Patent Application: US-54898505-A

Abstract:
a scintillator crystal = based gamma - ray camera system is described . the gamma - ray camera system includes a spectra processing component for providing improved energy resolution over that seen in conventional gamma - ray camera systems . the spectra processing component operates to deconvolve detector response functions from observed energy spectra on a pixel by pixel basis . the pixel dependent to detector response functions are obtained by a combination of theoretical simulation , and empirical calibration . by deconvolving pixel specific detector response functions , variations in response of a gamma - ray camera system across its image plane can be accounted for . this offers significant improvements in energy resolution and many of the problems associated with conventional gamma - ray camera systems are reduced . for example , the improved energy resolution allows better rejection of photons associated with compton scattering events occurring in a source being imaged . this is because a narrower energy window filter can be used without rejecting a significant fraction of non - compton scattered photons . the spectra processing component can be easily implemented with different types of gamma - ray imagers , for example anger - type cameras , and may also be retroactively fitted to existing gamma - ray camera systems .

Description:
fig5 schematically shows in vertical cross - section a gamma - ray camera system 32 according to a first embodiment of the invention . many of the features of the gamma - ray camera system 32 shown in fig5 are similar to and will be understood from the correspondingly numbered features shown in fig1 and described above . however , the gamma - ray camera system 32 additionally includes a spectra processing component 34 . the spectra processing component includes a store of detector response functions 36 , a spectra processor 38 and a data storage component 40 . the functionality of the spectra processing component in this example is provided by a suitably configured general purpose computer . however , an application specific integrated circuit ( asic ), a field programmable gate array ( fgpa ) or a digital signal processor ( dsp ) may also be employed . the spectra processing component 34 is arranged to read the observed data array i ( x , y , e ) from the data storage component 20 in the energy spectra accumulating component 12 . when an exposure is completed and an observed data array i ( x , y , e ) generated as described above , the observed data array is read by the spectra processor 38 in the spectra processing component 34 . the spectra processor is operable to deconvolve a detector response function from the individual spectra associated with each pixel in the observed data array i ( x , y , e ), the specific detector response function employed in the deconvolution is selected from a store of detector response functions 36 . the choice of detector response function is based on which detector pixel the spectrum currently being processed is associated with . once each of the energy spectra comprising the observed data array i ( x , y , e ) have been deconvolved , the resulting refined energy spectra are collated into a refined data array s ( x , y , e ). the refined data array is of the same form as i ( x , y , e ) and may be used in the same way to generate diagnostic images . with a gamma - ray camera system of the type shown in fig5 , the energy spectrum n ( e ) formed by the spectra accumulating component 12 for a detector pixel at position x 0 , y 0 in response to an incident gamma - ray spectrum a ( e ) is given by : n ⁡ ( e ) = ∫ 0 ∞ ⁢ ( r x 0 ⁢ y 0 ⁡ ( e , e ′ ) · a ⁡ ( e ′ ) + ɛ ⁡ ( e ) ) · ⅆ e ′ where r x 0 y 0 ( e , e ′) describes the detector response function for the detector pixel at position x 0 , y 0 and ε ( e ) is the noise contribution . this integral can be discretised as : [ n 1 n 2 . . . n l ] = [ r x 0 ⁢ y 0 11 r x 0 ⁢ y 0 12 . . . r x 0 ⁢ y 0 im r x 0 ⁢ y 0 21 r x 0 ⁢ y 0 22 . . . r x 0 ⁢ y 0 2 ⁢ m . . . . . . . . . . . . . . . . . . r x 0 ⁢ y 0 i ⁢ ⁢ 1 r x 0 ⁢ y 0 i ⁢ ⁢ 2 . . . r x 0 ⁢ y 0 im ] · [ a 1 a 2 . . . a i ] + [ ɛ 1 ɛ 2 . . . ɛ i ] where the detector response function is parametrized such that r x 0 y 0 lm describes the probability that a detected gamma - ray photon which generates a scintillation event in pixel x 0 , y 0 , and having an incident energy falling into energy bin 1 , will be actually detected as having an energy falling within bin m . a thorough description of techniques which can be used to deconvolve r x 0 y 0 lm from n l . . . l to determine the incident gamma - ray spectrum a l . . . l is given by berger & amp ; seltzer [ 10 ]. fig6 shows a typical energy spectrum which would be seen in one of the detector pixels of a gamma - ray camera system similar to that shown in fig5 . in this example , as with the example shown in fig2 , an isolated ( i . e . not embedded in a body ) gamma - ray point source is viewed to show the intrinsic energy resolution of a gamma - ray camera system including deconvolution . this allows the performance of the gamma - ray camera system shown in fig5 to be directly compared with that shown in fig1 . as with the data shown in fig2 , the gamma - ray source is cobalt - 57 emitting primarily at an energy e 0 = 122 kev . as with fig2 , count rate n ( e ) is plotted as a function of energy e for a detector pixel at position x 0 , y 0 . in this case , however , n ( e )= s ( x 0 , y 0 , e ), i . e . it is a deconvolved energy spectrum which is plotted and not the observed energy spectrum . the detector response function employed in the deconvolution is determined as detailed further below . in fig6 the deconvolved full width at half maximum ( fwhm dec ) of the peak corresponding to gamma - ray emission from the cobalt - 57 source is approximately 6 kev . accordingly , at an energy of 122 kev the gamma - ray camera system shown in fig5 has an energy resolution of around 5 %. this is a significant improvement on the energy resolution of around 20 % seen with previous gamma - ray camera systems such as shown in fig1 . the much improved