Magnetic resonance imaging(English:NuclearMagneticResonanceImaging，abbreviationNMRI）, also known asspin imaging(English:spin imaging）, also known asMagnetic resonance imaging（MagneticResonanceImaging，abbreviationMRI）, is usingnuclear magnetic resonance(Nuclear magnetic resonance, referred to as NMR) principle, according to the different attenuation of the released energy in different structural environments inside the materialgradientEmission from magnetic field detectionelectromagnetic wave, we can know that this object is composedNucleusThe position and type of the object can be drawn into the internal structure image of the object.
Using this technology to image the internal structure of the human body will produce a revolutionary medical diagnostic tool.The application of rapidly changing gradient magnetic field has greatly accelerated the speed of NMR imaging, making the technologyclinical diagnosisThe application of scientific research has become a reality, greatly promotingMedical Science、NeurophysiologyAnd the rapid development of cognitive neuroscience.
During the decades from the discovery of NMR phenomenon to the maturity of MRI technology, there were three research fields related to NMR（physics、Chemistry、PhysiologyorMedical Science）Obtained 6 times inNobel Prize, which is sufficient to illustrate the importance of this field and its derivative technologies.
April 2023At the 50th anniversary of the advent of magnetic resonance imaging (MRI) technology, the resolution of mouse brain images was increased by 64 million times[5]。
Magnetic resonance imaging (MRI) is a relatively new medical imaging technology, which was officially used in clinical practice in 1982.It uses static magnetic field and radio frequency magnetic field to image human tissues. During the imaging process, clear images with high contrast can be obtained without electron ionization radiation or contrast agent.It can reflect the disorder and early pathological changes of human organs from the inside of human molecules.It is superior to X-ray CT in many places.Although X-CT solves the problem of human image overlap, the image provided is still the spatial distribution image of tissue absorption of X-rays, which cannot provide the physiological status information of human organs.When the absorption coefficient of the diseased tissue is the same as that of the surrounding normal tissue, it cannot provide valuable information.It can only be found when the lesion has developed to change the shape, position and self enlargement of the organ to give people an abnormal feeling.In addition to the anatomical characteristics of X-ray CT, namely obtaining non overlapping proton density tomography images, the magnetic resonance imaging device can also usePrinciple of nuclear magnetic resonanceAccurate measurement of nuclear relaxation time ToneAnd TtwoCan reflect the information about chemical structure in human tissues.The computer reconstructed image of this information is a component image (chemical structure image), which is capable of representing different tissues with the same density and different chemical structures of the same tissue through image display.This makes it easy to distinguish between gray matter and white matter in the brainTissue necrosisThe early diagnosis of malignant diseases and degenerative diseases has great advantages, and its soft tissue contrast is more accurate.
As early as 1946,Harvard UniversityTwo research groups led by Edward Purcell of Stanford University and Felix Block of Stanford University discovered the phenomenon of nuclear magnetic resonance (NMR) of matter.Both of them were awarded in 1952Nobel Prize in Physics。After the discovery of nuclear magnetic resonance, a new frontier discipline was formed soon,Nuclear magnetic resonance spectroscopy。It enables people to determine various molecular structures by distinguishing NMR lines without destroying the samples.This provides favorable conditions for clinical medicine.In 1967, Jasper Jackson first measured signals from living animals, making it possible for NMR method to be used for anthropometry.In 1971,State University of New YorkRProfessor Damadian used the nuclear magnetic resonance spectrometer to study the nuclear magnetic resonance characteristics of normal tissue and cancerous tissue samples of miceoneThe values are significantly different.In the same year that X-CT was invented, in 1972, Paul C. Lauterbur of the State University of New York at Stony Brook in the United States made the first two-dimensional image with water as a sample, showing the possibility of nuclear magnetic resonance CT, namely spin density imaging.These experiments all use a limited non-uniform magnetic field. The typical method is to make the magnetic field intensity change linearly along the spatial coordinate axis to identify the NMR signals sent from different spatial positions.In 1978, the image quality of MRI has reached the initial level of X-ray CT, and has been carried out in hospitalsHuman body test。It was finally named Magnetic Resonance Imaging (MRI).[1]
