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Dual screen sandwich configurations for digital radiography

A. R. Lubinsky, Adrian Howansky, Hao Zheng, Wei Zhao

A. R. Lubinsky, Adrian Howansky, Hao Zheng, Wei Zhao, "Dual screen sandwich configurations for digital radiography," Proc. SPIE 10573, Medical Imaging 2018: Physics of Medical Imaging, 105735U (9 March 2018); doi: 10.1117/12.2293673 Event: SPIE Medical Imaging, 2018, Houston, Texas, United States Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 3/11/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

Du ual screen sandwiich configuration ns for diggital radioography A.R. A Lubinskky, Adrian Howansky, H H Zheng, Wei Hao W Zhao Departmeent of Radio ology, Stony Brook Univversity, Health Sciences Center L4-120, Stony Brook, B New York, USA, 117900-8460; AB BSTRACT Motivated byy recent advan nces in TFT arrray technologyy for display, this study devvelops a theoreetical treatmentt of dual granular scinntillating screen ns sandwiched around a lightt detector and applies a this to investigate i posssible improvem ments in imaging perrformance of indirect activve-matrix flat-ppanel imagerss (AMFPI’s) for x-ray appplications, whhen dual intensifying screen configu urations are used. Theoreticcal methods, based b on previious studies off granular inteensifying screens, are developed and d applied to caalculate modulaation transfer function f (MTF F), normalized noise power spectrum s (NNPS), Sw wank factor (A As), Lubberts function f L(f), and spatial frequency-depe fr endent detectivve quantum effficiency (DQE(f)) forr a variety of dual-screen d dettector configurrations. Single--screen front illluminated (FI)) and back illuuminated (BI) configurrations are also o included in the t analysis. DQE(f) D is used as a performaance metric to optimize and compare c the performaance of the variious configurattions. Large im mprovements inn performance over single-scrreen systems are a found possible, wheen the substratte layer betweeen the light sennsing array andd the intensifyiing screen is optically o thin. The T ratio of the thicknesses of the tw wo screens whicch optimizes DQE D performannce is generallyy asymmetric with w the thinneer screen facing the inccident flux, and d the ratio depends on the x-rray attenuationn length in the phosphor p mateerial. Keywords: flat-panel f detecctor, digital raddiography, indiirect detection,, Swank factor,, Lubberts

1. INT TRODUCTIO ON me common in clinical appplications off digital Indirect actiive-matrix flaat-panel imageers (AMFPI’ss) have becom radiography. Large area thin film transisttor array technnology has reacched an impressive level of cost/performannce after much investm ment by the display d industryy, and consideerable effort haas been expennded to facilitaate the developpment of detectors satiisfying the req quirements of medical m x-ray imaging.1 Thee arrays typicaally consist of pixels p which contain c a hydrogenatedd amorphous silicon s (a-Si:H) switching traansistor, coupled with an a-S Si:H photodiodde sensor. The array is placed in inttimate contact with a scintillaating screen which w absorbs x-rays x and prooduces light whhich is detected by the sensors of thhe array to gen nerate electricaal charge signaals. The chargees are stored onn the pixel cappacitances andd may be read out by sequentially sw witching rowss of TFT’s to an ON state, which w transferrs the charges to an array of charge sensitive ampplifiers, whosee signals are seent to an ADC which digitizees the signals, which w then cann be stored in memory. m An illustratioon of such a deetector is shownn in Figure 1.

Figure 1. Illusttrating a typical indirect active matrix m flat panel imager, using a single screen, exposed e in front illumination (FI) mode.

