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How to unlock the hidden potential of magnetic resonance with 180° pulses? Uncovering the mysterious echo in the magnetic field!
In magnetic resonance technology, a key phenomenon is the "spin echo", which is the refocusing signal of the spin magnetization due to the application of a resonant electromagnetic radiation pulse. This phenomenon plays an important role in modern nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). The NMR signal observed after the initial excitation pulse decays over time, mainly due to spin relaxation and inhomogeneity effects. These inhomogeneities cause the spins in the sample to precess at different rates, affecting the stability of the signal.
In the case of spin relaxation, the irreversible loss of magnetization leads to a decrease in the signal. However, by applying a 180° inversion pulse, these uneven dephasing effects can be eliminated.
Take the distribution of various magnetic field gradients and chemical shifts as examples, which are specific manifestations of the inhomogeneous effect. If, after a period of dephasing, an inversion pulse is applied, the inhomogeneous evolution can be rephased, thus producing an echo at time 2t.
Historical Background of Spin Echo
The spin echo phenomenon was first discovered by Erwin Hahn in 1950 and is now often referred to as the Hahn echo. In MRI and MRI, the most commonly used form of radiation is radio frequency radiation. In 1972, F. Mezei introduced the spin-echo neutron scattering technique, which can be used to study spin waves and phonons in single crystals. With the continuous advancement of technology, research by two teams in 2020 showed that when spin clusters are strongly coupled to a resonator, the Hahn pulse sequence can produce a series of periodic echoes. This discovery undoubtedly expands the field of spin echoes. application potential.
Principle of spin echo
The principle of spin echo originates from earlier experiments by Hahn, who discovered the appearance of an echo by applying two 90° pulses to observe the signal but without applying a measuring pulse. This phenomenon was described in detail in his 1950 paper and further generalized by Carr and Percher, who emphasized the advantages of using 180° inversion pulses.
We can better understand the process by simplifying the pulse sequence into its individual steps.
Spin echo decay
Hahn echo decay experiments can be used to measure the spin-spin relaxation time (T2). At different pulse intervals, the intensity of the echoes was recorded, reflecting the dephasing effect that was not refocused by the inversion pulse. In simple cases, echoes show an exponential decay, which is usually described by the T2 time.
Stimulus Echo
Hahn's 1950 paper also demonstrated another way to produce spin echoes, which was to apply three consecutive 90° pulses. In this process, after the first pulse is applied, the magnetization vector begins to expand to form a "pancake-shaped" structure, while the second pulse transforms the structure into three-dimensional space, and finally the stimulation echo is observed after the third pulse. .
Photon Echo
In addition to spin echoes, Hahn echoes can also be observed at optical frequencies. By applying resonant light to a material with inhomogeneous absorption resonance, the phenomenon of photon echoes can still exist even in zero magnetic field.
Fast Spin Echo
Rapid spin echo (such as RARE, FAISE, or FSE) is an MRI sequence that can significantly reduce scan time. In this sequence, the radio frequency pulses are refocused through 180° multiple times, with the phase encoding gradients briefly switched between each echo. This technology significantly improves imaging speed and becomes an important technological innovation in the field of MRI.
Future Outlook
With the evolution of technology, the application scope of magnetic resonance continues to expand, and the academic community continues to deepen its research on spin echo. This not only helps improve the accuracy of medical imaging, but also provides new ideas for the development of new materials and quantum technologies. So, how will we use these technologies to unlock more potential in the future?
