Abstract
Mobile X-ray device is widely employed because it is useful for diagnosis in patients having mobility difficulties and in medical emergencies. As various devices for X-ray generation have continued to be developed, X-ray devices can now be used more safely and effectively. However, mobile X-ray devices generate relatively low X-ray doses due to the limitation of the power input. Therefore, the use of mobile X-ray devices is limited to thin parts of body. In this study, a new device was designed in order to increase the usefulness of mobile X-ray devices by offsetting the weaknesses of the existing mobile X-ray devices, rendering them useable independently. A supercapacitor and battery were used as the internal power source for the X-ray generation in the manufactured device. The pulse width modulation (PWM) method is applied to control the tube voltage and current required for generating the X-ray, and the pulse frequency modulation (PFM) method is applied to the control to generate the high voltage in order to enhance the precision and efficiency. The manufactured X-ray device was used to evaluate the control signal, frequency, and output characteristics according to changes in tube voltage and current. Based on the results of X-ray generation, it is confirmed that precise control was achieved by X-ray generation increases linearly with increasing tube voltage and tube current. This means that precise control of the manufactured mobile X-ray device is passible. In addition, the study confirmed that stable output was achieved by checking the tube voltage, tube current and exposure rate during the exposure times by high power condition.
Introduction
The wavelength of the X-ray is a short electromagnetic wave which serves diverse purposes: substance analysis using the diffraction of the X-ray striking with the crystal, flaw detection considered in non-destructive object inspection, and medical measure for internally diagnosing living matters and destructing pathological tissues [1–4]. The diode-vacuum type X-ray tube is universally used to control the X-ray dose [5]. The X-ray tube generates the X-ray when the electron created in the negative pole accelerates and strikes the target constructed on the positive and negative poles due to the high-voltage, thereby creating the Bremsstrahlung. Accordingly, the X-ray generation device essentially requires a high-voltage direct current. In particular, the X-ray diagnosis device requires a stable supply of the input power used for creating the high-voltage required for quality images. As a result, most of the X-ray diagnosis devices are stationarily installed at locations where the power facilities are prepared [6]. The weakness of the stationary X-ray device is that the patient must travel to a designated place to receive the diagnosis.
To supplement such weakness, the mobile X-ray device is increasingly used to provide a more efficient service for patients with reduced mobility, for emergency situation, and in emergency rooms and hospitals with limited power facilities [7, 8]. Either the battery or the electrolytic capacitor is used as the internal power for the mobile X-ray device. In the case where the battery is used, the storable energy density is high, but the low charge and discharge efficiency limits the high output X-ray generation [9]. In the case where the electrolytic capacitor is used, the energy can be rapidly charged and discharged, but a number of capacitors must be combined in order to execute the high output generation, and this increases the volume of the storage device. In the case where the electrolytic capacitor is used, the energy can be rapidly charged and discharged, but a number of capacitors must be combined in order to execute the high output generation, and this increases the volume of the storage device. In addition, the power facility must be used to frequently execute the electric charge in order to continuously execute the X-ray exposure process.
In this research, a device using a battery and supercapacitor is manufactured to supplement the commercialized mobile X-ray device. Supercapacitor has a high capacitive reactance of 100 to 1000 times more than electrolytic capacitor. And the supercapacitor has shorter discharge duration and a greater output density than those of the battery the supercapacitor has been attracting interest in the area of high-power system research [10–12]. In this research, the electrical characteristics of the battery and supercapacitor are examined to assist in the manufacture of the internal power so that the mobile X-ray device can be independently operated. In addition, the pulse frequency modulation (PFM) method is applied to the development of the mobile X-ray device and the characteristics are analyzed so that the efficiency of the high voltage occurrence can be enhanced.
Materials and methods
In the case where the supercapacitor is used as the internal power for the mobile X-ray device, the battery is used so that only internal power is used without any separate power facility generating the high output X-ray. The two batteries (12 V 100 Ah) are used, and a AC 220 V is used for the charging process. The energy stored in the batteries been used to charge the supercapacitors and supply power to the internal electrical element. Supercapacitor block is 24 supercapacitors consisting of 400F 2.7 V capacitance are connected in series. Total 10 supercapacitor blocks to supply the high voltage required for generating the X-ray. Accordingly, 240 supercapacitors are connected in series. The energy stored in the battery is a structure that charges super capacitor at the same time as X-ray generation. It uses internal power source, therefore it is used to execute a number of repetitive shoots. The energy stored in the battery is converted to two AC 190 V for charging the supercapacitor through the primary transformer. It is converted to the DC voltages depending on the signal from the charge board and then charged into the respective each supercapacitor blocks.
The mobile X-ray device using supercapacitors consists of diverse safety and control circuits to secure the convenience and safety needed for mobile use. Figure 1 shows a block diagram of the mobile X-ray device.
