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[Fundamentals of Medical Engineering] Questions and Answers on Basic Theory of X-Ray Machines(2)

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Update time : 2023-09-18 11:20:11
X-ray technology has revolutionized the field of medicine by providing a non-invasive way to visualize the internal structures of the human body. Understanding the basic theory behind X-ray machines is crucial for medical engineers and practitioners alike. In this article, we will explore  questions and answers to shed light on the fundamental principles of X-ray technology.
1. What are the main parts of an ordinary X-ray machine? What are the core components?
Ordinary X-ray machines are mainly composed of a console, a high-voltage generator, a handpiece, a diagnostic bed and various mechanical devices. An X-ray tube is installed in the machine head. The high-voltage generator and the handpiece of the small X-ray machine are brought together to make it lightweight, and are called combined handpieces.
Because the X-ray machine is a device that converts electrical energy into X-rays, and this conversion is achieved through the X-ray tube, the X-ray tube has become the core component of an X-ray machine. Since the material and structure of each X-ray tube have been determined, the inter-electrode insulation: strength and anode heat capacity are subject to certain limitations. During operation, any combination of tube voltage, tube current and time of applying tube voltage should not exceed the tolerance level of the X-ray tube, otherwise there is a risk of immediate damage to the X-ray tube. The high-voltage part, control part, filament heating part, overload protection part, time limit part and other circuits in the X-ray machine are all set up to ensure the normal operation of the X-ray tube.
It can be seen that the X-ray tube occupies a core position in the X-ray machine and should be given special care during work.
2. What are the main parts of the X-ray tube? What are the main functions of each?
An ordinary x-ray tube usually consists of a cathode, an anode and a glass wall that seals the electrodes. The cathode is made of tungsten wire, also called a filament. When used, the filament is heated by electricity and provides free electrons through thermal emission. In addition, in order to make the electrons form an ideal electron beam, a metal collector cover is often added around the filament and combined with it. One end of the filament is connected and at the same potential as the filament. The anode is a smooth flat surface made of tungsten, commonly known as the anode target surface. The function of the target surface is to convert the kinetic energy of high-speed electrons into X-rays. In order to effectively radiate X-rays and form an effective focus surface as small as possible, the target surface is tilted at a certain angle. The target surface generates X-rays and also generates a large amount of heat. In order to dissipate the heat in time, the tungsten target is embedded in a copper base with good heat transfer. In addition, in order to prevent electron bounce and X-ray scattering, a copper protective cap is placed around the target surface of a common X-ray tube. The well-sealed glass wall can maintain a high vacuum inside the X-ray tube to reduce resistance to electron movement, ensure one-way conductivity and prevent oxidation of the filament and target surface.
3. What are the necessary conditions for an X-ray tube to produce X-rays?
In order for the X-ray tube to produce X-rays, firstly, the filament must be heated to the extent that it emits a sufficient number of free electrons; secondly, a certain amount of voltage must be applied between the cathode and the anode, with the cathode being negative and the anode being positive, so that the cathode and An electric field is formed between the anodes to accelerate electrons. The electrons gain energy in the electric field and then hit the tungsten target, converting part of their energy into X-rays in the tungsten atoms.
In short, the necessary conditions for an X-ray tube to produce X-rays are firstly heating the filament and secondly applying high voltage to the two poles, both of which are indispensable. When the X-ray machine does not produce X-rays, the reason generally needs to be found from these two aspects.
4. What does the value indicated by the milliammeter mean?
The milliamp meter and the X-ray tube are connected in series in a high-voltage circuit. The value indicated by the milliammeter is the current value passing through the X-ray tube. Therefore, the larger the milliamp number, the more electrons that hit the anode target surface per second. The more X-rays are produced. The total amount of X-rays generated over several seconds can be indirectly expressed by the product of milliamps and time. This product is usually called exposure. One milliampere of light exposure per second is called one milliampere second and is recorded as 1mAS. In a full-wave rectified X-ray machine, due to the distributed capacitance of the high-voltage cable and the secondary conductor of the high-voltage transformer to the ground, these distributed capacitances can be equivalent to an equivalent capacitance connected in parallel to the X-ray tube. Therefore, the value indicated by the milliammeter It is not all the current in the tube that generates X-rays. Several milliamperes of current pass through the distributed capacitance. This part of the current is called capacitive current. The small amount of capacitive current has no significant impact on the photo work, but because it is similar to the value of the current in the tube during fluoroscopy, it is easy to give people false impressions, so there are usually corresponding measures inside the machine to compensate. Small X-ray machines are all self-rectifying, and the mA measurement system does not include a rectifier. Therefore, the mA meter does not display the AC capacitance current value.
5. What does the value indicated by the kilovolt meter mean?
The kinetic energy of electrons in the X-ray tube is determined by the voltage value applied to the two poles of the X-ray tube (E=ev). The higher the voltage, the greater the kinetic energy of the electrons, and the stronger the X-ray energy generated, that is, the penetrating power. Therefore, the value indicated by the kilovoltmeter indirectly indicates the penetrating power of X-rays.
In order to ensure the safety of the equipment, the kilovolt value is pre-selected on the primary side of the high-voltage transformer. This measure is called kilovolt indication. It is strictly prohibited to adjust kilovolts during exposure.
6. What aspects mainly determine the life of the X-ray tube?
A perfect X-ray tube should produce a sufficient number of X-ray beams with satisfactory penetration in the direction of use within the allowed time, under the allowed tube voltage and filament heating voltage. Any X-ray tube that cannot meet these requirements at the same time The wire pipe can be considered to have reached the end of its life.
In long-term use, if the filament evaporates more and becomes thinner, resulting in insufficient electron emission, the amount of X-rays decreases. At this time, the X-ray tube has begun to age. In addition, no matter what the reason is, as long as the filament is broken, the parent line tube will become a waste tube. After the tube is inflated, the electronic resistance will increase, and the X-rays produced will be weak and lose their use value. In severe cases, X-rays will not be generated and high-voltage short circuit will occur. If the gas contains oxygen, the filament will burn out immediately. In short, the inflated tube is also a useless tube.
The anode target surface caused by overload is rough and uneven, cracks fall off, and it melts into bulges and depressions, etc., which disperses the direction of the X-ray and changes the effective focus area, so that satisfactory images cannot be obtained. At this time, the X-ray tube Although it can also produce X-rays, it has to be discarded. In short, the life of the X-ray tube is mainly determined by three aspects: filament, vacuum degree and target surface. If there is a problem with one of the three, the X-ray tube will not work properly. For rotating anode X-ray tubes, the anode must be able to rotate flexibly. Damage to the target surface and reduced vacuum are common during use.
7. In order to extend the life of the X-ray tube, what issues should be paid attention to during work?
Because the life of the X-ray tube is mainly determined by the anode target surface, vacuum degree and filament, these three aspects should be paid attention to during work.
Most damage to the anode target surface is caused by instantaneous overload. The so-called instant overload means that the total input energy after the combination of tube voltage, tube current and exposure time exceeds the usage specifications of the X-ray tube, causing the heat generated instantly to exceed the maximum storage capacity of the anode, causing local overheating. This local high temperature can cause the tungsten target to evaporate, causing roughness and unevenness, and can also cause uneven expansion and contraction throughout the target surface, causing cracks. Larger X-ray machines have overload protection measures, but small X-ray machines generally do not have such measures. Users must operate carefully to prevent instantaneous overload. When using larger X-ray machines, you should always pay attention to whether there is overload. If this happens, you should find out the cause immediately and adjust it normally. In addition, since the overload protection devices of larger X-ray machines are mostly one-time protection, when the work is at its maximum, although each exposure is not overloaded, the target surface will be damaged due to frequent exposure and high heat accumulation. . Therefore, it is necessary to maintain intermittent use and create good cooling conditions.
Poor vacuum is mainly caused by two reasons. On the one hand, the glass wall is cracked by the impact of the machine and air is introduced; on the other hand, the vacuum degree is reduced due to the gradual escape of the gas remaining inside the electrode and the evaporation of the electrode. Therefore, violent vibrations must be prevented during use. When using an X-ray machine that has been left for a long time, the X-ray tube must be trained to increase its vacuum degree. The main way to protect the filament is not to increase the filament voltage casually. In addition, the X-ray machine should be turned off immediately when the work is completed, the mA selector should usually be placed in the fluoroscopy position, and it should be turned on to preheat before working.
8. How to properly place the head of the X-ray machine?
When the X-ray machine is working, the anode of the It will settle downward, so that the anode is cooled quickly through the convection of the insulating oil, preventing damage to the machine head due to heat accumulation.
9. What are the advantages of rotating anode X-ray tubes? What issues should be paid attention to during use?
In order to improve the clarity of the image, the smaller the focus area of the X-ray tube is, the better. In order to reduce the blur caused by the patient's activities on the film, it is also hoped that the exposure can be completed in the shortest possible time. The larger the capacity of the X-ray tube, the better. It is difficult for fixed anode X-ray tubes to meet these two contradictory requirements at the same time. Rotating anode X-ray tubes were developed for this reason. Its anode is a tungsten circular cone with a very small height, and the inclined surface serves as the target surface. Since the anode rotates during operation, even if there is a powerful electron beam, it will not damage the target surface. In this way, the effective focus will be It can be made very small and can withstand strong current. It can be seen that the anode-converting X-ray tube is suitable for high milliamp short-time exposure.
When using a rotating anode X-ray tube, in addition to paying attention to the issues of fixing the anode X-ray tube, special attention should be paid to starting the anode before exposure, and then exposure after the anode rotation reaches full speed. Otherwise, it can be used during the tilt. The target surface is damaged. However, all x-ray machines using rotating anode Exposure, the fault should be eliminated immediately at this time, and the work must not be forced. In addition, during the anode rotation process, it is not advisable to rotate the machine head at the same time to prevent inertia torque from being generated on the anode shaft and causing serious failure.
10. What are the main high-voltage components of the X-ray machine? What issues must be paid attention to when inspecting the high-voltage parts?
High-voltage components mainly refer to components with a high potential to the ground. In X-ray systems, they mainly include high-voltage transformers, filament heating transformers, high-voltage rectifiers, high-voltage cables, high-voltage plugs and sockets, X-ray tubes, and the leads of these components. , High-pressure exchange gates in ball storage tube machines, etc.
When inspecting these components, if it is done under live conditions, the primary of the high-voltage transformer should be removed and short-circuited to ground at the end of the high-voltage transformer to ensure safety. If the cable needs to be unplugged for inspection, the core wires must be placed in contact with the casing. Remove high-voltage static electricity.
The high-voltage static electricity on the secondary distributed capacitance of the high-voltage transformer is grounded at the center. The static electricity can be discharged by itself after the mine high voltage stops. However, high-voltage static electricity may also be stored in the event of a fault. In order to ensure safety, a grounded wire should be used to touch the high-voltage generating area. Each component to be inspected so that static electricity can be completely discharged.
In addition, although the mA measurement part is at zero potential to the ground, if the central grounding is poor or the mA measurement unit has an open circuit fault, all measurement parts will be at high potential. Therefore, the operator needs to use a mA meter to check the tube current. You should ensure good circuit contact and keep your body away from the meter and wiring.
11. In a four-tube full-wave rectifier X-ray machine, what is the approximate relationship between the voltage of each cable core wire to ground and the value indicated by the kilovolt meter?
In the four-tube full-wave rectifier X-ray machine, since the secondary center of the high-voltage transformer is grounded, and the core wire of each high-voltage cable is always connected to one end of the secondary end of the high-voltage transformer through a rectifier tube in each half cycle, Therefore, if the voltage drop on the rectifier is not included, the voltage of each cable core to ground is actually equal to half of the kilovolts indicated by the meter. For example, the voltage of the cable core to ground is 30KV when the meter indicates 60KV. For an X-ray machine with a rated voltage of 100KV, each cable should withstand more than 60KV.
