The characterization of high-intensity charged-particle and photon beams at medical accelerators is often a time-consuming task. In this work, we discuss the possibility to use highly segmented CMOS imagers as a way to measure the ﬂuxes with high spatial precision and in a short time. Quite recently CMOS imagers, designed to collect visible light, have been used to detect ionizing radiation, either charged particles (electron, proton) or photons. These devices, due to the very low single pixel noise, have a very high detection eﬃciency, once the interaction between radiation and silicon has taken place, and act primarily as counting detectors. We will show how it is possible to extract a precise beam shape using as a test case a therapeutic electron beam delivered by an Elekta e-LINAC at the S. Maria Hospital in Terni (Italy), and as sensors commercial oﬀ-the-shelf (COTS) CMOS imagers

Alunni Solestizi, L.

Biasini, M.

Fabiani, S.

Italiani, M.

Kanxheri, K.

Servoli, L.

Tucceri, P.

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DOI 10.1393/ncc/i2018-18216-3Colloquia: SIRR 2018IL NUOVO CIMENTO 41 C (2018) 216On the use of highly pixellated CMOS imagers to measuretherapeutic beam profileL. Servoli(1)(∗), L. Alunni Solestizi(1), M. Biasini(1)(2), S. Fabiani(3),M. Italiani(3), K. Kanxheri(1) and P. Tucceri(1)(1) INFN, Sezione di Perugia - Perugia, Italy(2) Dipartimento di Fisica e Geoologia, Università di Perugia - Perugia, Italy(3) Azienda Ospedaliera S. Maria - Terni, Italyreceived 4 December 2018Summary. — The characterization of high-intensity charged-particle and photonbeams at medical accelerators is often a time-consuming task. In this work, we dis-cuss the possibility to use highly segmented CMOS imagers as a way to measure thefluxes with high spatial precision and in a short time. Quite recently CMOS imagers,designed to collect visible light, have been used to detect ionizing radiation, eithercharged particles (electron, proton) or photons. These devices, due to the very lowsingle pixel noise, have a very high detection efficiency, once the interaction betweenradiation and silicon has taken place, and act primarily as counting detectors. Wewill show how it is possible to extract a precise beam shape using as a test case atherapeutic electron beam delivered by an Elekta e-LINAC at the S. Maria Hospitalin Terni (Italy), and as sensors commercial off-the-shelf (COTS) CMOS imagers.1. – CMOS imagers as ionizing radiation detectorsThe characterization of charged particle beams, either for medical purposes or indus-trial ones, is an important subject, and the availability of new sensors is always sought toincrease the precision of the measurements and reduce the time and the effort needed. Inparticular flux measurement is essential to define the beam shape, including penumbraregions that are often difficult to define precisely due to the rapidly varying radiation fieldand the dimension of the sensor. In this work the capability of commercial off-the-shelf(COTS) CMOS imagers as detectors for measuring the flux of charged particles will beexplored.CMOS imagers, optimized for visible light collection, mainly produced by Aptina,have been used in the past years as ionizing radiation detectors either for photons or(∗) Corresponding author. E-mail: leonello.servoli@pg.infn.itCreative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 L. SERVOLI et al.Fig. 1. – CMOS imager signal formation: (top) visible light: each photon creates an electron-holepair; (bottom) a single charged particle creates many electron-hole pairs.for charged particles [1-6]. The reason for this growing interest lies in the very highS/N that a charged particle produces in the sensitive layer of the devices. Due to therequest of being capable of making pictures even in very poor light condition, (low signallevel), these types of sensors are forced to have a very low pixel noise, of the order of fewelectrons, to enhance the S/N ratio even in all cases.In fig. 1 the two different energy deposition mechanisms for the case of visible light andsingle charged particle crossing the sensitive layer are shown. In