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The new effective detector for digital scanning radiography
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 513 (2003) 57–60
The new effective detector for digital scanning radiography E.A. Babichev, S.E. Baru, V.R. Groshev*, A.G. Khabakhpashev, V.V. Leonov, V.A. Neustroev, V.V. Porosev, G.A. Savinov, L.I. Shekhtman The Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia
Abstract The new effective one-dimensional detector for digital scanning radiography is presented. The progress in the design of low-noise electronics allowed the Multistrip Ionization Chamber to be used as the X-ray detector. The detector has been designed for use in the Low-dose Digital Radiographic Device ‘‘Siberia’’ instead of a multiwire proportional chamber. The working gas is Xe under 12 atm pressure. The main detector parameters are as follows: total number of channels, 1024; channel size, 0.4 0.4 mm2; space resolution, 1.25 lp/mm; contrast sensitivity, 1%; dynamic range, 480; surface dose for chest image, 3 to 5 mR. r 2003 Elsevier B.V. All rights reserved. Keywords: X-ray detector; Ionization chamber; Digital radiography
1. Introduction In medical digital projection radiography, which is now rapidly developing and successfully replacing the standard investigation methods, two directions of evolution can be clearly seen. The first one is in good progress mainly in Western countries. This method uses two-dimensional detectors of X-ray radiation. The advantages of this method are high spatial resolution and short image acquisition time. Nevertheless, there are some significant disadvantages. One is the price, because the manufacture of high-quality detectors with a large detection area requires high-level technology; therefore, they are very expensive. The other disadvantage is the additional noise due to *Corresponding author. Tel.: +7-3832-394533; fax: +73832-342163. E-mail address: [email protected] (V.R. Groshev).
the scattered radiation background in a patient’s body. Another direction that is widespread in Russia is the creation of a scanning system. In spite of some faults of these systems, like a long image acquisition time, inefficient X-ray tube utilization and the necessity of a precision scanning system, there are some significant advantages: a lower price and high technical parameters that lead to lower patient irradiation dose. The scanning systems are not fully universal systems, but they can be used for prophylactic examinations of chest organs against tuberculosis; it is very important for Russia. More than 15 years ago, in the Budker Institute of Nuclear Physics the Low-dose Digital Radiography Device (LDRD) ‘‘Siberia’’ based on MWPC was created [1–4]. In 1996, we started work on the application of Multistrip Ionization Chamber (MIC) instead of MWPC [5].
0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.08.001
ARTICLE IN PRESS 58
E.A. Babichev et al. / Nuclear Instruments and Methods in Physics Research A 513 (2003) 57–60
2. Design of the MIC
3. Detector parameters
The MIC is a flat capacitor with a uniform anode and cathode consisting of the 1024 strip, width 200 mm and pitch 400 mm, placed in a highpressure cylindrical vessel. Each strip of 60 mm length is aligned with the focal spot of the X-ray tube. The cathode is produced with printed circuit board technology. The gap between the electrodes is 4 mm. The collimator on the entrance window has a width of 0.4 mm. The detector is filled with pure Xe at 12 atm. A high voltage of 1000 V is applied to the anode. The charge produced by Xray radiation is collected on the strips and its value is measured with special electronics situated inside the vessel case. In Fig. 1 a detector layout is presented. To improve the X-ray detection efficiency, the area between the entrance window and sensitive volume is filled with polyethylene embedment. The X-ray attenuation in the polyethylene is approximately 10% for 50 keV photons in comparison with 40% in Xe at 12 atm. The front-end electronics consists of 16 chips; each of them contains 64 readout channels. Each channel has capacitor connected to the strip and control circuit that performs subsequent readout of the charge collected on the capacitors. The electronic was designed and produced at plant ‘‘Vostok’’, Novosibirsk.
3.1. Plateau
Fig. 1. (a) General view of the MIC: 1—vessel case, 2—anode, 3—cathode, 4—entrance window, 5—electronics. (b) Schematic view of the electronics.
