SELF-REFOCUSED SPATIAL-SPECTRAL RF PULES FOR IMAGING CELLS LABELED WITH SPIOS

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Introduction: We have developed a self-refocused spatial-spectral (SPSP) pulse pair to achieve slice-selective, short-echo-time, spin-echo imaging of off-resonant spins. A self-refocused SPSP pulse pair is essentially a phase-matched 90 SPSP pulse and 180 SPSP pulse combined into one pulse through a series of approximations, resulting in a considerably shorter echo time than possible with two separate pulses. When used with standard imaging sequences, the SPIO nanoparticles lead to signal dephasing and act as a negative contrast agent. However, negative contrast agents cannot be distinguished from voids in the image and can suffer from partial volume effects. One of techniques proposed for positive contrast imaging of SPIO-labeled cells uses spectrally selective pulses to image off-resonant water near the labeled cells. The SPSP self-refocused pulse pair we have developed enables slice-selective imaging of off-resonant frequencies, hence eliminating background signal from sources of off-resonance outside the slice of interest. Methods and Discussion: A minimum-phase 180 pulse with a bandwidth (BW) of 164 Hz was designed using the Shinnar Le-Roux (SLR) algorithm [6]. The final self-refocused SPSP pulse pairs were comprised of 53 conventional small tip-angle subpulses scaled by the sampled values of the self-refocused pulse pair envelopes. Data were obtained from an agar phantom with varying concentrations of SPIO-labeled human stromal cells. In vivo data were obtained from a mouse, with 1 million and 3 million SPIO-labeled cells injected into the hind legs. Images showed positive contrast was obtained using the self-refocused SPSP spin-echo sequence.