What is the difference between CT and MRI?

Let me say a few words first. CT imaging is based on X-rays and uses computer technology to make overlapping X-rays scan clearly plane by plane. Magnetic * * * vibration is the signal that the nucleus vibrates in a strong magnetic field, and then it can be imaged on all planes of the human body. To put it bluntly, its imaging is related to the number of protons in the scanned part. Their differences are mainly in principle, equipment and imaging characteristics.

The basic principle of CT 1 CT imaging process

X-ray imaging is based on the principle that the human body selectively absorbs X-rays. When X-rays pass through the human body, images of tissues and organs are formed on the fluorescent screen or film, and CT imaging is similar.

The process of CT scanning is a process of 360-degree cross-sectional scanning around the examination part of the human body through a highly collimated X-ray beam. When the bed translates, X-rays irradiate the patient from different directions, and the X-ray beam passing through the human body is attenuated because some photons are absorbed by the human body. Unabsorbed photons pass through the human body, are collimated and received by the detector. The detector receives X-rays with different intensities after passing through human body, and converts them into its own signals, which are collected by DAS. A large number of received analog signal information is converted into digital signals by an analog-to-digital (A/D) converter, and input to a computer for processing and operation. After preliminary processing, it becomes the collected original data, which is called 6lteredrawdata after being curled and filtered. The digital-to-analog (D/A) converter is used to display on the display screen through different gray levels, so as to obtain the anatomical structure image of the cross section of this part, that is, the CT cross section image.

Therefore, the digital images obtained by CT examination reflect the distribution of human tissue structure, fundamentally overcome the defect of overlapping images of conventional X-ray examination, and make a qualitative leap in medical image diagnosis.

Second, the basic principle of CT imaging

Usually, the intensity of the radiation signal received by the detector depends on the tissue density of the human body on the cross section of this part. High-density tissues, such as bones, absorb more X-rays and the signals received by detectors are weak. Low-density tissues, such as fat and hollow organs, absorb less X-rays and the detector gets stronger signals. This property that different tissues have different X-ray absorption values can be expressed by the absorption coefficient μ of tissues, so the signal intensity received by the detector reflects the different μ values of human tissues. And CT is based on the attenuation characteristics of X-rays after penetrating human body.

The attenuation of X-rays after penetrating human body follows the exponential attenuation law I = i0e-μ d. ..

Where: I is the X-ray intensity attenuated after absorption by human body; I0 is the incident X-ray intensity; μ is the linear absorption coefficient of X-ray irradiated tissue; D is the thickness of the human tissue at the site to be examined.

The absorption coefficient of human tissue is listed by computer operation, which is distributed in the grid array of the synthetic image, that is, the square (element) of the matrix. Each array element on the matrix is equivalent to a pixel on the reconstructed image, which is called a pixel. The imaging process of CT is the process of finding the attenuation coefficient of each pixel. If the pixels are smaller and the number of detectors is more, the attenuation coefficient measured by computer will be more and more accurate and the reconstructed image will be clearer. At present, the matrices of CT machines are mostly 256×256, 565, 438+02× 565, 438+02, and the product is the number of pixels contained in each matrix.

MRI

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Magnetic resonance imaging (NMRI), also known as spin imaging, also known as Magnetic Resonance Imaging and MRI, is based on the principle of nuclear magnetic resonance (NMR). According to the different attenuation of energy released by different structural environments in the material, the emitted electromagnetic wave can be detected by applying gradient magnetic field, so that the position and type of the nucleus that constitutes this object can be known, and the structural image inside the object can be drawn accordingly.

Using this technology to image the internal structure of human body will produce a revolutionary medical diagnostic tool. The application of rapidly changing gradient magnetic field has greatly accelerated the speed of magnetic resonance imaging, made 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.

From the discovery of nuclear magnetic resonance phenomenon to the maturity of nuclear magnetic resonance technology, the research field of nuclear magnetic resonance has won six Nobel Prizes in three fields (physics, chemistry, physiology or medicine), which is enough to show the importance of this field and its derivative technologies.

