In this article we present a novel method of hit time and hit position
reconstruction in long scintillator detectors. We take advantage of the fact
that for this kind of detectors amplitude and shape of registered signals
depends strongly on the position where particle hit the detector. The
reconstruction is based on determination of the degree of similarity between
measured and averaged signals stored in a library for a set of well-defined
positions along the scintillator. Preliminary results of validation of the
introduced method with experimental data obtained by means of the double strip
prototype of the J-PET detector are presented

Bednarski, T.

Białas, P.

Bułka, J.

Czerwiński, E.

Gajos, A.

Kamińska, D.

Kapłon, L.

Kochanowski, A.

Korcyl, G.

Kowal, J.

Kowalski, P.

Kozik, T.

Krzemień, W.

Kubicz, E.

Moskal, P.

Niedźwiecki, Sz.

Pałka, M.

Raczyński, L.

Rudy, Z.

Rundel, O.

Salabura, P.

Sharma, N. G.

Silarski, M.

Smyrski, J.

Strzelecki, A.

Słomski, A.

Wieczorek, A.

Wiślicki, W.

Wochlik, I.

Zieliński, M.

Zoń, N.

Acta Physica Polonica A

English

arXiv

In this article we present a novel method of hit time and hit position reconstruction in long scintillator detectors. We take advantage of the fact that for this kind of detectors amplitude and shape of registered signals depends strongly on the position where particle hit the detector. The reconstruction is based on determination of the degree of similarity between measured and averaged signals stored in a library for a set of well-defined positions along the scintillator. Preliminary results of validation of the introduced method with experimental data obtained by means of the double strip prototype of the J-PET detector are presented

