Claustrophobia and Motion on a PET/CT
A 54-year-old woman with newly diagnosed lymphoma is undergoing a staging FDG PET/CT. She reports severe claustrophobia and became tearful in the scanner room during a prior MRI. She is 158 cm tall and 118 kg (BMI ~47). The PET/CT table has a weight limit of 200 kg, and the bore is 70 cm. The technologist plans a whole-body acquisition from skull base to mid-thigh with arms up. The patient's arms cannot be raised above her head due to shoulder pain, and she is unable to lie still for the planned 25-minute emission scan.
1. What acquisition and positioning adjustments can the technologist make to obtain a diagnostic-quality study?
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Optimize within the physics and the patient's limits.
- Arms down is acceptable if arms-up is impossible; document it, because arms-down produces beam-hardening and truncation artifacts on the CT and increased scatter on PET. Reconstruction with CT-based attenuation correction and review of the arms-down region for artifact are needed.
- Shorten the emission time per bed position and/or increase bed overlap, and consider time-of-flight reconstruction if available to preserve image quality with fewer counts. A higher administered activity within institutional limits can offset the shorter scan.
- Use the largest acceptable bore and table; confirm the 118 kg patient is within the 200 kg table limit (she is).
- Address claustrophobia with reassurance, a companion in the room if policy allows, music, and a warm blanket. If she truly cannot tolerate 25 minutes, split the acquisition into segments with breaks, or use a shorter protocol with physician approval.
2. The patient is tearful and cannot lie still. What is the appropriate next step before injecting FDG?
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Do not inject until the patient can cooperate or the physician approves an alternative.
- Motion during the 60-minute uptake and the emission scan degrades image quality and can mimic or obscure disease. The technologist should pause, involve the physician, and consider anxiolytic premedication ordered by the physician (with attention to sedation and transport safety) or rescheduling.
- Document the claustrophobia, the interventions tried, and the physician's decision. If the study proceeds, note the risk of motion artifact in the report.
- The technologist should not force the study or administer medication independently; scope of practice and institutional policy govern anxiolysis.
3. The CT is used for attenuation correction. How does the patient's body habitus affect the CT parameters and the PET interpretation?
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Habitus changes both the CT and the PET.
- For attenuation-correction CT, the technologist may need to increase mAs/kVp to penetrate the larger body, but should keep dose as low as reasonably achievable since the CT is not diagnostic. Truncation of the arms-down patient at the edge of the 70 cm bore can cause inaccurate attenuation correction and artifacts.
- Increased attenuation and scatter reduce PET count statistics; longer or overlapped bed positions, TOF, and higher activity help. The interpreting physician should be told about arms-down positioning and any truncation so artifacts are not misread as lesions.
- Document all deviations (arms down, shortened scan, any truncation) so the reader can interpret with appropriate caution.