Multi-band acceleration (e.g. [1]) has become an important tool to speed up acquisitions, in particular in functional neuroimaging. However, the T2*-weighted acquisitions usually used suffer from through-slice dephasing and related signal dropouts. Additional, so-called z-shim gradient pulses can be applied in the slice direction to compensate the dephasing effect and minimize signal losses (e.g., [2]). For small target regions like the spinal cord, a single, slice-specific moment may already reduce signal drop outs significantly without affecting the temporal resolution [2]. Here, it is shown that with an appropriate temporal shift of the different bands individual dephasing moments can be realized for the different slices of a multi-band RF pulse as is demonstrated for slice-selective and 2D-selective RF excitations [3, 4].

Figure 1 sketches the basic principles of the current RF pulse design for slice-specific z-shim moments. For conventional single-band RF excitations, the individual z-shim moments can be realized by manipulating the slice rephasing gradient pulse accordingly (Fig. 1a). For multi-band RF pulses the moments can be realized by a temporal shift of the corresponding RF envelope which requires to prolong the slice-selection gradient pulse to avoid truncation artifacts (Fig. 1b). The application of this approach to 2D-selective RF excitations with a fly-back blipped-planar trajectory, e.g. for inner-field-of-view imaging (e.g. [5]), is straightforward (Fig. 1c).

Multi-band RF pulses were calculated for an acceleration factor of 3 as a superposition of phase-modulated RF envelopes with an appropriate temporal shift if required. The pulses were implemented in an FID echo-planar imaging pulse sequence. To avoid excessive peak power, multi-band pulses were applied with longer pulse duration, i.e. a lower slice-selection gradient pulse. For single-band RF pulses, a manipulation of the slice rephasing gradient was used to realize the desired z-shim gradient moment as shown in Fig.1a.

Measurements were performed using a 3 T whole-body MR system (Magnetom TIM Trio) with a 32-channel head coil (image resolution 2.0×2.0 mm²). A slice thickness of 2.0 mm was used for slice-selective RF excitations. For the 2D-selective RF excitations the excitation profile was 5.0×40 mm² (slice thickness×field-of-view) which is in line with typical inner-field-of-view applications in the spinal cord [5] A water phantom and a cucumber cut into two pieces with an air-filled gap in-between to obtain susceptibility differences and related through-slice dephasing, were investigated.

Figures 2 and 3 show images acquired without and with arbitrary, slice–specific z-shim gradient moments with single- and multi-band RF excitations that are slice-selective and 2D-selective, respectively. The signal intensities for the slices acquired with multi-band RF excitations are very similar to those obtained for the corresponding single-band acquisitions which demonstrates that (i) the multi-band RF pulse envelopes were calculated correctly and (ii) the desired, slice-specific z-shim gradient moments can be realized as for the conventional, single-band acquisitions.

The results obtained for the cucumber phantom exhibiting different degrees of through-slice dephasing are presented in Fig. 4 and 5 for slice-selective and 2D-selective RF excitations, respectively. Without z-shim gradient pulses the slices close to the gap between the two cucumber pieces suffer from significant signal loss. Applying a fixed, averaged z-shim moment to all slices of a multi-band excitation, increases the signal intensity in some of these slices but is at the expense of a signal loss in slices far away from the gap that do not require a z-shim moment but share the same multi-band excitation. The best signal intensities are obtained when an individual moment is applied to all slices in a multi-band excitation yielding significantly increased signal intensities close to the gap without affecting the other slices. Compared to the measurements without z-shim or with an averaged z-shim moment, intensities are increased by up to 70% and 40%, respectively, for the slice-selective RF excitations, and by up to 130% and 80%, respectively, for the 2D-selective RF excitations.

For significant z-shim moments and related temporal shifts the multi-band RF excitation may be prolonged by several milliseconds which usually should not be crucial. However, in 2D-selective RF excitations multiple slice-selection gradient lobes are applied (see Fig. 1c). i.e. the prolongation may be come significant. To avoid excessive prolongations, the gradient amplitude of the outer k-space lines could be increased like in VERSE.

In conclusion, multi-band RF excitations can be designed to provide a slice specific z-shim gradient moment by applying a temporal shift of the corresponding RF envelope. Thus, application that benefit from slice-specific z-shimming like functional neuroimaging of the spinal cord, can be accelerated with multi-band imaging without additional signal loss.