Effect of Reconstruction Algorithms on Myocardial Blood Flow Measurement with 13N-Ammonia PET

The temporal acquisition capacity of PET allows for quantitative assessment of myocardial blood flow (MBF), which is not possible currently with SPECT. 13N-NH3 is frequently used in clinical cardiac PET. Tracer kinetic models have been developed to describe the exchange of NH3 between the vascular space and myocardial tissue to quantify MBF (1,2). MBF as determined by cardiac PET with 13N-NH3 has been validated against blood flow measurements by microspheres in animal experiments (3), by PET with 15O-water (4), and by argon inert gas in humans (5). Reconstruction is an essential step in the processing of PET images. Commonly used reconstruction algorithms for PET follow 2 main categories: (i) analytic, such as filtered backprojection (FBP); and (ii) iterative, such as ordered-subsets expectation maximization (OSEM) (6). FBP is the traditional method for 13N-NH3 PET (7). It provides a fast and direct solution and is well suited for clinical use. However, OSEM has gained popularity recently because of the noise reduction properties in regions of low uptake (8). However, it is known that image resolution and noise increase with an increasing number of iterations (9,10), but the relative trade-offs for both visualization and measurement tasks are unclear. Limited data exist on the effects of reconstruction algorithms on PET data in cardiac studies (9,11). Moreover, the data in cardiac studies pertain only to 18F-FDG PET studies. Lubberink et al. compared FBP and OSEM (16 subsets, 6 iterations) in cardiac 18F-FDG PET studies of 8 patients (11). Scatter plots of the 18F-FDG influx rate constants showed linear correlation between values obtained with FBP and with OSEM. Boellaard et al. compared 5 reconstruction algorithms in 5 cardiac 18F-FDG PET studies (9). The reconstruction algorithms included one FBP algorithm and several OSEM algorithms with varying iterations and subsets and varying amounts of smoothing. The authors found significant changes to the metabolic rate of glucose depending on the reconstruction algorithm and on the use of smoothing. The effects of reconstruction algorithms on quantitative MBF have not been tested. This study was undertaken because we observed a marked difference in the shape of the heart and in regional abnormalities when we compared images reconstructed using the equipment vendor’s default OSEM parameters (2 iterations, 28 subsets) with images reconstructed using FBP (Fig. 1). This raised questions about the effects of iterative reconstruction on qualitative and quantitative MBF. The goals of the current study were: FIGURE 1 Short-axis images reconstructed using FBP and OSEM with 28 subsets and either 2 or 6 iterations. Of interest is anterior–septal defect on 2-iteration image set that is not present on FBP or 6-iteration OSEM images. To define the effects of the number of OSEM iterations on absolute rest and vasodilator stress MBF and coronary flow reserve (CFR) estimated by 13N-NH3PET; To assess the effects of smoothing in OSEM on the estimation of absolute rest and vasodilator stress MBF and CFR by 13N-NH3 PET; and To compare the OSEM results with those obtained by the conventional FBP reconstruction algorithm.