CHARACTERIZATION OF THE COUNT RATE PERFORMANCE AND EVALUATION OF THE EFFECTS OF HIGH COUNT RATES ON MODERN GAMMA CAMERAS

CHARACTERIZATION OF THE COUNT RATE PERFORMANCE AND EVALUATION OF THE EFFECTS OF HIGH COUNT RATES ON MODERN GAMMA CAMERAS Michael Stephen Silosky, B.S. Supervisory Professor: S. Cheenu Kappadath, Ph.D. Evaluation of count rate performance (CRP) is an integral component of gamma camera quality assurance and measurement of system dead time (τ) is important for quantitative SPECT. The CRP of three modern gamma cameras was characterized using established methods (Decay and Dual Source) under a variety of experimental conditions. For the Decay method, input count rate was plotted against observed count rate and fit to the paralyzable detector model (PDM) to estimate τ (Rates method). A novel expression for observed counts as a function of measurement time interval was derived and the observed counts were fit to this expression to estimate τ (Counts method). Correlation and Bland-Altman analysis were performed to assess agreement in estimates of τ between methods. The dependencies of τ on energy window definition and incident energy spectrum were characterized. The Dual Source method was also used to estimate τ and its agreement with the Decay method under identical conditions and the effects of total activity and the ratio of source activities were investigated. Additionally, the effects of count rate on several performance metrics were evaluated. The CRP curves for each system agreed with the PDM at low count rates but deviated substantially at high count rates. Estimates of τ for the paralyzable portion of the CRP curves using the Rates and Counts methods were highly correlated (r=0.999) but with a small (~6%) difference. No significant difference was observed between the highly correlated estimates of τ using the Decay or Dual Source methods under identical experimental conditions (r=0.996). Estimates of τ increased as a power-law function with decreasing ratio of counts in the photopeak to the total counts and linearly with decreasing spectral effective energy. Dual Source method estimates of τ varied as a quadratic with the ratio of the single source to combined source activities and linearly with total activity used across a large range. Image uniformity, spatial resolution, and energy resolution degraded linearly with count rate and image distorting effects were observed. Guidelines for CRP testing and a possible method for the correction of count rate losses for clinical images have been proposed

Performance evaluation of two CZT gamma ray imaging systems

The purpose of this research was to evaluate the performance of the imaging characteristics of two versions of a cadmium zinc telluride (CZT) gamma radiation detector system called the Laboratory Radioactive Assay Tracer (LabRAT). The performance evaluation follows the National Electrical Manufacturers Association standards for pixellated detector cameras. The LabRAT detector system hardware was developed by Mosaic Imaging Technology, Inc. LabRAT is a portable nuclear medicine imaging detector system intended for small field of view applications such as small animal imaging, portable radioisotope imaging in emergency room or intensive care units, and as an instruction tool for radiology residents and physics students. The tests performed include the measurement of count rate performance, per-pixel and composite energy resolution, uniformity of detector response, extrinsic spatial resolution, linearity, and integral and differential uniformity. Prior to the performance evaluation acquisition software was developed to operate the detector, including initializing the detector, performing data acquisition and displaying images and energy spectra. One of the systems had a better composite energy resolution due to the fact that the locations of photopeak centers for the individual pixels in that detector were consistently more uniform than the locations for the other detector. The energy resolution attainable for individual pixels is good, but due to limitations in user control over tuning of individual pixels, the composite energy resolution values were higher than expected for both systems. In practice, energy windows must be applied on a per-pixel basis. Spatial uniformity is worse than for typical scintillator-based gamma cameras, while extrinsic spatial resolution is satisfactory

The influence of accurate attenuation correction on quantitative gamma camera imaging

Gamma camera systems are used in a variety of diagnostic applications to image and in some cases measure, the physiological uptake of a radioactive tracer within the body. A number of factors, particularly attenuation and scatter of photons within the body tissues can cause degradation of image quality and inaccuracies in the measurement of tracer uptake. Single photon emission tomography (SPECT) systems which incorporate an xray computed tomography (CT) facility have enabled accurate transmission images of the patient to be obtained. These ‘attenuation maps’ can be used to correct the SPECT images for the effects of attenuation. The aim of this project was to investigate the use of an x-ray CT based attenuation correction (AC) system in SPECT gamma camera imaging. The use of AC with other physical parameters of the imaging process including scatter was firstly examined in order to determine the optimum imaging parameters required to maximise image quality. The influence of attenuation, scatter and other imaging parameters on the accuracy of absolute and relative quantitative measurements was then investigated. The methodology involved using the GE Millenium Hawkeye gamma camera system to obtain images of a range of phantoms filled with various concentrations of radioactivity; from simple point sources to phantoms which simulate organs of the body. An attempt was made to establish SPECT sensitivity values that would allow accurate determination of activity in a region of interest. These sensitivity values were applied to all subsequent measurements and a measure made of quantitative accuracy. The results showed that the sensitivity value used for quantitative SPECT measurements must reflect the reconstruction method and corrections used in the acquisition. Attenuation correction proved to be more significant than scatter correction in quantitative accuracy, with activity results being within 30% of expected values in all cases where AC was used

