Alex Elliott

Comparison of repeatability of the multifocal electroretinogram and Humphrey perimeter.

Functional mapping of the retina by multifocal electroretinographic recordings is now possible. We compared the normal range, repeatability and response topography of this new technique with conventional static Humphrey perimetry to assess its suitability in clinical practice. The multifocal technique was performed on 60 age-matched controls. Measures of repeatability and reproducibility were obtained. Results were then compared with those obtained from a customized perimetry test. In both tests the coefficients of repeatability were found to decrease with eccentricity. The inherent measurement variation between techniques was comparable. Overall system variation indicates that the technique could be a useful tool at the clinical level.

A novel method for patient exit and entrance dose prediction based on water equivalent path length measured with an amorphous silicon electronic portal imaging device

In vivo dosimetry is one of the quality assurance tools used in radiotherapy to monitor the dose delivered to the patient. Electronic portal imaging device (EPID) images for a set of solid water phantoms of varying thicknesses were acquired and the data fitted onto a quadratic equation, which relates the reduction in photon beam intensity to the attenuation coefficient and material thickness at a reference condition. The quadratic model is used to convert the measured grey scale value into water equivalent path length (EPL) at each pixel for any material imaged by the detector. For any other non-reference conditions, scatter, field size and MU variation effects on the image were corrected by relative measurements using an ionization chamber and an EPID. The 2D EPL is linked to the percentage exit dose table, for different thicknesses and field sizes, thereby converting the plane pixel values at each point into a 2D dose map. The off-axis ratio is corrected using envelope and boundary profiles generated from the treatment planning system (TPS). The method requires field size, monitor unit and source-to-surface distance (SSD) as clinical input parameters to predict the exit dose, which is then used to determine the entrance dose. The measured pixel dose maps were compared with calculated doses from TPS for both entrance and exit depth of phantom. The gamma index at 3% dose difference (DD) and 3 mm distance to agreement (DTA) resulted in an average of 97% passing for the square fields of 5, 10, 15 and 20 cm. The exit dose EPID dose distributions predicted by the algorithm were in better agreement with TPS-calculated doses than phantom entrance dose distributions.

Issues in medical exposures

Medical exposures account, on average, for some 14% of the background ionising radiation exposure in the UK and form the great majority of the non-natural component. In the United States of America, medical exposures comprised over 50% of the total in 2006. This is due primarily to an increase in x-ray computed tomography (CT) and positron emission tomography (PET) procedures. This paper highlights the potential problems in the use of CT scanning to investigate the asymptomatic individual, where the traditional risk/benefit considerations are less clear-cut than in conventional clinical situations. It draws on a recent COMARE report which examined the use of CT for whole body, heart, lung and colon studies. The number of PET facilities is increasing rapidly in the UK and, in addition to considerations of radiation dose to subjects, careful planning is necessary to limit doses to staff. In non-ionising radiation, a topic of keen interest at present is the use of increasingly powerful sunbeds, particularly by those aged under 18. Legislation and regulation vary widely across Europe and the Scottish Parliament has recently introduced the first UK regulation. It is suggested that further research is required into the effects of current UV systems and the reasons why tanning is thought so desirable by Caucasians. Lastly, a number of issues requiring radiobiological and epidemiological input are considered and actions to satisfy these identified.

Evaluation of six TPS algorithms in computing entrance and exit doses.

