Value of Non-Attenuation Corrected (NAC) Images

Resolving Attenuation Correction Artifacts:

The CT images obtained during an exam are not only used for “anatomic” interpretation.  They are also used for “attenuation correction” (AC) of the PET images, generating the much more useful AC PET images used for interpretation.

Unfortunately, attenuation correction of PET images can result in falsely elevated metabolic activity in regions of high CT density (e.g. metallic devices, oral contrast, calcification).  

In such cases, these areas must be reviewed on the NAC (non-attenuation correction) images. If these regions are not also hypermetabolic on the NAC images, then their apparent increased FDG-uptake on the AC images is artefactual.

This phenomenon is almost exclusively seen with older PET/CT scanners, as most modern scanners utilize newer algorithms that do not “over-correct” in these areas of increased density.

Lung Nodule Identification:

NAC images can occasionally be useful in identifying mild to moderately FDG-avid lung nodules, especially those located in the lung periphery or lung bases.