Summary of warnings:
Two issues exist with Siemens HRRT PET images.
1. After analysis of the ADNI normal control PET subjects, it has become apparent that the baseline pattern of FDG metabolism is different for subjects scanned on HRRT PET scanners than on other scanners. This is not a resolution effect, since the differences are apparent on images smoothed to a common resolution (8 mm). In general, regions in the lower transaxial slices and in particular regions near the middle of these slices, show relatively lower metabolic activity in HRRT scans than in scans acquired on other scanners. For example, the value of the pons relative to the global mean is 12% lower for HRRT scans on average than for all other scans. While the cause of this is not certain, my best guess is that it is a related to scatter correction (or possibly attenuation correction or a combination of the two). The differences are consistent with the lower transaxial slices (base of the brain down into the neck) having greater scatter subtraction for HRRT scanners than other scanners. One possible reason would be that the axial FOV of HRRT scanners is much larger than most all other scanner models, and hence the correction can account for scatter which comes from the levels of the body (e.g. below the brain) imaged by the HRRT that are out of the axial field-of-view of other scanners. Thus, scatter that can be attributed to these below-brain slices imaged with the HRRT is being successfully removed by the scatter correction routine, resulting in relatively lower PET values in these inferior slices. Scatter from these areas of the body would not be accounted for and hence subtracted with other scanners since they are out of the axial FOV and not imaged.
This apparent difference between scanner types was not seen in the phantom studies, where the relative values in pons or cerebellar regions of the Hoffman phantom did not differ significantly between HRRT and other scanners. This result is also consistent with the idea of the difference being due to scatter correction. The Hoffman phantom scans did not have activity in these lower slices and hence there was not additional scatter in the field-of-view that could be removed. There was a torso phantom used with the Hoffman phantom scans (simulating out-of-field scatter), but this was outside the FOV for all scanners including the HRRT.
2. Some HRRT images have reconstruction (attenuation and/or scatter correction induced) artifacts. Some of these are quite subtle that were only noticed or confirmed when the 6 month or 12 month PET scans were obtained and overlaid with the baseline scans. This has resulted in some of the HRRT scans that had originally passed QC (and hence available) now being failed, and no longer available for download. LONIs advanced database search can be used to find the current HRRT scans that have passed QC. In some cases these artifacts have caused even greater differences in the relative metabolic values in lower image slices, compounding the issue raised above, where for example pons values are relatively lower than in other scanners.
Current Advice (May 8, 2007):
1. At present, scans from HRRT scanners should be considered to produce a different pattern of FDG uptake than other scanners (see below). Analyses that assume comparability across all scanners should consider excluding HRRT scans for the time being (e.g. comparison of individual scans to a normal database).
2. Check to see if any HRRT PET scans that you have downloaded are no longer available. Do not use any HRRT images that are now failed and not available through LONI.
Plan:
1. Now that all the baseline PET scans on normal controls have been acquired and uploaded to LONI, we are working on an empirical correction for scanner-specific differences in PET scans such as those described above. Notices will be provided here concerning progress on this front.
2. The HRRT sites have figured out how to deal with most of the artifacts mentioned above. All failed scans from HRRT sites are being reprocessed in an attempt to reduce or remove these artifacts. Re-reconstructed images will be uploaded and QCd. Notices will be provided here for all re-worked HRRT images that pass QC. After re-reconstruction and re-processing of these failed, other HRRT scans that are currently available will be re-reconstructed with the new processing stream, to make sure all subjects have optimal images over time.
— Bob Koeppe – 08 May 2007
