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Positron camera for range verification of heavy-ion radiotherapy
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Nuclear Instruments and Methods in Physics Research A 515 (2003) 840–849
Positron camera for range verification of heavy-ion radiotherapy Yasushi Isekia,b,*, Hideyuki Mizunoc, Yasuyuki Futamid, Takehiro Tomitanie, Tatsuaki Kanaif,b, Mitsutaka Kanazawae, Atsushi Kitagawae, Takeshi Murakamie, Teiji Nishiog, Mitsuru Sudae, Eriko Urakabef, Akira Yunokih, Hirotaka Sakaih a
Power and Industrial Systems Research and Development Center, Toshiba Corporation, 1, Toshiba-cho, Fuchu 183-8511, Japan b Department of Energy Sciences, Tokyo Institute of Technology, Nagatsuda-cho, Midori-ku, Yokohama, 226-8502, Japan c Department of Technical Radiology, Saitama Cancer Center, 818, Komuro, Ina-machi, Saitama 362-0806, Japan d Department of Proton Beam Therapy, Shizuoka Cancer Center, 1007, Shimonagakubo, Nagaizumi-cho, Shizuoka 411-8777, Japan e Department of Accelerator Physics and Engineering, National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage-ku, Chiba 263-8555, Japan f Department of Medical Physics, National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage-ku, Chiba 263-8555, Japan g Department of Radiology, National Cancer Center, Kashiwa 277-8577, Japan h Fuchu Operation-Power Systems, Toshiba Corporation, Fuchu 183-8511, Japan Received 17 February 2003; received in revised form 26 June 2003; accepted 1 July 2003
Abstract A positron camera, consisting of a pair of Anger-type scintillation detectors, has been developed to verify ranges by using positron emitter beams. Each detector head is equipped with a NaI(Tl) crystal (diameter: 600 mm; thickness: 30 mm) for high detection efficiency. To get a low counting rate for this application, the electric circuit was designed for flexibility in measurement and analysis by software. The energy and position were calibrated for high measurement accuracy. A spatial resolution of 8:6 mm in FWHM within a 750 mm region (field of view) and a linear response of a 0:3 mm standard deviation within a 7200 mm region were obtained. The camera was designed so as to measure the ranges within an accuracy of 1 mm under a dose limitation (about 100 mGyE) to reduce the safety margin for the irradiation field, and it met the required characteristics. r 2003 Elsevier B.V. All rights reserved. PACS: 29.40.MC Keywords: Positron camera; Positron emitter; Spatial resolution; Heavy ion; Radiotherapy; HIMAC
1. Introduction *Corresponding author. Address for correspondence: Power and Industrial Systems R&D Center, Toshiba Corporation, 1, Toshiba-cho, Fuchu 183-8511, Japan. E-mail address: [email protected] (Y. Iseki).
One excellent feature of heavy-ion radiotherapy is available dose concentration range, and therefore, the precise range control of irradiation beam is required. We are studying a radiotherapeutic use
0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.07.005
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of radioactive isotope beams at the Heavy Ion Medical Accelerator in Chiba (HIMAC) [1–3] to verify the ranges in patient bodies. The range verification system and the spot scanning irradiation system [4,5] were constructed in 2000, and now, we are checking the properties of these systems carefully [6]. The first attempt to verify the range verification by using a radioactive beam was carried out by a group at Lawrence Berkely Laboratory (LBL) [7]. They used a 19 Ne beam and obtained an accuracy of less than 1 mm to determine the center of the activity distribution with a low irradiation dose. As another approach, a group at Gesellschaft fur . Schwerionenforschung (GSI) in Darmstadt has applied projectile-fragmentation reactions to image the irradiation field [8]. This approach has the merit that it is able to use stable nucleus beams although it is difficult to image the irradiation field from the distribution of positron emitters. In our approach, the radioactive isotope beams which are selected by an isotope separator are be injected in the patient bodies as in situ range verification. Here, a positron camera is a key device which detects annihilation gamma pairs from the decay of the positron emitters and determines the end point of the beam trajectory. The positron camera is required to have not only high spatial resolution, but also high detection efficiency to prevent damage of normal tissue by the probing beam. The range should be measured with an accuracy of less than 1 mm under the limitation of about 100 mGyE (gray-equivalent dose) to reduce the safety margin of treatment planning. The number of positron emitters decaying was estimated to be on the order of 105 [9]. We have chosen a positron camera consisting of a pair of Anger-type scintillation detectors [10] although the groups at LBL and GSI used positron emission tomography (PET) detectors. The PET detectors are susceptible to a sampling problem and to have some ambiguity on the absolute position. Furthermore, the positron distribution obtained with PET detectors is sensitive to the density distribution of the body. Since the separation of background events is required for improvement of the spatial resolution, good
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energy resolution is an advantage of the Angertype detector. The camera has a thick crystal to increase the detection efficiency compared with the camera used in conventional diagnosis. The crystal size was determined by numerical simulation as having a diameter of 600 mm and thickness of 30 mm [9]. As a significant feature of range verification, the positron decay rate is very low. The positron emitters are injected into the body, forming an energetic beam, and thus, the fragmentation nuclei become one source of the background. Then, the software analysis should be flexible and provide the optimum algorithm for the position calculation which rejects the background and scattered events as well as improves the resolution. Since there is little need for fast data acquisition speed, all photomultiplier outputs are transferred and stored in the control computer. The positions where the gamma rays were deposited in the crystal are obtained by a centroid calculation of photomultiplier outputs in the Anger-type detector. There often are distortions in the line source image, especially near the edge of the crystal. In particular, a thick crystal detector is influenced more because the scintillation light spreads to a large area in the crystal. This distortion causes a nonlinear position response and the position resolution decreases. Thus, we developed a calibration method and prepared its software routine. We also developed the energy calibration routine; a good energy resolution is necessary to reduce the background and scattered events by setting an energy window. This paper presents design of the hardware, development of the event analysis routine and properties measured with a checking source. The measurement results indicate the positron camera has the designed properties for verifying the end point of positron emitter beams.