April 2023At the 50th anniversary of the advent of magnetic resonance imaging (MRI) technology, the resolution of mouse brain images was increased by 64 million times, the single voxel (3D pixel) in the new image is only 5 μ m[5]。
Imaging principle
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Nuclear spin, angular momentum.Since the nuclei are charged, their spins produce magnetic moments.When the atomic nucleus is placed in a static magnetic field, the originally randomly oriented bipolar magnet is in the same orientation with the magnetic field under the action of magnetic force.Taking proton as the main isotope of hydrogen for example, it can only have two basic states: orientation "parallel" and "reverse parallel", which correspond to low energy and high energy states respectively.The accurate analysis proves that the spin is not completely consistent with the magnetic field trend, but inclines an angle θ.In this way, the bipolar magnet begins to precess around the magnetic field.The frequency of precession depends on the strength of the magnetic field.It is also related to the type of nucleus.The relationship between them meets the Lamore relationship: ωzero=γBzero, i.e. precession angle frequency ωzeroIs the magnetic field strength BzeroAnd magnetic rotation ratio γ.γ is one of each nuclideBasic physical constants。The main isotope of hydrogen, proton, is abundant in the human body, and its magnetic moment is easy to detect, so it is most suitable for obtaining NMR images from it.
From a macro perspective, the phase is random in the set of precession magnetic moments.Their synthetic orientation forms macroscopic magnetization, which is expressed by magnetic moment M.It is this macroscopic magnetic moment that generates the NMR signal in the receiving coil.In a large number of hydrogen nuclei, a little more than half are in a lower state.It can be proved that there is a dynamic equilibrium between nucleons in two basic energy states, and the equilibrium state is determined by the magnetic field and temperature.When the number of nucleons transiting from a lower energy state to a higher energy state is equal to the number of nucleons transiting from a higher energy state to a lower energy state, the "heat balance" is reached.If the magnetic moment is applied with radio frequency energy in line with the Lamar frequency, and this energy is equal to the difference of magnetic field energy between the higher and lower basic energy states, the magnetic moment can jump from the "parallel" state with lower energy to the "reverse parallel" state with higher energy, and then co vibration occurs.
Since applying energy of Lamo frequency to the magnetic moment can make the magnetic moment resonate, then use an amplitude of BoneAnd the RF field synchronized (resonant) with the precession spin, when the RF magnetic field BoneDirection of action and main magnetic field BzeroVertically, the magnetization vector M can deviate from the rest position to make a spiral movement, or nutation, that is, the force of the RF field forces the macro magnetization vector to precession around it.If each duration can rotate the macro magnetization vector by 90 º, it will fall in the plane perpendicular to the static magnetic field.Transverse magnetization vector M can be generatedxy。If a receiving coil is placed in this transverse plane, the coil can cut the magnetic line of force to generate induced voltage.When RF magnetic field BoneAfter removal, the macro magnetization vector undergoes the action of static magnetic field, and then precesses around it, which is called "free precession".Since the frequency of precession is the Lamore frequency, the induced voltage also has the same frequency.Since the transverse magnetization vector is not constant, it decays to zero with a characteristic time constant. Therefore, the voltage amplitude induced by it also decays with time, which is represented by damped oscillation. This signal is called free induced decay signal (FID,Free Induction Decay)。The initial amplitude of the signal is proportional to the transverse magnetization, and the transverse magnetization is proportional to the number of excited nucleons in the tissue of a specific voxel. Therefore, the difference in hydrogen atom density can be identified in the magnetic resonance image.
