Continuous SWIFT

Abstract

This work describes our first efforts to implement SWIFT (SWeep Imaging with Fourier Transformation) in continuous mode for imaging and spectroscopy. We connected a standard quadrature hybrid with a quad coil and acquired NMR signal during continuous radiofrequency excitation. We utilized a chirped radiofrequency pulse to minimize the instantaneous radiofrequency field during excitation of the spin system for the target flip angle and bandwidth. Due to the complete absence of “dead time”, continuous SWIFT has the potential to extend applications of MRI and spectroscopy in studies of spin systems having extremely fast relaxation or broad chemical shift distributions beyond the range of existing MRI sequences.

Multi-Band-SWIFT

“…With standard gapped SWIFT, coil ring down and transmit/receive (T/R) switching times often restrict the practically obtainable bandwidth. Continuous SWIFT [19] and related schemes [20] allowing true simultaneous acquisition and excitation do not have this problem, but are not considered here because these approaches are still yet to be developed for in vivo use. Thus for various reasons, all existing sequences are limited to a maximum achievable bandwidth which may be insufficient to resolve all excited short T 2 signals and/or to avoid off-resonance artifacts (blurring) in radial imaging.…”

Solid-State Quantitative 1H and 31P MRI of Cortical Bone in Humans

“…short-T 2 *) 1 H and 31 P signal in bone, the time between signal excitation, encoding, and acquisition must be substantially reduced, relative to standard MRI pulse sequences. Three solid-state radial pulse sequences have emerged during the past two decades for imaging short- T 2 * tissues: ultrashort echo-time (UTE) 21 , zero echo-time (ZTE) 22-24 , and sweep imaging with Fourier transformation (SWIFT) 25,26 . As shown in Figure 1 , these solid-state MRI (SS-MRI) pulse sequences can capture these rapidly-decaying signals.…”

“…In SWIFT, the excitation, encoding, and acquisition steps are performed in a finely interleaved (gapped 25 ) or fully simultaneous (continuous 26 ) manner. This method, however, requires significant hardware modifications to completely eliminate the delay between excitation and acquisition for every k-space point.…”

Active decoupling of RF coils using a transmit array system

“…Methods proposed for the implementation of true CEA include geometric decoupling [18], sideband excitation [19], and hybrid coupler isolation [20,21]. In geometric decoupling, separate transmit and receive coils are placed orthogonally to each other around the imaging volume.…”