2. Design of positron camera 2.1. Head design The image of the range verification method is illustrated in Fig. 1. A probing beam of positron
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Y. Iseki et al. / Nuclear Instruments and Methods in Physics Research A 515 (2003) 840–849 Table 1 Camera head design Crystal material Crystal diameter Crystal thickness PMT size PMT number Head distance Solid angle Coincidence detection efficiency
NaI(Tl) 600 mm 30 mm 2 in: 109/head 500–700 mm 0.093–0:15 sr=crystal 0.18–0.21
Fig. 1. Range verification principle and system. PMTs ¼ photomultipliers:
multipliers are mounted is insensitive, the effective area is inside the diameter of about 500 mm: The geometry of the camera is summarized in Table 1. The distance between the two detectors is variable from 500 to 700 mm: The solid angle for a detector is from 0.093 to 0:15 sr as the distance decreases. Since the average gamma ray path length in the crystal depends on the incident angle, that is, on the detector distance, the coincidence detection efficiency is variable from 0.18 to 0.21 for the total absorption of 511 keV gamma rays. The detector distance is now set to 700 mm so as not to restrict placement of the patient. 2.2. Design of electric circuit Fig. 2. Arrangement of photomultipliers on a detector.
emitters, such as 11 C; is injected into a position of interest in a tumor, e.g., quite close to a critical organ. The positron camera detects pairs of annihilation gamma rays and determines the end point of the beam trajectory in the body. This camera consists of a pair of Anger-type scintillation detectors equipped with a NaI(Tl) crystal (diameter: 600 mm; thickness: 30 mm). In view of the good spatial resolution, the surface of the crystal was considered to be diffusively reflective at the front and absorbent at the edge [11]. The camera has 109 2-in. photomultipliers (PMTs) that are close-packed on the crystal, as shown in Fig. 2. The interval between the photomultipliers is 54 mm: Since the area where the outer photo-
Features required in the hardware of the camera are accurate signal outputting with a small dispersion, high efficiency for detecting real events, and small influence from background events. Unlike a conventional scintillation camera, a high-speed response is not necessary because the counting rate is less than 100 cps even including the background events in the range verification. The gains and offsets of all photomultiplier amplifiers are carefully measured with a radioisotope source and a current generator. The photomultiplier gains are adjusted with dividing resistors and an accurate calibration is carried out in a software routine. For reduction of the background events, the trigger signal is generated by hit logic; the trigger condition is written on a hit logic ROM and can be changed by exchanging the ROM. The optimum trigger condition, for example, the multiplicity of
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the photomultiplier hits, is one of the objects of our study. The outputs of all photomultipliers are transferred and stored in the control computer due to absence of any limit in the counting rate. Thus, the events can be analyzed in the software accurately. The background events are separated from the true events by some steps in the software. The position and energy of the deposited gamma rays can be calibrated flexibly to improve the position and energy resolution. The development of the calibration method is another object of our study. The positron camera can also measure the range of the positron emitters having a short half-life, such as 10 C ð19:3 sÞ: The short-lived nuclei are useful in the research involving study of the influence of blood flow as well as for measuring more than one point where probing beams are injected successively. The detection can be synchronized with the interval of the irradiation pulse by external inhibit signals so as to reduce the amount of transferred and saved data. The circuit has a clock and a counter so that events include information on the time of generation. The generation time can be used to reject background events by decay-curve analysis, and also to study blood flow.
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2.3. Signal treatment boards Signal treatment boards evaluate the photomultiplier pulse heights and generate timing signals. The boards are constructed as shown in Fig. 3. Each detector has seven boards, and each board treats signals from 16 photomultipliers. There are two lines on each board: one is a ‘fast line’, that is, generating the timing signal, and the other is a ‘slow line’, evaluating the pulse heights. In the fast line, a discriminator generates the timing pulse when the fast rising pulse is beyond the discrimination level, and outputs it to the coincidence board. The timing pulses are TTL signals having a 50 ns width; the width is variable in 10 ns steps by switch setting. In the slow line, the pulses are digitized through capacitanceresistance integrators, peak holds and 10-bit analog-digital converters. All of the digitized signals are stored in memory for every event, and the event number and the detecting time are added to the event data. The memory is a FIFO (fast-in fast-out) type and the digital signals are transferred to the control computer. The analog signals are summated and the summed signal is output to the coincidence board. This signal is used for the event selection.
