Safety evaluation of mobile X-ray equipment using super-capacitor

Abstract

In this study, we designed mobile X-ray equipment that generates high-power X-rays, using an internal power source by means of a super-capacitor, and evaluated its safety. The proposed X-ray equipment uses the charging voltage of a battery to store high density energy, supplementing the electric charge of the super-capacitor, which can instantly release a large amount of energy. Further, pulse frequency modulation was applied to produce high voltage and thereby improve energy efficiency. The developed mobile X-ray equipment enables to generate an output of 30 kW and, therefore, can be applied to many diagnostic fields. In addition, various devices and control circuits were employed to ensure convenience and safety of using the equipment in clinical applications. This study analyzed the error ranges regarding tube voltage, tube current, irradiation time, coefficient variation, half-value layer, and the output characteristics. The results showed that the proposed X-ray equipment was able to generate 800mR X-ray power under the condition of 30 kW. The coefficient variation was less than 0.05 at all measurement points, which indicates that it is possible to generate the equal amount of X-ray when the driving conditions are same. Results also showed 51.25% of transmittance at 3.5mmAL in the case of the wire, which is thicker than a common reference of 2.3mmAL and indicates that this new mobile equipment is possible to generate X-rays with relatively high permeability. In conclusion, the findings in this study suggest that the new equipment can generate consistent high-power X-rays and, therefore, can be used safely by minimizing unnecessary re-taking of images and radiation exposure.

Keywords

Mobile X-ray super-capacitor coefficient variation half-value layer

1. Introduction

X-ray equipment uses an electromagnetic wave generated by quickly colliding electrons with the target. The generated X-ray is widely used for the analysis of materials by diffraction and for non-destructive tests, such as flaw detection [1, 2], as well as widely used in the diagnostic tools and radiation therapy treatment equipment in the medical imaging and therapy fields [3, 4]. The X-ray device used for the diagnosis of the human body is utilized a lot as the most basic image diagnostic device. Also, a variety of devices have been developed and being used depending on the difference between the human body area and the diagnosis area [5 –7].

Diagnostic X-ray devices basically require a stable power supply for X-ray generation. Therefore, most X-ray equipment was used in a fixed form located where power facilities have been provided [8]. Because of this, there are restrictions on the use of X-ray in special circumstances, including emergency medical situations. Moving a patient to a specific location for diagnosis, may add to the patient’s discomfort [9]. To solve this problem, the use of X-ray equipment in mobile form is increasing [10].

Yet, most mobile X-ray equipment uses small amounts of internal power, limiting the capacity of the X-ray. Therefore, the mobile X-ray equipment is used for localized shots, such as of the hands and feet, where it is possible to diagnose at relatively low power. This study attempted to design and develop mobile X-ray equipment capable of generating a high-power X-ray. For the internal power supply to generate a high-power X-ray, a battery and a super-capacitor were applied. The density of the stored energy in the battery is high, but the charging and discharging efficiency is low, so it was used as the primary power source of the mobile X-ray equipment. In actual X-ray generation, a super-capacitor was applied. The super-capacitor allows energy to be charged and discharged at a rapid speed [11]. Thus, ensuring high output generation in a short amount of time. This paper investigates attempted to store energy using a battery and to design that the deficient electricity of the super-capacitor is charged. Consequently, if there was no separate power supply, mobile X-ray equipment could be manufactured to facilitate the generation of high-power X-rays using only internal power.

The X-ray used in the diagnosis of the human body is basically an electromagnetic wave with a short wavelength. If it is absorbed by the body, side effects can occur via interactions with the exposed organ. Therefore, when considering X-ray equipment used for human diagnosis, it is necessary to minimize unnecessary exposure. To reduce excessive exposure, the bio-reaction time of an X-ray should be minimized. In addition, for medical diagnostic X-ray equipment, it is a requirement to change the X-ray dose depending on the various diagnosis areas and body types. To that end, the diagnostic X-ray generator needs to be precisely controlled for tube voltage, tube current, irradiation time, etc., related to X-ray generation, and to generate a high-power X-ray whilst minimizing the exposure time of an X-ray in vivo. Thus, in this study, tube voltage, tube current, irradiation time accuracy, power characteristics, and the quality of radiation of mobile X-ray equipment using a battery and super-capacitors are investigated.

