Pernilla Norberg, Agnetha Gustafsson och Gudrun Alm- Carlsson, Linköping, samt Lars Jacobsson och Björn Bake, Göteborg.

In studies of the distribution of lung function, the image quality of lung single photon emission computed tomography (SPECT) is important and one factor influencing it is the reconstruction algorithm.

The purpose of this study was to systematically evaluate Ordered Subsets Expectation Maximisation (OSEM) and compare it with filtered back projection (FBP) for lung SPECT with Tc-99m. The evaluation of number of iterations used in OSEM was based on the image quality parameter contrast. The comparison between OSEM and FBP was based on trade- off plots between statistical noise and spatial resolution for different filter parameters, collimators and count- levels.

A Monte Carlo technique was used to simulate SPECT studies of a digital thorax phantom containing two sets of activity, one with a homogeneous activity distribution within the lungs and the other with superposed high- and low activity objects. Statistical noise in the reconstructed images was calculated as the coefficient of variation (CV) and spatial resolution as full width at half- maximum (FWHM).

The results show, for the configuration studied, that the OSEM reconstruction in combination with post-filtering should be used in lung SPECT studies with at least 60 MLEM equivalent iterations. Compared to FBP the spatial resolution was improved by about 1mm. For a constant level of CV, a four fold increase in count-level resulted in an increased resolution of 2 mm. Spatial resolution was also increased by using a low-energy, high-resolution collimator for CV-values above 3%. The choice of noise-reducing filter was not crucial.