The proton Computed Tomography (pCT) is a medical imaging

method, based on the use of proton beams with kinetic energy of the order of 250 MeV, aimed to directly measure the stopping power distribution of tissues thus improving the present accuracy of treatment planning in hadron therapy. A pCT system should be capable to measure tissue electron density with an accuracy better than 1% and a spatial resolution better than 1 mm. The blurring effect due to multiple Coulomb scattering can be mitigated by single proton tracking technique. As a first step towards pCT the PRIMA+ Collaboration built a prototype capable to carry out a single radiography and a tomographic image of a rotating object. This apparatus includes a silicon microstrip tracker to identify the proton trajectory and a YAG:Ce calorimeter to measure the particle residual energy

Bruzzi, M.

Bucciolini, M.

Cirrone, G. A. P.

Civinini, Carlo

Cuttone, G.

Lo Presti, D.

Pallotta, S.

Randazzo, N.

Scaringella, M.

Sipala, V.

Stancampiano, C.

Talamonti, C.

Tesi, M.

Vanzi, E.

Zani, M.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2013-11411-0Colloquia: IFAE 2012IL NUOVO CIMENTO Vol. 36 C, N. 1 Gennaio-Febbraio 2013PRIMA+: A proton Computed Tomography apparatusC. Civinini(1)(∗), M. Bruzzi(1)(2), M. Bucciolini(1)(3), G. A. P. Cirrone(4),G. Cuttone(4), D. Lo Presti(5)(6), S. Pallotta(1)(3), N. Randazzo(5),M. Scaringella(2), V. Sipala(7)(8), C. Stancampiano(5)(6), C. Talamonti(1)(3),M. Tesi(2), E. Vanzi(9) and M. Zani(3)(1) INFN, Sezione di Firenze - Via G. Sansone 1, Sesto Fiorentino (FI), Italy(2) Dipartimento di Energetica, Universita` di FirenzeVia S. Marta 3, Firenze, Italy(3) Dipartimento di Fisiopatologia Clinica, Universita` di FirenzeViale G. Pieraccini 6, Firenze, Italy(4) INFN, Laboratori Nazionali del Sud - Via S. Sofia 62, Catania, Italy(5) INFN, Sezione di Catania - Via S. Sofia 64, Catania, Italy(6) Dipartimento di Fisica e Astronomia, Universita` di CataniaVia S. Sofia 64, Catania, Italy(7) Dipartimento di Chimica e Farmacia, Universita` di SassariVia Vienna 2, Sassari, Italy(8) INFN, Sezione di Cagliari - S.P. Sestu, Monserrato (CA), Italy(9) SOD Fisica Medica, Azienda Ospedaliero Universitaria Careggi - Firenze, Italyricevuto il 31 Agosto 2012Summary. — The proton Computed Tomography (pCT) is a medical imagingmethod, based on the use of proton beams with kinetic energy of the order of250MeV, aimed to directly measure the stopping power distribution of tissues thusimproving the present accuracy of treatment planning in hadron therapy. A pCTsystem should be capable to measure tissue electron density with an accuracy betterthan 1% and a spatial resolution better than 1mm. The blurring effect due tomultiple Coulomb scattering can be mitigated by single proton tracking technique.As a first step towards pCT the PRIMA+ Collaboration built a prototype capableto carry out a single radiography and a tomographic image of a rotating object. Thisapparatus includes a silicon microstrip tracker to identify the proton trajectory anda YAG:Ce calorimeter to measure the particle residual energy.PACS 87.57.Q – Computed tomography.PACS 29.40.Wk – Solid-state detectors.PACS 07.20.Fw – Calorimeters.PACS 87.53.Bn – Dosimetry/exposure assessment.(∗) E-mail: Carlo.Civinini@fi.infn.itc© Societa` Italiana di Fisica 7374 C. CIVININI, M. BRUZZI, M. BUCCIOLINI, ETC.1. – IntroductionOne of the factors that drives the effectiveness of the hadron therapy is the precisionby which the dose is delivered to the target volume. The hadron energies should be tunedduring the treatment to allow the particle’s Bragg peak to completely cover the tumorleaving the nearby healthy tissues as much as possibly unaffected. To achieve this goalthe Stopping Power (SP) map for each patient should be measured before the treatmentand a detailed irradiation plan based on this information should be defined.Presently the SP maps are extracted from 3-dimensional images obtained by X-rayComputed