Raw Data Space Research Articles

Fast "real time" imaging with different k-space update strategies for interventional procedures.

Interventional procedures under MR guidance require the images to be acquired with a fast acquisition strategy, a rapid reconstruction algorithm for "real-time" imaging (ie, high temporal resolution), acquisition of at least three adjacent slices to track a tool reliably, and high tissue contrast to ensure safe positioning of interventional devices. Often times, the field strength for interventional MR-imaging units is limited by the open magnet design. This complicates the trade-off between scan time and image quality, particularly when applied during low field interventional MRI procedures. To minimize the impact of some of these trade-offs, a combination of keyhole techniques or modified k-space trajectories, in conjunction with a fluoroscopic (ie, continuous acquisition) mode and a real time reconstruction, permits rapid imaging in a low field system using standard (speed optimized) reconstruction hardware and standard gradient electronics. The purpose of this study was to design and describe different keyhole strategies that can be used in a real time mode to increase the image frame rate by a factor of up to 16. By updating the entire raw data space with our strategies, even small changes of the object could be recognized. Our results using these new strategies on two commercially available open magnet MR-imaging units (Siemens Magnetom Open 0.2T resistive magnet, Toshiba Access 0.064T permanent magnet) and a 1.5T superconductive solenoidal magnet design imager (Siemens SP) are presented to show the potential of these acquisition strategies in interventional MRI. Furthermore, these strategies may also be helpful for several other medical applications requiring high temporal resolution like contrast-enhanced breast imaging or functional brain imaging.

5543709 Magnetic resonance imaging method and apparatus for generating a plurality of single spin echoes

An object is repeatedly excited by a 90-degree pulse and a plurality of spin echoes are obtained by a 180-degree pulse each time the excitation is repeated. The spin echoes are differently phase-encoded and the spin echoes thus generated are assigned to a substantial half portion of a raw image data space in a phase-encoding direction, the remaining substantial half of data being estimated from the data in that portion. The data covering the raw image data space are used to produce a corresponding image of the object.

Reduced MR acquisition time with the CROCUS technique.

A technique to reduce magnetic resonance (MR) image acquisition time by approximately 50% is introduced. The conjugate reconstruction by off-center under-sampling (CROCUS) method samples only every other phase-encoded line in raw data space and uses the conjugate symmetry of the data to reconstruct a real image. If acquisition and image reconstruction were done in the simplest manner, this under-sampling would produce unacceptable image aliasing. However, if the phase-encoded lines are offset from the origin, the assumption of conjugate symmetry allows the image to be calculated without aliasing. Unfortunately, information from all practical MR systems contain phase-shift errors that produce deviations from conjugate symmetry. The CROCUS method incorporates a correction technique, similar to that used in other half-Fourier methods, which uses low-resolution phase-shift information obtained from a few extra lines of phase-encoded data. This paper provides a theoretical derivation of the reconstruction algorithm and correction technique and illustrates the results. Excellent image quality is obtained with no loss of spatial resolution. Image signal-to-noise ratio is reduced by a factor of approximately 1.4 because of the reduced acquisition time. When the imaging circumstances are such that a high signal level is available, CROCUS imaging can be an effective means of reducing imaging time.