Moskal, Paweł

Gupta-Sharma, Neha

Silarski, Michał

Bednarski, Tomasz

Białas, Piotr

Czerwiński, Eryk

Gajos, Aleksander

Kisielewska, Daria

Kapłon, Łukasz

Kochanowski, Andrzej

Korcyl, Grzegorz

Kowal, Jakub

Kozik, Tomasz

Krzemień, Wojciech

Kubicz, Ewelina

Niedźwiecki, Szymon

Pałka, Marek

Rudy, Zbigniew

Rundel, Oleksandr

Salabura, Piotr

Słomski, Artur

Smyrski, Jerzy

Strzelecki, Adam

Wieczorek, Anna

Zieliński, Marcin

Zoń, Natalia

Jagiellonian Univeristy Repository

Vol. 127 (2015) ACTA PHYSICA POLONICA A No. 5Proceedings of the II Symposium on Positron Emission Tomography, Kraków, September 2124, 2014Hit Time and Hit Position Reconstruction in the J-PETDetector Based on a Library of Averaged Model SignalsP. Moskala, N.G. Sharmaa,*, M. Silarskia, T. Bednarskia, P. Biaªasa, J. Buªkab,E. Czerwi«skia, A. Gajosa, D. Kami«skaa, L. Kapªona,c, A. Kochanowskid, G. Korcyla,J. Kowala, P. Kowalskie, T. Kozika, W. Krzemie«e, E. Kubicza, Sz. Nied¹wieckia, M.Paªkaa, L. Raczy«skie, Z. Rudya, O. Rundela, P. Salaburaa, A. Sªomskia, J. Smyrskia,A. Strzeleckia, A. Wieczoreka,c, W. Wi±lickie, I. Wochlikb, M. Zieli«skia and N. Zo«aaFaculty of Physics, Astronomy and Applied Computer Science, Jagiellonian University, 30-348 Kraków, PolandbDepartment of Automatics and Bioengineering AGH, Krakow, PolandcInstitute of Metallurgy and Materials Science of Polish Academy of Sciences, 30-059 Kraków, PolanddFaculty of Chemistry, Jagiellonian University, 30-060 Kraków, Polandewierk Computing Centre, National Centre for Nuclear Research, 05-400 Otwock-wierk, PolandIn this article we present a novel method of hit time and hit position reconstruction in long scintillatordetectors. We take advantage of the fact that for this kind of detectors amplitude and shape of registered signalsdepend strongly on the position where particle hits the detector. The reconstruction is based on determination ofthe degree of similarity between measured and averaged signals stored in a library for a set of well-dened positionsalong the scintillator. Preliminary results of validation of the introduced method with experimental data obtainedby means of the double strip prototype of the J-PET detector are presented.DOI: 10.12693/APhysPolA.127.1495PACS: 87.57.uk, 07.05.Kf1. IntroductionRecently a new concept of large acceptance Jagiel-lonian PET (J-PET) system was proposed [14]. Unlikeall the commercial PET devices using inorganic scintil-lators as radiation detectors [58] (usually these are theBGO, LSO or LYSO crystals) J-PET is based on poly-mer scintillators. This technique oers improvement ofthe time of ight (TOF) resolution and also constitutes apromising solution for a whole-body PET imaging. In thecase of J-PET annihilation gamma quanta are registeredby means of long scintillator strips read out from bothends by photomultipliers. This allows for the determina-tion of position and time of the gamma quanta reactionbased predominantly on the time measurement. There-fore, to fully exploit the potential of this solution, it re-quires the elaboration of a new hit position and TOFreconstruction methods [10]. Recently, one method ofreconstruction in scintillator detectors based on the com-parison of registered signals with respect to a library ofsynchronized model signals collected for a set of well-dened positions along the scintillator [11] is published.In this article we present similar method, however thecomparison of measured signal is done with averagedmodel signals determined as a function of position alongthe scintillator. This approach speeds up signicantly*corresponding author; e-mail: tanu8513@gmail.comPresently the best TOF resolution was achieved with LSOcrystals and amounts to about 400 ps [9].the reconstruction in comparison to the previously usedmethod.2. Library of synchronized model signalsIn order to create the library of model signals, we haveused the setup composed of two BC-420 [12] scintilatorswith dimensions 300×19×5 mm3 and Hamamatsu photo-multipliers R4998 [13] connected optically to their mostdistant ends via optical gel EJ-550 [14]. General schemeof experimental setup used to build the library of signalsis shown in Fig. 1.Fig. 1. Scheme of experimental setup where abbreva-tions PM and SDA denote photomultiplier and serialdata analyser (SDA6000A), respectively.The two detectors were irradiated with a collimatedbeam of annihilation quanta (FWHM ≈ 1.5 mm) in stepsof 3 mm. The 22Na source was placed inside a lead col-limator which could be moved along the scintillator byusing a dedicated mechanical system. For each positiona high statistics of sampled signals with interval of 100 pswas collected by means of SDA. A coincident