B1 field control in magnetic resonance imaging

A magnetic resonance imaging method for performing multi-slice magnetic resonance imaging of a region of interest of an associated imaging subject using a radio frequency coil arranged to generate a B1 magnetic field in the region of interest, the method comprising:

• determining a per-slice B1 field value for each slice that is representative of the B1 field over a selected area of the slice by computing the B1 field numerically using a model of the radio frequency coil and a non-homogeneous model of the imaging subject, the non-homogeneous model of the imaging subject employing different conductivity and permittivity values for different materials of which the imaging subject is formed, and computing an aggregate value of the numerically computed B1 field across the selected area of the slice;
• determining an adjusted per-slice radio frequency excitation for each slice that adjusts the B1 field value for the slice to a selected value;
• acquiring magnetic resonance imaging data for each slice using the adjusted per slice radio frequency excitation for that slice;
• and
• reconstructing the acquired magnetic resonance imaging data into a reconstructed image representation.

A magnetic resonance imaging method for performing multi-slice magnetic resonance imaging of a region of interest of an associated imaging subject using a radio frequency coil arranged to generate a B1 magnetic field in the region of interest the method comprising:

• determining a value of a B1 field figure of merit for each slice that is representative of the B1 field over a selected area of the slice;
• determining an adjusted per-slice radio frequency excitation for each slice that adjusts the B1 field figure of merit for the slice to a selected value;
• acquiring magnetic resonance imaging data for each slice using the adjusted per slice radio frequency excitation for that slice;
• and
• reconstructing the acquired magnetic resonance imaging data into a reconstructed image representation.

A magnetic resonance imaging method for performing multi-slice magnetic resonance imaging of a region of interest of an associated imaging subject using a radio frequency coil arranged to generate a B1 magnetic field in the region of interest, the method comprising:

• determining a per-slice B1 field value for each slice that is representative of the B1 field over a selected area of the slice;
• determining an adjusted per-slice radio frequency excitation for each slice that adjusts the B1 field value for the slice to a selected value that is substantially the same for a plurality of slices for which imaging data is acquired;
• determining a specific absorption rate based on the adjusted per-slice radio frequency excitations;
• conditional upon the determined specific absorption rate exceeding a regulatory safety limit, repeating the determining of an adjusted per-slice radio frequency excitation for each slice using one of a lower selected value of the per-slice B1 field value and adjustment of at least one other imaging sequence parameter to reduce the specific absorption rate;
• acquiring magnetic resonance imaging data for each slice using the adjusted per slice radio frequency excitation for that slice;
• and
• reconstructing the acquired magnetic resonance imaging data into a reconstructed image representation.

A magnetic resonance imaging method for performing multi-slice magnetic resonance imaging of a region of interest of an associated imaging subject using a radio frequency coil arranged to generate a B1 magnetic field in the region of interest, the method comprising:

• determining a per-slice B1 field value for each slice that is representative of the B1 field over a selected area of the slice;
• determining an adjusted per-slice radio frequency excitation for each slice that adjusts the B1 field value for the slice to a selected value;
• acquiring magnetic resonance imaging data for each slice using the adjusted per slice radio frequency excitation for that slice;
• reconstructing the acquired magnetic resonance imaging data into a reconstructed image representation;
• and
• moving the associated imaging subject in a direction transverse to the slices, the determining of a per-slice B1 field value, the determining of an adjusted per-slice radio frequency excitation, and the acquiring of magnetic resonance imaging data being repeated for a stationary slice position with the imaging subject moved relative to the stationary slice position between each repetition.

A magnetic resonance imaging apparatus for performing multi-slice magnetic resonance imaging of a region of interest of an associated imaging subject, the apparatus comprising:

• a radio frequency coil arranged to generate a B1 magnetic field in the region of interest;
• a means for determining a per-slice B1 field value for each slice that is representative of the B1 field over a selected area of the slice, the determining means comprising at least one of:
• an electromagnetic simulator receiving a digital model of the region of interest and a digital model of the radio frequency coil and estimating the B1 field generated across the region of interest, the digital model of the region of interest mimicking non-uniform dielectric and conductivity properties of the region of interest;
• and
• a table of adjusted per-slice radio frequency excitation versus slice position derived from the per-slice B1 field values;
• a means for determining an adjusted per-slice radio frequency excitation for each slice that adjusts the B1 field value for the slice to a selected value;
• a magnetic resonance imaging scanner configured to acquire magnetic resonance imaging data for each slice using the adjusted per-slice radio frequency excitation for that slice;
• and
• a reconstruction processor configured to reconstruct the acquired magnetic resonance imaging data into a reconstructed image representation.