The impact of Ga-68 positron range on the image quality of clinical PET

Objectives Ga-68 DOTATATE is being used routinely for the investigation of neuroendocrine tumours. Ga-68 has a higher positron range compared to F-18 due to it higher energy positron. There have been only been a small number of investigations into positron range correction for clinical PET imaging due its lower spatial resolution and the small positron range for F-18 in soft tissue. The impact of positron range on point spread function (PSF) and contrast was investigated experimentally for Ga-68 and F-18 in different tissue equivalent materials using a Siemens mCT PET scanner and routine clinical reconstruction parameters.

Methods Five point sources of Ga-68 and F-18 were positioned in the centre of the camera and imaged in air and tissue equivalent materials. A line profile was plotted and mean FWHM and FWTM calculated for the sinograms and PSF corrected reconstructed point sources. A NEMA NU-2 Image Quality Phantom was acquired using Ga-68 and F-18 for 8:1 and 4:1 contrasts and reconstructed using PSF correction and PSF plus time-of-flight (TOF) corrections.

Results Line profiles and measurements of spatial resolution showed only small differences for F-18 point sources in different tissue equivalent materials. There was a proportional decrease in spatial resolution with decreasing tissue equivalent material density for Ga-68 particularly for the reconstructed FWTM measurements, e.g. 14 and 17 mm for bone and lung respectively. A higher mean contrast was measured for hot and cold lesions for F-18 (PSF = 51%, PSF+TOF = 60%) compared to Ga-68 (PSF = 45%, PSF+TOF =54%) with both showing improvement when reconstructed with PSF plus TOF corrections compared to PSF only.

Conclusions Experimental measurements show potential for improvement in clinical PET image quality and quantification using positron range correction for Ga-68 and the benefit of TOF correction for image contrast for both Ga-68 and F-18 acquisitions.