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Excuse me, what is MRI?

MRI

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Human Brain Longitudinal section Nuclear Magnetic Resonance Imaging (NMRI), also known as spin imaging, also known as Magnetic Resonance Imaging (MRI for short) uses the principle of nuclear magnetic resonance (NMR) to detect the emitted electromagnetic waves by applying an external gradient magnetic field based on the different attenuation of the released energy in different structural environments inside the material. The position and type of the atomic nuclei that make up the object can be known, and an image of the internal structure of the object can be drawn based on this.

Using this technology to image the internal structures of the human body creates a revolutionary medical diagnostic tool. The application of rapidly changing gradient magnetic fields has greatly accelerated the speed of MRI, making the application of this technology in clinical diagnosis and scientific research a reality, and greatly promoted the rapid development of medicine, neurophysiology and cognitive neuroscience. .

In the decades from the discovery of nuclear magnetic resonance phenomena to the maturity of MRI technology, the research field related to nuclear magnetic resonance has been achieved in three fields (physics, chemistry, physiology or medicine) He has won 6 Nobel Prizes, which is enough to illustrate the importance of this field and its derived technologies.

Table of Contents[Hide]

1 Physical Principles

1.1 Overview of Principles

1.2 Mathematical Operations

2 System Composition

2.1 NMR experimental device

2.2 Composition of MRI system

2.2.1 Magnet system

2.2.2 Radio frequency system

2.2.3 Computer image reconstruction system

2.3 Basic methods of MRI

3 Technical applications

3.1 Application of MRI in medicine

3.1.1 Overview of principles

3.1.2 Advantages of magnetic resonance imaging

3.1.3 Disadvantages and possible harms of MRI

3.2 Application of MRI in the field of chemistry

3.3 Other advances in magnetic resonance imaging

4 Contributions of Nobel laureates

5 The future Outlook

6 Related items

6.1 Magnetization preparation

6.2 Imaging method

6.3 Medical physiological applications

7 References

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Physical Principles

Animation of a continuous slice obtained by scanning the human brain through an MRI, by Start at the top of the head and work your way to the base. [edit]

Principle Overview

Nuclear magnetic resonance imaging is a biomagnetic nuclear imaging that has developed rapidly with the development of computer technology, electronic circuit technology, and superconductor technology. Spin imaging technology. Taking into account patients' fear of "nuclear", doctors often call this technology magnetic resonance imaging. It uses magnetic fields and radio frequency pulses to nutate precessing hydrogen nuclei (i.e. H+) in human tissues to generate radio frequency signals, which are imaged through computer processing.

When the atomic nucleus precesses, it absorbs radio frequency pulses with the same frequency as the precession frequency of the atomic nucleus, that is, the frequency of the external alternating magnetic field is equal to the Larmor frequency, and the atomic nucleus undergoes vibrational absorption. After the radio frequency pulse is removed, The magnetic moment of the atomic nucleus emits part of the absorbed energy in the form of electromagnetic waves, which is called vibrational emission. The process of electromagnetic absorption and electromagnetic emission is called "nuclear magnetic resonance".

The "nucleus" of MRI refers to the nucleus of hydrogen atoms, because about 70% of the human body is composed of water, and MRI relies on hydrogen atoms in water. When an object is placed in a magnetic field, irradiated with appropriate electromagnetic waves to make it vibrate, and then the electromagnetic waves released by it are analyzed, the position and type of the atomic nuclei that make up the object can be known, and the object can be drawn based on this. Precise stereoscopic images of the interior.