registration(1495)1496 P. Moskal et al.of signals from both detectors allowed for the photomulti-pliers noise suppression and the selection of annihilationgamma quanta.Exemplary sampled signal is shown in Fig. 2a. To cre-ate a background-free library, rst we have corrected allthe signals for pedestal. For every signal the averagevalue of voltage was calculated in the noise region shownin Fig. 2a by encircled red area. This computed aver-age value was then used for pedestal correction for thatparticular signal. An exemplary signal after correction ispresented in Fig. 2b.Fig. 2. (a) Measured signal from the database. (b)The same signal after pedestal correction.Fig. 3. Distribution of number of photoelectrons ob-tained from signals measured irradiating the scintillatorstrip at central position (solid black histogram). Dashedred histogram represents theoretical distribution [15] t-ted to the experimental data using normalization andenergy calibration constants as free parameters. Detailsof the tting procedure are described in Ref. [16].After pedestal correction, signals building the librarywere selected based on number of registered photoelec-trons such that it is constructed from events with thehigh energy deposition.In Fig. 3 we show an example of such spectrum ob-tained at central position. For every position we havecut all the signals for which the number of registeredphotoelectrons is lower than half of the number of photo-electrons corresponding to the Compton edge for 511 keVgamma quanta.Finally, database signals were synchronized by shiftingtheir time scales in such a way that time of the gammaquantum hit inside the detector is the same for all eventsin the library.The shape of a model signal for each position from thedatabase is determined by averaging pedestal correctedand synchronized signals. More details on model signalsdetermination are given in the next section of this article.3. Determination of the shape of model signalsFunction describing the approximate shape of themodel signal was determined by averaging measured sig-nals for a given hit position. Calculated average for sig-nals registered simultaneously at both ends of the scin-tillator was treated as a reference in order to align all thedatabase signals measured for a particular position, as itis shown in Fig. 4a and b.Fig. 4. (a) Example of a database signal before align-ment to the average. Black curve represents the mea-sured signal, while the average signal is shown by reddashed curve. (b) The same signal after rescaling.Such alignment is necessary to account for the spreadof signals in amplitude and time. To perform signal'salignment in the database, we have dened for each mea-sured signal at a given position a χ2 value. It was com-puted by comparing leading edge of the database andaverage signals. Comparison was performed by takinginto account signal registered at the left and right side ofscintillator simultaneously. The χ2 was calculated as afunction of three parameters δt, αL and αR:χ2(δt, αL, αR) =n∑i=1tAvgL(Vi) − tdbL((αLVi) − δt)2n+m∑i=1(tAvgR(Vi) − tdbR((αRVi) − δt))2m. (1)δt is a time shift for sample points along time axisHit Time and Hit Position Reconstruction in the J-PET Detector. . . 1497and αL, αR are normalisation factors for both signals(left and right) registered at both ends of scintillator.tAvgL(Vi) and tAvgR(Vi) denote time for model signalcomputed at left and right side for voltage Vi at theirleading edge. tdbL(αLVi) and tdbR(αRVi) is the time com-puted for rescaled left and right signals at their leadingedge, respectively. By minimization of the χ2 value weobtained the best alignment between two compared sig-nals and each database signal was rescaled using t pa-rameters αL, αR and δt giving the lowest χ2. Next, aver-age of these rescaled signals was computed again leadingto the model signal determination and the whole pro-cedure was repeated until the changes were negligible.Example of model signals obtained at three dierent po-sitions is shown in Fig. 5.Fig. 5. Sample of model signals at three dierent po-sitions: solid black line represents the left side of thescintillator, dashed red line center and dotted blue linethe right side of scintillator.4. Validation and optimization of the time andposition reconstructionIn order to reconstruct hit position of gamma quantain the detector the measured signals are compared witheach model signal from the library. This is done by min-imizing χ2 value calculated at the leading edges of mea-sured and model signals. Validation and optimization ofthis reconstruction method has been performed by uti-lizing the signals gathered at known positions with thesame experimental setup which was used