What Is Nuclear Magnetic Resonance Imaging?

Nuclear Magnetic Resonance Imaging (English: Nuclear M agnetic R esonance I maging, referred to as NMRI ), also known as spin imaging (English: spin imaging ), also known as Magnetic Resonance Imaging ( M agnetic R esonance I maging, referred to as MRI ), Taiwan also It is called magnetic resonance imaging , and Hong Kong is also called magnetic resonance imaging . It uses the principle of nuclear magnetic resonance (NMR) to detect the emitted energy through different gradients in different structural environments inside the material. The electromagnetic waves can be used to know the position and type of the nucleus that constitutes this object, and based on this, it can be drawn as an image of the structure inside the object.

Magnetic resonance imaging is a newer medical imaging technology that was officially used clinically in 1982 internationally. It uses static magnetic field and radio frequency magnetic field to image human tissue. During the imaging process, it can obtain high-contrast and clear images without using electron ionizing radiation or contrast agent. It can reflect the abnormalities of human organs and early lesions from the inside of human molecules. It is superior to X-ray CT in many places. Although X-CT solves the problem of human body image overlap, because the provided image is still a spatial distribution image of tissues absorbing X-rays, it cannot provide information about the physiological state of human organs. When the lesion tissue has the same absorption coefficient as the surrounding normal tissue, it cannot provide valuable information. The lesions can only be discovered when the disease has developed to alter the organ shape, location, and enlargement to give people an abnormal feeling. In addition to the anatomical characteristics of X-ray CT, that is, to obtain non-overlapping proton density tomographic images, the magnetic resonance imaging device can also accurately measure the nuclear relaxation times T ₁ and T ₂ by means of nuclear magnetic resonance principle, which can transform human tissue The information about the chemical structure is reflected in it. The computer reconstructed image of this information is a component image (chemical structure image), which has the ability to characterize different tissues of 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 brain. It has a great advantage in the early diagnosis of tissue necrosis, malignant diseases and degenerative diseases, and its soft tissue contrast is more accurate.

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. The two were awarded the Nobel Prize in Physics in 1952. After the discovery of the NMR phenomenon, a new marginal subject, NMR spectroscopy, soon formed. It can enable people to determine various molecular structures through the difference of nuclear magnetic resonance lines without damaging the sample. This provides favorable conditions for clinical medicine. In 1967, Jasper Jackson measured signals from live animals for the first time, making NMR methods possible for anthropometric measurements. In 1971, Professor R. Damadidian of the State University of New York in the United States conducted a study on the nuclear magnetic resonance characteristics of normal and cancerous tissue samples of rats using an NMR spectrometer. 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 made the first two-dimensional image using water as a sample, showing the possibility of nuclear magnetic resonance CT, that is, spin density Imaging method. These experiments use a defined non-uniform magnetic field. The typical method is to make the magnetic field intensity linearly change along the spatial coordinate axis to identify the nuclear magnetic resonance signals emitted from different spatial locations. In 1978, the quality of MRI images had reached the initial level of X-ray CT, and human tests were performed in hospitals. And finally named magnetic resonance imaging (MRI). ^[1]

The nucleus spins and has angular momentum. Because the cores are charged, their spins generate magnetic moments. When an atomic nucleus is placed in a static magnetic field, a bipolar magnet that was originally randomly oriented is subjected to the force of a magnetic field and has the same orientation as the magnetic field. Taking protons as the main isotope of hydrogen as an example, it can only have two basic states: orientations parallel and antiparallel, which correspond to low-energy and high-energy states, respectively. Accurate analysis proves that the spin is not completely consistent with the magnetic field trend, but is inclined by an angle . In this way, the bipolar magnet starts to precess around the magnetic field. The frequency of precession depends on the strength of the magnetic field. Also related to the type of nucleus. The relationship between them satisfies the Lamor relationship: ₀ = B ₀ , that is, the precession angular frequency ₀ is the product of the magnetic field strength B ₀ and the magnetic rotation ratio . is a basic physical constant for each species. The main isotope of hydrogen, protons, is abundant in the human body, and its magnetic moment is easy to detect, so it is most suitable for obtaining nuclear magnetic resonance images from it.

