CA2231911A1 - Position calculation and energy correction in the digital scintillation camera - Google Patents
Position calculation and energy correction in the digital scintillation camera Download PDFInfo
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- G01T1/1642—Static instruments for imaging the distribution of radioactivity in one or two dimensions using one or several scintillating elements; Radio-isotope cameras using a scintillation crystal and position sensing photodetector arrays, e.g. ANGER cameras
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Abstract
For accurate position calculation of scintillation events in a gamma camera, photodetector signals are processed based on small groups of photodetectors surrounding the scintillations. Relative position correction and energy correction is earned out based on rough position values relative to the group of photodetectors, taking into consideration the number of the centre photodetector and the sum of all photodetector signals in the group.
Description
POSITION CALCULATION AND ENERGY CORRECTION
IN THE DIGITAL SCINTILLATION CAMERA
FIELD OF INVENTION
The present invention relates to energy-independent position calculation, position correction, and energy calculation and energy correction for scintillation events in the digital scintillation camera.
BACKGROUND OF THE INVENTION
Scintillation cameras are well known in the art, and are used for medical diagnostics.
A patient is injected, or ingests or inhales a small quantity of a radioactive isotope whose emission photons are detected by a scintillation medium in the camera. The scintillation is commonly a sodium iodide crystal, B~~O or other, which emits a small flash or scintillation of light, in re:~ponse to stimulating radiation. The intensity of the scintillation is proportional (but not linearly) to the energy of the stimulating gamma photon.
As known in the prior art the depth of interaction of the scintillation in the crystal is proportional to the energy of the gamlrca photons (true, theoretically, but never proven to have any measurable impact in practice). As a prior art this fact prevents Anger based gamma cameras from having linear positional response for different energies (this may be due to non-linearity in the crystal and the pmts). In order to produce a diagnostic medical image, scintillations :having an energy which .corresponds to the energy of the decay gamma photons of the radioactive isotope are detected and the intensity each scintillation in the crystal (or crystals for rrmlticrystal cameras) is measured.
Then, the position of the scintilllation calculated, and the calculated position is corrected for the scintillation. Similarly the energy is calculated and corrected. All the calculations are based on the energy intensity values of seven or more (although the computations may be done with only three) light detectors (n-tuplf;) coupled to the surface of the scintillation medium and surrounding the point of scintillation.
SUMMARY ~OF THE INVENTION
It is an obj ect of the present invention to improve scintillation camera image quality by providing a method of energy independent position calculation and corrections.
It is another obj ect of the present to improve scintillation camera image quality by providing the method of energy calculation and corrections.
It is yet another object of thf; present to improve uniformity of the image which consists of the multiplicity of the small images which are tiled, to form the image of the scintillation c;~mera.
IN THE DIGITAL SCINTILLATION CAMERA
FIELD OF INVENTION
The present invention relates to energy-independent position calculation, position correction, and energy calculation and energy correction for scintillation events in the digital scintillation camera.
BACKGROUND OF THE INVENTION
Scintillation cameras are well known in the art, and are used for medical diagnostics.
A patient is injected, or ingests or inhales a small quantity of a radioactive isotope whose emission photons are detected by a scintillation medium in the camera. The scintillation is commonly a sodium iodide crystal, B~~O or other, which emits a small flash or scintillation of light, in re:~ponse to stimulating radiation. The intensity of the scintillation is proportional (but not linearly) to the energy of the stimulating gamma photon.
As known in the prior art the depth of interaction of the scintillation in the crystal is proportional to the energy of the gamlrca photons (true, theoretically, but never proven to have any measurable impact in practice). As a prior art this fact prevents Anger based gamma cameras from having linear positional response for different energies (this may be due to non-linearity in the crystal and the pmts). In order to produce a diagnostic medical image, scintillations :having an energy which .corresponds to the energy of the decay gamma photons of the radioactive isotope are detected and the intensity each scintillation in the crystal (or crystals for rrmlticrystal cameras) is measured.
Then, the position of the scintilllation calculated, and the calculated position is corrected for the scintillation. Similarly the energy is calculated and corrected. All the calculations are based on the energy intensity values of seven or more (although the computations may be done with only three) light detectors (n-tuplf;) coupled to the surface of the scintillation medium and surrounding the point of scintillation.
