Factors affecting accuracy and precision in PET volume imaging.

Volume imaging positron emission tomographic (PET) scanners with no septa and a large axial acceptance angle offer several advantages over multiring PET scanners. A volume imaging scanner combines high sensitivity with fine axial sampling and spatial resolution. The fine axial sampling minimizes the partial volume effect, which affects the measured concentration of an object. Even if the size of an object is large compared to the slice spacing in a multiring scanner, significant variation in the concentration is measured as a function of the axial position of the object. With a volume imaging scanner, it is necessary to use a three-dimensional reconstruction algorithm in order to avoid variations in the axial resolution as a function of the distance from the center of the scanner. In addition, good energy resolution is needed in order to use a high energy threshold to reduce the coincident scattered radiation.

Positron camera with extended counting rate capability.

Positron emitters may be imaged using two opposing scintillation cameras without collimators. The counting rate limitation of this approach can be largely largely overcome by using graded absorbers to reduce scattered radiation from the patient and using not only photopeak events but Compton events in the scintillator as well. This increases the useful counting rate by more than a factor of 5. By combing this technique with the use of fast electronics, useful images have been obtained in the presence of scattering material at counting rates above 7,500 cps.

Treatment of axial data in three-dimensional PET.

Improved axial spatial resolution in positron emission tomography (PET) scanners will lead to reduced sensitivity unless the axial acceptance angle for the coincidences is kept constant. A large acceptance angle, however, violates assumptions made in most reconstruction algorithms, which reconstruct parallel independent slices, rather than a three-dimensional volume. Two methods of treating the axial information from a volume PET scanner are presented. Qualitative and quantitative errors introduced by the approximations are examined for simulated objects with sharp boundaries and for a more anatomically realistic distribution with smooth activity gradients.

A positron camera using position-sensitive detectors: PENN-PET.

A single-slice positron camera has been developed with good spatial resolution and high count rate capability. The camera uses a hexagonal arrangement of six position-sensitive NaI(Tl) detectors. The count rate capability of NaI(Tl) was extended to 800k cps through the use of pulse shortening. In order to keep the detectors stationary, an iterative reconstruction algorithm was modified which ignores the missing data in the gaps between the six detectors and gives artifact-free images. The spatial resolution, as determined from the image of point sources in air, is 6.5 mm full width at half maximum. We have also imaged a brain phantom and dog hearts.

Standards for performance measurements in scintillation cameras.

Performance measurements of scintillation cameras are made (a) by the manufacturer to test and specify the equipment he sells, (b) by the customer as part of his acceptance testing, and (c) by the user as part of a periodic quality-control program. This paper describes the NEMA (National Electrical Manufacturer's Association) standard, which was developed to provide a uniform criterion for the measurement and reporting of performance parameters for scintillation cameras, and by which a manufacturer may specify his device. The purpose of this paper is to familiarize the nuclear medicine practitioner with the standard, the measurement methods, the reasons for them, and the meaning of the reported values.

Correction for field nonuniformity in scintillation cameras through removal of spastial distortion.

A method to correct for the spatial distortions of gamma cameras has been developed. The method consists of two parts: measuring spatial distortions and repositioning events during accumulation. Distortions are measured using a pattern consisting of parallel slits on 15-mm centers with slit-pattern images obtained in two orthogonal orientations. Slit locations are used to determine X and Y displacements. In repositioning camera events, X and Y event coordinates are digitized and correction displacements added. The procedure is implemented in hardware that replositions each event in real time without introducing additional dead time. Distortion removal offers considerable advantage over other uniformity-improvement schemes, since it correctly compensates for the major cause of nonuniformity, spatial distortion. The method may be used for quantitative studies, because it does not change the number of detected events.

Continuous-slice PENN-PET: a positron tomograph with volume imaging capability.

