Why does PET not require a collimator?

PET does not require a physical collimator because it uses electronic collimation through coincidence detection. When a positron annihilates with an electron, it produces two 511 keV photons that travel in nearly opposite directions. If two detectors positioned around the patient detect these photons simultaneously, the system assumes the annihilation occurred somewhere along the straight line connecting the detectors.

PET uses coincidence detection as electronic collimation, eliminating the need for a physical collimator and greatly increasing sensitivity.

This line, known as the line of response (LOR), provides directional information without the need for lead septa or mechanical collimation. As a result, PET achieves much higher sensitivity than gamma camera systems used in SPECT imaging.

By eliminating the physical collimator, PET detects a much greater proportion of emitted photons, significantly improving count efficiency and image quality.

Understanding the physics

In conventional gamma camera imaging, photons are emitted in all directions. A physical collimator is required to restrict detection to photons travelling along specific paths, otherwise image localisation would be impossible. However, collimators absorb the vast majority of photons, drastically reducing sensitivity.

PET imaging is fundamentally different. Because positron annihilation produces two photons emitted in opposite directions, detecting both photons simultaneously provides intrinsic directional information. If two detectors register 511 keV photons within the coincidence timing window, the annihilation event must have occurred along the line joining those detectors.

This process is called electronic collimation. Instead of absorbing off-axis photons, PET uses timing information to infer geometry.

The absence of a physical collimator leads to:

Dramatically increased sensitivity
Improved signal-to-noise ratio
Higher count rates

However, PET resolution is still limited by factors such as positron range and non-collinearity, rather than by collimator geometry.

Where this matters clinically

The lack of a physical collimator explains why PET imaging is more sensitive than SPECT and why PET studies can achieve higher contrast resolution. It also explains differences in radiation dose and scanner design between PET and gamma camera systems.