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Mathematical operations

The atomic nucleus is positively charged and has spin motion. Its spin motion will inevitably produce a magnetic moment, which is called nuclear magnetic moment. . Research shows that the nuclear magnetic moment μ is proportional to the spin angular momentum S of the nucleus, that is,

where γ is the proportional coefficient, which is called the gyromagnetic ratio of the nucleus. In an external magnetic field, the spatial orientation of the nuclear spin angular momentum is quantized, and its projection value in the direction of the external magnetic field can be expressed as

m is the nuclear spin quantum number. According to the relationship between nuclear magnetic moment and spin angular momentum, the orientation of the nuclear magnetic moment in the external magnetic field is also quantized, and its projection value in the direction of the magnetic field is

For different nuclei, m takes an integer or a half number respectively. integer.

In an external magnetic field, the atomic nucleus with a magnetic moment has a corresponding energy, and its value can be expressed as

Where B is the magnetic induction intensity. It can be seen that the energy of the atomic nucleus in the external magnetic field is also quantized. Due to the interaction between the magnetic moment and the magnetic field, the spin energy is split into a series of discrete energy levels, and the difference between two adjacent energy levels is ΔE = γhB. Irradiate the atomic nucleus with electromagnetic radiation of appropriate frequency. If the electromagnetic radiation photon energy hν is exactly the difference ΔE between two adjacent nuclear energy levels, the atomic nucleus will absorb this photon. The frequency condition for nuclear magnetic resonance to occur is:

< p> where ν is the frequency and ω is the angular frequency. For a certain nucleus, the gyromagnetic ratio γ can be determined accurately. It can be seen that by measuring the frequency ν of the radiation field during nuclear magnetic resonance, the magnetic induction intensity can be determined; conversely, if the magnetic induction intensity is known, the nuclear vibration frequency can be determined.

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System composition

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NMR experimental device

Adopt frequency-adjusting method to achieve NMR. The coil emits electromagnetic waves to the sample, and the function of the modulated oscillator is to continuously change the frequency of the radiofrequency electromagnetic waves near the maximum oscillation frequency of the sample. When the frequency coincides with the nuclear magnetic resonance frequency, an absorption peak will appear in the output of the radio frequency oscillator, which can be displayed on an oscilloscope, and the frequency of the nuclear magnetic resonance at this time can be read immediately by the frequency meter. The NMR spectrometer is an instrument specially used for observing NMR. It mainly consists of three parts: magnet, probe and spectrometer. The function of the magnet is to generate a constant magnetic field; the probe is placed between the magnetic poles to detect nuclear magnetic resonance signals; the spectrometer amplifies, displays and records the nuclear magnetic resonance signals.

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Composition of MRI system

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Magnet system

Static magnetic field: Superconducting magnets currently used clinically have magnetic field strengths ranging from 0.5 to 4.0T, with 1.5T and 3.0T being common. There are also shim coils to help achieve high uniformity.

Gradient field: used to generate and control the gradient in the magnetic field to achieve spatial encoding of NMR signals. This system has three groups of coils, which generate gradient fields in the three directions of x, y, and z. The magnetic fields of the coil groups are superimposed to obtain a gradient field in any direction.

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Radio frequency system

Radio frequency (RF) generator: generates a short and strong radio frequency field, which is added to the sample in a pulse manner to cause The hydrogen nuclei in the sample produce NMR phenomena.

Radio frequency (RF) receiver: receives the NMR signal, amplifies it and enters the image processing system.

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Computer image reconstruction system

The signal sent from the radio frequency receiver passes through the A/D converter to convert the analog signal into a mathematical signal , based on the corresponding relationship with each voxel of the observation layer, the layer image data is obtained through computer processing, and then added to the image display through a D/A converter. According to the size of the NMR, the desired image is displayed with different gray levels. Observational level images.

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Basic methods of MRI

Slice gradient field Gz

Phase encoding and frequency encoding

< p>Image reconstruction

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Technical application

3D MRI[edit]

MRI applications in medicine

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Principle Overview

Hydrogen nuclei are the first choice for human body imaging: various tissues of the human body contain large amounts of water and hydrocarbons, so hydrogen nuclei The NMR has high flexibility and strong signal, which is why people prefer hydrogen nuclei as human body imaging elements. The NMR signal intensity is related to the density of hydrogen nuclei in the sample. If the water content ratio of various tissues in the human body is different, that is, the number of hydrogen nuclei is different, the NMR signal intensity will be different. This difference is used as a characteristic quantity to separate various tissues. , this is the NMR image of hydrogen nuclear density. The differences in hydrogen nuclear density and relaxation time T1 and T2 between different tissues of the human body and between normal tissues and diseased tissues are the most important physical basis for MRI to be used in clinical diagnosis.

