Why is there so much movement during an MRI? so long? What's going on in the machine?
Why is the movement so loud?
The concept of gradient field, simply put, the gradient field is a field generated by an electric field that changes with the strength of the magnetic field in space. To accurately locate in three-dimensional space, three coordinate axes are required, so that any part of the body can be positioned. , when the specific sequence is implemented, you can think like this, in the three-dimensional coordinate system, cut along an axis, you can locate a surface, if you locate it along a direction on this surface, you can locate a line, if With an additional orientation orientation, you can locate a point on the line. Then these three axes are simply, according to the doctor's instructions, locate each point of your body with abnormal parts, and image it. The functions of the three axes are interchangeable. The key reason for noise is that you can choose a different volume for each of your imaging points. For example: If you want to look at the 5cm thickness of the knee meniscus, then the size of your gradient is different from the size of the 3cm thickness.
And you have to locate each point, so you need to do it by constantly switching the size of the gradient and the time of the work, which is part of the concept of the sequence in MR. Because the magnitude of the gradient needs to change, the magnetic field generated by the electric field will change. In this process, an electric field that prevents the magnetic field from changing will be generated, which is called eddy current. The larger and faster the magnetic field changes, the larger the eddy current will be. According to high school physics, it can be known that the energized wire will be affected by the Lorentz force under the condition of the magnetic field. The gradient coil itself is the energized wire, so it is affected by the force. Although it has been fixed, it will still cause vibration, and the vibration of a certain frequency will bring noise. Because the structures of the gradient coils in the three directions are different and the direction of the force is different, the sound generated during the vibration will not be different. The added gradient size and duration will also bring about different sounds. However, magnetic resonance manufacturers generally reduce the noise to the range below 120db, which is harmless to the human body according to international standards. But the existing technology still can't completely eliminate the noise.
Why is it taking so long?
Magnetic resonance imaging determines tissue characteristics according to the energy level transition of H protons in water molecules in the human body. It is also a water molecule. Water in fat and water in muscle tissue have different energy levels due to different chemical environments. The transition will be fast and slow, that is, the relaxation time will be different. According to this characteristic, the character is acquired at different time points, and the characteristics of the energy level state of each tissue water molecule are displayed on the signal level, which also distinguishes the light and dark in the image. But sometimes the characteristics of the diseased tissue are very similar to the characteristics of the normal tissue in the human body, which requires different sequences to judge whether the tissue is a normal or abnormal signal.
Sometimes the diseased tissue has similar characteristics to fat or water, and a single imaging is easy to miss the diagnosis, so some sequences that inhibit water or inhibit fat are scanned. How to achieve it depends on the physical and physiological characteristics involved. So it usually takes a long time to scan. And to determine one of the main reasons for the long scanning time for each point, if you need to look more closely, the smaller the volume of each point, the longer the scanning time will be. To give you a simple example, the smaller the grid of a piece of paper, the time it takes for you to point once will definitely be slower than the one with only a few grids, but we have to make the point small enough in the MRI scan, because it is only small enough In order to accurately locate and find the lesion area.
Change positions every 2-3 minutes?
Generally, a regular sequence takes 2-3 minutes. Sometimes it is to scan sequences with different characteristics, not to change the position. Sometimes the direction of the selected level is indeed exchanged, such as the knee joint being cut from the sky to the ground, and the direction of the knee joint from the sky to the ground. When cutting from the foot to the head, the level seen is different. Due to the complex structure of the human body, it is necessary to confirm from multiple angles in order to accurately seek medical treatment.
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