The human connectome project (HCP) relies primarily on three complementary magnetic resonance (MR) methods. These are: 1) resting state functional MR imaging (rfMRI) which uses correlations in the temporal fluctuations in an fMRI time series to deduce ‘functional connectivity’; 2) diffusion imaging (dMRI), which provides the input for tractography algorithms used for the reconstruction of the complex axonal fiber architecture; and 3) task based fMRI (tfMRI), which is employed to identify functional parcellation in the human brain in order to assist analyses of data obtained with the first two methods. We describe technical improvements and optimization of these methods as well as instrumental choices that impact speed of acquisition of fMRI and dMRI images at 3 Tesla, leading to whole brain coverage with 2 mm isotropic resolution in 0.7 second for fMRI, and 1.25 mm isotropic resolution dMRI data for tractography analysis with three-fold reduction in total data acquisition time. Ongoing technical developments and optimization for acquisition of similar data at 7 Tesla magnetic field are also presented, targeting higher resolution, specificity of functional imaging signals, mitigation of the inhomogeneous radio frequency (RF) fields and power deposition. Results demonstrate that overall, these approaches represent a significant advance in MR imaging of the human brain to investigate brain function and structure.

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Purpose To develop high-resolution electrical properties tomography (EPT) methods and investigate a gradient-based EPT (gEPT) approach which aims to reconstruct the electrical properties (EP), including conductivity and permittivity, of an imaged sample from experimentally measured B1 maps with improved boundary reconstruction and robustness against measurement noise. Theory and Methods Using a multi-channel transmit/receive stripline head coil, with acquired B1 maps for each coil element, by assuming negligible Bz component compared to transverse B1 components, a theory describing the relationship between B1 field, EP value and their spatial gradient has been proposed. The final EP images were obtained through spatial integration over the reconstructed EP gradient. Numerical simulation, physical phantom and in vivo human experiments at 7 T have been conducted to evaluate the performance of the proposed methods. Results Reconstruction results were compared with target EP values in both simulations and phantom experiments. Human experimental results were compared with EP values in literature. Satisfactory agreement was observed with improved boundary reconstruction. Importantly, the proposed gEPT method proved to be more robust against noise when compared to previously described non-gradient-based EPT approaches. Conclusion The proposed gEPT approach holds promises to improve EP mapping quality by recovering the boundary information and enhancing robustness against noise.

The electric properties (EPs) of biological tissue provide important diagnostic information within radio and microwave frequencies, and also play an important role in specific absorption rate (SAR) calculation which is a major safety concern at ultrahigh field (UHF). The recently proposed electrical properties tomography (EPT) technique aims to reconstruct EPs in biological tissues based on B1 measurement. However, for individual coil element in multi-channel transceiver coil which is increasingly utilized at UHF, current B1-mapping techniques could not provide adequate information (magnitude and absolute phase) of complex transmit and receive B1 which are essential for EPT, electric field, and quantitative SAR assessment. In this study, using a 16-channel transceiver coil at 7T, based on hybrid B1-mapping techniques within the human brain, a complex B1-mapping method has been developed, and in-vivo EPs imaging of the human brain has been demonstrated by applying a logarithm-based inverse algorithm. Computer simulation studies as well as phantom and human experiments have been conducted at 7T. The average bias and standard deviation for reconstructed conductivity in vivo were 28% and 67%, and 10% and 43% for relative permittivity, respectively. The present results suggest the feasibility and reliability of proposed complex B1-mapping technique and EPs reconstruction method.

Purpose Higher SNR and improved contrast have been demonstrated at Ultra-high magnetic fields (≥7T) in multiple targets, often with multi-channel transmit B1+ methods to address the deleterious impact on tissue contrast due to spatial variations in B1+ profiles. When imaging the heart at 7T, however, respiratory and cardiac motion, as well as B0 inhomogeneity, greatly increase the methodological challenge. In this study we compare 2-spoke parallel transmit (pTX) RF pulses with static B1+ shimming in cardiac imaging at 7T. Methods Using a 16-channel pTX system, slice-selective 2-spoke pTX pulses and static B1+ shimming were applied in cardiac CINE imaging. B1+ and B0 mapping required modified cardiac triggered sequences. Excitation homogeneity and RF energy were compared in different imaging orientations. Results 2-spoke pulses provide higher excitation homogeneity than B1+ shimming, especially in the more challenging posterior region of the heart. The peak value of channel-wise RF energy was reduced, allowing for higher flip angle, hence increased tissue contrast. Image quality with 2-spoke excitation proved to be stable throughout the entire cardiac cycle. Conclusion 2-spoke pTX excitation has been successfully demonstrated in the human heart at 7T, with improved image quality and reduced RF pulse energy when compared to B1+ shimming.