SUMMARY ~OF THE INVENTION
It is an obj ect of the present invention to improve scintillation camera image quality by providing a method of energy independent position calculation and corrections.
It is another obj ect of the present to improve scintillation camera image quality by providing the method of energy calculation and corrections.
It is yet another object of thf; present to improve uniformity of the image which consists of the multiplicity of the small images which are tiled, to form the image of the scintillation c;~mera.
According to one aspect of the invention, in the method of using at least three light detectors (n-tuple), an image from the scintillation camera is composed (tiled) from many small images each of which corresponds to the area of calculation of at least three light detectors (n-tvple), may be adapted to more than three light detectors, if available. Energy calculation and correction are also done over the area of calculation of at least three light detectors (but may be adapted to more than three light detectors, if available).
This method can also be used to improve the position and energy correction for Anger based scintillation cameras.
According to a further aspect of the invention, there is provided the final image consisting of the multiplicity of the small images which are tiled in geometrical order to create the image of the area over the whole scintillation detector. In order to improve the efficiency, and, hence, the speed of the computations, a new method is proposed. The proposed method consists in thc; computation of a rough estimate of the energy, prior to the processing of the position and energy computation. If the rough estimate of the energy fall outside of a prescribed energy window, the event is. rejected altogether, not being submitted to the position and energy computations. Also, the mew positioning method is linked, but not limited to, a new calibration method. This method consists of finding the light detectors characteristics and making the positioning tables through the acquisition of one or more images with a selected hole phantom.
This method can also be used to improve the position and energy correction for Anger based scintillation cameras.
According to a further aspect of the invention, there is provided the final image consisting of the multiplicity of the small images which are tiled in geometrical order to create the image of the area over the whole scintillation detector. In order to improve the efficiency, and, hence, the speed of the computations, a new method is proposed. The proposed method consists in thc; computation of a rough estimate of the energy, prior to the processing of the position and energy computation. If the rough estimate of the energy fall outside of a prescribed energy window, the event is. rejected altogether, not being submitted to the position and energy computations. Also, the mew positioning method is linked, but not limited to, a new calibration method. This method consists of finding the light detectors characteristics and making the positioning tables through the acquisition of one or more images with a selected hole phantom.
According to the invention, thc,re is provided a method of producing a position value signal in a scintillation camera having; a scintillator, light detectors optically coupled to said scintillator for producing light detector signals, the method comprising the steps of:
(a) determining from said light detector signals a centre light detector receiving a maximum amount of light from a scintillation; (b) selecting a group of light detectors surrounding said centre light detector; (c) calculating firom said group of surrounding light detectors a rough estimate of the energy, which allows for the early rejection of events with non-desirable energy; (d) calculating based on values from said non-rejected group of surrounding light detectors a relative coordinate of a sciintillation relative to said centre light detector, using a transformation value table for a vector or matrix of positions of scintillations within a scope of each of said centre light detectors; (e) calculating from said relative coordinate and said rough estimate of energy a finer enerf;y estimate; and (f) calculating from said finer energy estimate and said group of surrounding light detectors a correction for the said group of surrounding liight detectors signals.
According to the present invention, there is further provided a method of producing and filtering a position value signal in a scintillation camera having a scintillator, light detectors optically coupled to said scintillato:r for producing light detector signals, the method comprising the steps of: (a) determining from said light detector signals a centre light detector receiving a maximum or near-maximum amount of light from a scintillation; (b) selecting a group of light detectors surrounding said centre light detector; (c) calculating based on values from each light detector from said group of surrounding light detectors, giving relative coordinates oi-' a scintillation relative to said centre light detector; (d) calculating a sum of said light detector signals from said group of surrounding light detectors and from said centre light detector; (e) providing an energy calculation value table for a matrix of positions of scintillations 'within a scope of each of said centre light detectors; (f) applying one of said correction values to said sum to obtain a calculated energy sum value, said one of said calculation values having a position in said matrix corresponding to said coordinate of said scintillation; ;end (g) filtering said relative coordinates for each scintillation based on said calculated energy sum value.
