Differences between high-frequency and power-frequency machines

  The power frequency machine is a traditional X-ray equipment that uses 50Hz AC as the power supply of High Voltage Generators. We know that the composition of X-ray machine (or X-ray equipment) can be roughly divided into two parts: X-ray Generator (X-ray source) and auxiliary equipment. The following discussion will confuse the x-ray machine with the X-ray generator without distinction.

     High-voltage power supplies of power frequency X-ray machines are generally in the form of single-phase self-rectification, single-phase half-wave rectification, single-phase full-wave rectification, three-phase full-wave rectification, three-phase twelve pulse (12 beats), etc. The voltage pulsation coefficient (ripple coefficient) applied on both ends of X-ray tubes varies greatly depending on the form of rectification. Among them, the single-phase half-wave rectification (including self-rectification) output of the Maximum voltage ripple coefficient (100%), while 12 beats minimum (3.4% ).

We know that if the voltage applied to both ends of the X-ray tube changes continuously from zero to the maximum value over time, then in a change cycle, only a small segment of voltage value near the peak voltage can be used to accelerate the free electrons emitted by the X-ray tube lamp system to obtain the energy (kVp) We have preset on the machine ), only in this way can the X-ray photon be useful for photography. The X-ray photon energy generated in the rest of the time is much lower than the preset energy. Such X-ray photon is useless for our photography purposes, simply increasing the patient's absorption dose.

     The working process of the high-frequency X-ray generator is as follows: after the AC Power Supply of 50Hz is rectified by the diode and the capacitor is smoothed to a constant DC voltage, the inverter is converted to a square wave voltage with a certain frequency, after the square wave voltage is increased by the transformer, and then rectified and filtered, it becomes a smooth DC high voltage and is applied to both ends of the X-ray tube. At the same time, the sampling circuit samples the pipe voltage and sends the sample value to the comparator to compare with the preset tube voltage value. If the sample value is lower than the preset value, the regulator sends an adjustment signal to increase the frequency of the inverter, or increase the pulse width of the inverter output until the actual pipe voltage sample value is equal to the preset value. If the sampling value is higher than the preset value, the regulating circuit reduces the inverter's working frequency or reduces the pulse width until the sampling value is equal to the preset value. Similarly, the pipe current is adjusted and controlled in the same way.
Next we will talk about the differences between power frequency X-ray machine and high frequency X-ray machine from different perspectives.

1,Accuracy of exposure parameter (Kv, Ma, time, or MAS) ControlThe exposure control accuracy of the power frequency X-ray machine varies greatly depending on the rectifying method. The middle (high) frequency X-ray generator uses voltage preset, feedback adjustment method, and high control accuracy. In this way, the soft radiation components in the X-ray photon bundle can be greatly reduced, the patient's radiation dose can be reduced, the exposure repeatability can be improved, and the quality of X-ray photos can be improved. Secondly, the output efficiency of X-ray tubes is also improved. This can be intuitively seen from the above;

2,Simple StructureBecause the kV and Ma values of the mid-and high-frequency X-ray generators are preset in pure electronic mode and the feedback is adjusted separately, there is no mechanical knob or switch, the KV compensation circuit and space charge compensation circuit are omitted;

3. Easy computer control and high automationThe middle-and high-frequency X-ray generators adopt pure electronic control and adjustment methods, and there is no mechanical servo mechanism, which facilitates computer control and improves automation;

4. Because there is no mechanical contact or knob, the response speed is fast and the output parameter accuracy is high, and Kv and MA can be adjusted during the exposure;
5. Because of the use of energy storage components, the power supply requirements for power supply are greatly reduced, and even direct current power supply can be used (large power frequency X-ray inspection equipment often require leased line power supply );

6. Due to high frequency, according to the transformer equation, the volume of high-voltage transformer can be greatly reduced, facilitating the miniaturization or miniaturization of high-power (large-capacity) X-ray generators;
7. Because the transformer is small in size, it not only saves the core and copper wire required for making the high-voltage generator, but also reduces the copper and iron consumption of the high-voltage generator.
  Because the single-phase power frequency X-ray generator produces many soft-ray components, the patient's absorption dose is high, and the photo quality is difficult to guarantee, production has been banned by developed countries such as Europe and America. However, the power frequency three-phase twelve-pulse X-ray generator is bulky, complex in structure, high requirement on power supply, low output parameter control accuracy, and low degree of automation, well-known international manufacturers no longer produce such X-ray generators.

