Comparison of Bremsstrahlung SPECT/CT and PET/CT for Y-90 Quantification

1. Comparison of Bremsstrahlung SPECT/CT and PET/CT for Y-90 Quantification Neal ClinthornePaul KisonJill RothleyYuni Dewaraja Department of RadiologyDivision of Nuclear Medicine The University of MichiganAnn Arbor, MI USA nclintho@umich.edu

2. Y-90 Imaging • Desirable for verifying distribution of Y-90 microspheres used in SIRT for unresectable liver cancer • Can potentially be used for dosimetry • Possible with both SPECT and PET • Goal was to compare performance of PET and SPECT using existing commercial acquisition and reconstruction

3. Phantom with Spheres Phantom having spheres with volumes ranging from 1.9ml (~1.5cm diameter) to 100ml (~5.7cm diameter) used for quantification studies 10000 ml 11.0 ml 63.0 ml 100.0 ml 1.9 ml

4. Initial Loading without Background • Spheres loaded with total of 341 MBq(9.2 mCi) of Y-90 chloride + EDTA • Y-90 assayed on Capintec CRC 15R using settings 55 (x10) for plastic syringe and 48 (x10) for glass vials • Scanned with PET and then SPECT 0 MBq 24.7 MBq 2.2 MBq/ml 199.0 MBq 2.0 MBq/ml 106.6 MBq 1.7 MBq/ml 5.3 MBq 2.8 MBq/ml

5. Spheres with Background 414.6 MBq 0.04 MBq/ml • One week (2.6 half-lives) later, 414.6 MBq (11.2 mCi) of Y-90 chloride was loaded into water surrounding spheres as background • Phantom rescanned with SPECT and then PET 4.1 MBq 0.37 MBq/ml 33.3 MBq 0.33 MBq/ml 17.8 MBq 0.28 MBq/ml 0.9 MBq 0.46 MBq/ml

6. SPECT Imaging Y-90 Energy Spectrum • Siemens Symbia T6 TruePoint SPECT/CT • High-energy collimators • Six contiguous energy windows 105–285 keV • 128x128 matrix 4.8mm pixels, OSEM (Flash 3D) 6 subsets, 20 iterations • No scatter correction • Attenuation correction from CT using center energy for each window Energy (keV)

7. PET Imaging • Siemens Biograph 6 extended FOV (21.6cm) LSO-based PET/CT • 435–650 keV window • Phantom imaged in one bed position • 3D OSEM reconstruction, 3 iterations, 21 subsets • No TOF • No copper or lead filter to moderate bremsstrahlung count rate • CT used for attenuation correction

8. Analysis • Spherical VOIs for spheres and background regions defined on CT scan for each dataset • For PET quantification, images were reconstructed to obtain radionuclide concentration in Bq/ml with F-18 calibration and corrected for Y-90 positron fraction • For SPECT, the largest (100 ml) sphere was used for calibration in each scan

9. SPECT 135–165 keV Window MIP 70 minute acquisition time

10. SPECT 135–165 keV Window

11. SPECT 255–285 keV Window MIP 70 minute acquisition time

12. SPECT 255–285 keV Window

13. PET No Background MIP 40 minute acquisition time (equivalent to SPECT with decay)

14. PET No Background

15. SPECT 135–165 keV Window + Bkgd MIP 120 minute acquisition time

16. SPECT 135–165 keV Window + Bkgd

17. SPECT 255–285 keV Window + Bkgd MIP 120 minute acquisition time

18. SPECT 255–285 keV Window + Bkgd

19. PET + Background MIP 120 minute acquisition time

20. PET + Background

21. Estimated Activity vs. Truth

22. SPECT Improvements • Shown in previous talk

23. PET Improvements • Time-of-flight • Shielding of low-energy bremsstrahlung? • More appropriate reconstruction • Better random coincidence modeling • Methods for reducing bias at low activities for datasets with few counts

24. Summary • Both PET and bremsstrahlung SPECT are usable for Y-90 imaging • SPECT with high-energy collimators has moderate resolution and suffers from partial volume effects for small sources but much higher countrate than PET • PET suffers from a low event rate, high background from bremsstrahlung, and significant positive bias in regions of low activity but has inherently higher resolution than SPECT • Both will likely be useful for quantitative Y-90 imaging if appropriate corrections are performed

25. PET without Activity?