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Purpose The goal of this study was to devise a gradient system for MRI in humans that reconciles cutting‐edge gradient strength with rapid switching and brings up the duty cycle to 100% at full continuous amplitude. Aiming to advance neuroimaging and short‐T2 techniques, the hardware design focused on the head and the extremities as target anatomies. Methods A boundary element method with minimization of power dissipation and stored magnetic energy was used to design anatomy‐targeted gradient coils with maximally relaxed geometry constraints. The design relies on hollow conductors for high‐performance cooling and split coils to enable dual‐mode gradient amplifier operation. With this approach, strength and slew rate specifications of either 100 mT/m with 1200 mT/m/ms or 200 mT/m with 600 mT/m/ms were reached at 100% duty cycle, assuming a standard gradient amplifier and cooling unit. Results After manufacturing, the specified values for maximum gradient strength, maximum switching rate, and field geometry were verified experimentally. In temperature measurements, maximum local values of 63°C were observed, confirming that the device can be operated continuously at full amplitude. Testing for peripheral nerve stimulation showed nearly unrestricted applicability in humans at full gradient performance. In measurements of acoustic noise, a maximum average sound pressure level of 132 dB(A) was determined. In vivo capability was demonstrated by head and knee imaging. Full gradient performance was employed with echo planar and zero echo time readouts. Conclusion Combining extreme gradient strength and switching speed without duty cycle limitations, the described system offers unprecedented options for rapid and short‐T2 imaging. Magn Reson Med 79:3256–3266, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Purpose The aim of this work was to provide parallel imaging capability for the human head in a gradient insert of 33-cm inner diameter within the related constraints of space, encoding ambiguity, and eddy current immunity. Methods Eddy current behavior of the 8-channel transmit-receive array coil was investigated via heating and field deviation measurements. RF performance was evaluated using S-parameters, noise statistics, B1 maps, and g-factor maps. In vivo images of a human head and knee were acquired with Cartesian readout and TE below 0.45 ms. Results Under intense gradient use, the shield was heated up to 55°C and other coil structures to 40°C. After standard preemphasis calibration, eddy current-related field distortions caused by the developed RF coil were smaller than for a commercial receive-only coil. In the ambiguous regions of the gradient, B 1 + is 20 dB lower than in the center of the FOV. Coupling between elements is below -15 dB, and noise correlation is less than 0.31 when the coil is loaded with a head. Power efficiency was 0.52 ± 0.02 μT/√W, and the SD of the flip angle was below 10% in central slices of the brain. 2D, up to fourfold acceleration causes less than 30% noise amplification. The RF coil can be used during full gradient performance. Conclusion Based on the described features, the presented coil enables parallel imaging in the high-performance gradient insert.

The goal of this study was to devise a gradient system for MRI in humans that reconciles cutting-edge gradient strength with rapid switching and brings up the duty cycle to 100% at full continuous amplitude. Aiming to advance neuroimaging and short-TA boundary element method with minimization of power dissipation and stored magnetic energy was used to design anatomy-targeted gradient coils with maximally relaxed geometry constraints. The design relies on hollow conductors for high-performance cooling and split coils to enable dual-mode gradient amplifier operation. With this approach, strength and slew rate specifications of either 100 mT/m with 1200 mT/m/ms or 200 mT/m with 600 mT/m/ms were reached at 100% duty cycle, assuming a standard gradient amplifier and cooling unit.After manufacturing, the specified values for maximum gradient strength, maximum switching rate, and field geometry were verified experimentally. In temperature measurements, maximum local values of 63°C were observed, confirming that the device can be operated continuously at full amplitude. Testing for peripheral nerve stimulation showed nearly unrestricted applicability in humans at full gradient performance. In measurements of acoustic noise, a maximum average sound pressure level of 132 dB(A) was determined. In vivo capability was demonstrated by head and knee imaging. Full gradient performance was employed with echo planar and zero echo time readouts.Combining extreme gradient strength and switching speed without duty cycle limitations, the described system offers unprecedented options for rapid and short-T

