In high field MRI, fast and robust in vivo B₁⁺ mapping is an essential prerequisite for many quantitative or parallel-transmit-based applications (1-3). However, most B₁⁺ mapping techniques are too slow for a seamless integration into the clinical workflow. Recently, the DREAM (Dual Refocusing Echo Acquisition Mode) B₁⁺ mapping approach has been introduced (4), allowing the acquisition of a 2D B₁⁺ map in a fraction of a second. Moreover, a volumetric DREAM B₁⁺ map can be acquired in a couple of seconds by successive acquisition of multiple slices. In the present study, the DREAM approach is further accelerated by employing simultaneous multi-slice imaging (5), allowing a volumetric B₁⁺ map to be acquired in a single shot. The new approach has been implemented on a clinical scanner, and basic feasibility has been shown on phantoms and in vivo.For multiband slice excitation, sinusoidally modulated RF pulses are employed, representing a superposition of two or more offset frequencies, each one corresponding to a certain imaging slice (Fig.1). Using parallel imaging, the different slices can be unfolded from the acquired data in reconstruction. Nevertheless, the strongly modulated broad-band RF pulses potentially suffer from long pulse duration due to the limited available peak B₁⁺ and SAR constraints at higher field strength. This turns out to be especially a problem for B₁⁺ mapping, because these sequences are generally based on large flip angles required for B₁⁺ encoding. On the other hand, some B₁⁺ mapping approaches separate the B₁⁺ encoding from the spatial encoding. For instance, the DREAM sequence uses a STEAM preparation sequence for B₁⁺ encoding and a low angle pulse train for spatial encoding (Fig.2a), were both, an FID and a STE, are measured as separate recalled gradient echoes under a single readout gradient lobe. Hence, simultaneous multi-slice DREAM B₁⁺ mapping can be implemented combining a non-selective STEAM preparation with a multiband imaging RF pulse train (Fig.2b).Phantom and in vivo experiments on the abdomen were performed on a clinical dual-transmit 3T MRI system (Ingenia, Philips Healthcare, Best, The Netherlands). DREAM Multiband was implemented in the scan and reconstruction software of the scanner and used for simultaneous B₁⁺ mapping of four equidistant slices (transversal orientation, slice thickness: 10 mm, slice gap: 40 mm, FOV = 530x460mm², 2D scan matrix = 64x56, STEAM/imaging flip angle = 50°/10°, TE_FID/STE=2.3/3.3 ms, TR = 4.5 ms, receive coil: 28-channel anterior-posterior coil array). To improve the stability of SENSE unfolding (g-factor), neighbouring slices were shifted by a quarter field-of-view, which was achieved by appropriate modulation of the the RF pulses (5). The B₁⁺ maps were acquired in a single shot (duration ≈ 250 ms), which allowed acquisition of the in vivo data during free-breathing.Figure 3 and 4 show the acquired B₁⁺ maps along with the underlying source images of the DREAM sequence for the water-filled body phantom and for the abdominal acquisitions, respectively. The B₁⁺ maps show the well-known shading pattern typical for quadrature excitation at 3T. The local torso SAR level was slightly increased from 9% (conventional multi-slice acquisition) to 13 % (simulataneous multi-slice), which is still very moderate, especially for a B₁⁺ mapping scan.Multiband DREAM allows for multi-slice B₁⁺ mapping in a fraction of a second. Thus, motion is frozen, and the acquisition can be performed during free breathing capable also to capture dynamic processes (like respiration). The presented approach represents an alternative to 3DREAM (6), which is based on a single shot 3D DREAM acquisition. Multiband DREAM provides the advantage of a reduced partial volume effect, because thinner slices may be chosen. Moreover, oversampling in z-direction is not required, increasing the sampling efficiency of the approach. Finally, the approach is more flexible with respect to optimizing the g-factor for the chosen coil array by selecting appropriate phase shifts between the slices.