An experimental implementation and first performance analysis of parallel spatially selective excitation with an array of transmit coils and simultaneous transmission of individual waveforms on multiple channels is presented. This technique, also known as Transmit SENSE, uses the basic idea of parallel imaging to shorten the k-space trajectories that accompany multidimensional excitation pulses, and hence shorten the duration of such pulses. So far, this concept has only been presented in simulations and semi-experimental studies since no hardware setup had been available for a full experimental realization. Interest in multidimensional, spatially selective excitation (1-3) is increasing in the field of MRI because it has a large number of useful applications. Simple slice selection is probably the most widely used form of selective excitation in one dimension, but there are also many applications that use localization in more than one dimension. These applications include volume-selective excitation for localized spectroscopy (4,5), reduced field of view (FOV) scanning of a region of interest (ROI) (6), imaging of specially shaped volumes following anatomical structures (7), or echo planar imaging (EPI) with reduced echo train lengths (8). With the trend in MRI to move continuously to higher field strengths, the possibility of compensating for transmit field inhomogeneities due to wave effects by means of spatially varying excitation is expected to gain great importance (9).Despite these numerous possible beneficial applications, the use of multidimensional, spatially selective excitation is restricted by technical limitations that arise when RF pulses are combined with gradient shapes for spatial selectivity. Since gradient performance is limited, the duration of such pulses becomes rather long because of the time required to traverse a certain k-space trajectory in order to achieve sufficient resolution and reasonably sized fields of excitation (FOXs). In general, these considerable pulse durations lead to undesirable effects, such as increased echo times (TEs), repetition times (TRs), and specific absorption rates (SARs), as well as sensitivity of the excitation profiles to off-resonance effects induced by main field inhomogeneities or varying susceptibility.To overcome these difficulties, the recently introduced concept of multiple-channel transmit (10) or Transmit Sensitivity Encoding (Transmit SENSE) (11,12) will most likely play an important role in the future. In Transmit SENSE the concept of parallel imaging is transferred from reception to transmission. The RF required for a spatially selective pulse is applied using a phased-array coil with spatially varying transmit sensitivities of the array elements. The array elements are driven by an equal number of independent RF channels with individual waveforms on each channel. In analogy to parallel reception, the spatial dependency of the sensitivities provides a localization effect that is complementary to the one induced by gradient action. This makes it...

Purpose: To propose and illustrate a safety concept for multichannel transmit coils in MRI based on finite-differences time-domain (FDTD) simulations and validated by measurements. Materials and Methods:FDTD simulations of specific absorption rate (SAR) distributions in a cylindrical agarose phantom were carried out for various radio frequency (RF) driving conditions of a four-element coil array. Additionally, maps of transmit amplitude, signal phase, and temperature rise following RF heating were measured by MRI.Results: Quantitative agreement was achieved between simulated and measured field distributions, thus validating the numerical modeling. When applying the same RF power to each element of the coil array but systematically varying the RF phase between its elements, the maximum of the SAR distribution was found to vary by a factor of about 15. Conclusion:Our results demonstrate that current RF safety approaches are inadequate to deal with the new challenge of multichannel transmit coils. We propose a new concept based on a systematic investigation of the parameter space for RF phases and amplitudes. In this way the driving conditions generating the highest local SAR values per unit power can be identified and appropriately considered in the RF safety concept of a given MRI system.

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This study evaluates the performance of the Bruker PET insert combined with a BioSpec 70/30 USR MRI scanner using the manufacturer acceptance protocol and the NEMA NU 4-2008 for small animal positron emission tomographs (PET). The PET insert is made of 3 rings of 8 monolithic LYSO crystals (50 x 50 x 10 mm 3 ) coupled to silicon photomultipliers (SiPM) arrays, conferring an axial and transaxial FOV of 15 cm and 8 cm. The MRI performance was evaluated with and without the insert for the following radiofrequency noise, magnetic field homogeneity and image quality. For the PET performance, we extended the NEMA protocol featuring system sensitivity, count rates, spatial resolution and image quality to homogeneity and accuracy for quantification using several MRI sequences (RARE, FLASH, EPI and UTE). The PET insert does not show any adverse effect on the MRI performances. The MR field homogeneity is well preserved (Diameter Spherical Volume, for 20 mm of 1.98 ± 4.78 without and -0.96 ± 5.16 Hz with the PET insert). The PET insert has no major effect on the radiofrequency field. The SNR measurements also do not show major differences. Image ghosting is well within the manufacturer specifications (<2.5%) and no RF noise is visible. Maximum sensitivity of the PET insert is 11.0% at the center of the FOV even with simultaneous acquisition of EPI and RARE. PET MLEM resolution is 0.87 mm (FWHM) at 5 mm off-center of the FOV and 0.97 mm at 25 mm radial offset. The peaks for true/noise equivalent count rates are 410/240 and 628/486 kcps for the rat and mouse phantoms, and are reached at 30.34/22.85 and 27.94/22.58 MBq. PET image quality is minimally altered by the different MRI sequences. The Bruker PET insert shows no adverse effect on the MRI performance and demonstrated a high sensitivity, sub-millimeter resolution and good image quality even during simultaneous MRI acquisition.

A technique for noninvasively quantifying the concentration of sodium (23Na) ions was applied to the study of ischemic stroke. 23Na‐magnetic resonance imaging techniques have shown considerable potential for measuring subtle changes in ischemic tissue, although studies to date have suffered primarily from poor signal/noise ratio. In this study, accurate quantification of tissue sodium concentration (TSC) was achieved in 23Na images with voxel sizes of 1.2 μL acquired in 10 min. The evolution of TSC was investigated from 0.5 to 8 h in focal cortical and subcortical ischemic tissue following permanent middle cerebral artery occlusion in the rat (n = 5). Infarct volumes determined from TSC measurements correlated significantly with histology (P = 0.0006). A delayed linear model was fitted to the TSC time course data in each voxel, which revealed that the TSC increase was more immediate (0.2 ± 0.1 h delay time) in subcortical ischemic tissue, whereas it was delayed by 1.6 ± 0.5 h in ischemic cortex (P = 0.0002). No significant differences (P = 0.5) were measured between TSC slope rates in cortical (10.2 ± 1.1 mM/h) and subcortical (9.7 ± 1.1 mM/h) ischemic tissue. The data suggest that any TSC increase measured in ischemic tissue indicates infarction (core) and regions exhibiting a delay to TSC increase indicate potentially salvageable tissue (penumbra). Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.