Other .advantages, objects and features of the present invention will be readily apparent to those skilled in the art from a review of the following detailed descriptions of preferred embodiments in conjunction with the accompanying drawings and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The embodiments of the invention will now be described with reference to the accompanying drawing, in which:
Figure 1 is a block schematic diagram of the scintillation camera according to the preferred embodiment.
DETAILED DESCRIPTION OF THF; INVENTION
With i:he reference to Figure 1. the scintillation camera system comprises the digital camera 40, energy rej ection circuit 42, relative position calculation 44, energy calculation circuit 46, absolute position calculation circuit 48.
In its preferred embodiment, the energy rejection calculation is digital and independent of the relative position calculation, which means that it can be performed, before, or in parallel with the relative position calculation. :If it is performed after relative position calculation then it becomes position dependent. If the energy correction is performed before the relative position calculation, events which are; outside the required energy window can be filtered earlier in the ~~rocess, which improves the efficiency, and hence the speed of the positioning.
In the preferred embodiment it is assumed that a tuning device exists, as described in commonly assigned application entitled "Photodetector Calibration in a Scintillation Camera Using a Single Light Source" Serial No. 08/354,546 .filed December 14, 1994 or as described in United States Patent No. 5,237, 173 but not limited to such devices, and that the tuning is done before l:he acquisition for the Energy information and positional information. The assumption is that before acquisition, tuning is performed on the detector head, which will normalize the responses of all the lighl: detectors. The assumption is that the detector head is digital, but not limited to being digital. (This energy correction method can be used with any detector head on the market, which can improve the characteristics of the detector heads.) After or instead of those tuning devicc;s, a new calibration is also performed based on a hole phantom imal;e acquisition.
Outputs from the digital detecl:or head as seen in Figure 1, are the following:
1. The label or sequential number associated with the light detector in the detector head T, with the highest response, or in the close neighbourhood of the detector with the highest response. The light detector with the highest response or in close neighbourhood will be called the centre light detector. Tlhe assumption is that the absolute coordinates of each light detector is known in the detector head.
2. The response signal of the central light detector of an n-tuple, defining the n-tuple as a group of the light detectors in the neighbourhood of the centre light detector.
3. The responses of all light detectors in the neighbouring n-tuple of the central light detector, defining the n-tuple as a group of the light detectors in the neighbourhood of the centre light detector.
Energy rej ection circuit 42, produces a sum signal of said n-tuple of light detector signals including the signal of the central light detector, ( E. Relative Position calculation circuit 44, produces x and y values for the particular n-tuple of the light detectors. Output from the position calculation is the associated label or sequential number T
of the centre light detector in th~~ n-tuple.
Energy rejection circuit 42 let pass the events with an energy within the peak energy window. For those events, relative position calculation and energy calculation are weakly dependent. >=;nergy calculation may give back an energy evaluation to the relative position calculation, which improves the precision of the position. This loop may be done zero, one or more times.
The energy calculation method consists of three well defined phases: first, acquisition of the energy information; second producing the energy calculation tables;
third, applying the energy calculation 46 in real time acquisitions.
Acquisition of energy information: For each of many n-tuples with corresponding central light detector in the preferred embodiment, N by M histograms are recorded which cover the area of calculation of one n-tuple. Each histogram consists of at least 256 bins.
Histograms are addressed by the highest n bits of the x position and the highest m bits of the y position. For each event with particular position x and y, particular histogram is chosen depending on position, and the counter of that histogram is increased, depending on the energy. The number of counts in each histogram has to be statistically sufficient. Acquisition is done with the known energy, and without any structured phantoms or collimators.
For producing the energy tablc;s; in the preferred embodiment, histograms should be filtered with a 3D filter for each n-tuple to smooth the response. It is known in the prior art that the response of the light detectors is higher in the centre, and it decreases towards the periphery of the light detector, and that the response is continuous.
Responses of the n-tuples are also smooth. For each n-tuple, the maximum response of each of the histograms is computed after filtering. For each histogram the factor should be computed so that the responses of all the light detectors are equal. For each n-tuple, a table of N
by M factors is stored in the energy table.
When applying the energy calculation 46 in real time, for each event, and depending on the central light detector of the n-tuple, address or label, and also depending on the first m bits of x coordinate and n bits of y coordinate, a particular address in the table is addressed.