Directory [hidden]

The physical principle of 1

1. 1 principle overview

1.2 mathematical operation

2 system composition

2. 1 nuclear magnetic resonance experimental device

2.2 Composition of Magnetic Resonance Imaging System

1 magnet system

Radio frequency system

2.2.3 Computer image reconstruction system

2.3 Basic methods of magnetic resonance imaging

3 technology application

3. 1 application of magnetic resonance imaging in medicine

3. 1. 1 principle overview

3. 1.2 Advantages of magnetic vibration imaging

3.1.3 Shortcomings and possible hazards of MRI

3.2 Application of MRI in the field of chemistry

3.3 Other Progress of Magnetic Vibration Imaging

Contributions of Four Nobel Prize Winners

5 Future prospect

6 related projects

6. 1 magnetization preparation

6.2 Image shooting method

6.3 Application of Medical Physiology

7 references

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Physical principle

The animation of continuous slices obtained by scanning the human brain with magnetic resonance imaging, starting from the top of the head to the bottom. [edit]

Overview of principle

Magnetic resonance imaging (MRI) is a kind of biomagnetism nuclear spin imaging technology developed rapidly with the development of computer technology, electronic circuit technology and superconducting technology. Considering the patient's fear of "nuclear", doctors usually call this technique magnetic resonance imaging. It uses magnetic field and RF pulse to nutate precession hydrogen nucleus (H+) in human tissue to generate RF signal, which is processed by computer and imaged.

When the nucleus precesses, it absorbs the RF pulse with the same precession frequency as the nucleus, that is, the frequency of the external alternating magnetic field is equal to the Mora frequency, and the nucleus will be absorbed by vibration. After removing the RF pulse, the nuclear magnetic moment will emit some absorbed energy in the form of electromagnetic waves, which is called * * * vibration emission. The process of absorption and emission of vibration is called "nuclear magnetic vibration".

The "nucleus" of magnetic resonance imaging refers to the hydrogen nucleus, because about 70% of the human body is composed of water, and magnetic resonance imaging relies on hydrogen atoms in water. By placing an object in a magnetic field, irradiating it with appropriate electromagnetic waves to make it vibrate, and then analyzing the electromagnetic waves released by it, we can know the position and type of the nucleus that constitutes this object, and draw an accurate three-dimensional image of the object.

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Mathematical operation

The nucleus is positively charged and has spin motion, and its spin motion will inevitably produce magnetic moment, which is called nuclear magnetic moment. The research shows that the magnetic moment μ of the nucleus 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 the external magnetic field, the spatial orientation of nuclear spin angular momentum is quantized, and its projection value in the direction of 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 nuclear magnetic moment in the external magnetic field is also quantized, and its projection value in the magnetic field direction is

For different cores, m is an integer or a semi-integer respectively. In the external magnetic field, the nucleus with magnetic moment has 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 nucleus in the external magnetic field is also quantized. Due to the interaction between magnetic moment and magnetic field, the spin energy is split into a series of discrete energy levels, and the difference between two adjacent energy levels is δ δδE =γhB. Irradiate the nucleus with electromagnetic radiation of appropriate frequency. If the photon energy hν of electromagnetic radiation is just the difference Δ e between two adjacent nuclear energy levels, the nucleus will absorb this photon, and the frequency conditions of nuclear magnetic resonance are as follows:

Where ν is the frequency and ω is the angular frequency. For a certain nucleus, the gyromagnetic ratio γ can be accurately determined. It can be seen that the magnetic induction intensity can be determined by measuring the frequency ν of the radiation field during nuclear magnetic resonance. On the other hand, if the magnetic induction intensity is known, the * * * vibration frequency of the nucleus can be determined.

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system composition

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Nuclear magnetic resonance experimental device

Nuclear magnetic resonance is realized by adjusting frequency. The coil emits electromagnetic waves to the sample, and the function of the modulation oscillator is to make the frequency of RF electromagnetic waves change continuously near the vibration frequency of the sample. When the frequency coincides with the nuclear magnetic resonance frequency, the output of the RF oscillator will have an absorption peak, which can be displayed on the oscilloscope, and the frequency meter will immediately read the frequency value of the vibration. Nuclear magnetic resonance spectrometer is an instrument specially used to observe nuclear magnetic resonance, which is mainly composed of 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 the nuclear magnetic vibration signal; The spectrometer amplifies, displays and records * * vibration signals.

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Composition of magnetic resonance imaging system

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Magnet system

Static magnetic field: At present, the magnetic field strength of superconducting magnets used in clinic is 0.5 to 4.0T, and the common ones are 1.5T and 3.0t. In addition, the shim coil helps to achieve high uniformity.

Gradient field: used to generate and control the gradient in the magnetic field and realize the spatial coding of nuclear magnetic resonance signals. The system has three groups of coils, which generate gradient fields in X, Y and Z directions. The magnetic fields of the coil groups are superimposed to obtain gradient fields in any direction.

