As the field strength and, therefore, the operational frequency in MRI is increased, the wavelength approaches the size of the human head/body, resulting in wave effects, which cause signal decreases and dropouts. Several multichannel approaches have been proposed to try to tackle these problems, including RF shimming, where each element in an array is driven by its own amplifier and modulated with a certain (constant) amplitude and phase relative to the other elements, and Transmit SENSE, where spatially tailored RF pulses are used. In this article, a relatively inexpensive and easy to use imaging scheme for 7 Tesla imaging is proposed to mitigate signal voids due to B þ 1 field inhomogeneity. Two time-interleaved images are acquired using a different excitation mode for each. By forming virtual receive elements, both images are reconstructed together using GRAPPA to achieve a more homogeneous image, with only small SNR and SAR penalty in head and body imaging at 7 Tesla. Magn Reson Med 64:327-333, 2010. V C 2010 Wiley-Liss, Inc.Key words: 7 Tesla; ultra high field; body imaging; parallel transmissionSince the beginning of magnetic resonance imaging (MRI), there has been a steady drive to higher magnetic field strengths to increase signal-to-noise ratio (SNR) and to achieve new contrasts. As operational frequency is proportional to field strength, severe problems are often encountered with today's high-field systems regarding homogeneity of the transmission field (1,2). As the operational frequency is increased, the wavelength approaches the size of the human head/body, resulting in wave effects which cause signal decreases and dropouts.Several multichannel approaches have been proposed to try to tackle these problems. The most straightforward approach is static RF shimming (3,4). Each element in an array is driven by its own amplifier and modulated with a certain (constant) amplitude and phase relative to the other elements; the pulse profile for each element remains, however, identical. By choosing the amplitudes and phases properly, a more homogeneous transmit field or signal improvement in a certain region of interest can be achieved (5). For this approach, one needs to know the transmission profiles of each element; furthermore, many channels are needed to achieve a satisfactory result (4).A more complicated approach is Transmit SENSE (6,7), where spatially tailored RF pulses are used. Amplitude and phase vary during transmission and, thus, different pulse profiles are played out for each element. This approach yields excellent results, but presupposes exact knowledge of element transmission profiles as well as expensive and complicated hardware.In this article, we propose a new imaging scheme based on multimode excitation and GRAPPA (8) parallel imaging reconstruction to mitigate signal voids due to B þ 1 field inhomogeneity that is relatively inexpensive and easy to apply. We designate this acquisition scheme Time-Interleaved Acquisition of Modes (TIAMO). The basic premise is to excite two (or more) diff...

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The sensitivity of proton MR Spectroscopic Imaging ((1)H-MRSI) of the prostate can be optimized by using the high magnetic field strength of 7 T in combination with an endorectal coil. In the work described in this paper we introduce an endorectal transceiver at 7 T, validate its safety for in vivo use and apply a pulse sequence, optimized for three-dimensional (3D) (1)H-MRSI of the human prostate at 7 T. A transmit/receive endorectal RF coil was adapted from a commercially available 3 T endorectal receive-only coil and validated to remain within safety guidelines for radiofrequency (RF) power deposition using numerical models, MR thermometry of phantoms, and in vivo temperature measurements. The (1)H-MRSI pulse sequence used adiabatic slice selective refocusing pulses and frequency-selective water and lipid suppression to selectively obtain the relevant metabolite signals from the prostate. Quantum mechanical simulations were used to adjust the inter-pulse timing for optimal detection of the strongly coupled spin system of citrate resulting in an echo time of 56 ms. Using this endorectal transceiver and pulse sequence with slice selective adiabatic refocusing pulses, 3D (1)H-MRSI of the human prostate is feasible at 7 T with a repetition time of 2 s. The optimized inter-pulse timing enables the absorptive detection of resonances of spins from spermine and citrate in phase with creatine and choline. These potential tumor markers may improve the in vivo detection, localization, and assessment of prostate cancer.