The PENN-PET scanner consists of six hexagonally arranged position-sensitive Nal(TI) detectors. This design offers high spatial resolution in all three dimensions, high sampling density along all three axes without scanner motion, a large axial acceptance angle, good energy resolution, and good timing resolution. This results in three-dimensional imaging capability with high sensitivity and low scatter and random backgrounds. The spatial resolution is 5.5 mm (FWHM) in all directions near the center. The true sensitivity, for a brain-sized object, is a maximum of 85 kcps/microCi/ml and the scatter fraction is a minimum of 10%, both depending on the lower level energy threshold. The scanner can handle up to 5 mCi in the field of view, at which point the randoms equal the true coincidences and the detectors reach their count rate limit. We have so far acquired [18F]FDG brain studies and cardiac studies, which show the applicability of our scanner for both brain and whole-body imaging. With the results to date, we feel that this design results in a simple yet high performance scanner which is applicable to many types of static and dynamic clinical studies.

Three-dimensional imaging characteristics of the HEAD PENN-PET scanner.

UNLABELLED: A volume-imaging PET scanner, without interplane septa, for brain imaging has been designed and built to achieve high performance, specifically in spatial resolution and sensitivity. The scanner is unique in its use of a single annular crystal of Nal(Tl), which allows a field of view (FOV) of 25.6 cm in both the transverse and axial directions. Data are reconstructed into an image matrix of 128(3) with (2 mm)3 voxels, using three-dimensional image reconstruction algorithms. METHODS: Point-source measurements are performed to determine spatial resolution over the scanner FOV, and cylindrical phantom distributions are used to determine the sensitivity, scatter fraction and counting rate performance of the system. A three-dimensional brain phantom and 18F-FDG patient studies are used to evaluate image quality with three-dimensional reconstruction algorithms. RESULTS: The system spatial resolution is measured to be 3.5 mm in both the transverse and axial directions, in the center of the FOV. The true sensitivity, using the standard NEMA phantom (6 liter), is 660 kcps/microCi/ml, after subtracting a scatter fraction of 34%. Due to deadtime effects, we measure a peak true counting rate, after scatter and randoms subtraction, of 100 kcps at 0.7 mCi for a smaller brain-sized (1.1 liter) phantom, and 70 kcps for a head-sized (2.5 liter) phantom at the same activity. A typical 18F-FDG clinical brain study requires only 2 mCi to achieve high statistics (100 million true events) with a scan time of 30 min. CONCLUSION: The HEAD PENN-PET scanner is based on a cost-effective design using Nal(Tl) and has been shown to achieve high performance for brain studies and pediatric whole-body studies. As a full-time three-dimensional imaging scanner with a very large axial acceptance angle, high sensitivity is achieved. The system becomes counting-rate limited as the activity is increased, but we achieve high image quality with a small injected dose. This is a significant advantage for clinical imaging, particularly for pediatric patients.

Positron emission tomography imaging--technical considerations.

Positron imaging instrumentation has improved rapidly in the last few years. Scanners currently under development are beginning to approach fundamental limits set by positron range and noncolinearity effects. This report reviews the latest developments in positron emission tomography (PET) instrumentation, emphasizing the development of coding schemes that reduce the complexity and cost of high-resolution scanners. The relative benefits of using time-of-flight (TOF) information is discussed as well.

Performance of positron imaging systems as a function of energy threshold and shielding depth.

The effect of energy discrimination and shielding on positron imaging data quality was investigated using a detector pair to simulate a ring positron emission tomograph. Formulas are presented relating the sensitivity, random fraction, and scatter fraction for a detector pair to the same parameters for a ring system. Data were fitted to detector pair expressions for the variation of the above parameters with shielding depth in order to obtain information on the effect of energy threshold level. These fitted curves were used to determine the sensitivity, random fraction, and scatter fraction, as well as an overall data quality factor as a function of energy threshold level and shielding depth. Data were obtained for both NaI(T1) and BGO detector types using activity levels in the range of 1.5 muCi/cm3. Results show that for NaI(T1) detectors, the lowest possible energy threshold level is optimal, with the corresponding optimal shielding depth determined by the level of activity to be imaged. For BGO detectors, a tradeoff exists between energy thresholds of 100-400 keV and shielding depths of 15-30 cm with smaller shielding depths requiring higher energy thresholds.

Accelerated iterative reconstruction for positron emission tomography based on the em algorithm for maximum likelihood estimation.