Fast proton magnetic resonance spectroscopic imaging (MRSI) using a multi-element head-coil array is examined with respect to three aspects: the coil design, the use of an appropriate signal combination method, and the design of the MRSI pulse sequence itself. An eight-element head-coil array has been developed to increase the signal-to-noise ratio (SNR) of MRSI in the human brain. The flexible wraparound design optimally fits different head sizes and thus provides high sensitivity. The signal combination of the individual coil elements is based on the approach proposed by Roemer et al. (Magn. Reson. Med. 16, 192 (1990)). An additional short prescan is performed to provide a good estimate of the complex coil sensitivity profiles, which are used in the signal combination procedure to correct the spectroscopic imaging data for the spatially varying intensity. The use of coil arrays in MRSI has some effect on the requirements for both water and lipid suppression. These techniques and a MRSI pulse sequence that provides a high spectroscopic resolution are described and discussed. Experimental results at 1.5 T show that metabolite maps of N-acetylaspartate (NAA), choline (Cho), phosphocreatine (PCr)/creatine (Cr) can be obtained within a 5-min acquisition time.

A multichannel spectrally resolved optical tomography system to image molecular targets in small animals from within a clinical MRI is described. Long source∕detector fibers operate in contact mode and couple light from the tissue surface in the magnet bore to 16 spectrometers, each containing two optical gratings optimized for the near infrared wavelength range. High sensitivity, cooled charge coupled devices connected to each spectrograph provide detection of the spectrally resolved signal, with exposure times that are automated for acquisition at each fiber. The design allows spectral fitting of the remission light, thereby separating the fluorescence signal from the nonspecific background, which improves the accuracy and sensitivity when imaging low fluorophore concentrations. Images of fluorescence yield are recovered using a nonlinear reconstruction approach based on the diffusion approximation of photon propagation in tissue. The tissue morphology derived from the MR images serves as an imaging template to guide the optical reconstruction algorithm. Sensitivity studies show that recovered values of indocyanine green fluorescence yield are linear to concentrations of 1 nM in a 70 mm diameter homogeneous phantom, and detection is feasible to near 10 pM. Phantom data also demonstrate imaging capabilities of imperfect fluorophore uptake in tissue volumes of clinically relevant sizes. A unique rodent MR coil provides optical fiber access for simultaneous optical and MR data acquisition of small animals. A pilot murine study using an orthotopic glioma tumor model demonstrates optical-MRI imaging of an epidermal growth factor receptor targeted fluorescent probe in vivo.

The idea of using parallel imaging to shorten the acquisition time by the simultaneous use of multiple receive coils can be adapted for the parallel transmission of a spatially-selective multidimensional RF pulse. As in data acquisition, a multidimensional RF pulse follows a certain k-space trajectory. Shortening this trajectory shortens the pulse duration. The use of multiple transmit coils, each with its own time-dependent waveform and spatial sensitivity, can compensate for the missing parts of the excitation k-space. This results in a maintained spatial definition of the pulse profile, while its duration is reduced. This work introduces the concept of parallel transmission with arbitrarily shaped transmit coils (termed "Transmit SENSE"). Results of numerical studies demonstrate the theoretical feasibility of the approach. The experimental proof of principle is provided on a commercial MR scanner. The lack of multiple independent transmit channels was addressed by combining the excitation patterns from two separate subexperiments with different transmit setups. Shortening multidimensional RF pulses could be an interesting means of making 3D RF pulses feasible even for fast T(2)(*) relaxing species or strong main field inhomogeneities. Other applications might benefit from the ability of Transmit SENSE to improve the spatial resolution of the pulse profile while maintaining the transmit duration.

In sensitivity encoding (SENSE), the effects of inhomogeneous spatial sensitivity of surface coils are utilized for signal localization in addition to common Fourier encoding using magnetic field gradients. Unlike standard Fourier MRI, SENSE images exhibit an inhomogeneous noise distribution, which crucially depends on the geometrical sensitivity relations of the coils used. Thus, for optimum signal-to-noise-ratio (SNR) and noise homogeneity, specialized coil configurations are called for. In this article we study the implications of SENSE imaging for coil layout by means of simulations and imaging experiments in a phantom and in vivo. New, specific design principles are identified. For SENSE imaging, the elements of a coil array should be smaller than for common phased-array imaging. Furthermore, adjacent coil elements should not overlap. Based on the findings of initial investigations, a configuration of six coils was designed and built specifically for cardiac applications. The in vivo evaluation of this array showed a considerable SNR increase in SENSE images, as compared with a conventional array. Magn Reson Med 45:495–504, 2001. © 2001 Wiley-Liss, Inc.