Investigation of causes of geometric distortion in 180 degrees and 360 degrees angular sampling in SPECT.

To investigate geometric distortion when 180 degrees or 360 degrees angular sampling techniques are used in single photon emission computed tomography (SPECT), a study of point sources imaged at 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 spread function (PSF), was used to investigate the contributions of attenuation, spatial resolution and scatter on distortion of the reconstructed PSFs. To study the geometric distortion in transverse (x-y plane), coronal (x-z plane), and sagittal (y-z plane) sections, the ratios of the full widths at half maximum (FWHM) and full widths at tenth maximum (FWTM) in the x/y, x/z, and y/z directions were calculated for the real and simulated PSFs. These results showed that, in an attenuating medium, there is more distortion of point sources into ovals for 180 degrees than for 360 degrees sampling. The simulation study indicated that the primary cause of 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 measured by the ratios of FWHMs and FWTMs for PSFs. Attenuation correction applied to acquired PSFs significantly reduced geometric distortion in both 180 degrees and 360 degrees studies. To investigate distortion in extended objects, an Iowa heart phantom was placed inside an Alderson body phantom and 201Tl heart SPECT studies acquired. The phantom images confirmed the conclusion that in transverse sections of 360 degrees studies with arithmetic averaging of opposite views, geometric distortion is reduced compared to 180 degrees. The coronal and sagittal sections were equally distorted in both, the 180 degrees and 360 degrees studies, and the 180 degrees studies yielded better contrast.