Block diagram of mobile X-ray device.
The mobile X-ray device consists of the following components: a touch-based control panel displaying the user-applied setting, a current system operating status and acquired images, a PC connected to digital radiography (DR) for acquiring diagnostic images, and a power board for generating the power to be applied to the respective each internal boards and rotors. In addition, the mobile X-ray device consists of the following equipment: two motors for mobile convenience, a motor control board for controlling these two motors, a charging board for charging the supercapacitor blocks, a filament board for controlling the thermoelectron release in the negative pole of the X-ray tube, and a field effect transistor (FET) board for controlling the signals related to the high voltage occurrences. The interface board distributes the control signals of the overall device depending on the X-ray generating conditions. The interface board uses the CortexTM-M3-based STM32F205ZE (STMicroelectronics, Switzerland) as well as the complexity programable logic device (CPLD)-based EPM1270-GT144C4 (Altera, USA). The two ICs used control the respective boards as well as the information related to the operational status of the following components: a high voltage transformer for generating the high-voltage direct current used for the X-ray tube, an X-ray tube for the X-ray emission, and a collimator for limiting the exposure scope of the generated X-ray.
The tube current and tube voltage are the most important factors related to the X-ray generation. The tube current determines the number of thermoelectrons to be released by the negative pole within the X-ray tube, and the tube voltage determines the strike speed of the thermoelectrons. This eventually determines the radiation quality of the X-ray as well as the exposure rate.
In this research, the pulse width modulation (PWM) method is applied to the mobile X-ray device to precisely control the tube voltage and current. The control signals for the tube voltage and current are generated in the STM32F205ZE of the interface board. The driving frequency of the STM32F205ZE is set to 120 MHz, and the PWM control signals, departmentalized into 4,096 steps, are generated according to the set point of the tube voltage and current entered in the control panel. The tube voltage and current related PWM control signals are converted into 0∼3.3 V analogue signals as they run through the resistor-capacitor(RC) filter fabricated on the frequency conversion circuit as well as on the filament driving circuit, thereby serving as the reference signals for generating the tube voltage and current. Figure 2 shows the flow chart of the control signal.
Flow chart of the control signals.
In this research, the PFM method is applied to the high voltage used for generating the tube voltage. The PWM signals related to the tube voltage generate the variable frequency signal in the frequency conversion circuit within the FET board. The generated frequency signal is switched in the FET block and generates the high voltage as it is applied to the primary side of the high voltage (HV) transformer along with input voltage from the supercapacitor block. The HV transformer uses the input voltage applied to the primary and increases the voltage to approximately 150 kV so that the voltage can be applied to the X-ray tube. The PWM signals related to the tube current are delivered to the negative pole of the X-ray tube through the filament driving circuit within the filament board, thereby generating the thermoelectrons.
The X-ray generation device used for the diagnosis must generate equal X-rays according to the set conditions in order to minimize unnecessary exposure to radiation and acquire quality images. To achieve this, the reference signals for generating the tube voltage and current used for the X-ray generation are analyzed. In this research, the duty rate changes indicated in the PWM signals generated in the STM32F205ZE within the interface board according to the tube voltage and current changes set by the users are measured. Figure 3 shows the duty rate changes according to the set tube voltage and current.
The change of duty rate according to the tube voltage and tube current.
In the experiment, the tube voltage ranges from 40 to 100 kV in a unit of 20 kV, and the tube current ranges from 50 to 300 mA in a unit of 50 mA. The oscilloscope is then used to measure the duty rate within each set value. The measurement result confirmed that the higher tube voltage and current values linearly increase the duty rate. This means that the higher set value increases the on-rate of the generated PWM signal. The reference voltage generated through the RC filter is thus then converted and used to control the tube voltage and current. The reference voltage is set to follow the set value through the comparison of the feedback signals between the actually generated tube voltage and tube current so that the equal X-rays are generated at all times.
The resonant inverter is applied to the high voltage used for the X-ray generation [13, 14]. The resonant inverter allows the electric charge to repeat the charge and discharge process in the capacitor and to force the coil to cause direct current voltage resonance, which then converts this direct current voltage into an alternating current voltage with a certain frequency to generate the high voltage through the transformer. This means that the higher oscillation frequency diminishes the reduction of power consumption during the high voltage generation process and also decreases the loss caused by the heat generation. Accordingly, it is highly efficient and the weight can also be reduced [9]. In this research, the PFM method is used to enhance the generation efficiency of the high voltage used for X-ray generation. Figure 4 show the converter circuit for controlling the driving frequency according to the set tube voltage and current.
Frequency conversion circuit.
