In vivo quantification of proton exchange rate in healthy human brains with omega plot

To implement omega plot method for in vivo mapping of proton exchange rates in human brain by taking into account the water direct saturation (DS) effect and multiple saturation transfer exchanging species in vivo.

Four Z-spectra were collected with chemical exchange saturation transfer (CEST) saturation power =1, 2, 3 & 4 µT. Water DS was estimated by fitting the Z-spectrum to a linear combination of multiple Lorentzian components and its contribution to the signal was subsequently removed. Exchange rate maps were derived by the omega plot, consisting of fitting the inverse of the signal intensity, M /(M ₀-M ), as a function of 1/(γB₁)².

The exchange rate values quantified with the DS removed omega plot were significantly higher in the GM region than in the WM region (616±29 vs. 575±20 s^-1, P<0.001). Phantom studies confirmed that the exchange rates from DS-removed plots varied linearly with pH (R²=0.998) for the pH range of 6.2 to 7.4, whereas exchange rates from conventional omega plots failed to show such linearity in the entire physiological pH range.

The calculated exchange rate with DS-corrected omega plot is a weighted average for all saturation transfer exchanging proton species which contribute to Z-spectral signal. The healthy brain exchange rate map provided by DS-removed omega plots may serve as a baseline for detecting any pathological changes.