A magnetic field measuring method in a magnetic resonance imaging (MRI) apparatus includes applying a radio frequency (RF) pulse to an object, acquiring first and second echo signals from a first readout gradient according to test gradients having different intensities, acquiring third and fourth echo signals from a second readout gradient according to the test gradients having different intensities, and determining a characteristic value of an eddy field based on an echo time (TE) of at least one of the first through the fourth echo signals.

Non-linearities of a gradient amplifier (1) for powering a gradient coil (16) are caused by the finite dead time of the amplifier and/or by a forward voltage drop. The gradient amplifier (1) includes a controllable full bridge (8) and an output filter (9). The full bridge (8) is controlled to provide a desired coil current (i.sub.c), including receiving a desired duty cycle (a.sub.eff) of the gradient amplifier (1), measuring an input current (i.sub.filt) and an output voltage (uc.sub.filt) of the output filter (9), evaluating an modulator duty cycle (a.sub.mod), and providing the modulator duty cycle (a.sub.mod) for controlling the full bridge (8). The gradient amplifier (1) powers a gradient coil (16) including at least two half bridges (10), each having at least two power switches (11) connected in series. An output filter (9) connected to a tapped center points of the half bridges (10) between two the power switches (11). A controller provides a desired duty cycle (a.sub.eff) of the gradient amplifier (1), a compensation block (5) for providing an modulator duty cycle (a.sub.mod). A modulator (6) controls the power switches (11) according to the modulator duty cycle (a.sub.mod).

Non-linearities of a gradient amplifier (1) for powering a gradient coil (16) are caused by the finite dead time of the amplifier and/or by a forward voltage drop. The gradient amplifier (1) includes a controllable full bridge (8) and an output filter (9). The full bridge (8) is controlled to provide a desired coil current (i.sub.c), including receiving a desired duty cycle (a.sub.eff) of the gradient amplifier (1), measuring an input current (i.sub.filt) and an output voltage (uc.sub.filt) of the output filter (9), evaluating an modulator duty cycle (a.sub.mod), and providing the modulator duty cycle (a.sub.mod) for controlling the full bridge (8). The gradient amplifier (1) powers a gradient coil (16) including at least two half bridges (10), each having at least two power switches (11) connected in series. An output filter (9) connected to a tapped center points of the half bridges (10) between two the power switches (11). A controller provides a desired duty cycle (a.sub.eff) of the gradient amplifier (1), a compensation block (5) for providing an modulator duty cycle (a.sub.mod). A modulator (6) controls the power switches (11) according to the modulator duty cycle (a.sub.mod).

A computer executes: calculating a first volume distribution (v.d.) of magnetic materials on a shim tray, based on a first magnetic field strength distribution (m.f.s.d.) in a magnetic field space (S3); acquiring a first composite distribution (c.d.) representing a volume by addition of volumes of magnetic materials for each region of the shim tray, and positions of the regions (S5); calculating a virtual m.f.s.d. created by magnetic materials supposed to be arranged as in the first c.d. (S8); calculating a second m.f.s.d. by addition of the first m.f.s.d. and the virtual m.f.s.d. (S9); calculating a second v.d. of magnetic materials on the shim tray, based on the second m.f.s.d. (S3); acquiring a second c.d. representing a volume by addition of volumes of magnetic materials for each region, and positions of the regions (S5); and displaying the positions of regions and the volumes in the first c.d. and second c.d. (S10).

A computer executes: calculating a first volume distribution (v.d.) of magnetic materials on a shim tray, based on a first magnetic field strength distribution (m.f.s.d.) in a magnetic field space (S3); acquiring a first composite distribution (c.d.) representing a volume by addition of volumes of magnetic materials for each region of the shim tray, and positions of the regions (S5); calculating a virtual m.f.s.d. created by magnetic materials supposed to be arranged as in the first c.d. (S8); calculating a second m.f.s.d. by addition of the first m.f.s.d. and the virtual m.f.s.d. (S9); calculating a second v.d. of magnetic materials on the shim tray, based on the second m.f.s.d. (S3); acquiring a second c.d. representing a volume by addition of volumes of magnetic materials for each region, and positions of the regions (S5); and displaying the positions of regions and the volumes in the first c.d. and second c.d. (S10).

