What determines spatial resolution in gamma camera imaging?

Spatial resolution in gamma camera imaging is determined primarily by the collimator design, as well as the intrinsic resolution of the detector system and the distance between the patient and the collimator.

For parallel-hole collimators, spatial resolution depends on hole diameter and hole length. Smaller holes and longer hole lengths improve resolution but reduce sensitivity. Resolution also worsens as the distance between the source and the collimator increases.

Spatial resolution in gamma camera imaging is mainly determined by collimator geometry and source-to-collimator distance, with intrinsic detector resolution playing a secondary role.

Although the scintillation crystal and electronics contribute to intrinsic resolution, in most clinical situations the collimator is the dominant factor limiting spatial resolution.

Understanding the physics

Spatial resolution refers to the system’s ability to distinguish two closely spaced sources of radioactivity as separate.

In gamma camera imaging, resolution has two main components:

Collimator resolution
Intrinsic resolution

Collimator resolution

The collimator restricts photon angles, but it cannot perfectly determine the exact origin of each photon. Photons passing through a hole may have originated anywhere along a small cone-shaped region defined by the hole geometry.

For a parallel-hole collimator, spatial resolution improves when:

Hole diameter is smaller
Hole length is longer
Source-to-collimator distance is reduced

Resolution degrades linearly as the source moves further from the collimator because the angular uncertainty translates into greater positional uncertainty.

This is why positioning the detector as close to the patient as possible improves image sharpness.

Intrinsic resolution

Intrinsic resolution is determined by:

Crystal thickness
Light spread within the crystal
Photomultiplier tube size and arrangement
Positioning electronics

However, intrinsic resolution is typically better than collimator resolution for common radionuclides such as Tc-99m, meaning the collimator is usually the limiting factor.

Combined system resolution

Overall spatial resolution is a combination of intrinsic and collimator resolution. In practice, collimator geometry dominates system performance.

Where this matters clinically

Poor spatial resolution reduces the ability to detect small lesions. Bringing the detector closer to the patient improves resolution. Collimator choice directly affects lesion detectability and image clarity in SPECT and planar imaging.