X-ray machine basics

I. Overview: On November 8, 1895, German physicist Roentgen accidentally discovered X-rays while studying gas discharge experiments in cathode ray tubes. X-rays were soon applied to medical imaging, creating a non-invasive imaging diagnosis method for internal organs. X-rays are electromagnetic waves with extremely short wavelengths and high energy. In addition to the general properties of visible light, they also have some of their own characteristics, namely penetration, ionization, photosensitivity, fluorescence and biological effects. The application of X-rays in medicine includes three aspects, namely X-ray diagnosis, X-ray therapy and X-ray protection. X-ray machines used for diagnosis are called diagnostic X-ray machines, which can be used for fluoroscopy and photography. X-ray fluoroscopy is mainly based on the penetration, differential absorption and fluorescence of X-rays; X-ray photography is based on the penetration, differential absorption and photosensitivity of X-rays. X-ray photography can be divided into several methods, such as ordinary photography, filter photography, and tomography.

II. Introduction to high-voltage components of X-ray machines

     (I) X-ray tubes X-ray generation requires three conditions: 

          (1) electron source; 

          (2) high-speed electron flow; 

          (3) target. X-ray tubes are the core devices designed and manufactured based on this to generate X-rays. X-ray tubes are mainly composed of three parts: cathode, anode and glass shell. The anode and cathode are encapsulated in a high-vacuum glass shell. The cathode (tungsten filament) can emit a sufficient number of electrons at high temperatures. These electrons are accelerated into high-speed electron streams under the action of high voltage at the positive and negative poles. When high-speed electrons hit the tungsten target, the electron kinetic energy is converted into two parts of energy: less than 1% of the energy is converted into X-ray energy; and more than 99% of the energy is converted into anode heat, which accelerates the temperature rise of the anode target surface. X-ray tubes are divided into fixed anode X-ray tubes and rotating anode X-ray tubes. The structural difference between the two is mainly reflected in the anode. The former is mainly composed of a tungsten target surface and a heat-transfer copper body, while the latter is mainly composed of a target surface, a rotor, a rotating shaft, a stator and a bearing. Rotating anode X-ray tubes are divided into low-speed tubes (3000 rpm) and high-speed tubes (9000 rpm). In order to reduce wear and prevent resonance, rotating anode X-ray tubes generally have brake devices. The X-ray tube is placed in the X-ray tube chamber, and the two are filled with transformer oil for insulation and cooling.

     (II) High-voltage generator The main function of the high-voltage generator is to supply the cathode and anode of the X-ray tube with DC high voltage and filament heating voltage. It is mainly composed of a high-voltage transformer, a filament transformer, a high-voltage rectifier, a high-voltage switch, transformer oil and a box that encapsulates the above components. The high-voltage primary voltage is stepped up by the high-voltage transformer and rectified by the high-voltage rectifier to become a DC high voltage, which is added to the cathode and anode of the X-ray tube through a high-voltage cable. The filament primary voltage is stepped down by the filament transformer and added to the filament of the X-ray tube through a cathode high-voltage cable. An X-ray machine often has two or three X-ray tubes, which work in time and are switched by a high-voltage switch. Transformer oil plays the role of insulation and heat dissipation.

     (III) High-voltage cable High-voltage cable is a device that connects the high-voltage generator and the X-ray tube. It is mainly used to transmit the cathode and anode high voltage of the X-ray tube and the filament heating voltage. High-voltage cables are divided into two categories: coaxial high-voltage cables and non-coaxial high-voltage cables. Their structure from inside to outside is conductive core wire, high-voltage insulation layer, semiconductor layer, metal shielding layer and protective layer. There are two types of cathode cables, three-core and two-core, which are used for dual-focus and single-focus X-ray tubes respectively. High-voltage plugs are installed at both ends of the high-voltage cable, which are connected to the high-voltage generator and X-ray tube equipped with high-voltage sockets respectively.

III. Composition of diagnostic X-ray machine Diagnostic X-ray machine mainly consists of two parts: host and peripheral equipment.

IV. Basic circuit of X-ray machine host system X-ray machine host system can be divided into the following 10 components in terms of its circuit:

          (1). Power supply circuit Provides the required power supply voltage for each part of the X-ray machine circuit, and its core component is the autotransformer.

          (2). High-voltage primary circuit Circuit that provides power to the high-voltage transformer. It is required that the high-voltage primary can realize the regulation of tube voltage, control of tube voltage and indication of tube voltage.

          (3). High-voltage secondary circuit Convert the AC high voltage output of the secondary of the high-voltage transformer into DC high voltage through a rectifier device and send it to both ends of the X-ray tube.

          (4). X-ray tube filament heating circuit The circuit that supplies power to the X-ray tube filament heating is divided into the filament primary circuit and the secondary circuit, and its basic requirements are stability and adjustability.

          (5). Tube current measurement circuit The tube current is usually measured using a milliammeter and a milliammeter. This meter is connected in series to the secondary neutral point of the high-voltage transformer and installed on the console to indicate the tube current during exposure.

          (6). Delay circuit The main function is to complete the waiting time required in the circuit, device, device and mechanical transmission process. For example, the rotating anode start-up time, the filament heating time, and the time required to switch from the perspective state to the photography state during spot film photography are all provided by the delay circuit.

          (7). Time limit circuit The circuit that controls the exposure time. The exposure time is usually controlled by controlling the on and off of the high-voltage contactor or the main thyristor. Common time limiters include mechanical time limiters, electronic time limiters, and automatic exposure time limiters.

          (8). Rotating anode start circuit The circuit that controls the rotation of the rotating anode. Usually there are the following requirements: first, it must be able to start quickly; second, there must be a delay; third, there must be a voltage reduction device; fourth, there must be a protection device; and fifth, there must be a braking device.

          (9). X-ray tube safety protection circuit Each X-ray tube has its own maximum capacity. When in use, if it exceeds its limit, the X-ray tube will be damaged. In order to ensure that each exposure is carried out under the maximum allowable load of the X-ray tube, many X-ray machines are equipped with X-ray tube safety protection circuits. However, this protection is a one-time preset protection, which only protects against one-time exposure overload, and does not protect against cumulative overload caused by continuous repeated exposure within the rated value. Therefore, when using the X-ray machine, there must be a sufficient time interval between two adjacent exposures to ensure that the X-ray tube is fully cooled.

         (10). Control circuit The control circuit refers to a comprehensive circuit that uses contactors and relays as the main actuators to control the occurrence and interruption of X-rays in accordance with clinical technical requirements, and can coordinate certain mechanical actions. The above 10 basic circuits constitute the main circuit of an X-ray machine, and they interact and control each other.