Gavin Poludniowski

Karolinska University Hospital | Karolinska · Medical Radiation Physics and Nuclear Medicine

Proton radiography and tomography have long promised benefit for proton therapy. Their first suggestion was in the early 1960s and the first published proton radiographs and CT images appeared in the late 1960s and 1970s, respectively. More than just providing anatomical images, proton transmission imaging provides the potential for the more accura...

Purpose: To develop an analytical model for bremsstrahlung production in a thick x-ray target (i.e., the x-ray tube anode) that takes into account the intrinsic bremsstrahlung angular distribution. Methods: X-ray spectrum models developed from theoretical principles have traditionally treated the angular distribution of the bremsstrahlung produc...

Purpose: SpekPy is a free toolkit for modeling x-ray tube spectra with the Python programming language. In this article, the advances in version 2.0 (v2) of the software are described, including additional target materials and more accurate modeling of the heel effect. Use of the toolkit is also demonstrated. Methods: The predictions of SpekPy a...

Introduction: Dose-response relationships for local control of lung tumours treated with stereotactic body radiotherapy (SBRT) have proved ambiguous, however, these have been based on the prescribed or planned dose. Delivered dose to the target may be a better predictor for local control. In this study, the probability of the delivered minimum dos...

Purpose: Conventional cone-beam computed tomography CT (CBCT) provides limited discrimination between low-contrast tissues. Furthermore, it is limited to full-spectrum energy integration. A dual-energy CBCT system could be used to separate photon energy spectra with the potential to increase the visibility of clinically relevant features and acqui...

Perhaps the most measured metric of beam quality is the half-value layer (HVL). The first and second half-value layers (HVL1 and HVL2), together, give a good indication of the penetrative characteristics of an x-ray beam. It is straightforward to obtain estimates of HVLs using the spectrum calculation software discussed in this book. When predictio...

In this chapter, some important features of x-ray tube design are explained, and those aspects are defined that are important for the modelling approaches presented later in the book. The text begins with a discussion of the evolution of the x-ray tube since Roentgen's discovery of x rays. It then introduces the essential parameters and terminology...

This chapter summarizes the physics of x-ray generation in an x-ray tube anode, including the penetration of electrons into the target, and the particle-atom interactions that generate x rays. The chapter begins with a discussion of the salient points of electron transport at energies corresponding to kilovoltage potentials, including inelastic col...

Given an x-ray beam, the dosimetry goal is to determine the absorbed dose at some specific point in a medium under so-called reference conditions using a calibrated suitable detector, usually an ionization chamber. The process is known as beam calibration and establishes the reference dosimetry against which any other condition refers to, or any ot...

In this chapter, the basic concepts and formulations entering into the determination of the absorbed dose to a patient—or to any material exposed to a beam of x rays—is provided. Basic quantities of different types forming the basis of radiation dosimetry are defined. The various coefficients and factors necessary for the dosimetry of an x-ray spec...

An advanced analytical model for the x-ray spectra emitted from an x-ray tube is described in detail in this chapter. The model incorporates a thorough treatment of the electron transport, utilizing Monte Carlo-simulated distributions of the number of electrons at depth in the target and the energy and direction at each depth. The cross-sections us...

The Monte Carlo method is a numerical technique that makes use of repeated random sampling to simulate the behaviour of a complex system. This chapter provides an overview of the Monte Carlo modelling of x-ray tube spectra and includes a summary of relevant theoretical and practical aspects. The chapter begins with an introduction to the general pr...

In this chapter, a simple figure-of-merit denoted the dose-normalized contrast-to-noise ratio (CNRD) is introduced to optimize the trade-off between image quality and radiation exposure in digital radiography. In the examples, the dose metric used for normalization is the air kerma incident on a patient. The two example tasks explored are those of...

This book provides a comprehensive review of the modelling of x-ray tube emissions, with a focus on medical imaging and radiotherapy applications. It begins by covering the relevant background, before discussing modelling approaches, including both analytical formulations and Monte Carlo simulation. Historical context is provided, based on the past...

Analytical models are empirical, semi-empirical or entirely theoretical models for predicting x-ray spectra, that, unlike the Monte Carlo method, do not use random sampling. This chapter summarizes some landmark contributions from a century of developments and takes a closer look at some of the models. It begins with a discussion of Kramers’ semina...

