What is MRI
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- publisher
- Evan
- Issue Time
- Apr 7,2020
Summary
Magnetic resonance imaging(MRI) is an image detection technology that uses the principle of nuclear magnetic resonance. It uses the characteristics of nuclear spin motion to generate a signal after an RF pulse in an external magnetic field. It is detected by a detector and then input into a computer. The image is displayed.
Principle of MRI
Nuclear magnetic resonance imaging, also known as magnetic resonance imaging, uses the principle of nuclear magnetic resonance to detect the electromagnetic waves emitted by an external gradient magnetic field, which can be drawn into an internal structure image of the object Has been widely used.
The application of magnetic resonance imaging technology in modern medicine is specifically to place the patient in a special magnetic field and use radio pulses to excite the hydrogen nuclei in the patient to resonate and absorb energy. After stopping the radio frequency pulse, the hydrogen nucleus in the patient will emit an electrical signal according to a certain frequency to release the energy it absorbs. The electronic receiver will capture these energy signals and then process the computer to obtain image data.
Human tissue cells contain very rich water. Different tissues have different water contents. If the distribution information can be detected, a relatively complete image of the internal structure of the human body can be drawn. The MRI technology is through Identify the distribution of hydrogen atom signals in water molecules, speculate the distribution of water molecules in the human body, and then detect the internal structure of the human body.
As early as 1946, two research groups led by Edward Purcell of Harvard University and Felix Block of Stanford University discovered the phenomenon of nuclear magnetic resonance of matter.
In 1967, Jasper Jackson first measured signals from live animals, making it possible for the NMR method to be used in human measurements.
In 1971, Professor R. Damadian of the State University of New York in the United States used a nuclear magnetic resonance spectrometer to study the nuclear magnetic resonance characteristics of normal and cancerous tissue samples in rats and found that the T1 value of water protons in normal and cancerous tissues was significantly different .
In 1972, Paul C. Lauterbur of the State University of New York at Stony Brook made the first two-dimensional image using water as a sample, showing the possibility of MRI CT.
In 1978, the image quality of MRI had reached the initial level of X-ray CT, and human trials were conducted in the hospital. And finally named magnetic resonance imaging (MRI).
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Type
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Description
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Advantages
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Disadvantages
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Permanent magnet type
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The main materials are aluminum nickel cobalt, ferrite and rare earth cobalt, which are composed of multiple pieces of material. The arrangement of the magnet block must not only meet the requirements of forming a certain imaging space, but also make its magnetic field uniformity as high as possible.
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low cost, field strength can reach 0.3T, low power consumption, low maintenance cost, can be installed in a relatively small room, open for intervention.
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Low magnetic field strength, poor uniformity, and susceptibility to external factors (especially temperature).
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Normally conductive type
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The normally-conducting magnet is designed according to the principle that the current generates a magnetic field, and its coil is usually wound with copper wire. Because copper has a certain resistivity, the magnet made of this kind of coil is called an impedance magnet.
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Simple structure and low cost, easy to manufacture and install, can reduce the radius or increase the coil current to improve the field strength of the magnet.
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The power consumption is large, the stability is poor, a perfect circulating water cooling device is needed, the uniformity is poor, and it is greatly affected by the environment.
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Superconducting type | Superconducting magnets make use of the characteristics of superconductors with zero resistance at absolute zero degrees, and generate strong magnetic fields through powerful currents on very small wires. | High field strength, good stability and uniformity, magnetic field strength can be adjusted, and the magnetic field can be turned off if necessary. | The superconducting coil must be immersed in the sealed liquid nitrogen dewar to work, the technology is complex, and the cost is high. |
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Hybrid type
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A magnet constructed using two or more magnet technologies. Common combinations of permanent magnet and normally conductive magnets.
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It can produce higher field strength, overcoming the disadvantages of permanent magnets being unstable, bulky and normally conducting magnets with large power consumption.
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The structure is complicated, low-temperature containers need to be installed, and the cost is also high.
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Multi-parameter imaging can provide rich diagnostic information
High-contrast imaging for detailed anatomical atlas
Arbitrary orientation faults make it possible for the medical community to observe the human body in three dimensions
Research on human energy metabolism, it is possible to directly observe the biochemical blueprint of cell activity
Not suitable for contrast agents, can observe the structure of the heart and blood vessels
No ionizing radiation, intervention treatment under certain conditions
No bone artifact interference, clear posterior fossa lesion