The EM method that was originally developed for maximum likelihood estimation in the context of mathematical statistics may be applied to a stochastic model of positron emission tomography (PET). The result is an iterative algorithm for image reconstruction that is finding increasing use in PET, due to its attractive theoretical and practical properties. Its major disadvantage is the large amount of computation that is often required, due to the algorithm's slow rate of convergence. This paper presents an accelerated form of the EM algorithm for PET in which the changes to the image, as calculated by the standard algorithm, are multiplied at each iteration by an overrelaxation parameter. The accelerated algorithm retains two of the important practical properties of the standard algorithm, namely the selfnormalization and nonnegativity of the reconstructed images. Experimental results are presented using measured data obtained from a hexagonal detector system for PET. The likelihood function and the norm of the data residual were monitored during the iterative process. According to both of these measures, the images reconstructed at iterations 7 and 11 of the accelerated algorithm are similar to those at iterations 15 and 30 of the standard algorithm, for two different sets of data. Important theoretical properties remain to be investigated, namely the convergence of the accelerated algorithm and its performance as a maximum likelihood estimator.

An Iterative Image Space Reconstruction Algorthm Suitable for Volume ECT.

The trend in the design of scanners for positron emission computed tomography has traditionally been to improve the transverse spatial resolution to several millimeters while maintaining relatively coarse axial resolution (1-2 cm). Several scanners are being built with fine sampling in the axial as well as transverse directions, leading to the possibility of the true volume imaging. The number of possible coincidence pairs in these scanners is quite large. The usual methods of image reconstruction cannot handle these data without making approximations. It is computationally most efficient to reduce the size of this large, sparsely populated array by back-projecting the coincidence data prior to reconstruction. While analytic reconstruction techniques exist for back-projected data, an iterative algorithm may be necessary for those cases where the point spread function is spatially variant. A modification of the maximum likelihood algorithm is proposed to reconstruct these back-projected data. The method, the iterative image space reconstruction algorithm (ISRA), is able to reconstruct data from a scanner with a spatially variant point spread function in less time than other proposed algorithms. Results are presented for single-slice data, simulated and actual, from the PENN-PET scanner.

Constrained Fourier space method for compensation of missing data in emission computed tomography.

A method is introduced to compensate for missing projection data that can result from gas between detectors or from malfunctioning detectors. This method uses constraints in the Fourier domain to estimate the missing data, thus completing the data set so that the filtered backprojection algorithm can be used to reconstruct artifact-free images. The image reconstructed from estimates using this technique and a data set with gaps is nearly indistinguishable from an image reconstructed from a complete data set without gaps, using a simulated brain phantom.

A method for reconstructing images from data obtained with a hexagonal bar positron camera.

This paper describes the algorithms and procedures developed for reconstructing images using the hexagonal bar positron camera. This camera has six continuous position-sensitive detectors which are completely stationary, and it has some special software requirements. In particular, spatial nonlinearities in the detectors must be removed in software, the large but sparsely populated data matrix must be reduced in size, and the gaps in the data from the intersections of the detectors must be compensated for. These problems have been investigated, and an appropriate algorithm for this system has been implemented. The effectiveness of this algorithm was evaluated by reconstructing both real and simulated data.

The countrate performance of the volume imaging PENN-PET scanner.

The UGM PENN-PET camera uses large position sensitive detectors and operates without septa. This design results in high sensitivity and 3-D imaging capability, but poses problems in high countrate situations. The maximum true countrates and random countrates have been measured, as a function of object size in the field-of-view. The countrate performance is understood in terms of the limiting process rates and event rejection in the camera. In addition, the camera is calibrated to generate absolute activity concentrations to within 5% by correcting for system deadtime with up to 3 mCi in the field of view in brain studies (50% deadtime at 3 muCi/mL). This allows the performance of a variety of brain and body studies, and accurate quantitation of the data over a wide range of imaging countrates, from (18)F-FDG brain studies to (15)O-water dynamic brain studies.

Effect of increased axial field of view on the performance of a volume PET scanner.

