Nuclear magnetic resonance (NMR) is a nuclear physics phenomenon. As early as 1946 Block and Purcell reported this phenomenon and applied to wave spectroscopy, Lauterbur 1973 published MR imaging technology, so that the nuclear magnetic **** vibration is not only used in physics and chemistry. It is also used in the field of clinical medicine. In recent years, nuclear magnetic *** vibration imaging technology has developed very rapidly, has become increasingly mature and perfect. The scope of the examination basically covers all systems of the whole body and has been popularized and applied worldwide. In order to accurately reflect the basis of its imaging, to avoid confusion with nuclide imaging, now renamed as magnetic *** vibration imaging. The factors involved in MRI imaging are more, more informative and different from the existing various imaging, which has great superiority and application potential in diagnosing diseases.
I, MRI imaging basic principles and equipment
(A) magnetic *** vibration phenomenon and MRI
Atoms with singular protons, such as hydrogen nuclei widely available in the human body, the protons have a spin motion, positively charged, resulting in magnetic moments, such as a small magnet (Figure 1-5-1). The arrangement of the spin axis of the small magnet is not necessarily regular. However, if in a uniform strong magnetic field, the spin axis of the small magnet will be rearranged according to the direction of the magnetic field lines of force (Figure 1-5-2). In this state, with a specific frequency of radio frequency pulses (radiofrequency, RF) for excitation, as a small magnet of the hydrogen nucleus absorbs a certain amount of energy and * * * vibration, that is, the phenomenon of magnetic * * * vibration occurs. Stop transmitting RF pulses, the excited hydrogen nucleus to the absorbed energy gradually released, its phase and energy level are restored to the state before the excitation. This recovery process is called the relaxation process (relaxationproces), and the time required to return to the original equilibrium state is called the relaxation time (relaxationtime). There are two kinds of relaxation time, one is the spin-lattice relaxation time (spin-lattice relaxationtime), also known as the longitudinal relaxation time (longitudinal relaxation time) reflecting the spin nucleus to the absorbed energy to the surrounding lattice of the time required, but also 90 ° rf pulse protons from the longitudinal magnetization to the transverse magnetization After the 90 ° RF pulse proton from the longitudinal magnetization to transverse magnetization and then return to the longitudinal magnetization excitation of the state before the time required, called T1. another is the spin - spin relaxation time (spin-spin relaxation time), also known as the transverse relaxation time (transverse relaxation time) reflecting the transverse magnetization attenuation, the loss of the process, that is, the transverse magnetization of the time maintained, called T2. T2 decay is caused by the *** vibration between the proton mutual magnetization, and T1, it causes a change in phase.
Figure 1-5-2 Normally, protons are in a haphazard arrangement. When placed in a strong external magnetic field, they change. They are aligned in only two directions, parallel or antiparallel to the external magnetic field
The T1 of normal and pathological tissues in different organs of the human body is relatively fixed, and there are certain differences between them, as well as the T2 (Table 1-5-1a, b). This difference in relaxation time between tissues is the basis of MRI imaging. There is the same reasoning as in CT, where the difference in absorption coefficient (CT value) between tissues is the basis of CT imaging. However, unlike CT, which has only one parameter, the absorption coefficient, MRI has several parameters such as T1, T2 and spin kernel density (P), of which T1 and T2 are particularly important. Therefore, by obtaining T1 (or T2) values for various tissues in a selected level, an image can be obtained that includes images of various tissues in that level.
The imaging method of MRI is also similar to that of CT. There is, for example, the division of the examination level into Nx, Ny, Nz ...... a certain number of small volumes, i.e., voxels, and the information is collected with a receiver, digitized, and fed into a computer for processing, and the T1 value (or T2 value) of each voxel is obtained and spatially encoded. A converter is used to convert each T-value to an analog grayscale while reconstructing the image.
