The most commonly used nucleus of magnetic resonance is hydrogen proton (1H), because its signal is the strongest and it is also widely found in human tissues. The factors that affect the magnetic resonance image include: (a) the density of protons; (b) the length of relaxation time; (c) the flow of blood and cerebrospinal fluid; (d) paramagnetic materials; Protein.
The characteristics of gray scale of magnetic vibration image are that the stronger the magnetic vibration signal, the greater the brightness, the weaker the magnetic vibration signal and the smaller the brightness, ranging from white, gray to black.
The gray features of MRI images of various tissues are as follows: adipose tissue and cancellous bone are white; Cerebrospinal fluid and bone marrow are white gray; Viscera and muscles are grayish white; Liquid, flowing at normal speed, blood is black; Bone cortex, air and air-containing lungs are all black.
Another characteristic of NMR vibration is that the flowing liquid does not produce signal, which is called flow effect or flow blank effect. So blood vessels are gray-white tubular structures, while blood is black and has no signal. This makes it easy to separate blood vessels from soft tissues. Normal spinal cord is surrounded by cerebrospinal fluid, which is black and has white dura mater set off by fat, which makes the spinal cord show white strong signal structure.
Magnetic resonance imaging (MRI) has been applied to imaging diagnosis of various systems in the whole body. The best effect is the brain, its spinal cord, heart blood vessels, joint bones, soft tissues and pelvic cavity. For cardiovascular diseases, we can not only observe the anatomical changes of heart cavities, great vessels and valves, but also analyze ventricles for qualitative and semi-quantitative diagnosis. We can also make multiple sections with high spatial resolution to show the whole picture of heart and lesions and their relationship with surrounding structures, which is superior to other X-ray imaging, two-dimensional ultrasound, nuclides and CT examinations. In the diagnosis of encephalomyelitis, coronal, sagittal and transverse images can be made.
The information provided by MRI is not only greater than many other imaging methods in medical images, but also different from the existing imaging methods. Therefore, it has great potential advantages in disease diagnosis. It can directly make cross-sectional, sagittal, coronal and various inclined planes without artifacts in CT detection; No need to inject contrast agent; No ionizing radiation, no adverse effects on the body. MRI is very effective in detecting common brain diseases such as intracerebral hematoma, extracerebral hematoma, brain tumor, intracranial aneurysm, arteriovenous malformation, cerebral ischemia, intraspinal tumor, syringomyelia, hydrocephalus, etc., and it is also very effective in diagnosing diseases such as lumbar disc herniation and primary liver cancer.
Purpose of examination: brain, spine and spinal cord diseases, five sense organs diseases, heart disease, mediastinal mass, bone and joint and muscle diseases, uterus, ovary, bladder, prostate, liver, kidney and pancreas diseases.
Advantages: 1 MRI has no ionizing radiation damage to human body;
2.MRI can obtain the original three-dimensional tomography, and multi-directional images can be obtained without reconstruction;
3. The soft tissue structure is clearly displayed, and the examination of central nervous system, bladder, rectum, uterus, vagina, joints and muscles is superior to CT.
4. Multi-sequence imaging and various image types provide more abundant image information for determining the nature of lesions.
Disadvantages: 1 Like CT, MRI is also an imaging diagnosis. Many lesions are still difficult to be diagnosed by MRI alone, unlike endoscopy, which can obtain both imaging and pathological diagnosis.
2. 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;
3. Gastrointestinal lesions are not as good as endoscopy;
4. The sensitivity of fracture diagnosis is not as good as that of CT and X-ray plain film;
People with metal objects in their bodies should not receive MRI.
6. Critically ill patients should not do this.
7. It is not recommended to do MRI examination within 3 months of pregnancy unless it is necessary.
8. People with pacemakers can't do MRI and can't get close to MRI equipment.
9. The examination space of most magnetic resonance equipment is relatively closed, and some patients can't cooperate to complete the examination because of fear.
10. The inspection takes a long time.
Matters needing attention
Because there is a very strong magnetic field in the MRI machine and MRI examination room, it is absolutely forbidden to do MRI examination for people with pacemakers, people who have left metal clips or metal stents after vascular surgery, or people who have undergone metal stent surgery such as coronary artery, esophagus, prostate and biliary tract. Otherwise, the metal will move due to the attraction of strong magnetic field, which may cause serious consequences and even endanger life. Generally, there are red or yellow eye-catching signs outside the MRI examination room of the hospital, indicating that MRI examination is absolutely forbidden.
If there are other metal foreign bodies that can't be taken out, such as metal internal fixings, artificial joints, metal dentures, brackets, silver clips, shrapnel and other metal residues, it is taboo to check them. When inspection is necessary, close observation should be made to prevent the metal from moving in a strong magnetic field during inspection, damaging adjacent large blood vessels and important tissues and causing serious consequences. If there is no special need, generally should not be nuclear magnetic resonance examination. Metal contraceptive rings and removable metal dentures must be removed before inspection.
Sometimes, the residual metallic iron ions in the body may affect the image quality and even the correct diagnosis.
Before entering the nuclear magnetic resonance examination room, remove the mobile phone, pager, magnetic card, watch, coin, key, lighter, metal belt, metal necklace, metal earrings, metal buttons and other metal ornaments or items. Otherwise, it may affect the uniformity of magnetic field, cause image interference and form artifacts, which is not conducive to the display of lesions; Moreover, due to the strong magnetic field, metal objects may be sucked into the nuclear magnetic vibration machine, thus damaging the very expensive nuclear magnetic vibration machine; In addition, mobile phones, pagers, magnetic cards, watches and other items may also be damaged by strong magnetic fields, causing unnecessary personal property losses.
zeugmatography
In recent years, with the progress and development of science and technology, many orthopedic internal fixators, especially spinal internal fixators, have begun to be made of titanium alloy or titanium metal. Because titanium is not attracted by the magnetic field, it will not move in the magnetic field. Therefore, it is safe for patients with titanium internal fixator to undergo MRI vibration examination; Moreover, titanium metal will not interfere with the MRI image. This is very valuable for patients with spinal diseases who need spinal internal fixation surgery. However, the internal fixation of titanium alloy and titanium metal is expensive, which affects its popularization and application to some extent.