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Transitory states of the brain, sometimes linked to particular mental disorders, can be identified by conducting a dynamic analysis of connectivity in functional magnetic resonance imaging (fMRI) data. Here, we investigated whether the connectivity between corticallaminae(in contrast to between corticalareas), can vary during the course of a resting-state scan and whether this could impact the identification of brain states in the healthy brain.We performed a sliding-window connectivity analysis of high-resolution fMRI data and found that the differential participation of the cortical layers in the overall cortical connectivity constitutes a more dynamic feature of the brain compared to region-to-region connectivity, with two main states (superficial vs. deep-laminae connectivity) fluctuating over time. Laminar connectivity restricted to the default mode network appeared relatively stable (superficial connectivity preferred), while the central executive and salience networks showed a fluctuating laminar preference. We anticipate that the dynamic analysis of connectivity focused on the different depths of the cerebral cortex could play an important role in characterizing healthy brain states as well as in identifying novel targets of psychiatric diseases.

Simultaneous MR-PET is an increasingly popular multimodal imaging technique that is able to combine metabolic information obtained from PET with anatomical/functional information from MRI. One of the key technological challenges of the technique is the integration of a PET-transparent MR coil system, a solution to which is demonstrated here for a double-tuned 1 H/ 31 P head coil at 3 T. Two single-resonant birdcage coils tuned to the 1 H and 31 P resonances were arranged in an interleaved fashion and electrically decoupled with the use of trap circuits. All high 511 keV quanta absorbing components were arranged outside the PET field-of-view in order to minimize count rate reduction. The materials inside the PET field-of-view were carefully evaluated and chosen for minimum impact on the PET image quality. As far as possible, the coil case was geometrically optimized to avoid sharp transitions in attenuation, which may potentially result in streaking artefacts during PET image reconstruction. The coil caused a count rate loss of just above 5% when inserted into the PET detector ring. Except for the anterior region, which was designed to maintain free openings for increased patient comfort, an almost uniform distribution of 511 keV attenuation was maintained around the circumference of the coil. MR-related performance for both nuclei was similar or slightly better than that of a commercial double-tuned coil, despite the MR-PET coil having a close-fitting RF screen to shield the PET and MR electronics from possible electromagnetic interferences.

Purpose To overcome existing challenges in dynamic B0 shimming by implementing a shim optimization algorithm which limits shim current amplitudes and their temporal variation through the application of constraints and regularization terms. Theory and methods Spherical harmonic dynamic B0 shimming is complicated by eddy currents, ill-posed optimizations, and the need for strong power supplies. Based on the fact that eddy current amplitudes are proportional to the magnitude of the shim current changes, and assuming a smoothness of the B0 inhomogeneity variation in the slice direction, a novel algorithm was implemented to reduce eddy current generation by limiting interslice shim current changes. Shim degeneracy issues and resulting high current amplitudes are additionally addressed by penalizing high solution norms. Applicability of the proposed algorithm was validated in simulations and in phantom and in vivo measurements. Results High-order dynamic shimming simulations and measurements have shown that absolute shim current amplitudes and their temporal variation can be substantially reduced with negligible loss in achievable B0 homogeneity. Whereas conventional dynamic shim updating optimizations improve the B0 homogeneity, on average, by a factor of 2.1 over second-order static solutions, our proposed routine reached a factor of 2.0, while simultaneously providing a 14-fold reduction of the average maximum shim current changes. Conclusions The proposed algorithm substantially reduces the shim amplitudes and their temporal variation, while only marginally affecting the achievable B0 homogeneity. As a result, it has the potential to mitigate the remaining challenges in dynamic B0 shimming and help in making its application more readily available.

Magnetic resonance in medicine : MRM 85(5), 2892-2903 (2021). doi:10.1002/mrm.28597, Published by Wiley-Liss, New York, NY [u.a.]

