CA1117228A

CA1117228A - Positron annihilation imaging device using multiple offset rings of detectors

Info

Publication number: CA1117228A
Application number: CA000341946A
Authority: CA
Inventors: Christopher J. Thompson
Original assignee: Montreal Neurological Institute
Current assignee: Montreal Neurological Institute
Priority date: 1979-08-27
Filing date: 1979-12-14
Publication date: 1982-01-26
Also published as: FR2464056A1; GB2058511A; JPS5636064A; SE8000911L; SE436938B; DE3007815A1

Abstract

ABSTRACT OF THE DISCLOSURE

Positron annihilation imaging device having rings of detectors arranged to detect more than one tomographic image simultaneously through different cross-sections of a patient, the device having two or more concentric circular arrays of detectors, wherein the detectors in one array are offset with respect to the detectors in the adjacent array.

Description

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BACKGROUND OF THE INVENTION

Field of the Invention This invention relates generally to positron emission tomography and more particularly to devices which use an array of scintillation detectors to detect the annihilation radiation from positron disintegration and use this information to recon-struct an image of the distribution of positron emitting iso~
topes within a body.

DESCRIPTION OF THE PRIOR ART

Positron emission tomography is a technique for measuring the concentration of a positron emitting isotope through a sectional plane through the body. NormaLly the isotope is used to label a substance which circulates with the blood and may be absorbed in certain tissues. The technique allows the actual concentration in the slice to be determined if the device is suitably calibrated.
Certain isotopes decay by emitting a positively charged particle with the same mass as the electron (positron) and the neutrino from the nucleus. In this process one of the protons in the nucleus becomes a neutron, so that its atomic number goes down while its atomic weight remains constant.
This positron is ejected with a kinetic energy of up to 2 MeV
depending on the isotope and loses this energy and collisions ~ ', while travelling a distance of up to a few mms in water. When -it has reached thermal energies it interacts with an electron and they mutually annihilate one another. The rest mass of the

2 particles is transformed into 2 gamma rays of 511 keV which are emitted at 180 in the 'center of mass' coordinates of the original particles. The 2 gamma rays may be detected by suit-able devices. If these devices measured the energy of the ~ ;, 7~Z28 gamma rays at 511 keV and register this energy almos-t simul-taneously it may be assumed that the origin of the radiation is on a straight line between the 2 detectors. Several de-tectors may be used in an arrangement so that many coincident events may be imaged during the same time interval. Then the information from these detectors is processed by a computer using image reconstruction technigues in order to find the location of distribution of positron emitting isotope.

COMPONENTS OF IMAGING DEVICE

A device for imaging positron annihilation radiation consists of the following basic parts: ;
(1) A number of detectors arranged in a precise geometrical pattern. These detectors are normally scintilla-tion detectors in one or several planes, and these detectors are normally arranged in a polygonal pattern or around the cir-cumference of a circle. Scintillation detectors emit a light flash each time they absorb gamma radiation which may or may not arise from the mutual annihilation of a positron and elec-tron. The intensity of the light flash is proportional to the gamma ray energy.
(2) The device must contain a means of converting the light flash to an electrical charge pulse. Its amplitude is proportional to the light intenisity.

(3) The device must contain a means of determining that the charge pulse could have arisen from a gamma ray whose energy was approximately equivalent to the mass of the elec-tron at rest (511 keV).

(4) The device must have an electric circuit capa~

ble of determining that 2 and only 2 detectors each recorded gamma rays of appropriate energy within a short time interval L722~

(coincidence resolving time). ~se detectors are said to have recorded a 'coincident event'.

(5) The device must have an electric circuit which determines which 2 detectors out of the many possible combina-tions recorded the so-called 'coincident event'.

(6) The device must have a memory in which it can record how often each pair of detectors record a 'coincident event'. The memory may be part of the random access memory of a general purpose computer.

(7) The device is required to use an algorithm through which the information in the memory may be transformed into an image of the distribution of positron annihilation per unit time in a cross-section surrounded by the detectors. The sequence of steps described by this algorithm may be programmed into a general purpose computer.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly it is the main object of this invention to provide a means for recording more than one tomographic ~ ;
image simultaneously through different cross-sections of a patient.
Another object of this invention is to use separate rings of detectors for every odd numbered slice and to use coincident events which occur between adjacent rings of de-tectors to provice a center or even numbered slice.
Another object of this invention is to offset the detector rings with respect to one another by half the angular separation of the detectors allowing an image to be reconstruct-ed from the central slice without the necessity of physically rotating the detector array while accumulating data.
In accordance with the foregoing objects, there is - ~ : ; : , "` 11~7~ZI~
provided:
Positron annihilation imaging device comprising:
(a) a first array of N detectors equally spaced around a first circle disposed in a first common plane;
~ b) a second array of N detectors equally spaced around a second circle disposed in a second common plane, the first and second circles being coaxial on a common axis, said first and second planes being parallel and spaced apart, the said detectors in the first array being offset with respect to said detectors in the second array by an angle substantially equal to 360/2N subtended at the said common axis, at least one additional array of N detectors equally spaced around and circularly disposed in one or more planes spaced from said common planes, the detectors in said additional array, being offset by 360/2N relative to the detectors in an adjacent one of said ~irst or second arrays of detectors~

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A preferred embodiment of the inven~ion wil~ bè
.. , . c described with reference to the accompanying drawings, on which:
Fig. 1 is an overall block diagram of the apparatus;
. Fig. 2 is a cross-sectional view showing how the coincident events are obtained from two rings of detectors to produce three independent tomographic images of consecutive cross-section through a patient;
Fig. 3 shows the detector placement in a two-ring system which is able to produce three cross-sectional slices;
Fig. 4 is a pictorial view of two rings of detectors, the detectors in one ring being offset in relation to the detectors in the outer ring~.