Fig. 3. Design of electric circuit. STB ¼ signal treatment board, CB ¼ coincidence board, Discri ¼ Discriminator; PH ¼ peak-hold, MPX ¼ multiplexa; ADC ¼ analog=digital convertor, Mem ¼ memory and Coin ¼ coincidence:
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2.4. Coincidence boards The coincidence boards judge if the event is true or not, and they generate a trigger signal when all conditions are satisfied. The hit logic judges the coincidence of 109 timing pulses. When the signals satisfy the coincidence condition which is memorized on the hit logic ROM, the timing logic outputs a timing signal to the signal treatment boards. The 16-channel summed signals are summed in the coincidence boards. The height of the summed signal corresponds to the deposited energy of the gamma ray, and is used to make a preliminary distinction of whether the event is true or not; accurate energy selection is achieved in the software routine. The coincidence between two detectors is checked on the board of one detector, DetectorA. The hit signal of Detector-B is transferred to the coincidence board of Detector-A. When the timing of the hit signals of the two detectors is coincident, the control signal is output to the memories of the signal treatment boards of the two detectors. The coincidence board in Detector-A has a counter that counts the number of the coincident hits between two detectors, and it also has a timer for memorizing the time that the coincident event is generated after the measurement starts. The trigger signal can be inhibited with the external gate signal. The time when the external signal is gotten is also saved on the memory. This function is useful especially for short-lived nuclei measurements. The measurement circuit provides a function for measuring the count rate of the random coincidence. The coincidence board in Detector-B generates a delayed timing signal after the hit signal is transferred to Detector-A for this purpose. 2.5. Software analysis routine The position and energy of an annihilation gamma ray deposited in each detector are evaluated in the following four steps. First, the uniformity of the photomultiplier gains is adjusted with a look-up table. The offset and response parameters were obtained beforehand with a current generator and a gamma ray source of
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Co ð122 keVÞ through an aperture of 1 mm diameter. In the second step, the deposited position and energy is pre-evaluated; the position (z as horizontal and y as vertical) is obtained by a centroid calculation. We tried some algorithms for the position calculation and found that the smallest distortion and localization appeared in the image for the case with the centroid calculation that had biased outputs of the photomultipliers [12]: Zpre ¼
109 X i
½zi pi =
109 X
( Pi
i
pi ¼ Pi Pb pi ¼ 0
if Pi > Pb ifPi oPb : ð1Þ
Here, zi is the position of the ith photomultiplier and Pi is its output. The bias level ðPb Þ was set to 7% of the peak value of the output distribution of the photomultiplier for which the deposit position of the 511 keV gamma rays was the closest. The energy is obtained by the summation of the photomultiplier outputs: ( 109 X pi ¼ Pi if Pi > Pd E¼ pi ð2Þ pi ¼ 0 if Pi oPd i where Pd is the discrimination level and is set to be equal to the bias level. In the third step, the energy dependent on the gamma ray incident position is calibrated with the pre-evaluated positions. The response function on the position was prepared from data for a positron emitter source (see Section 3). Lastly, the position distortion is calibrated with the pre-evaluated positions, Ypr;k and Zpr;k ; by Yk ¼ Ypr;k þ DYk ;
Zk ¼ Zpr;k þ DZk
ð3Þ
where the subscript k denotes Detector A or Detector B. The correction parameters ðDYk ; DZk Þ were listed in a look-up table of 250 250 matrix (1 mm pitch on two axes: y and z) and were obtained from the line source measurement (see Section 4). The positron positions, Y and Z; on the focal plane is calculated by the gamma ray positions on the two detectors and the beam position. For the
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horizontal position ðZÞ; Z¼
D 2X D þ 2X ZA þ ZB 2D 2D
ð4Þ
where ZA and ZB are the gamma ray positions on Detector A and Detector B, respectively, D is the distance between the detectors, and X is the horizontal position on the cross-sectional plane of the positron emitter beam. Some event selections, e.g., position window on each detector and the energy window there, are prepared. The energy window is used for rejecting the background events and the scattered gamma rays. As the numerical simulation in our previous paper indicated [9], the wide energy gate provides more accurate position measurements when there is little influence from the background events. However, we have understood that the window in the region of the total absorption is necessary to simplify the procedure for the energy and position calibration. The window for the gamma ray incident positions restricts the incident angle on the detector. This is effective for improving the spatial resolution if the detection efficiency is unimportant because the events deposited near the edge of the crystal are affected by the parallax effect.
Fig. 4. Energy distribution dependent on gamma ray deposition area. The numbers in brackets denote the lower and outer positions of the selection windows for radial position (50-mm width window).
3. Energy calibration Because the scintillation light escapes from the edge of the crystal, the summation of the photomultiplier outputs does not correspond to the deposition energy when the gamma ray is injected near the edge. The response function on the radial position for each detector was obtained from the coincident events with a 22 Na point source mounted at the center of the field of view of the positron camera. Here, the calculated position without calibration was used as the initial position because the position correction made following this needs the results of the energy calibration. Fig. 4 is the energy spectra on Detector A for the case where the events were selected with the window of the radial incident position having a 50-mm width. The photopeak energy of the
Fig. 5. Peak energy dependent on gamma ray deposition area. The solid line is a fitting curve to a polynomial function.
annihilation gamma rays became smaller and the width of the distribution, larger, as the incident position was closer to the edge of the crystal; the peak energy for the case of the outermost window of the effective area was half of that for the case at the center of the crystal. The dependence of the peak energy is shown in Fig. 5. The response function was given by fitting the peak energies to a polynomial function. Fig. 6 shows the change of the energy distribution after correction with the response function. The energy resolution was improved from 14% to 11% and the photopeak was clearly separated from the Compton scattering region. The camera had enough resolution to reject the scattered and
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Fig. 6. Energy distribution for full detection region. A source was set at the isocenter.
22
Na
background events. This was mainly due to use of the NaI crystal which has a large quantum efficiency. 4. Line image correction Distortions in images often appear when using the Anger-type detector. We considered two types of distortions here. One is seen in a cell defined by a row of photomultipliers. The position sensitivity near the central axis of the photomultipliers is worse than that near the boundary with the next photomultiplier, and consequently, the image is distorted. The other is seen near the edge of the crystal. For deposition near the edge, the number of photomultipliers outside of the gamma ray deposition position is less than the number inside. Thus, the calculated position moves toward the center of the crystal. We detected line images to get a correction routine for these distortions. The line images were measured with a 68 Ge line source collimated to a 1-mm gap. The line source was mounted with a driving stage immediately in front of the crystal. The measurements were carried out at intervals of 40 mm in the vertical and horizontal directions (on the y- and z-axis) for each detector. The line image is likely to depend on the deposited energy because the depth where the gamma ray deposits in the crystal depends on the energy. To simplify the position correction, the data were taken for the deposited energy within 711% of 511 keV; here, the window width corresponded to the energy resolution of 11% in FWHM.
Fig. 7. Line images. (a) Before correction, (b) after correction. The 68 Ge source was collimated with a 1-mm gap. Left: the source parallel to the y-axis; right: to the z-axis.
