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Fig. 5. (A) In the spin-echo sequence, a 90° RF pulse brings the M vector into the x-y plane. Different components of the transverse magnetization precess at slightly different rates owing to static/local field inhomogeneities. These will dephase (B) and cancel each other, resulting in a net zero vector if not for the 180° RF pulse that rephases them (C).

Fig. 5. (A) In the spin-echo sequence, a 90° RF pulse brings the M vector into the x-y plane. Different components of the transverse magnetization precess at slightly different rates owing to static/local field inhomogeneities. These will dephase (B) and cancel each other, resulting in a net zero vector if not for the 180° RF pulse that rephases them (C).

Fig. 6. T2* reflects the decay of echoes themselves with signal loss from both static and dynamic inhomogeneities. T2 decay represents the overall amplitude decay of the spin echo via dynamic inhomogeneities.

Fig. 6. T2* reflects the decay of echoes themselves with signal loss from both static and dynamic inhomogeneities. T2 decay represents the overall amplitude decay of the spin echo via dynamic inhomogeneities.

mor frequency matching the RF pulse will be excited (Fig. 8). The RF pulse is designed to consist of a defined range of frequencies that also corresponds to the slice thickness. By changing the RF frequency, different slices can be excited along the z-axis. The z-gradient is turned on at the same time as the 90° RF pulse, whereas the phase-encoding y-gradient is activated between the 90° and 180° RF pulse. The strength of the y-gradient is sequentially stepped up for each 90-180° encoding step, again usually from large negative to large positive values.

Fig. 7. A basic spin-warp or 90-180° pulse sequence. RF, radiofrequency pulse; z, the slice-select gradient; y, the phase encoding gradient; x, the frequency-encoding gradient.
Fig. 8. Any one of the x, y, or z linear gradients causes the magnetic field to be a function of position along this axis. The different positions of a and b allow each to experience a slightly different local magnetic field, with b at slightly higher frequency.

After the 90° pulse as the spins precess and undergo relaxation, the signal generated needs to be encoded in such a way as to reflect their spatial position accurately within the imaged tissue. Two techniques called frequency and phase encoding are employed for this purpose, with the x-direction often assigned as the frequency or readout gradient and the y-axis as phase-encoding gradient. The understanding of the readout x-gradient is similar to that of the aforementioned z-gradient whereby the spins further to the right of the gradient will have a higher frequency than the more left-sided ones and will fall into position

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