Because the Lamore frequency is proportional to the magnetic field intensity, if the magnetic field changes in a gradient along the X axis, the resonance frequency obtained is obviously related to the position of the volume element in the X axis.To get the signal projected on the two coordinate axes X-Y at the same time, the gradient magnetic field G can be added firstX, collect and transform the signal, and then use the magnetic field GYReplace GX, repeat the process.In reality, the signal is collected from a large number of spatial locations, and the signal is composed of many frequency combinations.Using mathematical analysis methods, such as Fourier transform, not only can each resonance frequency, that is, the corresponding spatial position, but also the corresponding signal amplitude, which is proportional to the spin density of a specific spatial position.All MRI methods are based on this principle.[2]
Relaxation process
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The image obtained by means of spatial encoding (positioning) of resonance signal with gradient magnetic field is essentially the density map of plasmons in human tissues.The transverse magnetization reflected by the magnetic resonance pixel value is not only related to the number of protons, but also to their motion characteristics, namely the so-called "relaxation time".
In the free precession phase, the magnetization vector returns to its original resting position through a process called relaxation.The characteristic of relaxation process is determined by the time constant ToneAnd TtwoDescription.In order to make a simple thermodynamic simulation, the concept of "spin temperature" is proposed.It is believed that the spin excited by the radio frequency magnetic field is "hot", and the environment of the nucleus is called "lattice", which can be understood as a container with large heat capacity, absorbing the excess energy of the nucleus through "hot" contact.The absolute "heat" of spin and lattice is very effective. The "heat" is transferred slowly, and the relaxation time is long.In pure water, at room temperature, the spin lattice relaxation time of the proton is about 3 seconds, and in biological tissues, it is between several hundred milliseconds and about 2 seconds.Spin lattice relaxation time ToneIs the longitudinal magnetization vector MZThe process of resetting is also called longitudinal relaxation time.The reset process follows the exponential law. After 90 º pulse, after ToneSeconds, reset to 63% of its static value.
After RF magnetic field excitation, except for the longitudinal magnetization component, the transverse magnetization component MXYAttenuation is also required to make the signal disappear gradually.If the magnetic field is ideal and uniform, that is, all nucleons completely experience the same magnetic field intensity, the transverse magnetization component is constant TtwoAttenuation, which is called transverse or spin spin relaxation time.Due to the nonuniformity of the actual magnetic field, the effective time constant T of the FID (free attenuation signal) attenuation processtwo*Than TtwoShort.
Since the FID (free attenuation signal) signal does not represent the longitudinal magnetization vector, nor can it correctly represent the actual time constant of the transverse magnetization component attenuation, the actual measurement is carried out indirectly by giving a certain pulse sequence (180 degree and 90 degree RF excitation pulses form a certain pulse sequence) to obtain ToneWeighted sum TtwoWeighted image.
By selecting different pulse sequences and different imaging time, magnetic resonance equipment can form proton density images, weighted images and weighted images.It is important to find out the characteristics of relaxation time difference between normal and diseased tissues.
System structure
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It is mainly composed of three basic components, i.eMagnet part、Magnetic resonance spectrometer、Data processing and image reconstruction.
Magnet part
The magnet is mainly composed of main magnet (generating strong static magnetic field), compensation coil (correction coil), RF coil and gradient coil.
The main magnet is used to provide a strong static magnetic field, and requires a large space range (to accommodate patients) to maintain a highly uniform magnetic field intensity.There are four standards to measure the performance of magnets: magnetic field strength, time stability, uniformity, and channel size.Increasing the static magnetic field intensity can improve the detection sensitivity, that is, the scanning time is shortened and the spatial resolution is improved.However, it will also reduce the penetration depth of the RF field.When the magnetic field intensity is 0.35T, a good spatial resolution can be obtained. The current higher magnetic field intensity used in clinical practice is 1.5T.
