Principle of magnetic resonance holographic imaging

Magnetic resonance imaging (MRI) is an imaging technique, which uses the signals generated by hydrogen nuclei in a magnetic field to reconstruct images.

Every hydrogen proton in the human body can be regarded as a small magnet, and the protons are arranged in disorder before entering the strong external magnetic field. When placed in a strong external magnetic field, they are only arranged in two directions parallel or anti-parallel to the magnetic field lines of the external magnetic field.

The protons parallel to the external magnetic field line are at the low energy level, while the protons anti-parallel to the external magnetic field line are at the high energy level, and the former is slightly more than the latter.

Under the excitation of a certain frequency RF pulse, some low-energy protons jump to a high-energy level. When the RF pulse stops, they return to their original state and release energy in the form of RF signals. These released RF signals are three-dimensionally encoded, received by external coils, and reconstructed into images after computer processing.

Classification description: 1, MRI angiography: the basic principle of MRI angiography. Magnetic resonance angiography (MRA) is a technique to display the characteristics of blood vessels and blood flow signals.

As a non-invasive examination, MRA has special advantages compared with CT and conventional radiology. It does not need to use contrast agent, but the flowing liquid is. As the inherent physiological contrast agent in magnetic resonance imaging, the commonly used MRA methods are time-of-flight (TOF) method and phase contrast (PC) method. However, in order to improve the image quality, blood vessels can also be displayed by contrast agents.

2. Diffusion imaging of magnetic resonance: The basic principle of diffusion imaging of magnetic resonance (DI) is imaging by Brownian motion (random thermal motion of molecules) in tissues. It can be used to check cerebral ischemia. Due to the ischemic changes of brain cells and different nerve bundles, the diffusion movement of water molecules is limited, which can be shown by diffusion-weighted imaging (DWI).

3. MR perfusion imaging: basic principle:

Perfusion imaging (PI) shortens the T 1 and T2 values of the imaged tissue by introducing paramagnetic contrast agent, and obtains the time resolution of imaging by ultra-fast imaging method. Through the change rate of T 1 and T2 value of peripheral tissue microcirculation after intravenous injection of paramagnetic contrast agent, the tissue blood perfusion function was calculated.

4. Magnetic resonance functional imaging: Brain activity functional imaging is an imaging technique that uses the change of the ratio of oxygenated hemoglobin to deoxyhemoglobin in the local blood of brain activity area to cause the change of local tissue T2, thus reflecting the local activity function of brain tissue on T2-weighted images.

This technique is also called MRI (BOLD MRI. It is obtained by stimulating peripheral nerves, activating corresponding cortical centers, increasing blood flow in the central area, and then causing changes in blood oxygen concentration and magnetic susceptibility.

(Medical Education Network)