DESCRIPTION OF A PREFERRED EMBODIMENT
. _ The preferred embodiment of this positron annihilation imaging device 1 is shown in outline form in Fig. 1~ It is seen that the device consists of two or more rings o detectors ~W~

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2, 2' which surround the ob~ect being imaged in two or more planes. The electrical signals from these detectors are amplified at 4a ... 4n, and their energy is measured at energy discriminators 5a ... 5n, and the outputs of each energy discriminator are processed by a coincidence circuit 6. The output of the coincident circuit is usad to incre~ent memory locations in a general purpose computer. The computer then reconstructs an image of the distribution of the positron imaging isotope in the cross-sections which were sc~nned.
Further details of the circuit of Fig. 1 will be found in copen ~ g patent application 341,945 filed December 14, 1979 by C.J. Thompson entitled "Coincidence Analysis Circuit for Positron Annihilation Imaging Device" and assigned to the Applicant of the present invention.
The preferred embodiment contains two rings o 64 trapezoidal shaped bismuth germanate detectors which are separated by thin tungsten septa. ~See Fig. 2).
For further details of the detec-tor shape and arrange-ment for a positron annihilation imaging devic~, reference may be made to ~ ~ng application 330,087 filed June 19, 1979 `
by C.J. Thompson entitled "De~ector Shape and Arrangement for Positron Annihilation Imaging Device" and assigned to the Applicant o~ the present invention.
These two rings of detectors are rotated with respect to one another by half the angular separation of the detectors (~ 8) `
In normal operation of a single slice positron annihilation detector ring it is desirable to rotate the sir.gle detector ring in an oscillatory motion to and fro by half the angular separation of the detectors. The purpose of this is to double the number of points in each parallel projection ~7~

which is used in the image reconstruction technique. The other purpose of this is to uniformly sample the object to be scanned in sueh a way that the sampling of the projeetion is done at points whieh are not greater than half the widths of the de-tector pair aperture function apart. This is necessary to prevent "aliasing errors" in the reeonstrueted image.
This oscillatory motion takes a finite time (in the preferred embodiment about one third of a seeond) whieh limits the rate at whieh eonseeutive images can be obtained to approxi-mately two thirds of a seeond per image. An image ean bereeonstrueted of a 'central slice' between the two plane of detectors when two rings of detectors are used as shown in Fig. 2. Since this central slice is viewed by twice the number of detectors, twice the number of counts per unit time are recorded by the detectors from this area. Because of this, an image can be reeonstructed from the center slice in half the time it takes to obtain data of the same statistical aecuracy from the outer slices. By offsetting the deteetor rings by half the angular separation between deteetors, it is seen that data is eollected from the same number of points as would be collected from a single ring in its normal and rotated posi-tions. Thus the imaged area is ~mPled finely enough to allow an image to be reconstructed from a eentral sliee without having to rotate the ring at all. This means in practiee that images ean be reconstructed from the central slice at a rate deter-mined only by the amount of isotope administered to the patient, the statistical aecuracyrequired in the final image, and the transfer of raw data to more permanent storage (magnetic disk).
Referring now to Fig. 3, there is shown a side eleva-tion of adjacent portions of two rings of detectors, Ul --- U64 in one ring and Ll --- L64 in the other ring.

7Z~8 The detector rings are separated by a thin tungsten septum 230 which prevents penetration of unwanted radiation from one detector ring to the other. The adjacent detectors in the same row are also separated by tungsten septa. An annular collimator is also required between the two detector ' arrays to prevent radiation ou-tside the slice being viewed to penetrate into one of the detectors. This is shown best in Fig. 2.
Figs. 3 and 4 show the 2 rings of detectors 302, 304 with respect to one another. The detectors 2, 2' are angularly separated by half the angle subtended by an individual detector. The individual detectors are separated by tungsten septa 306, 308 in each ring. There is also a tungsten sheet 230 between the 2 rings of detectors.
In another embodiment a number of detector rings greater than two could be employed in which the detector arrays were alternately staggered by half the angular separation of the detectors so that for N-rings of detectors (2N-l) cross-sectional images can be obtained.
2C The following advantages are made with regard to this invention.
More detectors view the radiation emanating from the patient to increase the use of the isotope administered to the patient, than if only one detector array was employed.
Encoding and processing data from coincident events involving detectors in adjacent slices allows the whole volume to be imaged eliminating blind spots which would occur if the cross-slice coincident events were not used.
Using separate detectors on each slide (as opposed to long detectors and electronically encoding the location of the event within a detector) allows the count rate capability to be increased with no loss in efficiency. This allows the device to operate over a wider range in count rate.
By rotating the detectors in adjacent rings by one-half the angular separation of the detectors, the imaging time in an even numbered slice can be improved by a factor of two.
This is so since (a) twice the number of detector pairs view the center slice and (b) it is normally necessary to rotate the detector array by half the angular detector separation to ' achieve the desired spatial resolution and sampling of the imaged plane. , Other embodiments falling within the lines of the appended claims will occur to those skilled in the art.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Positron annihilation imaging device comprising:
(a) a first array of N detectors equally spaced around a first circle disposed in a first common plane;
(b) a second array of N detectors equally spaced around a second circle disposed in a second common plane, the first and second circles being coaxial on a common axis, said first and second planes being parallel and spaced apart, the said detectors in the first array being offset with respect to said detectors in the second array by an angle substantially equal to 360/2N° subtended at the said common axis, at least one additional array of N detectors equally spaced around and circularly disposed in one or more planes spaced from said common planes, the detectors in said additional array, being offset by 360/2N° relative to the detectors in an adjacent one of said first or second arrays of detectors.