Patent Application: US-60128508-A

Abstract:
this invention relates to a coded aperture mask for use in diagnostic nuclear medicine imaging . the coded aperture mask consists of a sheet of radiation opaque mask material having a series of apertures extending therethrough . the thickness of mask material has an attenuation percentage of less than 75 % and , in a preferred embodiment , about equal to 29 %. the coded aperture mask also , in a preferred embodiment , has a lead attenuation tube and has a projection of the smallest hole occupying the same area as a single pixel of a detector . the invention extends to a diagnostic nuclear medicine imaging system which uses a 16 bit gamma camera as a radiation detector .

Description:
current thinking with respect to nuclear medicine imaging and percentage attenuation derives from the use of collimators which collimate rays but which cease to function correctly at lower percentages of attenuation of the rays and this has , to a large extent , militated against the use of coded aperture masks which have lower percentages of attenuation of the rays . this line of thinking has been carried over to imaging using coded apertures and there are two likely reasons for this : 1 . in coded aperture imaging , each point of the source projects the coded aperture pattern onto a detector . the shadow of the coded aperture is projected with the greatest contrast when the attenuation is at 100 %. decreasing the attenuation percentage increases the penetration of the coded aperture by the gamma - rays , and results in a lighter shadow , with less contrast . the encoded image becomes less visible . this is seen if one compares the shepp - logan phantoms illustrated in fig1 and 2 ; and 2 . a gamma detector is not continuous ( analogue ) in measurement , but rather has a specific number of values that can be measured ( it is discreet ). decreasing the contrast in the detected image results in a decrease in the number of values that can be measured , and an associated loss of accuracy . this is illustrated in the 2d plots in fig3 and 4 . central to the current invention is the theory that the current line of thinking as outlined above is not entirely correct . the reasons for this are as follows : 1 . the contrast of the projected image , which is encoded , is of little significance . this is because the decoded image , which is obtained by correlation , results from searching for the expected pattern . provided that the pattern is recognisable , there will be no loss of contrast in the decoded image . 2 . quantisation of the measured values decreases the quality of the encoded image . this decreases the likelihood of the pattern being recognisable , which results in noise in the decoded image . this is strictly dependent on the number of values ( related to the number of bits ) that can be measured . provided that there are a sufficient number of values available ( or enough discrete levels ), there should be no loss of quality in the decoded image . to test this thinking , a computer simulator based on ray tracing techniques , and capable of predicting image acquisition in the field of nuclear medicine , was developed . computer simulation results for a digital shepp - logan phantom [ 7 ] are presented . a high contrast digital shepp - logan phantom is clearly visible ( fig1 ). lowering the contrast reduces the visibility of the image ( fig2 ), as well as the viewer &# 39 ; s ability to discern features within the image . with respect to quantisation , a 2d signal is shown by way of example ( fig3 ). if the vertical axis has only a 5 - bit resolution available , the signal can no longer be represented with the same degree of accuracy ( fig4 ). in the results that follow , the only variation is the transparency of the coded aperture , and the number of bits of the gamma camera . that is aperture material , aperture type , dimensions of the holes , near - field geometry , and decoding procedure remain unaltered . the results are by way of example only , and are based on the energy of technetium - 99m ( 140 kev ), and a tungsten coded aperture . 8 - bit 97 % attenuation : the figure of 97 % corresponds to a material thickness of 1 mm . there is no loss of either contrast or image quality in the encoded image . the resultant decoded image ( fig5 ) is affected by near - field artifacts , and image quality is related to the coded aperture type . an 8 - bit gamma camera has a maximum of 256 measurable values . 8 - bit 29 % attenuation : the figure of 29 % corresponds to a material thickness of 100 μm . the low contrast encoded image results in a loss of accuracy . the projected coded aperture pattern is not sufficiently recognisable , and the decoded image is affected by noise ( fig6 ). 16 - bit 97 % attenuation : compared to an 8 - bit gamma camera , a 16 - bit camera has 256 × the number of measurable values . the attenuation of the coded aperture material is sufficient to prevent a loss of contrast , and the additional measurable values have little impact on the quality of the decoded image ( fig7 ). 16 - bit 29 % attenuation : the encoded image suffers from both a loss of contrast and a loss of accuracy . being a 16 - bit gamma camera , however , there are a sufficient number of measurable values for the projected coded aperture pattern to remain recognisable . as a result , the decoded image is not affected by noise ( fig8 ). there is also a simultaneous reduction of collimation artifacts . this image is acquired by incorporating the principals described in this invention . it is suggested that the above simulation results show that a highly transparent coded aperture adds noise to the image , when the gamma camera has an insufficient number of measurable values . a gamma camera with 2 × the number of bits appears to result in little improvement to image quality , when imaging with the prior art coded aperture . acquiring the image under the same conditions , but with the use of a highly transparent coded aperture ( such as is described in this invention ), together with a 16 - bit gamma camera , does not add noise to the image , and simultaneously reduces collimation artifacts . these results show that a highly transparent coded aperture can be used , without loss of image quality , when the gamma camera has a sufficient number of measurable values . at this time , gamma cameras typically feature 16 - bits ( as opposed to the 8 - bits of an earlier generation of gamma cameras ). an aperture that is 10 × thinner makes possible several ( previously problematic ) avenues of manufacture . of more significance is image resolution , as an aperture that is 10 × thinner automatically allows for a 10 × improvement in image resolution . in this case the gamma camera would become the limiting factor , but there are already some indications that gamma cameras may be able to follow suit [ 8 ]. the only change between the 16 - bit gamma camera test scenarios was that a high attenuation coded aperture was replaced with a highly transparent coded aperture . from a practical perspective , such a change can readily be made . it is anticipated that this will enhance the practicality of coded aperture imaging in the field of nuclear medicine . it should be noted that coded apertures with thicknesses in the μm range have been developed for imaging at lower radiation energies [ 8 , 9 ]. at these energies , the thickness of the coded apertures is such as to still yield a high percentage of attenuation , in accordance with the prior art . in addition to the above and when each of the multiple transparent elements of a coded aperture acts and is considered as an independent hole , with each casting a projection of the source onto the detector . as such , the partial volume effect is applicable to the recorded coded aperture image [ 6 ]. in order to decode the overlapping projections , it is necessary to consider the situation from another perspective , which is as each point of the source projecting the coded aperture pattern onto the detector [ 6 ]. provided that the pattern has specific properties , it is possible to uniquely decode the encoded data . the coded aperture pattern is a discrete binary array [ 1 ]. the decoding procedure therefore operates not in the continuous domain , but rather on the sampled measured data . this means that for a point source , a perfect reconstruction would be achieved if the projected coded aperture pattern were sampled as a single array of impulses or delta functions , all equal in amplitude , as indicated in fig9 . the closest approximation to projecting a set of impulses would be with the use of an idealised infinitely thin but completely opaque coded aperture , having infinitely small pinholes . consider three parallel planes representing the source , the coded aperture , and the detector . if the grids representing the discrete points of the source , the pinholes of the coded aperture , and the pixels of the detector are all perfectly aligned , a perfect detector will measure the desired sets of impulses in the pattern of the coded aperture . if these three grids are not perfectly aligned , the shift causes the impulses to fall away from the centre of each pixel . the resultant interpolation is equivalent to the measurement of overlapping impulse patterns . the impulses of an individual pattern will ideally have the same amplitudes , but this amplitude is reduced as a result of the partial volume effect . if the infinitely small pinholes are replaced with transparent coded aperture elements , such that each element illuminates a 1 × 1 pixel area of the detector , blur is then increased , but the system becomes less susceptible to the partial volume effect . regardless of the alignment of the three grids , the partial volume effect is completely removed by applying the prior art technique of illuminating a 2 × 2 pixel area of the detector [ 6 ]. however , the increased area of the projection results in the measurement of neighbouring impulse patterns , and thus in further blurring of the reconstructed image . gamma camera pixel size clearly sets the first limit on image resolution — this being the closest spacing at which samples can be obtained . the partial volume effect sets the second limit , as its solution requires the illumination of an area corresponding to 2 × 2 pixels of the detector . the point spread function ( psf ) of the detector further contributes to these limitations . transparent coded aperture elements are designed with respect to the pixel size of a specific gamma camera . sub - optimal patterns can be remedied without affecting the open fraction , as this remains constant for a given family of coded apertures . the dimensions of the transparent elements can be decreased for a higher resolution system , and the number of elements in the array can be increased , such that both the field - of - view and the open fraction of the material are maintained . there is no trade - off between resolution and imaging efficiency , provided that the illuminated area is not below that of a single detector pixel . at this sampling threshold the number of elements in the array can no longer be increased . the concept is illustrated in fig1 . apart from the partial volume effect , a coded aperture designed to illuminate a 1 × 1 pixel area of the detector gives a resolution that is optimal without compromising efficiency . consider the scenario of a source having a finite spatial extent , with the system maintaining ideality in all other respects . a finite source increases the illuminated area , and assists with countering the partial volume effect — as would be the case for the effectively continuous objects that are imaged in nuclear medicine . while a grid representing discrete points of the source may be useful for computational purposes , multiple grids of varying shifts would be necessary in order to represent continuity . our theory associated with the invention is that a realistic source , coupled with an optimal coded