US3683180A - Scintillation camera with removable subassembly - Google Patents

Abstract

A gamma imaging camera with a scintillation crystal and a plurality of phototubes. A light pipe optically couples the crystal and the tubes. The light pipe has, at its output end, a plurality of truncated pyramids each associated with one of the phototubes. As disclosed, the walls of the pyramids are hexagonal in cross-sectional configuration and flare outwardly to intersect adjacent walls, thus providing a continuous field of view. The disclosure also shows a lift-out detector head subassembly construction.

Description

United States Patent Martone et al.

SCINTILLATION CAMERA WITH REMOVABLE SUBASSEMBLY Inventors: Ronald J. Martone, Cheshire; Peter G. Mueller, Guilford; Richard J. Flis, Plantsville, all of Conn.

Picker Corporation, White Plains, N.Y.

Filed: June 16, 1969 Appl. No.: 833,552

Assignee:

U.S. Cl ..250/7l.5 S, 250/71.5 R, 250/79, 250/83.3 R, 250/239 Int. Cl ..G01t 1/20 Field of Search ..250/71.5 S, 71.5 R, 83.3 HP, 250/239, 77, 79, 80, 213; 313/65 S; 350/96 References Cited UNITED STATES PATENTS Anger ..250/71.5

[ 1 Aug. 8, 1972 Packard ..250/71.5 S Keck et a1 ..250/71.5 S

Primary Examiner-James W. Lawrence Assistant Examiner-Morton J. F'rome Attorney--Watts, l-loffmann, Fisher & Heinke [57] ABSTRACT A gamma imaging camera with a scintillation crystal and a plurality of phototubes. A light pipe optically couples the crystal and the tubes. The light pipe has, at its output end, a plurality of truncated pyramids each associated with one of the phototubes. As disclosed, the walls of the pyramids are hexagonal in cross-sectional configuration and flare outwardly to intersectadjacent walls, thus providing a continuous field of view.

The disclosure also shows a lift-out detector head subassembly construction.

6 Claims, 6 Drawing Figures CROSS REFERENCES TO RELATED PATENTS AND APPLICATIONS tector Indicating System issued Oct. 6, 1970 as Pat.

4. Application, Ser. No. 837,072, filed June 27, 1969 by Robert Hindel et al., entitled Radiation Image Apparatus.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention pertains to gamma imaging devices and more particularly to that class of device known as scintillation cameras.

In the diagnosis of certain illnesses, radioactive isotopes are administered to patients. Many administered isotopes have the characteristic of concentrating in certain types of tissue and either not concentrating in or concentrating to a lesser degree in other types of tissue. For example, iodine l3l collects in thy- I roid glands. A graphic image produced to show the spatial distribution and concentration of this isotope in the thyroid gland provides an image of the thyroid gland itself. This image is useful in diagnosing a patients physical condition.

2. Summary of the prior art Generally speaking, the devices used for producing graphic images of the distribution of an isotope in a subject are known as scanners and cameras. With a scanner, a scintillation probe is moved rectilinearly along a plurality of spaced parallel paths. The energy detected is utilized to make either a photographic or a dot image reflecting the spatial distribution and concentration of the isotope in the subject. A clinically successful scanner is described in greater detail in the above-referenced US Pat. RE 26,014 to J. B. Stickney et al.

The devices known as cameras remain stationary with respect to the patient as the graphic image of the spatial distribution of an isotope is developed. Many cameras use an instrument where a relatively large disc-like scintillation crystal is positioned to be bombarded by gamma radiation emitted by a patient. With most cameras, a collimator is interposed between the patient and the crystal. 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. In one class of camera, a thalium-activated sodium iodide crystal is typically utilized. Since sodium iodide is 6O highly hygroscopic, it is encapsulated within a hermeti- Signals emitted simultaneously by the camera phototubes are amplified and then conducted to electronic circuitry. The preferred circuitry is described in greater detail in the referenced applications. This circuitry includes a pulse-height analyzer to determine a whether or not the signals in question reflect the occurrence of a so-called photopeak event. Summing and ratio circuits are included which. result in the signal being sent to an oscilloscope to cause a light signal to be emitted by the oscilloscope. The objective is that the oscilloscope signals be displaced relatively each at a location corresponding to the location of a corresponding scintillation in the crystal.

In a scintillation camera of the described type, a material forming an optical coupling is between the crystal and the phototubes.