resolution makes it easier to distinguish compton scattered detection events from direct detection events when forming diagnostic images from the refined data array s ( x , y , e ). this is possible since a narrower energy widow around the peak energy may be used when generating diagnostic images so as to discard much of the compton scattered detection events , while maintaining most of the direct detection events . for instance , the width of the narrow energy window w 2 shown in fig3 is also marked on fig6 . while in fig3 this window excluded a significant fraction of the direct detection events , it can be seen from fig6 that after appropriate deconvolution , most of the direct detection events are included within the window . the broad energy distribution of the compton scattered detection events is inherent in their contribution to the incident gamma - ray spectrum a ( e ). this means that the deconvolution does not significantly affect the spectral distribution of the compton scattered detection events and they remain largely outside of the narrower energy window w 2 , as seen in fig3 . the difficulties in applying this technique to gamma - ray camera systems are the complications introduced by the fact that the response of a relatively large single scintillator crystal is not only a function of photon energy , but it is also a function of the location of the gamma - ray interaction . this means that it is necessary to construct a model of the detector response function not only for the way that gamma - rays interact in the scintillator crystal material itself , but it is also necessary to modify this model with reference to an empirical calibration of the overall response of the gamma - ray camera system on a detector pixel by detector pixel basis . the detector response function varies from one detector pixel to another as a consequence of non - uniformities in the scintillator crystal light - yield , and also in the light - collection efficiency on the gamma - ray camera imager . for example , the light collection efficiency will depend on detector pixel both due to differences in transfer function from different scintillation sites to the photo - multiplier tubes , and also to non - uniformities in the response of the photo - multiplier tubes . further variations in the detector response function for different detector pixels are introduced as a consequence of non - linearities in the response of the scintillation crystal as a function of energy deposited in a scintillation event . by using , for example , a monte carlo technique , it is possible to predict the way that the scintillator material responds to incident gamma - ray photons at energies within an energy range of interest , for instance between 50 kev and 500 kev . other modelling methods could also be used . this information may then be combined with empirical calibration data to provide information on how the gamma - ray imager actually responds at a number of discrete energies within the same region of interest . the calibration data may be acquired , for example , by observing a number of monochromatic radioactive sources emitting within the energy range of interest . by experimentally determining the response of each individual detector pixel to known incident gamma - ray energy spectra , the positional dependence of the detector is response function can be determined . with a point calibration source , data can be obtained for each pixel simply by removing the collimator and placing the source at a distance of say 50 cm from the scintillator crystal , or by mounting the calibration source on a translation stage such that it can be scanned across the field - of - view of the camera . it is most appropriate to determine how the detector response function varies with position at a spatial resolution comparable with that of the gamma - ray imager . the determined detector response functions are specific to an individual gamma - ray camera . it will also be appreciated that while in the above examples a monte carlo or similar simulation provides a base model which is modified according to empirically determined calibration data , in other examples the simulation may be dispensed with . in such cases purely empirical pixel specific detector response functions can be obtained from observations of the response of the gamma - ray imager to point calibration source . with accurate models for the way in which the gamma - ray imager responds as a function of incident gamma - ray photon energy , deconvolution can be performed on a detector pixel by detector pixel basis using the techniques noted above [ 10 ]. accordingly , following an exposure , when energy spectra for each detector pixel have been recorded in an observed data array i ( x , y , e ) as described above , the gamma - ray spectrum incident on each detector pixel can be recovered using the position - sensitive detector response functions in a standard deconvolution algorithm . fig7 schematically show how the same gamma - ray source leading to the idealized image shown in fig4 a would appear when obtained with a gamma - ray camera system shown in fig5 . in this example , the energy window shown in fig6 and marked w 2 is used in generating the diagnostic image . since the narrow energy window excludes a high fraction of the compton scattered detection events , there is no compton scattered halo surrounding the image as seen with a wide energy window , for example as shown in fig4 b . the improved spectral resolution provided by the spectra processing component also ensures that the narrow energy window includes almost all of the direct detection events . this leads to a higher signal - to - noise ratio than that seen in the image shown in fig4 c . accordingly , the gamma - ray camera system shown in fig5 is able to provide diagnostic images which are much more closely matched to the idealized image shown in fig4 a than prior art gamma - ray camera systems . fig8 is a flow chart which schematically details some of the operational steps performed within the spectra processing component 34 . in this example , the gamma - ray camera imager provides a square array of x