Optimising phosphor screens in dose detectors or imaging sensor designs is a cumbersome and time- consuming work normally involving specialised measuring equipment and advanced modelling. It is known that crucial optical parameters of the same phosphor may vary within a wide range of values. The aim of this work was to experimentally assess a simpl...

Purpose To validate the SpekPy software toolkit that has been developed to estimate the spectra emitted from tungsten anode X-ray tubes. The model underlying the toolkit introduces improvements upon a well-known semi-empirical model of X-ray emission. Materials and methods Using the same theoretical framework as the widely-used SpekCalc software,...

Purpose: To present and validate a complete x-ray emission model (bremsstrahlung and characteristic x-ray emission) for the energy range 20-300 kV. Methods: An analytical x-ray spectrum model that combines the bremsstrahlung emission model developed in Part I with a previously developed characteristic x-ray emission model is validated by compari...

Objectives: To estimate risk for exposure-induced cancer death (REID), organ-specific risks of exposure-induced cancer death (REIDHT) and associated conversion coefficients (CCREID:KAP=REID/kerma-area product (KAP), CCREIDHT:KAP=REIDHT/KAP) in paediatric cardiac catheterizations using data from radiation dose structured reports (RDSR). A novel ris...

One of the most commonly used imaging techniques for diagnosing pulmonary embolism (PE) is ventilation/perfusion (V/P) scintigraphy. The aim of this study was to evaluate the performance of the currently used imaging protocols for V/P single photon emission computed tomography (V/P SPECT) at two nuclear medicine department sites and to investigate...

Purpose: To quantify the impact of incorporating a detailed form of the intrinsic bremsstrahlung angular distribution in an analytical x-ray spectrum model. Methods: Most x-ray spectrum models developed from theoretical principles assume that the angular distribution of bremsstrahlung production is uniform. This assumption stems from the rationale...

Purpose: To estimate effective dose (E), equivalent organ doses (HT) and associated conversion coefficients (CC[E:KAP] = E/KAP, CC[HT:KAP] = HT/KAP; KAP = Kerma-area product) in paediatric cardiac interventions, using detailed exposure data from radiation dose structured reports (RDSR). These “RDSR dose estimations” have been compared with estimati...

An analytical model for the prediction of thick-target characteristic x-ray emission has been developed and validated for several commonly used x-ray tube target materials. The model is based on a detailed description of the depth distribution of x-ray fluorescence, which is provided both in the form of an analytical parameterization and as supplem...

Purpose: Proton CT is widely recognised as a beneficial alternative to conventional X-ray CT for treatment planning in proton beam radiotherapy. A novel proton CT imaging system, based entirely on solid-state detector technology, is presented. Compared to conventional scintillator-based calorimeters, positional sensitive detectors allow for multip...

Purpose: To determine organ doses from a proton gantry-mounted cone-beam computed tomography (CBCT) system using two Monte Carlo codes and to study the influence on organ doses from different acquisition modes and repeated imaging. Methods: The CBCT system was characterized with MCNP6 and GATE using measurements of depth doses in water and spati...

Purpose This study compares the predictions of three parameterization models used in previously published works, implementing the stoichiometric CT calibration for proton therapy, and a further two alternative parameterizations suggested here. Methods Stoichiometric calibrations of patient CT-number to stopping-power ratio (SPR) were performed for...

Different theories of the intrinsic bremsstrahlung angular distribution (i.e., the shape function) have been evaluated using Monte Carlo calculations for various target materials and incident electron energies between 20 keV and 300 keV. The shape functions considered were the plane-wave first Born approximation cross sections (i) 2BS [high-energy...

The purpose of this study was to evaluate the dosimetric impact of including the patient table in Monte Carlo CT dose estimates for both spiral scans and scan projection radiographs (SPR). CT scan acquisitions were simulated for a Siemens SOMATOM Force scanner (Siemens Healthineers, Forchheim, Germany) with and without a patient table present. An a...

Proton therapy is rapidly gaining importance in the field of radiotherapy, because of its potential to deliver the planned dose over a small depth range and sparing dose to healthy tissue, when compared to conventional radiotherapy. Proton therapy, however, makes the need of new imaging modalities for treatment planning, based on direct measurement...

Introduction: Geometrical uncertainties can result in a delivered dose to the tumor different from that estimated in the static treatment plan. The purpose of this project was to investigate the accuracy of the dose calculated to the clinical target volume (CTV) with the dose-shift approximation, in stereotactic body radiation therapy (SBRT) of lu...

