Respiratory Ordered Phase Encoding (ROPE)

This note explains and illustrates a technique of reducing artifacts produced by periodic motion without using physiologic gating. The method is simple and can be applied on any standard MRI unit. For periodic motion, effective gating can be attained by setting the product of the number of acquisitions (at each phase-encoding step), N, times the repeat time, TR, equal to the period of the motion, T. This pseudo-gating can be used with any TR but, if changes in the period occur, is most robust with short TR values. The method is also applicable to multislice and volume imaging. For fast field echo methods, the above rule can be used if the period of the motion is smaller than the total scan time. Otherwise, the scan need only be repeated N times to avoid artifacts and improve signal-to-noise. The method has been implemented to remove both respiratory and/or cardiac motion artifacts.

“…[2] to set the scan parameters. Results are shown for respiratory and cardiac motion and blood flow.…”

Section: Resultsmentioning

confidence: 99%

Pseudo‐gating: Elimination of periodic motion artifacts in magnetic resonance imaging without gating

Haacke

1987

“…A paradigm is respiratory motion which, if uncorrected, leads to ghosting in the phaseencoded direction ( 10). A reordering of the acquisition of the phase encodings using ROPE (3,11 ) or Exorcist techniques can suppress the ghosts, but this does not eliminate blurring.…”

Section: ) mentioning

confidence: 99%

Compensation for effects of linear motion in MR imaging

Korin

Farzaneh

Wright

et al. 1989

Various compensation methods for different types of motion during MR image acquisition have been proposed. Presented here is a postprocessing scheme for eliminating artifacts due to linear, intra-slice motion of known velocity. The data for each phase encoding or "view" acquired from a moving object are shown to differ from those which would be measured from the stationary object by a phase factor which depends on the object's displacement from a reference point. This derivation is then used to propose a correction scheme for linear motion in which all phase encodings measured at different positions of the moving object are collapsed onto the same reference position. After subsequent reconstruction, the object appears perfectly "focused." By selection of different reference positions, the method permits positioning of the compensated object as desired within the field of view of the image. This property allows the method to be extended to create sequences of corrected images with smooth object motion between frames of the sequence. The basic correction scheme and its variations were tested experimentally in phantom studies with velocities as large as 8 cm/s.

“…Some of these streaks result from motion of high-signal structures, while others are caused by pulsatile blood components. Streaks deriving from respiratory motion may be reduced further in PR with the use of respiratory ordered view angulation (ROVA) (3), which is a technique analogous to ROPE (4) or EXORCIST (5). In this technique, the view angle is chosen adaptively during the scan cognizant of the respiratory state of the subject in such a fashion that the apparent interview motion is made as smooth as possible.…”

Section: Introductionmentioning

confidence: 99%

Consistent projection reconstruction (CPR) techniques for MRI

Glover

Noll

1993

Motion of the subject causes inconsistencies in MR scan data which in turn produce streaks in projection reconstruction (PR) scans or ghosts in 2DFT imaging. It is shown that a data manifold consistent with a static object may be extracted from input scan data by applying consistency criteria based on moment or Fourier expansions of the object's projections. The criteria limit the spectrum of azimuthal variation relative to the radial moments or radial spectral components, and their application diminishes k-space inconsistencies from motion or flow by discarding unallowed expansion terms. This consistent projection reconstruction (CPR) algorithm was applied to the reconstruction of projection data from a moving phantom and from human abdominal sections, and in each case provided reduced motion streaks relative to conventional reconstructions. In addition, CPR was found to be effective when used with 2DFT data by resampling the Cartesian data as radial projections, but to a lesser extent.