In Fig. 2 the dependence of the collected charge on the applied high voltage for different counting rates is presented. At a voltage higher than 800 V, we found that the total charge collection and signal dependence from the input flux was linear at least up to 2.3 MHz. 3.2. Space resolution The factors influencing space resolution were photoelectron range, electrons diffusion under drifting and readout structure pitch. To measure space resolution the Modulation Transfer Function (MTF) technique was used. It is defined as gðx; yÞ Z NZ ¼ N
N
f ðe; ZÞhðx e; y ZÞ de dZ;
N
Gðu; vÞ ¼ F ðu; vÞHðu; vÞ; Hðu; vÞ MTFðu; vÞ ¼ Hð0; 0Þ
Fig. 2. Variation of the signal versus high voltage for different counting rates.
ARTICLE IN PRESS E.A. Babichev et al. / Nuclear Instruments and Methods in Physics Research A 513 (2003) 57–60
59
can be written as
DQE ¼
SNR2out : SNR2in
For MIC it takes the following form: DQE ¼
Fig. 3. MTF of MIC measurements and the Monte-Carlo simulation with Geant4.
where hðx; yÞ is the point spread function of the system, f ðx; yÞ is the source signal distribution and gðx; yÞ is the measured signal. Hðu; vÞ; F ðu; vÞ; Gðu; vÞ are Fourier transformations of h; f and g; respectively. In Fig. 3 the MTF of the detector measured with the edge method [6] and calculated from the channel shape is presented. For comparison, an ideal square channel MTF is shown. The space resolution limit is defined by Nyquist frequency (channel pitch), that is 1.25 lpm, but not the detector itself. The vertical space resolution of the system is defined by geometrical factors like focal spot size, collimator sizes, scanning velocity and adjusted to resolution in the horizontal direction.
1þ
ðs2s =s20 Þ
e þ ðs2el =a2x s20 eq0 Þ
where e is the quantum efficiency of the detector, s0 is the mean signal value per photon, ss is the signal fluctuation, s2el is the intrinsic electronic noise and q0 is the input g-quanta flux per detector’s channel size ax : The electronics noise is equal to a signal from four g-quanta, so the main factors influencing DQE are quantum efficiency and relative signal fluctuation. In Fig. 4 SNR2out for MIC, two types of stimulated phosphors and different film screen systems [7] are shown. It is clearly seen that MIC having a high quantum efficiency and a low level of intrinsic noise reaches the same value of SNR at less flux (doses) than the standard imaging systems. The parameters of LDRD ‘‘Siberia’’ with MIC are presented in Table 1.
3.3. Quantum efficiency end noise The quantum efficiency of detector depends on gas mixture, gas pressure and photon absorption in dead areas and in the entrance window of the detector. In our design a value close to 70% was reached. One of the most vivid characteristics of the detector is Detection Quantum Efficiency (DQE), which depends on quantum efficiency and noise. It
Fig. 4. SNR performance of a digital storage phosphor systems (ST—standard and HR—high resolution), some film screen combinations [7] and MIC.
ARTICLE IN PRESS 60
E.A. Babichev et al. / Nuclear Instruments and Methods in Physics Research A 513 (2003) 57–60
Table 1 Parameters LDRD ‘‘Siberia’’ with MIC Characteristic
Value
Number of channels in line Channel size (mm2) Width of image (mm) Resolution (lp/mm) X-ray detection efficiency (Eg =70 ke V) (%) Contrast sensitivity (%) Dynamic range Scanning time (for chest) (s) Surface dose for chest image (mR)
1024 0.4 0.4 410 1.25 70 1 480 2.5 3–5
4. Conclusions The new detector for scanning radiography Multistrip Ionization Chamber (MIC) is currently being used in the LDRD ‘‘Siberia’’ installation instead of MWPC. Now more than 60 installations with new detectors are operating in clinics in Russia and China. The low-noise level of MIC electronics allows the DQE value at a low counting rate close to that of the counting detector. The other parameters of MIC are superior to the MWPC ones. It has better space resolution and channel uniformity and a wide dynamic range. The MIC production is simpler and the detector is
more reliable. An additional advantage of the new detector is the possibility of creating images at same low doses as the old detector, in spite of the fact that the new detector has more than a factor two smaller pixel size. Currently, to improve spatial resolution, we are in the process of designing MIC with a total of 2048 channels of size 0.2 0.2 mm2.
