Collimator aperture
The collimator diaphragm or the collimator (collector) lets gamma radiation that z. B. in nuclear medicine comes from a tumor enriched with technetium , only coming from certain spatial directions through to the measuring device. Collimator apertures are usually made of lead or tungsten .
In the case of non-imaging radiation measuring devices (such as scintillation probes, whole-body counters or Geiger counter tubes ), the radiation is often collimated by simple conical tubes or lamellas. In the case of a gamma camera , on the other hand, the collimator looks more or less like a perforated board, with the individual “holes” being separated by partitions. They ensure that only the perpendicular rays are allowed through the collimator, while obliquely incident photons are absorbed .
The septa length, which corresponds to the height of the collimator, and the septa width determine the suitability for different photon energies (low, medium, high and ultra high energy).
The sensitivity (measurement yield) depends on the ratio of hole width to septum width.
The shaft ratio (septa length to hole width) determines the permitted entry angle and thus the spatial resolution. This is therefore limited by the thickness of the septum (required for the respective photon energy). In general, a compromise between sensitivity on the one hand and resolving power on the other is always necessary with collimators. The smaller and longer the holes, the higher the resolution, but the worse the sensitivity, and vice versa.
Parallel hole collimators produce a parallel projection. The measurement yield is almost independent of the distance to the object, but the spatial resolution deteriorates enormously with increasing detector distance. Parallel hole collimators are either folded hexagonally from lead or tungsten sheet and soldered, or drilled, or cast. The holes are filled with radiolucent plastic. Slanthole collimators are parallel- hole collimators with oblique holes, e.g. B. to be able to measure past the shoulders closer to the head of a patient.
Diverging collimators provide an enlarged image. In fan beam collimators, the holes only diverge in one spatial direction. Since a larger crystal area absorbs the radiation emanating from a small object, the counting yield increases.
Pinhole collimators operate on the principle of thepinhole camera: A single "pupil" provides a reversed and upside continuous image whose magnification is highly dependent on the object distance. They are used to image particularly small, radiating objects (carpal bones, test animals, possibly also thethyroid gland), as they allow a strong magnification relative to the collimators mentioned above. More recently, multipinhole collimators have also been used, which increase the sensitivity without reducing the spatial resolution.
In positron emission tomography , septum collimators are often used to reduce singles (individually arriving photons) and random coincidences ( random ), which means that coincidence lines that run approximately axially contribute to the measurement result, which reduces the counting yield ("2D method").