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High-frequency X-ray machine

 

I. R & D Background:

X-ray machine is mainly used in the medical field and is one of the most basic diagnostic devices in hospitals. At present, medical Xray machine has been widely used in clinical applications and has become the main Medical Device for imaging diagnosis and treatment. However, the widely used power frequency X-ray machine in China has the disadvantages of poor image quality, low exposure accuracy, low automation, low protection level, and large size and heavy structure. Its main source is the power frequency X-ray generator. In some western developed countries, the power frequency X-ray machine is prohibited from use. High Frequency X-ray machines are used in China. The usage of medium-and high-frequency X-ray machines is less than 3%, which basically depends on imports. In response to this situation, the State attaches great importance to the technical development of high-frequency generators. During the "", "" and "", the state has included the development of high-frequency Xray machines in key scientific and technological projects. The relevant organizations have also made some achievements, but compared with foreign countries, there is still a big gap in the development technology level of high-frequency Xray machines in China. Therefore, the development of high-frequency X-ray equipment is of great significance to promote the development of medical imaging diagnostic equipment in China.

Ii. Differences between high-frequency Xray machine and power frequency machine:

A high frequency generator is an X-ray machine with a working frequency greater than 20 kHz. A power frequency generator is an X-ray machine with a working frequency less than Hz. The power frequency machine converts a 50Hz Power Frequency Electrical Source into a high-voltage rectification, and then receives a 10% Hz positive string ripple. After filtering, there are still more than lines of Ripple. The working frequency of the high frequency machine is high, the voltage after high-voltage rectification is basically a constant DC, and the ripple can be less than 0.1%. Different high voltage values correspond to different energy electron beam, which produces X-rays of different wavelengths. The simpler the X-ray spectrum, the less scattering, And the clearer the imaging. The X-ray spectrum generated by the power frequency machine is complex. The number of X-rays at the same frequency is small, the number of stray rays is large, and the imaging is blurred. The X-ray spectrum of the high-frequency machine is simple, with less stray rays and clear imaging. This reduces the total X-ray output by more than 50% compared with the power frequency machine. Since 1990s, all imported Xray machines are high-frequency machines. This is an international development direction, and China is currently developing towards high-frequency machines.

The reconstruction of the old Xray machine has the characteristics of low X-ray radiation dose and clear image, which reduces the photographic conditions and prolongs the service life of X-ray tubes. Easy to operate and highly intelligent. A High Level A film rate increases the economic efficiency of hospitals. The CR effect is derived from the film shot by a high-frequency machine and conforms to the high-frequency Development Trend of the X-ray machine. Flexible digital interfaces enable users to upgrade in the future. The Rebuild of the old Xray machine provides automatic fault detection and display functions, fast after-sales service, and low maintenance costs.

Iii. advantages of high-frequency Xray machine:

In principle, the high-frequency high-voltage generator produces a small voltage ripple that is loaded on the X-ray tube anode, resulting in a stable X-ray (current) and less soft radiation on the scattering line, therefore, high-frequency Xray machines should have the following advantages:

1. The output dose is high, which is 50% higher than that of the power frequency X-ray machine. The photo conditions (Kv and MAS) are low.