Fig. 7(a) shows the line images before the correction for Detector A; the source was set twice, first parallel to the y-axis and then the z-axis. The distortion in a photomultiplier cell depends on the head geometry (crystal size, photomultiplier size and so on) and the position calculation algorithm. We optimized the arithmetic and adopted Eq. (1) because the distortion and localization were the smallest [12]. To prepare the correction routine of the distortion near the edge, correction parameters ðDYk ; DZk Þ were evaluated as the difference between the setting position of the line source and their calculation points. The parameters in the areas between two lines were obtained by interpolation. The corrected images for Detector A are shown in Fig. 7(b). The curved lines near the edge were modified well and became straight. Fig. 8 is the corrected position distribution projected on the z-axis on Detector A for the case where the source was set parallel to the y-axis. The widths of the distributions were almost constant, 5:270:5 mm:
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Fig. 8. Projected position distribution of line image. The distribution was for the case where the source was set parallel to the y-axis, and projected on the z-axis. Fig. 9. Spatial resolution (FWHM) dependent on the source position. The source was moved on the beam axis.
5. Positron camera properties The properties of the positron camera were measured with a 22 Na point source. Fig. 9 shows the change of spatial resolution dependence on the source position due to the line image correction. Here, the source was moved in the horizontal direction (the z-axis) of the mid-plane of the two detectors. Before the correction, the resolution became worse as the source was moved away from the center, but after the correction the resolution was almost constant. The resolution was 8:6 mm in FWHM within 750 mm (this is the therapeutic region) and almost constant within 7200 mm after the line image correction. Another important property is linearity. The differences between the setting positions and the measured ones are shown in Fig. 10. Before the correction, there was a disagreement between the measurement and setting positions; the measurement positions deviated toward the center from the setting positions. These positions, however, agreed after the correction with a standard deviation of 0:3 mm within 7200 mm region. Fig. 11 shows the variation of the position distributions on gamma ray deposition area of a crystal for the case where the point source was set 50 mm from the center. The detected events were selected with the window of the deposition position (horizontal position) on Detector A with a 50-mm width. While the peak position moved toward the center in the case without the line
Fig. 10. Position response. The source was moved on the beam axis.
image correction as the area window moved outside, this shift was prevented after the correction. As a result, the distribution for the case without the window selection became symmetrical and the peak position agreed with the source position. These improvements in the distribution led to good resolution and good linearity of the source position response.
6. Discussion The spatial resolution is dominantly influenced by the parallax effect: a spread due to the large incident angle of the gamma ray pairs. When the
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Fig. 11. Position distribution dependent on gamma ray deposition area on a head. The 22 Na source was set 50 mm from the center along the beam axis. The numbers in brackets denote the lower and outer positions of the selection windows in the horizontal direction on Detector A: (a) before correction, (b) after correction.
gamma ray incident area on Detector A was restricted within a 50-mm width in the horizontal direction and a 200-mm width in the vertical direction to restrict the incident angle, the resolution in the horizontal direction was improved from 8.6 to 4:7 mm: In our previous paper [9], we predicted the spatial resolution by numerical simulation. The simulation was carried out for a 500-mm distance between two heads. Then, we calculated it for a 700-mm distance again; the dispersion not including parallax was 5:8 mm and that including parallax was 7:6 mm: We thought that the disagreement between the measurement and the simulation for the case of position restriction was due to overestimating the gamma ray multiple reaction spread. However, the measured resolution was slightly worse than the simulated one for the case of no position restriction. This was probably
because we approximated to a Gaussian distribution in the simulation in spite of having a widerwidth shape, and because of inaccuracy in the position correction near the edge of the crystal. Karp and Muehllehner [13] developed a onedimensional Anger-type detector having an excellent spatial resolution of 4:7 mm in FWHM for the case without grooves. We obtained a width of 5:2 mm as the projected position distribution in the detector (Fig. 8). They differed from our detector by using a crystal having a thickness of 2:5 cm and they set a relatively higher bias level (30%). Considering the width of the projected distribution in the detector, it would be likely that the resolution could be improved if the line image correction were done more precisely. The parallax effect, however, dominated the spatial resolution for our application rather than the dispersion for the event incident at 90 ; and thus, we thought that more precise correction of the line image was unimportant. The resolution we obtained was almost equal to the resolution for the BGO-camera which Llacer [7] had developed (7:5 mm in FWHM). Our camera, however, had a much larger solid angle when it was set with a sufficient distance between the detector heads. The accuracy of the range measurement was dominated by the statistical accuracy and the position response. The number of detecting gamma ray pairs was estimated to be a few hundred under assumptions of the peak dose of 100 mGyE; 11 C beam radius of 3 mm; beam momentum spread of 0.8% and transmission rate in the body of 0.15 [9]. Regarding the resolution of 8:6 mm in FWHM and the response of 0:3 mm in standard deviation, the range could be measured within an accuracy of 70:5 mm from the detection of a hundred gamma ray pairs. This indicated that the positron camera had the desired design characteristics.
7. Conclusion The positron camera that consists of a pair of Anger-type scintillation detectors has been developed to verify ranges of positron emitter beams.
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The camera had a spatial resolution of 8:6 mm in FWHM within a 750 mm region (field of view) and a linear response of 0:3 mm standard deviation within a 7200 mm region. Thus, the camera had the desired design characteristics, and the ranges within an accuracy of 1 mm could be measured under a few hundred mGyE. The linear response in a wide measurement region meant that only an offset adjustment would be necessary for application to range verification. Furthermore, the camera would be useful to detect irradiation field images because it had linearity within 7200 mm: There are still some subjects to study before realizing the camera for use in heavy-ion radiotherapy: the effect of scattering in the human body, influence of background events, optimizing the beam properties, and so on. We are now studying in situ properties of the camera and the total system of range verification carefully. Measurements made in this work will be presented in the near future.
Acknowledgements We appreciate the encouragement from and valuable discussions with Dr. F. Soga, Dr. A. Yamada, Dr. M. Endo and members of the Division of Accelerator Physics and Engineering at NIRS. Three of us (Y. Iseki, A. Yunoki and H. Sakai) especially thank Dr. N. Kobayashi, Mr. S. Makino, Dr. Y. Kita and Mr. K. Sato of Toshiba Corp. for their support. This work was performed as part of a Research Project with Heavy-ions at NIRS-HIMAC.