In early cameras, mineral oil was used as this optical coupling. While this is an inexpensive procedure for providing a light coupling for an experimental machine, it has disadvantages. The disadvantages include: (a) it obviously requires careful sealing to prevent leakage onto the patient or elsewhere; (b) it presents filling problems to obtain a sufficient quantum of mineral oil in a cavity so that no matter what attitude the head is positioned in, there will be an effective light coupling between the crystal and each of the phototubes; and, (c) it has other disadvantages which will become apparent.

Other cameras have been built with light pipes of solid materials. These have been configured with frustoconical shaped portions each axially aligned with a different one of the photo-tubes. These frustrums are difficult and relatively expensive to machine. The array of conical frustrums results in some blank areas of generally triangular configuration. The boundaries of these blank areas were each defined by three of the frustrums or by two frustrums and the perimeter of the light pipe.

With these prior art devices, attempts have been made to assure the conduction of as much light energy as possible to the phototubes. In fact, the attempt has been to assure that each photopeak event occurring in the crystal was delivered to the phototubes and resulted in an output signal which passed through a pulse-height analyzer or discriminator as the case might be. To assist in assuring this total reproduction of all photopeak events, prior cameras have utilized reflective surfaces in these blank areas and around the perimeter of the light pipe.

SUMMARY OF THE PRESENT INVENTION With the present invention, the crystal envelope includes a polished glass window on the photomultiplier tube side of the crystal. The window is in tight, lightconducting, intimate contact with the crystal. A light pipe of thermoplastic poly (methyl methancrylate)- type polymers in cast sheet which is sold by Rohm & Haas Company, Philadelphia, Pa., under the trademark Plexiglas has a planar input surface which is optically coupled to the glass. The light pipe has an output surface opposite the glass and adjacent the phototubes. The output surface is machined or otherwise formed to provide a plurality of frustrums of pyramids. There is one pyramid frustrum for each phototube. Each pyramid frustrum is, in cross-sectional configuration, hexagonal eliminating blank areas among the frustrums characteristic of prior solid light pipes.

The side walls of the pyramids are juxtaposed. Thus, a central field of view of the light pipe is continuous and uninterrupted.

It has been discovered that producing light flashes on an oscilloscope which accurately reflect the true location of a scintillation in a crystal is dependent, especially near the periphery of the crystal, on the optical coupling between the crystal and the phototubes. All top and side wall surfaces of the light pipe, other than the tops of the pyramids which are juxtaposed with the phototubes, are covered with a black coating for light absorption. To prevent light absorption in the pyramids, and to provide reflection, a white coating is applied to the walls of the pyramids prior to the black coating application. With this technique, scintillations perimetrally of the field of view of the light pipe are absorbed rather than reflected back into the field.

While this light absorption results in enough light energy from certain photopeak events being absorbed that they are rejected by the circuitry, nonetheless, a very material advantage is produced. This material advantage is that distortions which have occurred in perimetral portions of prior art cameras of this type are reduced and therefore the useful field of view of the instrument is greatly enhanced. With a 13.5-inch diameter crystal, it has been found that the useful field of view of approximately 12 inches in diameter can be obtained through the use of the optical coupling of this invention as compared with useful filed of view of the order of 9 to inch diameter of prior art devices using crystals of l l to 12 inch diameter.

Another advantage of the present invention is the provision of a lift-out subassembly. This subassembly includes the crystal, the light pipe and the array of phototubes. Accordingly, when service is required such as for replacing phototubes of the array, the entire subassembly can be removed from the head and a new subassembly inserted. This permits the camera to be placed back in use with nominal delay and the replacement of the phototubes to be performed at another location and at leisure.

Other objects and a fuller understanding of the invention may be had by referring to the following description and claims taken in conjunction with the drawings.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a plan view of a camera and associated consoles utilizing the novel detecting head of this invention;

FIG. 2 is a fragmentary, somewhat schematic, top plan view of the detector subassembly of this invention;

FIG. 3 is a sectional view of the detector subassembly of this invention as seen from the planes indicated by the lines 33 of FIG. 2;

FIG. 4 is a perspective view of the novel and improved light pipe of this invention;

FIG. 5 is an enlarged fragmentary view of a portion of the light pipe and a phototube; and,

FIG. 6 is a sectional view, on another scale, of the detector head assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, a detector head is shown generally at 10. The head is adjustably mounted on a stand 1 1 for positioning adjacent a patient or other subject. Electrical signals from the head 10 are conducted to circuitry contained within a console shown generally at 12.

