Method and Apparatus for Correcting B1-Inhomogeneities in Nuclear Magnetic Resonance Imaging

A magnetic resonance imaging scanner comprising:

• a magnet for generating a static magnetic field for aligning nuclear spins of a body to be imaged along a magnetization axis;
• means for generating a transverse radio-frequency pulsed field, and for directing said radio-frequency pulsed field toward said body in order to flip said nuclear spins by a predetermined angle;
• and
• means for detecting a signal emitted by flipped nuclear spins within said body;
• characterized in that said means for generating a radio-frequency pulsed field comprise computing means for carrying out a method of performing nuclear magnetic resonance imaging said body, comprising:
• immerging said body in a static magnetic field for aligning nuclear spins along a magnetization axis;
• exposing it to a transverse radio-frequency pulsed field for flipping said nuclear spins by a predetermined angle;
• detecting a signal emitted by flipped nuclear spins;
• and reconstructing a magnetic resonance image of said body on the basis of the detected signal;
• the method being characterized in that it comprises the steps of:
• (i) determining a statistical distribution of the amplitude of said radio-frequency pulsed field within a volume of said body;
• and
• (ii) computing a set of optimal parameters of a composite radio-frequency pulsed field for jointly minimizing a statistical dispersion of the spin flip angles (FA) distribution within said volume of the body, and the errors between the actual spin flip angles and a predetermined target value thereof, wherein
• said radio-frequency pulsed field consists of a train of elementary pulses having a constant frequency and amplitude, and a continuous phase;
• and
• said parameters comprise:
• the number N of said elementary pulses, as well as the duration (τ1, τ2, τ3), amplitude (A1, A2, A3), frequency (ω1, ω2, ω3) and initial phase (φ1, φ2, φ3) of each of them.