With the increasing availability of ultra-high field MRI systems, studying non-proton nuclei (X-nuclei), such as 23Na and 31P has received great interest. X-nuclei are able to provide insight into important cellular processes and energy metabolism in tissues and by monitoring these nuclei closely it is possible to establish links to pathological conditions and neurodegenerative diseases. In order to investigate X-nuclei, a well-designed radiofrequency (RF) system with a multi-tuned RF coil is required. However, as the intrinsic sensitivity of non-proton nuclei is lower compared to 1H, it is important to ensure that the signal-to-noise ratio (SNR) of the X-nuclei is as high as possible. This review aims to give a comprehensive overview of previous efforts, with particular focus on the design concept of multi-tuned coils, predominantly for brain applications. In order to guide the readers, the main body of the review is categorised into two parts: state-of-the art according to the single or multiple design structures and emerging technologies. A more detailed description is given in each subsection relating to the specific design approaches of, mostly, double-tuned coils, including using traps, PIN-diodes, nested and metamaterial, together with explanations of their novelties, optimal solutions and trade-offs.

Prostate cancer is one of the most common cancers among men and its early detection is critical for its successful treatment. The use of multimodal imaging, such as MR-PET, is most advantageous as it is able to provide detailed information about the prostate. However, as the human prostate is flexible and can move into different positions under external conditions, it is important to localise the focused region-of-interest using both MRI and PET under identical circumstances. In this work, we designed five commonly used linear and quadrature radiofrequency surface coils suitable for hybrid MR-PET use in endorectal applications. Due to the endorectal design and the shielded PET insert, the outer face of the coils investigated was curved and the region to be imaged was outside the volume of the coil. The tilting angles of the coils were varied with respect to the main magnetic field direction. This was done to approximate the various positions from which the prostate could be imaged. The transmit efficiencies and safety excitation efficiencies from simulations, together with the signal-to-noise ratios from the MR images were calculated and analysed. Overall, it was found that the overlapped loops driven in quadrature were superior to the other types of coils we tested. In order to determine the effect of the different coil designs on PET, transmission scans were carried out, and it was observed that the differences between attenuation maps with and without the coils were negligible. The findings of this work can provide useful guidance for the integration of such coil designs into MR-PET hybrid systems in the future.

MRI volume coils can be represented by equivalent lumped element circuits and for a variety of these circuit configurations analytical design equations have been presented. The unification of several volume coil topologies results in a two-dimensional gridded equivalent lumped element circuit which compromises the birdcage resonator, its multiple endring derivative but also novel structures like the capacitive coupled ring resonator. The theory section analyzes a general two-dimensional circuit by noting that its current distribution can be decomposed into a longitudinal and an azimuthal dependency. This can be exploited to compare the current distribution with a transfer function of filter circuits along one direction. The resonances of the transfer function coincide with the resonance of the volume resonator and the simple analytical solution can be used as a design equation. The proposed framework is verified experimentally against a novel capacitive coupled ring structure which was derived from the general circuit formulation and is proven to exhibit a dominant homogeneous mode. In conclusion, a unified analytical framework is presented that allows determining the resonance frequency of any volume resonator that can be represented by a two dimensional meshed equivalent circuit.

Non-proton MRI has recently garnered gathering interest with the increased availability of ultra high-field MRI system. Assuming the availability of a broadband RF amplifier, performing multinuclear MR experiments essentially requires additional hardware, such as an RF resonator and a T/R switch for each nucleus. A double- or triple-resonant RF probe is typically constructed using traps or PIN-diode circuits, but this approach degrades the signal-to-noise ratio (SNR) and image quality compared to a single-resonant coil and this is a limiting factor. In this work, we have designed the required hardware for multinuclear MR imaging experiments employing six single-resonant coil sets and a purpose-built animal bed; these have been implemented into a home-integrated 9.4 T preclinical MRI scanner. System capabilities are demonstrated by distinguishing concentration differences and sensitivity of X-nuclei imaging and spectroscopy without SNR penalty for any nuclei, no subject interruption and no degradation of the static shim conditions.