References [1] S.E. Baru, A.G. Khabakhpashev, I.R. Makarov, G.A. Savinov, L.I. Shekhtman, V.A. Sidorov, Nucl. Instr. and Meth. A 238 (1985) 165. [2] S.E. Baru, A.G. Khabakhpashev, L.I. Shekhtman, Nucl. Instr. and Meth. A 283 (1989) 431. [3] E.A. Babichev, S.E. Baru, A.G. Khabakhpashev, G.M. Kolachev, O.A. Ponamarev, G.A. Savinov, L.I. Shekhtman, Nucl. Instr. and Meth. A 323 (1992) 49. [4] E.A. Babichev, S.E. Baru, V.R. Groshev, A.G. Khabakhpashev, V.V. Porosev, G.A. Savinov, L.I. Shekhtman, V.I. Telnov, Nucl. Instr. and Meth. A 419 (1998) 290. [5] E.A. Babichev, S.E. Baru, V.R. Groshev, A.G. Khabakhpashev, G.S. Krainov, V.V. Leonov, V.A. Neustroev, V.V. Porosev, G.A. Savinov, L.I. Shekhtman, Nucl. Instr. and Meth. A 461 (2001) 430. [6] V. Kaftandjian, Y.M. Zhu, G. Rozeiere, G. Peix, D. Babot, J. X-ray Sci. Technol. 6 (1996) 205. [7] W. Hillen, U. Schiebel, T. Zaengel, Med. Phys. 14 (5) (1987) 744.
Nuclear Instruments and Methods in Physics Research A 513 (2003) 57–60
The new effective detector for digital scanning radiography E.A. Babichev, S.E. Baru, V.R. Groshev*, A.G. Khabakhpashev, V.V. Leonov, V.A. Neustroev, V.V. Porosev, G.A. Savinov, L.I. Shekhtman The Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia
Abstract The new effective one-dimensional detector for digital scanning radiography is presented. The progress in the design of low-noise electronics allowed the Multistrip Ionization Chamber to be used as the X-ray detector. The detector has been designed for use in the Low-dose Digital Radiographic Device ‘‘Siberia’’ instead of a multiwire proportional chamber. The working gas is Xe under 12 atm pressure. The main detector parameters are as follows: total number of channels, 1024; channel size, 0.4 0.4 mm2; space resolution, 1.25 lp/mm; contrast sensitivity, 1%; dynamic range, 480; surface dose for chest image, 3 to 5 mR. r 2003 Elsevier B.V. All rights reserved. Keywords: X-ray detector; Ionization chamber; Digital radiography
1. Introduction In medical digital projection radiography, which is now rapidly developing and successfully replacing the standard investigation methods, two directions of evolution can be clearly seen. The first one is in good progress mainly in Western countries. This method uses two-dimensional detectors of X-ray radiation. The advantages of this method are high spatial resolution and short image acquisition time. Nevertheless, there are some significant disadvantages. One is the price, because the manufacture of high-quality detectors with a large detection area requires high-level technology; therefore, they are very expensive. The other disadvantage is the additional noise due to *Corresponding author. Tel.: +7-3832-394533; fax: +73832-342163. E-mail address: [email protected] (V.R. Groshev).
the scattered radiation background in a patient’s body. Another direction that is widespread in Russia is the creation of a scanning system. In spite of some faults of these systems, like a long image acquisition time, inefficient X-ray tube utilization and the necessity of a precision scanning system, there are some significant advantages: a lower price and high technical parameters that lead to lower patient irradiation dose. The scanning systems are not fully universal systems, but they can be used for prophylactic examinations of chest organs against tuberculosis; it is very important for Russia. More than 15 years ago, in the Budker Institute of Nuclear Physics the Low-dose Digital Radiography Device (LDRD) ‘‘Siberia’’ based on MWPC was created [1–4]. In 1996, we started work on the application of Multistrip Ionization Chamber (MIC) instead of MWPC [5].