Medical Imaging 2018: Physics of Medical Imaging, edited by Joseph Y. Lo, Taly Gilat Schmidt, Guang-Hong Chen, Proc. of SPIE Vol. 10573, 105735U © 2018 SPIE · CCC code: 1605-7422/18/$18 · doi: 10.1117/12.2293673 Proc. of SPIE Vol. 10573 105735U-1 Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 3/11/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

Indirect AMF FPI’s have gen nerally been fabbricated and opperated in a “ffront illuminateed” (FI) mode, i.e. the sensorr array is placed below w the intensify ying screen, annd x-rays are inncident from the t top. Some work includinng the developpment of 2,3 commercial devices d has also been done on the “backk illuminated” (BI) ( mode, in which w x-rays would w be incident from the bottom. This T state of affairs a is in coontrast to the prior p history off screen/film (S/F) ( radiograpphy, where tw wo-screen systems, com mprising a duall-emulsion film m sandwiched between b two inntensifying screens were a staandard operatinng mode for a numberr of decades. The overall thickness t of a single x-ray sccreen used for a particular appplication is historically chossen based on a tradeoff between x-raay absorption and spatial ressolution. Increeasing screen thickness t imprroves absorptioon and thus seensitivity (dose), but also a decreases resolution beecause of lightt scattering in the phosphorr layer, necesssitating a comppromise. Elementary considerations c suggest that a 2-screen connfiguration in which the sinngle screen is divided d into tw wo parts sandwiched around a lightt detector will improve perfoormance becauuse the averagge distance from m an x-ray abbsorption event to the detector is lesssened, thus miinimizing lightt scatter. Recennt developmennts in TFT arraay technology4 indicate the possibiliity of arrays manufactured m on thin, flexiible, and posssibly transpareent substrates, which motivvates the investigationn of performan nce improvem ments possible with dual-screeen AMFPI detector configurations. A scchematic illustration of a possible du ual-screen detecctor is shown in Figure 2.

Figure 2. Scheematic of a possiible dual screen radiographic r imager, showing frront screen (1), back b screen (2), light detector layyer, and thin transparennt substrate.

In the presennt work we stu udy potential im mprovements in i detective quuantum efficienncy (DQE) for a number of proposed p detector conffigurations, usiing theoretical methods for deescribing the x-ray x and opticaal interactions in intensifyingg screens developed byy numerous otther workers. Light L scatterinng in the phospphor material, which gives rise r to the moodulation transfer funcction (MTF) off the screen, is described by the t Swank5 moodel for light-sscattering phossphors, in the diffusion d limit. In the model, m the amo ount of light sppreading depennds strongly onn the depth of x-ray x interactioon within the phosphor p layer, and inn fact these deepth effects are experimentaally found to be b substantial for typical graanular screens such as GOS.20 The methodss used by Rabb bani et al.6 andd Nishikawa ett al.7 are used to t model the innteractions in thhe screen as a series of cascaded stochastic events,, leading to ann expression foor the DQE as a product of thhe x-ray quantuum efficiency and two noise factors, one of which quantifies the variation in thhe magnitude of o response to an a absorption event: e “Swank factor”,8 9 and one quaantifying the variation v in sppatial response to an event: “Lubberts Funnction”. The two noise facctors are calculated inn a depth-depen ndent model foor each proposed detector configuration, annd the results are a combined to t obtain the DQE, whhich is then useed to optimize the detector design d in terms of screen masss loadings (thiicknesses) and backing reflectances. Although staccks of screens and light detecctors have beenn studied prevviously for application to duaal energy radiography 10 11, and work kers have descrribed12 and chaaracterized13 the imaging perfformance of duual screen-film systems 14 and the possiibility of digitaal dual screen systems has beeen mentionedd in the patent literature l , theere appears nott to have been any preevious attempt to theoretically optimize thee performance of systems com mprising light--scattering scinntillating screens placeed on both sidees of a light dettecting layer.

Proc. of SPIE Vol. 10573 105735U-2 Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 3/11/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

2. METHODS 2.1 Single screen case DQE is the metric used to compare the performance of digital x-ray detector systems. We can write the spatial frequency-dependent DQE(f) as:7

DQE ( f ) = Aq As L ( f ) RN ( f )