to build thelibrary. In order to simulate the response of front-endelectronics we have determined the times correspondingto predened constant level thresholds [17]: 60, 120, 180, and 240 mV. Analogously, we have chosen also fourconstant fraction thresholds to sample signals at: 1/8Ai,2/8Ai, 3/8Ai, 4/8Ai, where Ai is the amplitude of sig-nal which is smallest among all four compared signals.To optimize the reconstruction we have considered twoposibilities: 1. when χ2 is a function of the time shift δtonlyχ2(δt) =4∑i=1[tMdL(Vi) − tdbL(Vi − δt)]2+4∑i=1[tMdR(Vi) − tdbR(Vi − δt)]2, (2)2. when χ2 depends on δt and normalisation factors αLand αR:χ2(δt, αL, αR) =4∑i=1(tMdL(Vi) − tdbL((αLVi) − δt))2+4∑i=1(tMdR(Vi) − tdbR((αRVi) − δt))2. (3)tMdL(Vi), tMdR(Vi) and tdbL(Vi), tdbR(Vi) denote timesdetermined at left and right side of the scintillator atthreshold Vi for model and registered signals, respec-tively. The reconstructed hit position is the position ofmost similar signal from the library with respect to mea-sured signal (i.e. model signal for which χ2 is minimal).The time of particle interaction is determined as a rel-ative time between the measured signal and the mostsimilar one from the library. This provides also determi-nation of the gamma quantum time of ight (TOF) [11]:tfStr = δtfStr, tsStr = δtsStr, TOF = tsStr − tfStr, (4)where δt denotes shift in time for which the computed χ2dened in Eqs. (2) and (3) is lowest.An example of the χ2 distribution calculated accordingto Eq. (2) for one of signals sampled with constant levelsand measured at central position of the strip is shownin Fig. 6. One can see a clear minimum corresponding toz ≈ 150 mm.Present version of reconstruction procedure do not takeinto account the measurement uncertainties. Therefore,the χ2 values are treated as arbitrary and errors of ttedparameters are determined directly from distributions ofdierences between reconstructed and true values of timeor position. Figure 7a and b shows distributions of dier-Fig. 6. Example of the χ2 distribution dened in Eq. 2.ences between the true and reconstructed position usingχ2 dened by Eq. 2 for constant fraction and constantlevels discrimination, respectively. Resolution of positionreconstruction is determined by tting a Gauss functionto the presented distribution and obtained results give:σz = 9.7 ± 0.3 mm for constant fraction sampling andσz = 9.6 ± 0.3 mm for constant levels.In case when χ2 is a function of δt, αL and αR(Eq. (2)) obtained position resolution amounts to σz =Since the degree of similarity is represented by the χ2 value.1498 P. Moskal et al.Fig. 7. Distribution of dierences between the trueand reconstructed position ∆z for signals measured atz = 150 mm for (a) constant fractions (b) constant lev-els.Fig. 8. Distribution of dierences between the trueand reconstructed position ∆z for signals measured atz = 150 mm for (a) constant fractions (b) constant lev-els.12.1 ± 0.4 mm and 10.9 ± 0.4 mm for constant fractionand constant level method, respectively. Corresponding∆z distributions are shown in Fig. 8. These resolutionswere determined for signals measured at several positionsalong the scintillator as it is shown in Fig. 9. The resultsindicate that this resolution does not change much withposition. Results for TOF reconstruction for signals mea-sured at z = 150 mm are shown in Fig. 10. CorrespondingFig. 9. Position resolution as a function of the posi-tion of gamma quantum interaction for the case when(a) χ2 is a function of δt only (b) χ2 is a function of δt,αL and αR.Fig. 10. Distribution of dierences between the trueand reconstructed TOF for events registered at z =150 mm. The reconstruction was done using χ2 as afunction of δt with signal sampling at (a) constant frac-tion and (b) at constant levels.Fig. 11. Distribution of dierences between the trueand reconstructed TOF for signals measured at z =150 mm. The reconstruction was done using χ2 as afunction of δt,αL, and αR, and with signal sampling at(a) constant fraction (b) constant levels.resolutions are in this case equal to σTOF ≈ 163 ps forconstant fractions sampling and σTOF ≈ 143 ps in case ofconstant levels. As it is shown in Fig. 11 results obtainedfor χ2(δt, αL, αR) amount to σTOF ≈ 132 ps (constantfractions) and σTOF ≈ 119 ps (constant levels).In principle true value of TOF should be equal to zerowhen source was positioned in the middle between de-Fig. 12. TOF resolution as a function of positionalong the scintillator: (a) χ2 = χ2(δt) and (b) χ2 =χ2(δt, αL, αR).Hit Time and Hit Position Reconstruction in the J-PET Detector. . . 