0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.08.001
ARTICLE IN PRESS 58
E.A. Babichev et al. / Nuclear Instruments and Methods in Physics Research A 513 (2003) 57–60
2. Design of the MIC
3. Detector parameters
The MIC is a flat capacitor with a uniform anode and cathode consisting of the 1024 strip, width 200 mm and pitch 400 mm, placed in a highpressure cylindrical vessel. Each strip of 60 mm length is aligned with the focal spot of the X-ray tube. The cathode is produced with printed circuit board technology. The gap between the electrodes is 4 mm. The collimator on the entrance window has a width of 0.4 mm. The detector is filled with pure Xe at 12 atm. A high voltage of 1000 V is applied to the anode. The charge produced by Xray radiation is collected on the strips and its value is measured with special electronics situated inside the vessel case. In Fig. 1 a detector layout is presented. To improve the X-ray detection efficiency, the area between the entrance window and sensitive volume is filled with polyethylene embedment. The X-ray attenuation in the polyethylene is approximately 10% for 50 keV photons in comparison with 40% in Xe at 12 atm. The front-end electronics consists of 16 chips; each of them contains 64 readout channels. Each channel has capacitor connected to the strip and control circuit that performs subsequent readout of the charge collected on the capacitors. The electronic was designed and produced at plant ‘‘Vostok’’, Novosibirsk.
3.1. Plateau
Fig. 1. (a) General view of the MIC: 1—vessel case, 2—anode, 3—cathode, 4—entrance window, 5—electronics. (b) Schematic view of the electronics.
In Fig. 2 the dependence of the collected charge on the applied high voltage for different counting rates is presented. At a voltage higher than 800 V, we found that the total charge collection and signal dependence from the input flux was linear at least up to 2.3 MHz. 3.2. Space resolution The factors influencing space resolution were photoelectron range, electrons diffusion under drifting and readout structure pitch. To measure space resolution the Modulation Transfer Function (MTF) technique was used. It is defined as gðx; yÞ Z NZ ¼ N
N
f ðe; ZÞhðx e; y ZÞ de dZ;
N
Gðu; vÞ ¼ F ðu; vÞHðu; vÞ; Hðu; vÞ MTFðu; vÞ ¼ Hð0; 0Þ
Fig. 2. Variation of the signal versus high voltage for different counting rates.
ARTICLE IN PRESS E.A. Babichev et al. / Nuclear Instruments and Methods in Physics Research A 513 (2003) 57–60
59
can be written as
DQE ¼
SNR2out : SNR2in
For MIC it takes the following form: DQE ¼
Fig. 3. MTF of MIC measurements and the Monte-Carlo simulation with Geant4.
where hðx; yÞ is the point spread function of the system, f ðx; yÞ is the source signal distribution and gðx; yÞ is the measured signal. Hðu; vÞ; F ðu; vÞ; Gðu; vÞ are Fourier transformations of h; f and g; respectively. In Fig. 3 the MTF of the detector measured with the edge method [6] and calculated from the channel shape is presented. For comparison, an ideal square channel MTF is shown. The space resolution limit is defined by Nyquist frequency (channel pitch), that is 1.25 lpm, but not the detector itself. The vertical space resolution of the system is defined by geometrical factors like focal spot size, collimator sizes, scanning velocity and adjusted to resolution in the horizontal direction.
