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.

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.

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(31)P MR spectroscopic imaging of the human prostate provides information about phosphorylated metabolites that could be used for prostate cancer characterization. The sensitivity of a magnetic field strength of 7 T might enable 3D (31)P MR spectroscopic imaging with relevant spatial resolution in a clinically acceptable measurement time. To this end, a (31)P endorectal coil was developed and combined with an eight-channel (1)H body-array coil to relate metabolic information to anatomical location. An extensive safety validation was performed to evaluate the specific absorption rate, the radiofrequency field distribution, and the temperature distribution of both coils. This validation consisted of detailed Finite Integration Technique simulations, confirmed by MR thermometry and B 1+ measurements in a phantom and in vivo temperature measurements. The safety studies demonstrated that the presence of the (31)P endorectal coil had no influence on the specific absorption rate levels and temperature distribution of the external eight-channel (1)H array coil. To stay within a 10 g averaged local specific absorption rate of 10 W/kg, a maximum time-averaged input power of 33 W for the (1)H array coil was allowed. For transmitting with the (31)P endorectal coil, our safety limit of less than 1°C temperature increase in vivo during a 15-min MR spectroscopic imaging experiment was reached at a time-averaged input power of 1.9 W. With this power setting, a second in vivo measurement was performed on a healthy volunteer. Using adiabatic excitation, 3D (31)P MR spectroscopic imaging produced spectra from the entire prostate in 18 min with a spatial resolution of 4 cm(3). The spectral resolution enabled the separate detection of phosphocholine, phosphoethanolamine, inorganic phosphate, and other metabolites that could play an important role in the characterization of prostate cancer.

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.