The computed energy, which is the sum of all the signals in the n-tuple of light detectors including the central light detector, is multiplied by the factor in the table. This produces the energy calculated value for that event, In the preferred embodiment, tl:le relative position calculation method consists of four well defined phases. First, acquisition of the position information; second, producing the position calculation tables for each light detector in the n-tuple and third applying the relative position calculation 46 in real time acquisitions. The fourth phase consists of adding the relative position of the n-tuple to the known geometric position of that n-tuple in the scintillation detector to create the absolute position 48. Assumption is that the detector head is capable of providing:
1. Associated label of the: light detector in the detector head, with the highest response, or i.n the close neighbourhood. We will call the light detector with the highest response in one event the centre light detector.
2. Assumption is that the absolute coordinate of each light detector is known in the detector head.
3. Responses of all the li~;ht detectors in the neighbouring n-tuple, defining the n-tuple as a group of the light detectors, in the vicinity of the centre light detector.
4. In preferred embodiment n-tuple is consisting of seven or more light detectors.
(a) determining from said light detector signals a centre light detector receiving a maximum amount of light from a scintillation; (b) selecting a group of light detectors surrounding said centre light detector; (c) calculating firom said group of surrounding light detectors a rough estimate of the energy, which allows for the early rejection of events with non-desirable energy; (d) calculating based on values from said non-rejected group of surrounding light detectors a relative coordinate of a sciintillation relative to said centre light detector, using a transformation value table for a vector or matrix of positions of scintillations within a scope of each of said centre light detectors; (e) calculating from said relative coordinate and said rough estimate of energy a finer enerf;y estimate; and (f) calculating from said finer energy estimate and said group of surrounding light detectors a correction for the said group of surrounding liight detectors signals.
According to the present invention, there is further provided a method of producing and filtering a position value signal in a scintillation camera having a scintillator, light detectors optically coupled to said scintillato:r for producing light detector signals, the method comprising the steps of: (a) determining from said light detector signals a centre light detector receiving a maximum or near-maximum amount of light from a scintillation; (b) selecting a group of light detectors surrounding said centre light detector; (c) calculating based on values from each light detector from said group of surrounding light detectors, giving relative coordinates oi-' a scintillation relative to said centre light detector; (d) calculating a sum of said light detector signals from said group of surrounding light detectors and from said centre light detector; (e) providing an energy calculation value table for a matrix of positions of scintillations 'within a scope of each of said centre light detectors; (f) applying one of said correction values to said sum to obtain a calculated energy sum value, said one of said calculation values having a position in said matrix corresponding to said coordinate of said scintillation; ;end (g) filtering said relative coordinates for each scintillation based on said calculated energy sum value.
Other .advantages, objects and features of the present invention will be readily apparent to those skilled in the art from a review of the following detailed descriptions of preferred embodiments in conjunction with the accompanying drawings and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The embodiments of the invention will now be described with reference to the accompanying drawing, in which:
Figure 1 is a block schematic diagram of the scintillation camera according to the preferred embodiment.
DETAILED DESCRIPTION OF THF; INVENTION
With i:he reference to Figure 1. the scintillation camera system comprises the digital camera 40, energy rej ection circuit 42, relative position calculation 44, energy calculation circuit 46, absolute position calculation circuit 48.
In its preferred embodiment, the energy rejection calculation is digital and independent of the relative position calculation, which means that it can be performed, before, or in parallel with the relative position calculation. :If it is performed after relative position calculation then it becomes position dependent. If the energy correction is performed before the relative position calculation, events which are; outside the required energy window can be filtered earlier in the ~~rocess, which improves the efficiency, and hence the speed of the positioning.
In the preferred embodiment it is assumed that a tuning device exists, as described in commonly assigned application entitled "Photodetector Calibration in a Scintillation Camera Using a Single Light Source" Serial No. 08/354,546 .filed December 14, 1994 or as described in United States Patent No. 5,237, 173 but not limited to such devices, and that the tuning is done before l:he acquisition for the Energy information and positional information. The assumption is that before acquisition, tuning is performed on the detector head, which will normalize the responses of all the lighl: detectors. The assumption is that the detector head is digital, but not limited to being digital. (This energy correction method can be used with any detector head on the market, which can improve the characteristics of the detector heads.) After or instead of those tuning devicc;s, a new calibration is also performed based on a hole phantom imal;e acquisition.
