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Slice Encoding

The three-dimensional space can be imaged according to what has been discussed above. However, often it is sufficient to image two-dimensional slices only. In order to do this, the magnetization needs to be excited in one slice only, while the rest of the sample does not give a signal in the experiment. This can be achieved if a gradient is applied alongside the excitation. In presence of a so-called slice-selecting gradient the resonance frequency is different in every point in the planes perpendicular to the gradient. With a narrow-band excitation, i.e. with an excitation with a well-defined frequency, the spins are excited in a thin slice only. The slice can then be imaged by phase and frequency encoding, thus it is not necessary to perform the phase encoding in the third direction.
What does narrow-band excitation mean? Our aim is to make the frequency profile of the excitation a square signal, which would result in the ideal slice profile. The Fourier transform determines what shape RF signal needs to be emitted to achieve that. The transform of the square function is sin(x)/x or sinc, therefore, the radio frequency signal needs to be modulated by a sinc function that has an appropriate frequency, which corresponds to the desired slice thickness.

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