2. Methods

The manufactured mobile X-ray equipment consisted of a battery and a super-capacitor to be able to generate high-power X-rays using only internal power even if no separate power supply exists. The results of developing the mobile X-ray equipment have been reported via the thesis of Dr. Kim Young-pyo in 2016. Our study was created using some data from this thesis [12] and recently published paper [13]. Figures 1 and 2 shows the images and block diagrams of the manufactured mobile X-ray equipment.

Fig. 1.

Picture of the manufactured mobile X-ray equipment.

Fig. 2.

Block diagram of the developed mobile X-ray equipment.

Manufactured mobile X-ray equipment features a structure capable of charging the battery using 220 VAC, for which an ADC (Analog to digital converter) was used. The battery used as the internal power supply has a 12 V, 100 mA capacity. Two batteries were configured in series connection. To generate the high voltage required for X-ray generation, 24 super-capacitors with a capacity of 2.7 V, 400 F were blocked in series connection, for which 10 blocks were designed. With respect to the charging energy of the battery, voltage is boosted from the 1st electric transformer to two 190 VAC via the inverter, and the control unit charges the super-capacitor block to supply energy. Based on this configuration, the utilization of the internal power was maximized by making up the missing charge of the supply super-capacitor with the battery.

The manufactured mobile X-ray equipment is composed of various devices to provide safety and convenience. The input section transmitted the drive conditions for X-ray generation to the control unit. The console was configured to be able to check the operation status of the mobile X-ray equipment. The PC was designed to allow a user to check the result of shooting in a moving form, thereby ensuring user convenience. The control unit consisted of various protection and control circuits to control the overall function of the mobile X-ray equipment. The control unit plays various roles in the charging and discharging of the super-capacitor block, generating the control signal of the tube voltage to produce high voltage, creating the tube current control signal to drive the filament constituting the cathode of the used X-ray tube, checking the actual irradiation conditions and the operation of the equipment, and stopping the generation of an X-ray if necessary.

The high-voltage transformer generates a DC high voltage applied to the X-ray tube, using the control signals of the tube current and tube voltage generated in the control unit. To generate a high-power X-ray, the X-ray tube (E7239, Toshiba Co., Japan) of the rotating target was used. The generated X-ray can adjust the irradiation scope through the collimator, so it was configured to minimize unnecessary radiation exposure other than in the diagnosis area.

3. Results and Discussion

An X-ray is an electromagnetic wave with a short wavelength and is classified as a type of radiation, widely used for the diagnosis in the human body. However, if the human body of the radiation is exposed to the X-ray for a long time, ionization power causes unnecessary effects, such as disorders and genetic malformations. Therefore, it is necessary to ensure uniform amounts of X-ray doses according to the diagnosis area. To safeguard the reliability of the X-ray dose, precise control is required over the tube voltage, tube current, and irradiation time. For diagnostic X-ray equipment applied to the human body, the allowable range of each of the separate performance test criteria should not be exceeded. Table 1 shows the performance test criteria for the tube voltage, tube current and irradiation time errors of the diagnostic X-ray equipment.

Table 1
Performance evaluation standards

No List of Measurement Test Standards

1 Tube voltage error range Transformer type: Not more than±10 %

Condenser type: Not more than±5 %

2 Tube current error range Short time rating: Not more than±15 %

Long time rating: Not more than±15 %

Inverter unit:

3 Exposure time error range Standard 1: T〈0.01 s: ±1 msec

Standard 2: 0.01 s≤T: 10 %

No	List of Measurement	Test Standards
1	Tube voltage error range	Transformer type: Not more than±10 %
		Condenser type: Not more than±5 %
2	Tube current error range	Short time rating: Not more than±15 %
		Long time rating: Not more than±15 %
		Inverter unit:
3	Exposure time error range	Standard 1: T〈0.01 s: ±1 msec
		Standard 2: 0.01 s≤T: 10 %

