[Usefulness of the collimator detector response (CDR) recovery and the scatter correction by the effective scatter source estimation (ESSE) method in myocardial SPECT study].

UNLABELLED: The aim of this study was to evaluate the usefulness of the collimator detector response (CDR) recovery and the effective scatter source estimation (ESSE) method which is the scatter correction method built into the ordered subsets expectation maximization (OSEM) method. METHOD: The SPECT quality evaluation phantom and the anthropomorphic torso phantom were used in this study, and image contrast and uniformity were evaluated. The effect of each image correction method on the quantification of absolute radioactivity was also assessed. RESULTS AND CONCLUSION: Image contrast and uniformity were improved with the combination of the CDR recovery and triple energy window (TEW) method or the ESSE method. The combination of the CDR recovery and the ESSE method was the best method for the estimation of absolute radioactivity. Image quality of the SPECT is improved by the combination of CDR recovery and scatter correction in addition to attenuation correction. CDR recovery in addition to attenuation and scatter correction is also useful for the quantification of absolute radioactivity.

[Effect of reconstruction parameters on a calculation of left ventricular volumes by OS-EM reconstruction algorithm including various image corrections].

UNLABELLED: The aim of this study was to evaluate the effect of reconstruction parameters on the measurement of global left ventricular (LV) volume and define the appropriate reconstruction parameters when using the ordered subsets expectation maximization (OS-EM) method and 3D OS-EM (Astonish) method including the collimator distance response (CDR) recovery (RC) for myocardial perfusion SPECT study. METHOD: An anthropomorphic torso phantom with a 56 ml LV part was used. The LV volume was calculated with QPS software by the Update number (iteration number x subsets number) of OS-EM and 3D OS-EM (Astonish). RESULTS AND CONCLUSION: LV volumes calculated with OS-EM and Astonish without attenuation and scatter corrections corresponded to the true obtained by the Update number about 32 times and 24 times when using the OS-EM and Astonish method with attenuation and scatter corrections, respectively. However, LV volumes have changed greatly in the Astonish method according to the change in the Update number. Appropriate numbers of iterations and subsets are the measurement of global LV volumes, especially when using the OS-EM algorithm with RC.

Optimum energy window setting on Hg-201 x-rays photopeak for effective Tl-201 imaging.

For more effective Tl-201 imaging, the location and width of the energy window set on the Hg-201 x-rays photopeak was investigated using Monte Carlo simulation and phantom experiments. We calculated energy spectra and investigated the amount of primary and scattered photons within various energy windows set on the x-rays photopeak. The energy resolution (ER) at 71 keV (the peak of the x-rays photopeak) was changed to 10%, 12%, 14% and 16%. The relationships between the energy window and the primary counts rate or the scatter fraction (= scattered counts/primary counts, SF) were obtained. By compromise between the primary counts rate and the SF for ER = 12%, the optimum energy window was determined as a wider off-peak window, 77 keV +/- 14.3% (66-88 keV). This off-peak window increased the primary counts rate by 12.5% and decreased the SF by -17% as compared with the conventional on-peak energy window (71 keV +/- 10%, 64-78 keV). When this off-peak widow acquisition was compared with the conventional on-peak window one on a gamma camera, planar and SPECT images using the off-peak widow clearly showed superior results qualitatively and quantitatively.

Accurate scatter correction for transmission computed tomography using an uncollimated line array source.

We investigated scatter correction in transmission computed tomography (TCT) imaging by the combination of an uncollimated transmission source and a parallel-hole collimator. We employed the triple energy window (TEW) as the scatter correction and found that the conventional TEW method, which is accurate in emission computed tomography (ECT) imaging, needs some modification in TCT imaging based on our phantom studies. In this study a Tc-99m uncollimated line array source (area: 55 cm x 40 cm) was attached to one camera head of a dual-head gamma camera as a transmission source, and TCT data were acquired with a low-energy, general purpose (LEGP), parallel-hole collimator equipped on the other camera head. The energy spectra for 140 keV-photons transmitted through various attenuating material thicknesses were measured and analyzed for scatter fraction. The results of the energy spectra showed that the photons transmitted had an energy distribution that constructs a scatter peak within the 140 keV-photopeak energy window. In TCT imaging with a cylindrical water phantom, the conventional TEW method with triangle estimates (subtraction factor, K = 0.5) was not sufficient for accurate scatter correction (micro = 0.131 cm(-1) for water), whereas the modified TEW method with K = 1.0 gave the accurate attenuation coefficient of 0.153 cm(-1) for water. For the TCT imaging with the combination of the uncollimated Tc-99m line array source and parallel hole collimator, the modified TEW method with K = 1.0 gives the accurate TCT data for quantitative SPECT imaging in comparison with the conventional TEW method with K = 0.5.

Attenuation correction using asymmetric fanbeam transmission CT on two-head SPECT system.

For transmission computed tomography (TCT) systems using a centered transmission source with a fan-beam collimator, the transmission projection data are truncated. To achieve sufficiently large imaging field of view (FOV), we have designed the combination of an asymmetric fan-beam (AsF) collimator and a small uncollimated sheet-source for TCT, and implemented AsF sampling on a two-head SPECT system. The purpose of this study is to evaluate the feasibility of our TCT method for quantitative emission computed tomography (ECT) in clinical application. Sequential Tc-99m transmission and Tl-201 emission data acquisition were performed in a cardiac phantom (30 cm in width) with a myocardial chamber and a patient study. Tc-99m of 185 MBq was used as the transmission source. Both the ECT and TCT images were reconstructed with the filtered back-projection method after scatter correction with the triple energy window (TEW) method. The attenuation corrected transaxial images were iteratively reconstructed with the Chang algorithm utilizing the attenuation coefficient map computed from the TCT data. In this AsF sampling geometry, an imaging FOV of 50 cm was yielded. The attenuated regions appeared normal on the scatter and attenuation corrected (SAC) images in the phantom and patient study. The good quantitative accuracy on the SAC images was also confirmed by the measurement of the Tl-201 radioactivity in the myocardial chamber in the phantom study. The AsF collimation geometry that we have proposed in this study makes it easy to realize TCT data acquisition on the two-head SPECT system and to perform quantification on Tl-201 myocardial SPECT.

Attenuation correction using combination of a parallel hole collimator and an uncollimated non-uniform line array source.

Attenuation correction is very important for quantitative SPECT imaging. We designed an uncollimated non-uniform line array source (non-uniform LAS) for attenuation correction based on transmission computed tomography (TCT) using Tc-99m and compared its performance with an uncollimated uniform line array source (uniform LAS) in a thorax phantom study. This non-uniform LAS was attached to one camera head of a dual-head gamma camera, and transmission data were acquired with another camera head with a low-energy, general purpose, parallel-hole collimator at 50 cm-distance apart from the source. The modified TEW using a subtraction factor of 1.0 was employed to correct scattered Tc-99m photons for transmission data. In the phantom experiment, eight TCT data were acquired with the scanning time changed from 2 minutes to 20 minutes for each LAS. The Tc-99m attenuation coefficient (mu) maps with the non-uniform LAS and uniform LAS improved the statistical count variation in the mediastinum filled with water as the scanning time got longer. The Tc-99m mu-map with the non-uniform LAS and 6 minutes of scanning time had equal quality at the center of the thorax phantom to that with the uniform LAS and 16 minutes of scanning time. In conclusion, for the TCT imaging with combination of the parallel hole collimator and uncollimated Tc-99m external source the non-uniform LAS can reduce the Tc-99m radioactivity or the TCT scanning time compared with the uniform LAS.