers. Measurements included relative output factors and dose profiles for square-field sizes of 1, 2, 3, 4, and 10 cm. A customized Solid-Water® phantom was employed, and the beads and fibres were placed at defined positions along the longitudinal axis to allow accurate beam profile measurement. Output factors and the beam profile parameters were compared against those calculated by BEAMnrc/DOSXYZnrc. The output factors and field width measurements were found to be in agreement with reference measurements to within better than 3.5% for all field sizes down to 2 cm2 for both dosimetric systems, with the beads showing a discrepancy of no more than 2.8% for all field sizes. The results confirm the potential of the beads and fibres as thermoluminescent dosimeters for use in small photon radiation field sizes.

Standardisation of positron-emitters in gas with the NPL primary gas counting system.

A primary standard for positron-emitters in gas has been developed. The method involves internal gas proportional counting and the use of the PENELOPE Monte Carlo code to determine corrections for counting losses. The development work was carried out using (11)C, although the method can be applied to other positron emitters. The results were compared with measurements of (11)C (in solution) carried out using a secondary standard re-entrant ionisation chamber previously calibrated with reference to absolute counting techniques.

A detailed Monte Carlo accounting of radiation transport in the brain during BNCT.

The collision type central to BNCT is (10)B(n, alpha)(7)Li, however, other types of nuclear reactions also take place in the patient. In addition to the major elements (H, C, N, O), minor elements such as Na, Mg, P, S, Cl, K, Ca and Fe present in body tissues also interact in neutron collisions. Detailed accounting of the above not only provides a better understanding of radiation transport in the human body during BNCT, but such knowledge affects the design of the facility, as well as treatment planning, imaging and verification for a given BNCT agent. Of the methods of investigation currently available, only Monte Carlo simulation could provide the detailed accounting and breakdown of the quantities required. We report Monte Carlo simulation of an anthropomorphic voxel phantom, the VIP-Man and show how these quantities change with different (10)B concentrations in the tumour, the blood and the remaining tissues. The (10)B biodistribution has been chosen to be the variable of interest, since it is not accurately known, is frequently approximated and is a crucial quantity upon which dose calculations are based.

Non-convergence of Geant4 hadronic models for 10 and 30 MeV protons in 18O and 14N.

We investigated Geant4 Monte Carlo simulation of protons interactions in (18)O and (14)N. With proton incident energies of 10 and 30 MeV, we compared the different Geant4 models available for proton inelastic collisions: the low-energy parameterisation, Bertini cascade and precompound models. The models produced different (1) combinations of secondary particles and (2) final states of secondary particles. The non-convergence suggests that the hadronic models, yet to be benchmarked for this energy range, are not ready for production-mode problem-solving.

Epigastric electrical impedance for the quantitative determination of the gastric acidity.

Electrical impedance measurements have been used by scientists since the 1980s to investigate the gastric function. In this work, these measurements were carried out using the epigastrograph, a device generating alternating current of 32 kHz and injecting it in the gastric area of the human body with surface electrodes, located around the abdominal area. Although the method has been used for about three decades the physiological interpretation of these measurements is still under research. This work states that the electrical impedance measurements from the gastric area depend on the conductivity of the gastric lumen, due mainly to gastric acid secretions and to the conductivity and chemical form of the ingested meal. By choosing the proper test meal the gastric acidity in the empty, healthy stomach was also estimated. The estimated value is in accordance with the literature. The method is non-invasive, relatively inexpensive, simple to medical technologists and subjects, and involves no radiation risk. The method may form the basis for the development of a non-invasive gastric pH meter.

Lithium-gadolinium-borate as a neutron dosemeter.

Lithium-gadolinium-borate (LGB) dispersed as microcrystals within the plastic scintillator BC-490 is a promising material for accurate neutron dosimetry in mixed n/gamma fields. Spectral information > 1 MeV is obtained by capture gating proton recoil events in the plastic scintillator to subsequent capture in (6)Li. Below 1 MeV, isolated capture events in either gadolinium or (6)Li give energy information in this region. Discrimination based on capture gating is used to reject false coincidences due to gamma rays or incorrectly gated neutron events. A detailed Monte Carlo model has been created in MCNPX that predicts the energy response of the LGB spectrometer in the capture-gated mode of operation. X-ray microtomography has been performed on the detector in order to obtain the LGB microcrystal distribution within the plastic scintillator, and this is incorporated into the model. The way in which the calculated response functions can be included in an unfolding procedure is outlined.

