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NEW LOW IMPEDANCE HIGH INTENSITY X-RAY GENERATOR USING FIELD EMISSION FOR
BIOMEDICAL DIAGNOSIS
H. Isobe, E. Sato, T. Yanagisawa
To cite this version:
H. Isobe, E. Sato, T. Yanagisawa. NEW LOW IMPEDANCE HIGH INTENSITY X-RAY GENERA-
TOR USING FIELD EMISSION FOR BIOMEDICAL DIAGNOSIS. Journal de Physique Colloques,
1987, 48 (C6), pp.C6-127-C6-132. �10.1051/jphyscol:1987621�. �jpa-00226824�
JOURNAL DE
PHYSIQUEColloque
C6,
suppl6ment aunOll,
Tome48,
novembre 1987NEW LOW IMPEDANCE HIGH
INTENSITYX-RAY GENERATOR
USINGFIELD
EMISSIONFOR BIOMEDICAL
DIAGNOSISH.
Isobe,E.
Sato andT.
~ana~isawa*Department of Physics, Iwate Medical University, 3-16-1 Honcho-dori, Morioka 020, Japan
*~e~artrnent of Radiology, Iwate Medical University, 19-1 Uchimaru, Morioka 020, Japan
Abstract
-
A new low impedance high intensity single x-ray generator using field emission and its applications to biomedical diagnosis are described.This generator consisted of the following essential components: a high voltage generator, a low impedance transmission line using an oil condenser of 120kV-0.15~. a high speed impulse switching system with a time resolu- tion of less than lus, and a new type of x-ray tube having two sets of electrodes using field emission. This tube was supported by a large scale electric tube holder, and it was possible to rotate, raise, and lower it corresponding to the radiographic objectives. The generator could be used for condenser charging voltages of 50 to 120kV and peak currents of 20 to 40kA. The maximum intensities for the Type A and B electrodes were about 20 and 40C/kg at lm/pulse, respectively. The exposure time was about lps.
The optimum output corresponding to the optimum x-ray quality could be con- trolled. The size of the effective focal spot primarily varied according to the electrode configurations and the insertion of metal filters, and ranged from 0.2 to 3.0mm in diameter. Various kinds of high speed radiography, (e.
g., continuous delayed radiography) were accomplished by controlling the source conditions concerning the x-ray intensity, the quality, the effective focal spot, and the delay time.
I
-
INTRODUCTIONRecently, obtaining substantial information concerning the inside of biomedical objects has been made possible by the developement of various useful radiation sources. Yet, it is not very easy to obtain the complete stoppage of biomedical objects when using these sources. In view of these problems, field emission x- rays with a short time interval are an especially appropriate source for high speed imaging and image analysis necessary for such a human application as a cardiovas- cular contrast study /I/.
In the application of the field emission x-ray source of the single shot type to high speed biomedical radiography, the optimum radiographic conditions concerning the x-ray intensity, the quality, the effective focal spot size, and the delay time are required in order to obtain clear radiographs /1-5/. In particular, an appropriate time delay switch with a high time resolution /6/ corresponding to the radiographic objectives is needed.
Generally, in order to obtain high speed stroboscopic radiographs, the repeti- tional-type of field emission x-ray source utilizing a high speed camera system is necessary. Yet, in the case of objects displaying uniform movement, the single shot type of high intensity field emission x-ray source utilizing a precision delay system with a short time resolution in conjunction with a digital radiography system is capable of obtaining delayed radiographs equivalent to stroboscopic images /6/.
In this paper, in order to obtain high speed and high resolution biomedical radiographs, we constructed a new low impedance and high intensity field emission x-ray generator utilizing two types of precise delay switches. We also succeeded in performing high speed radiography, including delayed radiography.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987621
C6-128 JOURNAL DE PHYSIQUE
I1
-
EXPERIMENTAL MATERIALSA block diagram of the single shot type of low impedance and high intensity field emission x-ray generator utilizing a precision delay system is shown in Fig.
