A multiresolution restoration method for cardiac SPECT imaging

Iterative three-dimensional expectation maximization restoration of single photon emission computed tomography images: Application in striatal imaging

Single photon emission computed tomography imaging suffers from poor spatial resolution and high statistical noise. Consequently, the contrast of small structures is reduced, the visual detection of defects is limited and precise quantification is difficult. To improve the contrast, it is possible to include the spatially variant point spread function of the detection system into the iterative reconstruction algorithm. This kind of method is well known to be effective, but time consuming. We have developed a faster method to account for the spatial resolution loss in three dimensions, based on a postreconstruction restoration method. The method uses two steps. First, a noncorrected iterative ordered subsets expectation maximization (OSEM) reconstruction is performed and, in the second step, a three-dimensional (3D) iterative maximum likelihood expectation maximization (ML-EM) a posteriori spatial restoration of the reconstructed volume is done. In this paper, we compare to the standard OSEM-3D method, in three studies (two in simulation and one from experimental data). In the two first studies, contrast, noise, and visual detection of defects are studied. In the third study, a quantitative analysis is performed from data obtained with an anthropomorphic striatal phantom filled with 123-I. From the simulations, we demonstrate that contrast as a function of noise and lesion detectability are very similar for both OSEM-3D and OSEM-R methods. In the experimental study, we obtained very similar values of activity-quantification ratios for different regions in the brain. The advantage of OSEM-R compared to OSEM-3D is a substantial gain of processing time. This gain depends on several factors. In a typical situation, for a 128 x 128 acquisition of 120 projections, OSEM-R is 13 or 25 times faster than OSEM-3D, depending on the calculation method used in the iterative restoration. In this paper, the OSEM-R method is tested with the approximation of depth independent resolution. For the striatum this approximation is appropriate, but for other clinical situations we will need to include a spatially varying response. Such a response is already included in OSEM-3D.

Impact of Wiener filter in determining the left ventricular volume and ejection fraction using thallium-201 gated SPECT

Patient morphology, as well as the acquisition and reconstruction parameters, may influence the evaluation of the left ventricular volume (LVV) and left ventricular ejection fraction (LVEF) using gated single-photon emission computed tomography (SPECT). The purpose of this study was to examine the influence of gender and reconstruction filter on the measurement of LVV and LVEF using 201Tl gated SPECT. Using a static torso phantom, a female shape was created by the addition of two saline solution-filled balloons fixed on the anterior rib cage. The following parameters were similar for all acquisitions: 90 degrees dual-head gamma camera; 32 projections; 64x64 matrix (pixel size=6.77x6.77 mm); two 20% energy windows centred at 70 and 167 keV. The following acquisition times were tested: 1.25, 10, 20, 30 and 40 s per projection, leading to a total of 10 successive acquisitions. The effect of over-sampling was tested by 2.5 post-acquisition zooming. All SPECT images were successively reconstructed using filtered back-projection with Butterworth and Wiener filters. The effect of gender and reconstruction filter was also studied in 30 patients (15 males and 15 females) with a low likelihood of coronary artery disease. LVVs were calculated using QGS software. By multivariate analysis, the following factors influenced the accuracy of phantom measurement using QGS software: zooming (F=49, P<0.0001), phantom shape (F=61, P<0.0001) and filter type (F=240, P<0.0001). LVV was underestimated in the female shape phantom, even when using the Wiener filter. In patients, LVV and LVEF measurements were independently influenced by gender (P<0.0001) and filter (P<0.0001), but not by zooming. In conclusion, it was demonstrated that LVV was significantly decreased in the female shape phantom, suggesting a significant impact of breast interposition. This underestimation was minimized by use of the Wiener filter. In patients, the impact of the Wiener filter on the assessment of LVVs and LVEF was powerful, but independent of gender, and failed to correct the underestimation of LVVs and the overestimation of LVEF in females.