Acquisition Protocol Considerations for Combined PET/CT Imaging

Acquisition Protocol Considerations for Combined PET/CT Imaging

Thomas Beyer, MD¹^,2, Gerald Antoch, MD², Stefan Müller, MD¹, Thomas Egelhof, MD², Lutz S. Freudenberg, MD¹, Jörg Debatin, MD, MBA² and Andreas Bockisch, MD, PhD¹

¹ Department of Nuclear Medicine, University Hospital Essen, Essen, Germany
² Department of Diagnostic and Interventional Radiology, University Hospital Essen, Essen, Germany

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FIGURE 1. Standard FDG-PET/CT imaging protocol. The patient is positioned on a common patient handling system in front of the combined gantry. First, a topogram is used to define the co-axial imaging range (orange). The spiral CT scan (gray box) precedes the emission scan (green box). CT images are reconstructed on-line and used for the purpose of automatic attenuation correction of the corresponding emission data (blue box). Black and blue arrows indicate acquisition and data processing streams, respectively.

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FIGURE 2. Optimization tasks of PET/CT imaging protocols relate to clinical and methodological concerns: What CT dose levels are adequate for diagnostic purposes and CT attenuation correction? Should CT contrast agents be used? Should CT standards be adopted for the position of the arms, respiration protocols, or for the administration of CT contrast agents? Will metal implants introduce artifacts into corrected PET images?

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FIGURE 3. (A) Topogram scans of 3 patients with different arm positions. Selected transverse CT images (center 50 Hounsfield units [HU], width 300 HU) at level of midliver are shown to illustrate magnitude of streaking artifacts. Streaks are reduced most in case when both arms are raised (right). (B) Male patient with lymphoma. Fused image shows involvement of retrocrural lymph node (arrow). Streak artifacts originating from positioning of patient with arms down degrade quality of CT and fused PET/CT images.

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FIGURE 4. Patient positioning with arm rest (A) and knee rest (B) for whole-body PET/CT studies. Arm and knee rests are made of low-density foam with CT attenuation close to that of air. No artifacts are generated from these positioning aids, as is illustrated in transverse CT images to right (window levels center, C, and width, W, in Hounsfield units).

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FIGURE 5. Difference (A) in maximum transverse field of view of CT (blue) and PET (orange) may lead to truncation artifacts in PET/CT images (B). CT images of large patients or patients with arms down appear truncated at sides, and derived attenuation maps appear to mask corrected emission activity. (B) CT, uncorrected, and corrected PET. (C) Truncated attenuation information may be recovered before CT-based attenuation correction.

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FIGURE 6. (A) Significant respiration mismatch between CT and PET results in severe artifacts in corrected emission images. (B) Free breathing during spiral CT scan may cause artifacts that propagate into corrected PET, which is acquired in free breathing over many respiration cycles. (C) Using special breathing instructions, respiration-induced artifacts and mismatches may be reduced in majority of cases.

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FIGURE 7. Alternative schemes of application of intravenous contrasts (A) and negative oral contrast agents (B) are being pursued to avoid contrast-induced artifacts on PET/CT images while providing acceptable image quality for radiologists.

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FIGURE 8. High-density implants generate streak artifacts on CT, which may translate into distortions and biases of recovered tracer activity. PET images without CT-based attenuation correction (CT-AC) may help to interpret metal-induced artifacts.

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FIGURE 9. (A) Without efficient positioning aids, misalignment from muscle relaxation and involuntary patient motion may be observed in head and neck. (B) Whole-body imaging ranges can be separated into neck and thorax scan ranges, each with its own optimized positioning, acquisition, and reconstruction parameters for improved imaging at little cost in examination time.