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IEEE IST 2004 Intcmational Workshop ~ on Imaging Systems and Tcchniqun Strcsa, Italy, 14 May 2004 A Comparative Evaluation of Scatter Correction Techniques in high Resolution Detectors Based on PSPMTs and Scintillator Arrays E. Karalil, G. Loudos', N. Sakelios', K. S. Nikita', N. Giokaris2 ' School of Electrical and Computer Engineering, National Technical University of Athens, 9 Iroon Polytechniou Str., Zografos, 15780, Athens, Greece, Phone:+30(2 10)7722285, Fax:t30(2 10)7723557, email:knikita@cc.ece.ntua,gr 21nstituteof Accelerating Systems and Applications, P.0.Box 17214, 10024, Athens, Greece 2NationalKapodistrian University of Athens, Panepistimiou 30, 10679, Athens, Greece Abstract - SPECT images suffer from low conlrast (1s a result o f Window Subtraction Technique (DEWST), the Convolution photons scatter. The standard method for excluding scatter Subtraction Technique (CST) and a Deconvolution component in pixelized scintillators b the application of an energy Technique. All techniques are compared to the standard window around the central photopeak channel o each crystal cell, f method. but small angle scattered photons still appear in the photopeak window and they are included in the reconstructed images. A number o scatter correction techniques have been proposed in f 11. MATERIALS AND METHODS order to estimate the scatter component but they have not yet been applied in pixelized scintillotors, where most groups use the A . Data acquisition standard one-photopeak windowfor scatter correction. In this work we have assessed three subtraction techniques that The gamma camera that was used for the evaluation of use a different approach in order to calculate the scatter component DEWST, CST, DT and the standard method is based on a and subtract itfrom the photopeak image: The Dual Energy Window Hamamatsu E 4 8 6 PSPMT and a pixelized CsI(TI) Subtraction Technique (DEWS?) the Convolution Subtraction scintillator, 4.6cm in diameter, 4mm thick, with cell size Technique (CS?) and a Deconvolution Technique (Or). AN these techniques are compared to the standard method 1.13x1.13mm2. The spatial resolution of the system has been measured and found <2mm in planar imaging [3]. Keyworrls -scatter correction, pixelized scintillators B. Dual Energy Window Subtraction Technique I. INTRODUCTION The Dual Energy Window Subtraction Technique Small field of view detectors based on Position Sensitive (DEWST) proposed by Jaszczak et al. [7] involves data Photomultiplier Tubes (PSPMTs) coupled to pixelized collection in a lower energy window, which provides a scintillators offer improved spatial resolution compared to the reasonable approximation for the scatter component in the standard Anger camera and have been used in the last decade photopeak window. Two images are used for scatter in dedicated small field of view systems for Single Photon compensation; the photopeak image P(u) and the lower Emission Computed Tomography (SPECT) [1][2]. Their () energy window image L u . If C(u) is tbe corrected image main applications are planar and tomographic imaging of then: C ( u ) = P ( u ) - U ( u ) (1) small animals in laboratory environment and clinical scintimammography for the early detection of small breast where U is the vector of image pixels coordinates and k is tumors [3]. However SPECT images still suffer from low the weighting factor. The average optimal value for k was contrast as a result of photons scatter, which affects the found to be equal to 0.5 [4]. quantification of SPECT images [4]. The standard method for excluding scatter component in pixelized scintillators is the C. Convolution Subtraction Technique application of an energy window around the central photopeak channel of each crystal cell, but small angle The Convolution Subtraction Technique (CST) proposed scattered photons still appear in the photopeak window and by Axehson et al. [5] assumes that the scatter component in they are included in the reconstructed images. A number of the photopeak window can be estimated as the convolution of scatter correction techniques [5][6][7][8][9] have been the measured photopeak data P(u) with a characteristic proposed in order to estimate the scatter component. These function f ( x ) that can be modeled as an exponential techniques have not yet been applied in pixelized function, which is derived Bom the system's Line Spread scintillators, where most groups use the standard one- Function (LSF) in a scattering medium. In order to determine photopeak window for scatter correction. In this work we f ( x ) a capillary I . l m inner diameter filled with a v c have assessed three subtraction techniques that use a different approach in order to calculate the scatter component and solution was placed in the center of a water filled cylindrical subtract it &om the photopeak image: The Dual Energy pot, 15cm in height and lOcm in diameter. Data were collected and line profile of the capillary activity was drawn. 0-7803-859 I-8/04/$20.00 2004 E E E 18 This line profile stands for the system's LSF. Using a least lOcm in diameter, containing a 3001111 99mTc solution, squares method, the one dimensional exponential function 2mCi/ml. The activity ratios of the hot spots to the that best fitted the theoretically linear part of log(LSF) was: background were 1O:l (SI) and 5:l (S2) respectively. The fix) = 0. 