Multiple‐spin‐echo imaging with a 2D Fourier method

Abstract

In 2D Fourier imaging the normal Carr-Purcell multiple-echo sequence generally leads to center line and mirror artifacts caused by imperfect rotations by the rf pulses. We describe a method to avoid these distortions using a phase alternating-phase shift (PHAPS) sequence which also allows multiple-slice and multiple-echo imaging at the same time. Measuring phantoms with calibrated T2 values, we have shown that the PHAPS imaging sequence leads to an accuracy of quantitative T2 determinations of better than 10%. Contrast-enhanced images are presented which we calculated from multiple-echo images and extrapolated to arbitrary echotimes, including negative ones. We believe that these improvements in T2 imaging will result in a significant reduction of patient investigation time in magnetic resonance imaging.

Elimination of transverse coherences in FLASH MRI

“…It has been speculated (5) that the band can be explained as a center-line artifact, which is caused by a part of the MR signal that is not affected by the phase encode gradient (10,11). The signal would therefore fail to reconstruct properly during the 2D Fourier transform and would be concentrated onto the line y = 0 (where y is the direction of phase encoding).…”

T 2 : The Transverse Relaxation Time

“…These components can lead to additional phase information being encoded in the final image, resulting in a centre line streak artefact similar to that generated by a DC offset. Graumann et al (1986) have shown that repeating the experiment with the phase of the 180…”

“…• , a nonzero z-component of magnetization is present immediately following its application (Graumann et al, 1986). This leads to the generation of additional transverse and longitudinal components of magnetization after the application of the 180…”

“…The following sections attempt to document some of the more common issues (Simmons et al, 1994;Tofts, 1994a;Crawley and Henkelman, 1987;Larsson et al, 1988Larsson et al, , 1989Miller et al, 1989Miller et al, , 1999Majumdar and Gore, 1987;Graumann et al, 1986;Miller and Joseph, 1986;Henkelman, 1985Henkelman, , 1986.…”

“…• pulses described in the PHAPS (phase-alternating phase shift) sequence (Graumann et al, 1986), which is an extension of the process described above to eliminate the centre line artefact caused by an imperfect 90…”