Rapid and quantitative chemical exchange saturation transfer (CEST) imaging with magnetic resonance fingerprinting (MRF)

Purpose: To develop a fast magnetic resonance fingerprinting (MRF) method for quantitative chemical exchange saturation transfer (CEST) imaging. Methods: We implemented a CEST-MRF method to quantify the chemical exchange rate and volume fraction of the N-alpha-amine protons of L-arginine (L-Arg) phantoms and the amide and semi-solid exchangeable protons of in vivo rat brain tissue. L-Arg phantoms were made with different concentrations (25-100mM) and pH (pH4-6). The MRF acquisition schedule varied the saturation power randomly for 30 iterations (phantom: 0-6 mu T; in vivo: 0-4 mu T) with a total acquisition time of <= 2 min. The signal trajectories were pattern-matched to a large dictionary of signal trajectories simulated using the Bloch-McConnell equations for different combinations of exchange rate, exchangeable proton volume fraction, and water T-1 and T-2 relaxation times. Results: The chemical exchange rates of the N-alpha-amine protons of L-Arg were significantly (P<0.0001) correlated with the rates measured with the quantitation of exchange using saturation power method. Similarly, the L-Arg concentrations determined using MRF were significantly (P<0.0001) correlated with the known concentrations. The pH dependence of the exchange rate was well fit (R-2 = 0.9186) by a base catalyzed exchange model. The amide proton exchange rate measured in rat brain cortex (34.8 +/- 11.7 Hz) was in good agreement with that measured previously with the water exchange spectroscopy method (28.6 +/- 7.4 Hz). The semi-solid proton volume fraction was elevated in white (12.2 +/- 1.7%) compared to gray (8.1 +/- 1.1%) matter brain regions in agreement with previous magnetization transfer studies. Conclusion: CEST-MRF provides a method for fast, quantitative CEST imaging.