CONTROL METHODS FOR THE FIELD EMISSION X-RAY SPECTRA AND THEIR APPLICATIONS

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CONTROL METHODS FOR THE FIELD EMISSION X-RAY SPECTRA AND THEIR APPLICATIONS

E. Sato, H. Isobe, T. Yanagisawa

To cite this version:

E. Sato, H. Isobe, T. Yanagisawa. CONTROL METHODS FOR THE FIELD EMISSION X-RAY

SPECTRA AND THEIR APPLICATIONS. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-

133-C6-138. �10.1051/jphyscol:1987622�. �jpa-00226825�

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C O N T R O L

METHODS FOR THE FIELD

EMISSION X - R A Y SPECTRA

AND THEIR

APPLICATIONS

E.

Sato,

H.

Isobe and

T.

~anagisawa*

Department of Physics, Iwate Medical University, 3-16-1 Honcho-dori, Morioka 020, Japan

*~e~artment of Radiology, Iwate Medical University, 19-1 Uchimaru, Morioka 020, Japan

Abstract

-

Control methods for the field emission x-ray spectra and their applications to biomedical imaging are described. The x-ray sburce used for this research was a single shot type and consisted of the following essential components: a high voltage generator, a simple low impedance pulser with a coaxial oil condenser of 0.2pF-100kV, an impulse switching system utilizing a light communication device, and two types of field emission tubes. The tubes were of the diode type and were connected to a turbo molecular pump which allowed operation at pressures of approximately less than 1 x 10-'Pa. The maximum intensity was about 3OC/kg at lm/pulse, and the exposure time was about lps. The bremsstrahlung spectra from this source were determined by means of intensity attenuation analysis using a new type of spectrum function (derived by the authors) closely fitting the field emission spectrum distribution. The peak intensity and the energy latitude of the spectra could be controlled. Various kinds of high speed radiography, (e. g., single shot dual energy subtraction radiography, and three dimensional image analysis) were accomplished by controlling the spectrum distribution.

I - INTRODUCTION

Recently several different types of field emission x-ray generators have been reported /la/, but up until now, there have been few applications of their spectra to biomedical radiography.

It is known that the field emission x-ray spectra have a distinctive distribution compared to that obtained with a conventional hot cathode generator. The spectra varied according to several factors: the condenser charging voltage, the anode-cathode (A-C) space, insertion of some filters, and the type of the x-ray tube. Generally speaking, for the distribution of field emission x-ray spectra under the condition of no filtering, the peak intensity was positioned at a lower photon energy region in comparison to one obtained by using a conventional generator with the same peak tube voltage / 5 / . Thus, it was difficult to obtain a smooth line spectrum when using Kramer's function and others /6,7/, since these functions closely fitted the heavily filtered distribution and their peak intensities usually were located in a comparatively higher energy region.

For this research, we constructed a high intensity field emission x-ray source used at a lower photon energy region of less than lOOkeV and using a new spectrum function analysed the properties of the bremsstrahlung spectrum distribution and its variations according to the kind of tube and other spectrum control functions utilized with this generator. With this generator we also succeeded in obtaining various shapes of the spectrum distribution and in performing digital flash radiography and image analysis.

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^GENERATOR

The block diagram of the high intensity field emission x-ray generator is illustrated in Fig. 1. This generator consisted of the following essential components: a high voltage coaxial oil condenser of 100kV-0.2p with a current capacity of more than 50kA, a compressed-gas (SF6) gas switch system operated by light through a fiber, and two evacuated field emission x-ray tubes, each of a different type. The condenser was usually operated in the range of 50 to lOOkV

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987622

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and was directly connected to the gas gap switch.

The field emission x-ray tubes were of the diode type (see Fig. 2) and were connected to a turbo molecular pump which allowed operation at pressures of less than 1 x 10-'Pa. Both tubes had conical anode tips made of tungsten for obtaining a large amount of tungsten bremsstrahlung components. The anode-cathode (A-C) and the anode-cathode plane (A-Cp) space were each controlled by an adjustor outside of the x-ray tube. A plasma protecting disk was placed on the lower part of the anode rod which was 50mm in diameter and was used for preventing the diffusion of the plasma inside of the tube. The internal output mouth of the x-ray was posi-

Fig. 1 Block diagram of the high intensity field emission x-ray generator.

