We present a simple single RF channel transmit solution for improving the homogeneity of slice selective spin echo images acquired on human ultra-high field MRI systems. We have designed a two-component serial transmit excitation pulse (STEP) that was used to significantly improve the homogeneity of spin-echo images of the human brain at 7T. Such an approach is highly flexible, is relatively trivial to implement, and does not require but can be used in conjunction with, parallel transmit coils to produce excellent quality spin-echo MR images at 7T.
Ultra-high field (7T or greater) human MRI systems offer significant improvements in signal and contrast over 3T systems. This can be used to improve image quality, acquisition speed, resolution or the detection low concentration biomolecules in vivo. However, the significant image inhomogeneity problem requires solving for it to become a standard neuroscience research tool or integrated into clinical practice. The problem is so severe that, in spin-echo generated images there is a complete loss of signal and contrast from various brain regions. The cause of this is the non-uniformity of the RF transmit (B1+) fields causing non-uniform excitation and refocusing of the MRI signal.
To date the best methods for overcoming these inhomogeneity issues utilize a combination very complex RF and gradient waveforms and often multiple parallel transmit RF Coils are required. We have designed a two-component serial transmit excitation and refocusing pulses (STEP 2 and STEP 3) that were used to significantly improve the homogeneity of 2D spin-echo images of the human brain at 7T.