Showing papers by "Habib Zaidi published in 2004"

Scatter modelling and compensation in emission tomography

Abstract: In nuclear medicine, clinical assessment and diagnosis are generally based on qualitative assessment of the distribution pattern of radiotracers used. In addition, emission tomography (SPECT and PET) imaging methods offer the possibility of quantitative assessment of tracer concentration in vivo to quantify relevant parameters in clinical and research settings, provided accurate correction for the physical degrading factors (e.g. attenuation, scatter, partial volume effects) hampering their quantitative accuracy are applied. This review addresses the problem of Compton scattering as the dominant photon interaction phenomenon in emission tomography and discusses its impact on both the quality of reconstructed clinical images and the accuracy of quantitative analysis. After a general introduction, there is a section in which scatter modelling in uniform and non-uniform media is described in detail. This is followed by an overview of scatter compensation techniques and evaluation strategies used for the assessment of these correction methods. In the process, emphasis is placed on the clinical impact of image degradation due to Compton scattering. This, in turn, stresses the need for implementation of more accurate algorithms in software supplied by scanner manufacturers, although the choice of a general-purpose algorithm or algorithms may be difficult.

Monte carlo simulation of x-ray spectra in diagnostic radiology and mammography using MCNP4C.

Abstract: The general purpose Monte Carlo N-particle radiation transport computer code (MCNP4C) was used for the simulation of x-ray spectra in diagnostic radiology and mammography. The electrons were transported until they slow down and stop in the target. Both bremsstrahlung and characteristic x-ray production were considered in this work. We focus on the simulation of various target/filter combinations to investigate the effect of tube voltage, target material and filter thickness on x-ray spectra in the diagnostic radiology and mammography energy ranges. The simulated x-ray spectra were compared with experimental measurements and spectra calculated by IPEM report number 78. In addition, the anode heel effect and off-axis x-ray spectra were assessed for different anode angles and target materials and the results were compared with EGS4-based Monte Carlo simulations and measured data. Quantitative evaluation of the differences between our Monte Carlo simulated and comparison spectra was performed using student's t-test statistical analysis. Generally, there is a good agreement between the simulated x-ray and comparison spectra, although there are systematic differences between the simulated and reference spectra especially in the K-characteristic x-rays intensity. Nevertheless, no statistically significant differences have been observed between IPEM spectra and the simulated spectra. It has been shown that the difference between MCNP simulated spectra and IPEM spectra in the low energy range is the result of the overestimation of characteristic photons following the normalization procedure. The transmission curves produced by MCNP4C have good agreement with the IPEM report especially for tube voltages of 50 kV and 80 kV. The systematic discrepancy for higher tube voltages is the result of systematic differences between the corresponding spectra.

Feasibility of a novel design of high resolution parallax-free Compton enhanced PET scanner dedicated to brain research*

Abstract: A novel concept for a positron emission tomography (PET) camera module is proposed, which provides full 3D reconstruction with high resolution over the total detector volume, free of parallax errors The key components are a matrix of long scintillator crystals and hybrid photon detectors (HPDs) with matched segmentation and integrated readout electronics The HPDs read out the two ends of the scintillator package Both excellent spatial (x, y, z) and energy resolution are obtained The concept allows enhancing the detection efficiency by reconstructing a significant fraction of events which underwent Compton scattering in the crystals The proof of concept will first be demonstrated with yttrium orthoaluminate perovskite (YAP):Ce crystals, but the final design will rely on other scintillators more adequate for PET applications (eg LSO:Ce or LaBr3:Ce) A promising application of the proposed camera module, which is currently under development, is a high resolution 3D brain PET camera with an axial field-of-view of approximately 15 cm dedicated to brain research The design philosophy and performance predictions based on analytical calculations and Monte Carlo simulations are presented Image correction and reconstruction tools required to operate this transmissionless device in a research environment are also discussed Better or similar performance parameters were obtained compared to other known designs at lower fabrication cost The axial geometrical concept also seems to be promising for applications such as positron emission mammography

Comparative assessment of different computational models for generation of X-ray spectra in diagnostic radiology and mammography

Abstract: The X-ray spectra predicted by different computational models used in diagnostic radiology and mammography energy range has been assessed by comparison with experimentally measured spectra The comparative assessment encompassed many figures of merit including qualitative assessment of the spectra shape, the difference in K X-ray yield, transmission curves, half value layers (HVLs) and absorbed dose and effective dose equivalent (EDE) imparted to the adult ORNL hermaphroditic phantom when using X-ray spectra generated with different models at different tube voltages The calculated spectra with X-raytbc and IPEM agreed well with measured spectra in diagnostic radiology and mammography energy range, respectively The student's t-test statistical analysis showed there is no statistically significant difference between measured and generated spectra for all computational models investigated in this study In addition, the MCNP4C-based Monte Carlo calculations showed there is no discernable discrepancy in the calculation of absorbed dose and EDE in the adult ORNL hermaphroditic phantom when using different computational models for generating the X-ray spectra

