Detection of different water populations in brain tissue using²H single- and double-quantum-filtered diffusion NMR spectroscopy

This paper presents the first simultaneous ²H single- and double-quantum (SQ and DQ, respectively) diffusion study of excised brain tissue. The apparent diffusion coefficients (ADCs) of the ²H SQ and DQ signals were measured at a fixed diffusion time (Δ - δ/3 = 21.3 ms) and as a function of the diffusion time to assess restricted diffusion [(Δ -δ/3) was changed from 21.3 to 271.3 ms]. As expected, the ADC of the SQ signal was higher than that of the DQ signal [0.53 ± 0.03 × 10^-5 (n = 3) and 0.30 ± 0.03 × 10^-5 cm² s^-1 (n = 4), respectively]. When the ADCs of the SQ and DQ signals were measured as a function of the diffusion time, two components, a fast and a slow component, were observed in each case. The ADCs for the SQ signal were 1.16 ± 0.2 × 10^-5 and 0.35 ± 0.06 × 10^-5 cm² s^-1 (n = 3) for the fast and the slow components, respectively. The ADCs for the DQ signal were 0.31 ± 0.05 × 10^-5 and ∼0.03 ± 0.03 × 10 5 cm² s^-1 (n = 2) with the slow component being relatively small. Interestingly, the slow-diffusion component of the SQ signal was found to have an ADC similar to that of the fast component of the DQ signal. These results suggest that brain water can be divided into at least three water populations and that the DQ signal originates from water molecules which interact with slow-diffusing structural components of the brain. The new insights that one can obtain using simultaneous SQ and DQ diffusion measurement and the ability to distinguish among water populations in biological tissues using the above approach are discussed.