Fig. 2 Construction of the field emission x-ray tubes for biomedical use: 1.

anode tip; 2. cathode tip (Type A), cathode ring (Type B); 3. anode rod; 4. cathode rod; 5. plasma protecting disk; 6. vacuum seal; 7. electric contact part; 8. anode- cathode distance adjustor; 9. center adjustor; 10. internal output mouth of x-ray;

11. Mylar window; 12. various filters; 13. external output mouth of x-ray (dia- phragm); 14. vacuum mouth; 15. glass insulator; 16. ceramic insulator; 17. epoxy resin insulator.

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the radiation space and were also used for cutting the scattering beam. Metal filters made of aluminum or copper were placed between the mylar window and the diaphragms and were used for absorbing the soft components of the spectra.

For the Type A tube with a conical cathode made of tungsten mounted on a cathode rod 5 0 m in diameter, two combinations of electrodes could be selected by controlling the electrode angles and the other factors: (1) for normal focus of 1.0- 3 . 0 ~ in diameter, and (2) for fine focus of 0.2-1.0~ in diameter. In contrast, for the Type B tube which had a ring cathode made of tungsten or molybdenum, a normal focus of 1.0-3.0mm in diameter was obtained.

The pulse widths for the Type A and B tube were almost constant, and their values were about 0.3 and 0.4ps, respectively. The effective exposure times were about l.Ops, and the maximum intensities for the Type A and B tube were about 15 and 3OC/kg at lm per pulse, respectively.

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ANALYSIS OF THE SPECTRA

The field emission x-ray spectra from this generator were calculated using intensity attenuation data. These data were obtained by using a set of aluminum steps positioned at lm from the source, while the x-ray detectors which were the elements of a thermoluminescence dosimeter were placed behind the steps.

In order to calculate the energy at the peak intensity, the energy latitude variations in the bremsstrahlung spectra, and the average spectrum distribution containing the characteristic x-rays, we used a new function for the smooth line spectrum # E (E) :

where Ernax is the maximum photon energy, E is the photon energy, C is the maximum value of the spectrum, and (a), (b) and (c) are constants. This function closely fit the field emission x-rays and unfiltered x-ray spectrum distribution. In an approximate narrow beam condition, the attenuation K(x) may be represented by the following equation:

where R ( E ) is the relative response of the TLD element determined by using some data points obtained by K. Araki et al. /8/ who used various characteristic x-ray spectra and employed a three dimensional least square method; ptr(E)/p is the mass energy transfer coefficient of the air since the dosimeter produced digital values for the x-ray dose; and p(E) is the linear attenuation coefficient of the absorber.

These last two coefficients were taken from Hubbel's data /9/. Using a computer loop and the least mean squares method, two coefficients were calculated, and the normalized spectra were obtained. Thus, these spectral distributions resemble the actual distributions without the characteristic x-rays.

Fig. 3 shows the calculated bremsstrahlung spectra achieved with the Type A tube at lm from the source with an anode angle of 120°and a cathode angle of 50°. The A- C space dependence of the spectra is shown in Fig. 3 (a). As the A-C space became smaller, the hard components of the spectra tended to decrease, and the peak intensities of the spectra tended to shift to the low energy region. The energy latitude became narrow possibly owing to a drop in the anode voltage due to the A-C impedance since very low components were absorbed by the air. Fig. 3 (b) shows the condenser charging voltage dependence of the spectra with an A-C space of 4mm.

and this result shows that the peak intensties of the spectra tended to shift to the high energy region, and the energy latitude became narrow according to in- creases in the condenser charging voltage. In the case of inserting aluminum filters used for absorbing the soft component, the peaks of the spectra tended to rapidly shift to the high energy region and the latitude became narrow (see Fig. 3

(c)).

