UK National Physical Laboratory Research Articles

Quantitative validation of Monte Carlo SPECT simulation: application to a Mediso AnyScan GATE simulation

BackgroundMonte Carlo (MC) simulations are used in nuclear medicine imaging as they provide unparalleled insight into processes that are not directly experimentally measurable, such as scatter and attenuation in an acquisition. Whilst MC is often used to provide a ‘ground-truth’, this is only the case if the simulation is fully validated against experimental data. This work presents a quantitative validation for a MC simulation of a single-photon emission computed tomography (SPECT) system.MethodsAn MC simulation model of the Mediso AnyScan SCP SPECT system installed at the UK National Physical Laboratory was developed in the GATE (Geant4 Application for Tomographic Emission) toolkit. Components of the detector head and two collimator configurations were modelled according to technical specifications and physical measurements. Experimental detection efficiency measurements were collected for a range of energies, permitting an energy-dependent intrinsic camera efficiency correction function to be determined and applied to the simulation on an event-by-event basis. Experimental data were collected in a range of geometries with ^{99text {m}}Tc for comparison to simulation. The procedure was then repeated with ^{177}Lu to determine how the validation extended to another isotope and set of collimators.ResultsThe simulation’s spatial resolution, sensitivity, energy spectra and the projection images were compared with experimental measurements. The simulation and experimental uncertainties were determined and propagated to all calculations, permitting the quantitative agreement between simulated and experimental SPECT acquisitions to be determined. Statistical agreement was seen in sinograms and projection images of both ^{99text {m}}Tc and ^{177}Lu data. Average simulated and experimental sensitivity ratios of (0.991 pm 0.011) were seen for emission and scatter windows of ^{99text {m}}Tc, and (0.897 pm 0.014) and (0.839 pm 0.014) for the 113 and 208 keV emissions of ^{177}Lu, respectively.ConclusionsMC simulations will always be an approximation of a physical system and the level of agreement should be assessed. A validation method is presented to quantify the level of agreement between a simulation model and a physical SPECT system.

The use of a parametric array source and nearfield scanning in the characterisation of panel materials for underwater acoustics

The properties of the materials used in underwater acoustics are important for applications such as acoustic windows, reflectors and baffles, acoustic barriers or screens, decoupling materials, and anechoic coatings. To characterise the performance of such materials at frequencies above 1 kHz, measurements are typically undertaken on samples of the material in the form of finite sized panels. Such measurements suffer from uncertainty due to the finite size of the panel (leading to contaminating signals from edge diffraction), and the difficulty in simulating the ideal plane-wave insonification. This paper describes work at the UK National Physical Laboratory to minimise these effects by use of: (i) a parametric array as a sound source that provides a directional beam and short broadband pulses; and (ii) nearfield scanning using a hydrophone to sample the complex sound pressure field interacting with the test sample, decomposing the sound field into its plane-wave components. Results are presented of these techniques applied to measurements in laboratory test tanks at frequencies between a few kilohertz and a few hundred kilohertz to determine the reflection and transmission performance of a range of test samples, including panels consisting of homogeneous polymers and materials with regular periodic structure.

On ISO 21909–1:2015 and the metrological performance of the Landauer Neutrak-T dosemeter

Personal dosimetry for ionizing radiation is a regulated activity in Europe. More particularly, in France Individual Monitoring Services (IMS) are required to be accredited against ISO/IEC Standard 17025 (ISO 17025:2017) for the personal dose equivalent measurements they perform. In the field of external personal dosimetry for ionizing radiations, there are no standardized methods for routine measurements of participants' personal dose equivalent. Instead, the standards focus on defining protocols and associated acceptable performance limits for testing the measurement process. ISO Standard 21909 is the one related to passive neutron dosimetry systems. This standard has been completely revised and its newest version was published in 2015. The two versions of the standard differ in terms of methodology and performance limits. At the time when the new version was written its effects on system compliance were unknown. Landauer provides individual monitoring services for ionizing radiation dosimetry worldwide. The service for neutrons is based on an internally developed dosemeter called Neutrak® which has two versions: Neutrak-J for fast neutrons and Neutrak-T for fast and thermal. The study described here focuses on the neutron dosimetry system as implemented at the Paris office. This system has been tested by the UK National Physical Laboratory (NPL) against ISO 21909–1:2015. The NPL is one of the few calibration laboratories able to cover the extensive series of tests required by the standard. The purpose of this paper is twofold. Firstly, it aims to present the differences between ISO 21909:2005 and ISO 21909–1:2015 for non-linearity, reproducibility, energy-angle dependency and fading. This illustrates the challenges arising from ISO 21909–1:2015 and should prove useful to other IMSs since, to the authors’ knowledge, the work described herein is the first of its kind. The second aim is to document the results of the type test of the LANDAUER neutron dose measurement process based on the Neutrak-T dosemeter.

