Monte Carlo Simulation for the ECAT HRRT using GATE

Transcription

1 Monte Calo Simulation fo the ECAT HRRT using GATE F. Bataille, C. Comtat, Membe, IEEE, S. Jan, and R. Tébossen Abstact The ECAT HRRT (High Resolution Reseach Tomogaph, CPS Innovations, Knoxville, TN, U.S.A.) is a 3D-only bain positon tomogaph made of height flat panels using LSO and LYSO scintillatos. This dual-laye octagonal geomety allows the scanne to measue depth-of-inteaction, peseving the good spatial esolution towad the edge of the tansvese fieldof-view. With the objective of optimizing potocols and acquisition paametes, we used the GATE (Geant4 Application fo Tomogaphic Emission) Monte Calo simulation platfom to build a ealistic model of this scanne. The aim of this pape is to compae the pefomance of the model against the eal system. In ode to emain the compaison being independent on the econstuction scheme used, we geneated the simulation data as the HRRT native 64-bit list-mode fomat befoe softwae histogamming and ebinning into sinogams. We pefomed fo the compaison thee sets of expeiments : intinsic spatial esolution, scatte faction and count ates. Real and simulated data wee found to be in good ageement. To illustate the use of this model we evaluated the cystal thickness influence fo a single laye HRRT on spatial esolution and sensitivity. II. METHODS A. HRRT geomety The HRRT is a bain positon tomogaph made of 8 detecto heads aanged in an octagon. Each head consists of 9 13 blocks of 8 8 dual-laye ( ) mm 3 LSO- LYSO cystals, in coincidence with five opposing heads. The head-to-head distance is 469 mm and the axial field-of-view is mm. A simulation of this geomety is epesented in Fig. 1. The phoswitch configuation allows the scanne to measue depth of inteaction, peseving the good spatial esolution towad the edge of the tansvese field-of-view. The data ae acquied in a 64-bit list-mode fomat and then softwae histogamed into sinogams. I. INTRODUCTION Positon emission tomogaphy epesents today an essential ole in moden medicine fo both diagnostic and teatment puposes. Gowing equiements in velocity and pecision imply the optimization of acquisition paametes and potocols. Monte Calo simulations ae essential tool to assist these developments, impoving data analysis and image quantification. GATE [1] (Geant4 Application fo Tomogaphic Emission) is such a simulation platfom, based on the Geant4 toolkit package. Dedicated to nuclea medical imaging, GATE allows to descibe time-dependent phenomena such as detecto movement o souce decay kinetics, thus allowing to simulate time cuves unde ealistic acquisition conditions. We used this tool to build a ealistic model of the ECAT HRRT [2] (High Resolution Reseach Tomogaph, CPS Innovations, Knoxville, TN, U.S.A) 3D-only dedicated bain scanne. In this pape, we compae the pefomance of the simulato against the HRRT scanne in ode to validate the use of the model fo system optimization. Manuscipt eceived Octobe 29, F. Bataille, C. Comtat, S. Jan, and R. Tébossen ae with the Fédéic Joliot Hospital Facility (SHFJ), CEA, Osay, Fance. Fig. 1: HRRT geomety simulated with GATE, whee the LSO laye is geen, the LYSO is yellow and the tungsten end-shielding is blue. B. GATE configuation Fo each detected photon, the hit position coesponds to the cystal with the maximum enegy deposition, assuming an enegy esolution vaying between 20 % and 30 % fo each cystal. GATE capability to synchonize all time-dependent components allows fo the explicit simulation of coincidence events. As fo the eal acquisitions, we used a 6 ns coincidence time window. We used the ROOT [3] output povided by GATE to exploit the simulation esults. This stuctued listmode file contains event-by-event chaacteistics fo thee acquisition levels : cystal hits, singles and coincidences. We developed a maco to convet this output into the native 64-bit list-mode equied by the HRRT embedded softwae. This method allowed us to econstuct eal and simulated data with the same schemes.