the first case, the num-ber of electron-hole pairs produced is proportional to the number of incoming photons,focused by the microlens over the pixel, arriving within the exposure time (integrationtime from now on), while in the latter case the number of created electron-hole pairs de-pends on the thickness of the sensitive layer, on the momentum of the incoming particleand on the portion of the track that lies below the single pixel. Given the typical thick-ness of few μm of the sensitive layer and the worst signal formation case, the minimumionizing particle (MIP), the number of pairs created is of the order of 50–200. Coupledwith the very low noise (∼2–3 electrons for standard 33 ms integration time), this pro-duces a S/N > 20 when the pixel photodiode collects all electrons. Another differenceis that, due to the production process, the layer containing the CMOS electronics (1μmthickness) has dead regions, while the epitaxial layer below (sensitive layer with few μmthickness) is homogeneous across all matrix area. Hence, given the thickness of the twolayers, this class of sensors has no dead space and a slightly variable sensitive volume. Ithas been demonstrated that the reduction in charge collection efficiency in the first layeris of the order of 25% [7,8], thus keeping a very high S/N in all cases.If the path of the incoming charged particle is close to the border of the pixel orif the amount of created electron-hole pairs is very high (low-energy electrons, protons, α),the electrons could be collected by more than one pixel at a level that is greater thanthe noise level for each pixel. See, for example, fig. 2, where a 450 MeV electron createsa cluster with few pixels having signal above the noise level (left) while a 3 MeV protonON THE USE OF HIGHLY PIXELLATED CMOS IMAGERS ETC. 3Fig. 2. – Image of isolated clusters. Left: minimum ionizing particle (450 MeV electron). Right:highly ionizing particle (3 MeV proton).(right) creates a cluster of ∼100 pixels over the noise. Because each pixel produces ananalog signal, with ADC conversion done directly on the sensor at the periphery of thepixel matrix, the signal division among adjacent pixels could be used to better identifythe incoming particle through several types of clustering algorithms that reconstructboth energy deposition and impact position of the charged particle [2].The very high pixel density (∼0.25 Mpixel/mm2) makes this class of detectors idealfor single-cluster counting for MIPs up to quite high fluxes. A single cluster has almostalways a pixel with more signal than the others, thus a topology of a local maximum,hence to count the number of such maxima in a frame, is a direct measurement of theflux of incoming charged particles if a detection efficiency close to 100% can be assumed,as is when a S/N > 20 is reached. The only problem lies in the superposition among twoor more different clusters that results in cluster merging (decrease of cluster number) orcreation of fake maxima when two or more contributions sum up on a pixel belonging tothe periphery of the clusters, causing a cumulative signal greater than the neighboringones (increase of the cluster number). These problems could cause a deviation fromlinearity of the relation between incoming particles and detected clusters.Taking advantage of the high pixel density and also of the high multiplicity of incomingbeams, a non-linear calibration procedure has been developed [9] allowing to reach aprecision better than 1% in flux measurement in a range of ∼10 MHz/cm2, dependingon the pixel size of the CMOS imager and on the thickness of the epitaxial layer.2. – Electron flux measurementA measurement campaign on a clinic accelerator (Electa e-LINAC) located at S. MariaHospital in Terni (Italy) has been carried out to study the CMOS imager capability tomeasure the electron flux. The horizontal position of the accelerator allowed an easierpositioning of the mechanical setup (fig. 3). The electron beam parameters were set at10 MeV, 50 MU, 160 s irradiation time, while the sensor was placed at the beam centerat a distance of 140 cm from the beam exit window. Two