IPEM code of practice for high-energy photon therapy dosimetry based on the NPL absorbed dose calibration service

The 1990 code of practice (COP), produced by the IPSM (now the Institute of Physics and Engineering in Medicine, IPEM) and the UK National Physical Laboratory (NPL), gave instructions for determining absorbed dose to water for megavoltage photon (MV) radiotherapy beams (Lillicrap et al ). The simplicity and clarity of the 1990 COP led to widespread uptake and high levels of consistency in external dosimetry audits. An addendum was published in 2014 to include the non-conventional conditions in Tomotherapy units. However, the 1990 COP lacked detailed recommendations for calibration conditions, and the corresponding nomenclature, to account for modern treatment units with different reference fields, including small fields as described in IAEA TRS483 (International Atomic Energy Agency (IAEA) , Vienna). This updated COP recommends the irradiation geometries, the choice of ionisation chambers, appropriate correction factors and the derivation of absorbed dose to water calibration coefficients, for carrying out reference dosimetry measurements on MV external beam radiotherapy machines. It also includes worked examples of application to different conditions. The strengths of the 1990 COP are retained: recommending the NPL2611 chamber type as secondary standard; the use of tissue phantom ratio (TPR) as the beam quality specifier; and NPL-provided direct calibration coefficients for the user’s chamber in a range of beam qualities similar to those in clinical use. In addition, the formalism is now extended to units that cannot achieve the standard reference field size of 10 cm × 10 cm, and recommendations are given for measuring dose in non-reference conditions. This COP is designed around the service that NPL provides and thus it does not require the range of different options presented in TRS483, such as generic correction factors for beam quality. This approach results in a significantly simpler, more concise and easier to follow protocol.

Cavitation-erosion measurements on engineering materials

The resistance of a material to cavitation erosion is assessed by measuring specimen weight change induced by the application of a high-power vibration horn close its surface. This paper describes a proof-of-concept study of a measurement technique for assessing the progression of cavitation erosion for commonly used engineering materials. UK National Physical Laboratory Cavitation Sensor enabled the generated acoustic signals produced by inertial cavitation collapse. Results suggest that acoustic emission monitoring shows promise as a tool which can be employed to quantitatively record the stability of the applied cavitation erosion stimulus.

Validation of ammonia diffusive and pumped samplers in a controlled atmosphere test facility using traceable Primary Standard Gas Mixtures

We report the determination of ammonia (NH3) diffusive sampling rates for six different designs of commercial diffusive samplers (CEH ALPHA sampler, Gradko diffusion tube, Gradko DIFRAM-400, Passam ammonia sampler, and ICS Maugeri Radiello radial sampler (blue and white turbulence barriers)), together with the validation test results for a pumped sampler (CEH DELTA denuder). The devices were all exposed in the UK's National Physical Laboratory's (NPL) controlled atmosphere test facility (CATFAC). For each of the seven diffusive sampler exposure tests there were traceable concentrations of ammonia (in the range 3–25 μg m−3) generated under well-defined conditions of temperature, relative humidity and wind speed, which are applicable to a variety of ambient monitoring environments. The sampler exposure time at each concentration was 28 days, except for the radial devices, which were exposed for 14 days. The work relied on the dilution of newly developed stable Primary Standard Gas Mixtures (PSMs) prepared by gravimetry in passivated gas cylinders as a method of improving the metrological traceability of ammonia measurements. The exposed diffusive samplers were sent blind to the participants for analysis and the reported NH3 concentrations were then compared against the known reference concentration. From the results for each sampler type a diffusive sampling rate was calculated and compared against the rate used routinely by the participants. Some measurement results were in good agreement with the known traceable reference concentration (particularly for one diffusive sampler design (ALPHA)), while other devices exhibited over-reading and under-reading (each with a clear bias). The new diffusive sampling rates determined in the laboratory study were then applied to measurements in a field comparison campaign, and this was found to deliver an improvement in agreement between the different devices deployed.