aperture , not only limits the partial volume effect , but also allows for the enhancement of system resolution . this thinking was tested by means of a ray - tracing computer simulator . the coded apertures were taken as being infinitely thin and completely opaque , with the aim of omitting the artifacts that are introduced by the use of realistic apertures . a perfect detector psf was used , unless stated otherwise . discrete point sources , both perfectly aligned and misaligned , were investigated for varying illumination areas of the detector , together with the simulation of distributed objects . a detector psf was also applied to the encoded distributed images , so as to allow for testing of the methodology under the presence of increased blur . the results are based on near - field imaging conditions . accorsi &# 39 ; s method for the reduction of near - field artifacts [ 4 ] was applied to all images . two digital point sources were positioned on the image diagonal . the source grid was perfectly aligned with respect to the rest of the system . a 1 × 1 area projection ( fig1 ( a )) gives a sharper image than a 2 × 2 area projection ( fig1 ( d )). the peak intensities are measured correctly in both cases . the worst - case partial volume effect is obtained by shifting only the upper source by half a pixel along both axes . the effect is clearly visible for a 1 × 1 area projection ( fig1 ( b )). the peak of the shifted source remains unaffected for a 2 × 2 area projection ( fig1 ( e )), but the peak of the stationary source is lower by comparison . finite sources were represented by superimposing a second point source grid over the first ; shifted one quarter of a pixel along both axes . the partial volume effect is less severe for a 1 × 1 area projection ( fig1 ( c )), relative to fig1 ( b ). for a 2 × 2 area projection ( fig1 ( f )) the peak of the stationary source has increased , relative to fig1 ( e ), but the blur remains . a two - dimensional slice of the digital shepp - logan phantom [ 7 ] was used for the simulation of distributed objects ( fig1 ). the phantom was represented computationally as a grid of point sources , shifted by half a pixel along both axes for worst - case alignment . the results are quantified by means of a root - mean - square error ( rmse ), which is computed over the entire image , and is based on the percentage by which pixels differ from the pixels of the phantom [ 11 ]. in the results that follow , the only variations are the dimensions of the coded aperture holes . that is aperture material , aperture type , aperture thickness , near - field geometry , and decoding procedure remain unaltered . the results are by way of example only . a theoretical aperture with infinitely small pinholes is not practical in terms of efficiency , but gives a reconstruction that is close to perfect ( fig1 ). with reference to the infinitely small holes , a 1 × 1 area projection is blurred ( fig1 ), but gives a sharper image and a lower rmse than a 2 × 2 area projection ( fig1 ). a blurring detector psf having σ = 1 . 27 pixels was then applied to the encoded images , prior to decoding . although the blur makes any resolution improvement difficult to discern visually , a 1 × 1 area projection ( fig1 ) gives a lower rmse than a 2 × 2 area projection ( fig1 ). the above described simulation results show that an idealised coded aperture having infinitely small pinholes , used in conjunction with a gamma camera having a perfect psf , does not give a perfect image . the suggested reasons are twofold . firstly a worst - case alignment of the system grids was used . secondly , a near - field imaging geometry means that for a single point source , the projected impulse array will no longer have impulses of equal amplitudes [ 5 ]. this is one cause of near - field artifacts . coded apertures having finite transparent elements make it possible to adjust image resolution without affecting system efficiency , provided that the elements illuminate an area that is not below that of a single detector pixel . this allows a 2 × 2 area projection ( the prior art coded aperture ) to be replaced with a 1 × 1 area projection ( such as is described in this invention )— a methodology that both enhances resolution , and reduces the rmse measurement . significant blur minimises the resolution improvement . nevertheless , the rmse indicates that a 1 × 1 area projection remains preferable . the simulation results show that resolution can be enhanced by illuminating a 1 × 1 pixel area of the detector . this has been quantified by a root - mean - square error measurement . furthermore , the partial volume effect has less influence on sources that are of finite dimensions , and the results show no significant influence on distributed sources such as those imaged in nuclear medicine diagnostics . the only change between the test scenarios was that a 2 × 2 pixel area projection coded aperture was replaced with 1 × 1 pixel area projection coded aperture . from a practical perspective , such a change can readily be made . it is anticipated that this will enhance the practicality of coded aperture imaging in the field of nuclear medicine . it should be noted that coded apertures with small holes have been developed for imaging at high resolution [ 8 ]. at the chosen imaging geometry , the size of the holes is such as to still yield a minimum of a 2 × 2 pixel area projection , in accordance with the prior art . the above theoretical investigation was tested experimentally using a coded aperture mask designed with respect to a philips axis — dual head variable angle gamma camera , for the energy of 99m tc ( technetium - 99m which has an energy of 140 kev ). a self - supporting pattern was used , from the no - two - holes - touching ( ntht ) modified uniformly redundant array ( mura ) family of coded apertures [ 6 ]. the pattern has an open fraction of 12 . 