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...

Magnetic particle imaging (MPI) is a novel tracer-based in vivo imaging modality allowing quantitative measurements of the spatial distributions of superparamagnetic iron oxide (SPIO) nanoparticles in three dimensions (3D) and in real time using electromagnetic fields. However, MPI lacks the detection of morphological information which makes it difficult to unambiguously assign spatial SPIO distributions to actual organ structures. To compensate for this, a preclinical highly integrated hybrid system combining MPI and Magnetic Resonance Imaging (MRI) has been designed and gets characterized in this work. This hybrid MPI-MRI system offers a high grade of integration with respect to its hard- and software and enables sequential measurements of MPI and MRI within one seamless study and without the need for object repositioning. Therefore, time-resolved measurements of SPIO distributions acquired with MPI as well as morphological and functional information acquired with MRI can be combined with high spatial co-registration accuracy. With this initial phantom study, the feasibility of a highly integrated MPI-MRI hybrid systems has been proven successfully. This will enable dual-modal in vivo preclinical investigations of mice and rats with high confidence of success, offering the unique feature of precise MPI FOV planning on the basis of MRI data and vice versa.

This work describes the first experimental realization of three-dimensional spatially selective excitation using parallel transmission in vivo. For the design of three-dimensional parallel excitation pulses with short durations and high excitation accuracy, the choice of a suitable transmit k-space trajectory is crucial. For this reason, the characteristics of a stack-of-spirals trajectory and of a concentric-shells trajectory were examined in an initial simulation study. It showed that, especially when undersampling the trajectories in combination with parallel transmission, experimental parameters such as transmit-coil geometry and off-resonance conditions have an essential impact on the suitability of the selected trajectory and undersampling scheme. Both trajectories were applied in MR inner-volume imaging experiments which demonstrate that acceptably short and robust three-dimensional selective pulses can be achieved if the trajectory is temporally optimized and its actual path is measured and considered during pulse calculation. Pulse durations as short as 3.2 ms were realized and such pulses were appropriate to accurately excite arbitrarily shaped volumes in a corn cob and in a rat in vivo. Reduced field-of-view imaging of these selectively excited targets allowed high spatial resolution and significantly reduced measurement times and furthermore demonstrates the feasibility of three-dimensional parallel excitation in realistic MRI applications in vivo.

Multidimensional spatially selective excitation (SSE) has stimulated a variety of useful applications in magnetic resonance imaging and magnetic resonance spectroscopy, which have regained considerable interest after the recent introduction of parallel excitation. For SSE, radiofrequency pulses are designed specifically for certain time-courses of spatially encoding magnetic fields (SEM) which are applied simultaneously with the radiofrequency pulses. However, experimental imperfections of gradient-systems and undesired SEM field contributions often prevent the correct co-action of radiofrequency pulses and gradient-waveforms and therefore degrade the fidelity of excitation patterns, especially for parallel excitation. To cope with such imperfections, a classical measurement of k-space-trajectories can be performed followed by an adaptation of the SSE-pulses. However, this method is limited to linear SEM field distributions, which are describable in the k-space-formalism. Hence, this work presents a more sophisticated method consisting in a spatially resolved measurement of the temporal phase evolution of the transverse magnetization. This exhaustive phase information can be incorporated into pulse-design algorithms to compensate even for undesired spatially nonlinear, dynamic SEM field contributions. Both approaches are assessed in various experimental scenarios and individual benefits and limitations are discussed. The adaptation of SSE-pulses to experimentally achieved calibration data turned out to be very beneficial, and especially the novel spatially resolved method exhibited high potential for robust SSE even in adverse experimental setups. Magn Reson Med 65:409-421,

With the recent proposal of using magnetic fields that are nonlinear by design for spatial encoding, new flexibility has been introduced to MR imaging. The new degrees of freedom in shaping the spatially encoding magnetic fields (SEMs) can be used to locally adapt the imaging resolution to features of the imaged object, e.g., anatomical structures, to reduce peripheral nerve stimulation during in vivo experiments or to increase the gradient switching speed by reducing the inductance of the coils producing the SEMs and thus accelerate the imaging process. In this work, the potential of nonlinear and nonbijective SEMs for spatial encoding during transmission in multidimensional spatially selective excitation is explored. Methods for multidimensional spatially selective excitation radiofrequency pulse design based on nonlinear encoding fields are introduced, and it is shown how encoding ambiguities can be resolved using parallel transmission. In simulations and phantom experiments, the feasibility of selective excitation using nonlinear, nonbijective SEMs is demonstrated, and it is shown that the spatial resolution with which the target distribution of the transverse magnetization can be realized varies locally. Thus, the resolution of the target pattern can be increased in some regions compared with conventional linear encoding. Furthermore, experimental proof of principle of accelerated two-dimensional spatially selective excitation using nonlinear SEMs is provided in this study.