Annihilation photons are produced in mutually orthogonal polarization states, resulting in correlated angular distributions if both photons Compton scatter. This may impact the total scatter distribution, but may also impact detector normalization values for scattered coincidences. Single scatters within the source object are a significant fraction or the total scatter distribution. Further, 53 (62)% of first interactions within BGO (LSO) will be Compton scatter for 511 keV photons. 30 (39)% of unpolarized photons interacting in a 4×8×30 mm BGO (LSO) crystal will exit an individual crystal element prior to photoelectric absorption, possibly resulting in lost or mis-coded events. Thus, angular correlations associated with polarization effects may affect the efficiency response of the individual crystal elements of a block structure. Relative crystal efficiencies for axial and transverse polarization conditions were measured using a GE Advance BGO block detector. Results show a systematic difference in polarized crystal efficiencies within the block structure. The data impose an upper limit of 4% on the crystal sensitivity variations, for the BGO block tested, between incident photons with axial and transverse polarization states. The SimSET simulation software has been modified to include polarization. Investigations with measured and simulated data are ongoing

SimSET (a Simulation System for Emission Tomography) is a public domain simulation of PET and SPECT. The recently released version 2.6 adds capabilities to simulate slat collimation for dual-headed coincidence imaging (DHCl) and cylindrical detectors for positron emission tomography (PET). The slat collimator module simulates axial slats of attenuating material in front of planar detectors. The collimator may be composed of multiple materials and be radially layered. The cylindrical detector module simulates an annulus of detector material. The detector may also be composed of multiple materials, and may vary axially and radially, but transaxial cuts are not simulated. Validation tests compared results from slat collimator and cylindrical detector simulations to analytic predictions and to simulations using other previously tested SimSET modules. Beta testers have used both modules extensively

The Differential Attenuation Method (DAM) was developed to simultaneously estimate activity and attenuation distributions from multi-emission projection data alone. Previously, DAM was shown to improve the quality and quantitative accuracy compared with filtered-backprojection without attenuation compensation. Here, that work is extended by comparing the performance of DAM to an iterative penalized-weighted least-squares reconstruction with: 1) uniform attenuation, and 2) the true attenuation. A single-slice numerical torso phantom was used to simulate data from Tl-201 cardiac studies. Noise-free projection and data with additive Poisson-distributed noise were simulated with and without a myocardial perfusion defect. Scatter was not simulated. Images reconstructed with DAM from data at realistic count densities were more accurate than those reconstructed with an assumed uniform attenuation distribution, and nearly as accurate as those obtained using the true attenuation distribution.

A penalized weighted least squares (PWLS) reconstruction algorithm is described which simultaneously estimates activity and attenuation distributions from emission sinogram data alone. This estimation technique is based on differential attenuation information and is applicable to any SPECT imaging isotope with emissions at two or more distinct energies, after compensating for Compton scatter. A rotation-based forward projector is used to efficiently model photon attenuation at multiple emission energies, as well as distance-dependent spatial resolution. The algorithm was tested using simulated scatter-free /sup 201/Tl projection data from a single-slice numerical cardiac phantom with a large cold myocardial defect. Poisson noise was added to the projection data to mimic clinically realistic count densities. The activity estimates resulting from the proposed method had fewer artifacts and were substantially more accurate than images reconstructed with filtered backprojection without compensation for attenuation. Several techniques were employed to reduce the time required for the iterative routine to converge and to improve the stability of the solution, including: (1) a preconditioning image variable transformation; (2) a coarse-to-fine grid initialization schedule; and (3) a convex hull image mask determined directly from the projection data. The combined effect of these techniques was a reduction in compute time by a factor of /spl sim/200.

The authors have modified SimSET to model positron range and annihilation radiation non-collinearity. Positron range in water is sampled using the empirical model developed by Palmer and Brownell (IEEE Trans. Med. Imag., vol. 11, p. 373-8, 1992). The positron is projected from the decay location in a random direction with adjustment for the density and effective atomic number and weight of intervening tissues. The positron range algorithm was validated by comparing simulated range distributions to the model and with data published by Derenzo (5th Int. Conf. Positron Annihilation, Sendai, Japan, 1979). Annihilation non-collinearity is simulated as a Gaussian-distributed variation from 180 degrees with a standard deviation of 0.5 degrees. Tests verify the simulated noncollinearity is Gaussian distributed and that the azimuthal angle is unbiased

The Differential Attenuation Method (DAM) simultaneously estimates SPECT activity and attenuation distributions from emission data. In this work, we extend the investigation of DAM to measured data and compare the results obtained with FBP without compensating for attenuation, and Penalized Weighted Least Squares (PWLS) with uniform and no attenuation. An anthropomorphic torso phantom with a heart insert containing a myocardial defect filled with In-111 is used to acquire data on a GE VG tomograph. An accurate scatter correction before reconstruction is crucial. Scatter is subtracted with an Energy Window Based Subtraction method (EWBS), with positions and widths of the energy windows selected using Monte Carlo simulated source distributions. The results show the potential benefits of DAM as the activity estimate has fewer artifacts and is more accurate than FBP without compensating for attenuation or PWLS with uniform attenuation. However, artifacts remain and additional constraints on the allowed attenuation values were required to produce acceptable images.

