Everything you want to know about MRI is here! Full analysis!

When you go to the hospital for a checkup, sometimes the doctor will ask you to do an MRI. We will find that there are very eye-catching warnings in front of the MRI examination room: No metal objects are allowed to enter. Some hospitals even require you to change clothes in the locker room and remove all metal objects. Some hospitals have metal detectors in the MRI examination room, similar to the airport security equipment, to check whether the metal objects are completely removed. When you finally lie on the examination bed for the examination, the doctor will usually put something like a "rack" on us, and then we will be sent to a cylindrical hollow "examination chamber". The examination noise is relatively loud and the examination time is relatively long, which may make people feel a little depressed.

At this point, you may be confused. Why is the MRI so strict about metal inspection? What is the "frame" on my body used for? What is the difference between a 1.5T and a 3.0T MRI? Does "MRI" really have no radiation? If I have a certain disease, I must do an MRI, but not a CT scan?

There are still a lot of questions, okay, in this issue let’s talk about what MRI is.

1. What is MRI

MRI is the abbreviation of Magnetic Resonance Imaging. Its Chinese name is magnetic resonance imaging, which is also known as "nuclear magnetic resonance examination". MRI uses the principle of nuclear magnetic resonance to perform imaging. It can not only show the morphological structure of tissues and organs, but also the functional status and biochemical information of certain organs. MRI is a safe imaging examination without ionizing radiation. In order to avoid confusion with nuclear medicine radiography, it was renamed "magnetic resonance imaging" from the previous "nuclear magnetic resonance imaging".

2. MRI Imaging Principle

Since MRI uses the principle of nuclear magnetic resonance for imaging, let us first understand what "nuclear magnetic resonance" is.

More than 70 percent of the human body is made up of water. A water molecule contains one oxygen atom and two hydrogen atoms. The nucleus of each hydrogen atom is a proton (positively charged). The charged protons are spinning all the time, thus generating a weak magnetic field, just like each proton is a small magnetic needle. We call the magnetic field contributed by such a spinning proton a "small nuclear magnet."

Normally, the arrangement of "small nuclear magnets" in the human body is random. However, when the human body is placed in a strong external magnetic field, all the "small nuclear magnets" in the human body will tend to arrange themselves in the direction of the magnetic field.

At this time, a radio frequency pulse of a certain frequency is emitted to the human body, and the "small nuclear magnets" in the body absorb energy due to resonance, thereby changing their arrangement. When the external radio frequency pulse disappears, the changed "small nuclear magnets" will return to their initial position and release energy, that is, emit an electromagnetic signal. This process is called nuclear magnetic resonance. The MR device receives the electromagnetic signal released by the nuclear magnetic resonance at this time, and after computer calculation, the final result is the MR image.

Don't understand? Let's put it simply. There are many "small nuclear magnets" generated by hydrogen proton spins in the human body. Originally, they are arranged in a disorderly manner. If they are placed in a strong magnetic field, their positions will be "relatively fixed". At this time, if they are continuously given energy, they will absorb energy and change their positions. When the energy supply is stopped, they will return to the fixed position again and emit energy. The equipment receives the energy signal emitted at this time, and after computer calculation, a magnetic resonance image is obtained.

Still don't understand? Let's use another vivid analogy: Nuclear magnetic resonance uses a strong external magnetic field to turn the human body into a musical instrument with taut strings. The external radio frequency signal is like a pair of invisible hands that play the instrument. The fingers pluck the strings (applying energy to deform the strings), and the strings vibrate to produce sound (the strings return to their shape and release energy). The "music" produced by the playing can be used to analyze the internal structure of the human body.

3. MRI Equipment Composition

Magnetic resonance imaging equipment consists of:

1. Main magnet: provides external magnetic field, divided into permanent magnet type and electromagnet, among which electromagnet is further divided into superconducting type and normal conductive type.

2. Gradient system: Its main function is to provide three-dimensional spatial positioning for magnetic resonance imaging. It is generally composed of three gradient coils: X, Y, and Z.

3. Radio frequency system: It stimulates magnetic resonance at the detection site and collects magnetic resonance signals. It consists of a radio frequency coil, a radio frequency generator and a receiver. The "frame" on our body is the radio frequency coil.

4. Spectrometer system: The central control system of the magnetic resonance equipment, responsible for generating and controlling each link of the sequence and coordinating the operation. Signal acquisition, data processing and image reconstruction are mainly completed by the spectrometer system.

5. Computers and auxiliary facilities, etc.: including main control computer, image display, examination bed and radio frequency shielding, magnetic shielding, UPS power supply, cooling system, etc. Its function is to ensure that the process from the beginning of the examination to the acquisition of MR images can be carried out in an orderly and accurate manner.

Here we will focus on the main magnet. The MRs we often hear about are 1.5T and 3.0T, of which 1.5T and 3.0T refer to the field strength of the main magnet, both of which are superconducting types. Generally speaking, the higher the magnetic field strength of the main magnet, the better the scanning time and image quality.

Many people may not have a concept of the unit "T". For comparison, the magnetic field strength of 3.0T is about 60,000 times that of the earth's magnetic field! In the MRI examination room, a small coin can fly like a "bullet" and hit a person with a "snap" sound, causing human injury; if it is a wheelchair, the impact force is comparable to a car accident scene; not to mention the absolute contraindications such as pacemakers. According to literature reports, there are no less than 10 deaths worldwide each year due to carrying metal objects into the MRI room. Therefore, you must cooperate with the doctor when doing an MRI examination, and you must not bring prohibited items.