Doped silica fibre thermoluminescence measurements of radiation dose in the use of 223Ra

Using tailor-made sub-mm dimension doped-silica fibres, thermoluminescent dosimetric studies have been performed for α-emitting sources of 223RaCl2 (the basis of the Bayer Healthcare product Xofigo®). The use of 223RaCl2 in the palliative treatment of bone metastases resulting from late-stage castration-resistant prostate cancer focuses on its favourable uptake in metabolically active bone metastases. Such treatment benefits from the high linear energy transfer (LET) and associated short path length (<100µm) of the α-particles emitted by 223Ra and its decay progeny. In seeking to provide for in vitro dosimetry of the α-particles originating from the 223Ra decay series, investigation has been made of the TL yield of various forms of Ge-doped SiO2 fibres, including photonic crystal fibre (PCF) collapsed, PCF uncollapsed, flat and single-mode fibres. Irradiations of the fibres were performed at the UK National Physical Laboratory (NPL). Notable features are the considerable sensitivity of the dosimeters and an effective atomic number Zeff approaching that of bone, the glass fibres offering the added advantage of being able to be placed directly into liquid. The outcome of present research is expected to inform development of doped fibre dosimeters of versatile utility, including for applications as detailed herein.

Printing: the future of THz

Researchers from Imperial College London, the UK's National Physical Laboratory (NPL), and the University of Tokyo, have demonstrated a 3D-printed metal-pipe rectangular waveguide operating at a record-breaking frequency of 1.1 THz. Stepan Lucyszyn and William Otter, two of the researchers behind the record-breaking waveguides. 3D printing has found uses in a huge number of industries and applications, and its strengths over traditional manufacturing techniques were quickly exploited by the communications community. William Otter and Stepan Lucyszyn, two of the researchers on the THz project, told us that at Imperial College London, ‘the initial research into 3D printing of radio frequency metal-pipe rectangular waveguides was for the microwave frequency range of 8 to 12 GHz (X-band), and the success of this work gave our team the confidence to move up into the millimetre-wave frequency band, of 75–110 GHz (W-band), where the small dimensions require a tighter manufacturing tolerance.’ 3D printing, also known as additive manufacturing, is the layer-by-layer addition of material to create a structure. It dramatically reduces the time and the cost of producing components, while allowing more design freedom. In comparison with traditional manufacturing, ever more complex geometries can be realised with 3-D printing as fewer mechanical constraints are imposed on the process. With large complex structures, this means reducing the number of individual components needed to create a subsystem, thereby reducing the number of tests required during the manufacturing process. This design freedom helped produce excellent initial results in the W-band, comparable with off-the-shelf commercial waveguides. However, as Otter and Lucyszyn explained, they did not stop there, ‘This high level of performance was also demonstrated at W-band with a 6th-order Chebyshev filter, which are notoriously sensitive to manufacturing tolerances.’ The next challenge was to demonstrate a mechanically tuneable component that was fully 3D-printed. This was achieved using an X-band dielectric-flap phase shifter. After these early successes the team kept increasing the frequencies of their components. ‘We were pushing the mechanical limits of all 3D printing processes, but, we set our sights specifically on breaking the record for metal-pipe rectangular waveguides,’ said Otter and Lucyszyn, ‘and this was achieved with an I-Q vector modulator operating up to 500 GHz.’ Overcoming the limitations of 3D printing, specifically in regards to THz technology, to achieve record frequencies is where the collaboration between the UK and Japanese began: ‘The main limitation for THz waveguides is the ability to 3D print with relatively large structures, having small feature sizes and high-quality surface finishes.’ For this reason, the prototype for a new class of 3D-printers called RECILS, and developed at the University of Tokyo was used. The new RECILS technology, through its innovative method of controlling the photocurable resin level and detachment of cured resin, provides the required finish without further post-processing.’ Now that the team have achieved inexpensive THz technology, what doors do they hope to have opened? The work is expected to find future applications in high-performance imaging systems for security monitoring, satellite payloads and medical diagnostic equipment. Thanks to the team's techniques, according to Otter and Lucyszyn, ‘these systems can be developed in a shorter time and at a significantly lower cost, and, as such, [the method] represents a paradigm shift in the manufacture of precision components using mass marketable 3D printers.’ From left to right: X-Band phase shifter, W-Band thru line, W-Band 6th-order Chebyshev filter, 500 GHz I-Q vector modulator. In the background is an X-band twist and the foreground shows the 1.1 THz waveguide. To move ever closer to such a reality, the group have been developing minimal-part subsystems that break conventional design rules by exploiting the multiple degrees-of-freedom offered by additive manufacturing. And this is, as Otter and Lucyszyn explained, the really exciting thing about the technology. ‘Within a decade, ultra-high resolution low-cost 3D printing will be ubiquitous. It will become the norm to replace parts by downloading CAD files and printing and spray coating metal onsite.’ This is particularly important for remote and inaccessible locations. Moreover, a printer offering multiple materials, perhaps even having exotic properties, will be capable of printing whole systems, the only limit being our imaginations.’