The calculated spectra achieved with the Type B tube are shown in Fig. 4. As the A-Cp space became smaller, the hard components of the spectra tended to decrease. However, the peak intensties seldom varied, and the energy latitude

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became narrow (see Fig. 4 (a)). The minus sign means the case of inserting the anode tip into the cathode hole. When increasing the charging voltage, the peak intensities slightly varied to the high energy region and the latitude became narrow (see Fig. 4 (b)). Finally, the peak intensties rapidly shifted to the high

-

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^Fig. ³ Variations in the calculated x-ray

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5

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(c)

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All these results come from the fact that the field emission spectra consist largely of soft components and usually have a wide energy latitude compared to that obtained by using a conventional hot cathode generator.

When using the Type A tube, for shifting the peak intensity to the higher energy region, it is necessary to make the charging voltage higher, to make the A-C space longer, and to insert metal filters. In contrast, in the case of obtaining a wider energy latitude, a higher voltage corresponding to the large A-C space and the absence of filtering are necessary.

For the Type B tube, in the case of shifting the peak intensity to the higher energy region, insertion of metal filters is the most effective factor. The energy latitude obtained by this tube is comparatively wider compared to one obtained by using the Type A tube due to the method of the electron conversion into the x-ray spectra at the tip of the anode electrode.

IV

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APPLICATION OF THE SPECTRA TO RADIOGRAPHY

Fig. 5 shows a soft radiograph of a flying parakeet (body length = 8cm) achieved with the Fuji Computed Radiography (FCR) system and the Type A tube and the radiographic conditions as follows: a condenser charging voltage (Vc) of 80kV, an A-C space of 2mm, a film-focus (F-F) distance of 1.h, and no filter. As shown in this figure, detailed information concerning the articulation of the bones was obtained. Since this biomedical object consisted of soft tissues, a clear image could not be obtained when using slightly hard x-rays.

The field emission x-ray spectrum distribution with a narrow latitude is very useful for image analysis since the linear attenuation coefficient corresponding to the absorber becomes a constant value and it is possible to calculate the thickness of the absorber from the film density. A positive radiograph of a jet of contrast medium (Isopaque 280) in a water phantom achieved with the Type A tube is shown in Fig. 6. The radiographic conditions were as follows: a Vc of 90kV, an A-C space of 2mm, an F-F distance of I.&, an aluminum filter thickness of 0.2mn. and linear contrast control for the positive radiograph. An image analysis of a jet of the contrast medium as in Fig. 6 is shown in Fig.7. This image analysis is equivalent to the three dimensional image analysis accomplished by using the microdensitome- ter. In this analysis, the approximate relative concentration of the contrast medium could be obtained by subtraction of the density caused by water. The extent and the dilution of a jet could be observed.

In addition, a wide energy latitude is needed for achieving single shot dual energy subtraction radiography /lo/ using a thin metal filter. Fig. 8 shows the

Fig. 5 A soft radiograph of the a flying parakeet achieved with the Type A tube.

Fig. 6 A positive radiograph of a jet of contrast medium in a water phantom achieved with the Type A tube.

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Fig. 7 Relative concentra-

tion of a jet as in Fig. 6 Fig. 8 Single shot dual energy subtraction equivalent to the three radiograph (muscle less) of a moving guinea pig dimensional image analysis. achieved with the Type B tube.

energy subtraction radiograph (muscle-less) of a moving guinea pig (body length = 20cm) achieved with the Typ9 B tube with a Vc of 94kV, an A-Cp space of 2mm, an F-F distance of l.Om, and a subtraction copper filter of 0.3mm.

V

-

DISCUSSION

We constructed a high intensity field emission x-ray source operated at low charging voltages of less than lOOkV with two types of x-ray tubes and utilized various kinds of radiography using digital radiography described in this paper in order to apply the distinctive field emission spectrum more effectively to high speed imaging and image analysis. In particular, using the field emission x-ray source, it has been very easy to obtain various spectrum distributions as described above.

We are now constructing repetitional and multiple tube field emission x-ray sources having variable spectra. Since these sources have such distinctive radiographic characteristics as variable spectra, short and variable time inter- vals, and others, there are many possible diagnostic applications.

ACKNOWLEDGMENTS

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 P m m o - tion Foundation, and the Ministry of Education, Science, and Culture in Japan.

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