(1) The structure of CT tube
The CT tube is mainly composed of several parts such as the vertical tube core, tube sleeve, high-voltage circuit, and circulation cooling system. The core of the tube is composed of components such as a cup-shaped cathode filament, a rotating anode molybdenum-based tungsten target, a high-speed bearing, and a subordinate heat dissipation device.(II) Working Principle of CT Tube
The CT tube is actually a large high-vacuum cathode ray diode, which is a system for generating X-rays. Its working process is as follows: A 12V current is supplied to the cathode filament for heating, generating a concentration of free electrons. When a high voltage of 40 to 150kV is applied to the anode and cathode, the potential difference increases sharply. Driven by the high-voltage strong electric field, the free electron beam in an active state strikes the molybdenum-based tungsten target at high speed from the cathode and undergoes energy conversion. About 1% of the electrical energy forms X-rays, which are emitted through the window. 99% of it is converted into heat energy and distributed by the cooling system.
(3) Selection of CT tubes
The tubes are consumables that need to be replaced regularly, and there must be spare tubes. The price of the original tubes is very high. To reduce costs, alternative tubes can be used, but the performance of different brands varies greatly.
Firstly, the physical properties of the ball tube, such as its external dimensions, weight, center of gravity, and fixation method, are important parameters for high-speed rotation.
(2) Heat capacity (MHtJ) is an indicator for measuring the size of specifications. The larger the heat capacity, the more heat energy it can carry, the stronger the performance and the longer the service life.
(3) The voltage of the senior management and the current of the large tube are related to the heat capacity.
(4) The heat dissipation rate (KHLr/min), that is, the cooling efficiency, is also an important parameter affecting the normal operation of the tubes.
(5) Parameters such as filament characteristics, focus size, focal length (FFD), and anode speed should be as consistent as possible with the equipment; otherwise, the operation will not be normal.
(6) The Angle between the high-voltage cable socket and plug, as well as between the window and the high-voltage cable socket, must match; otherwise, normal installation cannot be carried out.
(7) The criterion for weighing the price should be the average price per exposure and the number of times the tube is guaranteed.
(8) The after-sales service, technical support and reputation of the supplier are also very important.
(4) Maintenance of CT tube
The general service life of a cT tube is 100,000 to 200,000 times. If it is carefully maintained, its lifespan can be increased by 50%. So this is a good thing with a small investment but great returns.
1. Preparation of instruments and materials
(1) Instruments: X-ray leak detector, vacuum oil filling machine, high-voltage insulation testing machine, molecular vacuum pump, AC/DC transformer, forced convection drying oven, drying box (room), ultrasonic cleaning machine, deionizing water machine.
(2) Materials: anhydrous ethanol, 25# transformer insulating oil, non-woven fabric paper, polytetrafluoroethylene.
(3) Accessories: Oil pump, oil pipe, fan.
2. Inspection of the ball tube
(1) Carefully inspect whether the casing, oil pipe and heat exchanger are damaged.
(2) Check whether the contraction cavity is in the normal position.
(3) Use a multimeter to check whether the filament of the bulb tube is normal.
(4) Power on and test whether the sound of the rotating anode of the tube is normal.
(5) Power on to check whether the cooling oil pump and cooling fan are working and if they are damaged.
3. Disassembly of the ball tube
(1) Be careful to remove or transcribe all trademarks and factory serial numbers for safekeeping.
(2) Remove the window and pour out the insulating oil.
(3) Open the cathode end, draw the wiring diagram of the filament on the paper, and then remove the filament connection.
(4) Remove the high-voltage plug of the cathode.
(5) Open the anode terminal and remove the anode high-voltage plug.
(6) To remove the core fixing nut, two people should operate. One person should hold the glass tube steady with both hands, and the other should loosen the nut with a special wrench. The removed tube cores should be placed properly with sponge pads.
4. Cleaning and drying of components
(1) The frame, heat exchanger and fan are washed with water and naturally dried.
(2) The sleeves, including the covers, bolts, insulating sleeves, oil pipes, etc., shall be washed with deionized water and then cleaned with an ultrasonic cleaner. 80. Dry at high temperature for 24 hours and set aside.
(3) If the paint on the surface is peeling off, it should be re-sprayed and re-baked with electronic color matching, and then cleaned and dried.
(4) The core, skin, etc. are washed with alcohol (anhydrous ethanol) and dried at 80℃ for 24 hours
(5) If the insulating materials such as the cathode fixation frame of the core, joints, insulating boards, and insulating cylinder sleeves age or get damaged, the drawings can be drawn and processed and replaced with polytetrafluoroethylene materials.
5. Packaging of the ball tube
(1) The operator's hands must be washed thoroughly and then wiped with ethanol. All tools should be cleaned with ethanol once, and then special rubber gloves should be worn during operation.
(2) Install in reverse according to the disassembly sequence. Pay attention to correct wiring and high-voltage insulation.
(3) Special attention: The tightness of the core shaft and the fixed sleeve should be properly matched. If the fixed sleeve is worn, it can be processed with 45 # steel. The anode target surface of the core and the window of the tube sleeve must be aligned. Generally, a dedicated vertical test mirror is used for testing and adjustment. Its radial adjustment * rotating tube core, locking nut, and axial adjustment * with gasket. The cathode of the core is fixed with a ring sleeve processed from epoxy bakelite board, and then bonded with silicone at 3 to 4 points. Pay attention to the direction of the inlet and outlet devices of the high-voltage wiring sleeve.
(4) The installed ball tubes are placed in a high-temperature drying oven for 4 to 6 hours for drying.
(5) Heat the transformer oil to 80℃.
(6) Connect the oil filling nozzle (processed according to different models of ball tubes), fill with oil, and then turn on the oil pump, the micro-fan for circulation, and the vacuum pump for exhaust.
(7) When oiling and venting, shake the hose from time to time to ensure that all the air in every corner of the hose sleeve is completely expelled. Continue for 2 hours until there are no tiny bubbles left.
(8) Be careful to adjust the window cover and stretch the leather bag.
6. Leak detection and efficiency of ball tubes
(1) The encapsulated tubes should be subjected to X-ray leak detection and aging tests. The operation process should be carried out in a dry room, box with strict radiation protection or directly installed on a CT machine.
(2) Leak detection and testing should be carried out in accordance with the parameters provided on the tube label, gradually starting from low kV and small mA to high kV and large mA. Tests should be conducted in accordance with the tube installation and debugging procedures. If abnormal fluctuations in the current and voltage meters or abnormal sounds are found, power should be immediately cut off to identify the cause. For more serious problems, repackaging is necessary.