X-ray tubes and three important parameters

X-ray tubes serve as a specific type of energy converter. The electrons released after cathode excitation are accelerated by the electric field between the cathode and anode, and then bombard the anode target, transferring their energy to the atoms on the target. About 1% of the energy is converted into X-rays.

 

 

X-ray tubes serve as a specific type of energy converter. The electrons released after cathode excitation are accelerated by the electric field between the cathode and anode, and then bombard the anode target, transferring their energy to the atoms on the target. About 1% of the energy is converted into X-rays.The quality and the quantity of the x-ray are controlled by adjusting the electrical parameters (kV–tube voltage , mA–tube current ) and exposure time, usually a fraction of a second. 

 

As is widely recognized, there are three key parameters associated with X-ray tubes: focal point, tube voltage, and tube current.

 

1.Focus

Focus refers to the small area formed when an electron beam impacts the anode target in an X-ray tube. When electrons are accelerated from the cathode and directed towards the anode, they interact with the anode material, producing X-rays. The specific location where this interaction occurs is the focal point. The size of the focus has the greatest impact on the quality of X-ray imaging. The smaller the focus, the more concentrated the X-ray projection, and the clearer the edges of the projected image, which can form a clear image (or sharper image) for smaller objects.

 

However, the size of the focus is not necessarily the smaller the better. A focus that is too small will inevitably require lower radiation power, which directly leads to a decrease in radiation dose. In order to obtain sufficient X-ray dose, a longer exposure time is required. Moreover, the size of the focus is also limited by the design and manufacturing technology of X-ray tubes.

 

2.Tube Voltage

Voltage is a critical parameter for X-ray sources to produce X-rays, and it refers to the voltage applied between the cathode and anode. Tube voltage determines the energy level of the X-rays. The higher the voltage, the more X-ray energy is generated, resulting in shorter wavelengths and enhanced penetration capabilities, allowing for the detection of thicker material layers. However, excess voltage can cause image overexposure, enlarging the focal size, reducing spatial resolution, and compromising detection results. Hence, when selecting voltage, it is essential to consider comprehensively the material (density), structure, thickness, and required image quality of the object being examined.

 

Furthermore, prolonged use of excess voltage can impact the stability and lifespan of the X-ray source. Excessive voltage may cause the X-ray source filament to overheat, hastening its aging process and shortening its lifespan. Therefore, when operating the X-ray source, it is necessary to maintain a stable voltage within the equipment's rated range to ensure its long-term reliable operation.

 

3.Tube Current

The current, which is the number of electrons that travel from the cathode to the anode/target per unit time. The tube current determines the amount of X-rays (luminous flux). In an X-ray tube, as the current increases, the cathode releases more electrons, which in turn generates more X-ray particles, resulting in an increase in the luminous flux of X-rays. This means that more X-ray particles can pass through the object and be captured by the detector within the same time, thereby improving the sensitivity and efficiency of imaging. Therefore, in scenes that require fast image acquisition or improved image contrast, the intensity of X-rays can be enhanced by increasing the current.

 

Excessive current can cause internal overheating of the radiation source, accelerate equipment aging, and even damage the equipment. Moreover, current instability may also lead to fluctuations in X-ray intensity, affecting the stability and quality of imaging. Therefore, when adjusting the current, it is necessary to consider the heat dissipation performance of the equipment and the stability of the current, to ensure that the X-ray source can work in a safe and reliable state.