A Sensitivity Model for Multi-Pinhole SPECT
ABSTRACT Multi-pinhole single photon emission computed tomography (SPECT) has seen increasing use in small animal imaging, often with overlapping projections in order to improve the count sensitivity. Accurate image reconstruction requires knowledge of the pinhole sensitivity, which depends on source-to-pinhole distance, effective aperture diameter, keel length, and angle of incidence relative to the pinhole normal direction. A model is presented that accounts for these factors as a function of a set of pinhole-specific parameters (effective aperture diameter, normal direction vector, etc.). The model parameters are estimated by least squares analysis of point source data from a single tomographic acquisition. This method was applied to data acquired with a fourteen-pinhole small animal SPECT scanner with overlapping projections. The parameter estimation algorithm converged to a stable solution, and the count values predicted by the pinhole sensitivity model were generally within 10% of the measured count values. The estimated pinhole-specific parameters were of sufficient precision to detect subtle and otherwise unknown manufacturing differences between two batches of pinhole apertures. In reconstructed images, use of the measured parameters led to a noticeable reduction of artifacts and improvement of image quality. Overall, this method of characterizing pinhole sensitivity is robust and practical for multi-pinhole SPECT scanners.
- [Show abstract] [Hide abstract]
ABSTRACT: Multi-pinhole SPECT collimators can provide sub-millimeter resolution and improved sensitivity over single-pinhole and parallel-beam collimators. Attenuation and scatter will degrade the quantitative accuracy of reconstruction for lower energy emitters like I-125 and therefore both effects should be accounted for during reconstruction. We implemented an OSEM MAP reconstruction which incorporated attenuation, scatter and detector intrinsic resolution for a multi-pinhole detector designed for whole-body mouse imaging. The ray- driven projector/backprojector implemented considerably reduced the calculation of attenuation factors and decreased reconstruction time compared to a voxel-driven approach. The multi-pinhole SPECT system simulated consists of 2 or 4 cameras, with a 5-pinhole collimator plate for each. The attenuation map would be obtained from a CT system mounted on the same gantry. Scatter is estimated from scatter windows using a triple energy window (TEW) method and applied during the iterative reconstruction. A quadratic smoothness prior is implemented to control noise. Simulations with the MOBY mouse phantom show that modeling of the pinhole sensitivity, attenuation and detector intrinsic resolution results in a more accurate reconstruction. Preliminary Monte Carlo simulations showed the importance of determining and correcting the attenuation and scatter for I-125 imaging. Further investigations will be performed towards accurate estimation of the point-spread-function or sensitivity model of the multi-pinhole collimator plate.Nuclear Science Symposium Conference Record, 2007. NSS '07. IEEE; 01/2007
- [Show abstract] [Hide abstract]
ABSTRACT: In conventional reconstruction of single photon emission computed tomography (SPECT) data acquired with a single-pinhole or multipinhole system, the point spread function (PSF) may be either approximated by some analytical equations or substituted by the sensitivity function, which is the integral of the PSF. We have developed a method to numerically calculate the PSF for a pinhole system in order to improve image resolution over a sensitivity-function-based method. The method calculates the probability of photon penetration through the pinhole edges using a ray-tracing approach. To calculate the transmission by the collimator plate along each ray, we trace the ray through the collimator by analytical calculations. The PSF is calculated for only one detector angle, and a Gaussian rotator is used to rotate the image grid for other detector angles in the iterative reconstruction. To evaluate our method, we measured the sensitivities of four keel-edged single-pinhole plates and scanned an ultramicro Derenzo phantom on a single-pinhole system and a five-pinhole system and performed two mouse bone scans on the five-pinhole system using the 140 keV photons of Tc-99m. The numerical calculations of sensitivities for the single-pinhole plates agreed well with the measurements. Results for both types of data scans showed that modeling of the PSF improved image resolution. In conclusion, we found that modeling of the PSF by numerical calculations increases the resolution of reconstruction for single-pinhole and multipinhole SPECT imaging.IEEE Transactions on Nuclear Science 03/2010; DOI:10.1109/TNS.2009.2034656