1. Slice Selection

Nuclei residing in a two-dimensional slice can be selectively excited from a three-dimensional sample using the combination of a slice selection gradient and a selective RF pulse. Although slice selection may occur during any period in which an RF pulse is applied, it is common to select the size during the preparation phase of the imaging sequence. The slice selection requires the simultaneous application of a linear magnetic-field gradient and selective RF energy. In this example a cylindrical tube of water is placed parallel to the Z axis in a uniform magnetic field. Without an external magnetic-field gradient all of the protons in the water sample would have the same resonant frequency. A NMR spectrum taken from this sample would consists of a single narrow line. The application of linear magnetic-field gradient along the Z axis causes a linear dispersion of the resonant frequency of an individual proton is directly proportional to its position on the Z axis. A NMR spectrum of this sample would identically demonstrate a rectangular frequency profile in which the NMR signal is dispersed over a frequency range of several kilohertz. The actual range of frequencies covered by the sample is proportional to the strength of the applied field gradient. If a selective 90⁰ RF pulse is applied in the presence of the magnetic-field gradient, a cross-sectional size of nuclei perpendicular to the Z axis is excited. Recall that a selective RF pulse has a narrow frequency band width that typically covers several hertz. The position of the slice can be varied by changing the carrier frequency of the RF pulse. The slice thickness is dependent on the magnitude of the applied magnetic-filed gradient (dB/dz), and the frequency band width of the selective RF pulse. For a constant gradient strength, the thickness of the excited slice is varied by changing the length of the RF pulse (PW), which is inversely proportional to the bandwidth of the selective pulse.

2. Frequency Encoding

Once a slice has been selected, the excited nuclei must be spatially encoded in the remaining two dimensions. In one dimension the frequency of the NMR signal can be spatially encoded by applying a linear field gradient during the detection stage. This gradient directly encodes spatial information along the applied axis.

1. Phase Encoding

Spatial encoding of the second dimension in the selected slice occurs through phase-modulation of the NMR signal. A phase-encoding magnetic-field gradient is applied during the evolution stage of the sequence in which the nuclear spins are precisely in the XY plane.

Frequency

Gradient

Receiver

1. Spin-Echo Imaging

 

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The spin-echo imaging sequence is the imaging correliate of the standard spin-echo spectroscopy experiment described earlier. It forms the basis for many of the standard sequences used in clinical MRI. The complete spin-echo imaging sequence consists of the combined pulse sequences of slice selection, frequency encoding and phase encoding. The initial selective 90⁰ pulse excites the protons residing in the sample.

 

2. Gradient-Echo Imaging

In addition to refocusing the transverse magnetisation with RF energy, nuclear spins may be re-phased using bipolar magnetic field gradients. This forms the basis for the gradient-echo imaging sequence. Following the initial RF excitation pulse a negative-read gradient is applied that de-phases the transverse magnetisation. The direction of the read gradient is then reversed during signal acquisition to frequency encode the NMR signal. In the rotating frame of reference, the reversal of the read gradient causes the spins to move in the opposite direction and therefore refocus.