PET Scanner Calibration Using a Small Ga68/Ge68 Source Bing Bai1, Magnus Dahlbom2, James Bading3, Peter S. Conti1 and Peter D. Esser4 1 Molecular Imaging Center, Department of Radiology, University of Southern California, Los Angeles, CA, USA 2 Department of Molecular and Medical Pharmacology, University of California, Los Angeles, CA, USA 3 Department of Cancer Immunotherapy and Tumor Immunology, City of Hope, Duarte, CA, U.S.A 4 Department of Radiology, Columbia University, New York, NY, USA Introduction Ga68/Ge68 source with simultaneous estimation of activity and resolution from image Same advantages as method 2 but less affected by partial volume. Reconstructed image is modeled as the convolution of a scaled binary image and a point spread function (PSF), assumed to be Gaussian. 5 consecutive central slices used for estimation. Simultaneously estimate source image intensity and resolution (FWHM of Gaussian) by minimizing the mean square difference (MSD) between the convolution and reconstructed image. PET Scanner Cross Calibration Factor (CCF) 𝐶𝐶𝐹= 𝐶 𝑡𝑟𝑢𝑒 𝐼 𝑡𝑟𝑢𝑒 𝐶 𝑡𝑟𝑢𝑒 : source activity measured in dose calibrator / source volume. 𝐼 𝑡𝑟𝑢𝑒 : image intensity in the absence of partial volume effect. We have designed a small Ga68/Ge68 source for easy and accurate routine cross-calibration of clinical PET scanners and dose calibrators. Partial volume effect (PVE) is minimized by estimation of calibration factor and image resolution simultaneously. We tested the quantitative calibration of four clinical PET/CT scanners from two manufacturers in three institutions using this source. Results show that both quantitative calibration factor and image resolution can be estimated accurately using this method. Method Profile. Green: reconstructed image; Yellow: convolution Relative MSD as a function of image resolution Reconstructed image Convolution of binary image with PSF A Ga68/Ge68 Source for Dose Calibrator and PET Scanner Cross Calibration in PET Clinics PET Scanner Cross Calibration Large F18 cylindrical phantom Standard cross calibration method used in many clinics. Requires F18 activity and extra work (measure activity, fill phantom, measure region of interests). Calibration results depends on dose calibrator accuracy. Not affected by partial volume effect. Ga68/Ge68 source with manually drawn ROI Traceable standard sources for F18 & Ge68. Cross calibration of ion chambers with PET scanners with the same source. PET scanner calibration uses widely available phantom (with modified lid). Easy to use and transport, suitable for standardizing SUV measurement across multiple scanners and sites. Affected by partial volume effect. Indirectly traceable source: simulated F18 from (Ga68/Ge68) 10 cc Vial (RadQual BM06v) Dose Calibrators PET Scanners Biograph 6, 64, mCT and Discovery STE Ga68/Ge68 vial in phantom lid, showing ROI used for analysis Modified ACR PET phantom lid with 2 accessible locations Results Scanner Model CCF(Ge68 Est) / CCF(F18) Estimated Resolution (FWHM) Biograph 6 1.000 7.54 mm Biograph mCT 0.996 8.71 mm Biograph 64 1.033 6.36 mm Discovery STE 0.980 6.59 mm Dose Calibrator Measurements Using the Ga68/Ge68 Source 4 dose calibrators, 5 measurements on each. Phantom Scan Standard F18 20cm cylinder cross calibration 5-minute scan, FBP reconstruction, ROI D=10cm, L=10cm Ga68/Ge68 10cc vial cross calibration 5-minute scan, FBP reconstruction ROI D=5mm, L=10mm in the center of vial Dose Calibrator Max Measured Activity Min Measured Activity Average Measured Activity Calibrated Activity % Deviation 1 985.0 mCi 981.0 mCi 982.8 mCi 992.3 mCi -0.96 2 983.5 mCi 983.7 mCi 983.4 mCi -0.90 3 983.1 mCi 982.7 mCi 983.2 mCi 989.7 mCi -0.66 4 855.0 mCi 852.0 mCi 853.2 mCi 859.8 mCi -0.77 Conclusions A 10cc indirectly traceable Ga68/Ge68 source was tested for cross calibration of PET scanners and dose calibrators. Dose calibrator measurements using F18 settings were in close agreement with the calibration vial value (deviation < 1%). The calibrated source in the modified phantom lid generated similar results to standard PET scanner cross-calibration using a 20 cm cylinder. Difference in the cross calibration factor is within 4%. A procedure was developed to estimate cross calibration factor and image resolution simultaneously. Our results show that the PET scanner quantitative-calibration factor, with partial volume correction, and image resolution can be accurately estimated using a 10cc calibrated Ga68/Ge68 source in a modified phantom. Image Reconstruction Parameters Scanner Model Institute Image Volume Voxel Size (mm) Post-smoothing Filter Biograph 6 UCLA 168x168x55 2.04x2.04x5 All pass Biograph mCT 200x200x109 1.63x1.63x2 Biograph 64 USC 168x168x109 1.02x1.02x2 Gaussian 5mm FWHM Discovery STE City of Hope 128x128x47 2.0x2.0x3.27 Rect 4.8mm Acknowledgements The authors would like to thank Data Spectrum (Hillsborough, NC) for the modified ACR phantom lid and RadQual (Weare, NH) for the traceable Ga-68/Ge-68 source.