Deciphering adiabatic rotating frame relaxometry in biological tissues.

Purpose: This work aims to unravel the intricacies of adiabatic rotating frame relaxometry in biological tissues.

Theory And Methods: The classical formalisms of dipolar relaxation and were systematically analyzed for water molecules reorienting on "fast" and "slow" timescales. These two timescales are, respectively, responsible for the absence and presence of dispersion. A time-averaged or over an adiabatic pulse duration was recast into a sum of and , but with different weightings. These weightings depend on the specific modulations of adiabatic pulse waveforms. In this context, stretched hyperbolic secant ( ) pulses were characterized. Previously published , continuous-wave (CW) , and measures from 12 agarose phantoms were used to validate the theoretical predictions. A similar validation was also performed on previously published ( =1, 4, 8) and from bovine cartilage specimens.

Results: Longitudinal relaxation weighting decreases for pulses as increases. Predicted CW values from agarose phantoms align well with the measured CW values, as indicated by a linear regression function: . The predicted adiabatic and from cartilage specimens are consistent with those previously measured, as quantified by: .

Conclusion: This work has theoretically and experimentally demonstrated that adiabatic and can be recast into a sum of and , with varying weightings. Therefore, any suggestions that adiabatic rotating frame relaxometry in biological tissues could provide more information than the standard and warrant closer scrutiny.