M.S. Kaplan

A differential attenuation method for simultaneous estimation of SPECT activity and attenuation distributions

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.

Multiwindow scatter correction techniques in single-photon imaging

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.

Positron range and coincidence non-collinearity in SimSET

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.

Coherent scatter implementation for SimSET

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 data acquisition system for coincidence imaging using a conventional dual head gamma camera

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.

Coincidence imaging using a standard dual head gamma camera

Coincidence electronics and a data acquisition system were developed to explore coincidence detection using a 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//spl 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 /spl sim/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.

Performance of a dual headed SPECT system modified for coincidence detection

A NIM/CAMAC/Macintosh data acquisition system was developed to collect/sup /spl plusmn//X, /sup /spl plusmn//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 /spl sim/7.5% (using pulse clipping), at 511 keV. The system sensitivity is 70 cps//spl mu/Ci 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 /spl sim/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.

Scatter and attenuation correction for 111In based on energy spectrum fitting

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.

Differential attenuation method for simultaneous estimation of activity and attenuation in multiemission single photon emission computed tomography

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.

Slat collimation and cylindrical detectors for PET simulations using SimSET

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.