Physics, Techniques and ProceduresSlice selection
process used to select the imaging slice in two-dimensional
MR imaging.The method consists of applying a magnetic
gradient field perpendicular to the imaging slice. As a result, the net magnetic field varies linearly along this axis (
Fig.1). With a combination of gradient fields in all three spatial axes, the net magnetic field can be made to vary along any spatial axis, thus allowing for the selection of any oblique image
plane. If a
radiofrequency RF pulse with a given frequency range
Dw is sent into the patient, it can only excite those
spins that are positioned at a magnetic field strength which satisfies the
Larmor equation (Fig. 1). An increase or decrease in the frequency will shift the position of the slice towards increasing or decreasing magnetic field strength (Fig. 1).
The slice thickness is determined by the shape of the radiofrequency pulse. So called sinc pulses are used (excitation pulse (I), Fig. 1) to excite the slices, because a time domain sinc pulse uniformly excites all spins within a given frequency range as selected by the magnetic gradient field (Fig. 1). Wide sinc pulses excite narrow slices while narrow sinc pulses generate wide slices. Alternatively the steepness of the field gradient can be altered. In volume imaging the slice is made as wide as the limits of the three-dimensional volume.
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Slice selection in MR is achieved by applying a magnetic gradient field along an arbitrary axis (here craniocaudally).
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Slice selection, Fig.1 | |