Tomography; here the photon attenuation coefficients are translated into SPusing conversion tables introducing uncertainties which can be as large as 3mm [1]. Tomitigate this effect the SP map can be directly determined using a proton beam withkinetic energy larger than the one used for treatment. The proton detector systemshould partially overcome the problems introduced by the intrinsic effect of the MultipleCoulomb Scattering (MCS). Typical targets could be as thick as 20 cm water equivalent:in this case a 200MeV proton undergoes to a r.m.s. MCS angle of about 40mrad whichcorresponds to an r.m.s. of the projected displacement of 3.2mm [2]. With this numbers aproton projection radiography cannot extract the SP map with a sub-millimeter precision.A possible solution is to measure the trajectory of each proton both upstream anddownstream the target together with the residual proton kinetic energy. With this infor-mation the most likely proton path (MLP) could be estimated [3]. The maximum valueof the one sigma envelope of a transverse coordinate of the MLP of a 200MeV kineticenergy proton in 20 cm of water is of the order of 0.5mm [3]. Assigning the proton energyloss to each MLP and using appropriate algorithms [4] the 3-dimensional SP map can beobtained.2. – The proton Computed Radiography (pCR) systemThe pCR apparatus described in this paper [5] is made by a silicon microstrip trackerfollowed by a YAG:Ce calorimeter (fig. 1).Fig. 1. – Layout of the pCR apparatus: a) calorimeter; b) front-end board; c) microstrip sensor;d) digital board; f) phantom; g) beam pipe; p1-p4) tracker planes.PRIMA+: A PROTON COMPUTED TOMOGRAPHY APPARATUS 75Fig. 2. – A pCR tracker front-end board.2.1. The Tracker system. – The pCR tracker is made by four planes (fig. 1 (p1-p4))each one capable to measure the proton trajectory impact point as a 3-dimensionalposition complemented by information on timing and deposited charge in silicon. Asingle tracker plane is realized using two identical modules each one carrying a single-sided microstrip sensor at its center. Two modules are coupled back-to-back in orderto have the sensors completely overlapping with their strips mutually orthogonal. Themodule is made by one tracker front-end board (fig. 2) with a tracker digital board(fig. 1 (d)) plugged on it.The single-sided silicon microstrip sensors are of p-on-n type with a substrate 〈100〉orientation, a thickness of 200 ± 15μm, a strip pitch of 200μm and a total depletionvoltage equal or less than 75V. The total number of strips per sensor is 256 in an activearea of 51× 50.66mm2. The fraction of disconnected strips is 3.4× 10−3.The front-end ASIC for the microstrip read-out, designed in AMS 0.35μm CMOStechnology, consists of 32-channels, each equipped with a charge-sensitive amplifier, ashaper and a comparator that produces a digital output by comparison with an externalthreshold. For testing and calibration the ASIC can be pulsed to inject a known charge,via a bank of 0.5 pF capacitors, into the preamplifiers.The silicon microstrip sensor and its read-out ASICs are glued and micro-bondendonto the front-end board. The microstrip sensor is glued onto a square aperture locatedat the center of the board. The precision of the gluing operation is not very important:the absolute spatial alignment of the eight sensors of the tracker is done using trackswhen no phantom is present between the tracker planes. The front-end board hosts theDACs used to set the threshold voltage for each ASICs.The 256 digital signal lines are connected to the tracker digital board plugged on thefront-end board. The digital board mounts a Xilinx Spartan FPGA which samples inparallel the signal lines to find active strips. The FPGA operates in pre-trigger modecontinuously sampling at 50MHz the strip signals but acquiring data only in a timewindow of about 5.1μs around the trigger (4μs before and 1.1μs after). For each activestrip its number, the signal start time with respect