(1)

where Aq is the x-ray quantum absorption efficiency, As is the Swank factor, and L(f) is the Lubberts factor. RN(f) is the ratio of the quantum noise power to the total system noise power, which may include other effects like secondary noise, structure noise, detector noise, or electronic noise. Here we consider the case of prompt screens with high conversion gain, and assume that structure noise, electronic noise, and secondary noise are small compared to quantum noise. DQE(0) is then reduced by As, and L(f) describes the rolloff of DQE(f) with spatial frequency. The product AqASL(f) provides a measure of DQE prior to losses in the light detector, and is used for screen configuration optimization purposes. We will use methods for calculating physical quantities such as light escape efficiency and MTF, required for evaluation of performance parameters like As and L(f), which are appropriate for granular phosphor screens. Considering x-ray energies such that K-fluorescence escape can be neglected and also assuming the Poisson excess in photon amplification to be small, the Swank factor is due to optical effects in the screen, and can be calculated as: 15

{∫ w( z)ε ( z)dz} T

As =

2

0

T

C ∫ w( z )ε ( z ) 2 dz

(2)

0

where w(z)=exp(-μz) is a weighting function accounting for exponential x-ray attenuation, ߳(z) is the light escape efficiency,15 z is depth in the screen measured from the x-ray entrance side, µ is the linear attenuation coefficient, and C = (1- exp(-μT)) / μ is a normalizing factor. The Lubberts factor L(f) is due to the depth dependence of MTF in the screen and is given by:7,16

MTF 2 ( f ) L( f ) = NNPS ( f )

(3)

where NNPS(f) is the (normalized) quantum noise power, and MTF and NNPS are expressed as integrals over the depth of the screen:

∫ MTF ( f ) =

T

0

∫ NNPS ( f ) =

T

0

w( z )ε ( z )MTF ( z , f ) dz Nm

w( z )ε 2 ( z )MTF 2 ( z , f ) dz Nn

(4)

(5)


where MTF(z,f) is the MTF due to a line source at depth z, and T is the screen thickness. Nm and Nn are normalizing integrals for MTF and NNPS respectively:7 T

N m = ∫ w( z )ε ( z )MTF ( z, 0)dz 0

T

N n = ∫ w( z )ε 2 ( z )MTF 2 ( z, 0)dz 0

(6)

(7)

2.2 Dual screen case In this case the detector signal is comprised of parts originating in each of the two screens, as in Figure 2, and the integrals above correspondingly split into two parts. Equation (4) for MTF, for example, becomes

∫ MTF ( f ) =

zd

0

T

w1 ( z )ε1 ( z )MTF1 ( z , f )dz + ∫ w2 ( z )ε 2 ( z )MTF2 ( z , f )dz zd

Nm

(8)

where MTF1(z,f) is the MTF due to a line source at z in screen 1, є1(z) the escape efficiency and w1(z) the weighting function in the same region, and similarly for screen 2. We assume the two screens have the same material properties, and that the light detector is located at depth zd. Making a change of variable in the second integral and writing the w(z)’s explicitly, we have

∫ MTF ( f ) =

zd

0

e − μ z ε 1 ( z )MTF1 ( z , f ) dz + ∫

T − zd

0

e − μ (T − z )ε 2 (T − z )MTF2 (T − z , f ) dz

Nm

(9)

The Swank model5 provides simple expressions for the escape efficiency and for the depth-dependent line source MTF in the case when the light scattering coefficient in the phosphor is large and the optical absorption coefficient is small (“diffusion limit”), representative of an optically highly turbid granular phosphor screen. A more complicated model incorporating finite optical scattering and absorption coefficients for a particular screen could also be used, in further studies. For the case of reflective (white) backing boundary conditions, the escape efficiency is unity and the line source MTF is

MTF W ( z , f ) =

cosh(2π fz ) cosh(2π fT )

(10)

where T is the thickness of the phosphor and z is measured from the side opposite to the detector. In the case of absorptive (black) boundary conditions the escape efficiency is z/T and

MTF B ( z , f ) =

sinh(2π fz ) sinh(2π fT )

(11)

The resulting MTF for the dual reflective (white) backing screen case then becomes:


MTF W ( f ) =

∫

zd

0

T − zd ⎛ cosh(2π fz ) ⎞ ⎞ cosh(2π fz ) − μ (T − z ) ⎛ e− μ z ⎜ ⎟dz + ∫0 e ⎜ ⎟dz ⎝ cosh(2π fzd ) ⎠ ⎝ cosh(2π f (T − zd )) ⎠ Nm

