A dual-core NMR system for field-cycling singlet assisted diffusion NMR.

Long-lived singlet spin order offers the possibility to extend the spin memory by more than an order of magnitude. This enhancement can be used, among other applications, to assist NMR diffusion experiments in porous media where the extended lifetime of singlet spin order can be used to gain information about structural features of the medium as well as the dynamics of the imbibed phase. Other than offering the possibility to explore longer diffusion times of the order of many minutes that, for example, gives unprecedented access to tortuosity in structures with interconnected pores, singlet order has the important advantage to be immune to the internal field gradients generated by magnetic susceptibility inhomogeneities. These inhomogeneities, however, are responsible for very short T2 decay constants in high magnetic field and this precludes access to the singlet order in the first instance. To overcome this difficulty and take advantage of singlet order in diffusion experiments in porous media, we have here developed a dual-core system with radiofrequency and 3-axis pulsed field gradients facilities in low magnetic field, for preparation and manipulation of singlet order and a probe, in high magnetic field, for polarisation and detection. The system operates in field-cycling and can be used for a variety of NMR experiments including diffusion tensor imaging (both singlet assisted and not). In this paper we present and discuss the new hardware and its calibration, and demonstrate its capabilities through a variety of examples.

Radial magnetic resonance imaging (MRI) using a rotating radiofrequency (RF) coil at 9.4 T.

The rotating radiofrequency coil (RRFC) has been developed recently as an alternative approach to multi-channel phased-array coils. The single-element RRFC avoids inter-channel coupling and allows a larger coil element with better B1 field penetration when compared with an array counterpart. However, dedicated image reconstruction algorithms require accurate estimation of temporally varying coil sensitivities to remove artefacts caused by coil rotation. Various methods have been developed to estimate unknown sensitivity profiles from a few experimentally measured sensitivity maps, but these methods become problematic when the RRFC is used as a transceiver coil. In this work, a novel and practical radial encoding method is introduced for the RRFC to facilitate image reconstruction without the measurement or estimation of rotation-dependent sensitivity profiles. Theoretical analyses suggest that the rotation-dependent sensitivities of the RRFC can be used to create a uniform profile with careful choice of sampling positions and imaging parameters. To test this new imaging method, dedicated electronics were designed and built to control the RRFC speed and hence positions in synchrony with imaging parameters. High-quality phantom and animal images acquired on a 9.4 T pre-clinical scanner demonstrate the feasibility and potential of this new RRFC method.

Image Reconstruction for a Rotating Radiofrequency Coil (RRFC) Using Self-Calibrated Sensitivity From Radial Sampling.

The purpose of this study was to develop a practical magnetic resonance imaging (MRI) scheme for the latest rotating radiofrequency coil (RRFC) design at 9.4 T. The new prototype RRFC was integrated with an optical sensor to facilitate recording of its angular positions relative to the sequence timing. In imaging, the RRFC was used together with radial k-space trajectories. To recover the image, the radial spokes were grouped according to the coil locations. Using an Eigen-decomposition approach, an array of location-dependent sensitivity maps was extracted from the central regions of the segmented k-space, enabling parallel-imaging techniques for image recovery in a straightforward manner. When the RRFC angular velocity is carefully designed and accurately controlled according to the sequence timing, the encoding by means of varying RRFC sensitivity maps can be accurately calibrated for a faithful image recovery. Approximations were made to counteract the variations of the RRFC angular velocity, providing successful image reconstruction at 9.4 T. The current study demonstrated a new and practical imaging scheme for RRFC-MRI. It is able to extract the temporally varying sensitivity maps retrospectively from the k-space acquisition itself, without resorting to electromagnetic simulation or numerical interpolation. The proposed imaging scheme and the supporting engineering solutions of the RRFC prototype enable accurate image reconstructions. These new developments pave the way for routine applications of the RRFC, and bode well for its further development in providing simultaneous multinuclear imaging by incorporating, for example, independent X-nuclear coil elements into the rotating structure.

Revealing signal from noisy (19) F MR images by chemical shift artifact correction.

PURPOSE: The correction of chemical shift artifacts in MR images of fluorinated molecules with a multi-resonance spectrum is investigated. The goal is to find a deconvolution method which is capable of correcting the artifact, thereby enhancing signal-to-noise ratio (SNR) and revealing signal that vanishes in the noise in the original image. THEORY AND METHODS: Simulations for inspecting the influence of MRI acquisition parameters on the possibility to correct the artifact are performed. Artifact correction is studied on the spectrum of a perfluorocarbon compound by means of deconvolution of complex images with a measured or an optimized point spread function and by using lasso regularization. RESULTS: Distinct parameter settings for image acquisition are identified that should be avoided for successful deconvolution. With in vitro and in vivo images it is shown that the SNR of the corrected image can be increased significantly by 20-50% by a regularized deconvolution of the artifact image and (19) F signal can be revealed from noise. CONCLUSION: By deconvolution, SNR can be enhanced as compared to an image which only exploits the strongest peak of the spectrum. Thus, the limit of detection of the (19) F signal can be lowered by exciting all resonances and by means of correcting the chemical shift artifact.

