What is the resolution–sensitivity trade-off in gamma camera imaging?
The resolution–sensitivity trade-off in gamma camera imaging refers to the fundamental relationship in which improving spatial resolution reduces sensitivity, and improving sensitivity reduces spatial resolution. This trade-off is primarily determined by collimator design.
Improving spatial resolution in gamma camera imaging inevitably reduces sensitivity, because both are governed by the same collimator geometry.
Small collimator holes and long hole lengths improve spatial resolution by restricting photon angles more tightly. However, this allows fewer photons to reach the detector, reducing sensitivity. Conversely, larger holes increase sensitivity but permit greater angular uncertainty, degrading spatial resolution.
Because mechanical collimation absorbs most emitted photons, this trade-off is unavoidable in conventional gamma camera systems.
Understanding the physics
In planar and SPECT imaging, spatial localisation depends on mechanical collimation. A parallel-hole collimator only allows photons travelling approximately parallel to the hole axis to reach the detector.
To improve resolution, the angular acceptance must be narrowed. This is achieved by:
Decreasing hole diameter
Increasing hole length
Both measures restrict the range of accepted photon trajectories. However, narrowing the accepted angle dramatically reduces the number of photons that pass through the collimator.
Sensitivity depends on the geometric area through which photons can pass. Larger holes increase geometric acceptance and therefore increase count rate. But wider angular acceptance increases positional uncertainty, degrading image sharpness.
Mathematically, sensitivity increases roughly with the square of hole diameter, while resolution improves inversely with hole diameter. Because both depend on the same geometric parameters, they cannot be optimised simultaneously.
This is why different collimator types exist:
High-resolution collimators: small holes, long septa = sharper images, lower counts.
High-sensitivity collimators: larger holes = higher counts, blurrier images.
The choice depends on the clinical task.
Where this matters clinically
In low-count studies or large patients, higher sensitivity may be necessary to maintain acceptable noise levels. In small-lesion detection, higher resolution may be prioritised. Understanding this trade-off guides collimator selection and protocol optimisation.