Proton magnetic resonance imaging (MRI) can be combined with signals from non-proton nuclei (X-nuclei) to provide metabolic information. Double-resonant coils are often used for X-nuclei MR studies where the proton element is employed for scout imaging and B

With the increased commercial availability of high channel count MR coil arrays and the associated higher number of plugs in the patient bed, it has become a common practice to include switch matrices in the receive path of MR systems. These allow the arbitrary routing of a signal from any plug to any receiver in the console. While switch matrices are standard in systems at clinical field strength and have been developed for systems operating up to 4T, they have not yet been implemented at ultra-high field (UHF). Here, we present a switch matrix suitable for operation at UHF. Crossbar switches, which are the most frequently employed forms of a switch matrix, use RF switches to connect horizontal input lines with the desired vertical output line. This leaves transmission line stubs of variable length physically connected with the selected signal path, potentially resulting in elevated signal losses. While this can be tolerated at low frequencies, and only needs partial compensation at intermediate frequencies (4T), a full compensation is required at UHF. In this study, an RF crossbar switch, which uses switchable compensation elements in both horizontal and vertical transmission lines, was implemented for a 9.4T MRI scanner. The prototype developed was evaluated for single channel and multichannel receive performance and benchmarked against a fixed wire connection.

PURPOSE: To overcome existing challenges in dynamic B(0) shimming by implementing a shim optimization algorithm which limits shim current amplitudes and their temporal variation through the application of constraints and regularization terms. THEORY AND METHODS: Spherical harmonic dynamic B(0) shimming is complicated by eddy currents, ill-posed optimizations, and the need for strong power supplies. Based on the fact that eddy current amplitudes are proportional to the magnitude of the shim current changes, and assuming a smoothness of the B(0) inhomogeneity variation in the slice direction, a novel algorithm was implemented to reduce eddy current generation by limiting interslice shim current changes. Shim degeneracy issues and resulting high current amplitudes are additionally addressed by penalizing high solution norms. Applicability of the proposed algorithm was validated in simulations and in phantom and in vivo measurements. RESULTS: High-order dynamic shimming simulations and measurements have shown that absolute shim current amplitudes and their temporal variation can be substantially reduced with negligible loss in achievable B(0) homogeneity. Whereas conventional dynamic shim updating optimizations improve the B(0) homogeneity, on average, by a factor of 2.1 over second-order static solutions, our proposed routine reached a factor of 2.0, while simultaneously providing a 14-fold reduction of the average maximum shim current changes. CONCLUSIONS: The proposed algorithm substantially reduces the shim amplitudes and their temporal variation, while only marginally affecting the achievable B(0) homogeneity. As a result, it has the potential to mitigate the remaining challenges in dynamic B(0) shimming and help in making its application more readily available.

The combination of several imaging modalities into a hybrid device has enabled a straightforward and accurate solution for the investigation of multiparametric processes in vivo. All existing imaging modalities have both strengths and weaknesses. For instance, computed tomography (CT) is an excellent modality for imaging bones. Magnetic resonance tomography images the effects of a manifold of interaction mechanisms between the spin ensemble and/with its environment and other nuclei. The differences in these interactions depend much more on the tissue type, its microscopic environment and interactions at the quantum level and therefore MR images provide a wide variety of superior soft tissue contrasts. Gamma scintigraphy, single photon emission computed tomography and positron emission tomography detect single radioactive decays from tagged molecules and can infer the spatial position of the decays from a few thousand decays. Thus, the sensitivity of these modalities is very high, but the spatial resolution is very low. Instead of pushing CT and MRI technology towards molecular imaging and positron emission tomography (PET) technology towards structural imaging, a much more promising approach is to further optimise the strengths of these modalities by combining them into hybrid devices. The combination of MRI and PET has been the second bi-modality, which resulted in commercially available hybrid scanners for animal and human imaging. In this chapter, the most important mutual interferences of PET and MRI, together with technical solutions for minimising them, are presented and discussed.

This chapter introduces the basic building blocks of a clinical magnetic resonance imaging machine and illustrates their function during image generation. Topics covered include the main magnet, gradient and shim system, radiofrequency chain and coils, as well as additional system components. The advantages and disadvantages of different implementations of the individual units will be explained and current state-of-the-art designs are summarised.

The use of preclinical imaging in research spans several decades and developments in the field are continuing in a number of directions, with research using simultaneous hybrid magnetic resonance-positron emission tomography (MR-PET) systems at the forefront. This opens up new opportunities to access large amounts of diverse temporal and spatial information with high accuracy. Following on from the human MR-PET instrumentation chapter, this chapter deals with the particular challenges, such as ultra-high magnetic and gradient field strengths associated with the combination of preclinical MR and PET hardware. Furthermore, the development of several small animal systems for combined MR-PET applications are also introduced.