Applications of LaBr3(Ce) Gamma-ray Spectrometer Arrays for Nuclear Spectroscopy and Radionuclide Assay

An overview of the use of discrete energy gamma-ray detectors based on cerium- doped LaBr3 scintillators for use in nuclear spectroscopy is presented. This review includes recent applications of such detectors in mixed, 'hybrid' gamma-ray coincidence detection arrays such ROSPHERE at IFIN-HH, Bucharest; EXILL+FATIMA at ILL Grenoble, France; GAMMASPHERE+FATIMA at Argonne National Laboratory, USA; FATIMA + EURICA, at RIKEN, Japan; and the National Nuclear Array (NANA) at the UK's National Physical Laboratory. This conference paper highlights the capabilities and limitations of using these sub-nanosecond 'fast-timing', medium-resolution gamma-ray detectors for both nuclear structure research and radionuclide standardisation. Potential future application of such coincidence scintillator arrays in measurements of civilian nuclear fuel waste evaluation and assay is demonstrated using coincidence spectroscopy of a mixed 134,7Cs source.

Investigating dynamic measurement applications through modelling and simulation

Abstract Dynamic measurement challenges are ubiquitous in metrology and can be found in many measurement applications that are not conventionally regarded as dynamic. We present four recent examples of work at the UK's National Physical Laboratory that demonstrate how modelling and simulation can contribute to improved understanding of dynamic measurement tasks. The examples are (i) a software simulation of a lock-in amplifier, (ii) a simulation of a sensor network in which one of the sensors has insufficient bandwidth for the measurement task and Kalman filter based data fusion is used to aggregate the sensor outputs, (iii) a study of performance imperfections in a clock embedded in a wireless sensor node using a Monte Carlo based method for simulating counting errors, and (iv) a simulation of wave propagation in a shock tube in which the lattice Boltzmann method was used to study non-ideal behaviour of the shock tube. It is shown that simulation (both physically based and phenomenological) is useful in designing measuring systems and identifying and quantifying measurement uncertainties and that the development of simulation software requires the developer to have a clear understanding of the measuring system of interest.

