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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.
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]
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. When the phase encoding gradient is zero, the line scanned is the k x axis.
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.
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.
Measuring spin–spin relaxation by using long TR and TE times Higher signal for more water content [1] Low signal for fat in standard Spine Echo (SE), [1] though not with Fast Spin Echo/Turbo Spin Echo (FSE/TSE). FSE/TSE is the standard of care in modern medicine because it is faster.
The effect of a uniform magnetic field gradient in the z-direction on spin I, is considered to be a rotation around z-axis by an angle = γ I Gz; where Gz is the gradient magnitude (along the z-direction) and γ I is the gyromagnetic ratio of spin I. It introduces a phase factor to the magnetizations: Φ (z,τ) = (γ I)(Gz)(τ)