Method and apparatus for compensating for nonuniformity of static magnetic field in MRI system

A method of compensating for nonuniformity of a static magnetic field in a nuclear magnetic resonance (NMR) imaging system, comprising the steps of:

• (a) selecting one of X-, Y- and Z-axes of a Cartesian coordinate system as a phase encoding direction;
• (b) selectively exciting a region of field of view by applying a high frequency pulse of .alpha..degree. to an object under inspection;
• (c) measuring an NMR signal by applying a phase encoding gradient magnetic field and a frequency encoding gradient magnetic field after application of said high frequency pulse of .alpha..degree., the step (c) of measuring being performed a plurality of times while changing said phase encoding gradient magnetic field;
• (d) detecting a peak value of the measurement data obtained by step (c) and determining a phase encoding quantity at which said peak value can be obtained;
• (e) determining an intensity and a plurality of a linear component of nonuniformity components of the static magnetic field in said direction selected by obtaining an intensity and a polarity of said phase encoding gradient magnetic field corresponding to said peak value;
• and
• (f) executing a pulse sequence for measuring the NMR signal by applying a gradient field added with a compensating gradient magnetic field having the same intensity as said linear component and a polarity opposite to that of said linear component to a gradient magnetic field applied in the selected direction.