12. In a four-tube full-wave rectifier X-ray machine, how high a reverse voltage does each rectifier tube bear?
In a four-tube full-wave rectifier X-ray machine, the positive poles of a pair of rectifier tubes are connected and serve as the negative pole of the DC high-voltage power supply. The negative poles of the two rectifier tubes are connected to the secondary ends of the high-voltage transformer respectively. The negative poles of the other pair of rectifier tubes After being connected, it serves as the positive pole of the high-voltage DC power supply, and the positive poles of the two rectifier tubes are respectively connected to the secondary ends of the high-voltage transformer. When working, each of the two pairs of rectifier tubes is conductive. If the voltage drop on the conductive tube is not considered, the two non-conducting tubes will bear all the secondary voltage. It can be seen that the reverse voltage borne by each rectifier tube is That is the voltage selected for each operation. When selecting a rectifier, the maximum reverse voltage should be more than 125% of the machine's rated voltage. If the rated voltage is 100KV, the maximum reverse voltage of the rectifier should be more than 125KV.
In a four-tube full-wave rectified X-ray machine, how high is the secondary-to-ground voltage of each filament transformer? Whether it is a rectifier tube filament heating transformer or an X-ray tube filament heating transformer, the secondary-to-ground voltage is the same as the cable core wire-to-ground voltage. The voltage is the same, half of the kilovolts indicated on the meter.
13. Why can the protection component work when the milliamp measurement part is disconnected?
The milliampere measurement section in the console is derived from the secondary center of the high voltage transformer. Once the milliampere measurement part is disconnected, the entire high-voltage part can be equivalent to a constant current source of the circuit in the X-ray tube. This constant current source uses the disconnected part as the output end. The protective components are all connected across the circuit that measures milliamps. Therefore, the instantaneous break will bear all the high voltage and start working. After the protective components work, almost all of the high voltage drops to the X-ray tube and will not lead to The control panel thus plays the role of insurance holder.
14. What will happen if one of the four rectifier tubes is broken?
There are three common faults of the rectifier tube. The first filament is open circuit. At this time, the machine works in Bu-wave rectification mode, and the milliampere of each gear is reduced by half; the second tube is filled with air. At this time, C has a high-voltage short circuit (see question 391;). Frequency triple anode melts and perforates, which may reduce the vacuum degree of the rectifier tube in the mild case, or cause the tube to break in severe cases. At this time, when the machine is working on a large scale, the X-ray penetration is weakened or a high-voltage short circuit occurs, and the machine cannot work at all.
15. What will happen if the machine has two rectifier filaments broken?
There are three situations when two rectifier tubes burn out the filaments at the same time and are accompanied by three phases. First, the two rectifier tubes in the same half cycle break at the same time. At this time, the machine works in half-wave rectification mode, and the milliamps at each gear are halved. Second, if the two rectifier tubes connected to the same DC high-voltage output end are broken at the same time, the machine will have neither fluorescence nor milliamps (a weak capacitive current may appear). The third two rectifier filaments connected to the same AC high-voltage output end are disconnected at the same time. At this time, the machine photo is not sensitive to light.
16. What are the possibilities when the X-ray machine does not produce X-rays? How to check? What are the general rules for X-ray failures?
If the high-voltage relay does not work, the fault lies in the control circuit and power supply part. If the high-voltage relay still does not output the The rectifier tube in the secondary circuit is not working properly, and the positive connection of the high-voltage switching switch may be disconnected and other faults. The second is that the X-ray tube filament is not heated, which can be caused by faults such as the voltage regulator not outputting, the primary circuit of the filament heating transformer, poor contact in the plug and socket, and the core wire of the cathode cable being broken. The third is that the X-ray tube filament is disconnected or the target surface falls off.

