Markus Weiger

SENSE : sensitivity encoding for fast MRI

New theoretical and practical concepts are presented for considerably enhancing the performance of magnetic resonance imaging (MRI) by means of arrays of multiple receiver coils. Sensitivity encoding (SENSE) is based on the fact that receiver sensitivity generally has an encoding effect complementary to Fourier preparation by linear field gradients. Thus, by using multiple receiver coils in parallel scan time in Fourier imaging can be considerably reduced. The problem of image reconstruction from sensitivity encoded data is formulated in a general fashion and solved for arbitrary coil configurations and k‐space sampling patterns. Special attention is given to the currently most practical case, namely, sampling a common Cartesian grid with reduced density. For this case the feasibility of the proposed methods was verified both in vitro and in vivo. Scan time was reduced to one‐half using a two‐coil array in brain imaging. With an array of five coils double‐oblique heart images were obtained in one‐third of conventional scan time. Magn Reson Med 42:952–962, 1999.

Advances in sensitivity encoding with arbitrary k‐space trajectories

New, efficient reconstruction procedures are proposed for sensitivity encoding (SENSE) with arbitrary k‐space trajectories. The presented methods combine gridding principles with so‐called conjugate‐gradient iteration. In this fashion, the bulk of the work of reconstruction can be performed by fast Fourier transform (FFT), reducing the complexity of data processing to the same order of magnitude as in conventional gridding reconstruction. Using the proposed method, SENSE becomes practical with nonstandard k‐space trajectories, enabling considerable scan time reduction with respect to mere gradient encoding. This is illustrated by imaging simulations with spiral, radial, and random k‐space patterns. Simulations were also used for investigating the convergence behavior of the proposed algorithm and its dependence on the factor by which gradient encoding is reduced. The in vivo feasibility of non‐Cartesian SENSE imaging with iterative reconstruction is demonstrated by examples of brain and cardiac imaging using spiral trajectories. In brain imaging with six receiver coils, the number of spiral interleaves was reduced by factors ranging from 2 to 6. In cardiac real‐time imaging with four coils, spiral SENSE permitted reducing the scan time per image from 112 ms to 56 ms, thus doubling the frame‐rate. Magn Reson Med 46:638–651, 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.

2D SENSE for faster 3D MRI.

Sensitivity encoding in two spatial dimensions (2D SENSE) with a receiver coil array is discussed as a means of improving the encoding efficiency of three-dimensional (3D) Fourier MRI. it is shown that in Fourier imaging with two phase encoding directions, 2D SENSE has key advantages over one-dimensional parallel imaging approaches. By exploiting two dimensions for hybrid encoding, the conditioning of the reconstruction problem can be considerably improved, resulting in superior signal-to-noise behavior. As a consequence, 2D SENSE permits greater scan time reduction, which particularly benefits the inherently time-consuming 3D techniques.Along with the principles of 2D SENSE imaging, the properties of the technique are discussed and investigated by means of simulations. Special attention is given to the role of the coil configuration, yielding practical setups with four and six coils. The in vivo feasibility of the two-dimensional approach is demonstrated for 3D head imaging, permitting four-fold scan time reduction.

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.

Cardiac real-time imaging using SENSE

Sensitivity encoding is used to improve the performance of real‐time MRI. The encoding efficiency of single‐shot and segmented echo‐planar imaging is tripled by means of a 6‐element receiver coil array. The feasibility of this approach is verified for double oblique cardiac real‐time imaging of human subjects at rest as well as under physiological stress. Sample images are presented with scan times per image down to 13 msec at a spatial resolution of 4.1 mm, and 27 msec at a resolution of 2.6 mm. Moreover, multiple slice real‐time imaging is demonstrated at a rate of 38 double‐frames per second. Magn Reson Med 43:177–184, 2000.

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.

PRESTO-SENSE: An ultrafast whole-brain fMRI technique

A new ultrafast MR imaging method is proposed and tested, which enables whole‐brain fMRI with a true temporal resolution of 1 sec. The method combines a 3D PRESTO pulse sequence with the concept of sensitivity‐encoding with multiple receiver coils (SENSE). The so‐called PRESTO‐SENSE technique is demonstrated on a set of functional block‐type motor and visual experiments and compared with conventional functional imaging techniques, such as PRESTO and EPI. Comparable image quality and activation areas are found with all sequences. The noise characteristics of the proposed method are analyzed in detail and their implications for ultrafast fMRI studies are discussed. Magn Reson Med 43:779–786, 2000.

Sensitivity Encoded Cardiac MRI

Imaging speed is a key factor in most cardiovascular applications of magnetic resonance imaging. Recently, simultaneous signal acquisition with multiple coils has received increasing attention as a means of enhancing scan speed in MRI. Based on this approach, the sensitivity encoding technique SENSE enables substantial scan time reduction by exploiting the inherent spatial encoding effect of receiver coil sensitivity. This work studies the benefit of sensitivity encoding for cardiovascular MRI. SENSE is applied to accelerate common breath-hold imaging as well as real-time imaging by factors up to 3.2. In the breath-hold mode with ECG triggering, this speed benefit has been used both for reducing the breath-hold interval and for improving spatial resolution. In cardiac real-time imaging without triggering and breath control, the SENSE approach has enabled significantly enhanced temporal resolution, ranging down to 13 ms (77 frames/s). Cardiac real-time SENSE is demonstrated in several modes, including real-time imaging of three parallel slices at a rate of 25 triple frames per second.

MRI with zero echo time: Hard versus sweep pulse excitation

Zero echo time can be obtained in MRI by performing radiofrequency (RF) excitation as well as acquisition in the presence of a constant gradient applied for purely frequency‐encoded, radial centre‐out k‐space encoding. In this approach, the spatially nonselective excitation must uniformly cover the full frequency bandwidth spanned by the readout gradient. This can be accomplished either by short, hard RF pulses or by pulses with a frequency sweep as used in the SWIFT (Sweep imaging with Fourier transform) method for improved performance at limited RF amplitudes. In this work, the two options are compared with respect to T2 sensitivity, signal‐to‐noise ratio (SNR), and SNR efficiency. In particular, the SNR implications of sweep excitation and of initial or periodical acquisition gaps required for transmit‐receive switching are investigated. It was found by simulations and experiments that, whereas equivalent in terms of T2 sensitivity, the two techniques differ in SNR performance. With ideal, ungapped simultaneous excitation and acquisition, the sweep approach would yield higher SNR throughout due to larger feasible flip angles. However, acquisition gapping is found to take a significant SNR toll related to a reduced acquisition duty cycle, rendering hard pulse excitation superior for sufficient RF amplitude and also in the short‐T2 limit. Magn Reson Med, 2011.