Definition of the echo time for UTE imaging and short rectangular rf-pulses

Motivation: Currently, in UTE literature, there is inconsistency in defining echo time when short rectangular excitation pulses are used. Goal(s): Investigate whether echo-time should be defined from the center or the end of the short rectangular rf-pulse. Approach: Theoretical considerations, simulations, and measurements with a 3D-UTE sequence. Results: Two MR signal mechanisms were investigated: (I) relaxation and (II) off-resonance effects during the rf-pulse. In both cases, defining echo time from the rf-pulse center was more accurate. Impact: Results indicate that defining TE from the center of short rectangular RF pulses is more accurate, suggesting the community should adopt this TE definition for future UTE applications.