Time-of-flight PET measures the tiny difference in arrival time between the two photons from each positron annihilation and uses it to place the event along its line of response, instead of spreading it along the whole line as conventional PET does. The timing resolution of the scanner sets how tightly: at 500 picoseconds the event is localized to about 7.5 cm, at 214 picoseconds to about 3.2 cm. That localization turns into more usable signal per count, which means clearer images in large patients, shorter scans, lower injected doses, or any mix of the three. Time-of-flight, or TOF, arrived commercially in 2006, became standard on LSO and LYSO scanners within a few years, and is the minimum specification Medical Outfitters recommends for any refurbished PET/CT purchase.
This guide explains how time-of-flight PET works, what timing resolution means and how it has improved from the first commercial scanner to today’s digital systems, what TOF changes in the image and the workflow, and how to read the TOF specification when choosing a new or refurbished PET/CT scanner. It accompanies our PET/CT scanner cost guide and our PET/CT scanner buying guide.
- The physics: distance along the line equals the speed of light times the arrival-time difference, divided by two.
- Timing resolution by generation: about 585 picoseconds on the first commercial TOF scanner, 540 on the LSO and photomultiplier generation, 200 to 400 on today’s silicon photomultiplier digital scanners.
- The gain: grows with the patient’s size, which is why TOF helps most where conventional PET struggles.
- What to buy: TOF on LSO or LYSO is the floor for a refurbished scanner; digital TOF for the highest volumes and the smallest lesions; BGO scanners without TOF are a deliberate trade of timing for sensitivity and price.
How Time-of-Flight PET Works
A PET tracer such as FDG decays by emitting a positron, which travels a millimeter or two in tissue, meets an electron and annihilates into two 511 keV photons flying in opposite directions. The scanner’s ring of detectors catches both within a coincidence window of a few nanoseconds and draws a line of response between the two detector elements. In conventional PET that line is all the scanner knows: the annihilation happened somewhere along it, and the reconstruction has to sort out where from the millions of other lines that cross the same tissue.
Time-of-flight adds one measurement. The photon that has the shorter path arrives a little earlier, and the difference in arrival time tells the scanner where along the line the event happened: the distance from the midpoint equals the speed of light multiplied by the time difference, divided by two. Light covers about 30 cm in a nanosecond, so a time difference of 100 picoseconds corresponds to a 1.5 cm offset from the center of the line.
No scanner measures that time exactly. Each has a coincidence timing resolution, quoted in picoseconds as the width of the timing uncertainty, and that figure sets the length of the segment along the line to which the event is confined. At 585 picoseconds, the resolution of the first commercial TOF scanner, the segment is close to 9 cm; at 540, about 8 cm; at 375, about 5.6 cm; at 310, about 4.7 cm; at 214, about 3.2 cm; at 179, about 2.7 cm. Even the loosest of those is a large improvement over a line that crosses the whole patient.
Why Timing Resolution Matters in the Image
The benefit of TOF is usually expressed as a gain in effective sensitivity, and it scales with the size of the object divided by the length of the localization segment. In a 40 cm abdomen, a scanner that places each event within 8 cm behaves roughly as if it had collected several times more counts than a non-TOF scanner with the same crystals, and a scanner at 3 cm behaves as if it had collected many times more. That is why the gain is largest exactly where PET is hardest: in large patients, where attenuation and scatter cut the true counts and the lines of response are longest.
The gain can be spent in four ways. Image quality: lower noise and higher contrast at the same injected dose and scan time, which is what shows as better lesion detectability in the reading room. Scan time: the same image in fewer minutes per bed position, which is throughput. Injected dose: the same image from less tracer, which is patient dose and tracer cost. And robustness: TOF reconstructions converge faster and tolerate errors in attenuation correction and scatter better, because each event is already roughly in the right place before the algorithm starts. The 2015 review by Surti in the Journal of Nuclear Medicine, “Update on time-of-flight PET imaging”, remains the standard reference on all four.