0247e"01x1 (2) detector was placed at a lOcm distance from the bottom of the pot. The breast phantom was imaged for 16min and where x denotes distance in pixels from the center of the -790,000 counts were collected. camera's field of view (FOV). 111. RESULTS D. Deconvolufion Technique The results from the application of the three methods Deconvolution techniques (DT) [IO] assume that the (one-photopeak, DEWST, CST and DT) are presented in measured data P(u)derive from the convolution of the true Fig.1 and Tables 1-111. As it can be seen &om the images and data C(u)and a characteristic two-dimensional function the corresponding h e profiles, all the techniques offer h(u) that can be modelled as an exponential function. The significantly improved results when compared with the one- photopeak method. DEWST subtracts data corresponding to latter is derived from the system's Point Spread Function non photopeak photons and thus causes the highest reduction (PSF) in a scattering medium: to the S N R . CST and DT perform scatter correction at an * P(u)= C(u) h(u) (3) image level and assume a detector with as uniform response Various methods have been applied to solve (deconvolve) as possible. On the other hand, they both depend on the equation 3. We have used the blind deconvolution technique scattering medium and the distance between the camera and that is based on the Expectation Maximization (EM) the phantom. Algorithm [II], which assumes that the true data Ck+,(u),in a specific time k + 1 is expressed as: FFT(h(u))' with w(u) = FFT-'{ I I FFUh(u))2I. +Y . ... . where FFT denotes the Fast Fourier Transform and y is the squared noise-to-signal ratio (NSR). In our implementation of DT h(u)was defined as: h ( u ) = 0.0247e~0~071" (6) and the value of the parameter y was experimentally determined as y = 0.0002 for all cases. E. Phantom Data A hot, a cold and a breast phantom have been used for the assessment of the three methods. The hot phantom consisted of three capillaries 7cm long, with 1.5mm inner diameter and 1.6mm outer diameter, placed at 5mm and 7mm distances fiom each other and filled with a 9m"rc solution, 8mCi"l. The phantom was placed at a lOcm distance from the collimator and a pot filled with 300ml of water was placed between the phantom and the detector. The phantom was imaged for 150secs and -301,OOOcounts were acquired. The cold phantom was a metallic cylinder with 1.5cm outer diameter, 0.8cm inner diameter and 0.7cm height, placed at the bottom of a thin plastic pot, 6cm in diameter and 8cm high. The pot was filled with 30ml of a 99mTc solution, O.l4mCi/ml. The detector was placed at a lOcm distance ftom the bottom of the pot. The cold circular phantom was imaged for 3 minutes and -600,000 counts were acquired. The breast phantom consisted of two hot quantities of a Fig. I : Hot (left column), cold (middle column) and breast phantom %Tc solution, both 0.5ml in volume, placed under a pot, (1.4 column) imaging. (a) The image in the 20% photopeak window. 19 (b) The corrected image using DEWST. ( c ) The corrected image Although scatter correction methods have been used in using CST. (d) The corrected image using DT. (e) Normalized line SPECT imaging, the results presented here are limited in profiles. scintigrdphic mode. However, since these methods are usually applied prior to reconstruction [14] their use for Table I Image contrast using three different ratios for ROIs in the cold : (C) and background (B) region for the standard 20% photopeak scatter correction in planar mode seems a correct approach. window technique. DEWST, CST and DT. The DEWST uses the energy of each detected photon in order to determine weather it is located in the selected lower I Method I C/B I (B-C)/B I IB-CMB+C) I energy window or not. The assumption that this window can 20% 0.7498 0.2502 0.1430 be used in order to provide an estimate for the scattered DEWST 0.6466 0.3534 0.2146 photons in the primary photopeak window is independent from the acquisition mode (planar or tomographic). The 0.5399 0.4601 0.2988 DEWST, as it has been modified, is related to the pixelized 0.5547 0.4423 0.2839 scintillators physics since it uses the energy spechum of each crystal cell. All system’s non-uniformities do not affect the method since calibration is performed in pixel level and the method is applied in pixel level as well. Each crystal cell is treated as an independent detector with uniform response and in the case of the used crystal, the response of the system is Method SUB (B-S 1)/B (B-S l)/(B+S 1) considered to be uniform only within each pixel, which has an area of 1.13x1.13mm2. However this method subtracts 20% 1.3881 0.3881 0.1625 non-photopeak data from photopeak data thus reducing image DEWST 1.6191 0.6191 0.2364 quality, as it can been seen in Fig. I(%). CST I 1.5113 1 0.5113 I 0.2036 The CST uses a function f ( x ) that depends on the DT I 1.8191 I 0.8191 I 0.2906 properties of the detection system to be used. Since this function is desirable to be independent of the position of the Table 111: Image contmt using three different ratios far ROls in the source within the investigated object, Axelsson et al. [14] spot with ratio to background 5:l (S2) and the background (B) region have extensively investigated the determination of the most for the standard 20% photopeak window technique, the DEWST, CST suitable f ( x ) in