The main magnets are divided into three categories: Ordinary electromagnets, permanent magnets and superconducting magnets.Ordinary electromagnet uses strong DC current to generate magnetic field through coil.The power consumption for maintaining a main magnet magnetic field is about 100kW.Generally, the magnetic field can reach a stable state after several hours of power on.The large current flowing through the coil will generate a lot of heat, which needs to be dissipated by cooling water through the heat exchanger.After the permanent magnetic material is magnetized once by the external excitation power supply, the magnetic field strength can be easily kept stable after the excitation power supply is removed.Therefore, the magnet is easy to maintain and the maintenance cost is the lowest.Its disadvantage is that it is heavy, so it is difficult to reach 1T field strength.The current field strength is limited below 0.5T.Superconducting magnets are widely used at present.In the superconducting state, there is no resistance loss when the current flows through the conductor, so the conductor will not be heated up.Wires of the same diameter can pass greater current without damage under superconducting state.A coil made of superconducting material can generate a strong magnetic field by passing a strong current, and after the external current is cut off, the current in the superconducting coil remains unchanged, so the superconducting magnetic field is extremely stable.In order to maintain the superconducting state, the superconducting coil must be immersed in liquid helium in a dewar. The temperature of liquid helium is 4.7K.In order to reduce the evaporation consumption of liquid helium, liquid nitrogen (77.4K) buffer layer shall be set in the cylinder outside.Liquid helium and liquid nitrogen shall be supplemented timely during use.In recent years, due to the progress of vacuum insulation technology, the secondary cooling of liquid nitrogen can be eliminated, and only liquid helium can be used to maintain superconducting conditions.
The function of the compensation coil is to compensate the main magnetic field coil, so that the static magnetic field generated by it approaches the ideal uniform magnetic field.Due to the high precision requirement and the tedious calibration work, it is generally carried out with the aid of computer, and it requires multiple measurements, calculations and corrections to meet the requirements.Generally, coils of various shapes are adopted and different currents are connected according to the specific situation to compensate for the non-uniformity of the basic field.
RF coil is used to radiate RF electromagnetic waves of specified frequency and certain power to the human body to excite the resonance of atomic nucleus.This coil should be perpendicular to the main magnetic field, and form a more uniform RF field in the human body as far as possible, and make it as close to the human body as possible to make the transmission and reception process more efficient.Some RF coils include two parts: the transmitting coil and the receiving coil, and some are used for both receiving and transmitting.In addition, there are head receiving coil, limb coil, neck coil, spine coil, eye socket coil, chest coil and other special surface coils to improve conversion efficiency and image quality.
Gradient coils require a specific gradient power supply.It is strictly matched with the special gradient coil, and the power supply stability is required to be 1/10000.Gradient power supply and compensation power supply generally adopt water cooling.In addition, the fugitive magnetic field of the main magnetic field has a great impact on the surroundings, mainly affecting various disks, image displays, image intensifiers and patients wearing pacemakers.The external magnetic objects also have influence on the uniformity of the main magnet.
Magnetic resonance spectrometer
It mainly includes RF transmitter and a set of MRI signal receiving system.The launching part is equivalent to oneRadio transmitterIt is a single sideband transmitter with precisely adjustable waveform and spectrum, and its peak transmission power is adjustable from hundreds of watts to 15 kilowatts.The receiving system is used to receive the free induction attenuation signal reflected by the human body.Because this signal is extremely weak, the total gain of the receiving system must be very high and the noise must be very low.General spectrometer adopts superheterodyne receiving system, and its main gain can be taken asIF amplifier。Since the IF amplifier works in a different frequency band from the transmission system, it can avoid direct transmission interference.There is a receiving gate between the preamplifier and the intermediate amplifier, which is actually a RF switch. It is mainly closed when the transmission system is working to prevent powerful RF transmission signals from entering the receiving system.The amplitude of FID (free attenuation signal) signal after IF amplification is generally more than 0.5 V, which can be detected.After detection, the signal shall be amplified and filtered.
Data processing and image reconstruction
The magnetic resonance signal is first converted into a digital quantity through a converter and stored in a register.The image processor processes the original data according to the required method to obtain MR images with different parameters and store them in the image memory.This kind of image can undergo a series of post processing as required.Post processing content is divided into two categories: one is general image processing, and the other is image processing dedicated to magnetic resonance, such as calculating T1 value, T2 value and proton density.At least 32 digits shall be usedArray processor。The reconstructed images are sent to the high-resolution display device in turn, and can also be stored in the disk and made into hard copies through multiple cameras.
The console is generally composed of the main diagnostic console and the auxiliary diagnostic console, which can improve the flow of patients.There are also two displays, one is the character display, and the menu operation software is also displayed here.The other is a high-resolution large screen image display.
