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Pulse Sequence Development for In Vivo Hyperpolarized 13C MRI


Hyperpolarized (HP) $^{13}$C MRI with dissolution Dynamic Nuclear Polarization (DNP), with its sensitivity enhancements of $>$10,000 fold over thermal equilibrium values, enables non-invasive, spatially localized measurements of the enzymatic conversions of endogenous, nontoxic $^{13}$C-labeled probes. Unique challenges faced in HP $^{13}$C metabolic imaging include the non-recoverable nature of HP magnetization and the complex pattern of resonances that must be spectrally resolved. Rapid and efficient MRI pulse sequences are required to acquire data within the limited temporal window dictated by the rapid irreversible T$_1$ decay of HP magnetization. Previous HP $^{13}$C MR sequences have been limited either by low spatial resolution or being able to image only one metabolically inactive compound. This work presents pulse sequence design for fast imaging of multiple HP $^{13}$C metabolites with high spatiotemporal resolution based on balanced steady state free precession (bSSFP) sequence. One key feature of this new pulse sequence is a novel optimized short duration spectrally selective RF pulses with multiband profile specification, solved by convex optimization. A new method was proposed and tested to avoid bSSFP banding artifact by carefully choosing TR. Compared to previous methods, this new method is more insensitive to off-resonance and more SNR efficient with simple and robust reconstruction. Since image acquisition occurs in a transient state instead of steady state, a constant flip angle is unnecessary and usually leads to decaying signal with suboptimal SNR. In this work variable flip angle schemes were designed for bSSFP transient state HP $^{13}$C MRI to achieve a uniform signal profile, reduce image blurring and increase SNR. A new optimization approach was developed with improved off-resonance insensitivity. Finally, a hardware design project is presented to further increase image SNR by maintaining a sufficient field during sample transfer to preserve polarization.

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