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). The hybrid architecture is the clinical standard — standalone PET ("PET-only") has been almost completely retired in favor of PET / CT.

Physics

Positron emitter (F-18, Ga-68, Rb-82, C-11, Cu-64, Zr-89) is injected and accumulates per the tracer's biology. Positrons annihilate with electrons, producing two 511 keV photons back-to-back. A ring of scintillators around the patient detects coincident photon pairs; the line connecting detection events (line of response) passes through the annihilation site. Millions of events reconstruct a 3D functional image.

Time-of-Flight (ToF) PET measures the timing difference between the two photon detections, localizing the annihilation along the line of response. ToF improves image quality and small-lesion conspicuity at equivalent injected dose. Modern digital PET (SiPM detectors) achieves ToF timing resolution of ~178–250 ps.

History

1970s — first PET scanners at WUSTL and Brookhaven.
1998 — first commercial PET / CT prototype (David Townsend, Pittsburgh).
2001 — Siemens ships the first commercial PET / CT.
2006 — first ToF PET ship (Philips Gemini TF).
2011 — GE Discovery 690 — first GE ToF PET.
2014 — Philips Vereos — first commercial fully-digital SiPM PET / CT.
2018 — digital PET (SiPM detectors) replaces PMTs across the premium tier (Discovery MI, Biograph Vision).
2018 — UIH uEXPLORER total-body PET / CT (~194 cm axial FOV) reshapes the top end.
2023 — Biograph Vision.X pushes commercial ToF timing to ~178 ps.

Key specs

Crystal — LYSO / LSO (ToF capable), BGO (pre-ToF, legacy).
Detector readout — PMT (legacy LSO / LYSO chassis) vs SiPM (digital, current generation).
ToF timing resolution — ~178–500 ps depending on platform and generation.
Axial FOV — ~16 cm (legacy), ~25 cm (TrueV / current premium), 194 cm (uEXPLORER total-body).
CT base platform — slice count, iterative / DL reconstruction tier, cardiac gating.
Calibration sources — Ge-68 rod (decays, periodic replacement).
Reconstruction software — Q.Clear, OptisoHD, equivalents — license-tier variables.

Systems

GE Discovery 690, Discovery MI (digital), Discovery IQ
Siemens Biograph mCT, Biograph 64, Biograph Vision (digital), Biograph Vision.X
Philips Gemini TF, Vereos (digital)

Clinical applications

FDG PET Oncology
Cardiac Rubidium-82 PET
PSMA prostate, DOTATATE neuroendocrine, amyloid / tau dementia, NaF bone, theranostic Lu-177 / Ac-225 follow-up, infection / inflammation imaging.

Service and refurb reality

Nuclear regulatory overhead is the dominant operating cost driver — NRC license, RSO, Authorized Users, hot lab or radiopharmacy contract.
Tracer delivery and uptake rooms gate real-world patient flow, not scanner speed. Throughput is rarely scanner-limited.
Detector technology generation (PMT vs SiPM) is the largest refurb price-determining variable. PMT-era LSO / LYSO PET (Discovery 690, Biograph 64 / mCT) is value-tier; SiPM digital is current generation.
Software tier — Q.Clear, TrueX / HD·PET / UltraHD·PET (LSO era), OptisoHD (Vision era), syngo.via license — confirm in writing on refurb deals.
See Siemens Biograph Field Guide and Discovery 690 Field Guide.

Regulatory

NRC or Agreement-State byproduct license — required to receive, use, and dispose of positron-emitting tracers.
Authorized User physician credentialing per isotope class.
RSO with documented radiation-safety program.
Dose calibrator daily QC, area surveys, decay-in-storage waste.
ACR PET / CT accreditation for reimbursement in many U.S. payer contexts.