1. Monday Physics Case of the Day Answers (1 of 3) Paul E
Monday Physics Case of the Day Answers (1 of 3) Paul E. Kinahan, PhD Imaging Research Laboratory, Department of Radiology, University of Washington, Seattle.
Answers: By comparing with the FDG PET image without attenuation (which is not quantitatively accurate but can be a useful secondary check), the abnormal uptake regions are not evident, with the exception of a potential bone lesion (black two-headed arrow).
Coronal section of attenuation-corrected FDG PET image.
PET image (now in hot metal color scale) overlaid on CT image.
Coronal section of FDG PET image without attenuation correction.

2. Monday Physics Case of the Day Answers (2 of 3) Paul E
Monday Physics Case of the Day Answers (2 of 3) Paul E. Kinahan, PhD Imaging Research Laboratory, Department of Radiology, University of Washington, Seattle.
The lung and spine areas of increased uptake (horizontal arrows) are likely due to a lateral shift of the patient that occurred between the CT and PET scans. All PET/CT scanners use the CT images for attenuation correction of the PET data, and since attenuation is the dominant effect in PET image quality, any misalignment has the potential to introduce artifacts.
Coronal section of attenuation-corrected FDG PET image.
PET image (now in hot metal color scale) overlaid on CT image.
Coronal section of FDG PET image without attenuation correction.

3. Monday Physics Case of the Day Answers (3 of 3) Paul E
Monday Physics Case of the Day Answers (3 of 3) Paul E. Kinahan, PhD Imaging Research Laboratory, Department of Radiology, University of Washington, Seattle.
The abnormally decreased FDG uptake in diaphragm region (vertical arrows) is likely due to misalignment of the diaphragm due to respiratory motion differences between the PET and CT scans, as indicated by the horizontal green lines. Again, since attenuation is the dominant effect in PET image quality, any misalignment has the potential to introduce artifacts.
Coronal section of attenuation-corrected FDG PET image.
PET image (now in hot metal color scale) overlaid on CT image.
Coronal section of FDG PET image without attenuation correction.