precision linear stages (eachstep is 210 nm) were connected to obtain x-y movements in the plane orthogonal to thebeam direction and a plastic holder was used to support the CMOS imager under test.4 L. SERVOLI et al.Fig. 3. – experimental setup on the Elekta e-LINAC in S. Maria Hospital (Terni): beam comingfrom left (greeen arrow), precision x-y movements and CMOS imager on its holder.Several sensors from Aptina were used, as detailed in table I, to perform different tests.Each sensor was located in its evaluation board and readout using a DAQ board (AptinaDemo2 or Demo2X) with an FPGA to control the sensor and to receive and processthe data, to be sent via a USB 2.0 connection to a PC for frame storing ([10]). Themeasurement sequence for each experimental point is the following:• the DAQ is started before switching on the beam, to have some frames withoutbeam;• the beam is switched on and left running for 180 s;• the beam is switched off;• the DAQ is stopped.Then, for each frame, the number of relative maxima is found and plotted as a functionof the frame number (the time), then converted into flux measurement and used to buildthe distribution of the flux measurements for all frames (inset in fig. 4). This distributionis essentially Gaussian due to the randomness of the occupancy on the sensor surfaceTable I. – List of CMOS imagers under study and their main characteristics.Sensor MT9V011 MT9T031 MT9J003Number of pixels 307200 3145728 10068672Pixel area (μm2) 31.36 10.24 2.7889Pixel size (μm) 5.6 3.2 1.67Epi-region thickness (μm) 4.0 4.0 4.0ON THE USE OF HIGHLY PIXELLATED CMOS IMAGERS ETC. 5Fig. 4. – Nmaxima vs. number of frame for the MT9J003 sensor on 10 MeV, 50 MU, 3 × 3 cm2collimator electron beam from Elekta e-LINAC. Inset: distribution of flux measurements afterapplying non-linear calibration.and its variation is a measure of the precision of the flux measurements. In this case therelative uncertainty is 0.25%, while the accuracy is within the uncertainty of the signalmeasured by the control system, i.e., the silicon microstrip system [11].The first set of measurements, using MT9T031 and MT9J003 reported in [9], con-firmed the validity of the non-linear calibration technique to measure the electron flux,giving results compatible within uncertainties and compatible also with the control mea-surement performed using silicon microstrips.3. – Beam profile measurementTo measure the beam profile a position scan in both x-y coordinates (x has a 100 mmrange, y a 50 mm range) has been performed using the sensor MT9V011. The rowcoordinate of the sensor is along the y-axis and the column coordinate is along thex-axis. The measurement procedure was organized as follows:• for each point of the scan 15 frames were acquired (3 seconds);• the distance from two consecutive points is 5 mm;• beam and DAQ always on;• x scan from [−50, 0] position until [+50, 0];• y scan from [−25, 0] position until [+25, 0];The beam profile scan was completed in less than 5 minutes, all considered. In fig. 5the sequence of flux measurements is reported as a function of the frame number during6 L. SERVOLI et al.Fig. 5. – Electron flux vs. number of frame for MT9V011 during position scan.the x-coordinate scan. Then, for each measurement point, the average of the measure-ment distribution and its relative RMS have been extracted (fig. 6).The position of each point of the scan is fixed at the center of the sensor, henceeach measurement point has a width along the x-coordinate of ±1.79 mm, defining thegranularity of the measurement and its precision in that coordinate. The shape of thebeam is approximately Gaussian-like, as expected, and its width is ∼23 mm. Concerningthe relative uncertainties, we have obtained approximately the same results reported in [9]where an uncertainty of 1% in flux measurement was predicted for the MT9V011 sensorat 2.14 MHz/cm2 flux. The position scan in the y-coordinates, even if performed on a50 mm range instead than 100 mm, gave similar results with a width of the distributionof 24.6 mm, confirming the symmetry of the beam around the beam axis.4. – Sensor segmentationA further possibility given by the very high segmentation is to subdivide the matrixin N × M submatrices and perform N × M flux measurements at the same time in anarea of ∼10 mm2. The area is wide enough to record flux variations in position betweenone submatrix and the others. That would allow a much finer shape measurement at apossible price of increasing the uncertainty for each flux measurement. In fact, the uncer-tainty will depend on the number of maxima present in the submatrix area, dependingFig. 