To precisely evaluate the stability of the high-power mobile X-ray equipment, an experimental environment was created at a temperature of 24±2°C and humidity of 60±10%. Furthermore, to ensure the safety of the measurer, measurements were conducted using a Barracuda dose meter (Piranha, RTI Electronics AB, Sweden) in a separate area shielded with lead. To check the tolerance range for the tube voltage, tube current, and irradiation time of the manufactured mobile X-ray equipment, repeated measurements were performed three times using the Barracuda dose meter under each condition. The experiment was fixed at 100 kV tube voltage and 100 msec irradiation time, where the tube current increased in 50 mA increments, from 50 mA to 300 mA, to measure the error rate. The results of the performance test are shown in Table 2.

Table 2

Test result of performance

Condition of Set Point	Measuring Efficiency (%)
	kV	mA	Sec
100 kV, 50 mA, 100 msec	0.55 %	0.35 %	0.069 %
100 kV, 100 mA, 100 msec	0.012 %	0.144 %	0.048 %
100 kV, 150 mA, 100 msec	0.004 %	0.097 %	0.066 %
100 kV, 200 mA, 100 msec	0.138 %	0.272 %	0.063 %
100 kV, 250 mA, 100 msec	0.115 %	1.793 %	0.063 %
100 kV, 300 mA, 100 msec	0.012 %	0.243 %	0.059 %

As a result of the performance test, the tube voltage showed the error rate from the lowest 0.004 % to the highest 0.55 % for each test condition. The error rate of the tube current ranged from 0.35 % to a maximum error rate of 1.793 % under each test condition. Investigating the irradiation time, the error rate was from 0.069 to 0.48 %, showing the lowest error rate. It was thus confirmed that the tube voltage, tube current, and irradiation time all fall within the tolerance ranges of the performance test criteria.

The used X-ray generator generally adopts the resonant inverter method [14 –16]. In this method, the electric charge repeats charging and discharging operations to the capacitor and coil, causing resonance, and converting the resonating electric charge to the alternating voltage of a specific frequency. For the high voltage transformer to generate high voltage, the power consumption and the losses due to heat generation are reduced, as the oscillation frequency becomes higher. Therefore, the manufactured mobile X-ray equipment adopted the PFM (pulse frequency modulation) method that ensures that the oscillation frequency used to generate high voltage changes [9].

The PFM method was applied to increase the efficiency of the limited internal power supply and subsequently to improve the utilization of the mobile X-ray equipment. An analysis was conducted to see the output characteristics according to the change of the oscillation frequency and the change of the setting conditions of the high voltage generation of the manufactured mobile X-ray equipment. Figure 3 shows the results of the analysis of frequency variation and output characteristics.

For the investigation, the tube voltage was fixed at 100 kV and the irradiation time was set at 100 msec when the test was conducted by changing the tube current from 50 to 300 mA, and the driving frequency and irradiation dose were analyzed with an oscilloscope and a dose meter, respectively. As a result, the driving frequency decreased linearly as the set value of the tube current increased. The results suggest that maximizing internal power utilization is done by increasing efficiency using a higher driving frequency in generating a low-power X-ray and by using a relatively low driving frequency to raise an energy transfer rate in generating a high-power X-ray. In addition, the exposure increased as the tube current increased, suggesting that a stable X-ray is generated under the high-power condition of 30 kW.

Fig. 3.

Analysis results of frequency variation and output characteristics.

For the X-ray equipment used for diagnosis, the generation of a uniform exposure dose should be guaranteed at all times, according to the conditions. If the exposure dose is not uniform, a contrast difference occurs in the image and the diagnostic results may vary. To confirm the reproducibility of the exposure dose, the X-ray equipment is verified by calculating the CV (coefficient variation). Therefore, this study also assessed the reproducibility of the exposure dose by calculating the CV. Table 3 shows the results of the CV measurement.