Outputs from the digital detecl:or head as seen in Figure 1, are the following:
1. The label or sequential number associated with the light detector in the detector head T, with the highest response, or in the close neighbourhood of the detector with the highest response. The light detector with the highest response or in close neighbourhood will be called the centre light detector. Tlhe assumption is that the absolute coordinates of each light detector is known in the detector head.
2. The response signal of the central light detector of an n-tuple, defining the n-tuple as a group of the light detectors in the neighbourhood of the centre light detector.
3. The responses of all light detectors in the neighbouring n-tuple of the central light detector, defining the n-tuple as a group of the light detectors in the neighbourhood of the centre light detector.
Energy rej ection circuit 42, produces a sum signal of said n-tuple of light detector signals including the signal of the central light detector, ( E. Relative Position calculation circuit 44, produces x and y values for the particular n-tuple of the light detectors. Output from the position calculation is the associated label or sequential number T
of the centre light detector in th~~ n-tuple.
Energy rejection circuit 42 let pass the events with an energy within the peak energy window. For those events, relative position calculation and energy calculation are weakly dependent. >=;nergy calculation may give back an energy evaluation to the relative position calculation, which improves the precision of the position. This loop may be done zero, one or more times.
The energy calculation method consists of three well defined phases: first, acquisition of the energy information; second producing the energy calculation tables;
third, applying the energy calculation 46 in real time acquisitions.
Acquisition of energy information: For each of many n-tuples with corresponding central light detector in the preferred embodiment, N by M histograms are recorded which cover the area of calculation of one n-tuple. Each histogram consists of at least 256 bins.
Histograms are addressed by the highest n bits of the x position and the highest m bits of the y position. For each event with particular position x and y, particular histogram is chosen depending on position, and the counter of that histogram is increased, depending on the energy. The number of counts in each histogram has to be statistically sufficient. Acquisition is done with the known energy, and without any structured phantoms or collimators.
For producing the energy tablc;s; in the preferred embodiment, histograms should be filtered with a 3D filter for each n-tuple to smooth the response. It is known in the prior art that the response of the light detectors is higher in the centre, and it decreases towards the periphery of the light detector, and that the response is continuous.
Responses of the n-tuples are also smooth. For each n-tuple, the maximum response of each of the histograms is computed after filtering. For each histogram the factor should be computed so that the responses of all the light detectors are equal. For each n-tuple, a table of N
by M factors is stored in the energy table.
When applying the energy calculation 46 in real time, for each event, and depending on the central light detector of the n-tuple, address or label, and also depending on the first m bits of x coordinate and n bits of y coordinate, a particular address in the table is addressed.
The computed energy, which is the sum of all the signals in the n-tuple of light detectors including the central light detector, is multiplied by the factor in the table. This produces the energy calculated value for that event, In the preferred embodiment, tl:le relative position calculation method consists of four well defined phases. First, acquisition of the position information; second, producing the position calculation tables for each light detector in the n-tuple and third applying the relative position calculation 46 in real time acquisitions. The fourth phase consists of adding the relative position of the n-tuple to the known geometric position of that n-tuple in the scintillation detector to create the absolute position 48. Assumption is that the detector head is capable of providing:
1. Associated label of the: light detector in the detector head, with the highest response, or i.n the close neighbourhood. We will call the light detector with the highest response in one event the centre light detector.
2. Assumption is that the absolute coordinate of each light detector is known in the detector head.
3. Responses of all the li~;ht detectors in the neighbouring n-tuple, defining the n-tuple as a group of the light detectors, in the vicinity of the centre light detector.
4. In preferred embodiment n-tuple is consisting of seven or more light detectors.
5. Definition of the event: Event is one incidence of the gamma photon producing the scintillation effect in the crystal oiE the detector head. Detector head outputs the label T
of the centre light detector, and the values of the centre light detector and the intensity values of the light dcaectors in the neighbouring n-tuple.
of the centre light detector, and the values of the centre light detector and the intensity values of the light dcaectors in the neighbouring n-tuple.
6. Positional calculation is the translation of the events from the light detectors output to X, '~ and energy values.
In the acquisition of position information; acquisition consists of two parts.