4. Characteristics of MRI

Advantages of MRI over CT:

1. No ionizing radiation.

We often say that X-rays have radiation that is harmful to human health because the wavelength of X-rays is about 10-10 power m. The wavelength is very short and the energy is very large. It can interact with the human body to produce ionizing radiation and even when the energy is even greater, it can directly destroy the DNA structure.

The radio frequency electromagnetic waves used in MR have a very long wavelength and an intensity close to the radio waves in our lives. Such radio frequency electromagnetic waves do not cause ionizing radiation to the human body.

2. Excellent resolution of soft tissue.

MRI uses hydrogen protons, or water, to image. Therefore, soft tissues with high water content are the best imaging objects for MR. On the contrary, tissues with very little water, such as bones and calcifications, are not as good as CT in MR imaging.

For example, in the three pictures below, MR's presentation of brain tissue is closer to the actual brain tissue anatomy, but the skull appears black and can hardly be displayed. CT is much worse than MR in identifying brain tissue, but the skull can be displayed.

3. Multi-directional imaging.

We know that CT is a tomographic imaging, and the original image of CT is the transverse plane imaging of the human body. Through post-processing technology, other planes can be reconstructed and displayed. MRI can directly scan and image at any angle and any plane.

4. Multi-parameter imaging.

When checking MR films or reports, we often look at parameters such as T2WI, T2WI, STIR, DWI, MRA, etc., which are collectively called "scanning sequences". Different scanning sequences can be used to obtain magnetic resonance images with different focuses, such as T1, T2, PD, etc. that display tissue structure and pathological changes through relaxation rate; STIR, FLAIR, etc. that highlight pathological changes with water and fat suppression; DWI that reflects the activity level of water molecules; MRA, MRV, etc. that display arteries and veins; MRS that displays metabolic products, etc.

Disadvantages of MRI compared to CT:

1. The scanning time is long.

MR scanning takes a long time, and the scanning area cannot be moved during the entire process. For those who cannot maintain posture for a long time, such as infants and young children, sedatives may be used if necessary.

2. The display effect of bones and calcified tissues is not as good as CT.

The content of hydrogen protons in bone structure and calcification components is very low, and usually appears as low signal on MRI. The details shown are not as good as CT, but the display effect of bone marrow lesions is worse than CT.

3. There are relatively more absolute and relative contraindications.

During the MRI examination, any external metal devices such as oxygen cylinders, wheelchairs, beds, and ferromagnetic objects on the examinee, such as watches, mobile phones, magnetic cards, metal jewelry, and dentures, are not allowed to enter the examination room. People with metal implants or claustrophobia also need to pay attention and communicate with the examiner or nurse in advance. People with pacemakers are strictly prohibited from undergoing MRI examinations (see Examination Precautions for details).

5. Clinical Application of MRI

1. Neurological diseases: MRI is the most effective imaging diagnostic method for neurological diseases today. It can detect cerebral infarction, cerebral hemorrhage, brain tumors, inflammation, congenital malformations, trauma, etc. at an early stage.

2. Cardiovascular system diseases: It can be used for the diagnosis of atherosclerosis, vascular occlusion, heart disease, cardiomyopathy, pericardial tumors, pericardial effusion, mural thrombosis, arterial dissection, etc.

3. Chest lesions: It can be used for the diagnosis of tumors in the mediastinum, lymph nodes and pleural lesions.

4. Abdominal lesions: It can be used to diagnose abdominal viscera and retroperitoneal lesions, such as liver cancer, hepatic hemangioma, kidney cancer, pancreatic cancer, adrenal cancer, etc.

5. Pelvic lesions: It can be used for the qualitative and localization of uterine fibroids, endometrial cancer, cervical cancer, ovarian tumors, pelvic masses, and the diagnosis of rectal, prostate and bladder tumors.

6. Bone and joint lesions: It has high diagnostic value for the diagnosis of bone infectious lesions, bone tumors, and trauma. It has high diagnostic value for abnormalities of articular cartilage, ligaments, meniscus, synovium, synovial bursa, and bone marrow lesions.

7. Systemic soft tissue lesions: Regardless of whether they originate from tumors, infections, degenerative lesions of nerves, blood vessels, lymphatic vessels, muscles, or connective tissues, accurate positioning and qualitative diagnosis can be made.

6. Precautions for MRI examination

Due to its high magnetic field and the particularity of imaging methods, MRI examinations take a long time and have contraindications, requiring full cooperation from the examinee.

1. Make sure you do not have any pacemakers, hearing aids, ferromagnetic vascular clips, artificial eyes, artificial limbs, or other metal implants installed in your body.

2. Make sure there is no history of trauma with metal objects, such as bullets, nails, and metal fragments.

3. Before the inspection, you must remove any metal objects and electronic products you are carrying, including necklaces, earrings, rings, hairpins, bras, zippers, glasses, coins, keys, watches, magnetic cards, mobile phones, headphones, etc.

4. Pregnant women should inform the doctor or technician before the examination.

5. For infants and children, and those with claustrophobia, an assessment should be conducted in advance to determine whether they can successfully complete the examination, and sedation measures should be taken if necessary.

6. When women with IUDs undergo pelvic examination, it is recommended that this examination be performed after the IUD is removed.