A penalized weighted least squares reconstruction algorithm is described that simultaneously estimates activity and attenuation distributions from emission sinogram data alone. This estimation technique is based on differential attenuation information and is applicable to any single photon emission computed tomography imaging isotope with emissions at two or more distinct energies, after accurate compensation for Compton scatter. A rotation-based forward projector is used to efficiently model photon attenuation at multiple emission energies, as well as distance-dependent spatial resolution. The algorithm was tested using simulated scatter-free 201T1 projection data from a single-slice numerical cardiac phantom with and without cold myocardial defects. Poisson noise was added to the projection data to mimic clinically realistic count densities. The activity estimates resulting from the proposed method had fewer artifacts and were substantially more accurate than images reconstructed with filtered backprojection without compensation for attenuation. Several techniques were employed to reduce the time required for the iterative routine to converge and to reduce the sensitivity of the solution to noise in the projection data. These included: (1) a preconditioning image variable transformation; (2) a coarse-to-fine grid initialization schedule; and (3) a convex hull image mask determined directly from the data. The combined effect of these techniques substantially reduced the compute time required for the reconstruction.

The authors studied the effects of count density, count rate, and source geometry on a model based normalization using a GE Advance PET system. To study precision vs. count density the authors generated crystal efficiencies, /spl epsi/, from 20 time equivalent PVI scans acquired with a 50 /spl mu/Ci rotating /sup 68/Ge line source, and calculated the standard deviation, /spl sigma/, of the individual /spl epsi/ over the 20 realizations. The /spl sigma/ followed the Poisson estimate of 1//spl radic/N where N is the total number of counts involving a given crystal. For example, an average value of 2.7 counts per line of response in the central 22 cm field of view resulted in 5,000 counts per crystal and a /spl sigma/ of 1.5%. Using a 20 cm cylinder filled with F-18, the authors studied count rate effects by measuring the difference between /spl epsi/ generated from acquisitions ranging in block deadtime (BDT) from 0.6% to 11.4% and those from a low rate, high count reference scan. Rate induced errors increased linearly from 0.5% to 4.8% over the 1.2% to 11.4% BDT range. The authors studied cylinder, annulus (20 cm OD and 14 cm ID), and rotating rod /sup 68/Ge source geometries. Eight sets of /spl epsi/ were generated for each geometry, and compared cylinder to annulus and cylinder to rotating rod using the Student's t statistic. Elliptical phantom data were normalized by each source geometry and reconstructed using 3DRP.

At 140 keV, 3% of photon scatter interactions in human tissues are coherent scatter; at Tl-201 emission energies, this fraction increases to approximately 7%. However, since coherent scatter at these energies is sharply forward-peaked, it is often the dominant scatter interaction at small angles. SimSET (Simulation System for Emission Tomography), which previously modeled only photoelectric absorption and Compton scatter, has been extended to include coherent scatter. The current implementation uses form factor and anomalous scattering amplitude data from the Livermore Evaluated Photon Data Library. Interaction probability and angular distribution tables for several human tissues and common detector materials were calculated using the independent atoms approximation and human-tissue composition data from the ICRP Reference Man. These data were also used to generate new tables for photoelectric absorption and Compton scatter, significantly improving the accuracy of SimSET and extending its photon tracking capability to lower photon energy (from 50 keV to 1 keV). The form, content, and structure of the tables were carefully designed for efficient data storage, access, and use by the software. The derived data tables and implementation of coherent scatter were validated by comparing simulation results to published differential cross-section data.

A low cost data acquisition system (DAS) was developed to acquire coincidence data from an unmodified General Electric Maxxus dual head scintillation camera. A high impedance pick-off circuit provides position and energy signals to the DAS without interfering with normal camera operation. The signals are pulse-clipped to reduce pileup effects. Coincidence is determined with fast timing signals derived from constant fraction discriminators. A charge-integrating FERA 16 channel ADC feeds position and energy data to two CAMAC FERA memories operated as ping-pong buffers. A Macintosh PowerPC running Labview controls the system and reads the CAMAC memories. A CAMAC 12-channel scaler records singles and coincidence rate data. The system dead-time is approximately 10% at a coincidence rate of 4.0 kHz.