Attenuation compensation in cerebral 3D PET: effect of the attenuation map on absolute and relative quantitation

Abstract: It is generally well accepted that transmission (TX)-based non-uniform attenuation correction can supply more accurate absolute quantification; however, whether it provides additional benefits in routine clinical diagnosis based on qualitative interpretation of 3D brain positron emission tomography (PET) images is still the subject of debate. The aim of this study was to compare the effect of the two major classes of method for determining the attenuation map, i.e. uniform versus non-uniform, using clinical studies based on qualitative assessment as well as absolute and relative quantitative volume of interest-based analysis. We investigated the effect of six different methods for determining the patient-specific attenuation map. The first method, referred to as the uniform fit-ellipse method (UFEM), approximates the outline of the head by an ellipse assuming a constant linear attenuation factor (μ=0.096 cm−1) for soft tissue. The second, referred to as the automated contour detection method (ACDM), estimates the outline of the head from the emission sinogram. Attenuation of the skull is accounted for by assuming a constant uniform skull thickness (0.45 cm) within the estimated shape and the correct μ value (0.151 cm−1) is used. The usual measured transmission method using caesium-137 single-photon sources was used without (MTM) and with segmentation of the TX data (STM). These techniques were finally compared with the segmented magnetic resonance imaging method (SMM) and an implementation of the inferring attenuation distributions method (IADM) based on the digital Zubal head atlas. Several image quality parameters were compared, including absolute and relative quantification indexes, and the correlation between them was checked. The qualitative evaluation showed no significant differences between the different attenuation correction techniques as assessed by expert physicians, with the exception of ACDM, which generated artefacts in the upper edges of the head. The mean squared error between the different attenuation maps was also larger when using this latter method owing to the fact that the current implementation of the method significantly overestimated the head contours on the external slices. Correlation between the mean regional cerebral glucose metabolism (rCGM) values obtained with the various attenuation correction methods and those obtained with the gold standard (MTM) was good, except in the case of ACDM (R 2=0.54). The STM and SMM methods showed the best correlation (R 2=0.90) and the regression lines agreed well with the line of identity. Relative differences in mean rCGM values were in general less than 8%. Nevertheless, ANOVA results showed statistically significant differences between the different methods for some regions of the brain. It is concluded that the attenuation map influences both absolute and relative quantitation in cerebral 3D PET. Transmission-less attenuation correction results in a reduced radiation dose and makes a dramatic difference in acquisition time, allowing increased patient throughput.

Novel design of a parallax free Compton enhanced PET scanner

Abstract: Molecular imaging by PET is a powerful tool in modern clinical practice for cancer diagnosis. Nevertheless, improvements are needed with respect to the spatial resolution and sensitivity of the technique for its application to specific human organs (breast, prostate, brain, etc.), and to small animals. Presently, commercial PET scanners do not detect the depth of interaction of photons in scintillators, which results in a not negligible parallax error. We describe here a novel concept of PET scanner design that provides full three-dimensional (3D) gamma reconstruction with high spatial resolution over the total detector volume, free of parallax errors. It uses matrices of long scintillators read at both ends by hybrid photon detectors. This so-called 3D axial concept also enhances the gamma detection efficiency since it allows one to reconstruct a significant fraction of Compton scattered events. In this note, we describe the concept, a possible design and the expected performance of this new PET device. We also report about first characterization measurements of 10 cm long YAP:Ce scintillation crystals. r 2004 Elsevier B.V. All rights reserved.

Correction for image degrading factors is essential for accurate quantification of brain function using PET

Abstract: Suggestions for topics suitable for these Point/Counterpoint debates should be addressed to the Moderator: William R. Hendee, Medical College of Wisconsin, Milwaukee: whendee@mcw.edu. Persons participating in Point/Counterpoint discussions are selected for their knowledge and communicative skill. Their positions for or against a proposition may or may not reflect their personal opinions or the positions of their employers.