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Nuclear Instruments and Methods in Physics Research A 515 (2003) 840–849
Positron camera for range verification of heavy-ion radiotherapy Yasushi Isekia,b,*, Hideyuki Mizunoc, Yasuyuki Futamid, Takehiro Tomitanie, Tatsuaki Kanaif,b, Mitsutaka Kanazawae, Atsushi Kitagawae, Takeshi Murakamie, Teiji Nishiog, Mitsuru Sudae, Eriko Urakabef, Akira Yunokih, Hirotaka Sakaih a
Power and Industrial Systems Research and Development Center, Toshiba Corporation, 1, Toshiba-cho, Fuchu 183-8511, Japan b Department of Energy Sciences, Tokyo Institute of Technology, Nagatsuda-cho, Midori-ku, Yokohama, 226-8502, Japan c Department of Technical Radiology, Saitama Cancer Center, 818, Komuro, Ina-machi, Saitama 362-0806, Japan d Department of Proton Beam Therapy, Shizuoka Cancer Center, 1007, Shimonagakubo, Nagaizumi-cho, Shizuoka 411-8777, Japan e Department of Accelerator Physics and Engineering, National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage-ku, Chiba 263-8555, Japan f Department of Medical Physics, National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage-ku, Chiba 263-8555, Japan g Department of Radiology, National Cancer Center, Kashiwa 277-8577, Japan h Fuchu Operation-Power Systems, Toshiba Corporation, Fuchu 183-8511, Japan Received 17 February 2003; received in revised form 26 June 2003; accepted 1 July 2003
Abstract A positron camera, consisting of a pair of Anger-type scintillation detectors, has been developed to verify ranges by using positron emitter beams. Each detector head is equipped with a NaI(Tl) crystal (diameter: 600 mm; thickness: 30 mm) for high detection efficiency. To get a low counting rate for this application, the electric circuit was designed for flexibility in measurement and analysis by software. The energy and position were calibrated for high measurement accuracy. A spatial resolution of 8:6 mm in FWHM within a 750 mm region (field of view) and a linear response of a 0:3 mm standard deviation within a 7200 mm region were obtained. The camera was designed so as to measure the ranges within an accuracy of 1 mm under a dose limitation (about 100 mGyE) to reduce the safety margin for the irradiation field, and it met the required characteristics. r 2003 Elsevier B.V. All rights reserved. PACS: 29.40.MC Keywords: Positron camera; Positron emitter; Spatial resolution; Heavy ion; Radiotherapy; HIMAC
1. Introduction *Corresponding author. Address for correspondence: Power and Industrial Systems R&D Center, Toshiba Corporation, 1, Toshiba-cho, Fuchu 183-8511, Japan. E-mail address: [email protected] (Y. Iseki).
One excellent feature of heavy-ion radiotherapy is available dose concentration range, and therefore, the precise range control of irradiation beam is required. We are studying a radiotherapeutic use
0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.07.005
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of radioactive isotope beams at the Heavy Ion Medical Accelerator in Chiba (HIMAC) [1–3] to verify the ranges in patient bodies. The range verification system and the spot scanning irradiation system [4,5] were constructed in 2000, and now, we are checking the properties of these systems carefully [6]. The first attempt to verify the range verification by using a radioactive beam was carried out by a group at Lawrence Berkely Laboratory (LBL) [7]. They used a 19 Ne beam and obtained an accuracy of less than 1 mm to determine the center of the activity distribution with a low irradiation dose. As another approach, a group at Gesellschaft fur . Schwerionenforschung (GSI) in Darmstadt has applied projectile-fragmentation reactions to image the irradiation field [8]. This approach has the merit that it is able to use stable nucleus beams although it is difficult to image the irradiation field from the distribution of positron emitters. In our approach, the radioactive isotope beams which are selected by an isotope separator are be injected in the patient bodies as in situ range verification. Here, a positron camera is a key device which detects annihilation gamma pairs from the decay of the positron emitters and determines the end point of the beam trajectory. The positron camera is required to have not only high spatial resolution, but also high detection efficiency to prevent damage of normal tissue by the probing beam. The range should be measured with an accuracy of less than 1 mm under the limitation of about 100 mGyE (gray-equivalent dose) to reduce the safety margin of treatment planning. The number of positron emitters decaying was estimated to be on the order of 105 [9]. We have chosen a positron camera consisting of a pair of Anger-type scintillation detectors [10] although the groups at LBL and GSI used positron emission tomography (PET) detectors. The PET detectors are susceptible to a sampling problem and to have some ambiguity on the absolute position. Furthermore, the positron distribution obtained with PET detectors is sensitive to the density distribution of the body. Since the separation of background events is required for improvement of the spatial resolution, good
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energy resolution is an advantage of the Angertype detector. The camera has a thick crystal to increase the detection efficiency compared with the camera used in conventional diagnosis. The crystal size was determined by numerical simulation as having a diameter of 600 mm and thickness of 30 mm [9]. As a significant feature of range verification, the positron decay rate is very low. The positron emitters are injected into the body, forming an energetic beam, and thus, the fragmentation nuclei become one source of the background. Then, the software analysis should be flexible and provide the optimum algorithm for the position calculation which rejects the background and scattered events as well as improves the resolution. Since there is little need for fast data acquisition speed, all photomultiplier outputs are transferred and stored in the control computer. The positions where the gamma rays were deposited in the crystal are obtained by a centroid calculation of photomultiplier outputs in the Anger-type detector. There often are distortions in the line source image, especially near the edge of the crystal. In particular, a thick crystal detector is influenced more because the scintillation light spreads to a large area in the crystal. This distortion causes a nonlinear position response and the position resolution decreases. Thus, we developed a calibration method and prepared its software routine. We also developed the energy calibration routine; a good energy resolution is necessary to reduce the background and scattered events by setting an energy window. This paper presents design of the hardware, development of the event analysis routine and properties measured with a checking source. The measurement results indicate the positron camera has the designed properties for verifying the end point of positron emitter beams.