When a radio frequency pulse signal is applied, the energy state of the hydrogen nucleus changes. After the radio frequency passes, the hydrogen nucleus returns to the initial energy state, and the electromagnetic waves generated by the oscillation are emitted. Small differences in the vibration of atomic nuclei can be accurately detected. After further computer processing, it is possible to obtain a three-dimensional image reflecting the chemical structure of the tissue, from which we can obtain information including water differences in the tissue and the movement of water molecules. In this way, pathological changes can be recorded.

2/3 of the weight of the human body is water. Such a high proportion is the basis for magnetic resonance imaging technology to be widely used in medical diagnosis. The water in organs and tissues in the human body is not the same. The pathological processes of many diseases will cause changes in the shape of water, which can be reflected by magnetic resonance images.

The images obtained by MRI are very clear and precise, which greatly improves the doctor's diagnostic efficiency and avoids the need for thoracotomy or laparotomy for exploratory diagnosis.

Because MRI does not use X-rays that are harmful to the human body or contrast agents that can easily cause allergic reactions, there is no harm to the human body. MRI can image various parts of the human body from multiple angles and in multiple planes. It has high resolution, can more objectively and specifically display the anatomical tissues and adjacent relationships in the human body, and can better locate and characterize lesions. It is of great value in the diagnosis of diseases in various systems of the body, especially in the diagnosis of early tumors.

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The advantages of magnetic resonance imaging

Compared with ordinary X-rays that won the Nobel Prize in Physics in 1901 or the Nobel Prize in Physics in 1979 Compared with computerized tomography (CT), which won the Bell Medicine Prize, the biggest advantage of magnetic resonance imaging is that it is one of the few safe, fast, and accurate clinical diagnostic methods that does no harm to the human body. Today, at least 60 million cases worldwide are examined using MRI technology every year. Specifically, there are the following points:

No ionizing radiation damage to the human body;

Various parameters can be used for imaging, and multiple imaging parameters can provide rich diagnostic information. It makes medical diagnosis and research on metabolism and functions in the human body convenient and effective. For example, the T1 value of hepatitis and cirrhosis becomes larger, while the T1 value of liver cancer is even larger. T1-weighted images can distinguish benign tumors from malignant tumors in the liver;

The desired section can be freely selected by adjusting the magnetic field. . It can obtain images of parts that are inaccessible or difficult to access with other imaging technologies. For the intervertebral disc and spinal cord, sagittal, coronal, and cross-sectional imaging can be performed, and nerve roots, spinal cord, ganglia, etc. can be seen. It can obtain three-dimensional images of the brain and spinal cord, unlike CT (which can only obtain cross-sections perpendicular to the long axis of the human body) which scans layer by layer and may miss the lesion;

Can diagnose the heart For lesions, CT is incompetent due to its slow scanning speed;

It has excellent resolution of soft tissue. The examination of bladder, rectum, uterus, vagina, bones, joints, muscles and other parts is better than CT;

In principle, all nuclear elements with non-zero spin can be used for imaging, such as hydrogen (1H ), carbon (13C), nitrogen (14N and 15N), phosphorus (31P), etc.