The SG3525 IC by STM Microelectronics is used as the device for the frequency conversion. The SG3525 IC features frequency modulation from 100 hz to 400 kHz. When the reference voltage and the feedback signal are compared within the 6th RT terminal constructed on the SG3525, a converted analog signal is generated and applied in real time, and the oscillation frequency within the oscillator is converted.
The converted frequency signal is outputted through the out A and B terminals of the SG3525 IC, amplified through the IR4426 IC, and delivered to the FET block gate circuit. At this point, the IR4426 IC controls the A and B signals inputted to the FET block gate so that such signals operate normally without striking, and the FET block generates a high voltage generation signal. In this research, in order to confirm the frequency changes according to the set tube voltage and current during the high voltage generation process, an oscilloscope is used to confirm the frequency signal applied to the FET block. In the experiment, the tube voltage condition ranges from 40 to 100 kV in a unit of 20 kV, and the tube current ranges from 50 to 300 mA in a unit of 50 mA. The oscilloscope is then used to confirm the converted frequency. Figure 5 shows the frequency changes.
The frequency variation of the tube voltage and current according to the setting change.
As the experimental results, the frequency value applied to the FET block gate is lowered according to the higher set values of tube current and tube voltage. This result is due to the PFM. The PFM based output has a low driving frequency for a high amount of energy transfer and then the amount of energy delivered per hour increases. Therefore, if the set value of the tube voltage and the tube current is high, a low frequency is applied to the FET block gate and then it is possible to generate high power X-rays after increased the amount of energy delivered per unit time. In addition, the tube current and voltage set at a lower value applies a higher frequency. In this research, it is confirmed that the frequency conversion occurs according to the set values ranging from 100 to 250 kHz. To confirm the operation of the mobile X-ray device based on the actual PFM, the output characteristics according to the exposure conditions are analyzed.
In the experiment, the E7239 (Toshiba, Japan) is used as the X-ray tube for the X-ray. The X-ray exposure rate was measured by varying the tube voltage and tube current conditions while fixing the exposure time to 100 msec. The same tube voltage and current conditions as those applied to the frequency variation are applied in this experiment. An exposure dose meter (RTI Electronics Inc.) is used for measuring the exposure rate, and the average exposure rate is calculated through three repeated measurements. Figure 6 shows the measured results of the exposure rate.
Change of exposure rate according to tube voltage and current.
As a result, the exposure rate indicated a linear increase as the tube voltage and current increased. This signifies that the manufactured mobile X-ray device can create equal X-ray generation at all times according to the set tube voltage and current conditions. In addition, the changes in the tube current and voltage and exposure rate are confirmed under the high output condition (tube voltage 100 kV, tube current 300 mA, exposure time 100 msec). Figure 7 shows the output characteristics under the high output condition.
Analysis of output characteristics.
If the X-ray generator is not supplied enough power during the high output generation process, the value of the tube voltage which is closely related to high voltage generation rapidly decrease. Also, it is showed the same pattern on the tube current and exposure rate. The mobile X-ray device which is developed in this study is configured to use the electric charge of the super capacitor when the high voltage was generated. It is confirmed that the tube voltage and the tube current value did not change abruptly in the high power state during the exposure time. This demonstrates that a stable X-ray generation can be executed under a high output conditions.
In this study, we developed and characteristic analyzed a mobile X-ray device which can generate high power X-rays by using independent internal power source. It is used a battery and a supercapacitor as the internal power source of the mobile X-ray device. The PWM method is used as the signal for controlling the tube voltage and current serving as the important factors related to the X-ray generation. The set tube voltage and current value generates the PWM signal in the STM32F205ZE of the interface board. Such generated PWM signal is converted into an analog signal ranging from 0 to 3.3 V as it runs through the RC filter, and, thereby, serves as the reference signal for generating the tube voltage and current. As a result of confirming the duty rate of the reference signal according to the changes in the tube voltage and current, it is confirmed that the increased tube voltage and current value linearly increases the duty rate of the reference signal, meaning that the precise control can be executed according to the setting.
In this research, the PFM method is applied in order to enhance the generation efficiency of the high voltage applied to the X-ray tube, and it is confirmed that the higher set tube voltage and current value applies the lower frequency. As a result of examining the exposure rate of the actual X-ray tube based on such control signal, it is confirmed that the increased tube voltage and current linearly increase the exposure rate. This signifies that the manufactured mobile X-ray generation device can generate an equal X-ray at all times according to the set tube voltage and current conditions. In addition, as a result of confirming the changes in the tube voltage and current and exposure rate under the high output condition, it is confirmed that the stable output is executed during the exposure time. The mobile X-ray device which is developed in this study can be generated high power X-rays by using batteries and supercapacitor as internal power. Then, it is possible to acquisition X-ray diagnosis images. In addition, it is configured to increase the efficiency of the internal power source for high voltage generation by using a variable frequency. Through this study, it is expected to increase the utilization of mobile X-ray devices.