D. C. Oxley

Multivoxel fMRI analysis of color tuning in human primary visual cortex

We use multivoxel pattern analysis (MVPA) to study the spatial clustering of color-selective neurons in the human brain. Our main objective was to investigate whether MVPA reveals the spatial arrangements of color-selective neurons in human primary visual cortex (V1). We measured the distributed fMRI activation patterns for different color stimuli (Experiment 1: cardinal colors (to which the LGN is known to be tuned), Experiment 2: perceptual hues) in V1. Our two main findings were that (i) cone-opponent cardinal color modulations produce highly reproducible patterns of activity in V1, but these were not unique to each color. This suggests that V1 neurons with tuning characteristics similar to those found in LGN are not spatially clustered. (ii) Unique activation patterns for perceptual hues in V1 support current evidence for a spatially clustered hue map. We believe that our work is the first to show evidence of spatial clustering of neurons with similar color preferences in human V1.

Prospectus: Development of a Compton Camera for Medical Imaging

Single Photon Emission Computed Tomography (SPECT) is an established method of studying physiological functions. However, novel gamma-ray Compton camera systems which provide electronic collimation have the potential to greatly improve the sensitivity of this technique. Compton cameras have been employed in high energy applications but have not yet been fully implemented for clinical applications at low energies. This paper describes the optimization of imaging efficiency for the ProSPECTus medical imaging Compton camera system with 99mTc. Experimental factors which degrade the image quality will also be assessed and quantified.

Status and Performance of an AGATA asymmetric detector

High‐resolution gamma‐ray detectors based on high‐purity germanium crystals (HPGe) are one of the key workhorses of experimental nuclear science. The technical development of such detector technology has been dramatic in recent years. Large volume, high‐granularity, electrically segmented HPGe detectors have been realised and a methodology to improve position sensitivity using pulse‐shape analysis coupled with the novel technique of gamma‐ray tracking has been developed. Collaborations have been established in Europe (AGATA) [1] and the USA (GRETA/GRETINA) [2] to build gamma‐ray tracking spectrometers. This paper discusses the performance of the first AGATA (Advanced GAmma Tracking Array) asymmetric detector that has been tested at the University of Liverpool. The use of a fully digital data acquisition system has allowed detector charge pulse shapes from a selection of well defined photon interaction positions to be analysed, yielding important information on the position sensitivity of the detector.

Design Considerations Of A Compton Camera For Low Energy Medical Imaging

Development of a Compton camera for low energy medical imaging applications is underway. The ProSPECTus project aims to utilize position sensitive detectors to generate high quality images using electronic collimation. This method has the potential to significantly increase the imaging efficiency compared with mechanically collimated SPECT systems, a highly desirable improvement on clinical systems. Design considerations encompass the geometrical optimisation and evaluation of image quality from the system which is to be built and assessed.

Characterisation of Pixellated CdZnTe Detectors for Use in a Portable Gamma-Ray Spectrometer

The characterization of a pixellated CdZnTe detector will be discussed in the context of the development of a portable gamma-ray spectrometer device. The device will utilize the spectroscopic properties of CdZnTe to provide both isotopic identification and directional information. The CdZnTe sensors will be configured for use as a Compton camera, maximizing the efficiency for both spectroscopy and imaging of the source being assayed. In order to achieve this, pulse shape analysis algorithms are being developed to accurately determine the position of the interaction of a gamma-ray photon within the detector. A pixellated CdZnTe detector has been scanned by moving a highly collimated source in controlled steps across the surface of the detector and taking data for a set period of time at each position. In this work, a 57Co source has been used with a 1 mm tungsten injection collimator moved in 1 mm steps across the front face and side surfaces of the detector. The results from the preparatory work and the initial scans of the detector are discussed in this work.

Compton imaging with a planar semiconductor system using pulse shape analysis

Homeland security agencies have a requirement to locate and identify nuclear material. Compton cameras [1, 2] offer a more efficient method of gamma-ray detection than collimated detector systems. The resolution of the interaction positions within the detectors greatly influences the accuracy of a reconstructed Compton image. Utilizing digital electronics and applying pulse shape analysis [3] allows the spatial resolution to be enhanced beyond the pixel granularity in three dimensions. Analytically reconstructed Compton images from a range of radiation sources shall be presented with and without pulse shape analysis showing the improvements gained along with a discussion of our analysis methods.

Quantification of the experimental limitations of a semiconductor PET camera

An investigation of the applicability of semiconductors to small animal positron emission tomography has been conducted. The SmartPET collaboration has been responsible for successful experimental work demonstrating how these detectors could enhance small animal imaging devices. Highly encouraging experimental results present an improvement in image resolution to 2mm (full width at half maximum). In order to understand current experimental limitations and overcome these in future systems, we have carried out a Monte Carlo study to systematically quantify experimental image distortions in terms of the image resolution of a point source. With a full understanding of experimental sources of error, the full potential of the existing SmartPET system can be understood. This ground work will allow the imaging of more complex phantom geometries to assist a transition into preclinical trials. This work also permits an optimized system to be designed in future.

A Semiconductor‐Based Positron Emission Tomography System

This paper shall summarize the research conducted employing the high‐purity germanium based small animal imaging system, SmartPET (SMall Animal Reconstructive Tomograph for Positron Emission Tomography). Geant4 simulations of the experimental setup were carried out in order to derive novel analysis procedures and quantify the system limitations. In this paper, we will focus on a gamma ray tracking approach devised to overcome germaniums high Compton scattering cross‐section and on imaging challenging and complex phantom geometries. The potential of the developed tools and of the system itself will be discussed.

First Results with TIGRESS and Accelerated Radioactive Ion Beams from ISAC: Coulomb Excitation of 20,21,29Na

The TRIUMF‐ISAC Gamma‐Ray Escape Suppressed Spectrometer (TIGRESS) is a state‐of‐the‐art γ‐ray spectrometer being constructed at the ISAC‐II radioactive ion beam facility at TRIUMF. TIGRESS will be comprised of twelve 32‐fold segmented high‐purity germanium (HPGe) clover‐type γ‐ray detectors, with BGO/CsI(Tl) Compton‐suppression shields, and is currently operational at ISAC‐II in an early‐implementation configuration of six detectors. Results have been obtained for the first experiments performed using TIGRESS, which examined the A = 20, 21, and 29 isotopes of Na by Coulomb excitation.