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Radio frequency system

Radio frequency (RF) generator: It generates a short and strong RF field, which acts on the sample in a pulse way, causing the hydrogen nuclei in the sample to produce NMR phenomenon.

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

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Computer image reconstruction system

The signal sent by the RF receiver is converted into a mathematical signal by the A/D converter, and the slice image data is obtained by computer processing according to the corresponding relationship with each voxel in the observation layer, and then added to the image display by the D/A converter, and the image of the layer to be observed is displayed in different gray levels according to the magnitude of nuclear magnetic resonance.

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Basic methods of magnetic resonance imaging

Thin film selective gradient field Gz

Phase coding and frequency coding

image reconstruction

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technology application

3D magnetic resonance imaging [edit]

Application of magnetic resonance imaging in medicine

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Overview of principle

Hydrogen nuclei are the first choice for human imaging: various tissues of human body contain a lot of water and hydrocarbons, so the nuclear magnetic resonance of hydrogen nuclei has high flexibility and strong signal, which is why people choose hydrogen nuclei as the first choice for human imaging elements. The intensity of NMR signal is related to the density of hydrogen nuclei in the sample. When the water content ratio of various tissues in human body is different, that is, the number of hydrogen nuclei is different, the intensity of nuclear magnetic resonance signal is also different. Using this difference as a characteristic quantity, various tissues are separated, which is the nuclear magnetic resonance image of hydrogen nucleus density. The differences of hydrogen nucleus density, relaxation time T 1 and T2 between different tissues of human body and between normal tissues and diseased tissues in this tissue are the most important physical basis of MRI in clinical diagnosis.

When RF pulse signal is applied, the state of hydrogen nuclear energy changes. After RF, hydrogen nuclear energy returns to its initial state and emits electromagnetic waves generated by vibration. Small differences in nuclear vibration can be accurately detected, and after further computer processing, it is possible to obtain a three-dimensional image of the chemical structure of the reaction tissue, from which we can obtain information including the difference of water in the tissue and the movement of water molecules. In this way, pathological changes can be recorded.

Two-thirds of human body weight is water, and 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 human body is different, and the pathological process of many diseases will lead to the change of water morphology, which can be reflected by magnetic resonance images.

The images obtained by nuclear magnetic resonance are very clear and fine, which greatly improves the diagnostic efficiency of doctors and avoids thoracotomy or laparotomy. Because nuclear magnetic resonance does not use X-rays that are harmful to human body and contrast agents that are easy to cause allergic reactions, it is harmless to human body. MRI can perform multi-angle and multi-plane imaging of various parts of the human body with high resolution, which can more objectively and concretely display the anatomical tissues and adjacent relations in the human body, and can better locate and characterize the lesions. It is of great value for the diagnosis of systemic diseases, especially the diagnosis of early tumors.

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Advantages of magnetic resonance imaging

Compared with 190 1 year's ordinary X-ray and1year's computer tomography (CT), which won the Nobel Prize in medicine in 1979, the biggest advantage of magnetic resonance imaging is that it is a safe, rapid and accurate clinical diagnosis method that is harmless to human body. Today, at least 60 million cases worldwide are examined by magnetic resonance imaging technology every year. Specifically, there are the following points:

No radiation damage to human body;

Various parameters can be used for imaging, and various imaging parameters can provide rich diagnostic information, which makes medical diagnosis and study of metabolism and function in human body convenient and effective. For example, the T 1 value of hepatitis and liver cirrhosis is larger, and the T 1 value of liver cancer is larger. T 1 weighted images can be used to distinguish between benign and malignant liver tumors.

The required contour can be freely selected by adjusting the magnetic field. Images of parts that are inaccessible or difficult to access by other imaging technologies can be obtained. For intervertebral disc and spinal cord, sagittal, coronal and cross-sectional images can be made, and nerve roots, spinal cord and ganglia can be seen. Three-dimensional images of the brain and spinal cord can be obtained, unlike CT (which can only obtain cross-sectional views perpendicular to the human body's long axis), which may miss lesions;

Can diagnose heart diseases, CT scanning speed is slow and incompetent;

Excellent resolution for soft tissues. 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 (3 1P) and so on.

Coronal magnetic resonance imaging of human abdomen [edit]

Disadvantages and possible hazards of magnetic resonance imaging

Although MRI is not fatal to patients, it will still bring some discomfort to patients. Necessary measures should be taken to reduce this negative effect before MRI diagnosis. Its shortcomings mainly include:

Like CT, MRI is an anatomical imaging diagnosis. Many lesions are still difficult to be diagnosed by magnetic resonance imaging alone, unlike endoscopy, which can obtain imaging and pathological diagnosis at the same time.