2. Design of positron camera 2.1. Head design The image of the range verification method is illustrated in Fig. 1. A probing beam of positron
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Y. Iseki et al. / Nuclear Instruments and Methods in Physics Research A 515 (2003) 840–849 Table 1 Camera head design Crystal material Crystal diameter Crystal thickness PMT size PMT number Head distance Solid angle Coincidence detection efficiency
NaI(Tl) 600 mm 30 mm 2 in: 109/head 500–700 mm 0.093–0:15 sr=crystal 0.18–0.21
Fig. 1. Range verification principle and system. PMTs ¼ photomultipliers:
multipliers are mounted is insensitive, the effective area is inside the diameter of about 500 mm: The geometry of the camera is summarized in Table 1. The distance between the two detectors is variable from 500 to 700 mm: The solid angle for a detector is from 0.093 to 0:15 sr as the distance decreases. Since the average gamma ray path length in the crystal depends on the incident angle, that is, on the detector distance, the coincidence detection efficiency is variable from 0.18 to 0.21 for the total absorption of 511 keV gamma rays. The detector distance is now set to 700 mm so as not to restrict placement of the patient. 2.2. Design of electric circuit Fig. 2. Arrangement of photomultipliers on a detector.
emitters, such as 11 C; is injected into a position of interest in a tumor, e.g., quite close to a critical organ. The positron camera detects pairs of annihilation gamma rays and determines the end point of the beam trajectory in the body. This camera consists of a pair of Anger-type scintillation detectors equipped with a NaI(Tl) crystal (diameter: 600 mm; thickness: 30 mm). In view of the good spatial resolution, the surface of the crystal was considered to be diffusively reflective at the front and absorbent at the edge [11]. The camera has 109 2-in. photomultipliers (PMTs) that are close-packed on the crystal, as shown in Fig. 2. The interval between the photomultipliers is 54 mm: Since the area where the outer photo-
Features required in the hardware of the camera are accurate signal outputting with a small dispersion, high efficiency for detecting real events, and small influence from background events. Unlike a conventional scintillation camera, a high-speed response is not necessary because the counting rate is less than 100 cps even including the background events in the range verification. The gains and offsets of all photomultiplier amplifiers are carefully measured with a radioisotope source and a current generator. The photomultiplier gains are adjusted with dividing resistors and an accurate calibration is carried out in a software routine. For reduction of the background events, the trigger signal is generated by hit logic; the trigger condition is written on a hit logic ROM and can be changed by exchanging the ROM. The optimum trigger condition, for example, the multiplicity of
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the photomultiplier hits, is one of the objects of our study. The outputs of all photomultipliers are transferred and stored in the control computer due to absence of any limit in the counting rate. Thus, the events can be analyzed in the software accurately. The background events are separated from the true events by some steps in the software. The position and energy of the deposited gamma rays can be calibrated flexibly to improve the position and energy resolution. The development of the calibration method is another object of our study. The positron camera can also measure the range of the positron emitters having a short half-life, such as 10 C ð19:3 sÞ: The short-lived nuclei are useful in the research involving study of the influence of blood flow as well as for measuring more than one point where probing beams are injected successively. The detection can be synchronized with the interval of the irradiation pulse by external inhibit signals so as to reduce the amount of transferred and saved data. The circuit has a clock and a counter so that events include information on the time of generation. The generation time can be used to reject background events by decay-curve analysis, and also to study blood flow.
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2.3. Signal treatment boards Signal treatment boards evaluate the photomultiplier pulse heights and generate timing signals. The boards are constructed as shown in Fig. 3. Each detector has seven boards, and each board treats signals from 16 photomultipliers. There are two lines on each board: one is a ‘fast line’, that is, generating the timing signal, and the other is a ‘slow line’, evaluating the pulse heights. In the fast line, a discriminator generates the timing pulse when the fast rising pulse is beyond the discrimination level, and outputs it to the coincidence board. The timing pulses are TTL signals having a 50 ns width; the width is variable in 10 ns steps by switch setting. In the slow line, the pulses are digitized through capacitanceresistance integrators, peak holds and 10-bit analog-digital converters. All of the digitized signals are stored in memory for every event, and the event number and the detecting time are added to the event data. The memory is a FIFO (fast-in fast-out) type and the digital signals are transferred to the control computer. The analog signals are summated and the summed signal is output to the coincidence board. This signal is used for the event selection.
