PET/CT Artifact and Uptake Pattern in a Patient with a Hip Replacement
A 71-year-old woman with newly diagnosed diffuse large B-cell lymphoma undergoes an FDG PET/CT for staging. Her glucose is 142 mg/dL and she received her last chemotherapy 9 days ago. She has a right total hip arthroplasty (titanium) and a left breast implant. On the attenuation-corrected images there is a band of apparent intense FDG uptake adjacent to the right femoral stem, and a photopenic streak artifact through the left breast. The non-attenuation-corrected images show the femoral uptake is much less prominent. The referring oncologist asks whether there is osseous involvement at the prosthesis.
1. Explain why the femoral uptake appears on attenuation-corrected but not (or less) on non-attenuation-corrected images, and how you would determine whether it is real disease.
Read the worked answer
Metallic implants cause photon starvation and beam-hardening on the CT used for attenuation correction. The CT attenuation map is corrupted, so the AC algorithm assigns incorrect attenuation factors and can create spuriously high or low FDG signal near the implant - a classic metal artifact. The fact that the uptake is prominent on AC but much less on NAC images points to artifact rather than true uptake. To decide, compare AC and NAC side by side, review the CT for streak artifact, and inspect the PET for the characteristic linear band conforming to the implant rather than to marrow or cortical bone. True osseous involvement would usually persist on NAC images and follow anatomic bone/marrow distribution. Additional steps: review any prior imaging, consider a dedicated MRI or contrast CT of the hip, and discuss with the radiologist. Do not call osseous involvement on the basis of the AC image alone.
2. The glucose is 142 mg/dL and chemotherapy was 9 days ago. How do these factors affect interpretation, and what would you recommend?
Read the worked answer
FDG uptake competes with endogenous glucose; elevated serum glucose can reduce tumor-to-background ratio and cause false-negative or underestimated uptake, particularly in low-grade or small lesions. A glucose of 142 mg/dL is above the commonly used threshold (~150-200 mg/dL) for proceeding without discussion, but not automatically disqualifying - many protocols proceed with documentation and physician awareness. Chemotherapy 9 days ago raises the risk of stunning: post-treatment inflammatory changes and marrow suppression can alter uptake, and scanning too soon after therapy can produce false-positive inflammatory uptake or false-negative tumor suppression. For staging, the ideal is to image before therapy or wait an appropriate interval per protocol. Recommendation: document the glucose and treatment timing, consult the oncologist and nuclear medicine physician, and consider repeating the glucose or rescheduling if the result would change management. Interpret marrow and nodal uptake with caution given recent chemotherapy.
3. The left breast implant produces a photopenic streak. What is the practical consequence for staging, and how would you address it?
Read the worked answer
A photopenic streak from the implant can obscure adjacent breast tissue and axillary nodes on the AC images, risking a missed or overcalled lesion. The implant itself is not FDG-avid, but the artifact can create apparent defects or edge artifacts. Practical steps: review NAC images (which avoid CT-based artifact), window the PET appropriately, and correlate with the CT and any breast imaging. If the axilla or breast is critical to staging, recommend dedicated breast imaging or MRI. Document the artifact in the report so the oncologist knows the limitation. The general principle: when an artifact could change staging, do not rely on a single image set - use NAC, CT correlation, and if needed additional imaging.