The examination of lung is not superior to X-ray or CT, and the examination of liver, pancreas, adrenal gland and prostate is not superior to CT, but the cost is much higher.

Gastrointestinal lesions are not as good as endoscopy;

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

Because of the strong magnetic field, magnetic resonance is not suitable for special patients with magnetic metal or pacemakers in their bodies.

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

Strong static magnetic field: in the presence of ferromagnetic substances, whether implanted in patients or in magnetic fields, it may be a risk factor;

Time-varying gradient field: it can induce electric field in the subject and excite nerves or muscles. Peripheral nerve excitation is the upper limit index of gradient field safety. Under sufficient intensity, it can produce peripheral nerve excitement (such as tingling or tapping), and even cause heart excitement or ventricular fibrillation;

Thermal effect of RF field: large-angle RF field emission used in MRI focusing or measurement, its electromagnetic energy is converted into heat energy in the patient's tissue, which makes the tissue temperature rise. The thermal effect of radio frequency needs further discussion. Clinical scanners have a so-called "specific absorption rate" (SAR) limit on RF energy.

Noise: All kinds of noise generated during MRI operation may damage the hearing of some patients;

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

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Application of magnetic resonance imaging in chemistry field

The application of MRI in the chemical field is not as extensive as that in the medical field, mainly due to the technical difficulty and the difficulty of imaging materials. At present, it is mainly used in the following aspects:

In the field of polymer chemistry, such as the study of carbon fiber reinforced epoxy resin, the study of spatial orientation of solid-state reaction, the study of solvent diffusion in polymer, the study of polymer vulcanization and elastomer uniformity.

In cermets, porous structure is studied to detect sand holes in ceramic products;

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 the study of fluid distribution and fluidity in rocks, as well as the study of reservoir description and enhanced oil recovery mechanism.

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Other advances in magnetic resonance imaging

Nuclear magnetic resonance analysis technology is to analyze the molecular structure and properties of substances by measuring the characteristic parameters of nuclear magnetic resonance lines (such as line width, line outline shape, line area, line position, etc.). ). It will not destroy the internal structure of the tested sample, and it is a completely nondestructive testing method. At the same time, it has very high resolution and accuracy, and there are many cores that can be used for measurement, which are superior to other measurement methods. Therefore, nuclear magnetic resonance technology has been widely used in physics, chemistry, medical treatment, petrochemical industry, archaeology and other fields.

Magnetic resonance microscope (MRM/μ MRI) is a technology that developed later in MRI technology. The highest spatial resolution of MRM is 4μm, which is close to the level of general optical microscope images. MRM has been widely used in animal models of diseases and drugs.

In vivo magnetic resonance spectroscopy (Mrs) can measure the nuclear magnetic resonance spectra of designated parts of animals or human bodies, so as to directly identify and analyze their chemical components.

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Contributions of Nobel Prize winners

On June 6th, 2003, Karolinska Medical College in Sweden announced that the 2003 Nobel Prize in Physiology or Medicine would be awarded to American chemist paul lauterbur and British physicist peter Mansfield for their breakthrough achievements in the field of magnetic resonance imaging technology used in medical diagnosis and research.

Lauterpur's contribution is that an uneven magnetic field is added to the main magnetic field, and a gradient is introduced into the magnetic field, thus creating a visible two-dimensional structural image of the internal structure of matter that cannot be seen by other technical means. He described how to add a gradient magnet to the main magnet, and then you can see the cross section of a test tube filled with ordinary water immersed in heavy water. No other image technology can distinguish the images of ordinary water and heavy water. By introducing gradient magnetic field, the electromagnetic wave frequency of nuclear magnetic resonance can be changed point by point, and the signal source can be determined by analyzing the emitted electromagnetic wave.

Mansfield further developed the theory of using additional gradient magnetic field in stable magnetic field and promoted its practical application. He discovered the mathematical analysis method of magnetic vibration signal, which laid the foundation for the method to move from theory to application. This makes magnetic resonance imaging a practical method for clinical diagnosis after 10 years. He used the gradient of magnetic field to show the difference of vibration more accurately. He proved how to analyze the detected signal effectively and quickly and convert it into an image. Mansfield also proposed that instantaneous images can be obtained by extremely rapid gradient changes, that is, echo plane imaging (EPI) technology, which became the main means of functional magnetic resonance imaging (FMRI) research that began to flourish in the 1990s.

