Case Author(s): Michael Quinn, M.D. and Barry A. Siegel, M.D. , 09/11/97 . Rating: #D3, #Q4

Diagnosis: Attenuation-Correction Artifact on FDG-PET

Brief history:

This 47-year-old woman has a history of treated dermatofibrosarcoma and now has a lung nodule seen on CT. This study was requested to evaluate for metastatic disease.


Coronal, axial, and sagittal emission images from a whole-body FDG-PET study are shown.

View main image(pt) in a separate image viewer

View second image(pt). Coronal, axial, and sagittal images transmission images are shown.

Full history/Diagnosis is available below

Diagnosis: Attenuation-Correction Artifact on FDG-PET

Full history:

This 47-year-old woman has a history of a left shoulder dermatofibrosarcoma treated by resection. Radiographs and CT of the chest revealed a nodule which was considered suspicious for a metastatic focus in the right upper lobe. An FDG-PET study was ordered to evaluate for metabolic activity in this nodule.


F-18 fluorodeoxyglucose (FDG)


No abnormal activity was seen either in the site of the patient's prior primary tumor or at the site of the lung nodule. This suggests that the lung nodule is benign. However, the emission images are remarkable for a rim of increased activity along the periphery of the bladder. There are no other abnormalities.


At Mallinckrodt Institute of Radiology, all PET tumor studies are reconstructed with use of a computer algorithm that takes into account the attenuation of emitted photons by the patient's tissues. This "attenuation correction" is achieved by obtaining initial transmission images with the patient positioned between a rotating germanium source and the camera detectors. Regions of low photon attenuation (lungs) result in more activity from the source being detected by the scanner. Conversely, regions of increased attenuation (abdomen) result in fewer counts detected from the transmission source. These data are used to create an attenuation "map" corresponding to each cross-section through the patient. The subsequent emission images then use this map to take into account regional photon attenuation of administered radiopharmaceutical by various soft tissues. However, with the technique we employ, the FDG is administered before the patient is put into the scanner, and is thus "on board" at the time of transmission imaging. Regions with marked radiopharmaceutical accumulation, such as the myocardium and bladder, have resultant large numbers of photons detected on transmission scan. The detector cannot differentiate photons emitted by the FDG from those arising from the transmission source. These areas are thus interpreted as having little photon attenuation, because so many counts are detected. This causes subsequent under-correction of these same regions on the corresponding emission images. Such under-correction is usually limted to large, very intense regions like the bladder, where interpretation is rarely an issue.

We routinely place a Foley catheter for FDG-PET tumor imaging studies, and also administer intravenous fluids and furosemide to minimize the activity in the pelvicalyceal systems of the kidneys and in the urinary bladder. However, in this patient, there was a significant increase in bladder volume during the interval between transmission and emission imaging, presumably because the Foley catheter was not draining properly. On the emission images, the central bladder is under-corrected, as expected, whereas a surrounding rim of urine accumulating in the interval between scans, is accurately corrected for attenuation. This resulted in the erroneous appearance of an "abnormal" rim of activity at the periphery of the bladder.

Although unlikely, potential non-artifactual causes of the FDG distribution pattern seen in this patient would include cystitis and a diffuse mucosal or mural neoplasm of the bladder.


To confirm the artifactual nature of the finding, images were reconstructed from the emission data without use of attenuation correction. These showed more uniform activity in the enlarged bladder. The slightly greater peripheral than central activity is related to the the effect of attenuation of photons arising deeper within the body.

View followup image(pt). Coronal, axial, and sagittal emission images (without attenuation correction) from a whole-body FDG-PET study are shown

Major teaching point(s):

For accurate attenuation correction there must be no change in the object between emission and transmission images. The most likely source of artifacts due to such changes is patient motion, but changes in internal organ configuration (e.g., in distribution of bowel gas or, as in this case, in bladder volume) can also cause artifacts.

Differential Diagnosis List

As noted above, cystitis and a diffuse mucosal or mural neoplasm of the bladder could produce similar findings.

ACR Codes and Keywords:

References and General Discussion of PET Tumor Imaging Studies (Anatomic field:Lung, Mediastinum, and Pleura, Category:Other(Artifact))

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Case number: pt016

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