The signals, after processing by the circuitry, produce a graphic image of the subject under investigation on a monitor oscilloscope 13. A duplicate image is produced on a camera oscilloscope cathode ray tube, not shown, which is viewed and photographed by a camera 14.

The circuitry in the console 12 first produces analog signals in manners more completely described in the referenced applications. Assuming the analog signals represent photopeak events, they are digitized. The digital signals may be fed to a computer for analysis and diagnosis.

The digital information is also fed to a build-in digital data processor 15. This processor utilizes the digital information to generate either a variable width profile histogramof counts versus horizontal distance or a histogram of counts versus time. Such histograms are displayed on a monitor 17.

The digital information is also fed to a tape recording console shown generally at 19 for storage and subsequent utilization. The digital information is reconverted to analog to produce the images displayed on the monitor oscilloscope 13 and recorded by the camera 14.

The imaging subassembly in this invention is shown in FIGS. 2, 3, and 6. The subassembly is mounted in a housing 21. The subassembly includes a large scintillation crystal 20 of thallium-activated sodium iodide. A parallel hole collimator 22 is removably secured to the housing 21, FIG. 6. Either a parallel hole or a pinhole collimator is interposed between the crystal and the pa- 7 tient so that the gamma rays striking the crystal are all travelling in determinable directions. The housing and the perimetral portions of the collimator are formed of shielding material such as lead so that essentially the only radiation which reaches the crystal has travelled along a determinable path through the collimator.

An input window 23 is provided. The window 23 is opaque to light but substantially transparent to gamma radiation. This input window is typically an aluminum disc fixed to and hermetically sealed to a surrounding crystal supporting ring 24. A glass output window 25 is carried by the crystal supporting ring 24. The crystal 20 and the input and output windows 23, 25 and its supporting ring 24 constitute a component which is commercially available from the Harshaw Chemical Co. of Cleveland, Oh.

The window component is secured to a supporting ring 27 by suitable fasteners 28. The supporting ring 27 is connected to a crystal assembly support ring 29 by an annular spacer 30.

A light pipe 32 is provided. The light pipe 32 has a planar input face 33 that is optically coupled to a polished, planar output face 34 of the output window 25.

A plurality of. phototubes 35 are provided. The phototubes 35 are arranged in an array with a total of 19 such tubes being provided. The phototubes 35 have input windows 36 which are juxtaposed against the light pipe 32 in a manner which will be described in greater detail presently. Suitable conductors, not

shown, couple the phototubes 35 to the circuitry in the console 12.

Apertured tube locator and cover plates 38, 39 are provided. The tube locator plate 38 is interposed between and spaced from the cover plate and the light pipe. Spacers 40 are interposed between the cover and tube locator plates to maintain the appropriate spacing between them. Fasteners 41, FIG. 2, connect the plates 38, 39 together with the spacers 40 therebetween.

Annular bushings 42 are provided. Each of the bushings 42 surrounds a corresponding one of the phototubes. The bushings 42 are interposed between the cover plate 39 and the phototubes 35. The bushings are compressible and, on clamping of the assembly together in a manner which will be described presently, bias each of the tubes 35 into surface engagement with an appropriate portion of the light pipe 32. v

A spacer cylinder 43 and a plurality of studs 43a are provided. The studs 430 project through the cover plate 39 and are secured to the crystal assembly support ring 29 as by screws 44, one of which is shown in FIG. 3. The cover plate 39 and the crystal assembly support ring 29 are clamped against the spacer cylinder 43 by tightening down suitable nuts 46 on the studs 43a. This fixes the entire lift-out phototube assembly together with the phototubes 35 in closely juxtaposed relationship and good optical coupling with the light pipe 32 and the light pipe in turn optically coupled to the glass window 25.

To facilitate replacement of or repair of the phototube subassembly, a pair of lift-out handles are provided. One of these lift-out handles is shown at 48 in FIGS. 2 and 3. The lift-out handles are threaded onto selected pairs of studs 43a at diametrically opposite locations. When the subassembly is to removed, housing cover 21a is removed. Anchor studs 45, FIG. 6 are removed. Next the handles are raised to the position shown in FIG. 3 and the entire subassembly is lifted out of the housing 21.

Referring now to FIGS. 4 and 5, thelight pipe 32 is machined to provide a total of 19 frustrums of pyramids 52. The pyramids 52 each terminate at a pyramid output face 53. Each pyramid output face is flat and designed to optically couple with an associated input window 36 of a phototube 35. The pyramid output faces 53 are in a common plane parallel to the light pipe input face 33.