Fig. 3. Design of electric circuit. STB ¼ signal treatment board, CB ¼ coincidence board, Discri ¼ Discriminator; PH ¼ peak-hold, MPX ¼ multiplexa; ADC ¼ analog=digital convertor, Mem ¼ memory and Coin ¼ coincidence:
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2.4. Coincidence boards The coincidence boards judge if the event is true or not, and they generate a trigger signal when all conditions are satisfied. The hit logic judges the coincidence of 109 timing pulses. When the signals satisfy the coincidence condition which is memorized on the hit logic ROM, the timing logic outputs a timing signal to the signal treatment boards. The 16-channel summed signals are summed in the coincidence boards. The height of the summed signal corresponds to the deposited energy of the gamma ray, and is used to make a preliminary distinction of whether the event is true or not; accurate energy selection is achieved in the software routine. The coincidence between two detectors is checked on the board of one detector, DetectorA. The hit signal of Detector-B is transferred to the coincidence board of Detector-A. When the timing of the hit signals of the two detectors is coincident, the control signal is output to the memories of the signal treatment boards of the two detectors. The coincidence board in Detector-A has a counter that counts the number of the coincident hits between two detectors, and it also has a timer for memorizing the time that the coincident event is generated after the measurement starts. The trigger signal can be inhibited with the external gate signal. The time when the external signal is gotten is also saved on the memory. This function is useful especially for short-lived nuclei measurements. The measurement circuit provides a function for measuring the count rate of the random coincidence. The coincidence board in Detector-B generates a delayed timing signal after the hit signal is transferred to Detector-A for this purpose. 2.5. Software analysis routine The position and energy of an annihilation gamma ray deposited in each detector are evaluated in the following four steps. First, the uniformity of the photomultiplier gains is adjusted with a look-up table. The offset and response parameters were obtained beforehand with a current generator and a gamma ray source of
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Co ð122 keVÞ through an aperture of 1 mm diameter. In the second step, the deposited position and energy is pre-evaluated; the position (z as horizontal and y as vertical) is obtained by a centroid calculation. We tried some algorithms for the position calculation and found that the smallest distortion and localization appeared in the image for the case with the centroid calculation that had biased outputs of the photomultipliers [12]: Zpre ¼
109 X i
½zi pi =
109 X
( Pi
i
pi ¼ Pi Pb pi ¼ 0
if Pi > Pb ifPi oPb : ð1Þ
Here, zi is the position of the ith photomultiplier and Pi is its output. The bias level ðPb Þ was set to 7% of the peak value of the output distribution of the photomultiplier for which the deposit position of the 511 keV gamma rays was the closest. The energy is obtained by the summation of the photomultiplier outputs: ( 109 X pi ¼ Pi if Pi > Pd E¼ pi ð2Þ pi ¼ 0 if Pi oPd i where Pd is the discrimination level and is set to be equal to the bias level. In the third step, the energy dependent on the gamma ray incident position is calibrated with the pre-evaluated positions. The response function on the position was prepared from data for a positron emitter source (see Section 3). Lastly, the position distortion is calibrated with the pre-evaluated positions, Ypr;k and Zpr;k ; by Yk ¼ Ypr;k þ DYk ;
Zk ¼ Zpr;k þ DZk
ð3Þ
where the subscript k denotes Detector A or Detector B. The correction parameters ðDYk ; DZk Þ were listed in a look-up table of 250 250 matrix (1 mm pitch on two axes: y and z) and were obtained from the line source measurement (see Section 4). The positron positions, Y and Z; on the focal plane is calculated by the gamma ray positions on the two detectors and the beam position. For the
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horizontal position ðZÞ; Z¼
D 2X D þ 2X ZA þ ZB 2D 2D
ð4Þ
where ZA and ZB are the gamma ray positions on Detector A and Detector B, respectively, D is the distance between the detectors, and X is the horizontal position on the cross-sectional plane of the positron emitter beam. Some event selections, e.g., position window on each detector and the energy window there, are prepared. The energy window is used for rejecting the background events and the scattered gamma rays. As the numerical simulation in our previous paper indicated [9], the wide energy gate provides more accurate position measurements when there is little influence from the background events. However, we have understood that the window in the region of the total absorption is necessary to simplify the procedure for the energy and position calibration. The window for the gamma ray incident positions restricts the incident angle on the detector. This is effective for improving the spatial resolution if the detection efficiency is unimportant because the events deposited near the edge of the crystal are affected by the parallax effect.
Fig. 4. Energy distribution dependent on gamma ray deposition area. The numbers in brackets denote the lower and outer positions of the selection windows for radial position (50-mm width window).
3. Energy calibration Because the scintillation light escapes from the edge of the crystal, the summation of the photomultiplier outputs does not correspond to the deposition energy when the gamma ray is injected near the edge. The response function on the radial position for each detector was obtained from the coincident events with a 22 Na point source mounted at the center of the field of view of the positron camera. Here, the calculated position without calibration was used as the initial position because the position correction made following this needs the results of the energy calibration. Fig. 4 is the energy spectra on Detector A for the case where the events were selected with the window of the radial incident position having a 50-mm width. The photopeak energy of the
Fig. 5. Peak energy dependent on gamma ray deposition area. The solid line is a fitting curve to a polynomial function.
annihilation gamma rays became smaller and the width of the distribution, larger, as the incident position was closer to the edge of the crystal; the peak energy for the case of the outermost window of the effective area was half of that for the case at the center of the crystal. The dependence of the peak energy is shown in Fig. 5. The response function was given by fitting the peak energies to a polynomial function. Fig. 6 shows the change of the energy distribution after correction with the response function. The energy resolution was improved from 14% to 11% and the photopeak was clearly separated from the Compton scattering region. The camera had enough resolution to reject the scattered and
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Fig. 6. Energy distribution for full detection region. A source was set at the isocenter.
22
Na
background events. This was mainly due to use of the NaI crystal which has a large quantum efficiency. 4. Line image correction Distortions in images often appear when using the Anger-type detector. We considered two types of distortions here. One is seen in a cell defined by a row of photomultipliers. The position sensitivity near the central axis of the photomultipliers is worse than that near the boundary with the next photomultiplier, and consequently, the image is distorted. The other is seen near the edge of the crystal. For deposition near the edge, the number of photomultipliers outside of the gamma ray deposition position is less than the number inside. Thus, the calculated position moves toward the center of the crystal. We detected line images to get a correction routine for these distortions. The line images were measured with a 68 Ge line source collimated to a 1-mm gap. The line source was mounted with a driving stage immediately in front of the crystal. The measurements were carried out at intervals of 40 mm in the vertical and horizontal directions (on the y- and z-axis) for each detector. The line image is likely to depend on the deposited energy because the depth where the gamma ray deposits in the crystal depends on the energy. To simplify the position correction, the data were taken for the deposited energy within 711% of 511 keV; here, the window width corresponded to the energy resolution of 11% in FWHM.
Fig. 7. Line images. (a) Before correction, (b) after correction. The 68 Ge source was collimated with a 1-mm gap. Left: the source parallel to the y-axis; right: to the z-axis.