First, acquisition with the structured phantom in front of the scintillation camera (similar to Smith phantom), and second, acquisition wil:hout phantom, the so-called flood acquisition. Smith phantom is known in the art, and consists of a lead plate with lots of pinholes in a rectangular array. The preferred embodiment uses a hexagonal pattern of holes array, with cycle harmonized to the disposition of the Light detectors within the detector head.
A mechanism is added to the hexagonal lead plate such that, by manoeuvring one of three levers, the plate may be moved half a distance between two neighbouring holes, so that the resolution along the three axes defining the hexagon<~l pattern is doubled. Acquisition is done with the radioactive isotope having a known energy. For each of many n-tuples with a corresponding central light detector, in the preferred embodiment, image data is acquired.
The images are distorted depending on the geometric .arrangement or constellation of the light detectors, the light detector and electronic channel properties, and the method of the position calculation.
The position of each pinhole from the phantom is determined. The second acquisition of the flood is needed to determine that the uniformity criterion is satisfied. This means that the number of counts in each area in between the position determined by the image of the pinholes and bounded by the splines which connect all the positions of the pinholes in horizontal and vertical direction. The number of co~.mts in each image has to be statistically sufficient to determine the position of the pinholes, or to check if the uniformity criterion is satisfied.
To apply the relative position calculation 44 in real time; for each event, and depending on the central light detector of the n-tu.ple, address or label, and also depending on each light detector signal of the n-tuple, a parti~;,ular address in the table is addressed, which gives a distance from the scintillation to the lil;ht detector centre. This is done for each light detector, giving a n-tuple of said distances. Position calculation is performed by solving the linear system of distances. This produces the position calculated value for that event.
Circuit 48 calculates the absolute position correction in real time. For each event, after calculation of the relative addresses and depending on the central light detector of the n-tuple, address or label, the position of the n-tuple is added to the relative position inside the n-tuple to form the absolute address.
In the preferred embodiment, the position calculation method consist of three well defined phases. First, acquisition of 'the position information, with one radioactive isotope with lower energy (approximately 100 keV) and later with the radioactive isotopes in the medium (250 keV) and high energy ranges (511 keV). Second, producing the expansion correction factors in table form or function with interpolation for the energies between the acquired energies.
In the preferred embodiment, to improve the energy independent position correction method consist of three well defined phases. First, acquisition of the position information;
with one radioactive isotope with lower energy (approximately 100 keV), and later with the radioactive isotopes in the medium (250 keV) and high energy ranges (511 keV).
Second, producing the expansion correction factors in table form or function with interpolation for the energies between the acquired energic;s. In circuit 46, the expansion correction factors are applied to the X, Y values calculated in 44, together with the sum of the light detectors values ( E given by the energy rejection circuit 42. Although the preferred embodiment illustrates a purely digital camera, it is to be understood that the above described methods can be easily adapted to operate when analog position calculation is used.
As can be appreciated, many 'variations are possible within the scope of the present invention. Combinations of various embodiments is also possible and may be advantageous depending on the exact requirements of the camera desired.
Numerous modifications, variations and adaptations may be made to the particular embodiments of the invention described above without departing from the scope of the invention, which is defined in the claims.
In the acquisition of position information; acquisition consists of two parts.
First, acquisition with the structured phantom in front of the scintillation camera (similar to Smith phantom), and second, acquisition wil:hout phantom, the so-called flood acquisition. Smith phantom is known in the art, and consists of a lead plate with lots of pinholes in a rectangular array. The preferred embodiment uses a hexagonal pattern of holes array, with cycle harmonized to the disposition of the Light detectors within the detector head.
A mechanism is added to the hexagonal lead plate such that, by manoeuvring one of three levers, the plate may be moved half a distance between two neighbouring holes, so that the resolution along the three axes defining the hexagon<~l pattern is doubled. Acquisition is done with the radioactive isotope having a known energy. For each of many n-tuples with a corresponding central light detector, in the preferred embodiment, image data is acquired.
The images are distorted depending on the geometric .arrangement or constellation of the light detectors, the light detector and electronic channel properties, and the method of the position calculation.