Purpose: To investigate two methods of approximating the Modulation Transfer Function (MTF) and Noise Power Spectrum (NPS) in computed tomography (CT) for a range of scan parameters, from limited image acquisitions. Methods: The two methods consist of 1) using a linear systems approach to approximate the NPS for different filtered backprojection...

Objectives: The aim of this study was to make a comprehensive evaluation of how variable scan settings can affect the performance of the automatic tube current modulation (ATCM) on recent CT scanners from the four major manufacturers. Methods: A phantom was designed and manufactured for the purpose of evaluating ATCM. The phantom was scanned wit...

Purpose: Radiography and tomography using proton beams promise benefit to image guidance and treatment planning for proton therapy. A novel proton tracking detector is described and experimental demonstrations at a therapy facility are reported. A new type of proton CT reconstructing relative “scattering power” rather than “stopping power” is also...

Purpose:Advanced dosimetry in CT (e.g. the Monte Carlo method) requires an accurate characterization of the shaped filter and radiation quality used during a scan. The purpose of this work was to develop a method where half value layer (HVL) profiles along shaped filters could be made. From the HVL profiles the beam shaping properties and effective...

Purpose: To investigate the feasibility of deploying a novel method for the daily monitoring of x-ray Angiography (XA) equipment performance as a cloud-based service. Methods: From previous work, we have developed a novel image based method of testing the performance of XA equipment on a daily basis. A pre-programmed protocol on an XA system is use...

Although interventional x-ray angiography (XA) procedures involve relatively high radiation doses that can lead to deterministic tissue reactions in addition to stochastic effects, convenient and accurate estimation of absorbed organ doses has traditionally been out of reach. This has mainly been due to the absence of practical means to access dose...

A method of reconstructing a3-dimensionalimage in a proton transmission computerised tomography (CT) apparatus is disclosed. The method comprises the creation of a reconstruction matrix. The matrix is created by directing a plurality of particles to traverse the object; and for each particle, measuring the trajectory and energy of each particle bef...

Purpose: The aim of this work was to compare and validate various computed tomography (CT) number calibration techniques with respect to cone beam CT (CBCT) dose calculation accuracy. Methods: CBCT dose calculation accuracy was assessed for pelvic, lung, and head and neck (H&N) treatment sites for two approaches: (1) physics-based scatter correc...

The quantitative effects of assumptions made in the calculation of stopping-power ratios (SPRs) are investigated, for stoichiometric CT calibration in proton therapy. The assumptions investigated include the use of the Bethe formula without correction terms, Bragg additivity, the choice of I-value for water, and the data source for elemental I-valu...

The purpose of this study was to develop a method of performing routine periodical quality controls (QC) of CT systems by automatically analyzing key performance indicators (KPIs), obtainable from images of manufacturers' quality assurance (QA) phantoms. A KPI pertains to a measurable or determinable QC parameter that is influenced by other underly...

Since the first proof of concept in the early 70s, a number of technologies has been proposed to perform proton CT (pCT), as a means of mapping tissue stopping power for accurate treatment planning in proton therapy. Previous prototypes of energy-range detectors for pCT have been mainly based on the use of scintillator-based calorimeters, to measur...

The purpose of this work was to develop a method so that the noise power spectrum (NPS) can be approximated for arbitrary levels of mAs, from a single determination in CT. The NPS is factorized into 2 components, 1) a parameterized function representing the 1D normalized spatial frequency distribution and 2) a function to scale the magnitude of 1)...

Proton radiotherapy has demonstrated benefits in the treatment of certain cancers. Accurate measurements of the proton stopping powers in body tissues are required in order to fully optimise the delivery of such treaments. The PRaVDA Consortium is developing a novel, fully solid state device to measure these stopping powers. The PRaVDA Range Telesc...

An advantage of semiconductor-based dedicated cardiac single photon emission computed tomography (SPECT) cameras when compared to conventional Anger cameras is superior energy resolution. This provides the potential for improved separation of the photopeaks in dual radionuclide imaging, such as combined use of (99m)Tc and (123)I . There is, however...

A novel approach to proton CT reconstruction using backprojection-then-filtering (BPF) is proposed. A list-mode algorithm is formulated accommodating non-linear proton paths. The analytical form is derived for the deblurring kernel necessary for the filtering step. Further, a finite matrix correction is derived to correct for the limited size of th...

