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Fast spin echo (RARE, FAISE or FSE [10] [11] [12]), 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 echoes. [ 13 ]
Unlike spin echo, gradient echo does not need to wait for transverse magnetisation to decay completely before initiating another sequence, thus it requires very short repetition times (TR), and therefore to acquire images in a short time. After echo is formed, some transverse magnetisations remains.
For example, it becomes very easy (for physicists, in particular) to understand the role of phase encoding (the so-called spin-warp method). In a standard spin echo or gradient echo scan, where the readout (or view) gradient is constant (e.g., G), a single line of k-space is scanned per RF excitation.
Unlike spin echo, gradient echo does not need to wait for transverse magnetisation to decay completely before initiating another sequence, thus it requires very short repetition times (TR), and therefore to acquire images in a short time. [2] After echo is formed, some transverse magnetisations remains because of short TR. [2]
When the echo is recorded close to the middle of the interval (TE ≈ TR/2, as is usually the case), the final term e−TE/T2 depends on T2, not T2*. Thus, bSSFP sequences behave more like spin echo than gradient echo sequences in that they do not have T2*-dependence.
Each of the spin-echo signal is a sinc function of time, which can be described by = Where = + ¯ Here ¯ is the gyromagnetic ratio constant, and is the basic resonance frequency of the spin. Due to the presence of the gradient G , the spatial information r is encoded onto the frequency ω {\displaystyle \omega } .
Timing diagram for an MRI spin echo pulse sequence. Graphical representation of a pulse sequence for a homonuclear NOESY experiment. The three bars represent three 90° pulses. An INEPT NMR pulse sequence for a heteronuclear experiment. The thin bar denotes a 90° pulse, while the thick bar denotes a 180° pulse.
Typical gradient-echo EPI uses two gradient coils within a slice, and turns on first one coil and then the other, tracing a set of lines in k-space. Turning on both gradient coils can generate angled lines, which cover the same grid space faster. Both gradient coils can also be turned on in a specific sequence to trace a spiral shape in k-space.