Zuo Zhao

Iterative SPECT Reconstruction Using Matched Filtering for Improved Image Quality

SPECT images reconstructed from low count studies suffer either from high noise or poor resolution. We have developed an iterative reconstruction with matched filtering (IRMF) to control image noise while maintaining higher image resolution. IRMF involves filtering the measured projection and re-projection during iterative reconstruction with the same low-pass filter before the two are compared to generate an error projection. Another low-pass filter can be applied to the error projection before it is backprojected to update the current activity distribution estimate. The method is validated with a cardiac phantom filled with a clinical distribution of Tc-99m. A 1-second-per-frame scan was acquired to mimic a single gated segment. The image was reconstructed using ordered-subset expectation-maximization (OSEM) algorithm with depth-dependent resolution recovery. Reconstructions of similar spatial resolution with post-reconstruction Butterworth filtering (OSEM+F) and with matched filtering are compared visually and via standard deviation (SD) and signal-to-noise ratio (SNR) measurements. Results: Images reconstructed with IRMF show strong noise suppression in both the myocardium and background areas as compared to those reconstructed with OSEM+F. The SD in the background is reduced by ~30%, and the SNR is improved by ~100%. IRMF significantly improves image quality by suppressing noise in low count SPECT studies while maintaining higher image resolution.

CT-based attenuation correction in PET image reconstruction for the Gemini system

The Gemini system is a combined CT/PET imaging system newly developed by Philips Medical Systems. It has a unique open gantry design that allows for variable separation between the CT and PET gantries. The Gemini system incorporates CT-based attenuation correction (CT-AC) into a three-dimensional row-action maximum likelihood algorithm (RAMLA) for PET image reconstruction. It uses several unique techniques to achieve high accuracy while reducing patient X-ray dose. These new techniques include (1) using low-dose CT protocols to obtain CT images with adequate quality and quantitation for CT-AC while keeping patient X-ray dose low; (2) using a CT truncation compensation technique to improve the accuracy of CT-AC; and (3) using a generalized model for the conversion of CT images to attenuation maps at 511 keV. In this paper, we report the workflow and performance of Gemini CT-AC using phantom and patient studies. For comparison, attenuation maps obtained from PET transmission scans are also used for attenuation correction (TX-AC). Both phantom and patient studies show that PET images with CT-AC have image quality equivalent to or better than those with TX- AC.

Study of the effect of statistical fluctuations on defect detectability at clinical count levels in cardiac SPECT

Cardiac SPECT using Tl-201 suffers from low count statistics. Any statistical studies concerning the evaluation of a reconstruction algorithm, acquisition parameters, diagnostic confidence, etc., for clinical applications are impacted by the difficulty of obtaining data with multiple noise realizations. For this work, we acquired list-mode data of a Tl-201 cardiac phantom with very high counts in three configurations-with an anterior defect, an inferior defect, and no defect. The list-mode data were repartitioned to obtain statistically independent multiple data sets all with the same, clinically relevant noise level. Images were reconstructed from each of the resulting data sets using an iterative algorithm with attenuation correction. Reconstructed images were examined by four human observers, as well as analyzed quantitatively. The ability of observers to differentiate between normal scans and scans with defects varied substantially among datasets. There was correlation between the measured defect detectability and the visual assessment. The fact that the visibility of defects and the uniformity of normal scans varied significantly from one data set to the next, even when both were acquired at the same time, under identical conditions, indicates that the low statistics levels at clinical doses can have a measurable effect on diagnostic confidence.

Transmission attenuation map with measured downscatter correction

As the attenuation correction becomes clinical reality in SPECT, accurate estimation of emission downscatter to transmission during simultaneous emission-transmission acquisition becomes critical. In this study, we proposed a technique that directly measures the downscatter contamination from 140-keV emission photons to the 100-keV transmission window. The technique combines energy window discrimination and spatial window discrimination to separate emission (EM), transmission (TR) and downscattered (DS) photons. A spatial mask (S) aligned with the opposing scanning line source and a gap (G) at each end of S are first defined. Photons detected inside the S belong to TR if their energy fall inside the 100 keV window; those detected outside the S and G belong to either EM if their energy inside the 140 keV window, or DS if they are inside the 100 keV window. The TR is corrected from the downscatter contamination via subtraction a fraction of the DS. The effectiveness of the technique is evaluated based on the uniformity of the ROI in the reconstructed attenuation map from a series of phantom experiments. The technique is compared with another technique using an adjacent energy window measurement. For typical cardiac studies, the uniformity of the transmission map from both techniques are comparable in the heart area, while for the cardiac study with high concentration present in the gall bladder, the uniformity is significantly improved from the proposed technique.