A magnetic resonance imaging (MRI) system includes an MRI magnet (100) including a bore (101) and having a magnetic field and a gradient coil (400) disposed within the bore and having an isocenter (404). A first location within the MRI magnet is determined with respect to a first predetermined reference surface of the MRI magnet, the first location representing a center (104) of the magnetic field. A second location within the gradient coil is determined with respect to a second predetermined reference surface of the gradient coil, the second location representing the isocenter. When the gradient coil is installed within the bore, the second predetermined reference surface abuts the first predetermined reference surface. The first predetermined reference surface is adjusted to an adjusted position, the adjusted position being determined as a function of the first location and the second location and corresponding to a position of the first predetermined reference surface at which the first location coincides with the second location when the gradient coil is installed within the bore.

A magnetic resonance imaging (MRI) system includes an MRI magnet (100) including a bore (101) and having a magnetic field and a gradient coil (400) disposed within the bore and having an isocenter (404). A first location within the MRI magnet is determined with respect to a first predetermined reference surface of the MRI magnet, the first location representing a center (104) of the magnetic field. A second location within the gradient coil is determined with respect to a second predetermined reference surface of the gradient coil, the second location representing the isocenter. When the gradient coil is installed within the bore, the second predetermined reference surface abuts the first predetermined reference surface. The first predetermined reference surface is adjusted to an adjusted position, the adjusted position being determined as a function of the first location and the second location and corresponding to a position of the first predetermined reference surface at which the first location coincides with the second location when the gradient coil is installed within the bore.

A feeding circuit arrangement (18) supplies a radio frequency signal to a plurality of coil elements (14) of a magnetic resonance coil system (12). The circuit arrangement (18) includes a main line (20) for connecting a radio frequency signal source (16); a plurality of feeding lines (22), each feeding line (22) for connecting a corresponding coil element (14) of the coil system (14); a power divider (24) arranged between the main line (20) and the plurality of feeding lines (22) for distributing the signal on the main line (20) to each of the feeding lines (22). At least one of the feeding lines (22) includes a controllable switching circuit (26) with a switching element (28) for connecting/disconnecting of two resulting line sections (30, 32) of the feeding line (22), a first line section (30) on the divider side and a second line section (32) on the side connectable to the coil element (14). The switching circuit (26) further includes at least one connectable termination element (44) for line termination of the first line section (30) or the main line (20) includes a circulator device (60) interconnected with a termination (62).

A feeding circuit arrangement (18) supplies a radio frequency signal to a plurality of coil elements (14) of a magnetic resonance coil system (12). The circuit arrangement (18) includes a main line (20) for connecting a radio frequency signal source (16); a plurality of feeding lines (22), each feeding line (22) for connecting a corresponding coil element (14) of the coil system (14); a power divider (24) arranged between the main line (20) and the plurality of feeding lines (22) for distributing the signal on the main line (20) to each of the feeding lines (22). At least one of the feeding lines (22) includes a controllable switching circuit (26) with a switching element (28) for connecting/disconnecting of two resulting line sections (30, 32) of the feeding line (22), a first line section (30) on the divider side and a second line section (32) on the side connectable to the coil element (14). The switching circuit (26) further includes at least one connectable termination element (44) for line termination of the first line section (30) or the main line (20) includes a circulator device (60) interconnected with a termination (62).

Various embodiments are described herein for an apparatus and a method for measuring and characterizing geometric distortions for a region of interest in images obtained using magnetic resonance. The method comprises deriving a computed set of 3D distortion vectors for a set of points within a region of interest covered by a phantom by using harmonic analysis to solve an associated boundary value problem based on boundary conditions derived from a measured set of 3D distortion vectors. The characterized image distortions may be used for various purposes such as for image correction or for shimming, for example.