6. – Left: electron flux vs. position for MT9V011 during position scan. Right: relative fluxRMS for each measurement point.ON THE USE OF HIGHLY PIXELLATED CMOS IMAGERS ETC. 7Fig. 7. – Map of the flux measurement for a subdivision into 3 × 4 submatrices, 150 × 150 pixelwide at point 10 mm of the position scan along the x-coordinate.Fig. 8. – Beam shape along the x-coordinate for the MT9V011 sensor subdivided into 3 × 4submatrices, 150 × 150 pixel wide, using the central submatrices of the y-coordinate (row coo-ordinate) in fig. 7.hence on the number of pixels that form the area. Choosing a square submatrix with a150 pixels side, with 1% relative uncertainty, 12 submatrices are formed. Studying theflux measurements on them at a point in the scan where there is a rapid variation ofthe flux, for example the 5 mm point, the sensor capability to measure flux differencesis confirmed as shown in fig. 7.The variation in flux is present on the x-coordinate (column coordinate) (10% dif-ference over 3.5 mm, with 1% uncertainty on each point) but not on the y-coordinate(row coordinate) with 2% over 1.3 mm. The slope of the flux variation is essentially thesame on all the slices on the x-coordinate, thus confirming the capability to performmeasurements over a limited sensor surface (0.7 mm2) with an uncertainty of the orderof 1% (fig. 8).5. – ConclusionsHighly pixellated COTS CMOS imagers could be used to characterize therapeuticelectron beams provided a proper non-linear calibration is performed. The shape of thebeam could be precisely determined, with uncertainty in each measurement point of theorder of 1% or less.8 L. SERVOLI et al.Moreover, a subdivision of the sensor surface in smaller submatrices with at least a150 pixel side could provide multiple measurement points for a better shape mapping. Inprinciple, the submatrix area could be reduced if we use sensors with smaller pixel size,like 2× 2 μm2 to 0.4 mm2 keeping the same 1% uncertainty on the measurement points.Further investigations are needed to understand if the beam shape determination couldbe done in just one DAQ session varying also the z position of the sensor and to assessthe stability in time of the sensor response.∗ ∗ ∗This work is partially supported by Fondazione Cassa di Risparmio di Perugia:Project RIPARI n. 2017.0104.021.REFERENCES[1] Servoli L., Biagetti D., Passeri D. and Spanti Gattuso E., IEEE Nuclear ScienceSymposium Conference Record, NSS08 (IEEE) 2008, pp. 2484–2488.[2] Servoli L., Biagetti D., Passeri D. and Spanti Gattuso E., J. Instrum., 5 (2010)P07003.[3] Servoli L., Biagetti D., Meroli S., Placidi P., Passeri D. and Tucceri P., Nucl.Phys. B Proc. Suppl., 215 (2011) 228.[4] Castoldi A., Guazzoni C., Maffessanti S., Montemurro G. V. and Carraresi L.,J. Instrum., 10 (2015) C01002.[5] Perez M. et al., Nucl. Instrum. Methods A, 827 (2016) 171.[6] Holden W. M., Hoidn O. R., Seidler G. T. and DiChiara A. D., Rev. Sci. Instrum.,89 (2018) 093111.[7] Meroli S., Biagetti D., Passeri D., Placidi P., Servoli L. and Tucceri P., Nucl.Instrum. Methods A, 650 (2011) 230.[8] Meroli S., Passeri D. and Servoli L., J. Instrum., 6 (2011) P06013.[9] Servoli L. and Tucceri P., J. Instrum., 11 (2016) P03014.[10] http://www.aptina.com/.[11] Alpat B., Pilicer E., Servoli L., Menichelli M., Tucceri P., Italiani M.,Buono E. and Di Capua F., J. Instrum., 7 (2012) P03013.

On the use of highly pixellated CMOS imagers to measure therapeutic beam proﬁle

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On the use of highly pixellated CMOS imagers to measure therapeutic beam proﬁle

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