Measurement of B1+ Field Map Using Composite 90° RF Pulse for Quantitative Sodium MRI

Motivation: The quantitation of sodium MRI depends on the reference tubes located near the inner surface of the RF coil. Hence, the RF flip angle at the reference tubes could be off from the intended 90°, which will result in inaccurate quantitation. Goal(s): To develop an MR sequence to measure the RF field map while maintaining the short echo time of UTE and sensitivity of 90° excitation. Approach: A composite 90° RF pulse was chosen and implemented into a 3D UTE sequence. Results: The flip angle map was measured with enough sensitivity and accuracy, which was used in more accurate sodium quantitation. Impact: The composite 90° RF pulse can be easily added to an existing 3D UTE sequence, and it has the advantage of short echo time and sensitivity. The measured RF field map will be essential for an accurate sodium quantitation.