Table 1-5-1a T1 values (ms) of normal and diseased tissues in the human body
Liver
140-170
Meningocardial tumor
200-300
Pancreas
180-200
Hepatocellular carcinoma
300-450
Kidney
300 to 340
Hepatic hemangioma
340 to 370
Biliary
250 to 300
Pancreatic Cancer
275 to 400
Hematologic
340 to 370
Kidney Cancer
400 to 450
Fat
60 to 80
Lung abscess
400 to 500
Muscle
120 to 140
Bladder carcinoma
200 to 240
Table 1-5-1b T1 and T2 values of normal cranial brain (ms)
Organization
T1
T2
Callosal body
380
80
Pontine brain
445
75
Medulla oblongata
475
100
Cerebellum
585
90
Brain
600
100
Cerebrospinal fluid
1155
145
Scalp
235
60
Bone marrow
320
80
(II) )MRI equipment
The imaging system of MRI consists of two parts: MR signal generation and data acquisition and processing and image display.MR signal generation is from a large-aperture, three-dimensional spatially encoded MR spectrometer, while the data processing and image display part is similar to that of CT scanning devices.
MRI equipment includes magnets, gradient coils, power supply, RF transmitters and MR signal receivers, which are responsible for MR signal generation, detection and encoding; and analog converters, computers, disks, and tape drives, which are responsible for data processing, image reconstruction, display and storage (Figure 1-5-3).
There are three types of magnets: normal-conducting, superconducting, and permanent-magnet, which are directly related to magnetic field strength, uniformity, and stability, and affect the image quality of MRI. Therefore, it is very important. The type of magnet is usually used to describe the type of MRI equipment. Normal-conducting coils are wound with copper and aluminum wires, and the magnetic field strength can be up to 0.15-0.3T*, superconducting coils are wound with niobium-titanium alloy wires, and the magnetic field strength is generally 0.35-2.0T, cooled by liquid helium and liquid nitrogen; permanent-magnet-type magnets consist of magnetic tiles made of magnetic material, are heavier, and the magnetic field strength is on the low side, up to 0.3T.
Gradient coils, which modify the main magnetic field to produce a gradient magnetic field. Its magnetic field strength is only a few hundredths of the main magnetic field. However, the gradient magnetic field provides the possibility of three-dimensional encoding of spatial localization for human MR signals. The gradient field consists of three gradient magnetic field coils in X, Y, and Z, and has a driver in order to quickly change the direction and strength of the magnetic field during scanning, and to rapidly complete the three-dimensional encoding.
The RF transmitter and MR signal receiver are RF systems. The RF transmitter is designed to generate different pulse sequences for clinical examination purposes in order to excite the hydrogen nuclei in the body to produce MR signals. RF transmitter and RF coil is very much like a short-wave transmitter and transmitter antenna, to the human body to transmit pulses, the human body hydrogen nucleus is quite a radio to receive pulses. After the pulses stop transmitting, the hydrogen nucleus of the human body becomes a short-wave transmitter, and the MR signal receiver becomes a radio receiving the MR signal. The pulse sequence emission is completely under computer control.
MRI equipment in the data acquisition, processing and image display, in addition to image reconstruction by the Fourier transform instead of the inverse projection, and CT equipment is very similar
Two, MRI examination technology
MRI scanning technology is different from CT scanning. Not only do we need cross-sectional images, but we also often need sagittal or (and) coronal images, and we also need to obtain T1WI and T2WI. therefore, we need to choose the appropriate pulse sequence and scanning parameters. The spin echo (SE) technique with multiple layers and multiple echoes is commonly used. The scan time parameters are echo time (TE) and pulse repetition time (TR). T1WI is obtained using a short TR and a short TE, while T2WI is obtained using a long TR and a long TE. time is measured in milliseconds. Depending on the length of TE, T2WI can be categorized as severe, moderate or mild. The change in signal intensity of a lesion in different T2WI can help determine the nature of the lesion. For example, hepatic hemangioma has low signal in T1WI and high signal in mild, moderate and severe T2WI, and with the degree of aggravation, there is an increase in signal intensity, i.e., its signal is particularly strong in severe T2WI. Hepatocellular carcinoma, on the other hand, showed a slightly lower signal on T1WI, a slightly higher signal on mild and moderate T2WI, and a slightly lower signal intensity on severe T2WI than that on moderate T2WI. Combined with other clinical imaging manifestations, it is not difficult to distinguish the two.