A folded four-ring quadrature birdcage coil was designed and constructed with a double-tune configuration of an outer high-pass coil for 1H (400 MHz) and inner low-pass coil for 23Na (105.72 MHz at 9.4 T). The coil was evaluated on the bench and in the scanner, comparing its performance with that of single-tuned coils and a large four-ring coil. All coils were tuned and matched and the isolation between two quadrature ports was found to be better than −13.7 dB for 1H and −27 dB for 23Na. Signal-to-noise ratios (SNRs) were calculated and 23Na flip angle maps were acquired. 23Na SNR of the folded four-ring reached ∼93% of that obtained with the single-tuned coil. A set of in vivo 1H and 23Na axial images to cover the whole rat brain were obtained. The performance of the folded four-ring coil and its benefit for 23Na imaging experiments have been demonstrated. This proposed four-ring coil could avoid length restrictions, e.g. the shoulders, by folding the outer rings vertically. This facilitates the construction of double-tuned four-ring birdcage coils just to fit the head, leading to higher filling factors and better SNR.

Hybrid MR–PET data acquisition in simultaneous mode confers a number of advantages at 3 T and 9.4 T. From an MR perspective, the potential for ultra-high resolution structural imaging is discussed and example images of the cerebellum with an isotropic resolution of 320 μm are presented. Further, metabolic imaging is discussed and high-resolution images of the sodium distribution are demonstrated. Examples of tumour imaging on a 3 T MR–PET system are included and discussed.

This paper presents the conceptualization, design and simulation of a hybrid multi-frequency antenna system for Radio Frequency (RF) hyperthermia and Magnetic Resonance Imaging (MRI) applications with ultra-sharp hotspot. Two different bowtie antenna arrays have been designed, simulated and integrated. The first antenna array is designed as an RF coil for MR imaging. It is optimized to generate a highly homogeneous transmit equation field and is intended for 3 T MRI systems. This corresponds to a frequency of 128 MHz. The second array aims to induce heat by accumulating the electromagnetic energy at one location thereby forming a hotspot. The operating frequency of the second system is 600 MHz. The computed homogeneity of the B1+ field generated by the imaging antennas shows similar values compared to a birdcage coil at the same frequency. The size of the hotspot generated by the heating subsystem was 26 × 26 × 93mm3. The heating produced by the MR imaging antennas was negligible, compared to the RF hyperthermia applicator.

Given the absence of a body coil, the radio frequency screen of a whole-body 9.4T magnetic resonance imaging scanner can be used as a circular waveguide. In the unloaded case, the screen allows propagation of the dominant TE11- as well as the TM01-mode. In the first part of this study, the optimum coupling of a circular polarized TE11-mode was determined empirically for excitation and reception with a rectangular patch antenna. Employing full-wave simulations, two simulation models and two phantoms, different patch positions were tested to find the optimum position with respect to coupled power and homogenous excitation field. The best simulation results were validated with measurements. The second part of this study describes the design and measurements of a multimode excitation device. Using the parallel transmit system of the MR scanner, all propagable traveling wave modes could be excited and detected independently. The performance of the multimode device related to field of view, B1+-efficiency and radio frequency shimming was assessed by phantom measurements. Initial results show that three modes are sufficient to homogeneously excite regions of interest at 9.4 T.

Proton magnetic resonance imaging (MRI) can be combined with signals from non-proton nuclei (X-nuclei) to provide metabolic information. Double-resonant coils are often used for X-nuclei MR studies where the proton element is employed for scout imaging and B 0 shimming. This work describes the development of a new double-resonant coil capable of operating at both proton and X-nuclei frequencies. The proposed design extends the wheel-and-spoke coil, which allows for quadrature drive, by adding an extra ring outside the coil to achieve double-resonance. Furthermore, in order to maximise SNR by increasing the filling factor, the shape of the coil has been modified to a helmet style making it suitable for brain applications. The performance of the double-resonant helmet coil was evaluated by simulation and MR measurements. The helmet coil was successfully tuned to the 1H/31P resonance frequencies of a 3T MR scanner, with high isolation between the two quadrature ports. MR measurements of a phantom were carried out, and the averaged sensitivity of the double-resonant helmet coil over the whole phantom was found to be higher than that of the conventional double-tuned birdcage coil at both frequencies.