A high-resolution anthropomorphic voxel-based tomographic phantom for proton therapy of the eye.

Proton therapy is increasingly used in medical treatments for cancer patients due to the sharp dose conformity offered by the characteristic Bragg peak. Proton beam interactions with the eye will be simulated using the MCNPX Monte Carlo code and available nuclear cross-section data to calculate the dose distribution in the eye gel and surrounding organs. A high-resolution eye model will be employed using a 3D geometrical voxel-based anthropomorphic head phantom obtained from the Visible Human Project (female data). Manual segmentation of the eye, carried out by the Medical Physics group at the University of Surrey resulted in 15 identified structures. This work emphasizes the use of a realistic phantom for accurately predicting dose deposition by protons.

Radioactivity levels of staple foodstuffs and dose estimates for most of the Tanzanian population.

Levels of natural radioactivity in staple food products (maize and rice) from various localities of Tanzania have been studied. The average activity concentrations of 40K, 232Th and 238U in maize were 48.79 +/- 0.11, 4.08 +/- 0.01 and 13.23 +/- 0.10 Bq kg(-1), respectively. In rice the concentrations of 40K, 232Th and 238U were 24.67 +/- 0.03, 3.82 +/- 0.02 and 5.02 +/- 0.02 Bq kg(-1), respectively. 137Cs was detected in only one sample collected in Zanzibar. The sample, with activity concentration of 5.57 +/- 0.01 Bq kg(-1), had been imported from Thailand. The relatively high average concentrations of the radionuclides in maize compared to rice may be attributed to the extensive use of phosphate fertilizers in maize production in Tanzania. Total annual committed effective doses due to total 232Th and 238U intakes as a result of consumption of staple foodstuffs for infants, children and adults were 0.16, 0.29 and 0.36 mSv y(-1), respectively, which are lower than the annual dose guideline for the general public.

Neutron-fluence-to-dose conversion coefficients in an anthropomorphic phantom.

A set of fluence-to-effective-dose conversion coefficients has been calculated for neutrons with energies <20 MeV using a high-resolution anthropomorphic phantom (Zubal model) and the MCNPX code. The calculation used 13 monodirectional monoenergetic neutron beams in the energy range 10(-9) to 20 MeV, under three different source irradiation configurations: anterior-posterior, posterior-anterior and left lateral. Dose calculations were performed for 18 selected organs of the body, for which the International Commission on Radiological Protection and the International Commission on Radiological Units and Measurements have set tissue weighting factors for the determination of the effective dose. Another set of neutron-fluence-to-effective-dose conversion coefficients was also calculated with the proposed modification wR from ICRP Publication 92. From comparison between the dose results calculated and the data reported for the MIRD and VIPMAN models, it can be concluded that, although some discrepancies exist between the Zubal model and the two other models, there is good agreement in the left lateral irradiation geometry.

Development and testing of an active area neutron dosemeter.

This paper describes the design, development and testing of an active area neutron dosemeter (AAND). The classic moderator and central detector is retained but in AAND this arrangement is augmented by small thermal neutron detectors positioned within the moderating body. The outputs from these detectors are combined using an appropriately weighted linear superposition to fit both the ambient dose equivalent and the radiation weighting factor. Experimental verifications of both the modelled detector energy reponses and the overall AAND response are given. In the relatively soft D2O moderated 252Cf spectra, the AAND determined both the H*(10) and mean radiation weighting factor to better than +10%.

Introduction of a thermal response to the DSTL PADC personal neutron dosemeter.