The whole system is controlled by the main computer.When the system is working, the main computer controls the single-chip microcomputer system at the same time.
Technology application
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Application of MRI in medicine
Inspection purpose
Detection and diagnosis of heart disease, cerebrovascular accident and vascular disease
Detection and Diagnosis of Organ Diseases in Thoracic and Abdominal Cavities
Diagnosis, evaluation and tracking of tumors and functional obstacles
MRI is widely used in the diagnosis of sports related injuries. It can present clear images of near bone and surrounding soft tissues, including ligaments and muscles. Therefore, MRI is a very sensitive examination for spine and joint problems.
Because MRI has no risk of radiation exposure, it is often used in the detection and diagnosis of reproductive system, breast, pelvis and bladder diseases.
Principle overview
Hydrogen nucleus is the first choice for human body imaging: various tissues of the human body contain a large amount of water andhydrocarbonTherefore, the nuclear magnetic resonance (NMR) of the hydrogen nucleus has high flexibility and strong signal, which is the reason why people prefer the hydrogen nucleus as the human body imaging element.The NMR signal intensity is related to the density of hydrogen nuclei in the sample. If the proportion of water in various tissues of the human body is different, that is, the number of hydrogen nuclei is different, the NMR signal intensity is different. Use this difference as a characteristic quantity to separate various tissues, which is the nuclear magnetic resonance image of the density of hydrogen nuclei.The density of hydrogen nuclei between different human tissues, between normal tissues and diseased tissues in this tissueRelaxation timeThe difference between T1 and T2 is that MRI is used forclinical diagnosisThe most important physical basis.
When a radio frequency pulse signal is applied, the state of the hydrogen nucleus changes. After the radio frequency, the hydrogen nucleus returns to the initial energy state, and the electromagnetic wave generated by resonance is emitted.The tiny difference of nuclear vibration can be accurately detected. After further computer processing, it is possible to obtain three-dimensional images of reaction histochemical structure composition, from which we can obtain information including water difference in tissue and water molecule movement.In this way, pathological changes can be recorded.
Two thirds of the weight of the human body 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 human organs and tissues is not the same. The pathological process of many diseases will lead to changes in water morphology, which can be reflected by magnetic resonance images.
The images obtained by MRI are very clear and fine, which greatly improves the diagnostic efficiency of doctors and avoids the operation of exploratory diagnosis by thoracotomy or laparotomy.Since MRI does not use X-ray harmful to human body and contrast agents that are easy to cause allergic reactions, it is not harmful to human body.MRI can image various parts of the human body from multiple angles and planes with high resolution. It can more objectively and concretely display the anatomical tissues and adjacent relationships in the human body, and can better locate and characterize the lesions.Diagnosis of systemic diseases, especially in the early stagetumourThe diagnosis is of great value.
Advantages of magnetic resonance imaging
Compared with the ordinary people who won the Nobel Prize in Physics in 1901X-rayOr won the Nobel Prize in Medicine in 1979Computer tomography（computerized tomography,CT）In contrast, the biggest advantage of magnetic resonance imaging is that it is a safe, fast and accurate clinical diagnosis method that does not harm the human body at present.Today, at least 60 million cases are examined by MRI every year in the world.Specifically, there are the following points:
Various parameters can be used for imaging, and multiple imaging parameters can provide rich diagnostic information, which makes medical diagnosis and research on metabolism and function in human body convenient and effective.For example, the T1 value of hepatitis and cirrhosis is larger, while the T1 value of liver cancer is larger. The T1 weighted image can distinguish benign and malignant liver tumors;
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The desired profile can be freely selected by adjusting the magnetic field.It can obtain images of parts that are inaccessible or difficult to access by other imaging technologies.aboutintervertebral discandspinal cord, can be used asSagittal plane、Coronal plane、cross sectionImaging, visiblenerve root, spinal cord andganglionEtc.Unlike CT, it can only obtain the cross section perpendicular to the long axis of the human body;
In principle, all nuclear elements with non-zero spin can be used for imaging, such ashydrogen（H）、carbon（C）、nitrogen(N and N)phosphorus(P) Etc.
