Summary of warning:
There exists a software bug in OSEM reconstruction for Siemens BioGraph HiRez PET/CT scanners. Depending upon the circumstances, this artifact can be quite minor or can be quite substantial. Siemens is aware of this, has developed a patch for the problem, and is in the process of distributing the patch to the BioGraph HiRez sites. We will be asking all HiRez sites to re-reconstruct all of their PET images once the site has the software patch installed and tested.
The problem involves an error in the Siemens image reconstruction algorithm that may produce an artifactual asymmetry in the PET images. The actual asymmetry depends on how the patient is positioned in the scanner. These asymmetries may produce data analytic results that appear as if brain activity is higher in one hemisphere than another, potentially seriously affecting the ADNI results. The actual error for a particular scan depends upon a variety of physical factors and the specific software version the site used for that scan reconstruction.
Technical details of the problem:The problem is in the Siemens OSEM reconstruction, where the ordered subsets of the PET and CT data sets used by the EM algorithm are not correctly matched between emission (PET) and transmission (CT) data. In other words, the emission projections are being attenuation corrected with the wrong CT projections. The artifact appears as an asymmetry in the image that does not really exist. If the subject is low in the field-of-view, an image left > image right asymmetry occurs (mostly in posterior regions). If the subject is high in the FOV, the opposite occurs, but more in the frontal regions. The magnitude of the artifacts depends upon many factors, including physical factors and the particular software version and software parameters being used. These are summarized as follows. Physical factors: If the object being imaged is cylindrical and perfectly centered in the field-of-view, there is no artifact. As the object becomes more elliptical (such as a head), and as the object is located further from the center of the scanners FOV, the magnitude of the asymmetry increases. This error was not seen in the phantom studies that were done at the start of ADNI since the Hoffman phantom is cylindrical, and each site scanned the phantom very near to the center of the FOV.
Software version/parameter factors: The error is both software version and OSEM parameter dependent. The error is less noticeable for software versions 3.x and more noticeable in the newer software releases, 4.x. (see below for explanation). The ADNI protocol specifies 4 iterations and 16 subsets with reconstruction into a 168×168 grid using TRIM=2. In OSEM reconstruction, the number of projections has to be divisible by the number of subsets so that the number of projections per subset is constant. The number of angular projections for the BioGraph HiRez is 336. This is divisible by 16 (336/16=21 projections per subset). HOWEVER, a feature of Siemens software used to speed up reconstruction is to interpolate the number of rays per projection and the number of projections to the output image grid size. Thus, if the 336 projections are interpolated to 168 projections prior to reconstruction, 16 subsets cannot be used (168/16=10.5, and you cant have half a projection). The older software either forced the user to select 14 subsets (168/14=12), or actually used 14 subsets even when 16 subsets was selected. When the version 4.x software was released, the option for 14 subsets disappeared, and when the user selects 16 subsets the software rounds this UP, not DOWN to the nearest number of acceptable subsets, which happens to be 21 (168/21=8). Thus, even though the software says 16 subsets, 21 are being used with version 4.x software. This has two effects. The minor one is that the images are noisier than before since 50% more total sub-iterations are being used (4 x 21, instead of 4×14), but more importantly, the error due to the mismatch of PET and CT projections for attenuation correction has become considerably more pronounced with 21 subsets than 14 subjects, due to the particulars of the actual software bug in Siemens code.
Current Advice (May 8, 2007):
All BioGraph HiRez PET scans need to be interpreted with caution. While the majority of artifact-induced asymmetries are probably only a few percent or less, in some cases these will be greater. If during longitudinal analysis, one finds a change in the degree of asymmetry between scans, this finding in particular should be questioned. Final conclusions should wait until the re-reconstructions are uploaded to LONI. One can make an educated guess about the magnitude of the artifact for a given scan by considering the above information (i.e. version 3 vs. version 4, location within the FOV), but also by looking at the raw images to see whether the asymmetries are seen in the scalp. While scalp uptake of FDG may also be asymmetric, the artifact will be more apparent in regions farther from the center of the object (such as scalp). Hence, in many of the ADNI BioGraph HiRez images, one can see a noticeable scalp asymmetry. In my review of all BioGraph HiRez scans, most subjects are scanned a little below the center of the field-of-view, and hence in many images one scan see an image left (brain right) posterior increase in scalp activity.
Plan:
When the software patch is available and re-reconstructions are performed by the sites, and uploaded to LONI, we will provide the updates here concerning the availability of these re-processed BioGraph HiRez scans.
— Bob Koeppe – 08 May 2007