From a macro perspective, in the set of magnetic moments for precession, the phases are random. Their synthetic orientation forms macroscopic magnetization, which is represented by the magnetic moment M. It is this macroscopic magnetic moment that produces a nuclear magnetic resonance signal in the receiving coil. Of the large number of hydrogen nuclei, about half are slightly lower. It can be proved that there is a dynamic equilibrium between the nucleus in two basic energy states, and the equilibrium state is determined by the magnetic field and temperature. The "thermal equilibrium" is reached when the number of nuclei transitioning from a lower energy state to a higher energy state is equal to the number of nucleus from a higher energy state to a lower energy state. If you apply RF energy that conforms to the Larmor frequency to the magnetic moment, and this energy is equal to the difference between the magnetic field energy between the higher and lower basic energy states, you can make the magnetic moment jump from the "parallel" state with lower energy. Resonance occurs when the energy is "antiparallel".

Since the magnetic moment is resonated by applying the energy of the Larmor frequency to the magnetic moment, a radio frequency field with an amplitude of B ₁ and synchronized (resonant) with the precessing spin is used. When the action direction of the radio frequency magnetic field B ₁ is The main magnetic field B _{0 is} perpendicular, which can make the magnetization vector M deviate from the rest position to make a spiral movement, or nutation, that is, the force of the radio frequency field forces the macroscopic magnetization vector to precess around it. If each duration can rotate the macroscopic magnetization vector by an angle of 90º, he falls in a plane perpendicular to the static magnetic field. A transverse magnetization vector M _xy can be generated. If a receiving coil is placed in this transverse plane, the coil can cut the magnetic lines of force and generate an induced voltage. When the radio frequency magnetic field B _{1 is} removed, the macroscopic magnetization vector undergoes a static magnetic field and precesses around it, which is called "free precession". Because the frequency of the precession is the Larmor frequency, the induced voltage also has the same frequency. Because the transverse magnetization vector is not constant, it decays to zero with a characteristic time constant. For this reason, the voltage amplitude it induces also decays with time and manifests itself as a damped oscillation. This type of signal is called a free induction attenuation signal (FID, Free Induction). Decay). The initial amplitude of the signal is proportional to the transverse magnetization, which is proportional to the number of excited nuclei in the tissue of a particular voxel, so the difference in hydrogen atom density can be discerned in the magnetic resonance image.

Because the Larmor frequency is proportional to the strength of the magnetic field, if the magnetic field changes in a gradient along the X axis, the obtained resonance frequency is also obviously related to the position of the voxel on the X axis. To obtain the signals projected on two coordinate axes XY at the same time, the gradient magnetic field G _X can be added first, the signals obtained can be collected and transformed, and then the magnetic field G _{Y can be used} instead of G _X , and the process is repeated. In the actual situation, the signal is collected from a large number of spatial locations, and the signal is composed of many frequency composites. Using mathematical analysis methods, such as the Fourier transform, not only can each resonance frequency, that is, the corresponding spatial position, but also the corresponding signal amplitude, and the signal amplitude be proportional to the spin density of a specific spatial position. All MRI methods are based on this principle. ^[2]

The image obtained by spatially encoding (locating) the resonance signal with a gradient magnetic field is essentially a density map of protons in human tissue. The transverse magnetization reflected by the magnetic resonance pixel values is not only related to the number of protons, but also to their motion characteristics, the so-called "relaxation time".