Disadvantages of MRI and possible hazards
Although MRI has no fatal injury to patients, it still brings some discomfort to patients.Necessary measures should be taken before MRI diagnosis to minimize this negative impact.Its disadvantages mainly include:
one
Like CT, MRI is also an anatomical imaging diagnosis. Many lesions are still difficult to be diagnosed by MRI alone, unlikeEndoscopeBoth imaging and pathological diagnosis can be obtained at the same time;
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yeslungThe inspection of the department is not better thanX-rayorCTCheck, yesliver、pancreas、adrenal gland、prostateCT examination is not superior to CT, but the cost is much higher;
The scanning time is long, and the spatial resolution is not ideal;
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Due to the strong magnetic field, MRI can detect magnetic metal orpacemakerNot applicable to special patients of.
The factors that may cause harm to human body by MRI system mainly include the following aspects:
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Strong static magnetic field: in the presence of ferromagnetic substances, whether implanted in the patient's body or within the magnetic field, it may be a risk factor;
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Time varying gradient field: electric field can be induced in the subject to excite nerves or muscles.Peripheral nerve excitation is the upper limit index of gradient field safety.Under sufficient intensity, it can produce peripheral nerve excitation (such as tingling or percussion), and even cause heart excitation orVentricular fibrillation；
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RF field(RF)Pyrogenic effect: The electromagnetic energy of the large angle RF field emission used in the MRI focusing or measurement process is converted into heat energy in the patient's tissue, which increases the tissue temperature.The pyrogenic effect of RF needs further discussion. The clinical scanner has the so-called "specific absorption rate" (SAR) limit on RF energy;
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Noise: Various noises generated during MRI operation may cause hearing damage to some patients;
Application of MRI in Chemistry
MRI is not as widely used in the chemical field as in the medical field, mainly because of technical difficulties and difficulties in imaging materials. At present, MRI is mainly used in the following aspects:
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stayPolymer ChemistryFields, such as research on carbon fiber reinforced epoxy resin, research on spatial orientation of solid state reactionpolymerinsolventResearch on diffusion, polymer vulcanization and elastomer uniformity;
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staycermetThe trachoma in ceramic products is detected by studying the porous structure;
stayPetrochemistryFocus on researchfluidstayrockThe distribution state and circulation of the oil reservoir and the research on the reservoir description and enhanced oil recovery mechanism.
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 (such as line width, line contour shape, line area, line position, etc.).It can not damage the internal structure of the tested sample, and is a completely nondestructive testing method.At the same time, it has very highDiscriminative abilityandaccuracyMoreover, there are many cores that can be used for measurement, all of which are superior to other measurement methods.Therefore,Nuclear magnetic resonance technologyIn physics, chemistry, medical treatment, petrochemical industryArchaeologyAnd other aspects have been widely used.
magnetic resonance microscopy(MR microscopy, MRM/μ MRI) is a technology developed slightly later in MRI technology. The highest spatial resolution of MRM is 4 μ m, which is close to the level of general optical microscopic images.MRM has been widely used as animal models of diseases and drugs.
In vivo magnetic resonance spectroscopy(in vivo MR spectroscopy, MRS) can measure the NMR spectrum of a specific part of an animal or human body, so as to directly identify and analyze the chemical components.
The Korean research team has developed a new method that can use magnetic resonance imaging (MRI) to non invasively track the propagation of brain signals on a millisecond time scale.[4]
Future Outlook
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How the human brain thinks has always been a mystery.It is also an important topic that scientists pay attention to.The brain functional imaging using MRI will help us to study human thinking at the living and overall levels.Among them, the study on whether the hand of blind children can replace the eyes is a good sample.Normal people can see the blue sky and clear water, and then form images in their brains to form an artistic conception. However, blind children who have never seen the world can touch words with their hands, and words tell them the world. Can blind children also "see" it?Through functional MRI, experts scanned the brains of normal and blind children, and found that blind children, like normal people, have good activation areas in the visual cortex of the brain.From this, we can draw a preliminary conclusion that the hands of blind children can "see" the outside world instead of eyes through cognitive education.