An der RWTH Aachen wurde der erste offene und mobile Kernspintomograph entwickelt. Anders als die normalerweise riesigen und unbeweglichen Gerate besitzt er keine enge Magnetrohre. So kann er auch in grose Untersuchungsobjekte hinein schauen, ohne dass diese zerstort werden mussen. Wie eine Lupe erfasst er dabei ein begrenztes Volumen um den Aufsatzpunkt herum. Neben der Medizin ermoglicht der offene Tomograph viele neue Anwendungsgebiete, vor allem in der Materialprufung und Qualitatskontrolle. Der Aachener Prototyp kann auch das Fliesprofil und die Geschwindigkeitsverteilung stromender Flussigkeiten sichtbar machen. Sein offenes Prinzip erforderte neue technische Komponenten und modifizierte bildgebende Verfahren. Die Messzeit pro Bild kann heute schon unter einer Viertelstunde liegen.

The Class-E amplifier is a highly efficient tuned switching power amplifier with a linear drain amplitude modulation characteristic ideally suited for high-efficiency amplifier systems (e.g., Kahn-amplifiers). This brief describes a design procedure to determine the circuit elements for optimum operation of the amplifier using state space methods. Finite feed inductance and quality factor of the resonant circuit are taken into account. It is shown that high efficiency can be sustained for a wide variety of element combinations, allowing relaxed demands on the output capacitance of the active device. This is especially useful for high-frequency amplifiers where the maximum operation frequency of a given device is often limited by its intrinsic drain-source capacitance.

Objectives The aims of this study were to investigate the subjective discomfort and sensory side effects during ultrahigh field (UHF) magnetic resonance imaging (MRI) examinations in a large-scale study and to evaluate differences between magnetic resonance (MR) sites. Materials and methods Four MR sites with a 7-T MR system and 2 MR sites with a 9.4-T MR system participated in this multicenter study with a total number of 3457 completed questionnaires on causes of discomfort and sensations during the examination. For a pooled retrospective analysis of the results from the partially different questionnaires, all data were adapted to an answer option with a 4-point scale (0 = no discomfort/side effect, 3 = very unpleasant/very strong sensation). To differentiate effects evoked by the low-frequency time-varying magnetic fields due to movement through the static magnetic field, most questionnaires separated the manifestation of sensory side effects during movement on the patient table from manifestation while lying still in the isocenter. Results In general, a high acceptance of UHF examinations was found, where in 82% of the completed questionnaires, the subjects stated the examination to be at least tolerable. Although in 7.6% of the questionnaires, subjects felt discomfort during the examination, only 0.9% of the image acquisitions had to be terminated prematurely. No adverse events occurred in any of the examinations. Only 1% of the subjects were unwilling to undergo further UHF MRI examinations. Examination duration was the most complained cause of discomfort, followed by acoustic noise and lying still. All magnetic-field-related sensations were more pronounced when moving the patient table versus the isocenter position (19%/2% of the subjects felt unpleasant vertigo during the moving/stationary state). In general, vertigo was the most often stated sensory side effect and was more pronounced at 9.4 T compared with 7 T. However, the results varied substantially among the different sites. Conclusions The high levels of subjective acceptance found in this study lead to the conclusion that UHF MRI would be tolerated as a diagnostic tool in clinical practice. For more consistent data ascertainment, we propose a standardized questionnaire for subjective perception monitoring.

Parallel radiofrequency (RF) transmission, either in the form of RF shimming or pulse design, has been proposed as a solution to the B1(+) inhomogeneity problem in ultra high field magnetic resonance imaging. As a prerequisite, accurate B1(+) maps from each of the available transmit channels are required. In this work, four different encoding methods for B1(+) mapping, namely 1-channel-on, all-channels-on-except-1, all-channels-on-1-inverted and Fourier phase encoding, were evaluated using dual refocusing acquisition mode (DREAM) at 9.4 T. Fourier phase encoding was demonstrated in both phantom and in vivo to be the least susceptible to artefacts caused by destructive RF interference at 9.4 T. Unlike the other two interferometric encoding schemes, Fourier phase encoding showed negligible dependency on the initial RF phase setting and therefore no prior B1(+) knowledge is required. Fourier phase encoding also provides a flexible way to increase the number of measurements to increase SNR, and to allow further reduction of artefacts by weighted decoding. These advantages of Fourier phase encoding suggest that it is a good choice for B1(+) mapping in parallel transmit systems at ultra high field.