The SE pulse sequence commonly used in MRI has a long scan time and imaging time, so braking of the patient is very important. The use of respiratory gating and/or respiratory compensation, cardiac gating and peripheral gating, and presaturation techniques can reduce the interference of respiratory artifacts, blood flow artifacts, and cerebrospinal fluid fluctuation artifacts due to respiratory motion and blood flow, and can improve the image quality of MRI.
In order to overcome the main drawbacks of slow imaging speed and long examination time of SE pulse sequence in MRI, imaging techniques such as gradient echo pulse sequence and fast spin echo pulse sequence have been developed in recent years, which have achieved significant results and are widely used in clinics. In addition, finger fat suppression and water suppression techniques have been developed to further increase MRI information.
Another new technique for MRI is magnetic resonance angiography (MRA). The blood flowing in the vessels appears to be flow-emptying. Its MR signal intensity depends on the flow rate, and fast-flowing blood often has a low signal. As a result, there is significant contrast between the flowing blood and adjacent tissues, thus offering the possibility of MRA. It has been applied to the diagnosis of large and medium-sized vascular lesions and is constantly being improved.MRA does not require perforation of blood vessels and injection of contrast medium, and has good prospects for application. MRA can also be used to measure blood flow velocity and observe its characteristics.
MRI can also be contrast-enhanced, i.e., a paramagnetic substance that can shorten the relaxation time of protons is injected from a vein as a contrast agent to enhance MRI. The commonly used contrast agent is gadolinium - diethylenetriamine pentaacetic acid (Gadolinium-DTPA, Gd-DTRA). This contrast agent cannot pass through the complete blood-brain barrier, is not absorbed by the gastric mucosa, and is completely in the extracellular space as well as has no special target organ distribution, which is helpful for identifying tumor and non-tumor lesions. When MRI of the central nervous system is used for contrast enhancement, the degree of enhancement of the lesion is closely related to the amount of blood supply to the lesion and the degree of destruction of the blood-brain barrier, which is conducive to the diagnosis of central nervous system diseases.
MRI can also be used to film television and movies, mainly for dynamic observation and diagnosis of cardiovascular disease.
Based on MRI's study of blood diffusion and perfusion, cerebral ischemic changes can be detected early. It foretells a good application prospect.
People with pacemakers need to stay away from MRI equipment. Metal implants in the body, such as metal clips, not only affect the MRI images, but also can cause serious consequences for the patient, and also can not be MRI examination, should be noted.
Three, the clinical application of MRI
MRI diagnosis is widely used in clinical, although the time is short, but has shown its superiority.
The application of MRI in the nervous system is more mature. Three-dimensional imaging and flow-space effect make the lesion localization and diagnosis more accurate, and can observe the relationship between lesions and blood vessels. The brainstem, subcameral region, occipital foramen, spinal cord and intervertebral discs are significantly better than CT, and it is of high value for the diagnosis of demyelinating brain diseases, multiple sclerosis, cerebral infarction, brain and spinal cord tumors, hematoma, congenital anomalies of the spinal cord, and spinal cord cavernous disease.
Mediastinum on MRI, fat and blood vessels form a good contrast, easy to observe the mediastinal tumor and its anatomical relationship with blood vessels. It is also more helpful in the diagnosis of hilar lymph nodes and central lung cancer.
The morphology and dynamics of cardiac large vessels can be studied in a non-invasive examination because the inner lumen can be shown on MRI.
MRI is also of considerable value for abdominal and pelvic organs, such as the liver, kidneys, bladder, prostate and uterus, neck and breast. It is superior to CT in the early visualization of malignant tumors, invasion of blood vessels, and staging of tumors.
The bone marrow appears as a high-signal area on MRI, and lesions invading the bone marrow, such as tumors, infections, and metabolic disorders, can be clearly visualized on MRI. It also has advantages in showing intra-articular lesions and soft tissues.
MRI is limited in its ability to visualize bones and the gastrointestinal tract.
MRI also holds promise for the study of blood flow, biochemistry, and metabolic function, as well as for the early diagnosis of malignant tumors.
Within the range of magnetic field strengths used to perform MR imaging, there are no adverse effects on human health, making it a non-invasive test.
However, MRI equipment is expensive, the cost of inspection is high, the time required for inspection is long, and there are limits to the inspection of certain organs and diseases, therefore, it is necessary to strictly control the indications.
MRI is a non-invasive test.