Purpose: ImpactMC (CT Imaging, Erlangen, Germany) is a Monte Carlo (MC) software package that offers a GPU enabled, user definable and validated method for 3D dose distribution calculations for radiography and Computed Tomography (CT). ImpactMC, in and of itself, offers limited capabilities to perform batch simulations. The aim of this work was to...

Despite the early recognition of the potential of proton imaging to assist proton therapy (Cormack 1963 J. Appl. Phys. 34 2722), the modality is still removed from clinical practice, with various approaches in development. For proton-counting radiography applications such as computed tomography (CT), the water-equivalent-path-length that each proto...

A radiotherapy system comprising a support for a patient undergoing radiotherapy treatment, a gantry rotatable about an axis, a source of radiation mounted on the gantry and producing a beam of radiation directed towards a target region of the patient, a collimator coupled to said radiation source, the collimator comprising a plurality of movable,...

An algorithm for dynamic multileaf-collimator (dMLC) tracking of a target performing a known a priori, rigid-body motion during volumetric modulated arc therapy (VMAT), has been experimentally validated and applied to investigate the potential of the Agility (Elekta AB, Stockholm, Sweden) multileaf-collimator (MLC) for use in motion-compensated VMA...

Purpose: In Part 1 of this two-part work, predictions for light transport in powdered-phosphor screens are made, based on three distinct approaches. Predictions of geometrical optics-based ray tracing through an explicit microscopic model (EMM) for screen structure are compared to a Monte Carlo program based on the Boltzmann transport equation (BT...

Purpose: Monte Carlo methods based on the Boltzmann transport equation (BTE) have previously been used to model light transport in powdered-phosphor scintillator screens. Physically motivated guesses or, alternatively, the complexities of Mie theory have been used by some authors to provide the necessary inputs of transport parameters. The purpose...

The AccuLeaf mMLC featuring four multileaf-collimator (MLC) banks has been used for the first time for an experimental comparison of conventional two-bank with novel four-bank dynamic MLC tracking of a two-dimensional sinusoidal respiratory motion. This comparison was performed for a square aperture, and for three conformal treatment apertures from...

A VMAT model for calculation of static-tumour and dMLC tracking motion-compensation delivery was applied to five VMAT treatment plans, for three respiratory motion trajectories generated from patient fluoroscopic data acquired during lung radiotherapy treatment. The delivery time was calculated for static-tumour, 1D dMLC tracking and 2D dMLC tracki...

The potential of keV cone beam computed tomography (CBCT) for guiding adaptive replanning is well-known. There are impediments to this, one being CBCT number accuracy. The purpose of this study was to investigate CBCT number correction methods and the affect of residual inaccuracies on dose deposition. Four different correction strategies were appl...

In this paper, the effect on image quality of significantly reducing the primary electron energy of a radiotherapy accelerator is investigated using a novel waveguide test piece. The waveguide contains a novel variable coupling device (rotovane), allowing for a wide continuously variable energy range of between 1.4 and 9 MeV suitable for both imagi...

Artifacts in treatment-room cone-beam reconstructions have been observed at the authors' center when cone-beam acquisition is simultaneous with radio frequency (RF) transponder tracking using the Calypso 4D system (Calypso Medical, Seattle, WA). These artifacts manifest as CT-number modulations and increased CT-noise. The authors present a method f...

A model has been developed to simulate volumetric modulated arc therapy (VMAT) delivery for Elekta control systems. The model was experimentally validated for static-tumour VMAT delivery and has been applied to the investigation of motion compensation with dynamic multileaf collimator (dMLC) delivery tracking for a series of VMAT lung treatment pla...

Computed tomography images have been acquired using an experimental (low atomic number (Z) insert) megavoltage cone-beam imaging system. These images have been compared with standard megavoltage and kilovoltage imaging systems. The experimental system requires a simple modification to the 4 MeV electron beam from an Elekta Precise linac. Low-energy...

Purpose: To design and evaluate an optimised radiotherapyimaging beam, based on a prototype Elekta waveguide, with a novel coupling device (rotovane) allowing for a wide, continuously variable energy range. Methods: A waveguide test piece consisting of a buncher, rotovane and short relativistic section was used to investigate the performance of the...