A method is proposed to estimate attenuation from differential attenuation information (DAI) contained solely in SPECT emission data produced by /sup 201/Tl or other multiple emission isotopes. The total attenuation along each projection line is shown to be related to the difference in attenuation for two emissions at distinct energies (low energy gamma rays are attenuated more than high energy gamma rays); this difference is reflected by the relative gamma-ray intensity detected at the two energies after compensation for Compton scatter. The feasibility of attenuation compensation using DAI was investigated using synthesized scatter-free data from an anthropomorphic digital phantom. Two approaches were investigated: (1) filtered backprojection reconstruction of sinograms pre-corrected for attenuation using a point-source approximation; and (2) incorporating the DAI into an iterative regularized image reconstruction algorithm using a more realistic distributed source model. Both these techniques improved image quality and reduced attenuation artifacts visible in uncorrected filtered backprojection images. However, Poisson noise presents a significant obstacle to practical application of this technique and several enhancements are required before it can be successfully applied to realistic imaging data.

The increased sensitivity of 3D PET reduces image noise but can also result in a loss of contrast due to higher scatter fractions. Phantom studies were performed to compare tumor detectability in 2D and 3D qualitative whole body PET without scatter or attenuation correction. Lesion detectability was defined as: detectability=contrast/noise=( -)//spl sigma//sub liver/, where and are the average of lesion and liver regions of interest (ROIs), respectively. Liver, heart, and soft tissue sections of a Data Spectrum torso phantom containing a Teflon spine insert were filled with F-18 to match relative concentrations found in clinical FDG studies. Spherical lesions of 1.2 and 2.2 cm diameter were placed in the liver with a lesion to liver activity concentration ratio of 2:1. Resulting 2D and 3D images were compared for equivalent whole body acquisition times. Circular ROIs, half the diameter of the lesions, were placed on the tumors and the surrounding background. Background ROIs were normalized to account for the spatially variant bias caused by the absence of the scatter and attenuation corrections. Detectability was greater in the 3D images over the range of count densities and lesion sizes studied, although the difference in detectability between 2D and 3D decreases with decreasing lesion size. These results suggest that 3D imaging is preferable to 2D imaging for clinical qualitative whole body scanning without scatter or attenuation correction. Further studies representing a larger range of clinical applications are required.

Performance measurements of the General Electric Advance Positron Emission Tomograph operating with the septa retracted (3D mode) have been made. All reconstructions were done with the GE Advance 3D package. Performance tests were carried out with: the NEMA phantoms; a 3D Hoffman phantom; a Data Spectrum torso phantom with lung and cardiac inserts; and the "Utah" 3D evaluation phantom. Data collected included: transaxial and axial resolution, uniformity, recovery coefficients, count rate performance, dead time accuracy, and effect of scatter correction.

UNLABELLED The use of high-dose 131I antibody therapy requires accurate measurement of normal tissue uptake to optimize the therapeutic dose. One of the factors limiting the accuracy of such measurements is scatter and collimator septal penetration. This study evaluated two classes of energy-based scatter corrections for quantitative 131I imaging: window-based and spectrum-fitting. METHODS The window-based approaches estimate scatter from data in two or three energy windows placed on either side of the 364-keV photopeak using empirical weighting factors. A set of images from spheres in an elliptical phantom were used to evaluate each of the window-based corrections. The spectrum-fitting technique estimates detected scatter at each pixel by fitting the observed energy spectrum with a function that models the photopeak and scatter, and which incorporates the response function of the camera. This technique was evaluated using a set of Rollo phantom images. RESULTS All of the window-based methods performed significantly better than a single photopeak window (338-389 keV), but the weighting factors were found to depend on the object being imaged. For images contaminated with scatter, the spectrum-fitting method significantly improved quantitation over photopeak windowing. Little difference, however, between any of the methods was observed for images containing small amounts of scatter. CONCLUSION Most clinical 131I imaging protocols will benefit from qualitative and quantitative improvements provided by the spectrum-fitting scatter correction. The technique offers the practical advantage that it does not require phantom-based calibrations. Finally, our results suggest that septal penetration and scatter in the collimator and other detector-head components are important sources of error in quantitative 131I images.