Table 3

Test result of coefficient variation

		Tube Voltage
		40 kV	60 kV	80 kV	100 kV
Tube current	50 mA	0.005	0.001	0.001	0.0003
	100 mA	0.004	0.002	0.002	0.0002
	150 mA	0.001	0.003	0.002	0.001
	200 mA	0.001	0.003	0.003	0.0007
	250 mA	0.001	0.0003	0.003	0.001
	300 mA	0.005	0.006	0.004	0.001

Fig. 4.

Test result of HVL experiment.

In the experiment, the irradiation time was fixed at 100 msec and the tube voltage condition was divided into 20 kV increments from 40 to 100 kV. The tube current conditions were divided into 50 to 300 mA in 50 mA increments, to calculate the CV by conducting measurements three times at each set point. As a result, the CV was less than 0.05 at all set points, which is in the tolerated range. An X-ray generated in the X-ray tube showed an uneven continuous spectrum with the photon having low energy and long wavelength. The photons with low energy and long wavelengths are absorbed in the human body during diagnosis, acting as a factor to increase the radiation dose. To filter out these unnecessary photons, filter materials are used in the spinner of the X-ray tube.

The filtration capacity of the filtration material used determines the quality of radiation the human body is exposed to, so it is an important factor that must be checked in generating a medical X-ray. A method of checking the quality of X-ray diagnostic equipment includes an HVL (half value layer) experiment. The HVL reduces the exposure dose of the generated X-ray by 50% and means a reduced thickness of the required absorbent material, notated in an Aluminum equivalent weight (mmAl). In this study, the quality of the mobile X-ray equipment was analyzed using the HVL experiment. Figure 4 shows the results of the HVL experiment.

In this test, the tube voltage was fixed at 100 kV, the tube current was set at 200 mA, and the irradiation time was set at 100 msec. An aluminum filter of 0.5 mm was stacked one by one. The exposure dose reaching the X-ray dose meter from 0 to 5 mm was recorded. To obtain the results of the experiment, 0 mm without using the filter was calculated as 100 %, to yield the transmittance as the filter thickness increased. In the performance test criteria, X-ray diagnostic equipment with a tube voltage exceeding 70 kV should meet the 2.3 mmAl or more required, as per the minimum HVL. As a result of the half-life layer test of the mobile X-ray equipment, the transmittance was 51.25 % in 3.5 mmAl and 47.78 % in 4 mmAl. It was thus confirmed that the results are within the transmittance range of the performance test criteria.

4. Conclusion

For this study, mobile X-ray equipment capable of generating a high-power X-ray was manufactured taking advantage of the internal power, and the output characteristics, including stability, were analyzed. For the manufactured mobile X-ray equipment, the battery was charged using commercial power. The manufactured mobile X-ray equipment has a structure for replenishing the electric charge insufficient in the super-capacitor. In addition, to employ the limited internal power efficiently, a PFM method was applied so that the oscillation frequency used to generate high voltage can be varied. To confirm the stability of the manufactured mobile X-ray equipment, the main setting conditions of the tube voltage, tube current, and irradiation time were investigated three times to check the error range. Although the difference of the error rate depended on each setting condition, it was confirmed that the error range was within the tolerance range for all items. The aim of this study was to generate a high-power X-ray using internal power only. To confirm this, the changes in frequency and exposure dose were checked by increasing the set point of the tube current. As a result, the driving frequency decreased linearly. This means that the driving frequency changes uniformly according to the drive condition. Moreover, the linear increase in exposure dose suggests stable X-ray generation under high-power conditions of 30 kW. Based on irradiation time, the reproducibility of the exposure dose was evaluated by checking the CV for the changes in the tube voltage and tube current. The CV was significantly lower than the tolerance range of 0.05 in all conditions, ensuring uniform irradiation. The quality of the generated X-rays was also evaluated by the HVL experiments. As a result, a higher transmission was found than set in the performance criteria, confirming that the quality of radiation generated in the mobile X-ray equipment is excellent. Additionally, for this reason, it is thought that the availability of the high-power mobile X-ray equipment ensuring the effective utilization of the internal power can be very high.

References

K.A.

Irshad ,

N.V.

