Quantification of 90Y PET images in a TOF scanner for liver selective internal radiotherapy

Objectives To investigate the ability of a PET/CT scanner with time-of-flight (TOF) and point-spread-function-modeling (PSF) for quantitative 90Y PET imaging.

Methods Data were acquired on a Biograph mCT TrueV scanner. Spatial resolution was measured with a 90Y point source. Sensitivity was evaluated using the NEMA 70cm line source with 90Y. To evaluate the count rate performance, 90Y vials were measured in the center of the field of view (CFOV) covering the 0.03-3.6GBq range. Intrinsic radiation and energy spectrum were evaluated. Image quality with different reconstructions was studied using the Jaszczak phantom, filled with a 207kBq/mL 90Y concentration, and a 5:1 sphere-to-background ratio. Acquisition time was adjusted to simulate the quality of a PET acquisition of a patient treated with SIR-Spheres. PET scanner performance was compared to the 18F performance. The developed methodology was applied to 3 patients after treatment.

Results The energy spectrum showed the 90Y Bremsstrahlung radiation. The 90Y transverse resolution was 4.5mm in the CFOV and degraded to 4.9mm at 10cm off-axis. TOF and PSF inclusion improved resolution. 90Y absolute sensitivity was 0.40 and 0.39 kcps/MBq in the center and at 10cm off-center. Tendency of trues and random rates as a function of the 90Y activity could be accurately described using linear and quadratic models, respectively. At zero activity, curve fitting provided 988 random cps and 31 true cps. The background acquisition showed 1024 and 4.4 cps for random and true rates. Due to low count statistics in 90Y Jaszczak studies, the TOF+PSF standard oncological reconstruction was smoothed to allow detectability of all hot spheres and accurate quantification. Patient studies were quantified and confirmed the phantom studies.

Conclusions Quantitative 90Y PET imaging of patients following liver selective internal radiotherapy is feasible and images can be used in voxel-based dosimetry.

Research Support Siemens