BASIC SCIENCES Investigation of Causes of Geometric Distortion in 1800and 3600 Angular Sampling in SPECT

To investigate geometric distortion when 180°or 360°angular sampling techniques are used in singlephotonemissioncomputedtomography(SPECT),a studyof pointsourcesimagedat different positions in a water filled cylindrical phantom, and reconstructed using filtered back projection, was conducted. A simulation study, based upon a serial model of the system point spreadfunction(PSF),was usedto investigatethe contributionsof attenuation,spatial resolution and scatter on distortion of the reconstructed PSFs. To study the geometric distortionintransverse(x-yplane),coronal(x-zplane),andsagittal(y-zplane)sections,the ratiosofthefullwidthsat halfmaximum(FWHM)andfullwidthsat tenthmaximum(FWTM)in the x/y, x/z, andy/z directionswerecalculatedfor the realandsimulatedPSFs.Theseresults showedthat,inanattenuatingmedium,thereismoredistortionofpointsourcesintoovalsfor 180°thanfor 360°sampling.Thesimulationstudy indicatedthat the primarycauseof geometrical distortion in SPECT studies, is the inconsistency of projections due to variable attenuation and spatial resolution. The impact of scatter on geometric distortion was small as measuredby the ratiosof FWHMsandFWTMsfor PSFs.Attenuationcorrectionappliedto acquired PSFs significantly reduced geometric distortion in both 180°and 360°studies. T