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echoes. The FSE/TSE pulse sequence superficially resembles a conventional spin-echo (CSE) sequence in that it uses a series of 180º-refocusing pulses after a single 90º-pulse to generate a train of echoes. The FSE/TSE technique, however, changes the phase-encoding gradient for each of these echoes (a conventional multi-echo sequence collects all echoes in a train with the same phase encoding). As a result of changing the phase-encoding gradient between echoes, multiple lines of k-space (i.e., phase-encoding steps) can be acquired within a given repetition time (TR). As multiple phase-encoding lines are acquired during each TR interval, FSE/TSE techniques may significantly reduce imaging time.
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Due to local magnetic field inhomogeneities (variations in the magnetic field at different parts of the sample that are constant in time), as the net moment precesses, some spins slow down due to lower local field strength (and so begin to progressively trail behind) while some speed up due to higher
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Fast spin echo (RARE, FAISE or FSE), also called turbo spin echo (TSE) is an MRI sequence that results in fast scan times. In this sequence, several 180 refocusing radio-frequency pulses are delivered during each echo time (TR) interval, and the phase-encoding gradient is briefly switched on between
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In 2020 two teams demonstrated that when strongly coupling an ensemble of spins to a resonator, the Hahn pulse sequence does not just lead to a single echo, but rather to a whole train of periodic echoes. In this process the first Hahn echo acts back on the spins as a refocusing pulse, leading to
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Hahn's 1950 paper showed that another method for generating spin echoes is to apply three successive 90° pulses. After the first 90° pulse, the magnetization vector spreads out as described above, forming what can be thought of as a "pancake" in the x-y plane. The spreading continues for a time
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is included and each spin experiences perfect pulses during which the environment provides no spreading. Six spins are shown above and these are not given the chance to dephase significantly. The spin-echo technique is more useful when the spins have dephased more significantly such as in the
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when he applied two successive 90° pulses separated by short time period, but detected a signal, the echo, when no pulse was applied. This phenomenon of spin echo was explained by Erwin Hahn in his 1950 paper, and further developed by
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introduced spin-echo neutron scattering, a technique that can be used to study magnons and phonons in single crystals. The technique is now applied in research facilities using triple axis spectrometers.
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The vertical red arrow is the average magnetic moment of a group of spins, such as protons. All are vertical in the vertical magnetic field and spinning on their long axis, but this illustration is in a
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time, as shown in the animation below. The size of the echo is recorded for different spacings of the two pulses. This reveals the decoherence which is not refocused by the π pulse. In simple cases, an
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Progressively, the fast moments catch up with the main moment and the slow moments drift back toward the main moment. At some moment between E and F the sampling of the echo starts.
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who pointed out the advantages of using a 180° refocusing pulse for the second pulse. The pulse sequence may be better understood by breaking it down into the following steps:
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absorption resonance. Instead of using two spin states in a magnetic field, photon echoes use two energy levels that are present in the material even in zero magnetic field.
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at different rates. The first of these, relaxation, leads to an irreversible loss of magnetisation. But the inhomogeneous dephasing can be removed by applying a 180°
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Debnath, Kamanasish; Dold, David; Morton, John J. L.; Mølmer, Klaus (2020). "Self-Stimulated Pulse Echo Trains from
Inhomogeneously Broadened Spin Ensembles".
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J. E. Tanner & E. O. Stejskal (2003). "Restricted Self-Diffusion of
Protons in Colloidal Systems by the Pulsed-Gradient, Spin-Echo Method".
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Weichselbaumer, Stefan; Zens, Matthias; Zollitsch, Christoph W.; Brandt, Martin S.; Rotter, Stefan; Gross, Rudolf; Huebl, Hans (2020).
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Carr, H. Y.; Purcell, E. M. (1954). "Effects of
Diffusion on Free Precession in Nuclear Magnetic Resonance Experiments".
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A 180 degree pulse is now applied so that the slower spins lead ahead of the main moment and the fast ones trail behind.
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Hahn echos have also been observed at optical frequencies. For this, resonant light is applied to a material with an
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930:"Partial RF echo planar imaging with the FAISE method. I. Experimental and theoretical assessment of artifact"
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effects removed. Quite separately, return of the red arrow towards the vertical (not shown) would reflect the
977:"Partial RF echo-planar imaging with the FAISE method. II. Contrast equivalence with spin-echo sequences"
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T1-weighted turbo spin echo MRI confirms a fracture, as the surrounding bone marrow has low signal from
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vectors. Examples of inhomogeneous effects include a magnetic field gradient and a distribution of
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Melki, Philippe S.; Mulkern, Robert V.; Panych, Lawrence P.; Jolesz, Ferenc A. (May–June 1991).
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Mezei, F. (1972), "Neutron spin echo: A new concept in polarized thermal neutron techniques",
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How does MRI work?: An
Introduction to the Physics and Function of Magnetic Resonance Imaging
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Kurnit, N. A.; Abella, I. D.; Hartmann, S. R. (1964). "Observation of a photon echo".
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a third pulse is applied and a stimulated echo is observed after waiting for a time
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field strength and start getting ahead of the others. This makes the signal decay.
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A 90° pulse has been applied that flips the arrow into the horizontal (x–y) plane.
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observed following an initial excitation pulse decays with time due to both spin
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of dephasing, the inhomogeneous evolution will rephase to form an echo at time 2
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Animation of spin echo, showing the response of spins (red arrows) in the blue
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relaxation. 180 degrees is π radians so 180° pulses are often called π pulses.
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Melki, Philippe S.; Jolesz, Ferenc A.; Mulkern, Robert V. (August 1992).
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Melki, Philippe S.; Jolesz, Ferenc A.; Mulkern, Robert V. (August 1992).
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showing a suspected compressive subcapital fracture as a radiodense line
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which have been used in fields other than magnetic resonance including
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shows the same, atypical for a fracture since the cortex is coherent
1030:(2nd ed.). Springer Science & Business Media. p. 64.
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883:"Comparing the FAISE method with conventional dual-echo sequences"
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Complete refocusing has occurred and at this time, an accurate
563:(1979). "NMR population inversion using a composite pulse".
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Echoes were first detected in nuclear magnetic resonance by
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A Hahn-echo decay experiment can be used to measure the
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Several simplifications are used in this sequence: no
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in 1950, and spin echoes are sometimes referred to as
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Principles of Pulse
Electron Paramagnetic Resonance
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Spin
Dynamics: Basics of Nuclear Magnetic Resonance
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134:Echo phenomena are important features of coherent
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27:Response of spin to electromagnetic radiation
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1022:Weishaupt D, Köchli VD, Marincek B (2008).
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1052:"What is Fast (Turbo) Spin Echo imaging?"
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61:magnetisation by a pulse of resonant
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668:Hahn, E.L. (1950). "Spin echoes".
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270:echo can be measured with all
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565:Journal of Magnetic Resonance
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710:(2), pp. 146–160.
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156:Erwin Hahn
82:relaxation
78:NMR signal
69:(NMR) and
1098:. Wiley.
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