(12)

In the above case Nm = C. The results for As and NNPS can also be cast into sums of integrals for the various cases, in the same way as above:

{∫ C {∫

zd

As ( f ) =

e − μ zε1 ( z )dz + ∫

0

0

zd

0

NNPS ( f ) = W

∫

zd

0

T − zd

e

−μz

ε ( z )dz + ∫ 2 1

}

e − μ (T − z )ε 2 ( z )dz

T − zd

0

e

− μ (T − z )

2

e

−μz

2

}

(13)

ε ( z )dz 2 2

2

T − zd ⎛ cosh(2π fz ) ⎞ ⎞ cosh(2π fz ) − μ (T − z ) ⎛ ⎜ ⎟ dz + ∫0 e ⎜ ⎟ dz ⎝ cosh(2π fzd ) ⎠ ⎝ cosh(2π f (T − zd )) ⎠ Nn

(14)

2.3 Calculations The results for AS, MTF, NNPS are expressed as sums of integrals as above, and the integrals are evaluated analytically using the symbolic math kernel of Maple®, as implemented in MATLAB®. This results in algebraic expressions for As, MTF, NNPS in terms of x-ray attenuation coefficient, spatial frequency, detector position, and the thicknesses of the two screens. The algebraic expressions are rather lengthy and are not reproduced here, for brevity. The expressions are coded in functions which are called to evaluate the performance parameters for various detector configurations and screen types. The standard front and back illumination cases are included in the analysis as, for example, when the thickness of the “back” screen becomes zero.

3. RESULTS 3.1 Theoretical results First consider the simple case of a single 160 micron granular screen split into two halves, sandwiched around a thin light detector layer. Also assume that the x-ray absorption is (nearly) uniform with depth, and that the screens have reflective (white) backings. Figure 3 compares the MTF’s for the single vs. dual screen configurations (lower curves), and also the ratio of the MTF’s as a function of spatial frequency (upper curve). It is noted that the MTF at higher frequencies (near 5lp/mm) is roughly doubled in the dual-screen case. This behavior is similar when the Lubberts factors L(f) are compared. Further note that in the theory MTFdual(2f) = MTFsingle(f) (meaning that the dual screen MTF at 4 cy/mm, for example, is equal to the single screen MTF at 2 cy/mm) and similarly for L(f) as shown in Figure 4, for reflective (white) backing screens at uniform exposure. This indicates that the spatial bandwidth for information transfer from the screens to the light detector (DQE spatial frequency bandwidth) is doubled with dual screens, in this case.


Figure 3. Com mparing the MTF F for a single 1600 micron screen in FI mode withh the MTF for a dual screen systtem comprising two t 80 micron screens sandwiched arround a thin lightt detector layer. The solid curves are for the casee of uniform x-raay exposure, andd the dashed curves are for the case of an x-ray beam m with attenuation as illustrated in Figure 6.

mparing L(f) forr a single 160 micron m screen in FI mode with L(f) L for a dual screen s system coomprising two 80 8 micron Figure 4. Com screens sandw wiched around a thin t light detectoor layer.

If a TFT arraay is deposited d onto a glass substrate and light from thee back screen must m pass throough the opticaal gap to reach the ligght detector, th hen there will be additional spreading and loss of MTF due to light diffusion d in thee gap. 17 Figure 5 shoows the MTF improvement ratio with duual screens as function of the t thickness of o the glass suubstrate, assuming thee index of refraaction of the trransparent subbstrate is n = 1.65. It noted thhat in order to maintain a suubstantial improvementt factor, an opttically thin layeer is necessary: less than 20 or o 30 microns.


Figure 5. Illusstrating the effecct of an optical substrate layer with n = 1.65 on o the MTF of the dual screen system, as a fuunction of substrate thickkness.