In this perspectives article we highlight the advantages of simultaneous acquisition of electroencephalography (EEG) and functional magnetic resonance imaging (fMRI). As MRI moves towards using ultra-high magnetic fields in the quest for increased signal-to-noise, the question arises whether combined EEG–fMRI measurements are feasible at magnetic fields of 7 T and higher. We describe the challenges of MRI–EEG at 1.5, 3, 7 and 9.4 T and review the proposed solutions. In an outlook, we discuss further developments such as simultaneous trimodal imaging using MR, positron emission tomography (PET) and EEG under the same physiological conditions in the same subject.

Multi-modal MR-PET-EEG data acquisition in simultaneous mode confers a number of advantages at 3 T and 9.4 T. The three modalities complement each other well; structural-functional imaging being the domain of MRI, molecular imaging with specific tracers is the strength of PET, and EEG provides a temporal dimension where the other two modalities are weak. The utility of hybrid MR-PET at 3 T in a clinical setting is presented and critically discussed. The potential problems and the putative gains to be accrued from hybrid imaging at 9.4 T, with examples from the human brain, are outlined. Steps on the road to 9.4 T multi-modal MR-PET-EEG are also illustrated. From an MR perspective, the potential for ultra-high resolution structural imaging is discussed and example images of the cerebellum with an isotropic resolution of 320 μm are presented, setting the stage for hybrid imaging at ultra-high field. Further, metabolic imaging is discussed and high-resolution images of the sodium distribution are presented. Examples of tumour imaging on a 3 T MR-PET system are presented and discussed. Finally, the perspectives for multi-modal imaging are discussed based on two on-going studies, the first comparing MR and PET methods for the measurement of perfusion and the second which looks at tumour delineation based on MRI contrasts but the knowledge of tumour extent is based on simultaneously acquired PET data.

The simultaneous acquisition of electroencephalographic (EEG) and functional magnetic resonance imaging (fMRI) data has gained momentum in recent years due to the synergistic effects of the two modalities with regard to temporal and spatial resolution. Currently, only EEG-data recorded in fields of up to 7 T have been reported. We investigated the feasibility of recording EEG inside a 9.4 T static magnetic field, specifically to determine whether meaningful EEG information could be recovered from the data after removal of the cardiac-related artefact. EEG-data were recorded reliably and reproducibly at 9.4 T and the cardiac-related artefact increased in amplitude with increasing B0, as expected. Furthermore, we were able to correct for the cardiac-related artefact and identify auditory event related responses at 9.4 T in 75% of subjects using independent component analysis (ICA). Also by means of ICA we detected event related spectral perturbations (ERSP) in subjects at 9.4 T in response to opening/closing the eyes comparable with the response at 0 T. Overall our results suggest that it is possible to record meaningful EEG data at ultra-high magnetic fields. The simultaneous EEG-fMRI approach at ultra-high-fields opens up the horizon for investigating brain dynamics at a superb spatial resolution and a temporal resolution in the millisecond domain.

Given the absence of a body coil, the radio frequency screen of a whole-body 9.4T magnetic resonance imaging scanner can be used as a circular waveguide. In the unloaded case, the screen allows propagation of the dominant TE11- as well as the TM01-mode. In the first part of this study, the optimum coupling of a circular polarized TE11-mode was determined empirically for excitation and reception with a rectangular patch antenna. Employing full-wave simulations, two simulation models and two phantoms, different patch positions were tested to find the optimum position with respect to coupled power and homogenous excitation field. The best simulation results were validated with measurements. The second part of this study describes the design and measurements of a multimode excitation device. Using the parallel transmit system of the MR scanner, all propagable traveling wave modes could be excited and detected independently. The performance of the multimode device related to field of view, B1+-efficiency and radio frequency shimming was assessed by phantom measurements. Initial results show that three modes are sufficient to homogeneously excite regions of interest at 9.4 T.