The inspection can be carried out in two steps. The first step of visual inspection is to check whether the filament of the Secondly, while the high-voltage relay is working, use a screwdriver to push against the shell of the high-voltage generator (or the combined head of a small If there is a buzzing sound, it means that the high-voltage transformer is already working, otherwise the high voltage is not added. Before doing this kind of inspection, you must ensure that the machine shell is well grounded. The second step is to conduct an in-depth inspection to determine the scope of the fault based on the first step of inspection; turn on the machine and use a multimeter to check step by step to quickly find out where the fault is.
The common faults of an ordinary X-ray machine generally follow some rules. Generally, inflating one of the four tubes can extinguish all the filaments of the four tubes. Add fluoroscopic high voltage under darkroom conditions, and observe the high-voltage generator at the same time to find flashes. That tube is the inflated bad tube. Sometimes the flash may not appear due to slight inflation. At this time, the voltage can be raised to 80-90KV. Generally, the flash can be found: the principle of this method is that after the four tubes are extinguished, they form four capacitors. The capacitors are AC paths, so they can Causes the gas inside the inflatable tube to produce self-excited discharge and emit a weak flash. The more serious the inflation, the more obvious the flash.
17. What is the function of insulating oil? What is the minimum insulation strength?
The function of insulating oil is high-voltage insulation, and the second function is heat dissipation and cooling. Therefore, both the machine head and the high-voltage generator cannot work when there is a lack of oil, especially in the machine head. First, because the anode of the Oil can easily cause high-voltage discharge, both of which can affect the life of the X-ray tube.
The dielectric strength of the insulating oil should not be less than 25KV/mm. After a rectifier is broken, the machine works in half-wave rectification mode. Can the mA be adjusted to the original number by adjusting the x-ray tube filament voltage? Of course, the filament heating voltage can be adjusted It is possible to increase the mA to its original value, but this is generally not possible for two reasons. First, it may cause instantaneous overload. If the current value pointed out by the DC milliamp meter in the half-wave rectified power supply is equal to that in the full wave, then the current peak value in the half wave is almost twice the current peak in the full wave. This may cause The X-ray tube's usage specifications are exceeded in an instant and the target surface is damaged. The heating voltage of the second filament may exceed the rated value and cause the filament to evaporate faster. During half-wave rectification, a DC component is added to the secondary coil of the high-voltage transformer, which generates DC magnetic flux. During operation, the primary side can generate a strong alternating current. Cause the relay contacts to burn out.
18. What phenomena occur when high voltage is short circuited?
A high-voltage short circuit occurs when the high-voltage lead touches the ground, or because the lead is too close, or the high-voltage component breaks down, so that the two ends of the high voltage with opposite polarity are directly connected without a load (i.e., X-ray tube).
When the high voltage is short-circuited, a huge amount of electrical energy will be released, that is, the primary of the high-voltage transformer will produce a huge current exceeding the normal value, which will cause a series of abnormal phenomena. The power transformer and high-voltage transformer will make a loud buzzing sound, and the power relay will not be able to maintain operation and cause the machine to malfunction. If there is a power outage, if the short circuit lasts for a long time, the fuse may burst. Except for the grounded high-voltage line package and its lead wire being short-circuited to the ground, almost all other short-circuit faults will make the pointer of the safety meter appear abnormally green, and the pointer of the full-wave aiming indicator will be distorted. At full scale, the self-rectifying pointer will vibrate unstablely. When high-voltage cables break down, two kinds of gas will be produced. The plugs and sockets will break down and leave traces of sparks. There will generally be special discharge sounds, and sometimes smoke and flashes may be seen. In short, once a high-voltage short circuit occurs, it must be accompanied by some thrilling operation. Some types of industry should avoid riding and reduce the number of tests, so as not to cause damage to other components or make the fault continue to worsen.
19. What are the common faults in milliampere measurement circuits?
Faults in the milliamp measurement circuit generally do not affect the generation of X-rays. Faults are common in rectifiers and milliamp meters. The rectifier is generally a four-tube bridge type. A common fault is tube breakdown. Generally speaking, if one tube breaks down, the mA value will be close to half, and if two tubes break down at the same time, the mA value will approach zero. The main faults in the meter are the shunt circuit block and the disconnection of the meter coil. Among them, if the index is zero during the photo and is high when viewed through perspective, the resistor with the larger value among the shunt resistors will be blocked; if both the perspective and photo pointers reach full scale, the resistor with the larger value among the shunt resistors will be blocked. If the meter coil is disconnected, the meter pointer will always point to zero.
