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Principles behind Magnetic Resonance Imaging (MRI)
Published in Michael Ljungberg, Handbook of Nuclear Medicine and Molecular Imaging for Physicists, 2022
In principle, the FID, as described above, is the result of a pulse sequence consisting of a single α-pulse, without any delay between the end of the RF pulse and the signal collection. When a time delay TE is introduced in combination with pulse-sequence components related to spatial encoding (as further described below), this design is referred to as a gradient-echo (or gradient-recalled-echo, GRE) sequence (at this point, see top and bottom rows of Figure 32.8). The signal is collected at the echo time TE, but the mechanisms of echo formation differ from those of the SE, as will be further explained below. No refocusing of spin dephasing related to time-invariant magnetic field inhomogeneities occurs in GRE (no 180° pulse is present), so the amplitude of the gradient-echo signal at time TE is thus governed by T2* relaxation during the time period from zero to TE.
Quantitative imaging using MRI
Published in Ruijiang Li, Lei Xing, Sandy Napel, Daniel L. Rubin, Radiomics and Radiogenomics, 2019
David A. Hormuth, John Virostko, Ashley Stokes, Adrienne Dula, Anna G. Sorace, Jennifer G. Whisenant, Jared A. Weis, C. Chad Quarles, Michael I. Miga, Thomas E. Yankeelov
The contrast between tissues on conventional magnetic resonance imaging (MRI) stems primarily from the water content of tissue and its inherent relaxation processes following radiofrequency excitation. Chief among these are the recovery of longitudinal magnetization following excitation, which is quantified by the time constant T1, and the decay of transverse magnetization, which is quantified by the time constant T2. Longitudinal relaxation is also known as spin-lattice relaxation, as it reflects the loss of spin excitation as thermal energy into the surrounding environment. Transverse relaxation is also known as spin-spin relaxation, as it is caused primarily by interaction with neighboring spins leading to dephasing. In practical settings, transverse relaxation occurs more quickly than predicted from T2 due to the presence of inhomogeneity in the main magnetic field. This dephasing due to inhomogeneity is captured by the relaxation time T2*, which incorporates the effect of inhomogeneity as well as T2 relaxation. In contrast with the aforementioned relaxation times, proton density reflects the number of protons (i.e., water content) in the tissue in the absence of relaxation.
Nonequilibrium Properties: Comparison of Experimental Results With Predictions of the BCS Theory
Published in R. D. Parks, Superconductivity, 2018
Suppose the spin system is initially in thermal equilibrium with the lattice in a strong field, and the field is then turned precipitously to zero. Using the sudden approximation one can see that the nuclei now precess about their local fields which are randomly oriented; in a few precession periods the nuclei will be randomly oriented, and the magnetization will be zero. The characteristic time for this disorientation or dephasing is T2, the spin-spin relaxation time, which arises from the magnetic interaction among the nuclei themselves. Usually T2 ≪ T1 in metals at low temperatures. If the field is again turned on precipitously, the magnetization will be trapped at zero in the strong field since the magnetization will require a time of order T2 to align itself following any sort of field change. Thus under these circumstances no resonance can be seen, because the magnetization cannot be established in the strong field without spin-lattice relaxation. The resonance has been lost completely and the magnetization and demagnetization have not been performed reversibly.
Composite-pulse enhanced room-temperature diamond magnetometry
Published in Functional Diamond, 2022
Yang Dong, Jing-Yan Xu, Shao-Chun Zhang, Yu Zheng, Xiang-Dong Chen, Wei Zhu, Guan-Zhong Wang, Guang-Can Guo, Fang-Wen Sun
However, for a quantum magnetometry based on NV center ensembles as shown in Figure 1, the detection sensitivity is usually limited by quantum dephasing process of electron spin [15]. The typical electron spin dephasing time is less than 1 µs, which is primarily caused by the spectrum detuning () due to inherent or extrinsic inhomogeneous broadening [15–20]. Randomly localized nuclei spins, nitrogen impurities (P1 centers) [6, 21], other unknown spins, strain gradients, magnetic-field gradients, and temperature fluctuations contribute to NV center dephasing process. Moreover, for the near-surface NV center in bulk diamond or NV center in nano-diamond, the electric field noise from surface charge fluctuations can be another part of spin decoherence and induce a significant inhomogeneous broadening [22]. Beside those intrinsic dephasing effects, detunings, and inhomogeneous in the microwave frequency and amplitude also destroy the coherence of system. With increasing the number of imperfect quantum controls, more rapid control errors accumulate and even generate spurious signals in quantum sensing process [23–25].
Entanglement-enabled interferometry using telescopic arrays
Published in Journal of Modern Optics, 2020
Siddhartha Santra, Brian T. Kirby, Vladimir S. Malinovsky, Michael Brodsky
Finite-lifetime quantum memories. Another example of decoherence in the network that leads to an X-state resource is dephasing in quantum memories. To show this, we consider a quantum network which relies on quantum memories and entanglement swapping to distribute the state between the two telescopes as shown in Figure 6. The simplest scheme comprises two sources of entangled Bell-pairs of photons, a set of quantum memories at the telescope sites and an entanglement swapping setup at the middle station. One photon of each entangled pair is stored in a quantum memory at the telescope site before entanglement swapping is performed by a joint measurement on one photon from each pair at a central station (25, 26). The action of an imperfect memory on a qubit σ stored in the memory for a time t can be modelled as dephasing of the off-diagonal elements. Following (26), we describe a single-qubit dephasing using a super operator, , acting on the qubit density matrix, , as where , is the memory coherence time and is the spin Pauli operator along the z-direction.
How electronic dephasing affects the high-harmonic generation in atoms
Published in Molecular Physics, 2020
Our goal is understanding the response of the various harmonics in the HHG spectrum of hydrogen and argon atoms to an applied electronic dephasing, that erases the coherence of the quantum state of the system. Hydrogen atom has been chosen as the simplest one-electron system to study the effect of pulse parameters, as intensity and frequency. Given a value, one can see how many and which harmonics ‘survive’ in the HHG spectrum, when compared with the standard HHG spectrum obtained with a coherent propagation of the wave packet [17,19,41].