1. This generator consisted of the following components: a high voltage genera- tor, a simple low impedance transmission line with a high voltage coaxial oil condenser, a compressed-gas (SFs) gap switch with a small-sized trigger device operated by light through a fiber, a trigger delay system, an oil diffusion pump, and two evacuated field emission x-ray tubes, each of a different type (see Fig.
2).
The electric circuit employed the simplest type of coaxial transmission line with eight coaxial cables to reduce the pulser impedance for generating a stable pulse under low voltage. The condenser was usually operated in the range of 50 to l2OkV and was connected to the gas gap switch which was triggered by a high energy spark discharge system using a Krytron tube /7/ with a voltage of 15kV and a current of 300A.
The x-ray tubes were of the diode type and were connected to an oil diffusion pump which allowed operation at pressures of approximately less than 1 x 10-)Pa (see Fig. 3). They were supported by a large scale electric tube holder because of their comparatively heavy weight of about 70kg, and it was possible to rotate, raise and lower these corresponding to the radiographic objectives. The coaxial cables were connected to the anode rod by using the gas insulation connector. The pressure was observed on the Bourdon gauge and was regulated from 2.0 to 3 . 0 ~ lo-' Pa. Each tube had the same vessel and could easily be changed to the Type A or B tube by changing the cathode unit, and both tubes had conical anode tips made of tungsten which could easily be changed. The A-C space was regulated by an adjustor outside of the x-ray tube.
For the Type A tube which employed a conical cathode made of tungsten mounted on the cathode rod, two combinations of electrodes were selected as follows: (a) normal focusing and a high dose rate; (b) fine focusing and a low dose rate.
For the Type B tube which had a ring cathode made of tungsten positioned on the top of the internal output mouth, the vertex angle of the anode was 30-45Oin order to obtain a smaller focus without the pinch-off of the anode tip due to the high electron flow coming from the field emission.
The block diagram for obtaining a delayed radiograph utilizing a single shot type of field emission x-ray generator is shown in Fig. 4. When the radiographic object was exposed to the controlled x-rays under the optimum radiographic condi-
GAS FLASH
SUPWY IMPEDANCE INSULATION X-RAY
PULSER CONNECTOR TUBE
HOLDER
Fig. 1 Block diagram of a low Fig. 2 General view of the field impedance and high intensity x-ray emission x-ray generator: 1. high generator using field emission. voltage generator; 2. high voltage pulser; 3. combined x-ray tube.
tions, the permeating x-rays produced the radiographic image on the imaging plate.
The computerized image was output as a film copy after image management corre- sponding to the objectives. This system employed two types of delay switches: (a) direct switching, and (b) light switching using a laser beam. For the direct switching device, a short electric pulse (trigger pulse) was produced and trans- mitted to the trigger delay unit through a coaxial cable. When employing the light switching device, the trigger pulse was produced when shutting off the laser beam because of inversion amplification by a high speed operational amplifier. The time resolutions of these two systems had very small values (less than lps) even when employing a very slow motion object.
Fig. 3 Schematic drawing of the field emiision x-ray tubes: 1. anode tip; 2.
cathode tip (Type A ) , ring cathode (Type B); 3. anode rod; 4. plasma protecting disk; 5. internal output of the x-rays; 6. Mylar window; 7. filters; 8. dia- phragms; 9. vacuum mouth; 10. A-C space adjustor; 11. glass insulator; 12.
polyvinyl insulator; 13. glass insulator connector; 14. coaxial cables.
RADIOGRAPHY ( F C R )
C o N T K U I D FX\
U
DELAYED RA01CGWR-I
Fig. 4 Block diagram for performing the delayed radiography.
