Using bidirectional chemical exchange for improved hyperpolarized [¹³ C]bicarbonate pH imaging.

Area of Science:

• Medical Imaging
• Biophysics
• Magnetic Resonance Imaging

Background:

• Hyperpolarized 13C-bicarbonate imaging is sensitive to chemical exchange effects.
• Rapid chemical exchange between bicarbonate and CO2 can reduce signal-to-noise ratio (SNR) and pH measurement accuracy.

Purpose of the Study:

• To investigate chemical exchange effects on hyperpolarized imaging sequences.
• To identify optimal sequence parameters for high SNR and pH accuracy in hyperpolarized pH imaging.

Main Methods:

• Simulations analyzed exchange effects on pH quantification and SNR under various sampling schemes.
• Four pulse sequences, including a novel multiple-excitation 2D EPI (multi-EPI) sequence, were compared in phantoms.
• In vivo measurements in murine prostate tumors informed the selection of imaging parameters.

Main Results:

• A multiple-excitation scheme increased CO2 SNR by up to 2.7x.
• Phantom imaging confirmed high pH accuracy and SNR gains with the new sampling schemes.
• A 47% CO2 SNR gain is predicted in vivo based on measured exchange rates.

Conclusions:

• The novel multi-EPI pulse sequence enhances CO2 imaging signal in hyperpolarized 13C bicarbonate imaging.
• This technique minimizes pH bias, addressing a significant challenge in hyperpolarized pH imaging.