SPECT mode. In their work several and DT. positions of the used line source in the field of view and Method (B-S2)/B (B-S2)/(B+S2) variable distances from the detector were used. The function 20% 1.1326 0.1326 0.0622 f ( x ) was derived by the tails of LSF, when plotted in semi- DEWST 1.2319 0.2319 0.1039 logarithmic scale, which had minor changes in most positions. In our approach the capillary was placed at IO 1.1758 0.1758 0.0808 different distances from the detector. The used parameters of 1.3898 0.3898 0.1631 the function f ( x ) where the mean values of the 10 different parameters estimated for each corresponding distance. The IV. DISCUSSION obtained results were superior, when compared with the standard one-photopeak window method, as it is shown in The necessity for pixelized scintillators in dedicated tables I-Ill. However a unique function f ( x ) that could SPECT systems that are based on PSPMTs is uncontested since the thickness of the PSPMT glass window together with produce optimum results for different energy windows could a large intrinsic spread of charge distribution hamper the use not be determined. of planar scintillation crystals [12]. In addition the use of a DT seems to drastically reduce scatter component and thick crystal with good detection efficiency involves a large increase image contrast. Moreover DT compensates for any spread of light distribution with respect to the size of the factor that decreases image contrast and resolution, like the PSPMT that decreases spatial resolution [13]. However, collimator sensitivity. In Fig.2 we have applied DT in an calibration steps and data manipulation are more complicated image of a small animal (small mouse) head. As it can be than in standard Anger type cameras and in many cases seen fiom Fig.2b DT causes a significant scatter reduction processing and correction methods are still an open research and improves image contrast.. On the other band DT like field. Scatter correction has not been extensively investigated CST depends on the used energy window and system’s and the use of an energy window around the photopeak energy resolution, the density of the scattering medium and channel of each crystal cell is a simple technique, which does the measured total counts that determine the S N R not reject the scatter component that is included in the photopeak. 20 [7] R.J. Jaszcnuk, K.L. Grcer, C.E. Floyd, C.C. Harris, R.E. Colcman, “Improved SPECT Quantitication Using Compcnsation for Scattered Photons”, J. Nue. Mcd., Vol. 29, pp.893-900, 1984. [8] Buvat, M. Rodriguez-Villafucnc. A. Todd-Pokmpck, H. Benali, R. Di Pa&, "Comparative Asscssmcnt of Ninc Scancr Comction Methods Based an Spcctral Analysis Using M o m Carlo Simulations”, J Nuc. Med,Vol. 36 (S), pp.1476-1488, A u y s t 1995. [9] M.C. Gilardi, V. Bcltinardi, A. Todd-Pokropck, L. Milancsi, F. Funio, “Asscssmcnt and Comparison of Three Scancr Corrcction Techniqucs in Singlc Photon Emission Computed Tomography”, I. Nuc. Med., Val. 29 (12). pp.1971-1979, 1988. [IO1 M. Mignotlc, I. Mcunier, J. Sausy, C. Janicki, “Comparison of 8 dcconvolution tcchniqun using a dismbution mixurc parameter estimation: Application in singlc photon cmission computcd (a) (b) tomography imagcry”. Joumal of ElectraNc Imaging, Val. I I, No. 1, Jan. 2002. Fig. 2. Image of a small animal head. a)The image in the 20% [ I l l G. Kontaxakis, L. Straus, “Maximum Likelihood Algorithms for photopeak window, b) the comcted image using DT. Image Rcconsrmctian in Positron Emission Tomography”, Radionuclidcs for Oncolaw- C u r ” Stam and Future Asoects. r . V. CONCLUSION MEDITTERA Publishcn. 73-106, Athens, 1998. [I21 R. Paoi. R. Sea% R. Pcllcgrini, A. Soluri. G . Tratta, L. Indovina, M. N. Cinti and G. Dc Vinccntis, Scintillation arrays chanctcrizatian for The presented results indicate that the three used photon emission imaging, Nuclear lmtrumcnts and Methods in Physics techniques (DEWST, CST and DT) can play a very important Rescarch Scction A Aceelcrators, Spcctromctcrs, DCtCClOK and role in scatter rejection in scintillator array detectors. Associatcd Equipment, Volumc 477, lssucs 1-3, 21 January 2002, Scintimmamography is a research area where these methods Pages 72-76. [I31 J.H. Kim. Y. Choi, K.S. loa, B.S. Sihn, J.W. Chang, S.E. Kim, K.H. would improve image contrast and allow the early detection Lee, Y.S. Choc, B.T. Kim. “Devclopmcnt o f a MiniaNrc Scintillation of small tumors. All these techniques are for the present Camen Using an Nal(T1) Scintillator and PSPMT for being evaluated in SPECT mode and applied in clinical data. Scintimammagraphy”, Physics in Mcdicinc and Biology, Volumc 45, Finally the performance of these techniques can be explored Issue I I, pp.3481-3488, Novcmbcr 2000. [I41 M.C. Gilardi, V. Bettinardi, A. Todd-Pohpek, L. Milancsi, F. Fazio, in other array detectors, where each detector element is “Asscssmcnt and Comvarisan of Three Scatter Correction Tcchniaucs individually processed. CdTeZn cameras provide an ideal in Singlc Photon Emission Computed Tomography”, J. Nuc. Med., Vol. field where these techniques could be applied. 29(IZ),pp.1971-1979,1988. ACKNOWLEDGMENT The authors would like to thank S. Majewski and D. Weisenberger from “Jefferson Lab” and R. Pani from Univeristy of Rome “La Sapienza” for offering part of the used equipment and their experience. 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