1499tection modules. However, due to dierent time osetsproduced by electronics the reconstructed mean values ofTOF may be dierent from zero.Again, we have performed studies of the TOF resolu-tion as a function of position for both χ2 functions as itis shown in Fig. 12. One can see that the determinedresolutions is constant within the error bars over the fulllength of 30 cm long scintillator strip.5. SummaryThe preliminary results obtained during validation ofthe reconstruction method introduced in this article showthat it is possible to obtain a spatial resolution of about1.2 cm (σ) for the gamma quanta hit position, and TOFresolution of about 125 ps (σ). As regards the posi-tion resolution along the detector obtained result is fewtime worse than achieved by the commercial TOF-PETscanners, However as regards the TOF determination ob-tained result is better by about a factor of two with re-spect to resolutions achieved by the commercial TOF-PET tomographs characterized by typical eld of views ofabout 16 cm and TOF resolution of about 230 ps (σ) [18].A further improvement is expected in the future by in-cluding measurement uncertainties and possible correla-tions between the times measured at dierent thresholds.References[1] P. Moskal, Patents granted in 2014,Nos. EP2454612B1, WO2011008119, EP2454611,WO2011008118.[2] P. Moskal, P. Salabura, M. Silarski, J. Smyrski,J. Zdebik, M. Zieli«ski, Bio-Algorithms Med-Syst. 7,73 (2011).[3] P. Moskal, T. Bednarski, P. Biaªas, M. Ciszewska,E. Czerwi«ski, A. Heczko, M. Kajetanowicz,. Kapªon, A. Kochanowski, G. Konopka-Cupiaª,G. Korcyl, W. Krzemie«, K. ojek, J. Majew-ski, W. Migdaª, M. Molenda, Sz. Nied¹wiecki,M. Paªka, Z. Rudy, P. Salabura, M. Silarski,A. Sªomski, J. Smyrski, J. Zdebik, M. Zieli«ski,Nucl. Med. Rev. 15, C68 (2012).[4] P. Moskal, T. Bednarski, P. Biaªas, M. Ciszewska,E. Czerwi«ski, A. Heczko, M. Kajetanowicz,. Kapªon, A. Kochanowski, G. Konopka-CupiaªG. Korcyl, W. Krzemie«, K. ojek, J. Ma-jewski, W. Migdaª, M. Molenda, Sz. Nied¹wiecki,M. Paªka, Z. Rudy, P. Salabura, M. Silarski,A. Sªomski, J. Smyrski, J. Zdebik, M. Zieli«ski,Nucl. Med. Rev. 15, C81 (2012).[5] M. Conti, Phys. Med. 25, 1 (2009).[6] J.L. Humm, A. Rosenfeld, A. Del Guerra,Eur. J. Nucl. Med. Mol. Imaging 30, 1574 (2003).[7] J.S. Karp, S. Surti, M.E. Daube-Witherspoon,G. Muehllehner, J. Nucl. Med. 49, 462 (2008).[8] J.W. Townsend, Ann. Acad. Med. Singapore 22, 133(2004).[9] W.W. Moses, S.E. Derenzo, IEEETrans. Nucl. Sci. 46, 474 (1999).[10] L. Raczy«ski, P. Kowalski, W. Wi±licki, T. Bednarski,P. Biaªas, E. Czerwi«ski, . Kapªon, A. Kochanowski,G. Korcyl, J. Kowal, T. Kozik, W. Krzemie«,E. Kubicz, M. Molenda, I. Moskal, Sz. Nied¹wiecki,M. Paªka, M. Pawlik-Nied¹wiecka, Z. Rudy, P. Sal-abura, N.G. Sharma, M. Silarski, A. Sªomski,J. Smyrski, A. Strzelecki, A. Wieczorek, M. Zieli«ski,N. Zo«, Nucl. Instrum. Methods Phys. Res. A 764,186 (2014); L. Raczy«ski, P. Kowalski, W. Wi±licki,T. Bednarski, P. Biaªas, E. Czerwi«ski, . Kapªon,A. Kochanowski, G. Korcyl, J. Kowal, T. Kozik,W. Krzemie«, E. Kubicz, M. Molenda, I. Moskal,Sz. Nied¹wiecki, M. Paªka, Z. Rudy, P. Salabura,N.G. Sharma, M. Silarski, A. Sªomski, J. Smyrski,A. Strzelecki, A. Wieczorek, M. Zieli«ski, N. Zo«,Nucl. Instrum. Methods Phys. Res. A 786, 105(2015).[11] P. Moskal, N. Zo«, T. Bednarski, P. Biaªas, E. Cz-erwi«ski, A. Gajos, D. Kami«ska, . Kapªon,A. Kochanowski, G. Korcyl, J. Kowal, P. Kowalski,T. Kozik, W. Krzemie«, E. Kubicz, Sz. Nied¹wiecki,M. Paªka, L. Raczy«ski, Z. Rudy, O. Rundel,P. Salabura, N.G. Sharma, M. Silarski, A. Sªomski,J. Smyrski, A. Strzelecki, A. Wieczorek, W. Wi±lick,M. Zieli«ski, Nucl. Instrum. Methods Phys. Res. A775, 54 (2015).[12] Saint Gobain Crystals, http://www.crystals.saint-gobain.com.[13] Hamamatsu, http://www.hamamatsu.com.[14] Scionix, http://www.scionix.nl.[15] O. Klein, Y. Nishima, Z. Phys. 52, 853 (1929).[16] P. Moskal, Sz. Nied¹wiecki, T. Bednarski, E. Czer-wi«ski, . Kapªon, E. Kubicz, I. Moskal, M. Pawlik-Nied¹wiecka, N.G. Sharma, M. Silarski M. Zieli«ski,N. Zo«, P. Biaªas, A. Gajos, A. Kochanowski, G. Ko-rcyl, J. Kowal, P. Kowalski, T. Kozik, W. Krzemie«,M. Molenda, M. Paªka, L. Raczy«ski, Z. Rudy,P. Salabura, A. Sªomski, J. Smyrski, A. Strzelecki,A. Wieczorek, W. Wi±licki, Nucl. Instrum. MethodsPhys. Res. A 764, 317 (2014).[17] M. Paªka, T. Bednarski, P. Biaªas, E. Czerwi«ski,. Kapªon, A. Kochanowski, G. Korcyl, J. Kowal,P. Kowalski, T. Kozik W. Krzemie«, M. Molenda,P. Moskal, Sz. Nied¹wiecki, M. Pawlik, L. Raczy«ski,Z. Rudy, P. Salabura, N.G. Sharma, M. Silarski,A. Sªomski, J. Smyrski, A. Strzelecki, W. Wi±licki,M. Zieli«ski, N. Zo«, Bio-Algorithms Med-Syst. 10,41 (2014).[18] V. Bettinardi, M. Danna, A. Savi, M. Lecchi, I. Cas-tiglioni, M. C. Gilardi, H. Bammer, G. Lucignani,F. Fazio, Med. Phys. 38, 5394 (2011).