Fig. 7(a) shows the line images before the correction for Detector A; the source was set twice, first parallel to the y-axis and then the z-axis. The distortion in a photomultiplier cell depends on the head geometry (crystal size, photomultiplier size and so on) and the position calculation algorithm. We optimized the arithmetic and adopted Eq. (1) because the distortion and localization were the smallest [12]. To prepare the correction routine of the distortion near the edge, correction parameters ðDYk ; DZk Þ were evaluated as the difference between the setting position of the line source and their calculation points. The parameters in the areas between two lines were obtained by interpolation. The corrected images for Detector A are shown in Fig. 7(b). The curved lines near the edge were modified well and became straight. Fig. 8 is the corrected position distribution projected on the z-axis on Detector A for the case where the source was set parallel to the y-axis. The widths of the distributions were almost constant, 5:270:5 mm:
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Fig. 8. Projected position distribution of line image. The distribution was for the case where the source was set parallel to the y-axis, and projected on the z-axis. Fig. 9. Spatial resolution (FWHM) dependent on the source position. The source was moved on the beam axis.
5. Positron camera properties The properties of the positron camera were measured with a 22 Na point source. Fig. 9 shows the change of spatial resolution dependence on the source position due to the line image correction. Here, the source was moved in the horizontal direction (the z-axis) of the mid-plane of the two detectors. Before the correction, the resolution became worse as the source was moved away from the center, but after the correction the resolution was almost constant. The resolution was 8:6 mm in FWHM within 750 mm (this is the therapeutic region) and almost constant within 7200 mm after the line image correction. Another important property is linearity. The differences between the setting positions and the measured ones are shown in Fig. 10. Before the correction, there was a disagreement between the measurement and setting positions; the measurement positions deviated toward the center from the setting positions. These positions, however, agreed after the correction with a standard deviation of 0:3 mm within 7200 mm region. Fig. 11 shows the variation of the position distributions on gamma ray deposition area of a crystal for the case where the point source was set 50 mm from the center. The detected events were selected with the window of the deposition position (horizontal position) on Detector A with a 50-mm width. While the peak position moved toward the center in the case without the line
Fig. 10. Position response. The source was moved on the beam axis.
image correction as the area window moved outside, this shift was prevented after the correction. As a result, the distribution for the case without the window selection became symmetrical and the peak position agreed with the source position. These improvements in the distribution led to good resolution and good linearity of the source position response.
6. Discussion The spatial resolution is dominantly influenced by the parallax effect: a spread due to the large incident angle of the gamma ray pairs. When the
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Fig. 11. Position distribution dependent on gamma ray deposition area on a head. The 22 Na source was set 50 mm from the center along the beam axis. The numbers in brackets denote the lower and outer positions of the selection windows in the horizontal direction on Detector A: (a) before correction, (b) after correction.
gamma ray incident area on Detector A was restricted within a 50-mm width in the horizontal direction and a 200-mm width in the vertical direction to restrict the incident angle, the resolution in the horizontal direction was improved from 8.6 to 4:7 mm: In our previous paper [9], we predicted the spatial resolution by numerical simulation. The simulation was carried out for a 500-mm distance between two heads. Then, we calculated it for a 700-mm distance again; the dispersion not including parallax was 5:8 mm and that including parallax was 7:6 mm: We thought that the disagreement between the measurement and the simulation for the case of position restriction was due to overestimating the gamma ray multiple reaction spread. However, the measured resolution was slightly worse than the simulated one for the case of no position restriction. This was probably
because we approximated to a Gaussian distribution in the simulation in spite of having a widerwidth shape, and because of inaccuracy in the position correction near the edge of the crystal. Karp and Muehllehner [13] developed a onedimensional Anger-type detector having an excellent spatial resolution of 4:7 mm in FWHM for the case without grooves. We obtained a width of 5:2 mm as the projected position distribution in the detector (Fig. 8). They differed from our detector by using a crystal having a thickness of 2:5 cm and they set a relatively higher bias level (30%). Considering the width of the projected distribution in the detector, it would be likely that the resolution could be improved if the line image correction were done more precisely. The parallax effect, however, dominated the spatial resolution for our application rather than the dispersion for the event incident at 90 ; and thus, we thought that more precise correction of the line image was unimportant. The resolution we obtained was almost equal to the resolution for the BGO-camera which Llacer [7] had developed (7:5 mm in FWHM). Our camera, however, had a much larger solid angle when it was set with a sufficient distance between the detector heads. The accuracy of the range measurement was dominated by the statistical accuracy and the position response. The number of detecting gamma ray pairs was estimated to be a few hundred under assumptions of the peak dose of 100 mGyE; 11 C beam radius of 3 mm; beam momentum spread of 0.8% and transmission rate in the body of 0.15 [9]. Regarding the resolution of 8:6 mm in FWHM and the response of 0:3 mm in standard deviation, the range could be measured within an accuracy of 70:5 mm from the detection of a hundred gamma ray pairs. This indicated that the positron camera had the desired design characteristics.
7. Conclusion The positron camera that consists of a pair of Anger-type scintillation detectors has been developed to verify ranges of positron emitter beams.
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The camera had a spatial resolution of 8:6 mm in FWHM within a 750 mm region (field of view) and a linear response of 0:3 mm standard deviation within a 7200 mm region. Thus, the camera had the desired design characteristics, and the ranges within an accuracy of 1 mm could be measured under a few hundred mGyE. The linear response in a wide measurement region meant that only an offset adjustment would be necessary for application to range verification. Furthermore, the camera would be useful to detect irradiation field images because it had linearity within 7200 mm: There are still some subjects to study before realizing the camera for use in heavy-ion radiotherapy: the effect of scattering in the human body, influence of background events, optimizing the beam properties, and so on. We are now studying in situ properties of the camera and the total system of range verification carefully. Measurements made in this work will be presented in the near future.
Acknowledgements We appreciate the encouragement from and valuable discussions with Dr. F. Soga, Dr. A. Yamada, Dr. M. Endo and members of the Division of Accelerator Physics and Engineering at NIRS. Three of us (Y. Iseki, A. Yunoki and H. Sakai) especially thank Dr. N. Kobayashi, Mr. S. Makino, Dr. Y. Kita and Mr. K. Sato of Toshiba Corp. for their support. This work was performed as part of a Research Project with Heavy-ions at NIRS-HIMAC.
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