20. What are the reasons why the mA meter pointer reaches full scale?
There are several main reasons for the four-tube full-wave rectifier X-ray machine. The first is short circuit. The second capacitor current compensation is reversely connected. The filament heating voltage of the third X-ray tube is too high. The shunt current in the fourth milliamp meter is blocked; for the self-rectifying X-ray machine, the last two are true.
21. What emergency measures should be taken when the X-ray machine continues to expose?
Exposure of the X-ray machine is not only a very dangerous failure, it can also easily exceed the X-ray machine's usage specifications and cause damage to the target surface. The main reasons for this kind of failure include: failure of the direction limiter, damage to the hand switch, sticking of the relay contacts, or dirt on the core contact surface, which causes the core to be unable to maintain its function even after the power is cut off. It can be seen that this kind of failure is very common. This kind of out-of-control phenomenon can only avoid machine damage by shutting down the machine immediately or quickly pulling down the power supply.
22. Why do the filaments of the rectifier tube and X-ray tube become dark after they are inflated?
Both the rectifier tube and the , although the heating voltage remains unchanged, the heat can be transferred to the tube wall through the convection of the gas in the tube, causing the filament temperature to decrease and become dark. During work, the vacuum degree of the tube can be judged based on the brightness of the filament. Finally, if the tube is broken in the insulating oil and the filament is immersed in the oil, although the heating current in the filament does not change (it can even increase because it is in the cold oil), there will be no light when the filament is energized. Therefore, as long as the heating is sure If it is normal but the filament does not light up, the tube is probably broken.
23. What are the power frequency requirements for X-ray machines?
The X-ray machine has very strict requirements on the power frequency, that is, the high frequency cannot exceed Hz and the low frequency cannot be less than 49 Hz, otherwise the machine will not work properly. If the frequency is too high, the core of the inductive component in the machine will heat up more, which will affect the insulation strength. For X-ray machines equipped with magnetic saturation regulators, milliamps will increase and cause overload. In severe cases, the voltage will not be able to cool down. It is easy to damage the target surface of the X-ray tube during operation; if the disaster rate is too low, the coil in the inductance element will generate more heat, which will also affect the insulation degree. For X-ray machines equipped with magnetic saturation regulators, the mA will decrease. , in severe cases, the bed pressure cannot be adjusted and the electric bed cannot be started. It can be seen that the power supply frequency requirements of X-ray machines are much stricter and narrower than those of ordinary electrical appliances. They cannot be ignored during work. Especially for units that provide their own power supply, they must ensure that the power supply frequency is within the requirements of the machine. Otherwise, not only the work may be affected, but also the power supply frequency may be affected. may damage the machine.
24. What impact does the power supply capacity have on the X-ray machine?
The capacity of the power supply is determined by the product of the rated voltage and the rated current. The medium rated current is based on the internal heat generation and heat dissipation conditions of the power supply. The factors are indeed expensive. For power transformers, the heat generation is mainly copper resistance loss and eddy current loss. In other words, the greater the copper resistance, the smaller the rated current determined under certain heat dissipation conditions, and therefore the smaller the capacity of the power supply. On the contrary, the smaller the power supply capacity, the greater the steel resistance. When the internal resistance exceeds the X-ray machine When the requirements are met, the X-ray machine cannot work normally due to the excessive internal drop in the power supply. The impact is particularly significant when working with large milliamps. For generators, when the capacity is small, the supporting power machine has less power. When working with high current, the power machine immediately decelerates due to the instantaneous load increase, thus reducing the power frequency, further affecting the normal operation of the X-ray machine. It can be seen that the larger the capacity of the transformer or generator, the more beneficial it is for the work.
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