# Filament X-ray Tube Current Control Method Using Indirect Filament Temperature Estimation
## 1 Introduction  
Images or videos obtained through X-ray equipment are used to diagnose or operate on the affected areas of a patient. As depicted in Figure 1, the image acquisition procedure of the X-ray equipment comprises the following steps [1]:  
1. The filament is heated at a high temperature, and thermal electrons are emitted into a vacuum [2].
2. The emitted electrons are accelerated to an anode, generating a tube current that flows through the vacuum tube.
3. The accelerated electrons are converted into X-rays after their collision with the anode.
4. Certain generated X-rays pass through the patient and reach the detector [3, 4].
5. An image is obtained after the photons that reach the detector undergo image processing.  
Several types of filament heating methods are currently used: analog current control method [5], digital current control method [6, 7, 8], heating method with a constant gradient to the filament heating current [9], and a method that supplies a constant voltage to the filament. The analog control method configuration has been used in electric step-down transformers and variable resistors. The filament current depends on the resistance and the transformer turn ratio. However, these components increase the system volume and heat generation, and they can heat the filament only at a calculated current value. Due to the miniaturization trend of the industry, the above problems were overcome using the digital current control method. The filament heating current command increases along a constant slope, which is used to reduce the stress on the filament, thereby increasing the longevityof the X-ray tube. Additionally, for faster filament heating, DC is applied to the filament; however, the existing current control method is mainly used due to the filament longevity, cost-efficiency, and volume increase caused by using diodes for the voltage rectifier. In the conventional heating method, the filament is heated to a temperature at which electrical input energy and thermal loss are similar. The thermal energy is proportional to the square of the current flowing. In addition, the resistance of the filament and the thermal property of tungsten change non-linearly with temperature; thus, heat loss and calorific value have a non-linear relationship with temperature. As described above, if there is a difference between the input energy and the lost energy over time, the temperature changes, which in turn changes the amount of X-ray exposure (dose rate) to the patient's body. This resulted in scenarios where the temperature deviated from the regulated value [10]. Therefore, we aimed to solve the aforementioned problem in this study by controlling the temperature of the filament based on the change in the electrical resistance of tungsten considering its temperature. To validate the effectiveness of the proposed approach in comparison with that of the existing filament heating method, a comparative analysis was performed experimentally based on the changes in the dose rate and filament temperature over time. To compare the dose rate emitted by the X-ray tube, we measured the tube current, which is proportional to the dose rate [11]. Additionally, the filament temperature was observed over an empirical period that is suitable for the experimental conditions to verify that the emitted dose rate can be maintained constant by regulating the filament temperature.