Comparison of attenuation, dual-energy-window, and model-based scatter correction of low-count SPECT to 82Rb PET/CT quantified myocardial perfusion scores

Development and evolution of nuclear cardiology and cardiac PET in Canada

Gated radionuclide angiography and myocardial perfusion imaging were developed in the United States and Europe in the 1970's and soon adopted in Canadian centers. Much of the early development of nuclear cardiology in Canada was in Toronto, Ontario and was quickly followed by new programs across the country. Clinical research in Canada contributed to the further development of nuclear cardiology and cardiac PET. The Canadian Nuclear Cardiology Society (CNCS) was formed in 1995 and became the Canadian Society of Cardiovascular Nuclear and CT Imaging (CNCT) in 2014. The CNCS had a major role in education and advocacy for cardiovascular nuclear medicine testing. The CNCS established the Dr Robert Burns Lecture and CNCT named the Canadian Society of Cardiovascular Nuclear and CT Imaging Annual Achievement Award for Dr Michael Freeman in memoriam of these two outstanding Canadian leaders in nuclear cardiology. The future of nuclear cardiology in Canada is exciting with the expanding use of SPECT imaging to include Tc-99m-pyrophosphate for diagnosis of transthyretin cardiac amyloidosis and the ongoing introduction of cardiac PET imaging.

Single CT for attenuation correction of rest/stress cardiac SPECT perfusion imaging

Common practice is to use separate CT scans acquired during rest and stress for attenuation correction of SPECT myocardial perfusion imaging (MPI). We evaluated using a single CT scan to correct both rest and stress SPECT scans. Studies from 154 patients were reprocessed using one CT acquired at stress to correct both rest and stress scans (1CT) and compared to correction of each scan with its own CT (2CT). Two expert readers independently read the images and determined summed stress (SSS), rest (SRS), and difference (SDS) scores. The correlation in SRS between 2CT and 1CT was r ≥ 0.88. The concordance in SDS was ≥0.84 (kappa ≥ 0.62). The mean SDS difference between 2CT and 1CT for the averaged observer was not significantly different from zero (p > 0.31). 1CT images had a small but significant increase in SRS and an increase in SDS variability. However, the mean SDS difference was similar to the mean inter-observer SDS difference for the 2CT approach (-0.08 vs -0.23, p = 0.46) and had less uncertainty (1.02 vs 2.05, p < 0.001). Thus, the differences between 1CT and 2CT are unlikely to be clinically significant, and the 1CT approach is feasible for SPECT MPI.

Scatter correction improves concordance in SPECT MPI with a dedicated cardiac SPECT solid-state camera

PURPOSE

Correction for photon attenuation and scatter improves image quality with conventional NaI-based gamma cameras but evaluation of these corrections for novel solid-state dedicated cardiac cameras is limited. In this study, we assess the accuracy of dual-energy-window (DEW) scatter correction (SC) applied to clinically acquired (99m)Tc-tetrofosmin myocardial perfusion images obtained on a dedicated multi-pinhole camera with cadmium-zinc-telluride (CZT) detectors (GE Discovery NM530) compared to DEW scatter-corrected images from our conventional SPECT camera (GE Infinia Hawkeye 4; INF).

METHODS

A modified DEW SC method was formulated to account for the detection of primary photons in the lower energy window (120 keV ± 5%) with CZT detectors, in addition to estimating the scattered photons detected in the photopeak window (140 keV ± 10%). Phantom experiments were used to estimate the DEW correction parameters. Data from 108 patients, acquired using a standard rest/stress Tc-99m-tetrofosmin SPECT/CT protocol on both cameras, were reconstructed with no correction (NC), attenuation correction (AC), and AC with DEW-SC. Images were compared based on the summed stress/rest/difference scores (SSS/SRS/SDS) calculated by clinical software.

RESULTS

The correlation between SSS/SRS for the two cameras was excellent (r ≥ 0.94). The mean difference between cameras was <0.4 for SSS/SRS/SDS scores. Since datasets did not follow a normal distribution, non-parametric tests were used to show significant differences between datasets. Classification of disease (SSS) was highly correlated, as ranked by the two cameras (kendall's tau = 0.72, P < .001). AC significantly reduced the mean difference between the two cameras for SSS/SRS compared to NC. AC without SC on the CZT introduced a bias towards higher scores when compared to the INF, which was reduced after applying SC. Although SC increased noise, the scores for the AC/SC images were not significantly different between the two cameras (P > .1).

CONCLUSIONS

DEW-SC on the CZT camera was feasible and produced images that are not significantly different from those acquired on the INF camera. Although use of SC on CZT images does increase noise, the resultant noise does not introduce bias relative to the INF camera.

Comparative analysis of iterative reconstruction algorithms with resolution recovery for cardiac SPECT studies. A multi-center phantom study

BACKGROUND

This investigation used image data generated by a physical phantom over a wide range of count statistics to evaluate the effectiveness of several of the newer commercially available SPECT reconstruction iterative algorithms (IRR) in improving perfusion defect contrast and spatial resolution, while controlling image noise.

METHODS

A cardiac phantom was imaged using four different gamma cameras over a wide range of counts statistics (from 6 to 0.8 Mcounts). Images were reconstructed with FBP, OSEM, and the IRR available on site. IRR were applied without corrections (IRR NC), with attenuation correction (IRR AC), scatter correction (IRR SC), and attenuation + scatter corrections (IRR SCAC). Four image performance indices related to spatial resolution, contrast, and image noise were analyzed.

RESULTS

IRR NC always determined significant improvements in all indices in comparison to FBP or OSEM. Improvements were emphasized with IRR SC and IRR SCAC. Count reduction from 6 to 1.5 Mcounts did not impair the performances of any of the considered indices.

CONCLUSIONS

This is the first study comparing the relative performance of different, commercially available, IRR software, over a wide range of count statistics; the additional effect of scatter and attenuation corrections, alone or in combination, was also evaluated. Our results confirm that IRR algorithms produce substantial benefits with respect to conventional FBP or OSEM reconstruction methods, as assessed through different figures of merit, in particular when SC and/or SCAC are also included.