Phase compensation in single echo acquisition imaging. Phase effects of voxel-sized coils in planar and cylindrical arrays

Flexible, phase-matched, linear receive arrays for high-field MRI in monkeys

High signal-to-noise ratios (SNR) are essential for high-resolution anatomical and functional MRI. Phased arrays are advantageous for this but have the drawback that they often have inflexible and bulky configurations. Particularly in experiments where functional MRI is combined with simultaneous electrophysiology, space constraints can be prohibitive. To this end we developed a highly flexible multiple receive element phased array for use on anesthetized monkeys. The elements are interchangeable and different sizes and combinations of coil elements can be used, for instance, combinations of single and overlapped elements. The preamplifiers including control electronics are detachable and can serve a variety of prefabricated and phase matched arrays of different configurations, allowing the elements to always be placed in close proximity to the area of interest. Optimizing performance of the individual elements ensured high SNR at the cortical surface as well as in deeper laying structures. Performance of a variety of arrangements of gapped linear arrays was evaluated at 4.7 and 7T in high-resolution anatomical and functional MRI.

Highly parallel transmit/receive systems for dynamic MRI

Dynamic MRI continues to grow in interest and capability with the introduction of 64 and 128 channel receivers, and, more recently, 8 and 16 channel parallel transmitters. This talk will describe progress in developing a 64 channel transmitter and applications in high-speed MR imaging, reaching 1000 frames per second.

96-Channel receive-only head coil for 3 Tesla: design optimization and evaluation

The benefits and challenges of highly parallel array coils for head imaging were investigated through the development of a 3T receive-only phased-array head coil with 96 receive elements constructed on a close-fitting helmet-shaped former. We evaluated several designs for the coil elements and matching circuitry, with particular attention to sources of signal-to-noise ratio (SNR) loss, including various sources of coil loading and coupling between the array elements. The SNR and noise amplification (g-factor) in accelerated imaging were quantitatively evaluated in phantom and human imaging and compared to a 32-channel array built on an identical helmet-shaped former and to a larger commercial 12-channel head coil. The 96-channel coil provided substantial SNR gains in the distal cortex compared to the 12- and 32-channel coils. The central SNR for the 96-channel coil was similar to the 32-channel coil for optimum SNR combination and 20% lower for root-sum-of-squares combination. There was a significant reduction in the maximum g-factor for 96 channels compared to 32; for example, the 96-channel maximum g-factor was 65% of the 32-channel value for acceleration rate 4. The performance of the array is demonstrated in highly accelerated brain images.

Single echo acquisition MRI using RF encoding

Encoding of spatial information in magnetic resonance imaging is conventionally accomplished by using magnetic field gradients. During gradient encoding, the position in k-space is determined by a time-integral of the gradient field, resulting in a limitation in imaging speed due to either gradient power or secondary effects such as peripheral nerve stimulation. Partial encoding of spatial information through the sensitivity patterns of an array of coils, known as parallel imaging, is widely used to accelerate the imaging, and is complementary to gradient encoding. This paper describes the one-dimensional limit of parallel imaging in which all spatial localization in one dimension is performed through encoding by the radiofrequency (RF) coil. Using a one-dimensional array of long and narrow parallel elements to localize the image information in one direction, an entire image is obtained from a single line of k-space, avoiding rapid or repeated manipulation of gradients. The technique, called single echo acquisition (SEA) imaging, is described, along with the need for a phase compensation gradient pulse to counteract the phase variation contained in the RF coil pattern which would otherwise cause signal cancellation in each imaging voxel. Image reconstruction and resolution enhancement methods compatible with the speed of the technique are discussed. MR movies at frame rates of 125 frames per second are demonstrated, illustrating the ability to monitor the evolution of transverse magnetization to steady state during an MR experiment as well as demonstrating the ability to image rapid motion. Because this technique, like all RF encoding approaches, relies on the inherent spatially varying pattern of the coil and is not a time-integral, it should enable new applications for MRI that were previously inaccessible due to speed constraints, and should be of interest as an approach to extending the limits of detection in MR imaging.

A simple approach to overcoming mutual coupling effects in some transmit array coils for magnetic resonance imaging

Single Echo Acquisition (SEA) is a method of completely parallel MR imaging that uses coil elements for spatial localization during receive, replacing the need for phase encoding repetitions. In this receive-only form, SEA imaging requires the use of a phase compensation gradient, the value of which is dependent on coil geometry, imaging distance from the elements, and element orientation. Operation of the arrays in transmit-receive mode, while adding significant complexity, is one potential method of eliminating the restrictions imposed by the phase compensation gradient. This abstract examines a straightforward current-splitting technique to enable parallel transmission for studying the complicated field interactions of these array coils in transmit mode.

Single-point Dixon water-fat imaging using 64-channel single-echo acquisition MRI

This paper presents a method for high-speed water-fat imaging using Single-Echo Acquisition (SEA) with an array of 64 localized coil elements and single-point Dixon sequence. The method forms two-dimensional separate water and fat images from a single echo data. Specifically, a channel correlation and region-growing algorithm was developed to extract the phase information from the single echo data, eliminating the need for multiple data acquisition normally required for water/fat separation. Phantom studies on a 4.7 T scanner show that the method can handle large inter-channel and cross-channel phase variations, even at relative high data noise levels. Assume that the water and fat are spatially separated and they can be identified by the phase discontinuity caused by the chemical frequency shift, the new method can acquire separate water and fat images without reducing the high frame rates of the SEA imaging method. Although its capability is limited if there are large susceptibility artifacts, disconnected tissues, or pixels with mixed fat and water signals, the new method is potentially useful for dynamic imaging of small animals, where the SEA imaging can provide high imaging speed but may suffer from reduced contrast due to the strong fat signals at short repetition time.