PET/CT

Hybrid imaging — a PET ring detects coincident 511 keV photons from positron annihilation of an injected radiotracer, combined with a CT scanner on a shared patient couch. CT provides anatomic reference and attenuation correction; PET provides functional / molecular information (metabolism, receptor binding, perfusion).

Physics

Positron emitter (F-18, Ga-68, Rb-82) is injected. Positrons annihilate with electrons, producing two 511 keV photons back-to-back. Ring of scintillators around the patient detects coincident photon pairs; the line connecting detection events passes through the annihilation site. Millions of events reconstruct a 3D functional image.

Time-of-Flight PET measures the timing difference between the two photon detections, localizing the annihilation along the line of response. ToF improves image quality at equivalent dose.

History

• 1970s — first PET scanners at WUSTL + Brookhaven.
• 1998 — first commercial PET/CT prototype (David Townsend, Pittsburgh).
• 2001 — Siemens ships first commercial PET/CT.
• 2011 — GE Discovery 690 — first GE ToF PET.
• 2018 — digital PET (SiPM detectors) replaces PMTs in premium tier.

Key specs

• Crystal — LYSO / LSO (ToF capable), BGO (pre-ToF)
• ToF timing resolution — ~250–500 ps on modern systems
• CT base platform — slice count, dose reduction, cardiac gating
• Calibration sources — Ge-68 rod (decays; replacement)

Systems

• GE Discovery PET/CT 690, Discovery MI (digital)
• Siemens Biograph mCT, Biograph Vision (digital)
• Philips Gemini TF, Vereos (digital)

Service reality

Nuclear regulatory overhead: NRC license, RSO, authorized users. Hot lab or radiopharmacy contract. PET throughput is rarely scanner-limited — tracer delivery and uptake rooms gate real patient flow.

Regulatory

NRC or Agreement State materials license. Authorized User credentialing. RSO + documented radiation safety program. ACR PET/CT accreditation for reimbursement.