The effects of axial transmission resolution in PET

The effect of the axial resolution of transmission data for use in positron emission tomography (PET) attenuation correction is an important consideration for the design of transmission scanning systems. PET transmission data from five patient scans were used in conjunction with simulated uniform emission data to quantify the effect of degraded axial transmission resolution. PET patient emission data were also corrected for attenuation using transmission projection data that had various levels of axial resolution. Results from the uniform emission studies show that the effects of imperfect axial transmission resolution are always less than are those of comparable transaxial resolution. Conservative limits for the axial resolution requirements [Gaussian full width at half-maximum (FWHM)] for a transmission system were deduced. For quantitative work, a 1-cm resolution limit yields a root mean square (rms) voxel deviation of less than 5% in the lung region (the worst case) of a uniform tracer distribution. For qualitative studies, a 2-cm resolution limit yields an rms voxel-value deviation of less than 11%. Only small effects were observed on the patient emission image using transmission data with this resolution to produce an attenuation correction.