Chandra Shekar ,

T.R.

Ravindran ,

Srihari and

K.K.

Pandey , X-ray diffraction and Raman studies on Ho: EU2O3, Journal of Molecular Structure 1128 (2017) 325–329.

Banaee ,

Asgari and

H. A.

Nedaie , Comparison of penumbra regions produced by ancient Gamma knife model C and Gamma ART 6000 using Monte Carlo MCNP6 simulation, Applied Radiation and Isotopes 137 (2018) 154–160.

Ali ,

Alsbou ,

J.M.

Taguenang ,

Ahmad , Quantitative evaluation by measurement and modeling of the variations in dose distributions deposited in mobile targets, Journal of X-ray Science and Technology 25 (2017) 573–583.

Li ,

Wang ,

Luo ,

Zhao ,

Lin ,

Guo ,

Hu ,

Zhou , and

Zhao , Diagnostic value of single-source dual-energy spectral computed tomography for papillary thyroid microcarcinomas, Journal of X-ray Science and Technology 25 (2017) 793–802.

Y. M.

Shin , Optically controlled coherent X-ray radiations from photo-excited nanotubes, Nuclear Instruments and Methods in Physics Research B 407 (2017) 276–281.

Magnus

W.D. Hanson-Heine

Michael

W. George

Nicholas

A. Besley

. Density functional theory calculations of the non-resonant and resonant X-ray emission spectroscopy of carbon fullerenes and nanotubes, Chemical Physics Letters 696 (2018) 119–124.

Bardi ,

Jurewicza ,

A.K

King ,

M.A.

Alkhorayef ,

Bradley and

A.B.

Dalton , X-ray irradiation-induced structural changes on Single Wall Carbon Nanotubes, Radiation Physics and Chemistry 140 (2017) 34–37.

F. Y.

Hsu ,

W. F.

Lee ,

C. J.

Tung ,

J. S.

Lee ,

T. H.

Wu ,

S. M.

Hsu ,

H. T.

Su and

T. R.

Chen , Ambient and Personal Dose Assessment of a Container Inspection Site Using a Mobile X-ray System, Applied Radiation and Isotopes 70 (2012) 456–461.

Y.P.

Kim ,

T. G.

Kim ,

Y. P.

Park ,

M.W.

Cheon and

Y.S.

Lee , The Study on the Mobile Diagnostic X-ray Device of Frequency Modulation Method by PI Controlled, Lecture Notes in Electrical Engineering 164 (2012) 249–258.

10.

Y.P.

Kim ,

Y.P.

Park and

M.W.

Cheon , Charge and Output Power Characteristics Analysis of Mobile X-ray Device by Using Ultra Capacitor, Journal of Nanoscience and Nanotechnology 16 (2016) 1–4.

11.

J.G.

Shon and

H.E.

Jong , Series Voltage Compensation Systems for Voltage Sag by Using an Environmentally Friendly Ultra-capacitor, The Transactions of the Korean Institute of Electrical Engineers 54 (2009) 763–769.

12.

Y.P.

Kim , Development of High Power Mobile X-ray Equipment Using the Super Capacitor (Dongshin University, Korea, 2016) p. 90.

13.

Y.P.

Kim ,

Y.P.

Park ,

M.W.

Cheon , A study on the characteristics of mobile X-ray device using supercapacitor as internal power, Journal of X-ray Science and Technology 26 (2018) 777–784.

14.

S. S.

Kang ,

C. S.

Kim and

S. J.

Ko , Usefulness of Non-Invasive Measurement Tool on Performance Evaluation of Inverter Type X-ray Unit, Korean Society of Radiological Science 31 (2008) 123–127.

15.

Mohamed and

Mohamed , Optimization strategy of distortion rate control for high power transfer supplying an X-ray tube, Journal of Electrical Engineering 63 (2012) 81–87.

16.

A. M.

Pernia , Miguel

Prieto, Pedro J.

Villegas, Juan Diaz and

Juan A.

Martin-Ramos , LCC Resonant Multilevel Converter for X-ray Applications, Energies 10 (2017), 1573.