tot by y tot detector pixels . at s 1 , an observed data array i ( x , y , e ) obtained during an observation of interest is read from the data storage element 20 shown in fig5 . in s 2 , iteration parameters x and y are set to zero . at s 3 and s 4 , the iteration parameters x and y are respectively incremented by one . in s 5 , the spectrum corresponding to the detector pixel x , y is copied into a one - dimensional data array o ( e ). in s 6 , a detector response function r ( e , e 0 ) is retrieved from the detector response function store 36 . as noted above , the selected detector response function is selected based on the values of x and y . in s 7 , the selected detector response function is deconvolved from o ( e ) to provide a one dimensional data array a ( e ). a ( e ) accordingly represents a calculation of the incident gamma - ray spectrum falling on detector pixel x , y . in s 8 , the array a ( e ) is copied to the elements of a refined data array s ( x , y , e ) which correspond to the detector pixel currently being processed . in s 9 , the value of y is tested to determine whether it is equal to y tot . if y is less than y tot , the process flow returns to s 4 . steps s 4 - s 8 are repeated , with y being incremented at s 4 in each iteration , until y is equal to y tot . in s 10 , the value of x is tested to determine whether it is equal to x tot . if x is less than x tot , the process flow returns to s 3 . steps s 3 - s 9 are repeated , with x being incremented at s 3 in each iteration , until x is equal to x tot . in s 11 , the spectra processing is completed and the refined data array s ( x , y , e ) is written to the data storage component 40 . s ( x , y , e ) is then available for further use from the data storage component in the spectra processing component in the same way that the observed data array i ( x , y , e ) is available in prior art gamma - ray camera systems . because there are more than 2000 detector pixels present in the gamma - ray camera system , the process shown in fig8 can be very time consuming . if faster processing is required , a fast parallel - processor may be used to accelerate the process such that diagnostic images can be made available within a time period which is comparable with the exposure time required to acquire the observed data array . again , the functionality of the processing component 34 can be achieved by using a suitably configured general purpose computer with parallel processing capability , or an application specific integrated circuit . fig9 is a flow chart which schematically details some of the operational steps performed within a spectra processing component employed in a gamma - ray camera system according to second embodiment of the invention . as above , the gamma - ray camera imager provides a square array of x tot by y tot detector pixels . at t 1 , the observed data array i ( x , y , e ) corresponding to a prior observation is read from the data storage element 20 shown in fig5 . in this embodiment , the spectra processing component is configured to process the spectra associated with each detector pixel in parallel . a separate process thread operates for each individual detector pixel , this removes the need to iterate seen fig8 , and provides for significantly faster processing . in the first step of one process thread , labelled t 2 in fig9 , the detector pixel corresponding to x = 1 and y = 2 is selected . at the same time , threads corresponding to all other detector pixels are similarly instigated . in t 3 , the next step in the thread started at t 2 , the spectrum corresponding to the detector pixel x = 1 , y = 2 is copied into a one - dimensional data array o ( e ). in t 4 , a detector response function r ( e , e 0 ) is retrieved from a detector response function store similar to that described above for the gamma - ray camera system shown in fig5 . the selected detector response function is chosen based on the values of x and y corresponding to the thread being processed , in this case , 1 and 2 respectively . in t 5 , the detector response function is deconvolved from o ( e ) to provide a one dimensional data array a ( e ). a ( e ) accordingly represents a calculation of the incident gamma - ray spectrum falling on detector pixel x = 1 , y - 2 . in t 6 , the array a ( e ) is copied to the elements of a refined data array s ( x , y , e ) which correspond to the detector pixel x = 0 , y = 2 . in t 7 , each process thread is finished , and a completed refined data array s ( x , y , e ) is obtained . in t 8 , the spectra processing is complete and the refined data array s ( x , y , e ) is written to a data storage component similar to the one described above for the gamma - ray camera system shown in fig5 . s ( x , y , e ) is then available for further use in the same way that the observed data array i ( x , y , e ) is made available in prior art gamma - ray camera systems . in other examples , a combination of the methods shown in fig8 and 9 is could be employed . for example , an observed data array i ( x , y , e ) including energy spectra for a total of 2000 pixels might be processed using twenty parallel processor channels , with each process channel sequentially processing 100 energy spectra . this would allow , for example , faster processing of the observed data array than would be seen using the method shown in fig8 , but would not require the same level of computing power necessary for the method shown in fig9 . whilst in the above examples , individual detector response functions are determined and stored for each detector pixel , it will be appreciated that in some circumstances the variation of detector response on spatial scales comparable to the detector pixel size will be small . in such cases , it may be unnecessary to store a detector response function for each pixel . for instance , in a gamma - ray camera system with a 50 cm square scintillator crystal detector and an imaging resolution of 5 mm , detector response functions may be determined with a spatial resolution of , for example , 1 cm . in this way a family of detector response functions are calculated corresponding to an array of 