Medical Physics
Table of Contents
Reggie Taylor
Magnetic resonance spectroscopy (MRS) is a non-invasive way to measure metabolites in a localized area of the brain. It has been widely used to study certain neurotransmitters, such as glutamate and GABA, in psychiatric disorders. A new MRS technique is available that allows for the quantification of another metabolite, glycine. Glutamatergic neurotransmission relies on glycine as an agonist to its primary receptor. Glycine has potentially large implications in the manifestation of negative symptoms (asociality, anhedonia, etc) in psychiatric disorders and could be a factor in performance on cognitive tasks. The MRS technique to measure glycine has been created locally, but needs to be optimized going forward. This project would involve working with MRS spectra acquired from the PET/MRI at the Brain Imaging Centre in The Royal Ottawa Mental Health Centre to determine the best parameter set and post-processing techniques for reliably measuring glycine in the human brain.
Paul Johns
Project 1: (joint supervision with Tong Xu)
In dual-energy radiography, the object is imaged with beams of two different energies. The dependences of the photoelectric and Compton cross sections on atomic number and photon energy allow these data to be sufficient to characterize the object's x-ray transmission properties at all energies used in diagnostic radiology. Applications include beam-hardening artefact suppression in CT and removal of contrast from objects not of diagnostic interest in projection imaging. In this project a dual-energy imaging protocol will be implemented using an x-ray tube and a Perkin-Elmer flat panel image receptor in our lab. Dual-energy performance will be measured by imaging test objects composed of aluminum and different plastics. The project will entail experiment design and operation, and the implementation of a nonlinear software correction for the polyenergetic nature of the x-ray beams, based on the established dual-material basis decomposition approach from the literature.
Project 2: Mapping x-ray tube source distribution with a miniature detector
In x-ray imaging, the x-ray tube source geometry is a key determinant of an imaging system's resolution. The radiation source region ranges in size from 100 microns square to about 2 mm square. Pinhole radiography of the source is the standard measurement method. Traditionally x-ray film was used to record the distribution. Our goal is to configure a modern digital alternative.
In this project the student will work with a digital detector originally designed for dental intra-oral imaging. The detector has 20 micron pixels - perfect for recording the details of the x-ray tube emission pattern. The student will (i) characterize the detector for sensitivity, resolution, and artefacts, and (ii) configure a mounting system to align the detector, pinhole and x-ray tube to make a radiation source inspection camera. Then (iii) the camera will be tested on two general radiology tubes.
Project 3: Display algorithms for x-ray scatter imaging
Our lab's focus is to develop an x-ray imaging system that uses low-angle scattered radiation, below 10 degrees, in addition to the conventional primary radiation. It is not obvious how best to display the results. An ensemble of scatter images can be generated in one acquisition, corresponding to scatter at different angles. We also have the conventional primary image. There are several options besides displaying simple stacks of grey-scale images. Previous work in our lab looked at peak location, centroid, encoding the scatter radial profile as an RBG spectrum, and a mosaic display approach. Our moasaic work needs to be extended and other schemes investigated. For example, peak scatter angle could be encoded in colour and total scatter signal as intensity, or two non-overlapping colour scales could be used for primary and scatter simultaneously. This computational project will be a good way to learn the HSV vs RGB colour systems and will be done in Matlab.
Malcolm McEwen
Novel detectors for primary standards dosimetry
Air-filled ionization chambers have been used for more than 50 years as reference detectors in primary and secondary standard dosimetry laboratories. They have high sensitivity and very good stability but are not ideal detectors for all measurement situations due to there relatively large size and reliance on measuring ionization of air. In recent years a number of novel detectors based on different measuring processes have become available, and these offer potential advantages over ion chambers. These include liquid ion chambers, plastic scintillating fibres and diamond detectors. However, they have typically been developed for clinical medical physics, where accuracy requirements are not as stringent as for primary labs. The aim of this project is to investigate these detectors and establish their limits for precision and accuracy in a range of radiation facilities - from low energy (keV) x-rays to high energy (MeV) electron beams - maintained at the Ionizing Radiation Standards group of the NRC. Suitable detectors will then be used to address a number of measurement problems that are a current focus of the IRS group's research.
Sangeeta Murugkar
Raman Spectroscopy and Multimodal Raman Imaging
Raman spectroscopy is a non-invasive optical technique that is based on the inelastic scattering of light by vibrating molecules. Various projects involving applications of Raman spectroscopy and multimodal nonlinear optical imaging are available in the areas of skin cancer detection and treatment; understanding cellular response to ionizing radiation; and for developing applications in radiation microdosimetry.