The response of the Defence Science and Technology Laboratory (DSTL) PADC personal neutron dosemeter is strongly dependent upon neutron energy, with a range of 300-500 tracks per cm2 per mSv for energies between 1 and 5 MeV. Below 1 MeV the response drops off sharply. This lack of sensitivity is undesirable when the dosemeter is employed with the softened fission spectra encountered in the workplace. In order to incorporate a thermal response, a polypropylene converter doped with LiF has been placed directly in front of the PADC elements. Tritons produced in the thermal neutron reaction 6Li (n,t)alpha at 2.7 MeV will then penetrate the PADC, leaving a trail of damage. The reaction rate within the converter has been calculated using MCNP for thermal neutrons and a range of higher energies, while transport of the tritons is modelled using the SRIM/TRIM package to determine the resultant track density and depth distribution. The modelling and experimental work have demonstrated that a concentration of 0.2% natural lithium by weight results in a track density in a thermal field comparable with that produced per unit personal dose equivalent by neutrons greater than 1 MeV in the standard dosemeter. Additional MCNP modelling has demonstrated that the dosemeters' albedo response to intermediate energy neutrons can be enhanced considerably by placing a boron-doped shield in front of the converter and increasing its lithium concentration.

Response of a lithium gadolinium borate scintillator in monoenergetic neutron fields.

Accurate estimation of neutron dose requires knowledge of the neutron energy distribution in the working environment. Existing neutron spectrometry systems, Bonner spheres for example, are large and bulky, and require long data acquisition times. A portable system that could indicate the approximate neutron energy spectrum in a short time would be extremely useful in radiation protection. A composite scintillator, consisting of lithium gadolinium borate crystals in a plastic scintillator matrix, produced by Photogenics is being tested for this purpose. A prototype device based on this scintillator and digital pulse processing electronics has been calibrated using quasi-monoenergetic neutron fields at the low-scatter facility of the UK National Physical Laboratory (NPL). Energies selected were 144, 250, 565, 1400, 2500 and 5000 keV, with correction for scattered neutrons being made using the shadow cone technique. Measurements were also made in the NPL thermal neutron field. Pulse distributions collected with the digitiser in capture-gated mode are presented, and detection efficiency and energy resolution derived. For comparison, neutron spectra were also collected using the commercially available Microspec N-Probe from Bubble Technology Industries, which consists of an NE213 scintillator and a 3He proportional counter.

Whole body [O-15]water pharmacokinetics measured in blood.

A simple pharmacokinetic model to explain the time course of [O-15] water in human whole blood after bolus injection is described. The model has been derived from measurements in twelve healthy volunteers who were measured repeatedly, resulting in 67 datasets, made in the context of PET blood flow studies. In contrast to traditional volume of distribution estimates of total body water (TBW) which rely on measurements after many hours, the model and data provide insights into the fast uptake components in the distribution of water in the body. Data fitting shows that the volume of distribution of fast exchanging tissues is 21 l. TBW was calculated to be 37 l. Monte Carlo simulation showed that the expected inaccuracy of determination of parameters due to unsystematic sources in the measurement data was around 5% for most parameters. Our data show that water extraction to tissue is somewhat higher than would be predicted from the tabulated values, probably because skeletal blood flow is sensitive to physiological status and environmental conditions. The study provides valuable reference data on the distribution and kinetics of water in man. Using the parameters and model from this study, reference input time-activity curves can be calculated, e.g. for the Monte Carlo study of error propagation in PET studies.

Physiological interpretation of electrical impedance epigastrography measurements.

Measurement of the electrical impedance of the gastric region is carried out with the epigastrograph. This generates and applies alternating current around the abdominal area and measures the potential difference in order to determine the impedance externally, via electrodes. The change of epigastric impedance for a subject, given a meal after fasting, depends on the conductivity of the meal compared to the stomach and surrounding tissues. Typically a conductive meal has conductivity >7 mS cm(-1), non-conductive <2 mS cm(-1) and neutral about 4.5 mS cm(-1). Half-emptying times (T50s) from gastric emptying studies in volunteers using three test meals of 450 ml volume were obtained and found to be shorter than expected from the literature. The meals were a 10% glucose solution and two milk shakes of energy 1,300 kJ and 2,850 kJ, respectively. These electrical impedance epigastrography (EIE) measurements were carried out with scintigraphy. The T50 values of the latter were significantly longer. The direct comparison of the normalized experimental data obtained by both methods led to the concept that EIE measurements are mainly influenced by gastric secretion. Thus the EIE trace of a 'neutral' meal suggests the hypothesis that the volume of the meal is not the significant factor but is influenced by gastric acid secretions. Physiology of the gastric mucosa during the digestion of a meal and intragastric pH values also suggests this. Gastric function studies using EIE measurements may therefore reflect gastric ionic concentration rather than the volume of the contents of the stomach. In turn this could lead to the development of a non-invasive method for the continuous recording of gastric acid secretions.