Novel Geometrical Concept of a High Performance Brain PET Scanner : Principle, Design and Performance Estimates

Abstract: We present the principle, a possible implementation and performance estimates of a novel geometrical concept for a high resolution positron emission tomograph. The concept, which can for example be implemented in a brain PET device, promisses to lead to an essentially parallax free 3D image reconstruction with excellent spatial resolution and constrast, uniform over the complete field of view. The key components are matrices of long axially oriented scintillator crystals which are read out at both extremities by segmented Hybrid Photon Detectors. We discuss the relevant design considerations for a 3D axial PET camera module, motivate parameter and material choices, and estimate its performance in terms of spatial and energy resolution. We support these estimates by Monte Carlo simulations and in some cases by first experimental results. From the performance of a camera module, we extrapolate to the reconstruction resolution of a 3D axial PET scanner in a semi-analytical way and compare it to an existing state-of-the art brain PET device. We finally describe a dedicated data acquisition system, capable to fully exploit the advantages of the proposed concept. We conclude that the proposed 3D Axial concept and the discussed implementation is a competitive approach for high resolution brain PET. Excellent energy resolution and Compton enhanced sensitivity are expected to lead to high quality reconstruction and reduced scanning times.

Optimization of the effective light attenuation length of YAP:Ce and LYSO:Ce crystals for a novel geometrical PET concept

Abstract: The effective light attenuation length in thin bars of YAP:Ce and LYSO:Ce crystals has been studied for different coatings of the lateral surfaces of the scintillators This physical parameter plays a key role in a novel 3D PET concept based on axial arrays of long scintillator bars read out at both ends by Hybrid Photodetectors (HPD) The parameter influences the spatial, energy, and time resolutions of such a device The expected resolutions let the novel concept appear competitive compared to existing traditional PET devices In this paper we show that the effective light attenuation length can be tuned to the desired value by wrapping the lateral surfaces of the crystals or by coating them with layers of Cr or Au of different thickness The studies have been carried out with long YAP and LYSO scintillator bars, read out by standard photomultiplier tubes Even though the novel PET device is planned to use different scintillators and HPD readout, the results described here prove the feasibility of an important aspect of the concept and explore the potential resolution of the device

Towards optimal model-based partial volume effect correction in oncological PET imaging

Abstract: Quantification of tumor uptake in nuclear medicine requires the assessment of the tumor size. We propose a method based on simple ellipsoidal models which only require the reconstructed image data and scanner physical parameters. The method uses simple image and system models but is theoretically not sensitive to the reconstructed image sampling, provided that the local resolution parameters of the scanner are known. Interpolation is thus avoided and the degradation model is computed analytically. The proposed method uses the property that the blurring effect of the scanner spreads the activity around the lesion location and in the neighbor-voxels-even for sub-voxel sized lesions-and attempts to use this limited information to accurately restore lesion parameters. We propose solutions to avoid bias and reduce computation time. The uptake of simple simulated tumor objects could be restored and the influence of lesion's shape and neighborhood was investigated on real patient data for which convergence of the algorithm was demonstrated. This approach is a contribution towards in-depth investigation of reconstruction/post-reconstruction-based models for tumor activity restoration based on more complex image and system models.

An event driven read-out system for a novel PET scanner with Compton enhanced 3D gamma reconstruction

Abstract: The design of a data acquisition system (DAQ) for a novel positron emission tomography scanner is reported. The PET system, based on long axially oriented scintillation crystals, read out by hybrid photon detectors (HPD), allows 3D parallax-error free Compton enhanced gamma reconstruction. The DAQ system is composed of several read-out cards, each one associated with a module of the PET scanner, and of a main card that controls the whole system. Using fast triggering signals from the silicon sensor back-planes, the main card performs the coincidence analysis, and, in case of coincidence, it enables the read-out of the two modules involved. The other modules are left free to perform new acquisitions. This concept based on several independent, event driven and parallel read-out chains, drastically reduce the acquisition dead time. Each enabled readout card digitizes, encodes and stores data from the respective module. Data are stored in a local FIFO and then are transferred through a network into a single computer. The system is designed according to the specifications of IDEAS VaTaGP5 chip. Each read-out card is able to accommodate all the chip read-out modes and the test procedures and can be used as a standalone read-out system that allows reading out up to 16 daisy chained chips per channel. We describe a simplified DAQ system, designed for a two module demonstrator set-up, developed to study and optimize the essential design parameters.

High resolution 3d brain pet with hybrid photon detectors

Abstract: The design of a high resolution 3 dimensional brain PET is presented. Thanks to its novel geometry a 3D measurement of the gamma interaction point in the detector is possible. Each 3D PET camera module consists of a matrix of long scintillation crystals, axially oriented, readout on both sides by Hybrid Photon Detectors. This concept leads to an image reconstruction free of parallax error and provides a uniform spatial and energy resolution over the whole sensitive volume. Furthermore it allows to enhance sensitivity by reconstructing a substantial fraction of the gamma quanta that underwent Compton scattering in the detectors.