The performance of the PENN-PET 240H scanner from UGM Medical Systems is tested and compared to the prototype PENN-PET scanner built at the University of Pennsylvania. The UGM PENN-PET scanner consists of six continuous position-sensitive NaI(Tl) detectors, which results in a 50 cm transverse field-of-view and a 12.8 cm axial field-of-view. The fine spatial sampling in the axial direction allows the data to be sorted into as many as 64 transverse planes, each 2 mm thick. A large axial acceptance angle, without interplane septa, results in a high-sensitivity and low-randoms fraction, with a low-scatter fraction due to the use of a narrow photopeak energy window. This work emphasizes those performance measurements that illustrate the special characteristics of a volume imaging scanner and how they change as the axial length is increased.

Effect of resolution improvement on required count density in ECT imaging: a computer simulation.

The effects of changes in spatial resolution and total number of counts on image quality were investigated for positron and single photon emission computed tomography (ECT) systems. A variety of high contrast phantoms were generated in a computer simulation and count density and spatial resolution were varied independently over a wide range. As system spatial resolution is improved, significantly fewer counts are needed to give images of comparable visual quality. Using 100% object contrast, it was found that the number of counts could be reduced by a factor of four for a 2 mm improvement in spatial resolution over a wide range of parameters. This is due to the fact that image contrast increases rapidly with spatial resolution improvements in high contrast objects such as those used in this simulation and typically encountered in brain and cardiac ECT studies.

Performance of a position-sensitive scintillation detector.

The spatial resolution of a NaI(T1), 25 mm thick bar detector designed for use in positron emission tomography has been studied. The position along the 500 mm long detector is determined from the centroid of the light distribution in the crystal as measured by a linear array of photomultiplier tubes. A Monte Carlo computer simulation was performed to investigate the factors limiting the spatial resolution. The program allowed us to study the effect of various phototube configurations and crystal surfaces. Since the resolution is affected by the width of the light distribution, we studied the effect of sharpening the distribution by modifying the front crystal surface with grooves cut perpendicular to the long axis of the crystal and by using non-linear preamplifiers. The simulation predicts a spatial resolution (FWHM) of 3 mm with this crystal. Experimental measurements of spatial resolution were performed concurrently with the simulations. In particular, a modified grooved crystal was measured to have 4.0 mm spatial resolution, an improvement over the original crystal without grooves. With delay line pulse shortening, which increases the count rate capability of the detector, the grooved crystal was measured to have 5.5 mm spatial resolution.

Influence of hole shape on collimator performance.

The imaging properties of triangular and hexagonal hole shapes were compared for low energy scintillation camera collimators. The response of both hole shapes was calculated using a ray-tracing computer program, corresponding collimators were constructed and their performance was evaluated by measuring line spread functions and imaging bar patterns. The triangular hole shape was found to give better results above a spatial frequency of 1-5 cycles cm-1, below that frequency the hexagonal hole shape is superior. The differences, however, are small and the superior performance of the hexagonal hole collimator at clinically significant spatial frequencies is barely visible in bar pattern images.

Image quality assessment of LaBr3-based whole-body 3D PET scanners: a Monte Carlo evaluation.

The main thrust for this work is the investigation and design of a whole-body PET scanner based on new lanthanum bromide scintillators. We use Monte Carlo simulations to generate data for a 3D PET scanner based on LaBr3 detectors, and to assess the count-rate capability and the reconstructed image quality of phantoms with hot and cold spheres using contrast and noise parameters. Previously we have shown that LaBr3 has very high light output, excellent energy resolution and fast timing properties which can lead to the design of a time-of-flight (TOF) whole-body PET camera. The data presented here illustrate the performance of LaBr3 without the additional benefit of TOF information, although our intention is to develop a scanner with TOF measurement capability. The only drawbacks of LaBr3 are the lower stopping power and photo-fraction which affect both sensitivity and spatial resolution. However, in 3D PET imaging where energy resolution is very important for reducing scattered coincidences in the reconstructed image, the image quality attained in a non-TOF LaBr3 scanner can potentially equal or surpass that achieved with other high sensitivity scanners. Our results show that there is a gain in NEC arising from the reduced scatter and random fractions in a LaBr3 scanner. The reconstructed image resolution is slightly worse than a high-Z scintillator, but at increased count-rates, reduced pulse pileup leads to an image resolution similar to that of LSO. Image quality simulations predict reduced contrast for small hot spheres compared to an LSO scanner, but improved noise characteristics at similar clinical activity levels.