Selective excitation method for nmr imaging

Details Selective excitation method for nmr imaging Selective excitation method for nmr imaging

1988-09-27

1990-01-16

Tokar, Michael J.

Electricity: measuring and testing

Particle precession resonance

Using a nuclear resonance spectrometer system

G01R 3320

Patent

active

048946168

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to an improved selective excitation method of collecting data for NMR imaging that reconfigures a sectional image of a subject using scan data based on a nuclear magnetic resonance phenomenon collected from the subject.


BACKGROUND ART

Various selective excitation methods have already been developed. One method disclosed by Patent Disclosure No. Showa 60(1985)-185149 sets the selective excitation pulse sequence in two directions (x and y) as shown in FIG. 6. In other words, positional selection gradient magnetic field G.sub.x is applied in the first direction or direction x (slice axis) for example. Then a selective excitation of spin is effected by applying a RF pulse 90.degree. to the first direction (90.degree. excitation RF signal). Then after applying a spin warp in the second direction specified in an area that orthogonally intersects the first direction or in direction y (warp axis) for example, and applying positional selection gradient magnetic field G.sub.y, a selective inversion of spin is effected by applying an RF pulse 180.degree. in the second direction (180.degree. excitation RF signal). Then a spin echo signal is received by applying readout gradient magnetic field G.sub.z in the third direction that orthogonally intersects the first and second directions, respectively, or direction z (read axis). As a result, a spin echo signal in an area of delta x in thickness and delta y (width) in both directions (x and y) can be obtained.
There are methods for generating NMR signals other than that mentioned above. For example, there is a method by which a spin echo signal is generated by inverting a gradient magnetic field after exciting it by applying a 90.degree. RF pulse. There is another method by which a Free Induction Decay (FID) signal (generated immediately after 90.degree. RF pulse excitation) is used as the NMR signal. Because these methods use a 90.degree. RF pulse instead of a 180.degree. RF pulse for excitation, a selective excitation method using both types of excitation signals (90.degree. and 180.degree. RF pulse signals) in both directions (x and y) cannot be adopted. If selective excitation cannot be effected in two directions, a turn is made in the directions where there is no selectivity, and aliasing artifacts are created as shown in FIG. 7 if a measured NMR signal is used as a raw data for reconfiguring an image under Fourier transformation.


DISCLOSURE OF INVENTION

The object of this invention is to provide a selective excitation method applicable to NMR imaging, regardless of the NMR signal generation method used.
To apply our invention in the most preferred mode, we utilized a selective excitation method for NMR imaging that, after exciting a subject area other than that used for scanning with an RF signal and spoiling the excited area with a gradient magnetic field, creates an area that receives no other excitation RF signals. NMR signals are collected from unspoiled areas according to the conventional method.


BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the block diagram of an NMR imaging device to which the preferred application mode according to the excitation method provided by this invention is applied.
FIGS. 2 through 4 explain the operation of the application mode of this invention.
FIG. 5 explains the operation of another application mode of this invention.
FIGS. 6 and 7 explain operation according to the prior art procedures.


BEST MODE FOR CARRYING OUT THE INVENTION

An example of the preferred application mode of this invention is described in detail by referring to the drawings as follows.
In FIG. 1, "1" indicates a magnet assembly having an inner space section for inserting a subject. The space section is surrounded by a static magnetic field coil that applies a constant static magnetic field to a subject, gradient magnetic field coils (x, y, and z) for generating a gradient magnetic field, an RF transmission coil for providing high-frequency pulses to excite the spinning of specified nucleuses inside the body syste

REFERENCES:
patent: 4535290 (1985-08-01), Post et al.
patent: 4558277 (1985-12-01), Post et al.
patent: 4733182 (1988-03-01), Clarke
patent: 4737714 (1988-04-01), Hanawa

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