1þ
ðs2s =s20 Þ
e þ ðs2el =a2x s20 eq0 Þ
where e is the quantum efficiency of the detector, s0 is the mean signal value per photon, ss is the signal fluctuation, s2el is the intrinsic electronic noise and q0 is the input g-quanta flux per detector’s channel size ax : The electronics noise is equal to a signal from four g-quanta, so the main factors influencing DQE are quantum efficiency and relative signal fluctuation. In Fig. 4 SNR2out for MIC, two types of stimulated phosphors and different film screen systems [7] are shown. It is clearly seen that MIC having a high quantum efficiency and a low level of intrinsic noise reaches the same value of SNR at less flux (doses) than the standard imaging systems. The parameters of LDRD ‘‘Siberia’’ with MIC are presented in Table 1.
3.3. Quantum efficiency end noise The quantum efficiency of detector depends on gas mixture, gas pressure and photon absorption in dead areas and in the entrance window of the detector. In our design a value close to 70% was reached. One of the most vivid characteristics of the detector is Detection Quantum Efficiency (DQE), which depends on quantum efficiency and noise. It
Fig. 4. SNR performance of a digital storage phosphor systems (ST—standard and HR—high resolution), some film screen combinations [7] and MIC.
ARTICLE IN PRESS 60
E.A. Babichev et al. / Nuclear Instruments and Methods in Physics Research A 513 (2003) 57–60
Table 1 Parameters LDRD ‘‘Siberia’’ with MIC Characteristic
Value
Number of channels in line Channel size (mm2) Width of image (mm) Resolution (lp/mm) X-ray detection efficiency (Eg =70 ke V) (%) Contrast sensitivity (%) Dynamic range Scanning time (for chest) (s) Surface dose for chest image (mR)
1024 0.4 0.4 410 1.25 70 1 480 2.5 3–5
4. Conclusions The new detector for scanning radiography Multistrip Ionization Chamber (MIC) is currently being used in the LDRD ‘‘Siberia’’ installation instead of MWPC. Now more than 60 installations with new detectors are operating in clinics in Russia and China. The low-noise level of MIC electronics allows the DQE value at a low counting rate close to that of the counting detector. The other parameters of MIC are superior to the MWPC ones. It has better space resolution and channel uniformity and a wide dynamic range. The MIC production is simpler and the detector is
more reliable. An additional advantage of the new detector is the possibility of creating images at same low doses as the old detector, in spite of the fact that the new detector has more than a factor two smaller pixel size. Currently, to improve spatial resolution, we are in the process of designing MIC with a total of 2048 channels of size 0.2 0.2 mm2.
References [1] S.E. Baru, A.G. Khabakhpashev, I.R. Makarov, G.A. Savinov, L.I. Shekhtman, V.A. Sidorov, Nucl. Instr. and Meth. A 238 (1985) 165. [2] S.E. Baru, A.G. Khabakhpashev, L.I. Shekhtman, Nucl. Instr. and Meth. A 283 (1989) 431. [3] E.A. Babichev, S.E. Baru, A.G. Khabakhpashev, G.M. Kolachev, O.A. Ponamarev, G.A. Savinov, L.I. Shekhtman, Nucl. Instr. and Meth. A 323 (1992) 49. [4] E.A. Babichev, S.E. Baru, V.R. Groshev, A.G. Khabakhpashev, V.V. Porosev, G.A. Savinov, L.I. Shekhtman, V.I. Telnov, Nucl. Instr. and Meth. A 419 (1998) 290. [5] E.A. Babichev, S.E. Baru, V.R. Groshev, A.G. Khabakhpashev, G.S. Krainov, V.V. Leonov, V.A. Neustroev, V.V. Porosev, G.A. Savinov, L.I. Shekhtman, Nucl. Instr. and Meth. A 461 (2001) 430. [6] V. Kaftandjian, Y.M. Zhu, G. Rozeiere, G. Peix, D. Babot, J. X-ray Sci. Technol. 6 (1996) 205. [7] W. Hillen, U. Schiebel, T. Zaengel, Med. Phys. 14 (5) (1987) 744.