A NIM/CAMAC/Macintosh data acquisition system was developed to collect±X, ±Y, and E information from a GE Maxxus SPECT system. A high impedance signal pick-off circuit was used so the data could be collected without disturbing normal operation of the camera. The measured coincidence timing resolution for the system is 8 msec FWHM. The energy resolutions for the detector heads are ~7.5% (using pulse clipping), at 511 keV. The system sensitivity is 70 cps/μCi for a point source centered in the field of view (450-575 keV energy window, 62.6 cm detector spacing). The intrinsic spatial resolution of the detector heads are ~3.7 mm FWHM in the focal plane using linear tomography. Linear tomography image reconstruction and a volume image reconstruction based on the Kinahan-Rogers approach have been implemented. Images of a line source phantom, the Data Spectrum torso phantom, and the 3D Hoffman brain phantom

Coincidence electronics and a data acquisition system were developed to explore coincidence detection using conventional dual head gamma camera. A high impedance pick-off circuit provides position and energy signals without interfering with normal camera operation. The signals are pulse-clipped to reduce pileup effects. Thin lead-tin-copper filters are used to reduce the flux of low energy photons to the detectors. The data are stored in list mode format.The measured coincidence timing resolution for the system is 9 nsec FWHM (450 kcps/detector) and the energy resolution is 11% (650 kcps/detector). The system sensitivity is 46 kcps/mu Ci/cc for a 20 cm diameter (18 cm length) cylindrical phantom centered in the field of view. A scatter fraction of 31% was measured using the 20 cm cylindrical phantom. The sensitivity and scatter fraction measurements were made using a 450-575 keV energy window, 63.0 cm detector spacing, and 1 mm thick lead filters. The maximum recommended singles rate (full spectrum) for coincidence imaging is similar to 800 kcps per detector.The 3D reprojection algorithm has been implemented. Example images of the 3D Hoffman brain phantom and patient tumor images are shown.

A combined scatter and attenuation correction that does not require a transmission scan is proposed for 111In imaging. Estimates of the unscattered intensity at both 171 and 245 keV are obtained by fitting the observed energy spectrum at each pixel or region of interest using the measured scatter‐free spectrum and a simple model for scatter. The scatter model for the 171 keV peak accounts for scatter contributed by both the 171 and 245 keV emissions. After correcting for scatter, the attenuation is estimated from the observed ratio of photopeak intensities using the known difference in attenuation at the two emission energies and a model based on a point source in water. Accurate scatter correction is a prerequisite for the success of this method because scatter from the higher energy emission will otherwise contaminate the lower photopeak. This differential attenuation method (DAM) of estimating attenuation is demonstrated and calibrated using a series of point source measurements with a wedge‐shaped attenuator. The observed absolute and differential attenuation are in good agreement with the narrow‐beam linear attenuation coefficients for water. Estimates of precision suggest a depth resolution of 1.0–2.5 cm for realistic count densities over the clinically relevant depth range (0–25 cm). The accuracy of DAM in a more realistic attenuation environment is assessed using a hot sphere inside the anthropomorphic data spectrum torso phantom viewed from several angles (with differing attenuation). Finally, the potential of DAM for SPECT attenuation correction was investigated by computer simulation using the SIMSET Monte Carlo software. Preliminary results based on measured planar data and simulated SPECT data indicate that DAM can improve the quality and quantitative accuracy of 111In images. In one SPECT simulation study, the average error in tumor to soft‐tissue ratios was reduced from 32% for uncorrected data to 8% for data corrected with DAM. However, the technique is susceptible to significant noise amplification and can cause substantial streak artifacts in low‐count SPECT studies if sufficient smoothing of the depth estimates is not performed.

We studied the performance of linear scatter correction methods for single-photon imaging with Tc-99m and Tl-201, using a numerical model of the Rollo phantom and measurements with a gamma camera modified to record position and energy information in list mode form. We compared the performance of these methods to per-image optimized linear methods and to locally adaptive linear methods, and developed estimates of the limits on accuracy of scatter correction imposed by the presence of Poisson noise. For both Tc-99m and Tl-201 imaging at a fixed depth, particularly at low count rates, the performance of dual-window methods, or of adaptive methods, is near the best possible for linear methods. Smoothing of the scatter estimate results in minor improvement for Tl-201. Substantial gaps between the performance of any of these linear methods and the limits imposed by Poisson noise remain and are due primarily to bias, with the gap for Tl-201 being larger than that for Tc-99m.

Energy information is used to generate emission (/sup 99m/Tc, E/spl gamma/=140 keV) and transmission (/sup 57/Co, E/spl gamma/=122 keV) images from a combined projection data set. These components are separated from each other and from Compton scatter by fitting the spectra at each pixel with a simple model. The method is tested using a cylindrical phantom containing inserts with differing attenuation. Emission and transmission position and energy (xyE) data were acquired in list mode and later summed to simulate simultaneous acquisition. Therefore, the true emission and transmission images are known. The quality and quantitative accuracy of the technique are evaluated by comparing images extracted from the combined data set to the "true" scatter-corrected transmission and emission images, respectively. These results are presented and the potential of this technique for accurate attenuation correction in SPECT is discussed.< >