Hit time and hit position reconstruction in the J-PET detector based on a library of averaged model signals

arXiv.org e-Print Archive

Hit time and hit position reconstruction in the J-PET detector based on
  a library of averaged model signals

P. Moskal

N.G. Sharma

M. Silarski

T. Bednarski

P. Białas

J. Bułka

E. Czerwiński

A. Gajos

D. Kamińska

L. Kapłon

A. Kochanowski

G. Korcyl

J. Kowal

P. Kowalski

T. Kozik

W. Krzemień

E. Kubicz

Sz. Niedźwiecki

M. Pałka

L. Raczyński

Z. Rudy

O. Rundel

P. Salabura

A. Słomski

J. Smyrski

A. Strzelecki

A. Wieczorek

W. Wiślicki

I. Wochlik

M. Zieliński

N. Zoń

Crossref

Hit Time and Hit Position Reconstruction in the J-PET Detector Based on a Library of Averaged Model Signals

In this article we present a novel method of hit time and hit position reconstruction in long scintillator detectors. We take advantage of the fact that for this kind of detectors amplitude and shape of registered signals depend strongly on the position where particle hits the detector. The reconstruction is based on determination of the degree of similarity between measured and averaged signals stored in a library for a set of well-defined positions along the scintillator. Preliminary results of validation of the introduced method with experimental data obtained by means of the double strip prototype of the J-PET detector are presented

Sharma, N.

Biblioteka Nauki - repozytorium artykuÅÃ³w

https://ruj.uj.edu.pl/xmlui/bitstream/handle/item/16776/moskal_et-al_hit_time_and_hit_2015.pdf?sequence=4&isAllowed=y

Hit time and hit position reconstruction in the J-PET detector based on a library of averaged model signals

Abstract

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Jagiellonian Univeristy Repository

arXiv.org e-Print Archive

Crossref

Biblioteka Nauki - repozytorium artykuÅÃ³w

Hit time and hit position reconstruction in the J-PET detector based on a library of averaged model signals

Abstract

Similar works

Full text

Available Versions

Jagiellonian Univeristy Repository

arXiv.org e-Print Archive

Crossref

Biblioteka Nauki - repozytorium artykuÅÃ³w

Biblioteka Nauki - repozytorium artykuÅÃ³w