Standardization of Small Lesion Contrast in PET Imaging
Quantitative measurements in PET imaging have recently become more widespread as a way to diagnose and stage many types of malignant cancer. Currently patients need to have follow-up scans performed on the same PET system due to technical factors. Multi-clinic studies using quantitative PET measurements are also confounded by these technological factors. This work aims to evaluate the use of commonly available phantoms to cross-calibrate processing parameters to equalize small lesion quantitation. The method was verified using an abdomen phantom with small hot sphere inserts, as well as a smaller phantom with small hot sphere inserts.
Methods: A GE Discovery 690 and STE were used. Both time-of-flight (TOF) and non-TOF images were used from the D690. Jaszczak phantoms with hot rod and cold rod inserts were scanned on both systems consecutively for 20 minutes. Images were reconstructed with a range of iterations and post-smoothed (PS) with 2-10 mm of smoothing. Automated analysis of the images used the CT images to find rods and then calculate a rod to background ratio for each rod sector, PET image variant, and scanner. A target rod contrast could then be chosen and parameters determined for both systems separately to equalize rod contrast. Iteration-based resolution control and PS were both evaluated. To verify, an abdomen phantom was filled with a low background activity and ten 10-mm diameter spheres filled with FDG and CT contrast. In order to evaluate any size dependence, six 10-mm diameter spheres filled with FDG and CT contrast were placed inside a Jaszczak container filled with low background activity. An automatic CT-based analysis of the spheres was performed, obtaining mean and maximum values across the spheres.
Results: Small sphere quantitation differed substantially for similar processing between systems. However, sphere quantitation matched well when cross-calibrating the DSTE and non-TOF D690 Jaszczak phantom images by independently limiting iterations. Doing the same process with post-smoothing yielded similar results, with high iteration PS performing slightly better than PS at iterations used clinically at Duke for twenty-minute scans. Equalizing TOF images from the D690 with DSTE images with spheres placed in an abdomen phantom resulted in relatively poor correlation, but correlated well with spheres placed inside the Jaszczak phantom. Shorter scan durations behaved similarly to the twenty-minute scans.
Conclusions: Both Jaszczak phantoms worked well for cross-calibrating processing parameters to equalize quantitation in small lesions for non-TOF imaging. Iterations and PS could both be used to control resolution. It appears the best method is to use PS to fine-tune the resolution. The size dependence of TOF, and PET in general, seems to be an issue.
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