0
JOURNAL DE PHYSIQUE
I11 - FIELD EMISSION X-RAYS
Assuming that the tube current and the voltage and the voltage are the constant values of J and V, respectively, the intensities of the continuous (10) and the characteristic spectra (Ik) are approximated by two emprical equations of the form:
where X is the atomic number of the anode material. Vk is the critical, T is the duration time, A and B some factors, m
+
2.0, and n+
1.5. Thus, the values of 10 and Ik can be expressed by the following two equation:where Vc is the condenser charging voltage, Z is the A-C impedance due to the electrode configuration, Zb is the body impedance of the high voltage pulser, Zc is the cable impedance per length used for the low impedance transmission line, % is the cable length, and n is the cable number. Thus. 10 and Ik vary according to several factors as described above, and T tends to increase when increasing the A-C impedance /I/. Since the impedance values of Zb and Zc%/n are usually constant values, the two values of 10 and Ik tend to have maximum values when increasing Z for a constant exposure time.
IV
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RADIOGRAPHIC CHARACTERISTICSFig. 5 shows the output of the field emission x-rays obtained by using both the Type A and B tubes, and measured by using a PIN diode made by Hamamatsu Photonics K.K.. These results come from the fact that the pulse width tended to decrease when the x-ray quality was made harder by the insertion of aluminum filters. In the field emission x-ray generator used at low charging voltages of less than 120kV, the source without filtering had a large amount of soft components from the plasma x-ray source produced at the end of the x-ray pulse wave form. The duration time of the light caused by the tungsten vapour corresponded to the duration of the tube current.
The x-ray quality was primarily determined by the average value of the A-C voltage /4/, and the filtering. Thus, the x-ray quality tended to become hard when increasing the charging voltage and the A-C impedance, and with the insertion of metal filters. The A-C impedance primarily increased according to increases in the A-C space and the A-Cp space.
The effective focal spot for the Type A tube primarily varied according to the electrode angles and the A-C space. The focal spot tended to become smaller when decreasing the A-C space and the electrode angles. When using this tube, it was necessary to make the cathode angle smaller compared to the anode angle, because of the vacuum field emission. For the Type B tube, a small cathode diameter and a
TYPE A TYPE B
CONDENSER VOLT. 8 0 kV A-C SPACE 3 -n ANODE ANGLE 100 ' CATHODE ANGLE 4 0 ' A1 F l LTER
-
NON-PI L T i R--
1 . 8 a nCONDENSER VOLT. 80 kV A-Cp SPACE 0 nn ANODE ANGLE 40 ' CATHODE D I A n . 18 n m
Al F I L T E R
-
NON-FI LTER----
1.0 mn---- 3 . 0 m n - - 3 . 0 nn
M I+--%
0 . 5 pe 0.5 ps
Fig. 5 Output of the field emission x-rays according to insertion of aluminum filters obtained by using both the Type A and B tube at the indicated conditions.
small anode angle were desired to obtain a smaller focus. Fig. 6 shows the typical focal spots for two combinations of electrodes: (a) fine focus achieved with the Type A tube with an angle of 600, a cathode angle of 30; a Vc of 80kV, and an A-C space of lmm; (b) normal focus obtained by the Type B tube with an angle of 45: a cathode diameter of lOmm, a Vc of 80kV, and an A-Cp space of Omm. For those two conditions, the effective focal spots tended to become smaller when the soft components of the spectra were absorbed using aluminum filters, since the spectrum distribution at the anode tip had the highest intensity and the highest photon energy.
V
-
FLASH RADIOGRAPHYFig. 7 shows two radiographs of a bouncing tennis ball with half of the air volume filled with water achieved with a normal focus of about 3mm in diameter at the following conditions: a Vc of 100kV, an A-C space of 3mm, a film-focus (F-F) distance of l.Om, no filter (a), a 0.2mm copper filter (b)
,
an anode angle of 100:a cathode angle of 50: a normal and linear contrast control, and a delay time of 30ms after hitting the time delay switch (direct switching). As shown in Fig. 7
(a) (soft image), there was hardly any image of the water inside of the ball since the x-ray quality was soft and the large amount of soft components of the spectra was absorbed by the ball. However, when using slightly hard x-rays, the image of the water could be observed inside wall of the ball (see Fig. 7 (b)).