The position of each pinhole from the phantom is determined. The second acquisition of the flood is needed to determine that the uniformity criterion is satisfied. This means that the number of counts in each area in between the position determined by the image of the pinholes and bounded by the splines which connect all the positions of the pinholes in horizontal and vertical direction. The number of co~.mts in each image has to be statistically sufficient to determine the position of the pinholes, or to check if the uniformity criterion is satisfied.
To apply the relative position calculation 44 in real time; for each event, and depending on the central light detector of the n-tu.ple, address or label, and also depending on each light detector signal of the n-tuple, a parti~;,ular address in the table is addressed, which gives a distance from the scintillation to the lil;ht detector centre. This is done for each light detector, giving a n-tuple of said distances. Position calculation is performed by solving the linear system of distances. This produces the position calculated value for that event.
Circuit 48 calculates the absolute position correction in real time. For each event, after calculation of the relative addresses and depending on the central light detector of the n-tuple, address or label, the position of the n-tuple is added to the relative position inside the n-tuple to form the absolute address.
In the preferred embodiment, the position calculation method consist of three well defined phases. First, acquisition of 'the position information, with one radioactive isotope with lower energy (approximately 100 keV) and later with the radioactive isotopes in the medium (250 keV) and high energy ranges (511 keV). Second, producing the expansion correction factors in table form or function with interpolation for the energies between the acquired energies.
In the preferred embodiment, to improve the energy independent position correction method consist of three well defined phases. First, acquisition of the position information;
with one radioactive isotope with lower energy (approximately 100 keV), and later with the radioactive isotopes in the medium (250 keV) and high energy ranges (511 keV).
Second, producing the expansion correction factors in table form or function with interpolation for the energies between the acquired energic;s. In circuit 46, the expansion correction factors are applied to the X, Y values calculated in 44, together with the sum of the light detectors values ( E given by the energy rejection circuit 42. Although the preferred embodiment illustrates a purely digital camera, it is to be understood that the above described methods can be easily adapted to operate when analog position calculation is used.
As can be appreciated, many 'variations are possible within the scope of the present invention. Combinations of various embodiments is also possible and may be advantageous depending on the exact requirements of the camera desired.
Numerous modifications, variations and adaptations may be made to the particular embodiments of the invention described above without departing from the scope of the invention, which is defined in the claims.
Claims (11)
1. A method of producing a position value signal in a scintillation camera having a scintillator, light detectors optically coupled to said scintillator for producing light detector signals, the method comprising the steps of:
(a) determining from said light detector signals a centre light detector receiving a maximum amount of light from a scintillation;
(b) selecting a group of light detectors surrounding said centre light detector;
(c) calculating from said group of surrounding light detectors a rough estimate of the energy, which allows for the early rejection of events with non-desirable energy;
(d) calculating based on values from said non-rejected group of surrounding light detectors a relative coordinate of a scintillation relative to said centre light detector, using a transformation value table for a vector or matrix of positions of scintillations within a scope of each of said centre light detectors;
(e) calculating from said relative coordinate and said rough estimate of energy a finer energy estimate; and (f) calculating from said finer energy estimate and said group of surrounding light detectors a correction for the said group of surrounding light detectors signals.
(a) determining from said light detector signals a centre light detector receiving a maximum amount of light from a scintillation;
(b) selecting a group of light detectors surrounding said centre light detector;
(c) calculating from said group of surrounding light detectors a rough estimate of the energy, which allows for the early rejection of events with non-desirable energy;
(d) calculating based on values from said non-rejected group of surrounding light detectors a relative coordinate of a scintillation relative to said centre light detector, using a transformation value table for a vector or matrix of positions of scintillations within a scope of each of said centre light detectors;
(e) calculating from said relative coordinate and said rough estimate of energy a finer energy estimate; and (f) calculating from said finer energy estimate and said group of surrounding light detectors a correction for the said group of surrounding light detectors signals.
2. The method as claimed in claim 1, further comprising the step of adding a base coordinate of said centre light detector to said position value signal to obtain a global position value signal.
3. The method as claimed in claim 1, wherein said step (d) comprises the steps of acquiring scintillation data using, either a structured phantom in front of said camera to obtain point position data for a known structure fixed array, or moving a point source at known positions in front of the detector, and collecting light detector signals corresponding to points of said array surrounding each position in said matrix as well as distances between each position in said matrix and the surrounding points of said array.
