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. Author manuscript; available in PMC: 2018 Aug 15.
Published in final edited form as: Neuroimage. 2017 Jun 7;157:341–350. doi: 10.1016/j.neuroimage.2017.06.007

Fig. 1. B1–tuning of the CEST contrast: Bloch simulations.

Fig. 1

(A) MTRasym was simulated as a function of exchange rate (k) for two γB1 levels of 21 and 84 Hz. Note that MTRasym is tuned to k = 2πγB1. Assuming a decrease of k from 600 to 200 s−1 (pink arrow), simulation results show that the MTRasym measured with γB1 = 21 or 84 Hz would increase or decrease, respectively. If k decreases from 30 to 10 s−1 (green arrow), MTRasym decreases for both B1 levels. (B) The change of MTRasym at 3.6 ppm induced by tissue acidosis as a function of γB1 shows a decrease of amide-CEST signal for nearly all the γB1 range. (C) The change of MTRasym at 2 ppm during acidosis as a function of γB1 shows an increase of guanidyl-CEST signal. (D) The pH contrast can be enhanced with pHenh which combines the amide- and guanidyl-CEST effects. To balance the direct water saturation at two different frequencies, different power levels are used.