Summary
Highlights
Collimators are crucial in gamma cameras to ensure photons from radiopharmaceuticals enter the camera perpendicularly, allowing for accurate image reconstruction of the tracer distribution within the patient. Without collimation, determining the origin of detected signals would be extremely difficult, leading to unclear images.
Collimators are typically made from lead foils or cast lead. The walls between the holes are called 'septa.' The design aims to prevent septal penetration, where photons pass through the septa instead of the holes. More than 95% of events should be stopped by the septa. Different manufacturing methods exist, such as shaping and attaching lead foils.
This lecture focuses on parallel-hole collimators, which are the most common in nuclear medicine imaging. Other types like converging, diverging, and pinhole collimators will be discussed later. The design parameters include hole diameter, septal thickness, and septal length. These parameters critically influence the sensitivity and spatial resolution of the imaging system.
Improving resolution (making it smaller) typically involves using smaller diameter holes, which decreases sensitivity due to increased lead and more rejected events. Conversely, increasing sensitivity (e.g., by using wider or shorter holes) degrades spatial resolution. Resolution and sensitivity are competing factors in collimator design. Higher energy isotopes require thicker septa to prevent penetration, which further impacts sensitivity.
Collimator blurring occurs due to a narrow acceptance angle for photons. While some events are properly collimated, others are absorbed by the septa or scatter within them, leading to undesirable events. Septal penetration, although minimized by design, can also occur. The length of the septa directly influences the acceptance angle and thus sensitivity.
The distance of the source from the collimator (f) significantly impacts resolution; a larger 'f' worsens resolution. Getting closer to the collimator improves resolution. Furthermore, smaller hole diameters and longer collimator lengths (l) lead to better (smaller) collimator resolution. Septal thickness primarily serves to block photons from penetration rather than directly improving resolution.
Collimator sensitivity is influenced by hole diameter (d), septal length (l), overall crystal diameter, intrinsic efficiency, and septal thickness (s). Sensitivity is roughly proportional to d^2 and inversely proportional to l^2. Increasing hole diameter, decreasing septal length, or decreasing septal thickness generally improves sensitivity. Unlike resolution, sensitivity in parallel-hole designs does not significantly change with source distance (f).
For higher energy isotopes, septal thickness must be significantly increased to prevent penetration. To compensate for the resulting loss in sensitivity, hole diameters are typically made larger. However, this then degrades resolution, leading to a complex interplay of design choices to balance sensitivity and resolution for specific applications. High-energy collimators are also heavier due to more lead.
Collimators are typically made of lead with hexagonal holes and septa between them. Parallel collimators project the same object size on the camera, and the field of view does not change with distance. However, camera resolution degrades with distance. Higher energy radionuclides necessitate high-energy collimators with thicker septa.
A poll question illustrates the inverse relationship between resolution and sensitivity. If one collimator has twice worse spatial resolution than another, its sensitivity will be approximately four times better. This highlights the constant compromise engineers face when designing collimators.
Limited injected activity and scan duration in clinical settings are practical deterrents to achieving superb spatial resolution in gamma cameras. While these factors don't directly impact resolution, they indirectly dictate the design of collimators to prioritize decent sensitivity, which inherently restricts the achievable spatial resolution. If unlimited counts or scan times were available, collimators could be designed for much higher resolution.