The pyramid frustrums 52 are hexagonal in crosssectional configuration with six planar wall surfaces 55.

These walls 55 are formed by milling grooves 56 in a surface of the light pipe 32. While the grooves 56 can be formed such that the surfaces 55 intersect to delineate a sharp V; for convenience of manufacture and for optical reasons which appear to produce improved performance for reasons that are not fully understood, the grooves are milled such that the base of each groove is curved in configuration. Thus, each pyramid wall has a portion 55a which is at an obtuse angle with respect to its associated output face 53. Each wall 55, other than perimetral ones of said walls, parallels an adjacent wall 55 of another pyramid. The connection of these adjacent walls is by a curved portion 55b which is concave in configuration as viewed from the phototube side of the light pipe. Thus, the walls of the pyramid, while they may be curved in a plane of cross section normal to the input face 33, are straight in planes of cross section paralleling the input face. Expressed another way, each pyramid wall flares outwardly until it intersects an adjacent pyramid wall to provide total coverage in the field of view of the pyramids. The pyramids are uniformly spaced and sized.

The surfaces 55 delineating the sides of the 19 pyramids are coated with a white reflective material indicated at 57, FIG. 5. Side wall 581 of the light pipe and the entire top surface of the crystal 32, other than the pyramid faces 53, are then coated with a black coating indicated at 59. For clarity of illustration, The white and black coatings 57, 59 are shown in greatly exaggerated thickness in FIG. 5.

The dimension from wall to wall, diametrically across the top of an output face 53 of a pyramid, as measured perpendicularly to the walls, is equal to the diameter of a photocathode, not shown, of any one of the phototubes. This assures that the entire photocathode of each phototube is exposed to light transmitted through the associated pyramids.

Since each phototube is of a diameter larger than its photocathode, the phototube itself overlies the entire contiguous output face 53 of a pyramid. Thus, light passing through one of the pyramids passes only into the associated phototube. A further assurance against false signals due to stray light is provided by wrapping each phototube with black tape 65 or other light-impervious covering.

With the pyramids providing a continuous, total coverage over the field of view, and the specifically described arrangement, each scintillation occurring in the crystal 20 results in an output signal from each of the phototubes 35. The total of all phototube signals are summed in the circuitry within the console 12. If the sum of all signals is within a predetermined and desired energy level, as determined by a pulse-height analyzer, a signal will be reproduced on the monitor oscilloscope 13 or other output device in use.

With prior art light pipes and their surrounding reflective surfaces, attempts have been made to utilize all photopeak events. These reflections confused the circuitry, resulting in improperly located oscilloscope images. Also, the phototube signals of these prior devices tended on occasion to indicate that perimetral ones of the scintillations occurred more closely to the center than in fact was the case because of the lack of output signals diametrically outward from such scintillations. The resulting oscilloscope signals representative of such perimetral scintillations tended to be produced inwardly of their proper locations. The result was an excessive concentration of signals at the perimeter of an oscilloscope image.

This peripheral concentration with prior art constructions resulted in a useful field of view, in an 11 to 12 inch crystal, of the order of 10 inches in diameter or less. A 10-inch diameter field of view is about 69 percent of the total area of a l2-inch crystal. With the camera of this invention, almost percent of the area of a 13.5-inch diameter crystal is utilized in producing a useful image. This outstanding advantage is obtained because some scintillations occurring near the perimeter of the crystal 20, even when they are so-called photopeak events, well not produce signals on the monitor oscilloscope 13 or other output device. Signals resulting from such perimetral scintillations do not occur because such a large percentage of the light energy is absorbed by the black coating on the perimetral surface 58 and on perimetral portions 60 of the light pipe.

Although the invention has been described in its What is claimed is: 15

1. In a gamma imaging device, the improved detector head comprising:

a. a housing;

b. a collimator connected to said housing c. a lift-out subassembly within the housing and removable as a unit;

(1. means for lifting the subassembly; and,

e. said subassembly including a scintillation crystal, a plurality of phototubes, an optical coupling means between said crystal and said phototubes, and means to maintain the subassembly in an assembled condition as it is inserted in and removed from the housing.

2. The device of claim 1 additionally including:

an apertured cover plate around the phototubes;

and,

spacers maintaining the plates in predetermined spaced relationship.

3. The device of claim 2 wherein bushings are around each of said phototubes and interposed between said cover plate and said phototubes and wherein the means to maintain the subassembly in assembled condition includes clamping means clamping said bushings against said phototubes and said crystal housing against said optical coupling means whereby the phototubes and the optical coupling means are maintained in close juxtaposed relationship to maintain an optical coupling from said crystal to said phototubes.

