Effect of gradient strength and bandwidth on slice thickness

Slice thickness is dependent on gradient strength and bandwidth, to attain a slice thickness, a range of frequencies must be transmitted to produce resonance across the whole slice. This range of frequencies is the transmitter RF (radiofrequency) bandwidth (tBW). Slice thickness is determined by the slope of the slice select gradient and the transmit bandwidth (it affects in-plane resolution).

Thin slices require a steep gradient and narrow tBW. This results in increased resolution. Thick slices require a shallow gradient and broad tBW, resulting in decreased resolution. The slice gap/skip is the space between slices. Too small a gap in relation to the slice thickness can lead to cross-excitation artifact.

A slice is thus excited by transmitting an RF pulse with a center frequency corresponding to the middle of the slice and a bandwidth and gradient slope according to the thickness required. The slice selection gradient is switched on during the delivery of the RF excitation pulse and the 180 degree rephasing pulse in spin echo (SE) sequences so that the rephasing pulse can be delivered specifically to the selected slice.