5 % and is based on a 61 × 61 mosaic , which is pattern centered and anti - symmetric . this allows for near - field artifact reduction by rotation of the aperture [ 4 ]. technical and practical challenges are associated with coded aperture construction . materials with high attenuation characteristics include uranium , platinum , gold , tungsten and lead . the aperture pattern requires square holes with vertical walls . the opaque aperture was constructed from tungsten of thickness 1 mm , corresponding to an attenuation of 97 %. a photograph of this aperture is shown in fig1 . the pattern was obtained by laser drilling a tungsten sheet . for a highly transparent aperture ( fig1 ), tungsten foil of thickness 100 μm was used , corresponding to an attenuation of 29 %. the pattern was obtained by laser ablating a thin frame on the border of each hole . mechanical strength was provided by an aluminium backing plate . to facilitate mounting and alignment of the coded apertures , a specialized aluminium gamma camera frame was designed ( fig2 ). the frame matched the mounting mechanism of the gamma camera , and allowed for the attachment of separate carriages . each carriage was rotatable through 90 ° and supported a coded aperture and lead shielding tubes . the carriages allowed the coded apertures to be centered with respect to the axis of rotation , and to be set parallel to the crystal of the gamma camera . the rotational technique for reducing near - field artifacts was applied to all coded aperture images and , in the below - described figures , the maximum pixel value and acquisition time refers to the encoded image , prior to rotation . for the simulation count statistics were implemented by altering the number of counts that were acquired by a given pixel of the encoded image , in accordance with the poisson distribution . this results in a projected aperture pattern that is less recognizable , and can be expected to degrade the highly transparent coded aperture images of our previous work [ 12 ]. a line set at 45 ° to the horizontal was used as a digital phantom , in order to allow for comparison with the experimental data . all simulation factors were held constant , apart from the acquisition bit - depth ( fig2 a , b and c ). count statistics require an increased bit - depth in order for the snr to not be adversely affected . nevertheless , the simulation results show that the concept of high - transparency coded apertures remains applicable . for all measurements , a 99m tc 1 ml syringe source was positioned at a distance of 20 cm from the philips axis crystal . the sensitivity of the system — comprising the gamma camera and the highly transparent coded aperture — was examined in terms of count rate measured in counts per second ( cps ), as a function of source activity . the results are presented in table 1 below and provide an indication of the region of optimal count rate , which was of greater interest than optimal sensitivity . the opaque coded aperture results in some noise at a maximum pixel count of 286 . this is evident in fig2 while , at a maximum pixel count of 3270 the noise is reduced ( fig2 ), but near - field artifacts remain evident . coded aperture resolution is superior to that of the lehr collimator , as is particularly evident where the syringe tapers on the upper right hand side of the image . note that both the collimator and the coded aperture images were acquired at identical geometries , however lehr resolution would be expected to improve as the source approaches the crystal . the highly transparent coded aperture results in significant noise at a maximum pixel count of 285 ( fig2 ). the noise clearly decreases at a maximum pixel count of 8248 ( fig2 ) and is comparable to the improvement predicted by the simulations . the experimental results support the concept of high - transparency coded apertures . it is possible not only to obtain an image with a highly transparent coded aperture , but also to acquire an image of a quality that approaches that of an opaque coded aperture based purely on the bit - depth or in other words count statistics of the acquisition . the practicality of highly transparent coded apertures must be viewed in terms of source activity and image acquisition time . the optimal count rate for the philips axis gamma camera and the 29 % attenuation coded aperture occurs in the 22 . 2 mbq ( 600 μci ) range , for a source at a distance of 20 cm from the crystal . if not limited by the gamma camera , a source in the 222 mbq ( 6 mci ) range would , for example , allow for practical 16 - bit image acquisition times , but this may not be clinically realizable . with the use of a higher sensitivity gamma camera , highly transparent coded apertures could be used without a decrease in the snr . coded aperture manufacture would be greatly simplified , and thickness artifacts reduced . alternatively , opaque coded apertures allow for the rapid acquisition of low source activity images , but thickness artifacts remain . in conclusion , planar phantom study results , for a 24 . 4 mbq ( 660 μci ) syringe source positioned at a distance of 20 cm from the philips axis crystal , show that a coded aperture with an attenuation of 29 % allows an image to be acquired , and that the image approaches that of an opaque coded aperture as the bit - depth of the acquisition increases . the result is not only comparable to that predicted by the simulations , but also serves to support the novel concept of highly transparent coded apertures in diagnostic nuclear medicine . furthermore , a coded aperture which has a projection of the smallest hole occupying the same area as a single pixel of the detector can also be applied to a coded aperture which has an attenuation percentage that is low relative to the state of the art , such as 50 % or less . in fact , the combination of the two inventions may give the optimum result for the application of coded apertures in nuclear medicine imaging . accorsi , r ., gasparini , f ., and lanza , r . a coded aperture for high - 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