A PET/CT Attenuation Correction Validation Study Using Microspheres

1993 Objectives A phantom study was developed to measure the minimum lesion detection limits of clinical PET/CT scanners. As a byproduct of this study a method was developed for validating the accuracy of the attenuation correction of the image reconstruction algorithms. This validation method was performed on the Philips Vereos and Gemini TF, Discovery 710 and Siemens Biograph mCT PET/CT scanners to determine the accuracy of the attenuation correction for small objects. Methods Seven microspheres procured from Data Spectrum ranging in inner diameter from 3.95mm to 15.43mm were imaged in a Jazczak ECT phantom, in air and filled with cold water. The microspheres were filled with a concentration of 30Kbq/cc at the start of the first scan with the spheres being scanned “in air”. The phantom volume was then filled with cold non-radioactive water and scanned again 30 minutes later for a second scan. The scan time for both scans was 8 minutes. The images were reconstructed using the PET/CT scanner’s standard iterative reconstruction using default image reconstruction clinical settings. This includes TOF, point spread function and any post filtering available to the reconstruction algorithms as provided by the manufacture and included in the standard clinical reconstruction. Furthermore, the images were reconstructed to the final image voxel size of 1x1x1mm, 2x2x2mm and 4x4x4mm or to the closest voxel size allowed by the image reconstruction system. Using MIM software’s image viewing and analysis software package, threshold contours were drawn around each of the 7 spheres using a lower threshold of 5%. The total activity was measured within the 5% contour and recorded. The total activity in bq/ml was then compared between the in air scan and the decay corrected cold water scan. The total activity for each sphere was plotted against the sphere volume and a linear regression analysis as performed and the slope and R2 was recorded for a linearity test. Results Two of the four scanners had successful phantom scans done. These were the Gemini TF and Siemens Biograph mCT. The Gemini TF showed variations is total activity between in air and in cold water scans of an average of 6%. The largest deviation was 14% for the smallest microscphere. The Biograph mCT has an average variation of 2% for the 1x1x1mm and 2x2x2mm images and 4% for the 4x4x4mm images. The R2 measured from the linear regression analysis was better than 0.99 for the Discovery 710, Gemini TF and Biograph mCT. The linearity was not as good for the Vereos with an R2 of 0.95 for the 1x1x1mm and 2x2x2mm images and 0.72 for the 4x4x4 mm images. Conclusions The method proved useful in determining the limits of imaging quantitation accuracy for various PET/CT scanners by scanning microsphere in air, in which there is minimal attenuation media present in the FOV and in cold water which simulates the gamma ray absorption of the human torso. Although bone was not accounted for in this study and its effect on the accuracy of attenuation correction, being able to measure activity with and without attenuation media is a direct validation of the attenuation correction and the limits imposed on the absolute quantitation accuracy of the activity measured in the FOV of the PET/CT scanner in the PET images. In general for the Gemini TF, this provides an empirical upper limit on the accuracy of small lesion measurement at about 6% and about 2-4% depending on voxels size for the Biograph mCT. The other test of attenuation correction is to see if the linearity is preserved with and without attenuation media. This was tested by measuring the R2 correlation value of the linear regression analysis done on the sphere activity vs sphere volume. The Discovery 710, Gemini TF and Biograph mCT all showed to have good linearity. The Philips Vereos linearity with the smaller of the microspheres affected the linear regression analysis. Research Support: Funded by the NCI Contract No. HHSN261200800001E.