Determination of cadmium and zinc in Alzheimer's brain tissue using inductively coupled plasma mass spectrometry.

In this work, brain tissue was taken from Alzheimer's Disease (AD) subjects (n=11), 'normal' subjects (n=10) and from subjects with senile involutive cortical changes (SICC) (n=6). Concentrations of Cd and Zn were determined in all samples, using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The brain tissue was selected and obtained from the Netherlands Brain Bank. Samples were taken in each case, from both hemispheres of the superior frontal gyrus, the superior parietal gyrus, the medial temporal gyrus, the hippocampus and the thalamus of the same brain.Cd which is known to have no essential role in the brain was found to follow, as expected, a lognormal distribution of concentrations in 'normal' subjects (Shapiro-Wilk's test (0.98) (p<0.18)). For the Alzheimer's Disease subjects and SICC subjects, the data tends to follow a lognormal distribution, rather than a normal distribution, but is still significantly different from it (Shapiro-Wilk's test (0.97) (p<0.03); (0.93) (p<0.0067), respectively)). In the case of Zn concentrations, the data tends to follow a normal distribution for the 'normal' subject group, even though the data is significantly different from it (Shapiro-Wilk's test (0.95) (p<0.001)). Whereas in the Alzheimer's Disease and SICC subject groups, the data follows a normal distribution (Shapiro-Wilk's test (0.98) (p<0.21); (0.97) (p<0.2002), respectively)). When comparing age-matched groups, for all regions and both hemispheres, no significant differences (p>0.1) for Cd were found between 'normals' and Alzheimer's Disease subjects and Alzheimer's Disease subjects and SICC but at a low level of significance, lower concentrations of Cd were found in the SICC group compared to the 'normals'. For all regions and both hemispheres, Zn was found to be significantly decreased in the Alzheimer's Disease group, compared to the 'normal' and SICC groups. Zn concentrations were also found to be significantly decreased in the 'normals' compared to the SICC group. It is also of interest that Cd negatively correlates with the scale of tangles in both 'normals' (p<0.001) and Alzheimer's Disease subjects (p<0.01). In the SICC subjects Cd correlates negatively with the tangles but not significantly so (p>0.1).

Variance in parametric images: direct estimation from parametric projections.

Recent work has shown that it is possible to apply linear kinetic models to dynamic projection data in PET in order to calculate parameter projections. These can subsequently be back-projected to form parametric images--maps of parameters of physiological interest. Critical to the application of these maps, to test for significant changes between normal and pathophysiology, is an assessment of the statistical uncertainty. In this context, parametric images also include simple integral images from, e.g., [O-15]-water used to calculate statistical parametric maps (SPMs). This paper revisits the concept of parameter projections and presents a more general formulation of the parameter projection derivation as well as a method to estimate parameter variance in projection space, showing which analysis methods (models) can be used. Using simulated pharmacokinetic image data we show that a method based on an analysis in projection space inherently calculates the mathematically rigorous pixel variance. This results in an estimation which is as accurate as either estimating variance in image space during model fitting, or estimation by comparison across sets of parametric images--as might be done between individuals in a group pharmacokinetic PET study. The method based on projections has, however, a higher computational efficiency, and is also shown to be more precise, as reflected in smooth variance distribution images when compared to the other methods.

Improvements in the determination of radionuclide distribution in the analysis of irradiated samples using double-differentiation method.