During the free precession phase, the magnetization vector returns to its original resting position through a process called "relaxation". The characteristics of the relaxation process are described by the time constants T ₁ and T ₂ . For simple thermodynamic simulation, the concept of "spin temperature" is proposed. It is believed that the spins excited by the RF magnetic field are "hot", and the environment of the nucleus is called "lattice". It can be understood as a container with a large heat capacity, and the excess energy of the nucleus is absorbed through "thermal" contact. The adiabatic "heat" of spin and lattice is very effective. The "heat" transfer is slow and the relaxation time is long. In pure water, at room temperature, the spin lattice relaxation time of a proton is about 3 seconds, and in biological tissues, it is between about 2 milliseconds and about 2 seconds. The spin lattice relaxation time T ₁ is the process of resetting the longitudinal magnetization vector M _Z , so D is also called the longitudinal relaxation time. The reset process follows the exponential law. After a 90º degree pulse, after ₁ second of T, it resets to 63% of its resting value.

After the RF magnetic field excitation, in addition to the longitudinal magnetization component to be restored, the transverse magnetization component M _{XY is} also to be attenuated, so that the signal gradually disappears. If the magnetic field is ideally homogeneous, that is, all nuclei are completely subjected to the same magnetic field strength, this transverse magnetization component decays with a constant T ₂ , which is called the lateral or spin-spin relaxation time. Due to the actual non-uniform magnetic field, the effective time constant T ₂ * of the FID decay process is shorter than T ₂ .

Since the FID signal does not represent the longitudinal magnetization vector, nor can it accurately represent the actual time constant of the transverse magnetization component attenuation. Therefore, the actual measurement is based on the given pulse sequence (180-degree and 90-degree RF excitation pulses form a certain pulse sequence). Perform indirect measurements to obtain T1-weighted and T2-weighted images.

By selecting different pulse sequences and different imaging times, magnetic resonance equipment can form proton density images, weighted images, and weighted images. It is important to characterize the difference in relaxation time between normal and diseased tissue.

It consists of three basic components, namely the magnet part, magnetic resonance spectrometer part, data processing and image reconstruction part.

Magnet part

The magnet is mainly composed of a main magnet (generating a strong static magnetic field), a compensation coil (correction coil), an RF coil and a gradient coil.

The main magnet is used to provide a strong static magnetic field, and requires a large spatial range (capable of accommodating patients), maintaining a highly uniform magnetic field strength. There are four criteria for measuring the performance of a magnet: magnetic field strength, time stability, uniformity, and channel size. Increasing the strength of the static magnetic field can increase the detection sensitivity, that is, shorten the scanning time and improve the spatial resolution. However, it also reduces the penetration depth of the RF field. When the magnetic field strength is 0.35T, a good spatial resolution can be obtained. The higher magnetic field strength currently used in clinical practice is 1.5T.

There are three main types of magnets: ordinary electromagnets, permanent magnets, and superconducting magnets. Ordinary electromagnets use a strong direct current to generate a magnetic field through a coil. The power consumption to maintain the magnetic field of a main magnet is about 100kW. It usually takes several hours for the magnetic field to reach a stable state. The large current flowing in the coil will generate a lot of heat, which will be cooled by the cooling water through the heat exchanger. After the permanent magnet material is once magnetized by an external excitation power source, the excitation power source is removed and maintained for a long time and magnetic, and the magnetic field strength is easily maintained stable. Therefore, the maintenance of the magnet is simple and the maintenance cost is the lowest. The disadvantage is that the weight is large, so it is difficult to achieve 1T field strength. The current field strength is limited to below 0.5T. Superconducting magnets are currently used more often. In the superconducting state, there is no resistance loss when the current flows through the conductor, so that the conductor does not heat up. A wire of the same diameter can pass a larger current without damage in a superconducting state. A strong magnetic field can be generated by applying a strong current to a coil made of a superconducting material, and the current in the superconducting coil remains unchanged after the current is cut off, 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 tank. The temperature of the liquid helium is 4.7K. In order to reduce the evaporation consumption of liquid helium, a buffer layer of liquid nitrogen (77.4K) is also provided in the outer cylinder. Liquid helium and liquid nitrogen should be added in time during use. In recent years, due to advances in vacuum insulation technology, secondary cooling of liquid nitrogen can be omitted, and 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 produced by it closes to the ideal uniform magnetic field. Due to the high accuracy requirements and the extremely tedious calibration work, it is generally computer-aided and requires multiple measurements, calculations and corrections to meet the requirements. Generally, coils of various shapes are adopted and different currents are passed according to the specific conditions to compensate for the unevenness of the basic field.