Rowan Thomson
Radiation transport simulations are broadly used to study many aspects of physics related to radiotherapy treatments for cancer. A variety of projects involving computational and theoretical studies of the interactions of radiation with matter are possible and may be tailored to the student's strengths. Some projects involve use of egs_brachy, a fast Monte Carlo dose calculation for brachytherapy (developed in the Carleton Laboratory for Radiotherapy Physics), to investigate questions in brachytherapy physics (brachytherapy is a type of radiation treatment in which radioactive sources are placed next to or inside a tumour). One project involves an investigation of approaches to model the biological effects of radiation. Other research directions include studies of cell dosimetry using Monte Carlo simulations, with applications to skeletal dosimetry.
Ruth Wilkins
Biological dosimetry is a method of measuring the amount of ionizing radiation received by an individual using biological material. This type of dosimeter is essential when an individual is accidentally exposed and no physical dosimetry is available. Currently, the accepted method of biological dosimetry is the dicentric chromosome assay which involves examining chromosomes for characteristic damage caused by ionizing radiation. This is a very tedious and time consuming assay, requiring weeks to process a sample from one individual. For large scale radiological events where thousands of individuals might be exposed to ionizing radiation, a biological dosimeter is desirable that could analyse many samples in a timely manner. Our laboratory is currently exploring new techniques for biological dosimetry. This project would involve testing an image analysis system for detecting biological damage in DNA.
Tong Xu
Accurate geometric calibration of a linac-mounted kilovoltage x-ray system (joint supervision with Elsayed Ali and Rolf Clackdoyle)
Cone Beam CT (CBCT) scanners is becoming a popular imaging tools in health care. Equipped with big area imaging detectors, CBCT can acquire a full 3D scan within a single rotation. It is widely used in image guided radiation therapy and surgeries. However, in these applications, as the heavy detector and x-ray source rotate around the patient, the gravity will cause the supporting structures of the scanner to deform. This deformation, if unaccounted for, will cause image blurring and artifacts. This study will investigate how a new 3D geometry calibration can improve image quality of CBCT. Furthermore, we will also study the feasibility of quantify the actual deformation using proposed calibration method.
The measurements are carried out at The Ottawa Hospital Cancer Center and the student will get exposure to state-of-the-art clinical system and the field of medical physics. It is a well balanced project that includes experimental design, code development/refinement, computer simulation, image processing and reconstruction.
Emily Heath
Monte Carlo modeling of an electronic portal imaging detector (EPID)
(joint supervision with Bryan Muir, NRC)
Radiation therapy uses high energy x-ray beams generated in a linear accelerator to deliver a high dose of ionizing radiation to the cancer cells. Modern linacs used in radiation therapy come with a flat panel detector (EPID) that measures the radiation beam that is transmitted through the patient. This EPID can be used in dose verification. The goal of this project is to model the EPID using the EGSnrc Monte Carlo software in order to predict the measured response to irradiation. The model will be compared with experimental measurements obtained on a linac-EPID system at the NRC.
Glenn Wells
Heart disease is one of the leading causes of death in Canada and the world. Myocardial perfusion imaging (MPI) is an essential tool in evaluating patients with known or suspected heart disease. SPECT is the clinical workhorse for performing MPI. Recent innovations in the design of cardiac SPECT cameras allow dynamic imaging and thus absolute measures of blood flow in the heart. This technique potentially allows more accurate identification of patients with multi-vessel disease, patients whose disease is underestimated in 50% of cases with current methods. Projects include work on the optimization and evaluation of protocols for SPECT blood flow measurement and/or algorithms for dynamic SPECT reconstruction. The student will work with anonymized patient datasets using a combination of commercial and in-house software. Interested students should contact Dr. Glenn Wells at the University of Ottawa Heart Institute for more information.
Lindsay Beaton
Biological and Physical Dosimetry
Biological dosimetry uses biological material to determine the amount of ionizing radiation received by an individual. This is often assessed by measuring characteristic damage in blood cells, which increases with increasing dose. Our lab is currently developing more sophisticated biological dosimetry methods, using samples irradiated in vitro with x-rays. It is important to have rigorous physical dosimetry when establishing these dose-calibration curves. This project will involve characterizing a robust physical dosimetry system (including ionization chambers, gafchromic film and optically stimulated luminescence (OSL) detectors).
Avery Berman
Functional magnetic resonance imaging (fMRI) has revolutionized the study of the organization of the human brain by providing a non-invasive means of imaging brain activity without the use of ionizing radiation or contrast agents. Despite its widespread use, most fMRI is based on an indirect measure of brain activity arising from a combination of vascular and metabolic changes. My laboratory seeks to improve the interpretation of fMRI signals through high-resolution imaging to better resolve the activity from small anatomical structures and to provide quantitative biomarkers for studying mental health and neurological disorders. Projects in my lab are primarily modelling or data analysis based. Students may simulate fMRI signals from the vasculature using biophysical modelling to help inform our imaging, analysis, and interpretation. Students may also analyze experimental MRI data to develop more accurate quantitative measurements of brain physiology.