What is a collimator in nuclear medicine?
A collimator is a lead device mounted in front of the gamma camera detector that allows only gamma photons travelling in specific directions to reach the scintillation crystal. Its purpose is to provide spatial localisation of detected photons.
A collimator mechanically filters gamma photons by direction, enabling spatial localisation at the expense of sensitivity.
Because gamma photons are emitted in all directions from within the patient, the detector alone cannot determine their origin. The collimator acts as a directional filter, blocking most photons and permitting only those travelling approximately parallel to its holes to pass through.
Although essential for image formation in SPECT and planar imaging, collimators dramatically reduce system sensitivity because the vast majority of emitted photons are absorbed.
Understanding the physics
Gamma photons are electrically neutral and cannot be focused by lenses or magnetic fields. Therefore, mechanical collimation is required to determine the direction of incoming photons.
A typical parallel-hole collimator consists of many small, closely packed holes separated by lead septa. Only photons travelling nearly parallel to the holes can pass through and reach the detector. Photons arriving at oblique angles strike the septa and are absorbed.
This directional filtering allows the gamma camera to infer the approximate location of photon emission within the patient.
Collimator performance is defined by several physical characteristics:
Hole diameter
Hole length
Septal thickness
Hole geometry (parallel, converging, diverging, pinhole)
There is a fundamental trade-off between spatial resolution and sensitivity:
Smaller holes and longer hole length improve spatial resolution because they restrict angular acceptance.
However, this reduces sensitivity because fewer photons pass through.
Larger holes improve sensitivity but reduce resolution.
Collimators are typically optimised for specific photon energies. For example, low-energy high-resolution (LEHR) collimators are designed for Tc-99m at 140 keV. Higher-energy photons require thicker septa to prevent septal penetration.
Where this matters clinically
Collimator selection significantly influences image sharpness, noise, and acquisition time. Choosing the wrong collimator for a radionuclide can degrade image quality or increase radiation dose. Understanding collimator design is central to optimising SPECT imaging.