Presentation
Staging for lymphoma.
Patient Data
F-18 FDG PET CT axial images show:
fused images show a fusiform focus of intense FDG avidity in the right supraclavicular region
AC PET image again demonstrates the intense FDG avidity
contrast-enhanced CT image shows FDG avidity corresponds to a dense bolus of IV contrast in the right subclavian vein
NAC PET image shows no FDG avidity, confirming the attenuation correction artifact
Case Discussion
With PET/CT, the attenuation correction (AC) is performed using the CT portion of the exam, rather than having to perform a separate transmission scan, which is necessary on a standalone PET system. Using the CT for AC is faster and may provide useful anatomic data depending on the acquisition technique.
Because CT scans are usually obtained using 40–150 keV and a PET photon is 511 keV, the attenuation of these the CT photons and PET photons is not one-for-one and a transformation process must be performed on the CT data to correct for this difference.
AC algorithms work well for most situations; however, they tend to overcorrect when applied to high-density objects such as metal, concentrated IV contrast material (as in this case), and bone cement such as used in vertebral augmentations. In such cases, the PET avidity is falsely elevated, given the appearance of hypermetabolism when imaging with F-18 FDG and potentially mimicking tumors or infections.
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