What is time-of-flight (TOF) PET?

Time-of-flight (TOF) PET is an advanced PET technique that improves image quality by using the small difference in arrival times of the two annihilation photons to better localise the annihilation event along the line of response.

Time-of-flight PET uses precise timing differences between annihilation photons to better localise events along the line of response, improving signal-to-noise ratio.

In conventional PET, when two photons are detected in coincidence, the system knows the event occurred somewhere along the line connecting the detectors, but not exactly where. In TOF PET, extremely precise timing measurements allow the system to estimate the position of the annihilation point along that line.

This improves signal-to-noise ratio and image contrast, particularly in larger patients.

Understanding the physics

When a positron annihilates with an electron, two 511 keV photons are emitted in nearly opposite directions. In ideal geometry, if the annihilation occurs exactly midway between two detectors, both photons arrive simultaneously.

However, if the annihilation occurs closer to one detector, that photon will arrive slightly earlier. The difference in arrival time is proportional to the difference in distance travelled.

Because photons travel at the speed of light, these time differences are extremely small, typically on the order of hundreds of picoseconds. Modern PET detectors and electronics are capable of measuring these tiny timing differences.

The relationship between timing difference and position is:

Δx = (c⋅Δt)/2

where:

c is the speed of light,
Δt is the measured arrival time difference,
Δx is the positional offset from the midpoint of the detectors.

Although TOF does not pinpoint the exact annihilation site, it reduces the uncertainty along the line of response. This improves localisation and reduces statistical noise propagation during reconstruction.

The benefit of TOF increases with patient size because positional uncertainty contributes more significantly to image degradation in larger volumes.

Where this matters clinically

TOF PET improves image contrast and signal-to-noise ratio, particularly in oncology imaging and in larger patients. It can allow shorter acquisition times or lower administered activity while maintaining image quality.