Case Author(s): Akash Sharma, M.D. and Barry A. Siegel, M.D. , September 20, 2004 . Rating: #D3, #Q4

Diagnosis: Attenuation-correction artifact from residual barium in colon

Brief history:

55-year-old woman with newly diagnosed colon cancer presents for initial staging.


Selected coronal PET images

View main image(pt) in a separate image viewer

View second image(pt). Selected fused PET-CT images

View third image(pt). Non-attenuation-corrected (NAC) and attenuation-corrected (AC) coronal PET images

Full history/Diagnosis is available below

Diagnosis: Attenuation-correction artifact from residual barium in colon

Full history:

55-year-old woman with newly diagnosed colon cancer presented for initial staging. The patient underwent a colonoscopy with biopsy at Barnes-Jewish Hospital. The patient also had a remote history of left breast cancer treated with left mastectomy and axillary dissection, as well as radiation and chemotherapy. She also had a left oophorectomy and appendectomy in the past.


F-18 FDG i.v.


There is moderately intense uptake in the region of the rectum in a contiguous segment which is corroborated by an eccentric rectal mass of equal length (approximately 5 cm) noted on the noncontrast CT images. A second focus of increased FDG uptake is noted just superior to the primary lesion. This focus certainly could be due to uptake in draining regional lymph nodes.

The corresponding CT images in this second region demonstrate barium in the colon, likely a residual from a previous CT study. The apparently increased FDG uptake in this region on the attenuation-corrected (AC) images is secondary to an attenuation-correction artifact. No other foci of abnormal FDG uptake are noted in the abdomen or pelvis. No adenopathy is evident. Incidental mild uptake in the supraclavicular fat and the deep cervical fat at the skull base is noted as a normal variant.

The artifactually increased "activity" in the second focus is not seen on the Non-attenuation-corrected (NAC)images.


Although non-attenuation-corrected (NAC) images were often the only images obtained for oncologic whole-body PET studies in the past, it is now standard practice to perform attenuation correction of both conventional PET and PET-CT image.

In hybrid PET-CT scanners, attenuation correction typically is performed using the CT data. By comparison, in a conventional PET scanner, this is done using the data from a transmission scan performed with a Ge-68/Ga-68 (511 keV) or Cs-137 (667 keV) source. Because of the lower average photon energy of the CT x-ray beam, performing the attenuation correction requires that a transformation of the CT attenuation map to that expected at 511 keV. This is accomplished usiing a bilinear, with one scaling factor for soft tissues and another for bone. In this calculation, artifacts can occur when materials that are much denser than bone, e.g., metal or concentrated barium, are present within the patient's body. This has been described in the following article:

"...high-density oral contrast can produce an artifact of apparent increased tracer uptake. The radiodensity of this dense barium is near that of metallic objects, which also can produce artifacts of increased activity on emission PET if CT-corrected emission data are used. These artifacts are likely caused by the energy differences between photons used for CT scanning and the 511-keV photons used for 68Ge transmission scanning. The use of low-energy photons for transmission imaging results in increased attenuation coefficients in the presence of materials of high atomic number (metallic objects, high-density contrast material) compared with the use of high-energy 511-keV photons. This increased attenuation for low-energy x-rays is caused by an increased probability of photoelectric interaction of low-energy photons with material of high atomic number. Mathematic algorithms ideally should scale the attenuation coefficients obtained with low-energy photons to the energy level of 511-keV photons. However, it appears that current and widely applied commercial scaling algorithms are not appropriate for high-density materials, causing an overestimation of attenuation coefficients in their presence and thus an overcorrection of the emission data that then produces an artifact of increased apparent tracer activity. The third phantom experiment shows that the CT attenuation correction algorithm produces an increasing overcorrection of the emission activity in the presence of increasing density of high- atomic-number contrast materials. The overestimation of tracer activity begins to appear for materials as the measured density rises between 93 and 629 HU. The exact concentration threshold at which artifacts of increased activity can be expected will vary according to the material used for contrast and its quantity." *

*Excerpt from: Cohade C, Osman M, Nakamoto Y, Marshall LT, Links JM, Fishman EK, Wahl RL. Initial experience with oral contrast in PET/CT: phantom and clinical studies. J Nucl Med. 2003 Mar;44(3):412-6.


Evaluation of the NAC images during interpretation can allow one to differentiate truly increased FDG uptake from an attenuation-correction artifact related to residual high-density oral contrast materials or metal that may be present within a patient.

View followup image(pt). Coronal fused PET-CT images with "false-positive" and "true-positive" foci.

Major teaching point(s):

Use of lower-density oral contrast and use of a "saline" chaser for the IV contrast bolus have been suggested as possible solutions to eliminate attenuation correction artifacts in PET-CT imaging.

ACR Codes and Keywords:

References and General Discussion of PET Tumor Imaging Studies (Anatomic field:Gasterointestinal System, Category:Other(Artifact))

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Case number: pt118

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