Simultaneous Short T2 Excitation and Long T2 Suppression RF Pulses

Introduction: Ultrashort echo time (UTE) MRI requires specialized pulse sequences to overcome the short T2 relaxation of the MR signal encountered in tissues such as ligaments, tendon or cortical bone. Imaging short T2 tissues is achieved in UTE by acquiring the Free Induction Decay (FID) of the MR signal as soon after the end of the RF excitation pulse as possible. This is typically accomplished by using a radial center-out k-space trajectory and data sampling of only a few hundred microseconds in duration. Magnitude images are then reconstructed from the re-gridded k-space data. In order to achieve a better delineation of short T2 tissues, several long T2 suppression techniques have been developed, including dual echo subtraction techniques [1], and long T2 preparation clusters using either long-duration hard pulses [2] or adiabatic pulses [3] to saturate or to invert and null long T2 tissues. We present a specialized RF technique based on applying a 180° RF excitation pulse that can achieve short T2 tissue excitation and long T2 tissue suppression simultaneously. Theory: The classical notion of a flip angle θ = γB1τ is derived from the Bloch equations while ignoring the T2 transverse relaxation during the RF pulse. For tissues with rapid transverse relaxation, the intrinsic T2 can be on the same order as the RF duration τ, so that the signal decay during the RF pulse may no longer be ignored, resulting in an altered magnetization trajectory [4]. This altered trajectory can be used to selectively excite only short T2 tissues. Our technique is based on applying 180° RF pulses, which adequately invert only the longer T2 tissues (which therefore generate no MR signal) while leaving short T2 tissues partially in the transverse plane (generating MR signal) as illustrated in Fig.1. For a spoiled hard RF pulse train, the steady state transverse magnetization Mss including T2 decay is given by [5]: