Imaging description

The process of attenuation correction at PET/CT is applied to the PET images in order to account for differences in tissue density. For example, lung is less attenuating and so is made relatively “colder” after correction. However, the process depends on accurate co-registration between the CT and PET images. Because CT images are generally acquired in held inspiration and PET images are acquired during quiet respiration, there is frequently a mismatch between the two datasets. This occurs particularly near the diaphragm, such that the liver is more superior on the PET images than on the CT images. As a result, portions of the liver are corrected as if they were lung, and become too “cold” (Figure 8.1). The net result is that “hot spots” in the liver may initially appear to be in the lung when reviewing the attenuation corrected images. Review of the non-corrected images is the key to recognizing this artifact (Figure 8.2) [1,2].

Importance

Incorrect localization of hepatic FDG “hot spot” foci to the lungs could have several adverse consequences. At a minimum, the error is confusing, since there will be no anatomic correlate for the foci of increased FDG uptake in the lungs on CT. Worse, a patient may be incorrectly assumed to have pulmonary metastases so that, for example, a patient with resectable colorectal hepatic metastases could be denied surgery because of apparent extrahepatic disease.

Typical clinical scenario

This artifactual misregistration typically occurs when a patient has metastases in the superior portion of the liver; attenuation over-correction of these parts of the liver results in an appearance that simulates “hot spots” in the lung.

Differential diagnosis

Review of the non-corrected PET images allows confident identification of this artifact, and there is no real differential.