The spatial response of an 8 x 4 block detector made up of 5.6-mm-wide, 12.9-mm-high, 30-mm-thick individual detector crystals to a collimated line source of 511 keV annihilation photons was examined. The response of each crystal showed a spread around the average positioning values and distributions from adjacent crystals overlapped as the collimated source scanned the individual detectors. This leads to possible errors in the event assignment. The implementation of double differentiation or the second derivative method was proposed for the removal of scattered photons so as to reduce the overlap and, hence, avoid mis-positioning. This method is a mathematical solution implemented when analysing the results. A curve in a spatial spectrum could be considered to be a function f(x), where x is the position. When double differentiation of f(x) is carried out, then the normalized curve d2f(x) appears with some reduction in the wings. It was shown that a reduction of the scattering contribution in the tails without overestimating the contribution of scattered events could be achieved by implementing a double-differentiation process.

Determination of trace elements in porcine brain by inductively coupled plasma-mass spectrometry, electrothermal atomic absorption spectrometry, and instrumental neutron activation analysis.

Methods have been developed for the analyses of trace metals in various areas of porcine brains, (temporal, parietal, frontal cortex, both right and left hemispheres). Determinations were carried out using inductively coupled plasma-mass spectrometry (ICP-MS) and electrothermal atomic absorption spectrometry (ETAAS). The elements investigated were Li, Mn, Cu, Zn, Cd, Hg, and Pb by ICP-MS and Cu, Cd, and Mn by ETAAS. For determination by ICP-MS, a method of standard additions calibration coupled with internal standards was used, and for ETAAS, standard additions calibrations were prepared. The accuracy of all methods was determined using NIST and IAEA certified reference material. A small number of pig brains were analyzed by instrumental neutron activation analysis for Cr, Co, Cs, Fe, Rb, Se, Sc, Sb, and Zn using the comparator method of analysis. Four separate NIST standard reference materials have been used to examine the validity of the comparator method.

A comparative study in Alzheimer's and normal brains of trace element distribution using PIXE and INA analyses and glucose metabolism by positron emission tomography.

The onset of Alzheimer's disease has been shown to affect trace element concentrations in the brain when compared to "normal" subjects in ex vivo samples. The techniques used to determine trace element concentrations were proton-induced X-ray emission and instrumental neutron activation analysis. With these methods, significant differences are seen between lobes within a hemisphere and between the same lobes of opposing hemispheres for "normal" brains. The change observed in trace element concentrations may indicate a possible alteration in the function of the blood-brain barrier, the effect of which can be investigated in vivo using the imaging technique of positron emission tomography (PET). A PET study was performed on nine female and nine male subjects to determine whether the regional metabolic rate of glucose (rCMRGlu) varied between hemispheres and sex in the Alzheimer diseased brain as was seen in the trace element study. Glucose metabolism was measured using [F-18]-fluorodeoxyglucose (18F-FDG). Hemispherical differences were observed for the frontal, occipital, parietal lobes, and the temporolateral region in both males and females for rCMRGlu. Variation was also seen between sexes, where the frontal lobe had a lower rCMRGlu in females compared to that of males.

Elemental composition changes between breast tissue with and without silicone gel sheeting and hypertrophic scar tissue.

Hypertrophic scars occur after dermal trauma and are characterized by being elevated above normal skin level as a result of an abundance of collagen. The application of silicone gel sheeting (SGS) has been found to be an effective method of treatment, causing them to regress much quicker than they would do naturally. Normal skin and hypertrophic scar tissue were characterized using proton-induced X-ray emission (PIXE). Skin tissue that had been covered in SGS was also analyzed. For each element and sample type, the concentrations in the epidermis were plotted against the dermis. By considering the concentrations of breast tissue with and without SGS, it could be seen if the SGS changed the compositional structure of the skin. It was found that for the elements P, S, Cl, and K the SGS has no effect on the structure of the skin, as both breast types (with and without SGS) have regression lines that overlap. However, this work shows that there are significant differences for P in the dermis and Cl in the epidermis between the breast tissue with SGS and its control. Therefore, this work shows that the effect the SGS has on concentration occurs similarly for both the epidermis and dermis.