The delayed radiographs of the left leg of an ambulating human being (adult) is
no f i l t e r i\ 1 Irnm A L 2mm
n o f i l t e r A 1 l r n m A: 2mm
5mm
Fig. 6 Typical focal spot for two combinations of electrodes achieved by the insertion .af aluminum filters: (a) fine focus obtained by using the Type A tube;
(b) normal focus achieved with the Type B tube.
(b)
Fig. 7 Delayed radiographs of a bouncing tennis ball with half of the air volume filled with water achieved with the Type A tube: (a) soft image; (b) slightly hard image.
C6-132 JOURNAL DE PHYSIQUE
Fig. 8 Continuous delayed radiography of the left leg of an ambulating human being at various delay times: (a) 0.0s; (b) 0.4s; (c) 0.8s.
shown in Fig. 8. These images were achieved with the Type B tube and laser switching with a Vc of 80kV. an A-Cp space of O m , F-F distance of lm, an aluminum filter thickness of 0.5m, an anode angle of 300, a cathode diameter of lOmm, uniform time intervals of 0.4s, a slightly soft contrast control, and a delicate frequency enhancement. Using this radiography (equivalent to the stroboscopic radiography) the images of the bones and muscles were clearly obtained.
IV
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DISCUSSIONAs described in this paper we developed a low impedance high intensity x-ray generator using field emission and applied it to high spped biomedical imaging. In particular, using a single shot of pulsed x-rays, we were able to perform high intensity radiography even when utilizing a regular film-screen combination commonly used in medical radiography. By combining this system with a digital radiography system, continuous delayed radiography and its application to image analysis was achieved because the radiographic characteristics and delay times could be controlled.
Now, we are constructing a multiple tube type of field emission x-ray source having variable spectra /8/. When this generator is combined with the repetiti- onal generator /9/ and the other high-intensity single generators /I/, many future possible diagnostic applications will be realized.
ACKNOWLEDGMENT
The authors wish to thank P. Langman, K. Nakadate and R. Ishiwata of Iwate Medical University for helpful support in this research, and Miss E. Tanifuji for typing. This work was supported by Grants-in-Aid for Scientific Research from the Iwate Medical University-Keiryokai Research Foundation, the Private School Promo- tion Foundation, and the Ministry of Education, Science, and Culture in Japan.
REFERENCES
/1/ Sato, E., Isobe, H., Fujiyama, T., et al., Jpn. Radiol. Phys.? (1987) 7.
/2/ Isobe, H., Sato, E., Hayasi, Y., et al., SPIE 491 (1984) 168.
/3/ Sato, E., Isobe, H., Nakadate, K., et al., Proc. 13th Congr. Int. Commission for Optics, Sapporo (1984) 268.
/4/ Sato, E., Isobe, H. and Hoshino, F., Rev. Sci. Instrum.,
57
(1986) 1399./5/ Isobe, H., Sato. E., Tanifuji, E., et al., Jpn. Med. Imaging and Information Sci.
2
(1986) 145./6/ Isobe, H., Sato. E. and Yanagisawa, T., Annual Report of Iwate Medical Univer- sity, School of Liberal Arts and Sciences
21
(1986) 1./7/ Friingel, F.B.A., In: High Speed Pulse Technology 3, New York Academic Press Inc. (1976) 52.
/8/ Kawasaki, S., Sato, E., Isobe, H. and Yanagisawa, T., Proc. 18th Int. Congr.
on High Speed Photography and Photonics, Xian (1988) in preparation.
/9/ Sato, E., Isobe. H., Oikawa. S., et al., Jpn. Med. Imaging and Information Sci.
3 (1986) 62.
