Optimal variable flip angle schemes for dynamic acquisition of exchanging hyperpolarized substrates

Department of Radiology and Biomedical Imaging, University of California - San Francisco, San Francisco, CA, United States.
Journal of Magnetic Resonance (Impact Factor: 2.51). 06/2013; 234C:75-81. DOI: 10.1016/j.jmr.2013.06.003
Source: PubMed


In metabolic MRI with hyperpolarized contrast agents, the signal levels vary over time due to T1 decay, T2 decay following RF excitations, and metabolic conversion. Efficient usage of the nonrenewable hyperpolarized magnetization requires specialized RF pulse schemes. In this work, we introduce two novel variable flip angle schemes for dynamic hyperpolarized MRI in which the flip angle is varied between excitations and between metabolites. These were optimized to distribute the magnetization relatively evenly throughout the acquisition by accounting for T1 decay, prior RF excitations, and metabolic conversion. Simulation results are presented to confirm the flip angle designs and evaluate the variability of signal dynamics across typical ranges of T1 and metabolic conversion. They were implemented using multiband spectral-spatial RF pulses to independently modulate the flip angle at various chemical shift frequencies. With these schemes we observed increased SNR of [1-(13)C]lactate generated from [1-(13)C]pyruvate, particularly at later time points. This will allow for improved characterization of tissue perfusion and metabolic profiles in dynamic hyperpolarized MRI.

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    • "MRSI experiments were conducted using a centric 8×8 phase-encoded matrix spanning a 1.25×1.25 cm2 field of view, beginning 10 s after hyperpolarized bolus injections. Ramped-flip angle [46] slice-selective Gaussian pulses were used to obtain free induction decays of 512 time points over 100 ms acquisition times (4 kHz spectral bandwidth); as the acquisition time equaled the phase-encode repetition time, total scan times were 6.7 s. MRSI data sets were reconstructed on Matlab and jMRUI-5 [47] by spatial zero-filling to 16×16 and time domain zero filling to 1024 points followed by 15 Hz Lorentzian apodization. "
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