A Calibration Facility for Dew Point in Air up to 1 MPa

The provision of primary dew-point standards for humidified air and nitrogen at atmospheric pressure is well established, and measurement traceability to these standards provides confidence in a vast number of air humidity measurements. However, hygrometers are used industrially at a wide range of pressures. Both the performance of hygrometers and the properties of humid gases are known to vary with gas pressure. The pressure-dependence of gas non-ideality for air–water mixtures (water-vapor enhancement factor) is well enough known at moderate pressures, but there remains a need to characterize hygrometers at the pressure of use. To address this, a humidity calibration capability of wider scope is under development at the UK National Physical Laboratory (NPL). As an initial stage in the development of this capability, a humidity standard generating air or nitrogen in the dew-point range from $$-60\,^{\circ }\hbox {C}$$ to $$+10\,^{\circ }\hbox {C}$$ , at pressures up to 1 MPa (10 bar) has been validated for the calibration of hygrometers. The expanded uncertainty of the dew-point generator in this range with a coverage factor $$k= 2$$ is $$\pm 0.07\,^{\circ }\hbox {C}$$ .

Development of NANA: A Fast-Scintillator, Coincidence Gamma-ray Array for Radioactive Source Characterisation and Absolute Activity Measurements at the UK National Physical Laboratory

A multi-detector modular coincidence gamma-ray spectrometer is being designed and constructed for use at the UK's National Physical Laboratory (NPL) for use in direct measurement and metrological standardisation of nuclear decay activities. In its first generation, the NPL National Nuclear Array (NANA) will consist of twelve individual halide scintillation detectors placed in a high-efficiency geometry around a well-defined central point source position. This brief conference paper provides details of the measured detector module and coincidence energy and timing responses for the LaBr3(Ce) detectors which will be used in the NANA array. Preliminary GEANT4 simulations of the array's full energy peak efficiency and expected gamma-ray coincidence response are also presented.

Progress towards the production of the 236gNp standard sources and competing fission fragment production

Abstract The isobaric distribution of fission residues produced following the bombardment of a natural uranium target with a beam of 25 MeV protons has been evaluated. Decay analysis of thirteen isobarically distinct fission residues were carried out using high-resolution γ-spectrometry at the UK National Physical Laboratory. Stoichiometric abundances were calculated via the determination of absolute activity concentrations associated with the longest-lived members of each isobaric chain. This technique was validated by computational modelling of likely sequential decay processes through an isobaric decay chain. The results were largely in agreement with previously published values for neutron bombardments on 238 U at energies of 14 MeV. Higher yields of products with mass numbers A ~110–130 were found, consistent with the increasing yield of these radionuclides as the bombarding energy is increased.

Roos and NACP-02 ion chamber perturbations and water-air stopping-power ratios for clinical electron beams for energies from 4 to 22 MeV

Empirical fits are developed for depth-compensated wall- and cavity-replacement perturbations in the PTW Roos 34001 and IBA / Scanditronix NACP-02 parallel-plate ionisation chambers, for electron beam qualities from 4 to 22 MeV for depths up to approximately 1.1 × R50,D. These are based on calculations using the Monte Carlo radiation transport code EGSnrc and its user codes with a full simulation of the linac treatment head modelled using BEAMnrc. These fits are used with calculated restricted stopping-power ratios between air and water to match measured depth-dose distributions in water from an Elekta Synergy clinical linear accelerator at the UK National Physical Laboratory. Results compare well with those from recent publications and from the IPEM 2003 electron beam radiotherapy Code of Practice.

Just doing our DUTy!—An international project on dosimetry for ultrasound therapy

This presentation will describe a large international project which aims to develop the metrological infrastructure for the determination of ultrasound exposure and dose to tissue. Ultimately this will improve treatment planning and risk assessment. The 3-year project started in June 2012. It is co-ordinated by the UK National Physical Laboratory and is funded in part by the European Metrology Research Programme of the European Union. The other project partners are INRIM (IT), PTB (DE), UME (TR), CSIC (ES), ICR (UK), Moscow State University (RU), NIM (CN), and University of Merseburg (DE). The scientific work is divided into six Work Packages: —Quantities and definitions will review dose and in situ exposure quantities proposing alternatives where necessary; —Laboratory dosimetry standards will develop reference methods for thermal and non-thermal therapeutic dose parameters; —Dose modeling and validation will develop theoretical and computer modelling procedures to calculate acoustic and thermal dose quantities with well understood uncertainties; —Intercomparison of methods will formally compare measurement methods and models developed; —Dosimetry Transfer Standards will develop phantoms and test systems for the assessment of clinical ultrasound equipment, transferring laboratory standards to end-users; —Application to clinical treatment will provide a direct link between the methods developed and the clinical use of therapeutic ultrasound with the long-term aim of improving treatment planning.

