Conventional medical gamma-ray camera systems utilise mechanical collimation to provide information on the position of an incident gamma-ray photon. Systems that use electronic collimation utilising Compton image reconstruction techniques have the potential to offer huge improvements in sensitivity. Position sensitive high purity germanium (HPGe) detector systems are being evaluated as part of a single photon emission computed tomography (SPECT) Compton camera system. Data have been acquired from the orthogonally segmented planar SmartPET detectors, operated in Compton camera mode. The minimum gamma-ray energy which can be imaged by the current system in Compton camera configuration is 244keV due to the 20mm thickness of the first scatter detector which causes large gamma-ray absorption. A simulation package for the optimisation of a new semiconductor Compton camera has been developed using the Geant4 toolkit. This paper will show results of preliminary analysis of the validated Geant4 simulation for gamma-ray energies of SPECT, 141keV.

Image reconstruction from Compton camera data is a complex problem requiring investigation. Generally reconstruction is conducted using iterative reconstruction methods such as Maximum Likelihood - Expectation Maximization (MLEM). However, iterative reconstruction into volumetric grids is a computational burden. Analytic methods of image reconstruction have been proposed which relieve the computational expense. However, such methods usually have unrealistic sampling assumptions or are not directly extendable to situations where the scattering detector is extended when compared to the detector-source distance. Starting from a standard inversion technique, a generic method of filtering inversion co-efficients is developed. By dynamically assigning the strength of co-efficients on an event-by-event basis, artifacts arising from the difference between assumed and actual forward transform may be reduced. Dynamic assignment allows inversion of single cone-surfaces, so that the generic technique may be implemented in volumetric reconstruction. Results are compared to direct-back-projection in a limited-angle tomography context. The ability to apply event-by-event analytic image reconstruction provides many advantages when compared to standard iterative techniques.

The SmartPET project is the development of a prototype small-animal imaging system based on the use of Hyperpure Germanium (HPGe) detectors. The use of digital electronics and application of Pulse Shape Analysis (PSA) techniques provide fine spatial resolution, while the excellent intrinsic energy resolution of HPGe detectors makes the system ideal for multi-nuclide imaging. As a result, the SmartPET system has the potential to function as a dual modality imager, operating as a dual-head Positron Emission Tomography (PET) camera or in a Compton Camera configuration for Single Photon Emission Computed Tomography (SPECT) imaging. In this paper, we discuss how the use of simple PSA techniques greatly improves the position sensitivity of the detector yielding improved spatial resolution in reconstructed images. The PSA methods presented have been validated by comparison to data from high-precision scanning of the detectors. Results from this analysis are presented along with initial images from the SmartPET system, which demonstrates the impact of these techniques on PET images.

Small Animal Reconstruction PET (SmartPET) is a project funded by the UK medical research council (MRC) to demonstrate proof of principle that Germanium can be utilised in Positron Emission Tomography (PET). The SmartPET demonstrator consists of two orthogonal strip High Purity Germanium (HPGe) planar detectors manufactured by ORTEC. The aim of the project is to produce images of an internal source with sub mm3 spatial resolution. Before this image can be achieved the detectors have to be fully characterised to understand the response at any given location to a γ-ray interaction. This has been achieved by probing the two detectors at a number of specified points with collimated sources of various energies and strengths. A 1mm diameter collimated beam of photons was raster scanned in 1mm steps across the detector. Digital pulse shape data were recorded from all the detector channels and the performance of the detector for energy and position determination has been assessed. Data will be presented for the first SmartPET detector.

In this paper we discuss the Smart Positron Emission Tomography (PET) imaging system being developed by the University of Liverpool in conjunction with CCLRC Daresbury Laboratory. We describe the motivation for the development of a semiconductor-based PET system and the advantages it will offer over current tomographs. Details of the detectors and associated electronics are discussed and results of high precision scans are presented. Analysis of this scan data has facilitated full characterization of the detector response function and calibration of the three-dimensional position sensitivity. This work presents the analysis of the depth sensitivity of the detector.

The evolution of germanium detectors over the last decade has lead to the possibility that they can be used in medical imaging and security scanning. The potential of increased sensitivity and energy resolution that germanium affords takes away the necessity of mechanical collimators that would be required in a gamma camera. Without mechanical collimation the resulting increase in statistics leads to the possibility of decreased patient dose or increased system throughput. In terms of security imaging segmented germanium provides directionality and excellent spectroscopic information for nuclide identification.

