1. Tuesday Case of the Day Physics History: A B MIP
Osama Mawlawi, PhD
Department of Imaging Physics, M. D. Anderson Cancer Center, Houston. A B
History:
A 62-year-old male who completed radiation with chemotherapy for recurrent nasopharyngeal carcinoma approximately 1 year ago. A whole-body PET/CT scan was done approximately 1 hour after the intravenous injection of 18 mCi of 18F-labeled FDG. PET scan was done in 2D mode.
MIP
Figure A. Sagittal view of PET scan showing multiple areas of increased uptake (arrows).
Figure B. MIP display of the same patient. Areas of increased uptake can be clearly seen.
The most likely reason for the increased uptake indicated by the arrows and visible on the MIP display is ______.

2. Answer:
The acquired PET data are corrected for multiple degrading factors (such as attenuation, scatter, randoms, etc) during image reconstruction. One of these corrections is normalization. This correction makes up for any differences between the detector responses so that differences in the detected signal are truly due to differences in the radioactivity distribution in the patient and not due to detector performance. This correction consists of a map of normalization factors that are stored in memory and applied during the image reconstruction process. If a wrong normalization map is applied to the acquired PET data, the detector response will no longer represent the true measured signal (activity concentration). Normalization factors are generated from “flood” scans, which are acquired infrequently. Figures C and D show a good normalization and a suboptimal normalization, respectively, for a single PET slice. C D

3. Answer (continued): SUVmax = 4.5
If the normalization in Figure D is applied to the PET data, artifacts similar to those in Figure E will be present. If the normalization in Figure C is applied to the PET data, the image will exhibit no artifacts (Fig F). Normalization artifacts appear at a specific location in an image corresponding to the detector pair with the wrong correction factor. In whole-body PET imaging, the location of the artifact will be repeated in every bed position, as seen in Figure E. This artifact can result in high measured activity concentrations (SUV of 4.5, Fig E), which might lead to wrong image interpretation.
SUVmax = 4.5
Increased activity due to error in normalization E F