Magnetic resonance imaging of coronal section of human abdomen[edit]

Disadvantages and possible harms of MRI

Although MRI is not fatal to patients Damage, but still caused some discomfort to the patient. Necessary measures should be taken before MRI diagnosis to minimize this negative impact. Its main disadvantages are:

Like CT, MRI is also an anatomical imaging diagnosis. Many lesions are still difficult to diagnose by MRI examination alone. Unlike endoscopy, both imaging and pathology can be obtained at the same time. diagnosis;

The examination of the lungs is no better than X-ray or CT examination, and the examination of the liver, pancreas, adrenal glands, and prostate is no better than CT examination, but the cost is much higher;

< p>It is not as good as endoscopy for gastrointestinal lesions;

The scanning time is long and the spatial resolution is not ideal;

Due to the strong magnetic field, MRI is not suitable for detecting magnetic fields such as those in the body. It is not applicable to special patients with metal or pacemakers.

The factors that may cause harm to the human body from the MRI system mainly include the following aspects:

Strong static magnetic field: In the presence of ferromagnetic substances, whether it is implanted in the patient's body or Within the scope of magnetic fields, they may be dangerous factors;

Gradient fields that change with time: can induce an electric field in the subject's body to excite nerves or muscles. Peripheral nerve excitation is the upper limit indicator of gradient field safety. At sufficient intensity, it can produce peripheral nerve excitement (such as tingling or percussion sensation), and even cause cardiac excitement or ventricular fibrillation;

Thermal effect of radiofrequency field (RF): focused or measured in MRI The large-angle radio frequency field used in the process is emitted, and its electromagnetic energy is converted into heat energy in the patient's tissue, causing the tissue temperature to increase. The heating effect of RF needs to be further explored. Clinical scanners have so-called "specific absorption rate (SAR)" limitations on radiofrequency energy;

Noise: Various noises generated during MRI operation , which may damage the hearing of some patients;

Toxic and side effects of contrast agents: The contrast agents currently used are mainly gadolinium-containing compounds, and the incidence of side effects is 2%-4%.

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Application of MRI in the field of chemistry

The application of MRI in the field of chemistry is not as extensive as that in the medical field, mainly because of technical difficulties and Difficulties in imaging materials are currently mainly used in the following areas:

In the field of polymer chemistry, such as research on carbon fiber reinforced epoxy resin, research on the spatial orientation of solid-state reactions, and solvents in polymers Diffusion research, polymer vulcanization and elastomer uniformity research, etc.;

In cermets, blisters present in ceramic products are detected through the study of porous structures;

In rocket fuel, it is used to detect defects in solid fuel and the distribution of fillers, plasticizers and propellants;

In petrochemistry, it mainly focuses on studying the distribution of fluids in rocks. and flowability as well as studies on reservoir characterization and enhanced oil recovery mechanisms.

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Other progress in magnetic resonance imaging

NMR analysis technology is based on the characteristics of NMR spectral lines Measurement of parameters (such as spectral line width, spectral line profile shape, spectral line area, spectral line position, etc.) to analyze the molecular structure and properties of substances. It does not destroy the internal structure of the sample being tested and is a completely non-destructive testing method. At the same time, it has very high resolving power and accuracy, and more nuclei can be used for measurement, all of which are superior to other measurement methods. Therefore, NMR technology has been widely used in physics, chemistry, medicine, petrochemical industry, archaeology, etc.

Magnetic resonance microscopy (MRM/μMRI) is a slightly later developed technology in MRI technology. The highest spatial resolution of MRM is 4μm, which is close to that of general optical microscope images. level. MRM has been very commonly used as an animal model for disease and drug research.

In vivo MR spectroscopy (MRS) can measure the NMR spectrum of a specified part of an animal or human body, thereby directly identifying and analyzing the chemical components.

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Contributions of Nobel Laureates

On October 6, 2003, the Karolinska Institute in Sweden announced that the 2003 Nobel Prize The Bell Prize in Physiology or Medicine is awarded to American chemist Paul C. Lauterbur and British physicist Peter Mansfield for their use of nuclear magnetism in the field of medical diagnosis and research* **A breakthrough achievement in the field of vibration imaging technology.