In situ diagnostics for PEM fuel cells at the National Physical Laboratory

The UK's National Physical Laboratory (NPL) has developed a range of advanced in situ diagnostic techniques to support research into mitigation strategies for critical PEM fuel cell (PEMFC) degradation mechanisms. Recent innovations include a new reference electrode configuration that for the first time allows mapping of the electrode potential across the active area of an operating PEMFC; a novel technique for simultaneous measurement of active catalyst area in each cell in a fuel cell stack; a world-first measurement of the relative humidity profile in the gas channels of a working PEMFC; and a non-invasive thermal imaging technique for detection of hotspots. In addition to providing valuable data on performance and durability, these in situ measurement techniques are being used to validate a 3D fuel cell model under development at NPL, a powerful design optimisation tool that will be available to industry in an accessible software platform.

CCTV gait recognition offers ‘natural’ interaction

The UK National Physical Laboratory (NPL) has developed a walking gait recognition system that, in combination with other tools, can help track an individual though a CCTV monitored area by analysing the way that they walk.

NPL's new absorbed dose standard for the calibration of HDR 192Ir brachytherapy sources

A novel graphite calorimeter for absorbed dose rate measurements close to high dose rate (HDR) 192Ir brachytherapy sources has been designed and built at the UK National Physical Laboratory (NPL). The graphite calorimeter allows a more direct calibration of HDR 192Ir sources in terms of absorbed dose rate to water at a distance of 1 cm, , compared with the current air kerma-based calibration method, where is determined as the product of the measured air kerma strength and the dose rate constant, Λ, resulting in overall standard uncertainties of up to 5%. With the new absorbed dose standard, can be measured directly with standard uncertainties between 0.7% and 1.0%, depending on the operating mode of the calorimeter. A Nucletron microSelectron-v1 Classic HDR 192Ir source was calibrated with the calorimeter in terms of . The same source was also calibrated with NPL's HDR 192Ir air kerma primary standard in terms of reference air kerma rate. Combining both measurements yielded the experimentally determined dose rate constant of the 192Ir source.

Automatic laser control for a 88Sr+ optical frequency standard

Optical frequency standards, based on the 674 nm 2S1/2–2D5/2 transition in a single cold trapped 88Sr+ ion, are being characterized at the UK National Physical Laboratory. Routine operation is required for measurement campaigns over several days and there is also a long-term requirement for unattended operation in remote environments, such as on a satellite or at a spacecraft tracking ground station. Software algorithms have been developed which enable the automatic relocking of multiple extended cavity diode lasers to optical cavities following out-of-lock errors. Additional algorithms are used to monitor and control auxiliary distributed feedback diode lasers by means of a wavemeter. Using this control system, unattended operation for more than 60 h has been demonstrated. Preliminary results are presented giving a single-trap relative frequency stability of 2.5 × 10−16 at τ = 4000 s. This capability paves the way for a reliable Sr+ ion-trap-based optical standard capable of operating remotely for extended periods.

Digital Image Correlation

Strain and displacement are critical parameters within engineering and construction projects. However, measuring these parameters outside of the lab requires a difficult choice between conventional techniques, as accuracy, simplicity and cost must all be balanced. Digital Imaging Correlation (DIC) is a technique which may prove to be ideally suited for the study of crack propagation and material deformation in real-world applications, as it has the potential to become a cheap, simple yet accurate solution. This is the motivation for perfecting the technique for researchers based at the UK's National Physical Laboratory (NPL). NPL is also expanding its use into challenging areas, such as enabling DIC to be used over a period of time without the need for a permanent set up, or in hard-to-reach areas via small unmanned helicopters carrying camera and relocation equipment.