Quantification and correction of respiration induced dynamic field map changes in fMRI using 3D single shot techniques.

PURPOSE: Respiration induced dynamic field map changes in the brain are quantified and the influence on the magnitude signal (physiological noise) is investigated. Dynamic off-resonance correction allows to reduce the signal fluctuations overlaying the blood oxygenation level dependent signal in T2*-weighted functional imaging. THEORY AND METHODS: A single-shot whole brain imaging technique with 100 ms temporal resolution was used to measure dynamic off-resonance maps that were calculated from the incremental changes of the image phase. These off-resonance maps are then used to dynamically update the off-resonance corrected reconstruction. RESULTS: A global resonance offset and a pronounced gradient in head-foot direction were identified as the main components of the change during a respiration cycle. On average, correction for these fluctuations decreases the magnitude fluctuations by around 30%. CONCLUSION: Single shot 3D imaging allows for a robust quantification of dynamic off-resonance changes in the brain. Correction for these fluctuations removes the physiological noise component associated with dynamic point spread function changes.

Single shot whole brain imaging using spherical stack of spirals trajectories.

MR-encephalography allows the observation of functional signal in the brain at a frequency of 10 Hz, permitting filtering of physiological "noise" and the detection of single event activations. High temporal resolution is achieved by the use of undersampled non-Cartesian trajectories, parallel imaging and regularized image reconstruction. MR-encephalography is based on 3D-encoding, allowing undersampling in two dimensions and providing advantages in terms of signal to noise ratio. Long readout times, which are necessary for single shot whole brain imaging (up to 75 ms), cause off-resonance artifacts. To meet this issue, a spherical stack of spirals trajectory is proposed in this work. By examining the trajectories in local k-space, it is shown that in areas of strong susceptibility gradients spatial information is fundamentally lost, making a meaningful image reconstruction impossible in the affected areas. It is shown that the loss of spatial information is reduced when using a stack of spirals trajectory compared to concentric shells.

Tracking dynamic resting-state networks at higher frequencies using MR-encephalography.

Current resting-state network analysis often looks for coherent spontaneous BOLD signal fluctuations at frequencies below 0.1 Hz in a multiple-minutes scan. However hemodynamic signal variation can occur at a faster rate, causing changes in functional connectivity at a smaller time scale. In this study we proposed to use MREG technique to increase the temporal resolution of resting-state fMRI. A three-dimensional single-shot concentric shells trajectory was used instead of conventional EPI, with a TR of 100 ms and a nominal spatial resolution of 4 × 4 × 4 mm(3). With this high sampling rate we were able to resolve frequency components up to 5 Hz, which prevents major physiological noises from aliasing with the BOLD signal of interest. We used a sliding-window method on signal components at different frequency bands, to look at the non-stationary connectivity maps over the course of each scan session. The aim of the study paradigm was to specifically observe visual and motor resting-state networks. Preliminary results have found corresponding networks at frequencies above 0.1 Hz. These networks at higher frequencies showed better stability in both spatial and temporal dimensions from the sliding-window analysis of the time series, which suggests the potential of using high temporal resolution MREG sequences to track dynamic resting-state networks at sub-minute time scale.

Single shot concentric shells trajectories for ultra fast fMRI.

MR-encephalography is a technique that allows real-time observation of functional changes in the brain with a time-resolution of 100 ms. The high sampling rate is enabled by the use of undersampled image acquisition with regularized reconstruction. The article describes a novel imaging method for fast three-dimensional-MR-encephalography whole brain coverage based on undersampled, single-shot concentric shells trajectories and the use of multiple small receiver coils. The technique allows the observation of changes in blood oxygenation level dependent signal as a measure of brain physiology at very high temporal resolution.

Fast undersampled functional magnetic resonance imaging using nonlinear regularized parallel image reconstruction.

In this article we aim at improving the performance of whole brain functional imaging at very high temporal resolution (100 ms or less). This is achieved by utilizing a nonlinear regularized parallel image reconstruction scheme, where the penalty term of the cost function is set to the L(1)-norm measured in some transform domain. This type of image reconstruction has gained much attention recently due to its application in compressed sensing and has proven to yield superior spatial resolution and image quality over e.g. Tikhonov regularized image reconstruction. We demonstrate that by using nonlinear regularization it is possible to more accurately localize brain activation from highly undersampled k-space data at the expense of an increase in computation time.