4. In a gamma imaging device, the improved detector 5 head comprising:

vi. bushings around each of the phototubes and interposed between the cover plate and the h t t be d; viiPcia n iing m ans clamping the bushings against the phototubes and the crystal housing against the optical coupling means whereby to maintain the phototubes and the optical coupling means in close juxtaposed relationship and whereby to maintain an optical coupling from said crystal to said phototubes.

5. The head of claim 4 wherein the lifting means comprises a pair of handles removably connected to said subassembly at diametrically opposite locations.

6. In a gamma imaging device, the improved detector head comprising: a. ahousing; b. a collimator connected to the housing; c. a removable assembly within said housing comprising:

i. a scintillation crystal carried by the housing inwardly of said collimator;

ii. a plurality of phototubes within the housing and adapted to emit electric signals in response to light scintillations occurring in the crystal;

iii. a crystal housing around the crystal and including an output window between the crystal and the phototubes;

iv. a light pipe optically connecting said window to the phototubes;

an apertured locator plate around the phototubes and maintaining the phototubes in predetermined spaced relationship;

vi. an apertured cover plate around the phototubes;

vii. spacers maintaining the plates in predetermined spaced relationship;

viii. bushings around each of the phototubes and interposed between the cover plate and the phototubes; and,

ix. clamping means clamping the bushings against the phototubes and the crystal housing against the light pipe whereby to maintain the phototubes and the light pipe in close juxtaposed relationship and whereby to maintain an optical coupling from said crystal to said phototubes.

Claims (6)

1. In a gamma imaging device, the improved detector head comprising: a. a housing; b. a collimator connected to said housing c. a lift-out subassembly within the housing and removable as a unit; d. means for lifting the subassembly; and, e. said subassembly including a scintillation crystal, a plurality of phototubes, an optical coupling means between said crystal and said phototubes, and means to maintain the subassembly in an assembled condition as it is inserted in and removed from the housing.

2. The device of claim 1 additionally including: an apertured cover plate around the phototubes; and, spacers maintaining the plates in predetermined spaced relationship.

3. The device of claim 2 wherein bushings are around each of said phototubes and interposed between said cover plate and said phototubes and wherein the means to maintain the subassembly in assembled condition includes clamping means clamping said bushings against said phototubes and said crystal housing against said optical coupling means whereby the phototubes and the optical coupling means are maintained in close juxtaposed relationship to maintain an optical coupling from said crystal to said phototubes.

4. In a gamma imaging device, the improved detector head comprising: a. a housing; b. a collimator connected to the housing; c. a removable assembly within said housing comprising; i. a scintillation crystal and a plurality of phototubes carried by the housing and adapted to emit electric signals in response to light scintillations occurring in the crystal; ii. a crystal housing around the crystal and including an output window between the crystal and the phototubes; iii. an optical coupling means connecting said window to the phototubes; iv. an apertured cover plate around the phototubes; v. spacers maintaining the plate in predetermined spaced relationship; vi. bushings around each of the phototubes and interposed between the cover plate and the phototubes; and; vii. clamping means clamping the bushings against the phototubes and the crystal housing against the optical coupling means whereby to maintain the phototubes and the optical coupling means in close juxtaposed relationship and whereby to maintain an optical coupling from said crystal to said phototubes.

5. The head of claim 4 wherein the lifting means comprises a pair of handles removably connected to said subassembly at diametrically opposite locations.

6. In a gamma imaging device, the improved detector head comprising: a. a housing; b. a collimator connected to the housing; c. a removable assembly within said housing comprising: i. a scintillation crystal carried by the housing inwardly of said collimator; ii. a plurality of phototubes within the housing and adapted to emit electric signals in response to light scintillations occurring in the crystal; iii. a crystal housing around the crystal and including an output window between the crystal and the phototubes; iv. a light pipe optically connecting said window to the phototubes; v. an apertured locator plate around the phototubes and maintaining the phototubes in predetermined spaced relationship; vi. an apertured cover plate around the phototubes; vii. spacers maintaining the plates in predetermined spaced relationship; viii. bushings around each of the phototubes and interposed between the cover plate and the phototubes; and, ix. clamping means clamping the bushings against the phototubes and the crystal housing against the light pipe whereby to maintain the phototubes and the light pipe in close juxtaposed relationship and whereby to maintain an optical coupling from said crystal to said phototubes.

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