2 C. Setups Thee sets of measuements and simulations wee pefomed to evaluate the pefomance of the scanne model: intinsic spatial esolution, scatte faction and count ates. Exactly the same setup was used fo the measuement and fo the simulation. The spatial esolution was measued using a 5 cm height and 1 mm inne diamete glass capillay filled with a 1 mm height F-18 aqueous souce. This souce was placed at 7 diffeent adial positions between 1 and 13 cm and axially centeed. Images wee econstucted using 2 iteations and 16 subsets of the ANW-OSEM3D (Attenuation Nomalization Weighted Odeed Subsets Expectation Maximization) algoithm, peseving the good esolution of the scanne and avoiding the need to compensate fo the missing data in the gaps between the heads. Othogonal pofiles though the econstucted images of the point souces wee analyzed in tems of full width at half maximum (FWHM) in adial and tangential diections. In ode to epoduce with GATE the intinsic esolution of the scanne, we intoduced a gaussian spatial bluing step on the cystal identification at the list-mode encoding level. Acolineaity and positon ange wee simulated. Scatte Faction evaluation was pefomed using a 20 cm diamete and 20 cm height cylinde centeed on the field-ofview and unifomly filled with an aqueous F-18 souce. Afte delayed coincidences substaction, the 3D sinogams wee summed in the pola and azimuthal diections, esulting in a 2D pojection matix poj(, in the adial and axial diections as shown in Fig. 2. Fig. 2 : Repesentation of the 2D pojection matix poj(,. C epesents the cylinde ROI and O and O Z the ROIs used to estimate the scattes outside the cylinde. Unde the assumption that the scatteed coincidences distibution sct(, in poj(, is a sepaable function sct(, = α( S ( ), we can compute the adial distibution S() fom the end axial slices that extended outside the cylinde (O z ) S () = poj(, Z O Z and the axial distibution α(z) by fitting the adial distibution on the tails O : α( Z ) = O poj(,. S() O The scatte faction SF( was obtained fo each axial slice as follows : SF ( = α( Z ) S(). poj(, Note that the initial sepaability assumption could intoduce a cetain amount of bias in the final SF evaluation. Howeve, the aim of this study was to compae eal and simulated systems, so that absolute measuement of the SF was not equied. Count ate pefomances wee studied using a 3 cm diamete and 7 cm height cylinde centeed on the FOV. This so-called mouse phantom was filled with a 350 MBq C-11 aqueous souce and a 3 hou acquisition was pefomed with an enegy window set to kev. Fo each second, the ates of pompt and delayed coincidences and single events wee ecoded at the coincidence pocesso level in a head-cuve file. Fo the simulation, we pefomed 11 one second uns with an activity unifomly distibuted between 10 and 300 MBq. In ode to faithfully epoduce the acquisition chain, we should conside that single events count ates ae affected by a paalyzable dead-time τ, due in pat to the integation delay, the electonics eset time, and the pulse shape descimination fo the phoswitch configuation. Although the simulation of dead-time fo the single events and the computation of pompt coincident events can be done inside GATE, we choose to output all single events in a oot list-mode file and use a oot pogam to simulate dead-time on the single events and build pompt and delayed coincident events. The set of single events was soted to model dead-time as follows: fo each block, a single event occuing less than τ seconds afte the pevious one (ejected o not) was ejected. This model coesponds to a suvival pobability of exp(-sτ), whee S is the single events count ate at the block level [4]. A softwae coincidence builde associated coincidences out of the set of singles that suvived dead-time ejection, poviding pompt coincidences ates. By the explicit simulation of coincidence events, both tue and andom coincidences ae geneated. In addition, we also explicitly simulated a 1 µs delayed coincidence window. We did not simulate the dead-time of the coincidences tansfe

3 and stoage to disk but add the bandwidth limit of the coincidence pocesseu. compaison of the measued data (aveage value of 45.6 %) against the simulated data (aveage value of 45.7 %). III. RESULTS A. Spatial esolution Figue 3 epesents the esults of spatial esolution measuements fo eal data, unblued simulated data and blued simulated data. Fig. 4 : Scatte adial distibution S() detemined using the end slices. Fig. 3: FWHM of adial (top) and tangential (bottom) pofiles though the econstucted point souces. We obseved that esolutions wee always lowe fo the simulated souces without spatial bluing on the cystal identification (about 12 %). These esults can be explained by the fact that only enegy depositions ae simulated and not the scintillation pocesses o the light collection, in paticula the light shaing of the block detecto geomety, esponsible fo the main pat of the degadation of the esolution. Using additional analytical bluing this effect was compensated and the aveage diffeence was deceased to 4 %. B. Scatte faction Figue 4 shows the scatte adial distibution S() fo expeimental and simulated data. The adial and axial pofiles of the cylinde sinogam afte azimuthal and pola summation ae epesented in Fig. 5. Figue 6 shows the scatte faction Fig. 5: Radial (at mid-slice) and axial (fo the centeed adial position) pofiles of the summed 2D pojection poj(, of the cylinde.