The CT half of the scanner is unchanged by any of this. It still provides the attenuation map and the anatomy, and it is specified the way any CT is, as our guide to CT scanner types by slice count explains.
Time-of-Flight PET/CT Scanners: From 585 to Under 200 Picoseconds
| Scanner | Detector | Timing resolution | Note |
|---|---|---|---|
| Philips Gemini TF (2006) | LYSO, photomultipliers | 585 ps | First commercially available TOF PET/CT; performance published in the Journal of Nuclear Medicine, 2007 |
| Siemens Biograph mCT and mCT Flow | LSO, photomultipliers | About 540 ps | The workhorse TOF generation on the refurbished market |
| Canon Celesteion | LYSO, photomultipliers | About 400 ps | Large-bore TOF system |
| GE Discovery MI | LYSO, silicon photomultipliers | About 375 ps | GE’s digital TOF platform |
| Philips Vereos | LYSO, digital silicon photomultipliers | About 310 ps | First digital PET/CT |
| Canon Cartesion Prime and Prime SP | LYSO, silicon photomultipliers | 263 ps typical; 238 ps on the SP | Canon’s digital TOF platform |
| Siemens Biograph Vision | LSO, silicon photomultipliers | About 214 ps | Published performance evaluation, 2019; the Vision.X variant reaches about 179 ps |
| GE Discovery IQ and Omni Legend | BGO | No TOF | BGO is too slow for TOF; the Omni Legend uses deep-learning reconstruction to recover part of the TOF benefit |
Timing resolutions are the figures published by the manufacturers or in peer-reviewed performance evaluations and are typical values; individual scanners are calibrated and verified at installation. Two trends run through the table. The crystal changed once, from BGO, which is sensitive but slow, to LSO and LYSO, which scintillate fast enough to time; and the light sensor changed once, from photomultiplier tubes to silicon photomultipliers, which is what the industry calls digital PET and what took timing from around 500 picoseconds to the 200s.
What Time-of-Flight Means When You Buy a PET/CT
On the refurbished market, TOF is the floor. The LSO and LYSO photomultiplier generation, the Siemens mCT family and the GE and Philips scanners of the same period, is deep, well supported and priced at a fraction of a new digital system, and its 500-picosecond-class TOF is a large step over the BGO scanners that preceded it. Our PET/CT buying guide makes the same point from the other direction: a refurbished scanner without TOF is a scanner from before 2006, and its reconstruction, detectors and CT belong to that era too.
Digital TOF is a volume and case-mix decision. Timing in the 200s, with the smaller detector elements of silicon photomultiplier designs, shows most in small lesions, in large patients and in programs that want to cut scan time or dose. A community program scanning a dozen patients a day on FDG gets most of what it needs from the photomultiplier generation; a high-volume oncology center, a theranostics program or a site with a research agenda will see the digital difference every day.
BGO without TOF is a deliberate trade. GE’s Discovery IQ and Omni Legend use BGO crystals for their sensitivity and their price, give up TOF because BGO cannot time, and lean on deep-learning reconstruction to recover image quality. They are new scanners with new-scanner warranties and current software, and for some programs that is the right answer; it is a different answer from a refurbished TOF system, and the two should be compared on the images and the throughput they deliver, not on the presence of a single acronym.
Check that the TOF actually works. On a used scanner, ask for the most recent timing calibration and the daily quality control records; timing resolution drifts with detector temperature and electronics age, and a scanner that was sold as TOF can be running well outside its specification. Medical Outfitters verifies the detector and timing calibration on every PET/CT it refurbishes and services, and our CT and PET/CT service team covers the detector blocks, the CT tube and the cooling that keeps both in specification.
Philips Gemini TF PET/CT mobile removal from Cleveland Clinic Weston, Florida, filmed by the Medical Outfitters crew
The scanner in that video is a Gemini TF, the first commercial time-of-flight PET/CT, coming out of a mobile unit in Weston, Florida, on a Medical Outfitters de-installation. Twenty years on, its design decisions, fast crystals and timing electronics, are in every digital PET/CT sold.
