Optimization of PET for ⁹⁰Y microspheres imaging and comparison to quantitative SPECT

Objectives ⁹⁰Y labeled microspheres are used for the radioembolization of liver lesions.Their bio-distribution is normally measured via gamma-camera imaging of the Bremsstrahlung photons,which is difficult to quantitate and yields poor resolution.Several proof-of-principle studies have recently shown the feasibility of imaging the low abundance annihilation photons of 90Y with PET.The aim of this study was to establish suitable parameters for the clinical PET imaging and to compare PET with quantitative SPECT.

Methods Three sets of data were acquired: (i) an IEC phantom with spherical inserts filled with ⁹⁰Y, (6.9MBq/cc) (ii) an ACR phantom with a 25cc lesion-like insert (12.3MBq/cc), (iii) ten ⁹⁰Y patients undergoing ⁹⁰Y liver radioembolization (1-3GBq injected).For (i) and (ii) the activity was uniformly distributed in the volumes.For (i)-(iii),two sets of images were obtained: (a) 1hr PET/CT,using GE D690 TOF scanner and (b) Bremsstrahlung SPECT/CT using Philips Precedence system,with MEGP collimators and energy windows at 75keV +- 54% and 190keV +- 99% and 40sec per gantry stop (32 stops,64 angles).All phantom and patient PET/CT data were replayed into 3,5,10,15,30,45,and 60min runs. Each of these was reconstructed with 2-10 iterations,at each of 18-48 subsets. Study (i) was used to obtain recovery coefficients and (ii) to optimize the acquisition and reconstruction parameters.These results were used in the the patient data reconstruction.SPECT quantitative images were created with Frey et al’s reconstruction algorithm.

Results PET reconstruction with 2 iterations and 8 subsets provided the least standard deviation in the target ROI.Acquisitions 20 minutes or longer had a <10% error in the measured activity for experiment (ii).Images were evaluated visually and quantitatively.

Conclusions Similar length PET and SPECT acquisitions yielded grossly similar images.Mean values within a target ROI agreed within about 15%.However,PET images showed more apparent structure but were noisier.SPECT images were grossly smoother