Lauterbull's contribution was to add an inhomogeneous magnetic field to the main magnetic field and introduce the gradient into the magnetic field, thereby creating a visible interior of matter that cannot be seen using other technical means. 2D structural image of the structure. He described how a gradient magnet could be added to the main magnet and a cross-section of a test tube filled with ordinary water immersed in heavy water could be seen. No other imaging technique can distinguish images between ordinary water and heavy water. By introducing a gradient magnetic field, the frequency of nuclear magnetic resonance electromagnetic waves can be changed point by point. By analyzing the emitted electromagnetic waves, the source of the signal can be determined.

Mansfield further developed the theory regarding the use of additional gradient magnetic fields in a stable magnetic field, promoting its practical application. He discovered the mathematical analysis method of magnetic resonance signals, which laid the foundation for the method to move from theory to application. This makes magnetic resonance imaging a realistic and feasible method for clinical diagnosis 10 years later. He used gradients in magnetic fields to show more precisely the differences in vibrations. He demonstrated how to efficiently and quickly analyze detected signals and convert them into images. Mansfield also proposed that extremely rapid gradient changes can obtain fleeting images, that is, echo-planar imaging (EPI) technology, which became a functional MRI that began to flourish in the 1990s. The main means of imaging (functional MRI, fMRI) research.

Raymond Damatian's "Apparatus and Methods for Cancer Tissue Detection" is worth mentioning that the 2003 Nobel Prize winners in physics made great contributions to the theory of superconductors and superfluids. His pioneering contributions provided a theoretical basis for the development of MRI scanners by two scientists who won the 2003 Nobel Prize in Physiology or Medicine, paving the way for MRI technology. Thanks to their theoretical work, breakthroughs were made in MRI technology, making high-definition images of the human body's internal organs possible.

In addition, in the "New York Times" and "Washington Post" on October 10, 2003, a full-page advertisement by Fonar Company appeared at the same time: "Raymond... Raymond Damadian, should share the 2003 Nobel Prize in Physiology or Medicine with Peter Mansfield and Paul Lauterbull. Without him, there would be no Nobel Prize in MRI technology. The committee caused widespread controversy for "falsifying history."

In fact, the issue of the ownership of the invention rights of MRI has been debated for many years, and the dispute is quite fierce. In the eyes of academic circles, Damatian is more of a businessman than a scientist.

[Editor]

Future Outlook

How the human brain thinks has always been a mystery. And it is an important topic that scientists pay attention to. Functional brain imaging using MRI helps us study human thinking at the living and holistic levels. Among them, the study on whether blind children's hands can replace their eyes is a good example. Normal people can see the blue sky and clear water, and then form images and artistic conceptions in their brains. However, a blind child who has never seen the world can touch words with his hands. The words tell him about the world. Can the blind child also "see" it? Woolen cloth? Experts used functional MRI to scan the brains of normal and blind children and found that blind children also have good activation areas in the visual cortex of the brain just like normal people. From this, we can preliminarily conclude that through cognitive education, blind children can "see" the outside world with their hands instead of their eyes.

The research and application of fast scanning technology will shorten the time for scanning patients with classic MRI imaging methods from several minutes or more to a few milliseconds, making the impact of organ motion on the image negligible; MRI Blood flow imaging uses the flow void effect to clearly present the shape of blood vessels on MRI images, making it possible to measure the flow direction and velocity of blood in blood vessels; MRI spectrum analysis can use high magnetic fields to achieve spectrum analysis technology of local tissues of the human body, thereby Increase information to help diagnosis; functional brain imaging, using high magnetic field MRI to study brain function and its mechanisms is the most important topic in brain science. There is reason to believe that MRI will develop into a mind reader.

From the mid-20th century to the present, information technology and life sciences are the two most actively developing fields. Experts believe that MRI technology, as a combination of the two, will continue to develop towards microscopic and functional examinations, which will be helpful in revealing The mysteries of life will play a greater role.

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