The performance of Compton imaging systems is limited by the angular uncertainty arising from the detector geometry and spatial resolution [Ordonez et al., 1997 amd 1999], When closely spaced multiple interactions are incorrectly recorded as a single event [Solomon and Ott, 1988], termed "interaction packing", the system response has considerable angular and directional uncertainty. In this situation, we test two methods for assigning the event location; binning the combined interaction to a central location, and sub-sampling to an energy weighted centroid. We considered a dual layer camera geometry containing two SmartPET detectors [Hall et al., 2003], and employed Geant4 [Agostinelli et al., 2003] to simulate the response to 662 keV photons. A cone-intersection algorithm [Wilderman et al., 2000] is utilised to reconstruct the activity distribution. The major findings are as follows. Interaction packing in the scatter detector leads to an increase in reconstructed background levels. Interaction packing in the absorption detector leads to a broader point spread function, but this is only observed using the energy centroid approximation. While both of these effects are small for the SmartPET based geometry, they may lead to image degradation when using different detection geometries, such as more closely spaced detection volumes.

Conventional neuropathological analysis for brain malignancies is heavily reliant on the observation of morphological abnormalities, observed in thin, stained sections of tissue. Small Angle X-ray Scattering (SAXS) data provide an alternative means of distinguishing pathology by examining the ultra-structural (nanometer length scales) characteristics of tissue. To evaluate the diagnostic potential of SAXS for brain tumors, data was collected from normal, malignant and benign tissues of the human brain at station 2.1 of the Daresbury Laboratory Synchrotron Radiation Source and subjected to data mining and multivariate statistical analysis. The results suggest SAXS data may be an effective classifier of malignancy.

The goal of this project is to determine the feasibility of utilizing colloidal gold as a marker for C6 glioblastoma cells implanted into rat brain as an appropriate model for volumetric measurements of tumors using absorption edge subtraction (AES). Phase sensitive X-ray imaging is combined with KES to give good soft tissue contrast. Current methods for volumetric measurements of implanted C6 glioblastoma tumors in rat brains using MRI technology are inadequate due to the small size of the tumor (2.5–4mm in diameter) and the thickness of the MRI slice (1–1.5mm). Previously, our group has shown that AES detection of colloidal gold labeled C6 glioblastoma cells implanted into a rat brains may be feasible. The long-term goal for this project is to establish a method, which would allow the researcher to monitor the development of a tumor over time. Most importantly, this technique should allow researchers to accurately determine the potency of a treatment on the size and growth rate for a C6 implanted tumors. In addition, we plan to challenge the hypothesis that tumors of the glioma type do not metastasize outside of the brain. A sensitive technique for the detection of C6 cells, such as that using colloidal gold and AES/DEI, should enable researchers to detect C6 cells, which have metastasized and migrated to different areas of the body. The ability to detect implanted C6 cells followed by the development of the tumor, the possible migration of the cells and the ability to accurately measure the effects of treatments on the volume of the tumor would be of the utmost importance to brain tumor research.

Glioblastoma multiforme (GBM) is the commonest and most aggressive primary brain tumor in humans. The high rate of tumor recurrence results in a poor prognosis despite multimodality treatment. One reason for high rate of recurrence is the invasive nature of the tumor into the surrounding normal brain tissue or multifocal occurrence at sites remote from that of the primary tumor establishment. Existing imaging demonstrates the primary tumor but fail to show the residual tumor microaggregates left behind following initial treatment. In this study, we employed diffraction-enhanced imaging (DEI) in an attempt to find an imaging modality that will provide visualization of residual disease that is not be apparent on MRI or CT scans.

Developments in position-sensitive spectroscopic detector systems resulting from work undertaken in the fields of astrophysics, high energy physics and medical imaging provide an opportunity for the synchrotron physics community to look at possible improvements to synchrotron studies. Specifications of the detector designed by the smartPET collaboration are outlined; a brief comparison with current options presented and possible applications to X-ray imaging, protein and macromolecular crystallography and X-ray fluorescence holography are introduced.

The concept of tracking the interactions of a gamma ray through a solid state detector is being pursued by several groups in the nuclear physics community. Here the ability to rid the detectors of the escape suppression shields and decrease the Doppler broadening of the spectroscopic lines will enable new physics measurements. Our group has embarked upon a project to apply this technology to medical imaging. We report on the progress in developing a planar germanium detector with gamma ray tracking ability, for positron emission tomography (PET). The prototype detector is a planar high purity germanium unit with X-Y readout electrodes. Using digital signal processing the timing and depth of individual interactions are recorded for each gamma ray. Compton scattering information for some of the events can be used to reconstruct the angle of the gamma ray to the first interaction point. In a similar technique to the Compton camera, this information can be used to reduce random coincidence rates in the PET system and provide the possibility for highly efficient use of events in the tomography reconstruction.