Gamma camera with improved resolution

Cross-references to the related articles and patents

1. Optik and Atomphysik - von Pohl, Berlin - Goettingen - Heidelberg, 1963.

2. U.S. Patent No. 3,859,531.

3. Fiber Optics by Lisitsa, Israel Program for Scientific Translations Ltd.

N.Y. - Jerusalem - London, 1972.

4. Nuclear Medicine for Technicians by Robert C. Lange. Year Book

Medical Publishers, Inc.1973.

Field of the Invention

Background of the Invention

This invention relates generally to a gamma ray scintillation camera head and more specifically to an improved light guide pipe designed to increase spatial resolution in a radiation detecting apparatus.

Scintillation camera is commonly used to take pictures of the internal structure of patients or׳ articles being diagnosed.

It provides an image representing the radio-nuclide distribution of the selected radiation - emanating material administered to a patient to be diagnosed.

The head of a typical gamma camera comprises a scintillation crystal, a plurality of photodetectors, for example, an array of photomultiplier tubes and a light guide pipe which optically couples the crystal and the photodetectors.

A relatively large disc - like scintillation crystal is positioned to be bombarded by gamma radiation emitted by a patient. The crystal converts the gamma ray energy impinging on it to light energy. This light energy is in the form of light flashes or scintillations. Conventionally a thalium activated sodium iodide crystal is typically utilized. Since sodium iodide is highly hygroscopic, it is encapsulated within a hermetically sealed envelope. The ' light photons produced in the crystal are conducted through a light guide pipe to the photomultipliers' cathode. A plurality of photomultiplier tubes are positioned near the crystal for detecting the scintillationsand emitting an electrical signal in response. Then signals emitted simultaneously by the camera photodetectors are amplified and conducted to electronic circuitry.

..12

One of the limitations of the gamma camera is its spatial resolution, which refers to the ability to distinguish in the display system each of two close isolated scintillating points. Insufficient resolution in the output display of the camera is caused mainly by the statistical distribution process during photoelectrons generation, acceleration and multiplication in any photomultiplier tube caused by a scintillation event. The statistical distribution contributes some spread to the size of the resulting output voltage pulse from each photomultiplier tube. This pulse size is directly proportional to the amount of energy deposited in the crystal and depends on the distance between a scintillation event and any photomultiplier tube.

In a conventional gamma camera scintillation crystal is made of material having index of refraction n_?ry5 = 1.84. The crystal is glued to the glass light pipe having index of refraction of about 1.5.

The difference between two indices of refraction in the conventional gamma camera causes that considerable part of the light generated in the crystal will be scattered to distant photomultiplier tubes.

It is known to those skilled in the art that the more : light photons will be conducted to the photomultiplier's cathode the more resulting resolution of the gamma camera can be obtained.

Thus by avoiding the scattering of light from the event to the distant photodetectors and forwarding as more as possible light photons to the photomultiplier tubes adjacent to the event one can increase the amount of light entering the adjacent photomultiplier tubes. Note that the total amount of light generated by a single scintillation is a parameter (with a statistical distribution) dependent on the.energy of the specific x-ray, λ-ray, fast and thermal neutrons, protons, a and β-radiation and other particles.

Summary of the Invention

An object of the present invention is to overcome the defects of the prior art, such as indicated above and to provide a new and improved gamma camera which makes it possible to significantly increase the spatial resolution of the gamma camera.

According to the present invention the modified light coupling means is interposed between the photosensitive surfaces of an array of photodetectors and a scintillator crystal assembly.

The crystal is glued to the light coupling means which is made of a material having index of refraction n! which is no less than about 1.6 and preferably substantially higher. Note that glue may be chosen with an index of refraction n₃ about the same as of the light coupling means, i.e. n^ ־־ n₃.

Alternatively an adhesive and transparent substance as glue, such as optical compounds, optical grease, optical epoxy cement, mixtures of gradients of the above identified materials can be chosen.

The provision of the material for the light guide pipe having an index of refraction higher than in the conventional gamma cameras serves to alter the direction of light from a light event in the crystal, that is reduces the scattering of light to distant photomultiplier tubes thus enhancing the total amount of light reaching adjacent photomultiplier tubes.

Other and further objects of the present invention will be apparent from the following description and claims and are illustrated in the accompanying drawings which, by way of illustration, show > preferred embodiments of the present invention and the principles thereof and what we now consider to be the best mode in which we have contemplated applying these principles.

