研究目的
To describe two sequences that make more efficient use of the 13C polarization than previously described single‐shot 3D sequences for metabolic imaging of tumors in vivo.
研究成果
The described single-shot 3D FSE pulse sequences make more efficient use of the spin polarization and double the nominal z-direction resolution compared with a similar sequence employing a DSE design. An alternative sequence regains the SNR lost due to the improved spatial resolution. Both sequences depend on the use of echoes formed by unpaired adiabatic refocusing pulses, where the quadratic phase twist is minimized by removing slice-selection gradients from the refocusing pulses.
研究不足
The sequences are prone to off-resonance effects due to B0 field inhomogeneity and eddy currents, which can lead to blurred and rotated images. The fringe-field effect, where the B1 field drops rapidly at the ends of the transmit coil, results in imperfect adiabatic pulses that quickly deplete spin polarization in these areas.
1:Experimental Design and Method Selection:
The study employed two sequences (FSE-I and FSE-II) that utilize spectral-spatial pulses and spiral readouts from a series of echoes created using a fast-spin-echo sequence with adiabatic 180° pulses. The third dimension was encoded with blipped gradients applied in an interleaved order to the echo train.
2:Sample Selection and Data Sources:
Tumor-bearing mice were used following intravenous injection of hyperpolarized [1-13C]pyruvate.
3:List of Experimental Equipment and Materials:
A 7 T MRI scanner (Agilent, Palo Alto, CA, USA) with a 42 mm diameter bird-cage volume coil for 1H transmission and reception, and a similar volume coil for 13C transmission and a 20 mm diameter surface coil for 13C detection (Rapid Biomedical, Rimpar, Germany).
4:Experimental Procedures and Operational Workflow:
The sequences were tested on tumor-bearing mice, with images acquired at 7 T following the injection of hyperpolarized [1-13C]pyruvate. The acquisition of signal from both odd and even echoes was enabled by removing slice selection gradients from the refocusing pulses.
5:Data Analysis Methods:
Images were reconstructed in MATLAB, with phase errors between spirals in the acquisition train corrected using reference scans.
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