PET/CT Scanners From Medical Outfitters
Medical Outfitters sources, refurbishes, installs and services PET/CT scanners from GE, Siemens and Philips for hospitals and imaging centers across the United States, Puerto Rico and the Caribbean, with the detector calibration, the CT tube and the siting handled by our own engineers. The nuclear medicine page lists the current systems alongside our gamma detection equipment, the PET/CT scanner cost guide covers the price bands by detector generation, and the Neoprobe cost and buying guide covers the gamma probes used alongside PET in radio-guided surgery. For a scanner matched to your case mix and your timing requirement, contact Medical Outfitters.
- PET/CT scanner cost and price guide (2026)
- PET/CT scanner buying guide: detectors, time-of-flight, siting and refurbished options
- Nuclear medicine: PET/CT scanners and gamma detection
- CT and PET/CT service, repair and maintenance
- CT scanner types by slice count
- CT scanner cost and price guide (2026)
- Neoprobe cost and gamma probe buying guide
- What is a Neoprobe?
- Fujifilm SCENARIA View Focus Edition CT
- Used imaging equipment inspection checklist
- MO Select refurbished imaging systems
- Ask about a PET/CT
Time-of-Flight PET: Common Questions
What is time-of-flight in PET?
Time-of-flight PET measures the difference in arrival time between the two 511 keV photons from each positron annihilation and uses it to place the event along its line of response, instead of treating every point on the line as equally likely. The distance from the midpoint equals the speed of light times the time difference, divided by two.
What is timing resolution, and why does it matter?
Timing resolution is the uncertainty, in picoseconds, in the scanner's measurement of the arrival-time difference, and it sets the length of the segment to which each event is confined: about 8 cm at 540 picoseconds and about 3 cm at 214. The shorter the segment, the more usable signal each count contributes, so better timing means clearer images, shorter scans or lower doses.
Does time-of-flight reduce the dose in a PET scan?
It can. The gain in effective sensitivity that TOF provides can be spent on image quality, on shorter scan time or on a lower injected activity, or on a mix of the three. Many sites use TOF scanners to cut scan time per bed position and injected dose together while keeping image quality constant.
Is TOF PET better than non-TOF PET?
For image quality at a given dose and time, yes, and the advantage grows with the size of the patient, which is why TOF helps most where conventional PET struggles. The exception is price and sensitivity: BGO scanners without TOF, such as GE's Discovery IQ and Omni Legend, trade timing for crystal sensitivity and cost and use deep-learning reconstruction to recover part of the difference.
Which PET/CT scanners have time-of-flight?
Every LSO and LYSO scanner since the Philips Gemini TF of 2006: the Siemens Biograph mCT family at about 540 picoseconds, the Canon Celesteion at about 400, and the digital generation with silicon photomultipliers, the GE Discovery MI at about 375, Philips Vereos at about 310, Canon Cartesion Prime at 263 and the Siemens Biograph Vision at about 214. BGO scanners, including GE's Discovery IQ and Omni Legend, do not have TOF.
What is digital PET?
A PET scanner whose crystals are read by silicon photomultipliers instead of photomultiplier tubes. The silicon sensors are smaller, faster and matched one to one with the crystals, which is what took timing resolution from the 500 picosecond class to the 200s and improved spatial resolution at the same time.
Do I need time-of-flight on a refurbished PET/CT?
Medical Outfitters' advice is that TOF on LSO or LYSO crystals is the floor for any refurbished PET/CT purchase. A scanner without it dates from before 2006, and its reconstruction, detectors and CT belong to that era too. The photomultiplier TOF generation is deep, well supported and priced at a fraction of a new digital system.
What does BGO mean on a PET scanner?
Bismuth germanate, the scintillator crystal used in PET scanners before LSO and LYSO and still used in GE's Discovery IQ and Omni Legend. BGO is sensitive and inexpensive but scintillates too slowly to time the photons, so BGO scanners cannot do time-of-flight; the current ones rely on deep-learning reconstruction instead.
