Magnetic Resonance Imaging (MRI)

Magnetic resonance imaging is a new type of high-tech imaging examination method in recent years. It is a new medical imaging diagnostic technology that was applied to the clinic in the early 1980s. It has non-ionizing radiation (radiation) damage, no bony artifacts, multi-directional (transverse, coronal, sagittal section, etc.) and multi-parameter imaging, high soft tissue resolving power, showing vascular structure without the use of contrast agents And so on. Basic Information Specialist Category: Neurological Classification: Nuclear Magnetic Resonance Applicable gender: whether men and women apply fasting: fasting Tips: Patients with head and neck should wash their hair one day before the test, and do not wipe any hair care products. Normal value normal. Clinical significance Indications: Neurological diseases including tumors, infarction, hemorrhage, degeneration, congenital malformations, infections, etc. are almost a means of diagnosis. In particular, lesions of the spinal cord, such as tumors of the spine, atrophy, degeneration, and traumatic disc disease, have become the preferred method of examination. Lesions of the great vessels of the heart, lesions of the mediastinum in the lungs. Abdominal pelvic organs examination, biliary system, urinary system, etc. are significantly better than CT. For soft tissue lesions of the joints, it is very sensitive to aseptic necrosis of bone marrow and bone. The lesions are found earlier than X-ray and CT. Precautions Before inspection: 1. Explain to the technician the following: (1) whether there is a history of surgery; (2) whether any metal or magnetic substance is implanted in the body including a metal birth control ring; (3) whether there are dentures, electronic ear, orthodontic eye, etc. (4) Whether there is drug allergy; (5) There are no metal foreign objects splashing into the body in the near future. 2, do not wear underwear with metal materials, patients with head and neck should wash their hair one day before the inspection, do not wipe any hair care products. 3, before the inspection, you need to remove all the clothes outside the underwear, and replace the special clothing for the magnetic resonance room. Remove the metal items such as necklaces, earrings, watches and rings. Remove cosmetics and dentures, prosthetic eyes, glasses and other items on your face. 4. The time of magnetic resonance examination is long, and the environment in which the patient is located is dark and the noise is large. Be mentally prepared, don't be impatient, don't be afraid, keep your position under the guidance of a doctor. Patience. 5, before the examination to the doctor to provide all medical history, inspection data and all X-ray films, CT films. Inspection process Nuclei containing singular protons, singular neutrons, or both are singular with spin and magnetic moment properties and rotate in a specific manner around the direction of the magnetic field. This rotation is called precession or precession. Excitation of the examined nuclei with a radio frequency pulse of the same frequency as the precession frequency will cause resonance, ie magnetic resonance. After the RF excitation is stopped, the phase and energy level of the relevant nucleus return to the pre-excitation state, a process called relaxation. The signals produced by these energy level changes and phase changes can be measured for the sample being tested or the receiver near the human body. Clinically used MRI is proton imaging. The protons in different physical and chemical states have different relaxation times after RF excitation and cessation of excitation. The relaxation time is divided into two types, T1 and T2. The T1 relaxation time, also known as the longitudinal relaxation time, is the time required for the material to be placed in the magnetic field to generate magnetization, that is, the time required to return to the longitudinal magnetization after the 90-degree RF pulse is converted from the longitudinal magnetization to the transverse magnetization. The T2 relaxation time is also called the transverse relaxation time or spin-spin relaxation time, which is the time that the transverse magnetization is maintained in a completely uniform external magnetic field. That is, after a 90 degree RF pulse, the resonant protons remain coherent or remain in phase for precession. The intensity of MR radiation photons is very weak. To improve the signal-to-noise ratio of the MR signal, the pulse program that generates the spin echo signal must be reused. The interval between repeated excitations is called repetition time, referred to as IR. It can be arbitrarily chosen. The time between the first 90 degree RF pulse and the detection of the spin echo signal, ie the echo delay time, referred to as the echo time or TE, is also related to the strength of the measured MR signal. TE can also be arbitrarily selected by the operator. By selecting different program indicator times, the T1, T2 and proton density of the substance can be distinguished or measured. For short TE and long TR, the image reflects the difference in proton density, called proton-weighted image; as TR becomes shorter, the T1 imaging factor increases, ie short TE short TR (eg TE=28ms, TR=0.5s) A T1-weighted image is generated, and when a long TE and a long TR (such as TE>56 ms, TR=2 s) are used, a T2-weighted image is generated. Depending on the program being designed, signals can be acquired from the entire inspection volume, or signals can be acquired from one of the volumes, and these signals can be used to reconstruct the image with computer aid. 1. T1 Weighted Image In the spin echo (SE) sequence, a short TR is applied to enhance the effect of the T1 value on the image, while a short TE is applied to attenuate the effect of the T2 value on the image. That is, short TR short TE (TR/TE ≤ 1000/40ms, such as TR500ms/TE15ms), which is biased towards the image showing the difference of T1, that is, the difference in tissue contrast in the image is mainly due to the difference in T1 values ​​between tissues. The long T1 appears as a low signal on the magnetic resonance image, such