2. Stable X-ray output and high repeatability. Improve image quality.

3. High quality X-rays can be obtained, and the same spectrum of rays will be monochrome to avoid the damage of soft rays to the human body. At the same time, the scattering line is reduced to make the image clearer;

4. Compact and small-sized structure, reducing the volume of the console and high-voltage generator.

5. Low requirements on power supply conditions.

6. It is conducive to intelligent development and creates conditions for the digitization of X-ray technology.

 

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Comparison Analysis 

 

	High FrequencyXMachine	Traditional Power FrequencyXMachine
Shape and size	Small	Very large
Stability	Adjustable and stable	Pre-adjustment, unstable
Active ingredient	High	Low
Imaging Quality	High	Low
Skin dose	Medium	Large
Environmental Pollution	Low	High
Repeatability	<0.02	<0.05
Minimum exposure time	0.001 seconds	0.02 seconds

 

 

High-frequency Xray machines have the following features:

1.1   Environmental protection

Because the output high-voltage pulsation is very small and similar to DC, the output X-Ray has a good color, the scattering line is reduced, and the radiation dose absorbed by the patient is greatly reduced;

1.2   High Output Efficiency

For example, in a single-phase power frequency machine, the duration of a high-voltage pulse waveform is 10 ms, and the duration of a peak value greater than 0.707 times is about 5 ms, while that of a high-frequency machine is similar to DC, therefore, if the high-frequency machine needs to be exposed for a dose of 10 ms (equivalent to half a cycle of high-voltage power frequency pulse), the full-wave rectifying power frequency machine must be exposed for at least 20 ms (one cycle is required ). The exposure time is doubled, which will inevitably increase the dynamic blur of the image, seriously affecting the imaging quality;

1.3   Closed-loop feedback control and real-time adjustment using electronic methods

Since there is no mechanical contact or knob, the adjustment speed is fast. Furthermore, the method of independent preset and feedback adjustment of tube voltage and tube current not only saves the space charge compensation circuit of Kv compensation and tube current, but also improves the control accuracy. KV and MA can be adjusted separately even during the exposure period.

1.4   Stable Output and good repeatability

In high-frequency machines, the set values of the kV and Ma closed-loop control circuits can be very precise, and the detection circuits can also be very stable. Therefore, no matter how many factors affect kV and ma, as long as the changes are within a certain allowable range, the output of each exposure of the high-frequency machine can be consistent;

1.5   Shorter exposure time

The shortest exposure time of the X-ray machine is determined by the high-pressure rising edge. The rising edge of the high-voltage waveform of the high-frequency machine is steep, generally dozens to dozens of μs, so the shortest exposure time can reach 0.01 s. However, the high-voltage waveform of the Power Frequency machine changes slowly according to the sine wave, for example, in a single-phase power frequency machine, one cycle wave is 10 ms, while the effective voltage (greater than 0.707kvp) accounts for only 5 ms. Each exposure requires at least some effective voltage; otherwise, such an exposure is meaningless. According to the calculation, the minimum effective exposure of the Power Frequency machine should be greater than 3 ms.

1.6   The volume can be greatly reduced

According to the transformer equation, V = f × n ×. When the output voltage of the transformer is certain and the input voltage frequency f is increased, the area A of the transformer core and the turns N of the transformer winding can be reduced. The input frequency of the high-frequency high voltage generator is generally greater than 20 kHz, which is far greater than 50Hz power frequency. Therefore, the transformer volume is greatly reduced. The reduction not only saves a lot of copper wires and cores, but also greatly reduces copper and iron consumption. Secondly, it can enable the integration of the original split-type cabinet and head, so that large-capacity X-rays can be miniaturized or miniaturized, which is conducive to the X-ray inspection under the conditions of war and disaster recovery.

1.7   Intelligent

As high-frequency machines have all been electronically implemented, it is easy to introduce the microcomputer technology. The Application of Microcomputer will enable various performance of The Xray machine, such as landing load, automatic exposure, real-time control and monitoring display, fault alarm, automatic processing, data storage, and real-time control, it has been upgraded to a brand new level, creating conditions for X-ray digitization and intelligence.

1.8   Energy Saving

Because the energy storage devices are used in the input circuit, the power supply requirements for the power supply are greatly reduced, energy consumption is reduced, and DC power supply can be used directly, solving the problem of power supply conditions. This is of special significance for scenarios that lack AC power, such as remote areas, disaster relief and field conditions.