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SPECT Image Quality and Quantification

Accurate reconstruction methods for SPECT are a pre-requirement for absolute tracer quantification. Exact activity numbers can be extracted from SPECT images by use of iterative reconstruction with appropriate corrections for attenuation, scatter and detector resolution. We have developed an analytical model for SPECT image generation, including attenuation, scatter and reconstruction by filtered back-projection (FBP). Image quality is described in terms of signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). MatLab simulations were added to include an evaluation of ordered-subset expectation maximization (OSEM) as an example of statistical reconstruction, and for a quantitative assessment of lesion signal recovery. As expected, classical reconstruction without attenuation or scatter correction provides incorrect contrast values, and standard corrections like the Chang method cannot mitigate this problem. Statistical reconstruction allows implementing all corrections and results in better image quality than FBP. Our analysis shows, however, that for adequate signal recovery the contrast-to-noise ratio for OSEM is the same as for FBP. We explain the trade-off between SPECT image quality and quantification by describing reconstruction in terms of signal power spectra (SPS), noise power spectra (NPS) and detective quantum efficiency (DQE).