Other embodiments of the invention embodying the same or equivalent principles may be used and structural changes may be made as desired by those skilled in the art without departing from the present invention and the appended claims. .

Brief Description of the Drawings

Fig^ lisa diagrammatic illustration of a radiation image head of the type described;

Referring now to Fig. 2 there is shown a conventional cross-section of the major components of Anger-type detector head structure;

Fig. 3 is the same cross-section of the head but showing the embodiment of the present invention.

Detailed Description of the Invention

The detailed explanation of the invention is as follows :

Scintillation camera head consists of scintillator crystal 2, light coupling means in a form of light guide 1 and an array of photodetectors 3, for example, nineteen photomultiplier tubes arranged in a 3-4-5-4-3 hexagonal pattern as in conventional Anger-type gamma cameras (P8, P9, PIO, Pll, P12 in Fig. 2 and Fig, 3),

Light'coupling means 1 in the form of light guide pipe shown in Figures 2 and 3 as a single layer of material for the purpose of more easily illustrating the invention although it should be understood that in an actual gamma camera the light coupling means 1 may be in the form of a plurality layers of material of glass and plastic.

Light guide 1 is optically coupled to the photomultipliers' windows 6 by means of optical joint 7. In the embodiment shown in Fig. 2 the material which is chosen is the optical grease 7. In an alternate arrangement optical joint 7 can be of any thick, oily substance or lubricant.

Using simple considerations we will analyze the behaviour of the light propagating through different media and reaching the windows 6 of photomultiplier tubes 3.

Scintillation events occur in every part of the scintillation crystal's volume 2. Suppose that event A occures in the crystal 2 at the place which is adjacent to the photomultiplier tube 10; light emerges from point A to all directions at different angles of incidence. If the index of refraction of the light guide pipe 1 is less than the index of refraction of the crystal 2, then the light is spread basically as occurs in a diverging spherical lens (see Fig. 2). Up to the critical angle a_c, as is shown in Fig. 1, all the rays (for example, a ray having angle of incidence a!) will diverge further from the vertical axis Y when entering the light guide 1 medium. On the other hand part of the light rays with angles of incidence greater than a_c, for example, «2 in Fig. 1, will undergo total internal reflection, hit the lower surface of the crystal, and from there spread at all directions by the special diffusion paint made on the lower side of the crystal 2. So most of this light (what is observed in the conventional case) will pass eventually to the more distant photomultiplier tubes, for instance, to tube 12 in Fig. 2.

In the embodiment shown in Fig. 3 the material of the light guide 1 has an index of refraction higher than the index of refraction of the usual light coupling means, for example, the index of refraction is 1.7 instead of 1.5.

According to this modification of the head structure two phenomena occurs :

a. The spread of the light in the light guide 1 will be less than in the conventional case and more light will enter the adjacent photomultiplier tubes (10 in Fig. 3).

b. The critical angle of refraction a_c = arc sin becomes higher and so less light will undergo total internal ² reflection.

Thus it can be stated that the scattering of the light to the more distant photomultiplier tubes would be smaller than one can observe in the conventional gamma camera.

In an alternate arrangement the upper surface 5 of the light guide pipe 1 is roughened and optical grease 7 having index of refraction ng le^s than index of refraction n! of the light coupling means 1 is used (nc < n!). In this case light will diffuse and part of the light reaching the pnotomultiplier tube window with an angle of incidence higher than the critical angle will overcome the total internal reflection and penetrate to the tubed

Due to another embodiment of the present invention the index of refraction of the light guide 1 is even higher than that of the crystal 2 (for instance, n! = 1.95 and n2 = 1.84). The light will converge in the light guide pipe 1 (see Fig. 3) as basically occurs in converging spherical lens. In this case no total internal reflection occurs when light propagates from the crystal 2 to the light guide pipe 1, Note that in this case the index of refraction n^ of the photomultipliers' windows 6 and the index of refraction n5 of the optical grease 7 are conventional.

Due to another embodiment of the present invention the photomultipliers' windows 6 are made of material having index of refraction higher than of the conventional material. Then no total reflection from photomultiplier surfaces occurs and this modification gives the possibility to shift the light from the distant photomultiplier tubes to adjacent ones thus increasing the total amount of light reaching the photomultiplier tubes adjacent to the scintillation event.