Magnetic resonance imaging apparatus using shim coil correction

Claim 1

An imaging apparatus (100) for examining an object (P) having a longitudinal axis (Z) and a cross-sectional slice (S) perpendicular thereto, by nuclear magnetic resonance, characterized by comprising: magnetic means (1;2) for applying to said object (P) a steady magnetic field (H o ) along said longitudinal axis (Z); first coil means (3;4), which is arranged along said longitudinal axis (Z), for applying to said object (P) a first gradient field (G z ), which in conjunction with said steady magnetic field (H o ) provides a predetermined field in said cross-sectional slice (S) of said object (P), the field direction of said first gradient field (G z ) being parallel to that of said steady magnetic field (H o ) and the absolute value of the field strength value of said first gradient field (G z ) increasing linearly in opposite directions from said cross-sectional slice along said longitudinal axis; probe head coil means (6) for applying RF pulses to said cross-sectional slice (S) in a direction perpendicular to said longitudinal axis (Z) so as to excite nuclei therein which are being subjected to said predetermined field, and for detecting nuclear magnetic resonance signals derived from said cross-sectional slice (S); second coil means (3;4) for applying a second gradient field (G x ;Gy) to said cross-sectional slice (S) so as to define a projection angle of said nuclear magnetic resonance signals, the field direction of said second gradient field (G x ;Gy) being perpendicular to said steady magnetic field (H o ) and the strength of said second gradient field (G x ;Gy) varying linearly, orthogonal to said first gradient field (G z ); signal processing means (8) for receiving said nuclear magnetic resonance signals from said probe head means (6), and for processing said received nuclear magnetic resonance signals by use of a two-dimensional Fourier transformation to produce an image of said cross-sectional slice (S); and, correction coil means (10;12;14) having at least first and second coil pairs (20;30;60), said first coil pair (20;30) generating in conjunction with said steady magnetic field (H o ) a first correction magnetic field so as to correct homogeneity of magnetic field components of said steady magnetic field (H o ) having even-numbered higher order terms in said longitudinal axis (Z), and said second coil pair (30;60) generating in conjunction with said steady magnetic field (H o ) a second correction magnetic field so as to correct uniformity of magnetic field components of said steady magnetic field (H o ) in a direction (R) perpendicular to said longitudinal axis (Z), and also uniformity of magnetic field components of said first coil pair in said direction (R) perpendicular to said longitudinal axis (Z).