Entrance and exit doses are commonly measured in in vivo dosimetry for comparison with expected values, usually generated by the treatment planning system (TPS), to verify accuracy of treatment delivery. This report aims to evaluate the accuracy of six TPS algorithms in computing entrance and exit doses for a 6 MV beam. The algorithms tested were: pencil beam convolution (Eclipse PBC), analytical anisotropic algorithm (Eclipse AAA), AcurosXB (Eclipse AXB), FFT convolution (XiO Convolution), multigrid superposition (XiO Superposition), and Monte Carlo photon (Monaco MC). Measurements with ionization chamber (IC) and diode detector in water phantoms were used as a reference. Comparisons were done in terms of central axis point dose, 1D relative profiles, and 2D absolute gamma analysis. Entrance doses computed by all TPS algorithms agreed to within 2% of the measured values. Exit doses computed by XiO Convolution, XiO Superposition, Eclipse AXB, and Monaco MC agreed with the IC measured doses to within 2%‐3%. Meanwhile, Eclipse PBC and Eclipse AAA computed exit doses were higher than the IC measured doses by up to 5.3% and 4.8%, respectively. Both algorithms assume that full backscatter exists even at the exit level, leading to an overestimation of exit doses. Despite good agreements at the central axis for Eclipse AXB and Monaco MC, 1D relative comparisons showed profiles mismatched at depths beyond 11.5 cm. Overall, the 2D absolute gamma (3%/3 mm) pass rates were better for Monaco MC, while Eclipse AXB failed mostly at the outer 20% of the field area. The findings of this study serve as a useful baseline for the implementation of entrance and exit in vivo dosimetry in clinical departments utilizing any of these six common TPS algorithms for reference comparison. PACS numbers: 87.55.‐x, 87.55.D‐, 87.55.N‐, 87.53.BnEntrance and exit doses are commonly measured in in vivo dosimetry for comparison with expected values, usually generated by the treatment planning system (TPS), to verify accuracy of treatment delivery. This report aims to evaluate the accuracy of six TPS algorithms in computing entrance and exit doses for a 6 MV beam. The algorithms tested were: pencil beam convolution (Eclipse PBC), analytical anisotropic algorithm (Eclipse AAA), AcurosXB (Eclipse AXB), FFT convolution (XiO Convolution), multigrid superposition (XiO Superposition), and Monte Carlo photon (Monaco MC). Measurements with ionization chamber (IC) and diode detector in water phantoms were used as a reference. Comparisons were done in terms of central axis point dose, 1D relative profiles, and 2D absolute gamma analysis. Entrance doses computed by all TPS algorithms agreed to within 2% of the measured values. Exit doses computed by XiO Convolution, XiO Superposition, Eclipse AXB, and Monaco MC agreed with the IC measured doses to within 2%-3%. Meanwhile, Eclipse PBC and Eclipse AAA computed exit doses were higher than the IC measured doses by up to 5.3% and 4.8%, respectively. Both algorithms assume that full backscatter exists even at the exit level, leading to an overestimation of exit doses. Despite good agreements at the central axis for Eclipse AXB and Monaco MC, 1D relative comparisons showed profiles mismatched at depths beyond 11.5 cm. Overall, the 2D absolute gamma (3%/3 mm) pass rates were better for Monaco MC, while Eclipse AXB failed mostly at the outer 20% of the field area. The findings of this study serve as a useful baseline for the implementation of entrance and exit in vivo dosimetry in clinical departments utilizing any of these six common TPS algorithms for reference comparison. PACS numbers: 87.55.-x, 87.55.D-, 87.55.N-, 87.53.Bn.

Measurement of Pulse Wave Velocity using Magnetic Resonance Imaging

Arterial stiffness has a strong relationship with cardiovascular disease. Pulse wave velocity (PWV) is increasingly used as a measure of arterial stiffness. The calculation of PWV requires accurate measurement of blood flow velocity and aortic length. Using magnetic resonance imaging (MRI), we are able to accurately acquire blood flow velocity and to virtually measure the aortic length inside the central aorta. Manual measurement of aorta length is inaccurate and subjective. In this study, we set out to develop a method which automatically locates the thoracic aorta and measures aortic length, allowing noninvasive measurement of PWV derived from the aortic flow. Our method is novel, efficient and robust. This offers a reliable and convenient tool for PWV measurement allowing detailed non invasive assessment of cardiovascular risk.

Determination of left ventricular long-axis orientation using MRI: changes during the respiratory and cardiac cycles in normal and diseased subjects

Background:  It has previously been shown that magnetic resonance imaging (MRI) can be used to accurately determine left ventricular (LV) long‐axis orientation in healthy individuals. However, the inter‐ and intra‐observer variability in patients with acute coronary syndrome (ACS) and chronic heart failure (CHF) has not been explored. Furthermore, the changes in LV long‐axis orientation because of respiration and during the cardiac cycle remain to be determined.

