System for installing a collimator in a gamma camera

Details System for installing a collimator in a gamma camera System for installing a collimator in a gamma camera

1991-06-07

1993-05-25

Howell, Janice A.

Radiant energy

Invisible radiant energy responsive electric signalling

With or including a luminophor

378147, G01T 1164, A61B 600

Patent

active

052142876

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention relates to a system for installing a collimator in a gamma camera. It is intended for use particularly in medical imaging. A gamma camera is an assembly of equipment: detector, stand, and console, enabling nuclear medicine examination to be performed. The collimator of a gamma camera is fixed to the detector.
2. Discussion of the Background
A gamma camera is conventionally used as follows. Patients subjected to examination on such apparatus have a tracer containing a radioactive isotope injected into their bodies. The isotope fixes itself preferentially in a particular organ depending on the type of tracer injected. The function of the gamma camera is to form the image of the plane projection of the gamma radioactivity fixed by said organ, thus revealing its functional state. The image is obtained by detecting gamma radiation coming from the organ under examination and emitted in a single direction so as to project the concentration of the isotope in the organ in said direction. A gamma camera thus comprises a gamma radiation detector constituted, in particular, by a large-area scintillator crystal covered with a network of photomultipliers. When a gamma ray passes through the scintillator, it produces local scintillation which is detected by a plurality of photomultiplier tubes situated in the vicinity of the scintillation. The photomultipliers provide electrical current proportional to the received scintillation. This makes it possible to locate the impact of gamma rays in the scintillator crystal by evaluating the center of gravity of the signals delivered by the photomultipliers.
A device called a "collimator" situated immediately in front of and against the detector is used to select for detection only those gamma rays that are emitted in a single direction, e.g. a direction perpendicular to the inlet face of the scintillator.
In outline, a collimator is an absorbent plate, generally made of lead, which is pierced by a multitude of ducts that pass gamma rays only over a very small solid angle whose axis is parallel to the desired projection direction for the image of radioactive isotope distribution that is to be produced.
In practice, collimators are designed as a function of the energy of the gamma radiation emitted by the isotope injected into the patient and as a function of the desired transparency and spatial resolution characteristics that are to be obtained. In general terms, spatial resolution and transparency depend on the size of the solid angle of the collimator ducts, and it is clear that these two characteristics vary in opposite directions. Depending on the types of examination performed using nuclear medicine, use is made of substances that emit gamma rays at different energies, or else specific resolution and transparency characteristics are required for the collimators. Under such circumstances, a gamma camera is generally fitted with a family of collimators having different geometrical characteristics. In routine operation of the gamma camera, it must be possible to swap the collimator associated with the detector of the camera for a particular examination with another collimator at the request of the operator performing the examination.
The thickness, or in practice the weight, of the collimator depends on the energy of the radiation to be detected and on the looked-for solid angle characteristics. In addition, detectors are generally cantilevered out from a bracket. The unbalance due to the cantilevered detector is normally compensated by counterweights disposed in the mechanism for moving or handling the detector. Since it is not easy to change these counterweights, it has become the practice to associate peripheral masses with lightweight collimators (collimators for low-energy radiation), thereby ensuring that they weigh as much as heavier collimators. When the working weight of the collimators may lie in the range 20 kg to 60 kg, this technique consists in providing collimators which, together with their asso

REFERENCES:
patent: 3982133 (1976-09-01), Jupa et al.
patent: 4629893 (1986-12-01), Hanz et al.

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