Echo-volumar imaging using 180 degree RF pulses

Details Echo-volumar imaging using 180 degree RF pulses Echo-volumar imaging using 180 degree RF pulses

1992-12-30

1994-11-08

Arana, Louis

Electricity: measuring and testing

Particle precession resonance

Using a nuclear resonance spectrometer system

324307, G01V 300

Patent

active

053630360

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The present invention relates to magnetic resonance imaging systems and, more particularly, to Echo Volumar Imaging (EVI) using 180.degree. RF pulses.


BACKGROUND AND SUMMARY OF THE INVENTION

Magnetic Resonance imaging using the echo-volumar imaging approach (P. Mansfield, A. M. Howseman and R. J. Ordidge, Volumar Imaging (EVI) at 0.1 T, J. Phys. E. 22, 324-330 (1989) allows the simultaneous examination of several planes within a defined thick slice to be observed in the same time as would be necessary for the examination of a single plane in echo-planar imaging (P. Mansfield, A. M. Howseman and R. J. Ordidge, Volumar Imaging using NMR Spin Echoes, J. Phys. C. 10, L55 (1977) EPI. Following an initial thick slice selection process, the spin system is encoded along three principal axes, using suitably modulated x, y and z linear magnetic gradients. This type of echo-volumar imaging works well for homogenous specimens at high field and for less homogenous specimens at low field. The operating field strength comes into question since bulk susceptibility artefacts are induced, causing a degradation of the static field homogeneity. If this static field homogeneity is degraded too far, EVI as described above becomes impractical with the gradient strengths and switching rates currently available.
It is an object of the present invention to provide a means of overcoming the induced local field effects arising from susceptibility effects. The EVI modification uses 180.degree. RF pulses instead of gradient reversals. A similar modification has been described for use with Echo Planar Imaging EPI in British Patent Application 8918105.1.
The present invention therefore provides a method of producing a magnetic resonance image of a defined region of an object by an echo volumar imaging process including subjecting the object to an initial selection process to select a thick slice within the object to produce an active volume of spin magnetisation characterized in that said method further includes subjecting the active volume so defined to a combination of 180.degree. RF pulses and suitably modulated x, y and z gradients to produce an echo volumar image.


BRIEF DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:
FIG. 1 shows a timing diagram for the initial thick slice selection phase. Also included are the pre-phasing pulses for G.sub.x, G.sub.y and G.sub.z (shaded);
FIG. 2 shows a timing diagram for the modified EVI sequence using 180.degree. RF pulses. (Note the gradient G.sub.x is monopolar with small gaps or notches allowing a window for the application of the non-selective RF selective pulses and also the application of G.sub.y, and G.sub.z gradient blips).
FIG. 3(a) shows the k-space trajectory for the modified EVI sequence shown in FIG. 2. (Note that the effect of the 180.degree. RF pulse is to take the k-trajectory to a conjugate point in k-space);
FIG. 3(b) shows an alternative scanning procedure to FIG. 3(a) in which the scan starts in the central z=0 and steps out subsequently through pairs of conjugal planes finishing at the .+-.z plane.
FIG. 4(a) shows a timing diagram for a modified hybrid EVI sequence in which occasional 180.degree. pulses are used on conjunction with a bipolar G.sub.x gradient;
FIG. 4(b) shows an alternative timing diagram for a modified hybrid EVI sequence using 180.degree. RF pulses.
FIG. 5 shows the k-space trajectory for the hybrid EVI sequence shown in FIGS. 4(a) and 4(b),


DETAILED DESCRIPTION OF THE INVENTION

The modified EVI sequence employs short 180.degree. RF pulses instead of gradient reversal. In one version described below, the 180.degree. RF pulses are non-selective, but variants of the new sequence are possible in which short selective 180.degree. pulses are used instead. In this section we concentrate on the use of non-selective 180.degree. pulses.
The timing diagram for the initial thick slice selection is shown in FIG. 1. Here a sh

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