as high water content, bone, calcification, etc.; short T1 appears as a high signal on the magnetic resonance image, such as fat, methemoglobin and the like. 2. T2-weighted image In the spin echo (SE) sequence, a long TE is applied to enhance the effect of the T2 value on the image, while a long TR is applied to attenuate the effect of the T1 value on the image. That is, the length of the long TR is IE (TR/TE1000/40ms, such as TR2000ms/TE90ms), which is biased towards the image showing the difference of T2. The long T2 appears as a high signal on the magnetic resonance image, such as high water content; the short T2 appears as a low signal on the magnetic resonance image, such as hemosiderin, melanin, calcification, and the like. 3. Proton density Like in the spin echo (SE) sequence, the long TR is applied to weaken the effect of the T1 value on the image, and the short TE is applied to weaken the effect of the T2 value, that is, the image obtained by the short TR short TE, TR2000ms/ IE15ms, which focuses on images that exhibit differences in proton density. 4. Enhanced scanning of the commonly used contrast agent GD-DTPA (rolling-diethylene pentamine acetic acid), which has paramagnetic properties and is distributed in the intercellular fluid, which mainly changes the magnetic action of hydrogen protons and its relaxation time, shortening T1 and T2 can cause high-signal on the T1-weighted image of the lesion and the damaged part of the blood-brain barrier to achieve the purpose of strengthening. Enhanced scan only T1 scan, to determine whether the image is enhanced according to changes in the nasal mucosa, pituitary, cavernous sinus, lateral ventricle choroid plexus. GD-DTPA is administered intravenously and does not require an allergy test. Enhanced scanning can identify the number of lesions and can find lesions that cannot be found by plain scan, identify tumors and surrounding edema, and facilitate the qualitative diagnosis of lesions. 5. Magnetic resonance angiography (MRA) is an effective means for non-invasive methods to display human blood vessels, and has been widely used in clinical practice. The principle of MRA is to use the characteristics of blood flow in the blood vessels, use different scanning sequences, improve the signal in the blood vessel, make it highly contrast with the surrounding tissue, use computer processing, remove the non-high signal tissue shadow to form blood vessels. image. It can measure blood flow velocity, observe blood flow characteristics, and display arteries or veins, respectively. The most commonly used technical means: 1 time flying over the law. 2 phase contrast method. Both methods MRA can be used for two-dimensional stacked face imaging or three-dimensional imaging. The time-flight method utilizes the fly-by time and inflow-enhancing effect: the blood that is excited by the corresponding segment is marked at a certain moment, and a fully relaxed proton flows into the blood of the imaging region, forming a high signal of blood in the blood vessel. The position of the corresponding blood group has changed between the mark and the detection, so it is called the time of flight. Method: First, a saturation pulse is used in the area to be contrasted, so that all tissues in the scanning range are saturated, that is, magnetic resonance signals are no longer generated. As the blood continues to flow, the saturated blood will flow out and flow into the unsaturated blood, which will produce a higher magnetic resonance signal, while the surrounding stationary tissue signal is low, thereby increasing the blood signal and inhibiting the surrounding tissue. signal. After reconstruction by computer, blood vessel morphology can be displayed. Phase contrast method: During the blood flow, the phase of the hydrogen protons can change, and this phase change does not occur in the stationary tissue. Therefore, phase contrast angiography uses blood flow induced phase changes to form a contrast between flowing protons and resting tissue, which distinguishes blood flow from surrounding tissue and completely eliminates signals from surrounding tissues. This method allows blood flow. The slow small blood vessels are enhanced to facilitate the display of microvessels. Three-dimensional inflow method: Using the flow enhancement effect, using three-dimensional global sampling, the excitation is divided into adjacent thin layers, so that the blood flow in the volume to be detected appears different from the MR high signal of other tissues, and the maximum intensity projection algorithm is used. Processing, a high resolution MRA image can be formed in the scan volume. Two-dimensional inflow method: When scanning, using a single thin layer sampling, a relatively strong inflow enhancement effect can be obtained, regardless of the selection of the direction of the layer selection, which can effectively cover a large range, and the same coverage can be obtained by superimposing two-dimensional volume. Range, but spatial resolution is not as good as the latter. In general, two-dimensional is used to observe large areas, sensitive to slow blood flow, and is only used to evaluate the degree of vascular stenosis; while three-dimensional technology provides a finer resolution image, sensitive to rapid blood flow, and arteriovenous malformation , intracranial aneurysms and other extremely diagnostic value. Although the value of MRA in intracranial vessels, neck vessels, and limb blood vessels is similar to conventional angiography, lesions with very slow blood flow may be missed, and spatial resolution is lower than DSA. With the continuous improvement of high-field magnetic resonance technology, MRA has gradually replaced the trend of intervention in DSA inspection. More concealed vascular lesions can be found using the contrast agent GD-DTPA in MRA. Not suitable for the crowd Pregnant women within 3 months of pregnancy, those with magnets in their bodies, such as those equipped with cardiac pacemakers, aneurysms, etc., prosthetic valves, people with metal foreign bodies remaining beside important organs. Adverse reactions and risks A few people can have a brief facial flush.