Klaas P. Pruessmann

Improved diffusion-weighted single-shot echo-planar imaging (EPI) in stroke using sensitivity encoding (SENSE)

Diffusion‐weighted single‐shot EPI (sshEPI) is one of the most important tools for the diagnostic assessment of stroke patients, but it suffers from well known artifacts. Therefore, sshEPI was combined with SENSitivity Encoding (SENSE) to further increase EPIs potential for stroke imaging. Eight healthy volunteers and a consecutive series of patients (N = 8) with suspected stroke were examined with diffusion‐weighted SENSE‐sshEPI using different reduction factors (1.0 ≤ R ≤ 3.0). Additionally, a high‐resolution diffusion‐weighted SENSE‐sshEPI scan was included. All examinations were diagnostic and of better quality than conventional sshEPI. No ghostings or aliasing artifacts were discernible, and EPI‐related image distortions were markedly diminished. Chemical shift artifacts and eddy current‐induced image warping were still present, although to a markedly smaller extent. Measured direction‐dependent diffusion‐coefficients and isotropic diffusion values were comparable to previous findings but showed less fluctuation. We have demonstrated the technical feasibility and clinical applicability of diffusion‐weighted SENSE‐sshEPI in patients with subacute stroke. Because of the faster k‐space traversal, this novel technique is able to reduce typical EPI artifacts and increase spatial resolution while simultaneously remaining insensitive to bulk motion. Magn Reson Med 46:548–554, 2001.

Contrast-enhanced 3D MRA using SENSE

Sensitivity encoding (SENSE) was used to improve the performance of three‐dimensional contrast‐enhanced magnetic resonance angiography (3D CE‐MRA). Utilizing an array of receiver coils for sensitivity encoding, the encoding efficiency of gradient‐echo imaging was increased by factors of up to three. The feasibility of the approach was demonstrated for imaging of the abdominal vasculature. On the one hand, using a SENSE reduction factor of two, the spatial resolution of a breath‐hold scan of 17 seconds was improved to 1.0 × 2.0 × 2.0 mm3. On the other hand, using threefold reduction, time‐resolved 3D CE‐MRA was performed with a true temporal resolution of 4 seconds, at a spatial resolution of 1.6 × 2.1 × 4.0 mm3. CE‐MRA with SENSE was performed in healthy volunteers and patients and compared with a standard protocol. Throughout, diagnostic quality images were obtained, showing the ability of sensitivity encoding to enhance spatial and/or temporal resolution considerably in clinical angiographic examinations. J. Magn. Reson. Imaging 2000;12:671–677.

Specific coil design for SENSE: A six-element cardiac array

In sensitivity encoding (SENSE), the effects of inhomogeneous spatial sensitivity of surface coils are utilized for signal localization in addition to common Fourier encoding using magnetic field gradients. Unlike standard Fourier MRI, SENSE images exhibit an inhomogeneous noise distribution, which crucially depends on the geometrical sensitivity relations of the coils used. Thus, for optimum signal‐to‐noise‐ratio (SNR) and noise homogeneity, specialized coil configurations are called for. In this article we study the implications of SENSE imaging for coil layout by means of simulations and imaging experiments in a phantom and in vivo. New, specific design principles are identified. For SENSE imaging, the elements of a coil array should be smaller than for common phased‐array imaging. Furthermore, adjacent coil elements should not overlap. Based on the findings of initial investigations, a configuration of six coils was designed and built specifically for cardiac applications. The in vivo evaluation of this array showed a considerable SNR increase in SENSE images, as compared with a conventional array. Magn Reson Med 45:495–504, 2001.

Travelling-wave nuclear magnetic resonance

Nuclear magnetic resonance (NMR) is one of the most versatile experimental methods in chemistry, physics and biology, providing insight into the structure and dynamics of matter at the molecular scale. Its imaging variant—magnetic resonance imaging (MRI)—is widely used to examine the anatomy, physiology and metabolism of the human body. NMR signal detection is traditionally based on Faraday induction in one or multiple radio-frequency resonators that are brought into close proximity with the sample. Alternative principles involving structured-material flux guides, superconducting quantum interference devices, atomic magnetometers, Hall probes or magnetoresistive elements have been explored. However, a common feature of all NMR implementations until now is that they rely on close coupling between the detector and the object under investigation. Here we show that NMR can also be excited and detected by long-range interaction, relying on travelling radio-frequency waves sent and received by an antenna. One benefit of this approach is more uniform coverage of samples that are larger than the wavelength of the NMR signal—an important current issue in MRI of humans at very high magnetic fields. By allowing a significant distance between the probe and the sample, travelling-wave interaction also introduces new possibilities in the design of NMR experiments and systems.

Parallel Imaging Performance as a Function of Field Strength - An Experimental Investigation using Electrodynamic Scaling

In this work, the dependence of parallel MRI performance on main magnetic field strength is experimentally investigated. Using the general framework of electrodynamic scaling, the B0‐dependent behavior of the relevant radiofrequency fields at a single physical field strength of 7 T is studied. In the chosen implementation this is accomplished by adjusting the permittivity and conductivity of a homogeneous spherical phantom. With different mixing ratios of decane, ethanol, purified water, N‐methylformamide, and sodium chloride, field strengths in the range of 1.5 to 11.5 T are mimicked. Based on sensitivity maps of an eight‐coil receiver array, the field‐dependent performance of parallel imaging is assessed in terms of the geometry factor g, which reflects noise enhancement in parallel imaging reconstruction. At low field strengths the SNR penalty was nearly independent of B0 and favorably low for 1D reduction factors up to between 3 and 4. At higher field strengths the transition between favorable and prohibitive parallel imaging conditions was found to shift toward higher feasible reduction factors. These findings are in good agreement with previous theoretical predictions. From this agreement it is concluded that parallel MRI at high B0 benefits specifically from onsetting far‐field behavior of the involved radiofrequency fields. Magn Reson Med 52:953–964, 2004.