Purpose To develop a new parallel transmit (pTx) pulse design for simultaneous multi-band (MB) excitation in order to tackle simultaneously the problems of transmit B1 (B1+) inhomogeneity and total RF power, so as to allow for optimal RF excitation when using MB pulses for slice acceleration for high and ultrahigh field MRI. Methods With the proposed approach, each of the bands that are simultaneously excited is subject to a band-specific set of B1 complex shim weights. The method was validated in the human brain at 7T using a 16-channel pTx system and was compared to conventional MB pulses operating in the circularly polarized (CP) mode. Further numerical simulations based on measured B1 maps were conducted. Results The new method improved B1+ homogeneity by 60% when keeping the total RF power constant and reduced total RF power by 72% when keeping the excitation fidelity constant, as compared to the conventional CP mode. Conclusion A new pTx pulse design formalism is introduced targeting slice-specific B1+homogenization in MB excitation while constraining total RF power. These pulses lead to significantly improved slice-wise B1+ uniformity and/or largely reduced total RF power, as compared to the conventionally employed MB pulses applied in the CP mode.

Purpose Two-spoke parallel transmission (pTX) RF pulses have been demonstrated in cardiac MRI at 7T. However, current pulse designs rely on a single set of B1+/B0 maps that may not be valid for subsequent scans acquired at another phase of the respiration cycle, because of organs displacement. Such mismatches may yield severe excitation profile degradation. Methods B1+/B0 maps were obtained, using 16 transmit channels at 7T, at three breath-hold positions: exhale, half-inhale and inhale. Standard and robust RF pulses were designed using maps obtained at exhale only, and at multiple respiratory positions, respectively. Excitation patterns were analyzed for all positions using Bloch simulations. Flip-angle homogeneity was compared in-vivo in cardiac CINE acquisitions. Results Standard 1- and 2-spoke pTX RF pulses are sensitive to breath-hold position, primarily due to B1+ alterations, with high dependency on excitation trajectory for 2-spokes. In-vivo excitation inhomogeneity varied from nRMSE=8.2% (exhale) up to 32.5% (inhale) with the standard design; much more stable results were obtained with the robust design with nRMSE=9.1% (exhale) and 10.6% (inhale). Conclusion A new pTX RF pulse design robust against respiration induced variations of B1+/B0 maps is demonstrated and is expected to positively impact cardiac MRI in breath-hold, free-breathing and real-time acquisitions.

Following rapid technological advances, ultra-high field functional MRI (fMRI) enables exploring correlates of neuronal population activity at an increasing spatial resolution. However, as the fMRI blood-oxygenation-level-dependent (BOLD) contrast is a vascular signal, the spatial specificity of fMRI data is ultimately determined by the characteristics of the underlying vasculature. At 7T, fMRI measurement parameters determine the relative contribution of the macro- and microvasculature to the acquired signal. Here we investigate how these parameters affect relevant high-end fMRI analyses such as encoding, decoding, and submillimeter mapping of voxel preferences in the human auditory cortex. Specifically, we compare a T* weighted fMRI dataset, obtained with 2D gradient echo (GE) EPI, to a predominantly T weighted dataset obtained with 3D GRASE. We first investigated the decoding accuracy based on two encoding models that represented different hypotheses about auditory cortical processing. This encoding/decoding analysis profited from the large spatial coverage and sensitivity of the T* weighted acquisitions, as evidenced by a significantly higher prediction accuracy in the GE-EPI dataset compared to the 3D GRASE dataset for both encoding models. The main disadvantage of the T* weighted GE-EPI dataset for encoding/decoding analyses was that the prediction accuracy exhibited cortical depth dependent vascular biases. However, we propose that the comparison of prediction accuracy across the different encoding models may be used as a post processing technique to salvage the spatial interpretability of the GE-EPI cortical depth-dependent prediction accuracy. Second, we explored the mapping of voxel preferences. Large-scale maps of frequency preference (i.e., tonotopy) were similar across datasets, yet the GE-EPI dataset was preferable due to its larger spatial coverage and sensitivity. However, submillimeter tonotopy maps revealed biases in assigned frequency preference and selectivity for the GE-EPI dataset, but not for the 3D GRASE dataset. Thus, a T weighted acquisition is recommended if high specificity in tonotopic maps is required. In conclusion, different fMRI acquisitions were better suited for different analyses. It is therefore critical that any sequence parameter optimization considers the eventual intended fMRI analyses and the nature of the neuroscience questions being asked.