Cerebral 3D time of flight (TOF) angiography significantly benefits from ultra high fields, mainly due to higher SNR and to longer T1 relaxation time of static brain tissues, however, SAR significantly increases with B0. Thus, additional RF pulses commonly used at lower field strengths to improve TOF contrast such as saturation of venous signal and improved background suppression by magnetization transfer typically cannot be used at higher fields. In this work we aimed at reducing SAR for each RF pulse category in a TOF sequence. We use the VERSE principle for the slab selective TOF excitation as well as the venous saturation RF pulses. Additionally, MT pulses are implemented by sparsely applying the pulses only during acquisition of the central k-space lines to limit their SAR contribution. Image quality, angiographic contrast and SAR reduction were investigated as a function of VERSE parameters and of the total number of MT pulses applied. Based on these results, a TOF protocol was generated that increases the angiographic contrast by more than 50% and reduces subcutaneous fat signal while keeping the resulting SAR within regulatory limits.

PurposeA 32-channel parallel transmit (pTx) add-on for 7 Tesla whole-body imaging is presented. First results are shown for phantom and in-vivo imaging.MethodsThe add-on system consists of a large number of hardware components, including modulators, amplifiers, SAR supervision, peripheral devices, a control computer, and an integrated 32-channel transmit/receive body array. B1+ maps in a phantom as well as B1+ maps and structural images in large volunteers are acquired to demonstrate the functionality of the system. EM simulations are used to ensure safe operation.ResultsGood agreement between simulation and experiment is shown. Phantom and in-vivo acquisitions show a field of view of up to 50 cm in z-direction. Selective excitation with 100 kHz sampling rate is possible. The add-on system does not affect the quality of the original single-channel system.ConclusionThe presented 32-channel parallel transmit system shows promising performance for ultra-high field whole-body imaging.

As the magnetic field strength and therefore the operational frequency in MRI are increased, the radiofrequency wavelength approaches the size of the human head/body, resulting in wave effects which cause signal decreases and dropouts. Especially, whole-body imaging at 7 T and higher is therefore challenging. Recently, an acquisition scheme called timeinterleaved acquisition of modes has been proposed to tackle the inhomogeneity problems in high-field MRI. The basic premise is to excite two (or more) different B þ 1 modes using static radiofrequency shimming in an interleaved acquisition, where the complementary radiofrequency patterns of the two modes can be exploited to improve overall signal homogeneity. In this work, the impact of time-interleaved acquisition of mode on image contrast as well as on time-averaged specific absorption rate is addressed in detail. Time-interleaved acquisition of mode is superior in B þ 1 homogeneity compared with conventional radiofrequency shimming while being highly specific absorption rate efficient. Time-interleaved acquisition of modes can enable almost homogeneous high-field imaging throughout the entire field of view in PD, T 2 , and T 2 *-weighted imaging and, if a specified homogeneity criterion is met, in Key words: 7 Tesla; ultra-high field; body imaging; parallel transmission; RF shimmingSince the early days of magnetic resonance imaging, a steady drive to higher field strengths has been apparent. At higher field strengths, an increased signal-to-noise ratio and new contrasts can be obtained; however, high static field strengths of 7 T and above lead to severe radiofrequency (RF) homogeneity problems (1,2), because the operational frequency is proportional to the static field strength, and hence, the wavelength is shortened. Especially, whole-body imaging at 7 T and higher is therefore challenging (3). Multichannel transmit approaches to tackle these problems have been proposed in the literature. The most notable are static RF shimming (4,5) and Transmit SENSE (6).In RF shimming, each element in an array is driven with its own constant amplitude and phase, while the pulse profile is identical for all elements. By choosing a suitable set of amplitudes and phases, the resulting B þ 1 can be shaped within certain limitations to achieve a more homogeneous field excitation in an extended area or a localized field of view (5,7).Transmit SENSE and equivalent approaches (8) use spatially tailored RF pulses. The amplitude and phase of each element in a transmit array are varied during transmission, leading to element-dependent pulse profiles. This approach yields excellent results but presupposes precise knowledge of element transmission profiles as well as expensive and complicated hardware.Recently, an acquisition scheme called time-interleaved acquisition of modes (TIAMO) (9) has been proposed to tackle the inhomogeneity problems in high-field MRI. The basic premise is to excite two (or more) different B þ 1 modes using static RF shimming in an interleaved acquisition, whe...

The results demonstrate the potential to acquire 32 accurate single-channel B1+ maps for large field-of-view body imaging within only a single breath-hold of 16 s at 7T UHF MRI. Magn Reson Med 79:2652-2664, 2018. © 2017 International Society for Magnetic Resonance in Medicine.