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Challenges with PET/CT
The artifacts most commonly seen on PET/CT images are due to metallic implants, respiratory motion and contrast medium 10. The coregistration of CT and PET images can produce artifacts not only caused by patient movement, particularly of the head-neck, but also by different breathing conditions during the two acquisitions 2. The CT images can be acquired with a single breath hold after maximum expiration or after maximum inspiration, whereas during the long acquisition time of a PET scan, the patient breathes freely. The final PET images are hence an average of many breathing cycles. The artifact is due to the discrepancy between the chest positions on the CT and PET images 11, 12. Artifacts can be reduced, particularly in relation to the regions immediately above and below the diaphragm, by performing the CT scan with a single breath hold after normal expiration. Due to the possible presence of artifacts, a careful and separate examination of the emission and transmission images is advisable 2.
One problem which still has not been completely solved regards the use of iodinated or barium-containing contrast materials to enable the visualization of the bowel loops, particularly those of the small bowel, and the vascular structures 13, 14. In this regard it is worth noting that while some PET centers frequently use contrast media, most centers tend to use them only in selected cases and in well defined protocols 2. High contrast concentrations result in high CT numbers and streaking artifacts on CT images as well as high PET attenuation coefficient, leading to an overestimation of tracer uptake, thereby producing false-positive PET results 15, 16, 17. Clearly there is a need to establish before the event in which cases oral contrast media should be used, bearing in mind that the media are usually administered in two doses, one immediately before and one 30 min after the radiotracer injection, so as to obtain a satisfactory visualization of the proximal and distal bowel loops 2.
As already stated the transmission CT images are used for the correction of the attenuation of the PET emission images. There is therefore need to avoid the use of oral contrast media causing artifacts in the correction procedure of the emission images. It has been shown that these artifacts can be completely avoided if very low density oral contrast materials are used (1.2-2.1 vol%). The visualization of the bowel loops is useful only when a PET/CT scanner is used and when those abdominal cancers are studied which give rise to peritoneal diffusion and lymph node involvement, such as ovarian and colon cancer. A more complex issue is the use of intravenous iodinated contrast media, both with regard to the need for close cooperation between the nuclear medicine physician and the radiologist and because the CT scans with contrast medium need to be performed after the PET acquisitions. Clearly then the use of a CT scan with intravenous contrast medium appears justified only in selected cases, when the visualization of the vessels and the perivascular anatomical structures are capable of providing additional useful information for significantly improving the interpretation of the findings, or when a reduction in the radiation dose or avoiding additional anesthesia is involved, as in the case of cancer in pediatric PET/CT 2.
Metalic implants result in high CT numbers and generate streaking artifact on CT images because of their high photon absorption. This increase in CT numbers results in correspondingly high PET attenuation coefficients, which lead to an overestimation of the PET activity in that region and thereby to a false positive PET findings 10. As a result, technologists should ask patients to remove all metallic objects before imaging and should document the location of nonremovable metallic objects to minimize or identify such artifacts.
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