Application of novel corrections for quantification of 123I SPECT

Introduction: The quantification of clinical images provides a useful adjunct to visual assessment in the differentiation of disease processes. In nuclear medicine imaging, the accurate quantification of Single Photon Emission Computed Tomography (SPECT) data is challenging due to limited spatial resolution and the corrections required for photon attenuation and scatter. Specific radionuclides used in SPECT imaging, such as Iodine-123 (123I), pose additional challenges to quantification due to their complex decay schemes. 123I has a predominantly low-energy photon emission of 159keV. However, 123I also has high-energy emissions which, due to septal penetration, are detected within the imaging window. Consequently, absolute quantification of 123I SPECT is not current clinical practice and remains a specialist task. A novel reconstruction correction scheme has been developed by Hermes Medical Solutions which incorporates Monte Carlo simulation of photon interactions in both the patient and the detector system. This Collimator and Detector Response Modelling (CDRM) algorithm has the potential to enhance image quality and, therefore, the quantitative accuracy of 123I SPECT studies. This thesis aims to optimise 123I SPECT quantification using advanced reconstruction algorithms and, furthermore, to assess the clinical applications of these optimised techniques. Method: With the ultimate aim of optimising quantification of 123I SPECT, work was undertaken to assess SPECT spatial uniformity, spatial resolution, contrast recovery, noise and scatter suppression. This work was used to specify the optimum collimator and reconstruction parameters required for accurate quantification. Using these parameters, absolute quantification was then assessed for accuracy with regard to neurology and oncology studies. The utility of Standardised Uptake Values (SUVs) was evaluated in 123I-DaTSCAN patient studies. Furthermore, human observer studies were used to verify the findings of the quantitative assessment. Results: Phantom studies demonstrated that Low Energy High Resolution (LEHR) collimators provide superior image quality for neurology applications where spatial resolution is essential. However, when imaging the torso, this work showed that Medium Energy General Purpose (MELP) collimators, with advanced reconstruction, can improve contrast recovery, noise characteristics and scatter suppression when compared with LEHR data. The accuracy of quantifying activity concentration for neurology studies was optimised using the novel CDRM correction scheme (measured activity concentration within +/-10% of true concentration). However, the accuracy of quantification in torso studies was shown to vary with lesion location in the Field of View (FOV). Therefore, neurology studies were identified as the best candidates for absolute quantification. In a subsequent evaluation of patient studies, measuring the mean SUV of the putamen in 123I-DaTSCAN studies marginally outperformed Hermes Medical Solutions BRASS automated analysis application with regard to the differentiation of normality. Direct quantitative assessment has the advantage that it removes the requirement for a normal database. Furthermore, the evaluation of clinical patient 123I-DaTSCAN studies by human observers demonstrated almost perfect agreement in diagnosis for the novel CDRM reconstruction correction scheme (Kappa coefficient=0.913). Image quality for the CDRM scheme rated significantly higher than current clinical practice (p-value<0.01). The torso phantom observer study suggested that optimised reconstruction of MELP data demonstrated superior image quality and lesion detectability when compared with LEHR reconstructions. Conclusions: For 123I-mIBG oncology studies, including quantification of serial studies, data should be acquired with MELP collimators and reconstructed with advanced corrections for attenuation, scatter and depth-dependent spatial resolution. However, quantification of 123I SPECT body section images for inter-patient comparison is not feasible due to variable accuracy with lesion location in the FOV. Absolute quantification of 123I-DaTSCAN studies, acquired with LEHR collimators, can be performed routinely with sufficient accuracy using the novel CDRM algorithm

Report on active and planned spacecraft and experiments

Information concerning active and planned spacecraft and experiments is reported. The information includes a wide range of disciplines: astronomy, earth sciences, meteorology, planetary sciences, aeronomy, particles and fields, solar physics, life sciences, and material sciences. These spacecraft projects represent the efforts and funding of individual countries as well as cooperative arrangements among different countries