4. The method as claimed in claim 3, wherein said step (d) further comprises the steps of acquiring flood image intensity data, and adding to said transformation values a uniformity factor calculated based on a local gradient of said intensity data such that said transformation values also help to make bright spots to look less bright by rejecting some of the events that occur there, so as to reduce intensity anomalies in an image based on said position values.
5. The method as claimed in claim 4, further comprising the step of adding a base coordinate of said centre light detector to said position value signal to obtain a global position value signal.
6. The method as claimed in claim 1, wherein said step (d) comprises the steps of acquiring flood image intensity data, and generating a weighting value computed by the ratio of the mean intensity to the intensity at the pixel, with said weighting value being added to the acquired image, such as to reduce intensity anomalies in an image based on said position values.
7. The method as claimed in claim 1, wherein said step (d) comprises providing a transformation value table for a plurality of scintillation energies for a vector or a matrix of positions of scintillations within a scope of each of said centre light detectors, and said step (e) comprises, using the said relative coordinate to find out an energy estimate from said energy table to obtain a value for generating said energy value signal.
8. One method as claimed in claim 5, wherein said step (d) comprises providing a transformation value table for a plurality of scintillation energies for a vector or matrix of distances of scintillations within a scope of each of said centre light detectors, and transforming said relative distances of said scintillation using linear systems solving algorithm, to obtain a value for generating said position value signal.
9. A method of producing and filtering a position value signal in a scintillation camera having a scintillator, light detectors optically coupled to said scintillator for producing light detector signals, the method comprising the steps of:
(a) determining from said light detector signals a centre light detector receiving a maximum or near-maximum amount of light from a scintillation;
(b) selecting a group of light detectors surrounding said centre light detector;
(c) calculating based on values from each light detector from said group of surrounding light detectors, giving relative coordinates of a scintillation relative to said centre light detector;
(d) calculating a sum of said light detector signals from said group of surrounding light detectors and from said centre light detector;
(e) providing an energy calculation value table for a matrix of positions of scintillations within a scope of each of said centre light detectors;
(f) applying one of said correction values to said sum to obtain a calculated energy sum value, said one of said calculation values having a position in said matrix corresponding to said coordinate of said scintillation; and (g) filtering said relative coordinates for each scintillation based on said calculated energy sum value.
(a) determining from said light detector signals a centre light detector receiving a maximum or near-maximum amount of light from a scintillation;
(b) selecting a group of light detectors surrounding said centre light detector;
(c) calculating based on values from each light detector from said group of surrounding light detectors, giving relative coordinates of a scintillation relative to said centre light detector;
(d) calculating a sum of said light detector signals from said group of surrounding light detectors and from said centre light detector;
(e) providing an energy calculation value table for a matrix of positions of scintillations within a scope of each of said centre light detectors;
(f) applying one of said correction values to said sum to obtain a calculated energy sum value, said one of said calculation values having a position in said matrix corresponding to said coordinate of said scintillation; and (g) filtering said relative coordinates for each scintillation based on said calculated energy sum value.
10. The method as claimed in claim 9, wherein said step (e) comprises the steps of acquiring scintillation data with a gamma ray source of known energy without use of phantoms or collimators to obtain a statistically large number of scintillation data for each position within said matrix, recording the energy of said scintillation data for each said position in the form of an energy histogram, and computing said energy correction value for said matrix using said histograms.
11. The method as claimed in claim 9, wherein said step (a) comprises choosing the light detectors with the maximum signals among those which are not at the edge of the detector, so that this reference light detector is always surrounded by neighbouring detectors, and is less affected by any kind of detector head edge effect.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2231911 CA2231911A1 (en) | 1998-03-11 | 1998-03-11 | Position calculation and energy correction in the digital scintillation camera |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2231911 CA2231911A1 (en) | 1998-03-11 | 1998-03-11 | Position calculation and energy correction in the digital scintillation camera |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2231911A1 true CA2231911A1 (en) | 1999-09-11 |
Family
ID=29409426
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2231911 Abandoned CA2231911A1 (en) | 1998-03-11 | 1998-03-11 | Position calculation and energy correction in the digital scintillation camera |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2231911A1 (en) |
-
1998
- 1998-03-11 CA CA 2231911 patent/CA2231911A1/en not_active Abandoned
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