The radio frequency coil is used to radiate radio frequency electromagnetic waves of a specified frequency and a certain power to the human body to excite the resonance of the atomic nucleus. This coil should be perpendicular to the main magnetic field, and form a more uniform RF field in the human body as much as possible, and make it as close to the human body as possible to make the transmitting and receiving processes have higher efficiency. Some radio frequency coils include two parts: a transmitting coil and a receiving coil. In addition, there are various special surface coils such as head receiving coils, limb coils, neck coils, spine coils, eye socket coils, and chest coils to improve conversion efficiency and image quality.

The gradient coil requires a specific gradient power supply. It is strictly matched with a dedicated gradient coil, and the power supply stability is required to be one ten thousandth. Gradient power and compensation power are generally water-cooled. In addition, the escaped magnetic field of the main magnetic field has a great influence on the surroundings, mainly affecting various magnetic disks, image displays, image intensifiers, and patients with pacemakers. External magnetic objects also affect the uniformity of the main magnet.

Magnetic resonance spectrometer

It mainly includes a radio frequency transmitting part and a receiving system for a magnetic resonance signal. The transmitting part is equivalent to a radio transmitter. It is a single-sideband transmitting device with precisely adjustable waveform and frequency spectrum. Its peak transmitting power is adjustable from several hundred watts to fifteen 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 is required to be high, and the noise must be low. Generally, a spectrometer uses a super-heterodyne receiver system, and its main gain can be an intermediate frequency amplifier. Because the IF amplifier works in a different frequency band from the transmitting system, direct interference from the transmission can be avoided. There is a receiving door between the preamplifier and the mid-amplifier, which is actually a radio frequency switch. It is mainly closed when the transmitting system is working to prevent powerful RF transmitting signals from entering the receiving system. The amplitude of the FID signal amplified by the intermediate frequency generally exceeds 0.5 volts, which can be detected. After detection, the signal is amplified and filtered.

Data processing and image reconstruction

The magnetic resonance signal is first converted into a digital quantity by a converter and stored in a temporary register. The image processor processes the raw data according to the required method, obtains different parameter images of magnetic resonance, and stores them in the image memory. This image can be subjected to a series of post-processing as required. The 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 a 32-bit array processor should be used. The reconstructed images are sent to a high-resolution display device in turn, and can also be stored on a disk and made into hard copies by multiple cameras.

The console is generally composed of a main diagnostic console and an auxiliary diagnostic console. Two consoles can improve patient throughput. There are also two displays, one is a character display, and the menu-type operation software is also displayed here. The other is a high-resolution large-screen image display.

The entire system is controlled by the host computer. When the system is working, the host computer controls the work of a single-chip computer system at the same time.

How the human brain thinks has always been a mystery. And it is an important subject that scientists pay attention to. The use of MRI for brain functional imaging will help us study human thinking at the living and holistic level. Among them, the research on whether the hand of a blind child can replace the eyes is a good sample. A normal person can see blue sky and clear water, and then form an image in the brain to form a mood. A blind child who has never seen the world can touch text with his hands. The text tells him that the world can see the blind child. How about it? Experts have scanned the brains of normal and blind children through functional MRI, and found that blind children, like normal people, have good activation areas in the visual cortex of the brain. From this, it can be initially concluded that through cognitive education, the blind child can "see" the outside world instead of the eyes.