A high sensitivity dual-detector shadow-shield whole-body counter with an 'invariant' response for total body in vivo neutron activation analysis

The development of a dual-detector shadow-shield whole-body counting system of high sensitivity is described. The sensitivity and performance of the counter are analysed, with specific references to total body in vivo neutron activation analysis, and shown to be at least comparable with those of many conventional shielded-room counters.

Investigation of 18F-FDG 3D mode PET image quality versus acquisition time

ObjectiveThree-dimensional (3D) mode positron emission tomography (PET) is being used increasingly for clinical PET imaging. However, as yet, optimal acquisition parameters have not been established. The aim of this study was to investigate the effect of varying acquisition time on 3D image quality using standard clinical activities of 18F-fluorodeoxyglucose (FDG). Methods18F-FDG phantom and patient PET images were acquired with varying acquisition times on a GE Discovery-STE PET/CT system. The NEMA Image-Quality phantom was imaged with four hot lesions in a uniform background. Images were acquired for 1, 2, 3, 4, and 5-min frames with three different lesion-to-background contrast ratios. Patient data were investigated using list mode acquisition to obtain comparable 2, 3, and 4-min frames. Qualitative analysis involved grading image quality and lesion detectability. Quantitative analysis of phantom images involved assessing the coefficient of variation (COV) of background areas as a measure of noise, and lesion over background variability as a measure of image quality. Patient data were also assessed using COV analysis of liver uptake. ResultsQualitative and quantitative analysis showed no significant difference in image quality between 4 and 5-min acquisition frames for 3D mode 18F-FDG PET imaging with standard clinical activities. The observers noted no difference in perceived image quality. This finding was supported by COV analysis. ConclusionThis study shows that GE Discovery-STE acquisition frame time can be reduced to 3 min for standard 3D mode imaging at standard clinical activities of 18F-FDG.

A restatement of the natural science evidence base concerning the health effects of low-level ionizing radiation

Exposure to ionizing radiation is ubiquitous, and it is well established that moderate and high doses cause ill-health and can be lethal. The health effects of low doses or low dose-rates of ionizing radiation are not so clear. This paper describes a project which sets out to summarize, as a restatement, the natural science evidence base concerning the human health effects of exposure to low-level ionizing radiation. A novel feature, compared to other reviews, is that a series of statements are listed and categorized according to the nature and strength of the evidence that underpins them. The purpose of this restatement is to provide a concise entrée into this vibrant field, pointing the interested reader deeper into the literature when more detail is needed. It is not our purpose to reach conclusions on whether the legal limits on radiation exposures are too high, too low or just right. Our aim is to provide an introduction so that non-specialist individuals in this area (be they policy-makers, disputers of policy, health professionals or students) have a straightforward place to start. The summary restatement of the evidence and an extensively annotated bibliography are provided as appendices in the electronic supplementary material.

Reconstructing the abundance of Dounreay hot particles on an adjacent public beach in Northern Scotland

Following the discovery of a number of hot particles in the offshore environment of Dounreay on the North Coast of Scotland in 1997, the Dounreay site operator was required to introduce rapid and extensive beach monitoring. Since the introduction of vehicular based beach monitoring in 1999 there have been two further generations of beach monitoring equipment, developed to satisfy regulatory requirements for particle detection and in response to the recommendations of the Dounreay Particles Advisory Group (DPAG). This paper reports the results of DPAGs review of beach monitoring capabilities, evaluating the factors influencing detection capability, assessing the likely monthly particle abundance and whether there has been any real change in particle arrivals with time. The incorporation of real time kinematic GPS has enabled changes in beach elevation to be mapped, and thus allowed the assessment of whether particles detected have recently arrived or may have been buried undisturbed for extended periods of time. The results focus on Sandside Beach from which, between 1984 and December 2009, 150 particles have been recovered. This is by far the largest number of particles found on a Caithness Beach with the exception of the Foreshore of the Dounreay site. The results suggested that there is no evidence for a change in the rate of particle arrivals and DPAG estimated that there is a 1 in 20 million chance of encountering a relevant particle via contact with the skin on Sandside Beach.