4 Fig. 6: Scatte faction C. Count Rate Pefomance Figues 7 show single event count ates fo expeimental data and fo simulated data with and without a 800 ns paalyzable dead-time model. A compaison is made between the explicit model (using the ejection potocol descibed above) and an analytical model (using the suvival pobability fomula), which eveals the efficiency of the single events list teatment pocess. When no dead-time model is used the simulation count ate excess eaches 10 % at 300 MBq. This maximum eo value is deceased to less than 3 % when we intoduce the 800 ns deadtime model. Note that the LSO emission backgound (300 counts/s/cm 3 ) was not simulated. Pompt and delayed coincidences count ates at coincidence pocesso output ae epesented espectively in Fig.8 and 9 fo both the eal and the simulated systems. The expeimental cuves ae well epoduced even when a discontinuity appeas, due to the bandwidth limit of the coincidence pocessing electonics. Fig. 8: Pompt coincidences count ate Fig. 7: Single events count ates. Top: compaison between the explicit simulation of dead-time and an analytical application of the suvival pobability. Bottom: compaison between measued and simulated (with and without dead-time) data. Fig. 9: Delayed coincidences count ate The main consequence of the bandwidth effect is a satuation occuing in the pompt plus delayed cuve epesented in Fig. 10.

5 Fig. 10 : Pompts plus delayed coincidences count ate. D. Illustation In positon emission tomogaphy, a compomise has to be made between spatial esolution and sensitivity. An essential paamete to tune both chaacteistics is the cystal thickness. To illustate a possible use of the scanne model pesented in this pape, we used it to evaluate the cystal thickness influence fo a LSO single laye HRRT on spatial esolution and sensitivity. Simulation setup consisted of a 1 mm diamete sphee, filled with a 4 MBq F-18 aqueous souce, axially centeed and placed at a adial position of 75 mm. Acolineaity and positon ange wee simulated. Enegy window was set to kev. As HRRT ebinning pocess is dedicated to the phoswitch system geomety, it couldn t be exploited to geneate the sinogams fo these simulations. Theefoe we diectly used the GATE event-by-event analysis output to build the sinogams. Fo each LSO thickness configuation, we fist computed the aveage depth of inteaction povided by the single events global positions. This allowed us to calculate the lines of esponse (LORs) coodinates using fo stat and aival point the absolute coodinate of the cystal face shifted fo aveage depth of inteaction. Images wee then econstucted using 16 subsets and 2 iteations of the ANW-OSEM2D algoithm (only diect sinogams). Cystal position bluing utility was not exploited pefoming this study. Figue 11 epesents FWHM of adial and tangential pofiles though the econstucted point souce and table I shows the coesponding sensibilities elative to the phoswitch one. As we expected, esolution degades and sensitivity augments when cystal thickness inceases. Fig. 11 : FWHM of the adial and tangential pofiles though the econstucted point souce image (dotted lines indicate dual-laye HRRT values) TABLE I CRYSTAL THICKNESS AND SENSITIVITY AT 7.5 CM IV. DISCUSSION AND CONCLUSION Sets of expeiments allowed us to validate the geometical model of the HRRT scanne built with GATE fo intinsic spatial esolution, scatte faction and count ates at the coincidence pocesso level. The convesion tools we have developed to tansfom simulation output into native HRRT 64-bit list-mode allowed the compaison to emain independent of the econstuction scheme. Theefoe we could faithfully epoduce the main pat of the acquisition chain, fom scintillato to coincidence pocesso. Futue developments ae focusing on dead-time modelization, especially on coincidences count losses that occu afte coincidence pocessing level, mainly due to tansfe and stoage dead-time at disk level. V. BIBLIOGRAPHY [1] S. Jan et al., GATE: a simulation toolkit fo PET and SPECT, Phys. Med. Biol., vol. 49, pp , [2] K. Wienhad et al., " The ECAT HRRT: Pefomance and Fist Clinical Applications of the New High Resolution Reseach Tomogaph, IEEE Tans. Nucl. Sci., 49, PP; , [3] R. Bun and F. Rademakes, ROOT An object oiented data analysis famewok, Nucl. Inst. Meth A, 389, pp , [4] L.Simon et al., Simulation of time cuves in small animal PET using GATE, Poc.2 nd ITBS Conf., Athens and Milos, May 2003.