Summary
Highlights
The lecture continues the discussion on gamma cameras, focusing on different collimator types and their applications in nuclear medicine imaging. The main collimators to be covered are pinhole, multi-pinhole, converging, and diverging collimators, with a comparison of their spatial resolution, sensitivity, and magnification.
Different collimators result in varying magnification ratios of the image to the object. Parallel-hole collimators maintain the ratio, while pinhole and converging collimators magnify the image (ratio > 1). Pinhole collimators also invert the image. Diverging collimators minify the image.
Pinhole collimators, originally used for small objects like the thyroid, had limited sensitivity with a single hole. However, multi-pinhole systems have significantly improved sensitivity and are now used for small animal, dedicated brain, and cardiac imaging, enabling tomography. Examples include the NM530c scanner with semiconductor detectors and dedicated brain imaging systems.
Converging collimators magnify the image, similar to pinhole collimators, but traditionally had limited sensitivity due to focusing on a small area. They were less suitable for whole-body imaging due to time constraints. There are two types: fan beam (convergence in one dimension) and cone beam (convergence in both dimensions). Sensitivity can be higher than parallel-hole, but only for a small region.
Diverging collimators minify the image. Their primary use was in older systems with small detectors to visualize a large object, such as the whole body, in a single scan. However, they suffer from limited spatial resolution, and with larger modern cameras, their use has diminished.
Resolution generally degrades (increases in full width at half maximum) for all collimator types as the object-to-collimator distance increases. Sensitivity behaves differently: it's constant for parallel-hole, improves with distance for converging, and worsens with distance for diverging and pinhole collimators. Field size changes are also observed: constant for parallel-hole, decreasing for converging, and increasing for diverging with distance. Magnification increases for pinhole and converging, and decreases for diverging.
Converging collimators produce a magnified image with a decreasing field of view with distance. Diverging collimators produce a minified image with an increasing field of view. Pinhole collimators generate a magnified, inverted image. High-sensitivity collimators have wider holes, while high-resolution collimators have smaller or longer holes.