Nuclear Magnetic Resonance (NMR) is a powerful technique that utilizes the behavior of atomic nuclei in a magnetic field to study the structure and properties of matter. Since its inception, NMR has played an extremely important role in multiple fields such as chemistry, biology, medicine, and materials science. Below, we will explore in detail the working principle of NMR.



1、 The basic principles of nuclear magnetic resonance
The basic principle of nuclear magnetic resonance is that hydrogen nuclei (i.e. 1H) in the human body under a strong external magnetic field have different relaxation times under specific radio frequency (RF) pulses.
1. The human body generates longitudinal magnetic vectors and 1H precession within a strong external magnetic field:
1H rotates around its own axis while also making conical movements around the direction of the external magnetic field, known as precession, and the frequency of precession is proportional to the strength of the external magnetic field.

2. Emission of specific RF pulses causes magnetic resonance phenomenon:
RF pulses of a specific frequency are emitted to the human body within a strong external magnetic field, and energy is absorbed for 1 hour, resulting in magnetic resonance phenomenon.

3. After stopping the RF pulse, it returns to its original state and generates a signal for 1 hour.
After stopping the emission of the RF pulse, it quickly returns to its original equilibrium state for 1 hour. This process is called the relaxation process, and the required time is called the relaxation time. The time for longitudinal magnetic vector recovery is the longitudinal relaxation time, also known as T1 relaxation time; The decay and disappearance time of the transverse magnetic vector is called the transverse relaxation time, also known as the T2 relaxation time.

4. Collecting, processing MR signals and reconstructing them into MRI images:
For MR signals that reflect the T1 and T2 values of human tissue structure, a series of complex processes such as acquisition, encoding, and calculation can be performed to reconstruct them into MRI grayscale images.



2、 The process of nuclear magnetic resonance
Preparation stage: Firstly, place the sample in a strong static magnetic field (usually referred to as B0 field). This magnetic field is strong enough to align the magnetic moment of the atomic nucleus with the direction of the magnetic field.
Excitation stage: Next, use a radio frequency pulse (RF pulse) perpendicular to the B0 field to excite the atomic nuclei in the sample. When the frequency of the RF pulse matches the Zeeman splitting frequency of the atomic nucleus in the B0 field, the nucleus absorbs energy and transitions from one energy level to another.
Signal generation: When the RF pulse stops, the atomic nucleus will begin to return to its original energy level and release energy. The process of releasing this energy will generate a weak electromagnetic signal, known as an NMR signal. This signal can be detected and recorded by the receiver.
Signal analysis: By analyzing NMR signals, we can obtain information about the types and quantities of atomic nuclei in the sample, as well as how they interact with other atomic nuclei. This information can be used to infer the molecular structure and properties of the sample.

3、 The application of nuclear magnetic resonance
NMR technology has been widely applied in multiple fields. In chemistry, NMR is used to determine the structure and composition of molecules; In biology, NMR is used to study the structure and function of biological macromolecules; In medicine, NMR is used as an imaging technique (MRI) to obtain non-invasive information about the human body's interior; In materials science, NMR is used to study the physical and chemical properties of materials.

Nuclear magnetic resonance is a powerful technique for studying the structure and properties of matter based on the behavior of atomic nuclei in a magnetic field. It generates NMR signals by utilizing the Zeeman splitting of atomic nuclei in a magnetic field and the excitation of RF pulses, and obtains information about the sample by analyzing these signals. NMR technology has extensive applications in various fields such as chemistry, biology, medicine, and materials science, and is one of the indispensable tools in modern scientific research.