Light output optimization

Details Light output optimization Light output optimization

2000-09-12

2002-04-09

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

With or including a luminophor

Reexamination Certificate

active

06369391

ABSTRACT:

FIELD OF THE INVENTION
The present invention pertains to the field of medical diagnostic imaging and more particularly to the optimization, both quantitatively and qualitatively, of light output from scintillators used in such imaging.
BACKGROUND OF THE INVENTION
Many medical imaging devices such as gamma cameras use a scintillation crystal to detect the radiation that is processed to provide images of the interior of patients. Radiation, such as gamma radiation, that has passed through or out of a patient being imaged impacts the scintillator generating scintillations, i.e. flashes of light. Light sensors such as photomultiplier tubes (PMTs) are optically coupled to the scintillation crystal, generally through transparent means such as a glass plate. The flashes of light which are sometimes referred to as “an event”, are detected by the PMTs and converted into electrical pulses. The electrical pulses are processed to provide the images. Thus the scintillation generated light is at the heart of the imaging by devices such as gamma cameras.
In practice, when an event occurs, the light from the scintillation crystal strikes and illuminates an area of the glass coupling the scintillation crystal to the PMTs. The location of the event needed to provide an image is computed with algorithms that use the outputs of the PMTs contiguous to illuminated areas. The accuracy of the location determination, which also effects the image uniformity, is maximized when a plurality of PMTs are illuminated by the event. However, for those portions of the image that are over a single PMT, the events will appear to be bunched at the center of the PMT. For those portions of the image that are between PMTs, light is lost. Both of these effects result in reduced energy and/or position resolution and/or increased noise.
It has been and still is an object of scientists in the field to capture as much of the light as possible and to assure that the light is spread so as to illuminate a plurality of PMTs. However, it is important that the light be distributed in a manner that allows for the determination of the position of the event that caused it.
In many applications in which high energy gamma rays are utilized, the absorbance of the rays by the crystal is poor. To increase the gamma ray capture, a thicker crystal is used. However, such a thicker crystal results in reduced resolution in the image.
When the crystal scintillates, photons of light are transmitted in all directions. Accordingly, much of the light is never sensed by a PMT and is lost. This is exacerbated by the fact that the high refractive index of the crystal results in its acting as a light guide which distributes the light far from the nearby PMTs. To increase the amount of light that is sensed by the PMTs, in the prior art, reflective materials are placed on the surface of the crystal opposite to the surface coupled to the PMTs. The reflective materials capture the light that is emitted in the direction away from the PMTs and transmit it back to the PMTs. In general, this is done by reflection surfaces that are attachments to the crystal. In general, these materials may be specular or diffusive reflectors and are sometimes distributed to improve the distribution of light from events such that better resolution of the positions of the events can be maintained.
Nevertheless, the thickness of scintillator crystals is limited by resolution effects such that much of the radiation in high energy gamma imaging is lost.
U.S. Pat. No. 5,763,887 also shows reflection surfaces that are tailored to cause the light to be reflected towards the PMTs in a manner which improves the event location property of the detector. This patent describes the use of holographic reflectors or light directors that are situated on one or both sides of the crystal. Due to the difficulty in coupling light out of the surface of the crystal, this solution appears to be of limited utility.
It is known from “Performance of a position-sensitive scintillation detector” by J. S. Karp, et al. (Phys. Med. Biol. 1985, Vol. 30, No. 7, pp. 643-655, to provide, in one dimensional detectors, one dimensional surface depressions uniformly spaced on the surface of a scintillation crystal to increase the amount of light transferred to the detectors and to increase the resolution of the detector. However, despite the nearly 14 year that have passed since the publication of this paper, no practical utility in either commercial detectors or in two dimensional detectors has been made.
BRIEF DESCRIPTION OF THE INVENTION
To overcome the faults of the light directing plates that are presently being used, one aspect of some preferred embodiments of the present invention comprises operating on the scintillation crystal per se to intrinsically provide the light direction control desired, using combinations of reflection, refraction, diffraction, or transparency. As used herein, the term “intrinsic” when applied to a scintillator of to functions of a scintillator, means that at least part of the function or structure referred to is part of the body or surface of the scintillator and not completely external to the outer surface of the scintillator.
Thus, some preferred embodiments of the invention provide scintillator crystals that includes intrinsic light direction controllers for optimizing the light output of the scintillator quantitatively and qualitatively. More particularly the light direction controllers include directive reflection surfaces, such as grooves, on the surfaces of the scintillation crystal, pyramids integral to the surfaces of the crystals, cones intrinsic to the surface of the crystals, or crystal surfaces for providing reflection from the crystal surfaces. The crystal surfaces are preferably shaped for directing the light by machining, etching, embossing or forging the surfaces of the crystal.
Thus, this aspect of the invention contemplates modifying the scintillation crystal itself so that it provides the desired light direction control. To this end the surfaces of the crystals are tailored so that the light illuminated areas of the detector are not directly beneath a PMT. The area directly under the PMTs is tailored to reflect light or to diffuse the light so that a greater portion of it strikes the surrounding PMTs rather than being captured for the most part by a single PMT. Thus in general the crystal is modified so that it directs the light towards the side of crystal facing the PMT and in addition directs the light to a group of PMTs rather than to single PMTs.
Furthermore, to avoid loss of light between PMTs, in some preferred embodiments of the invention, the surface of the crystal is formed to redistribute light which would have been lost between the PMTs so that the light is captured by a group of PMTs.
There are certain cases where it is desired to isolate the light so that its spread is reduced, so that it strikes only a small group of PMTs. The invention contemplates modifying the scintillator crystal in the manner which will also accomplish this isolation of the emitted light. For example, when a thick crystal is used focusing can reduce the light spread such that the spread is reduced to that of a scintillator half as thick. This allows for the use of thicker scintillator crystals (with higher capture efficiency) with the resolution of thinner crystals.
The direction of the light is controlled by machining such as by engraving or by pressing, or by embossing or even by etching the crystal to have a multiplicity of intrinsic small pyramids or rings or other forms on the surfaces thereof to direct the light as desired. Thus, for example, the pyramids can be located in defined areas and absent from other defined areas or replaced by grooves or cones in other defined areas. Thus, the pyramids can be located in a checkerboard type configuration wherein certain sections of the surfaces of the crystal have pyramids thereon other sections have simple diffraction or reflection type surfaces, and still other surfaces enable light to pass directly therethrough.
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