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A P Crawley

Publications and source records attributed to A P Crawley.

9 recordsLinked to original sources
PubMed1992

Elimination of oblique flow artifacts in magnetic resonance imaging.

We present an analysis of how flow oblique to the frequency-encoding direction generates displacement artifacts in MR imaging and show that for flow which has constant velocity between the start of the phase encoding and the center of the echo it is possible to eliminate these artifacts by gradient moment nulling in the phase-encoding direction. However, unlike the standard moment nulling calculations for flow compensating the frequency-encode and slice-selection gradients, the phase-encoding first moment must be nulled specifically with respect to the echo center. Limitations of this method imposed by finite gradient strengths are analyzed. In 3D volume acquisitions with two axes phase encoded it is possible to correct for oblique flow in all directions, and this is demonstrated in images of a human volunteer. Correction for oblique flow displacement artifacts may be particularly useful in quantitative flow and angiographic applications.

Artifacts↗
PubMed1988

A comparison of one-shot and recovery methods in T1 imaging.

Spin-lattice (T1) relaxation times are conventionally estimated using inversion recovery or saturation recovery sequences. Such "recovery" methods are limited in magnetic resonance imaging by the long times required to collect multiple points along the T1 relaxation curve. This problem can be overcome by the use of "one-shot" methods, which collect all points along the relaxation curve in a single excitation. We have compared the relative efficiency of these methods, on the basis of the signal-to-noise ratio obtained in the calculated T1 image from an exam time of fixed duration. We have found that a one-shot method using stimulated echoes has a poor efficiency. However, a method based on a technique first proposed by Look and Locker has an efficiency that is almost equal to the inversion recovery method and therefore possesses highly favorable properties for T1 imaging.

Magnetic Resonance Imaging↗
PubMed1988

Elimination of transverse coherences in FLASH MRI.

Fast low-angle shot (FLASH) imaging enables T1-weighted scans to be acquired in a few seconds. However, the diagnostic image quality is severely compromised by the appearance of artifactual bands parallel to the frequency encode direction. We show that the band structure arises from differences in the ability of the phase encode gradient to spoil transverse coherences that build up between successive repetition intervals. A theoretical understanding of the mechanisms involved leads to a comparison between various methods of spoiling the unwanted echoes throughout the whole field of view. Spoiler gradients whose amplitudes change linearly with phase encode step number are treated in detail. The theory predicts that the spoilers will rotate and rescale the band structure and these results are confirmed experimentally. The effect of the spoilers at a given location along the gradient is equivalent to the effect on the entire field of view of an incremented phase shift applied to the radiofrequency pulse. An appropriate rf phase shift scheme should therefore provide ideal spoiling characteristics for FLASH imaging.

Magnetic Resonance Imaging↗
PubMed1987

Errors in T2 estimation using multislice multiple-echo imaging.

Accurate T2 images calculated from multiple-echo sequences are difficult to obtain over a number of contiguous slices due to the presence of unwanted echoes that are generated at the slice edges. This problem is similar to problems encountered in single-slice imaging in the presence of rf pulse imperfections. In this paper, we use computer simulations to show that all unwanted echoes can be eliminated by defocusing them using additions to the slice-selection gradients. Accurate T2 images may then be calculated from the remaining echoes. We also present a method of experimentally displaying the slice shapes of unwanted echoes generated by a multislice imaging sequence.

False Positive Reactions↗
PubMedDate not supplied

Single-shot magnetic resonance imaging: applications to angiography.

Recently developed technologies that allow the collection of magnetic resonance imaging (MRI) in as little as 26 msec have been explored in their application to angiography. Advantages are demonstrated in scan time reduction, insensitivity to patient motion (especially in abdominal applications), flow quantification, and temporal resolution. We demonstrate that because such single-shot techniques are inherently resistant to flow dephasing during acquisition that allow for sustained high signal intensities to be achieved when images must be combined through the cardiac cycle. Such high temporal resolution scans may be utilized for the collection of time-resolved angiograms. With these techniques we demonstrate the collection of complete MR angiograms in the course of reasonable 10-25 sec breath holds. The relative simplicity of the technique, coupled with its overall short acquisition time, allows us to incorporate angiography into other imaging protocols without adding significant time burdens. Results to date are promising for further improvements in spatial resolution, without extension of scan time.

Algorithms↗
PubMedDate not supplied

Very slow in-plane flow with gradient echo imaging.

It is well appreciated that gradient-echo imaging techniques with short recovery times are highly sensitive to flow. This article analyzes the effect of in-plane flow in gradient recalled acquisition in the steady state (GRASS). It is shown that there is loss of signal due to velocity-dependent dephasing effects at velo cities as slow as 0.2 mm per second. It is also shown that striations appear in GRASS images of flow phantoms. This effect, which has not been previously described, arises from a modulation of K-space in the phase-encoding direction during the transient approach to steady state. Although these bands can give the appearance of flow lines, they are completely artifactual and not readily interpretable. Thus, the appearance of in-plane fluid movement in clinical GRASS images is a complex combination of signal loss due to dephasing and artifactual banding. Therefore, the interpretation of flow in GRASS images should be attempted only with caution.

Humans↗
PubMedDate not supplied

A stimulated echo artifact from slice interference in magnetic resonance imaging.

When multiple slices are imaged with a short time between slice acquisitions, a disturbing line artifact along the direction of frequency encoding is often seen across the center of the images. The artifact consists of alternating bright and dark pixel intensities. In this paper, we show that the artifact is due to slice interference, and is caused by stimulated echoes that are produced in the regions of overlap between slices. A theoretical analysis of the formation of these stimulated echoes leads to ways of reducing the artifact, which are verified experimentally. The artifact can be suppressed most conveniently by extending the duration of the read gradient beyond the sampling window.

Humans↗
PubMedDate not supplied

Effect of multislice interference on image contrast in T2- and T1-weighted MR images.

Multislice imaging markedly degrades the contrast of T2-weighted MR images as the separation between slices is reduced. Image contrast was measured clinically at 1.5 T and experimentally at 0.15 T as a function of interslice gap width and shown to be in agreement with calculations based on known relaxation times and excitation profiles. Thus, the cause of T2 contrast degradation in multislice sequences is demonstrated. Contrast in T1-weighted sequences is shown to be minimally affected or even slightly enhanced. Selective excitation pulses with better spatial definition will diminish these contrast changes. Since perfect slice profiles can never be achieved, the clinical implications of these findings are discussed for MR imaging. The choice of slice gaps is an important operator-selected parameter in reducing contrast degradation in T2-weighted sequences.

Adult↗