Raymond Damati's Equipment and Methods of Cancer Tissue Detection is worth mentioning. The groundbreaking contributions made by the winners of the 2003 Nobel Prize in Physics to the theory of superconductors and superfluids provided a theoretical basis for the two scientists who won the 2003 Nobel Prize in Physiology or Medicine to develop MRI scanners and paved the way for MRI technology. Because of their theoretical work, magnetic resonance imaging technology has made a breakthrough, which makes it possible to have high-definition images of internal organs of the human body.

In addition, in The New York Times and Washington post on June 10, 2003, a full-page advertisement of Fonal appeared at the same time: "Raymond Damadian should share the 2003 Nobel Prize in Physiology or Medicine with peter Mansfield and paul lauterbur. Without him, there would be no magnetic resonance imaging technology. " Accusing the Nobel Committee of "tampering with history" caused widespread controversy. In fact, the ownership of the invention right of nuclear magnetic resonance has been debated for many years, and the controversy is quite fierce. In academic circles, Damati Ann is not so much a scientist as a businessman.

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Future prospects

How the human brain thinks has always been a mystery. And it is an important topic that scientists pay attention to. Brain functional imaging with MRI is helpful for us to study human thinking in vivo and at the whole level. Among them, the study on whether blind children's hands can replace their eyes is a good sample. Normal people can see the blue sky and clear water, and then form images and artistic conception in their brains, while blind children who have never seen the world can touch the words with their hands and tell them the world. Can blind children "see" it? Experts scanned the brains of normal and blind children by functional magnetic resonance imaging, and found that blind children, like normal people, have a good activation area in the visual cortex of their brains. From this, a preliminary conclusion can be drawn: 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 of scanning patients with classical MRI imaging methods from several minutes to several milliseconds, thus ignoring the influence of organ movement on images. MRI blood flow imaging clearly shows the shape of blood vessels on MRI images by using void effect, which makes it possible to measure the flow direction and velocity of blood in blood vessels. MRI spectrum analysis can use strong magnetic field to realize the spectrum analysis technology of human local tissues, thus increasing information to help diagnosis; Brain functional imaging, using strong magnetic field vibration imaging to study brain function and its mechanism, is the most important subject in brain science. There is reason to believe that MRI will develop into mind reading.

Since the middle of the 20th century, information technology and life science have been the two most active fields. Experts believe that nuclear magnetic resonance technology, as a combination of the two, will continue to develop into microscopy and functional examination, and play a greater role in revealing the mysteries of life.

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Related projects

nuclear magnetic resonance

radio frequency

radio-frequency coil

Gradient magnetic field

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Magnetization preparation

Reverse recovery (reverse recovery)

Saturation recovery

Drive balance

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Image shooting method

Spin echo

Gradient echo (gradient echo)

Parallel imaging (parallel imaging)

Echo plane imaging (EPI)

Steady-state free precession imaging (SSFP)

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Application of medical physiology

magnetic resonance angiography

Magnetic resonance cholangiopancreatography

Diffusion weighted image

Diffusion tensor image

Perfusion weighted image

Functional magnetic resonance imaging (fmri)

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refer to

Fu Jieqing, "Nuclear Magnetic Resonance —— A Scientific Subject with the Most Nobel Prizes", Journal of Nature, 2003, (06):357-26 1.

Bie Yeguang, Lu Hua, "Re-discussion on the application of nuclear magnetic resonance in medicine", Physics and Engineering, 2004, (02): 34,665,438+0.

Jin Yongjun, Ai Yanbao, Nuclear Magnetic Resonance Technology and Application, Physics and Engineering, 2002, (01): 47-48,50.

Li Xianyao, Sun, "Magnetic Resonance Imaging", College Physics, 1997, (10): 36-39,29.

Ruan Ping, Magnetic Resonance Imaging and Its Medical Application, Guangxi Physics, 1999, (02): 50-53,28.

Lauterpur Publishing Company Nature,1973,242:190

Huang Weihua, Approaching Nuclear Magnetic Resonance, Medicine and Health Care, 2004, (03): 15.

Ye Chaohui, New Progress of Magnetic Resonance Imaging, Physics, 2004, (0 1): 12- 17.

Tian Jianguang, Liu Maili, Xia, Safety of magnetic resonance imaging. Journal of Spectroscopy, 2002, (06):505-5 1 1.

Jiang Zijiang, Application of NMRI in Chemical Field, Chemical World, 1995, (1 1): 563-565.

Fan Qingfu, Magnetic Resonance Imaging and Nobel Prize, Shanghai Biomedical Engineering, 2003, (04): Feng San.