Figures 6 andd 7 show the results r of a seriies of calculatiions in which a single intensiifying screen laayer is subdiviided into two parts of different d relative thicknessess, sandwiched around a a photoosensor layer, assumed a to havve negligible thhickness, at the positioon shown on th he x axis. The total thicknesss of phosphor is i 160 micronss. The screens have h reflectivee (white) backings, andd the performaance parameterrs are evaluatedd at 5 lp/mm. Under U these coonditions the zero-frequency DQE(0) will be the same for all configurations, but b the higher-frequency MTF TF and SNR peerformance willl vary, as indicated by the calculatedd MTF(f) and DQE(f). D

Figure 6. (a) Showing S the MT TF and NNPS off dual screen syystems, at 5 cy/m mm, as a functioon of front screeen thickness. Thhe screens have perfectlyy reflective (whitte) backings. Thhe total screen thhickness is 160 microns. m X-ray beam b attenuationn vs. depth is also shown for reference. Zero Z thickness corresponds c to a BI single screenn, and 160 microons thickness corrresponds to a FII single screen.

The front illuuminated (FI) configuration c c corresponds to the rightmost point p (x=160 microns), m and a back illuminaated (BI) single screenn system would d correspond too the leftmost point p (x=0). In figure 3a, the x-ray x photon number n attenuaation profile as a function fu of depth into the phoosphor layer is shown for refeerence, for an x-ray x beam havving the same linear l attenuation as a an RQA5 speectrum (70kVpp, 21 mm of All), incident on the x=0 side. The T MTF and NNPS N (normaliized noise power spectrum) are shown, for eacch detector connfiguration. It is noted that thee MTF at the optimal o relativee r imp proved over thhat in the FI sysstem, by a factoor of more thann 3. The thicknness ratio which thickness is remarkably


maximizes MTF M is asymmeetrical with thee front (top) scrreen being mucch thinner, aboout 25% of the total single-scrreen thickness.

Figure 7. Shoowing the Lubb berts function L(f) and the DQE E of reflective (white) backingg dual screen syystems, at 5 cy/m mm, as a function of froont screen thickness. The total screen thicknesss is 160 micronss. Zero thicknesss corresponds too a BI single screen, and 160 microns thhickness corresp ponds to a FI singgle screen.

In Figure 7, the DQE(f) an nd L(f) are shoown, for each detector conffiguration. It iss noted that thhe DQE at the optimal relative thickkness is much improved oveer that in the FI F system, by a factor of morre than 2. Thee thickness ratiio which maximizes DQE(f) D is asym mmetrical with a thinner frontt (top) screen, about 37% off the total singlle-screen thickkness. To maximize siggnal-to-noise raatio, the DQE would w take preecedence in settting the optimaal front layer thhickness. The relative thickness t conffiguration that maximizes m DQ QE(f) depends on o the x-ray atttenuation profiile through the detector stack, which in turn depen nds on the ratioo of x-ray attennuation lengthh to phosphor thickness. t The calculations shown s in Figure 7 weere repeated for fo a number of different x-ray x attenuatioon lengths to find the optiimal configuraation for maximizing DQE D at 5 cy/m mm. Figure 8 shhows the frontt screen fractional thickness which w maximizzes DQE, as a function of the total phosphor p thickn ness in units of HVL. The thhinner screen always a faces thhe incident x-raay flux. It is nooted that when the atteenuation length h is large (smaall T/HVL, highh energy x-rayy beam), the opptimal configurration approachhes 50% front screen thickness ratio o, and when thhe attenuation length l becomees small compaared to the phoosphor thickness (large T/HVL, low energy x-ray beam), b a relativvely thinner froont screen is opptimal.


Figure 8. Rattio of front screeen thickness to total thickness which w maximizees DQE at 5 cy//mm vs. total thickness in units of HVL. Thinner screenn always faces in ncident x-ray fluux.

For the casee of screens with w an absorpptive (black) backing, b the optical o compoonent of the Swank S factor becomes b important and must be conssidered. The Sw wank factor As was calculated for a variety of detector configurations, and a these calculations were w then repeeated for a num mber of x-ray attenuation a lenggths. Figure 9 shows s the (opttical) Swank faactor, for a range of deetector configurrations and atteenuation lengthhs. Here the sccreens have abssorptive (blackk) backings.