Scatter in a detector and its housing can result in image degradation. Typically, such scatter leads to a low-spatial frequency 'glare' superimposed on the primary signal. We infer the glare-spread function (GSF) of an amorphous-silicon flat-panel detector via an edge-spread technique. We demonstrate that this spread (referred to as 'scatter-glare'...

Volumetric-modulated arc therapy (VMAT), a form of intensity-modulated arc therapy (IMAT), has become a topic of research and clinical activity in recent years. As a form of arc therapy, portal images acquired during the treatment fraction form a (partial) Radon transform of the patient. We show that these portal images, when used in a modified glo...

In this note it is shown how the use of a rotate-translate methodology employing only jaws, which move dynamically with the beam continuously on, can lead to a delivery of a two-dimensional intensity-modulated beam wherein the modulation is spatially slowly varying. All that is necessary is that a pair of jaws sweep across the face of an accelerato...

IMRT can be delivered by jaws only (JO) provided some compromises are accepted. In this letter it is shown how the use of a rotate-translate methodology (ROTJO), also employing only jaws, can lead to the delivery of a two-dimensional intensity-modulated beam wherein the modulation is spatially slowly varying.

Monte Carlo simulation is the gold standard method for modelling scattering processes in medical x-ray imaging. General-purpose Monte Carlo codes, however, typically use the independent atom approximation (IAA). This is known to be inaccurate for Rayleigh scattering, for many materials, in the forward direction. This work addresses whether the IAA...

A software program, SpekCalc, is presented for the calculation of x-ray spectra from tungsten anode x-ray tubes. SpekCalc was designed primarily for use in a medical physics context, for both research and education purposes, but may also be of interest to those working with x-ray tubes in industry. Noteworthy is the particularly wide range of tube...

A new method is proposed for scatter-correction of cone-beam CT images. A coarse reconstruction is used in initial iteration steps. Modelling of the x-ray tube spectra and detector response are included in the algorithm. Photon diffusion inside the imaging subject is calculated using the Monte Carlo method. Photon scoring at the detector is calcula...

Purpose: To develop a free and easy‐to‐use software program to model the x‐ray spectra emitted from x‐ray tubes. Such a program is useful for the education of physicists, technicians and technologists working in radiation physics but also as a research tool. Method and Materials: A Graphical User Interface (GUI) was created using REALbasic (REAL so...

Purpose: To develop a fast Monte‐Carlo‐based scatter‐correction algorithm for clinical keV cone‐beam CT(CBCT)images.Method and Materials: Estimates of the scatter in the projection‐views of a CBCT scan were obtained by an iterative process, each step consisting of: (1) a coarse CBCT reconstruction; (2) simulation of photon histories for projections...

The penetration characteristics of electron beams into x-ray targets are investigated for incident electron kinetic energies in the range 50-150 keV. The frequency densities of electrons penetrating to a depth x in a target, with a fraction of initial kinetic energy, u, are calculated using Monte Carlo methods for beam energies of 50, 80, 100, 120...

A new approach to the calculation of the x-ray spectrum emerging from an x-ray tube is proposed. Theoretical results for the bremsstrahlung cross section appearing in the literature are summarized. Four different treatments of electron penetration, based on the work presented in Part I, are then used to generate bremsstrahlung spectra. These spectr...

Diffractive photoproduction of rho, phi and J/psi was studied in the BFKL approach to hard colour singlet exchange. Differential cross sections, the energy dependence and spin density matrix elements were calculated and compared to data from HERA. The overall description of data is reasonably good, except of the single flip amplitude which has the...

We summarise recent progress in the computation of helicity amplitudes for diffractive vector meson production at large momentum transfer and their comparison to data collected at the HERA collider.

Diffractive vector meson photoproduction accompanied by proton dissociation is studied for large momentum transfer. The process is described by the non-forward BFKL equation which we use to compare to data collected at the HERA collider. Comment: 39 pages, 29 figures

Diffractive vector meson photoproduction accompanied by proton dissociation is studied for large momentum transfer. The process is described by the non-forward BFKL equation, for which a complete analytical solution is found. The scattering amplitudes for all combinations of helicity are presented.

We explore QCD calculations for the process gammap --> VX where V is a vector meson, in the region s much greater than -t and -t much greater than Lambda(QCD)(2). We compare our calculations for the J/psi, phi and rho mesons with data from the ZEUS Collaboration at HERA and demonstrate that the BFKL approach is consistent with the data even for lig...