1 cm square elements spanning the scintillator crystal . when selecting a suitable detector response function to deconvolve form the observed data array corresponding to detector pixel at x , y , the detector response function corresponding to the 1 cm square element which includes detector pixel x , y is chosen . while the above described examples of gamma - ray camera systems have used anger - type gamma - ray cameras , it will be appreciated that in other embodiments spectra processing components of the type described above may be used in conjunction with other gamma - ray camera types . fig1 schematically shows in vertical cross - section a gamma - ray camera system 58 according to a further embodiment of the invention . many of the features of the gamma - ray camera system 58 shown in fig1 are similar to and will be understood from the correspondingly numbered features shown in fig5 and described above . however , in the example shown in fig1 a modified gamma - ray camera 60 is used . in this example , a discrete detector element gamma - ray camera imager 50 replaces the gamma - ray camera imager 14 shown in fig5 . in place of the single scintillator crystal 24 , light guide element 26 and family of photo - multiplier tubes 28 seen in fig5 , a detection plane comprising discrete scintillator crystal elements 52 coupled by discrete light guide elements 54 to individual photo - detectors 56 are used . this is type of gamma - ray imager is similar to the digirad “ 2020tc imager ” camera discussed above . the photo - detector signals are coupled to a read - out component 62 which is configured to provide a list mode output to the spectra accumulating component 12 similar to that described above . the spectra accumulating component 12 and spectra processing component 34 function as described above . as noted above , the detector response functions associated with the different detector pixels in a gamma - ray camera system are specific to each particular gamma - ray camera system . accordingly , appropriately calculated detector response functions may be provided along with a gamma - ray camera system at first supply to an end user . in cases where the functionality of the spectra processing component is to be provided by a general purpose computer , the calculated detector response functions and software operable to configure the general purpose computer may be supplied together or separately on a computer program product , for instance a cd - rom or other data - storage medium . in cases where the functionality of the spectra processing component is to be provided in firmware or hardware , for example by an asic , a fpga or a dsp , the calculated detector response functions can , for example , be stored in rom on an integrated chip along with the firmware . in addition to being shipped together at first supply , spectra processing components similar to those described above can be coupled to existing gamma - ray camera systems as an ‘ after - market ’ add - on for improving energy resolution . to provide the best possible improvements in energy resolution , an existing user of a gamma - ray camera system may provide a third party with access to the camera system , such that the third party can fully calculate the detector response functions using the techniques described above . alternatively , the existing user might supply the third party with appropriate calibration data from the gamma - ray camera from which the third party calculates the appropriate detector response functions to be supplied to the existing user . as above , the detector response functions may be supplied alone , for example on a data - storage medium , or along with a computer program product bearing machine readable instructions for implementing the functionality of a spectra processing component . in other cases , a hardware add - on may be supplied to an existing user . the hardware add - on including , for example , an appropriately configured interface for interfacing with the detector read - out component of the existing gamma - ray camera system , firmware or hardware , for example an asic , a fpga or a dsp , for providing the functionality of a spectra processing component , and a memory , for example rom or a replaceable data - storage medium , for storing appropriate detector response functions . while in the above examples , the detector response functions are calculated for individual gamma - ray camera systems , improvements in spectral resolution may also be achieved by deconvolving more generic pixel dependent detector response functions . for example , for commonly used configurations of gamma - ray camera system , detector response functions may be calculated for each detector pixel in a manner dependent only on properties of the scintillator material and the geometric configuration of the gamma - ray camera imager . this could be done , for example , by performing a monte carlo modelling simulation of a commonly used gamma - ray camera imager configuration without subsequently modifying the model according to specific calibration data obtained with an individual example gamma - ray camera imager . by disregarding effects which are specific to individual gamma - ray camera imagers , for example non - uniformities in the response of the actual scintillator material and / or photo - detectors employed , more widely applicable detector response functions may be provided . while the use of more generic detector response functions does not provide the maximum possible improvement in energy resolution , it does provide a way to quickly and easily improve the performance of a large number of similarly configured gamma - ray systems without requiring individual calibration . another way to obtain more generic detector response functions for a particular gamma - ray camera imager configuration would be to determine the average of a number of previously determined detector response functions seen in examples of the imager configuration of interest . k . b . parnham , r . k . davies , s . vydrin et al ., ‘ development , design and performance of a cdznte - 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