Figure 9. (a) Swank S factor Ass vs. front screenn thickness for dual d screen systeems with absorpptive (black) bacckings. Various values of T/HVL ratio are a shown, corressponding to diffe fering degrees off attenuation of thhe x-ray beam.

In the limit of o large attenu uation length (ssmall T/HVL),, when the totaal phosphor thhickness in uniits of HVL appproaches zero, AS = 0.75, as originallly pointed outt by Swank him mself.5 As the x-ray attenuatiion length decrreases (larger T/HVL), T AS increasinggly depends on n detector conffiguration. Thee single-screen back illuminaated (BI) configguration is alw ways best for Swank faactor, and the FI configuration is worst. This T is becausee in BI, absorpption events neear the detector, which have the highhest escape effficiency and thhe most photonns detected are weighted the heaviest, and the t opposite iss true for the FI case. The Lubberts factor, Swan nk factor, and DQE(f) D have also a been calcuulated as a funnction of detecctor position inn various dual-screen configurations c for the case where the scrreens have abssorptive (blackk) backings. Both B the MTF and the


Lubberts factor are found to improve with absorptive (black) backing screens, but the lower Swank factor causes the DQE(f) at the optimal configuration to be lower than that for reflective (white) backing screens. The low-frequency DQE will also be lower for absorptive (black) backings because of the Swank factor loss. Experimental verification of the theoretical results using a “bare” TFT array optically coupled to phosphor screens of various types and thicknesses and x-ray illuminated from both (FI and BI) directions is in progress. Preliminary results indicate good agreement between experimental results and theory for both MTF and DQE. There is also an initial indication of improved DQE performance for thick dual granular screen combinations over several commercial AMFPI’s for high energy (RQA9-type) applications.

4. DISCUSSION The simple (neglecting K-fluorescence and x-ray scatter effects) model calculations above illustrate the potential for using the strong depth dependence of the point spread function and MTF in granular screen materials to achieve improvements in imaging performance with optimized dual-screen type indirect detector configurations. Other advantages that might also accrue with the successful development of such imagers include low cost because of the lack of expensive vacuum deposited layers of scintillator or photoconductor, the possibility of a flexible detector to minimize geometric blur, the absence of x-ray scattering from heavy elements in a thick glass substrate18, and the additional design freedom (relative thicknesses) to optimize for different applications (x-ray energies). In order to realize the advantages in detected information theoretically possible in the dual-screen approach, however, the light detector array must be made capable of efficiently sensing and recording this information. This presents a number of challenges. It was noted above that in order to maintain a substantial improvement factor in MTF and Lubberts factor, an optically thin layer is necessary: less than 20 or 30 microns. Such a thin layer might be achieved in practice by adhering a thin detachable layer to a rigid substrate such as glass during array manufacture, then releasing the array and thin substrate layer before placing them in contact with a scintillating screen.4 A second method would be to manufacture the photosensor array directly on the surface of the thicker (bottom) screen (with a possible thin transparent protective layer) by, for example, ink jet printing means.19 In early S/F technology, the potential performance improvements with dual screens were not fully realized due to “crossover”, i.e. the penetration of light photons through the film emulsion, followed by reflection by the screen on the opposite side. In the case of a dual-screen digital detector, light must accordingly not be allowed to penetrate the photodiode layer and reflect back from the opposite screen. Also, care must be taken to minimize light scatter within the pixels, to prevent light from reaching the photosensor along any path other than directly from the adjacent screens. Finally, the sensing element must also have high sensitivity to light incident from both directions.

5. CONCLUSIONS An analytical theory for the imaging performance of granular x-ray screens sandwiched around a light detecting layer was presented. The theory was used to study alternative indirect AMFPI detector configurations including backilluminated and dual-screen types. Optimized dual-screen configurations were found to give large potential improvements in higher-frequency MTF(f) and DQE(f) over the standard single-screen configuration with the same total phosphor thickness and x-ray absorption. Alternatively, higher values of DQE(0) can be achieved with the same higherfrequency SNR response as the standard, using the dual-screen approach.

Acknowledgements The authors gratefully acknowledge financial support from the NIH (R01EB2655). They are grateful to John Rowlands for help and advice.

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