The first 10.5 Tesla (10.5T) whole body human scanner has recently become operational, which holds promise to a wide range of biomedical applications. The objective of this study was to assess the performance of the installed 16-channel parallel RF transmission (pTx)^1-3 system by designing 2D Transmit SENSE¹ RF pulses. Experiments were conducted on a 10.5T whole body scanner (magnet bore diameter 88 cm) equipped with 16 independent RF channels (2 kW amplifier/channel), with a whole body gradient coil (Siemens SC72, 70 mT/m maximum amplitude, 200 T/m/s maximum slew rate), and with second- and third-order B0 shim coils. A 16-element strip-line head array, consisting of 11-cm long resonance elements but otherwise similar to⁴, was used for transmission and reception. A doped spherical phantom of 15 cm in diameter was imaged. To collect complex transmit B1 (B1+) maps (absolute magnitude and relative phase) on 16 channels at 450 MHz, the use of a hybrid technique⁵, merging a large flip angle map (all channel transmitting with complex inter-channel B1+ interferences in full play) with a series of small flip angle data (one transmit channel at a time, absence of B1+ interference), proved to be an accurate and robust approach; this technique allowed for deriving absolute B1+ maps (B1+_Abs) over the entire Field of View (FOV) for each channel while alleviating the need for high RF power typically required with conventional, single-channel based B1+ mapping. An additional challenge however occurred: whereas at 7T it is often feasible⁶ to find a CP mode-like set of B1+ shim phases to measure B1+_Abs (all coils transmitting), we could not find at 10.5T a single set of B1+ shim phases that would not result in local signal void(s) incompatible with B1+_Abs mapping (Fig. 1a). This issue was readily addressed by collecting a second B1+_Abs (all coils transmitting) with another set of B1+ phase shim chosen to complement the signal void(s) of the first B1+_Abs map (Fig. 1b). The final 16 B1+ maps (Fig. 1c) were obtained by merging absolute and relative B1+ maps and interference patterns. The pulse design was formulated in the spatial domain⁷, including ΔB0 maps (measured after applying third order B0 shimming), as a minimization problem and assumed small tip angle excitation. A nominal spiral trajectory of 2.43 ms in duration was designed to cover a 4-fold under-sampled excitation k-space; actual gradient waveforms were measured as in⁸. Two excitation targets (rectangle and “M” logo) were considered (Fig. 2a). RF pulses were calculated using conjugate gradient iterations. Three-dimensional GRE images were acquired to evaluate the excitation pattern, using a pulse sequence accepting arbitrarily-shaped RF and gradient waveforms for excitation; these GRE images were divided by receive B1 maps³ to un-bias excitation patterns. All computations were performed in Matlab (MathWorks, Natick, MA, USA).Directly applying RF pulses designed with nominal gradients and timing resulted in excitation pattern distortions dominated by a characteristic rotation (Fig. 2b). This rotary distortion was effectively corrected (Fig. 2c) by compensating for a time delay (a few microseconds) between RF and gradients events; however residual background excitation was still noticeable. Designing RF pulses using the measured gradients further improved the excitation fidelity (Fig. 2d) by suppressing the background excitation to a level comparable to what was predicted by Bloch simulations (Fig. 2e).We have demonstrated using 16-channel pTx methods that 2D arbitrarily-defined excitation patterns with 4-fold under-sampled excitation k-space can be achieved with high excitation fidelity on a whole body scanner at a field as high as 10.5T. Critical to this achievement is a hybrid multichannel B1+ mapping method allowing to merge large flip angle maps in different B1+ shim phase settings with small flip angle series. Correction for gradient/RF timing errors and for k-space trajectory deviations is also necessary to achieve best excitation fidelity. This success in 2D Transmit SENSE provides a strong basis to further investigation on other pTx methods at 10.5T, including pTx multi-spoke RF pulse design for slice-selective homogeneous excitation.