Method of designing a pulse sequence for parallel-transmission mri, and a method of performing parallel-transmission mri using such a pulse sequence

A method of performing parallel-transmission magnetic resonance imaging of a subject using a parallel-transmission magnetic resonance imaging apparatus comprising a set of gradient coils (GC), a plurality of transmit channels (RFC1-RFC8) and a plurality of receive channels, the method comprising the steps of:

• i) using the receive channels of the apparatus for performing measurements representative of spatial distributions of radio-frequency fields generated by respective transmit channels within a region of interest comprising a body part of the subject;
• ii) using results of said measurements, estimating a linear transformation (L), called adjustment transformation, converting amplitude maps of radio-frequency fields generated within the region of interest by respective transmit channels of the magnetic resonance imaging apparatus into respective standardized maps, the adjustment transformation being chosen in such a way as to minimize a cost function representative of an averaged difference between the standardized maps for the subject and respective reference amplitude maps for a reference subject;
• iii) inverting the adjustment transformation, and computing a set of subject-specific radio-frequency waveforms (P′) by applying the inverted adjustment transformation to a set of predetermined reference radio-frequency waveforms;
• and
• iv) applying each subject-specific radio-frequency waveform to a respective transmit channel while applying a magnetic field gradient waveform to the gradient coils, and using the receive channels to receive parallel-transmission magnetic resonance imaging signals.

A parallel-transmission magnetic resonance imaging apparatus comprising a set of gradient coils (GC), a plurality of transmit channels (RFC1-RFC8), a plurality of receive channels and a data processor (CP), wherein:

• it further comprises a memory device (DB) storing data defining a set of predetermined reference radio-frequency waveforms and a magnetic field gradient waveform;
• and in that
• the data processor is programmed or configured for:
• driving the receive channels to perform measurements representative of spatial distributions of radio-frequency fields generated by the respective transmit channels within a region of interest comprising a body part of the subject;
• using results of said measurements, estimating a linear transformation (L), called adjustment transformation, converting amplitude maps of radio-frequency fields generated within the region of interest by respective transmit channels into respective standardized maps, the adjustment transformation being chosen in such a way as to minimize a cost function representative of an averaged difference between the standardized maps for the subject and respective reference amplitude maps for a reference subject;
• inverting the adjustment transformation and computing a set of subject-specific radio-frequency waveforms by applying the inverted adjustment transformation to a set of predetermined reference radio-frequency waveforms;
• and
• driving the transmit channels to play respective subject-specific radio-frequency waveforms, the gradient coils to play a magnetic field gradient waveform and the receive coil to receive parallel-transmission magnetic resonance imaging signals.