Sensitivity-encoded spectroscopic imaging.

Sensitivity encoding (SENSE) offers a new, highly effective approach to reducing the acquisition time in spectroscopic imaging (SI). In contrast to conventional fast SI techniques, which accelerate k‐space sampling, this method permits reducing the number of phase encoding steps in each phase encoding dimension of conventional SI. Using a coil array for data acquisition, the missing encoding information is recovered exploiting knowledge of the distinct spatial sensitivities of the individual coil elements. In this work, SENSE is applied to 2D spectroscopic imaging. Fourfold reduction of scan time is achieved at preserved spectral and spatial resolution, maintaining a reasonable SNR. The basic properties of the proposed method are demonstrated by phantom experiments. The in vivo feasibility of SENSE‐SI is verified by metabolic imaging of N‐acetylaspartate, creatine, and choline in the human brain. These results are compared to conventional SI, with special attention to the spatial response and the SNR. Magn Reson Med 46:713–722, 2001.

Reduced field‐of‐view MRI using outer volume suppression for spinal cord diffusion imaging

A spin‐echo single‐shot echo‐planar imaging (SS‐EPI) technique with a reduced field of view (FOV) in the phase‐encoding direction is presented that simultaneously reduces susceptibility effects and motion artifacts in diffusion‐weighted (DW) imaging (DWI) of the spinal cord at a high field strength (3T). To minimize aliasing, an outer volume suppression (OVS) sequence was implemented. Effective fat suppression was achieved with the use of a slice‐selection gradient‐reversal technique. The OVS was optimized by numerical simulations with respect to T1 relaxation times and B1 variations. The optimized sequence was evaluated in vitro and in vivo. In simulations the optimized OVS showed suppression to <0.25% and ∼3% in an optimal and worst‐case scenario, respectively. In vitro measurements showed a mean residual signal of <0.95% ± 0.42 for all suppressed areas. In vivo acquisition with 0.9 × 1.05 mm2 in‐plane resolution resulted in artifact‐free images. The short imaging time of this technique makes it promising for clinical studies. Magn Reson Med 57:625–630, 2007.

Array compression for MRI with large coil arrays

Arrays with large numbers of independent coil elements are becoming increasingly available as they provide increased signal‐to‐noise ratios (SNRs) and improved parallel imaging performance. Processing of data from a large set of independent receive channels is, however, associated with an increased memory and computational load in reconstruction. This work addresses this problem by introducing coil array compression. The method allows one to reduce the number of datasets from independent channels by combining all or partial sets in the time domain prior to image reconstruction. It is demonstrated that array compression can be very effective depending on the size of the region of interest (ROI). Based on 2D in vivo data obtained with a 32‐element phased‐array coil in the heart, it is shown that the number of channels can be compressed to as few as four with only 0.3% SNR loss in an ROI encompassing the heart. With twofold parallel imaging, only a 2% loss in SNR occurred using the same compression factor. Magn Reson Med 57:1131–1139, 2007.

Spatiotemporal magnetic field monitoring for MR

MR experiments frequently rely on signal encoding by the application of magnetic fields that vary in both space and time. The accurate interpretation of the resulting signals often requires knowledge of the exact spatiotemporal field evolution during the experiment. To better fulfill this need, a new approach is presented that enables measuring the field evolution concurrently with any MR sequence. Miniature NMR probes are used to monitor the MR phase evolution around the object under investigation. Based on these data, a global phase model is calculated that can then be used as a basis for processing the actual image or spectroscopic data. The new method is demonstrated by MRI of a phantom, using spin‐warp, spiral, and EPI trajectories. Throughout, the monitoring results enabled highly accurate image reconstruction, even in the presence of massive gradient imperfections. Magn Reson Med 60:187–197, 2008.

Optimizing spatiotemporal sampling for k-t BLAST and k-t SENSE: Application to high-resolution real-time cardiac steady-state free precession

In k‐t BLAST and k‐t SENSE, data acquisition is accelerated by sparsely sampling k‐space over time. This undersampling in k‐t space causes the object signals to be convolved with a point spread function in x‐f space (x = spatial position, f = temporal frequency). The resulting aliasing is resolved by exploiting spatiotemporal correlations within the data. In general, reconstruction accuracy can be improved by controlling the k‐t sampling pattern to minimize signal overlap in x‐f space. In this work, we describe an approach to obtain generally favorable patterns for typical image series without specific knowledge of the image series itself. These optimized sampling patterns were applied to free‐breathing, untriggered (i.e., real‐time) cardiac imaging with steady‐state free precession (SSFP). Eddy‐current artifacts, which are otherwise increased drastically in SSFP by the undersampling, were minimized using alternating k‐space sweeps. With the synergistic combination of the k‐t approach with optimized sampling and SSFP with alternating k‐space sweeps, it was possible to achieve a high signal‐to‐noise ratio, high contrast, and high spatiotemporal resolutions, while achieving substantial immunity against eddy currents. Cardiac images are shown, demonstrating excellent image quality and an in‐plane resolution of ∼2.0 mm at >25 frames/s, using one or more receiver coils. Magn Reson Med 53:1372–1382, 2005.