to the trigger and its duration, areformatted together event building ancillary information. Data is then moved to a RAMmemory (4 × 16Mbit chips, able to store up to 5 × 105 events). The board hosts anethernet module used for data and command communication.76 C. CIVININI, M. BRUZZI, M. BUCCIOLINI, ETC.Fig. 3. – The tomography phantom: 20mm external diameter with 4mm and 6mm diameterholes.3. – Calorimeter systemThe pCR calorimeter is made by four YAG:Ce (Yttrium Aluminum Garnet activatedby Cerium) scintillating crystals assembled in an optically decoupled 2× 2 matrix. Eachcrystal has a 3× 3 cm2 cross-section and a depth of 10 cm to stop up to 230MeV kineticenergy protons.The calorimeter measures the residual proton energy and generates the trigger signal.To use the calorimeter close to the stray magnetic field always present near a beamline the selected crystal should have a light emission wavelength fitting the sensitivityspectrum of silicon photodiodes. Furthermore the light emission decay constant should befast enough to be used in a high particle flux environment. The YAG:Ce crystals matchthese requirements having an emission ligth peak at 550 nm and a light decay constantof 70 ns with an integrated light output relative to NaI of 40% (35000 photons/MeV)almost insensitive to operating temperature. The material is not hygroscopic and has adensity of 4.56 gcm−3. Each crystal is glued to a Hamamatsu S3204 photodiode. Theread-out electronics consists of a charge-sensitive amplifier and a 1μs peak time shaper.The analog signals are sampled by an acquisition board hosting 14 bit 10MHz Flash-ADCs. The analog signals are also sent to comparators whose logical or output is usedfor triggering. The eigth least significal bits of the trigger counter are used to produce aGlobal Event Number tag to synchronize tracker and calorimeter data.Fig. 4. – FBP reconstructed image.PRIMA+: A PROTON COMPUTED TOMOGRAPHY APPARATUS 774. – pCT imagesThe pCR system has been tested at INFN-LNS in Catania using 60MeV protons.In particular a cylindrical phantom made of PMMA (fig. 3) has been mounted betweenplane 2 and 3 of the tracker and data have been taken rotating the object at 36 differentangles in steps of 10 degrees. On average about 2 × 106 events per angle have beencollected. The Filterd back-Projection (FBP) algorithm implemented in [6] has beenused to reconstruct the tomographic image of the phantom. The effect of the MCS onthe proton trajectories has been reduced selecting protons which do not deviate morethan ±4mrad from the nominal beam direction. Figure 4 shows a picture of the FBPreconstructed image.5. – ConclusionsA system for pCR has been developed and tested as a first step towards a protontomography equipment to be used in pre-clinical studies. The apparatus is made bya silicon tracker followed by a YAG:Ce calorimeter and collect data in single protonmode. The system has been tested using a 60MeV proton beam at the Catana line ofINFN-LNS. Collected data have been used to reconstruct radiographies and tomographicimages to extensively test the device.The PRIMA+ collaboration is presently working on the development of a similarpCT equipment with an enlarged field of view (5× 20 cm2); this system will reconstructtomographies of objects with dimensions close to the ones of clinical interest.REFERENCES[1] Schaffner B. and Pedroni E., Phys. Med. Biol., 43 (1998) 1579.[2] Mustafa A. A. M. and Jackson D. F., Phys. Med. Biol., 26-3 (1981) 461.[3] Williams D. C., Phys. Med. Biol., 49 (2004) 2899.[4] Li T. et al., Med. Phys., 33-3 (2006) 699.[5] Cirrone G. A. P. et al., Nucl. Instrum. Methods A, 576 (2007) 194.[6] CTSim is Copyright c©1983-2009 by Kevin M. and Rosenberg M. D. and is open-sourcesoftware under the GNU Public License.

PRIMA+: A proton Computed Tomography apparatus

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PRIMA+: A proton Computed Tomography apparatus

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