Saturation Power Dispersion Analysis for the Imaging of Amide Proton Transfer (APT) and Nuclear Overhauser Effect...

APT and NOE MRI signals are usually obtained from a Z-spectrum, but the separation of background relaxation effects has certain limitations. We propose to obtain APT and NOE signals from a power dispersion spectrum.

Theoretical analysis showed that in the power dispersion spectrum of the apparent relaxation ratio (ARR), APT and NOE signals can be separated by a double-Lorentzian model. Simulation, phantom, and in vivo rodent stroke experiments were performed to evaluate the feasibility of this method.

Simulation and phantom results showed that the ARR of various signal contributions are Lorentzians with different magnitudes and linewidths, and the linewidth of the Lorentzian from APT or NOE is much narrower than those from other relaxation effects. In rat brain, the power dispersion spectra were acquired in the 0.3-4.2 μT range, where the ARR for APT at 3.6 ppm and for NOE at -3.6 ppm can both be effectively approximated by a double Lorentzian model. The Lorentzian linewidth for APT and NOE was 18% and 20% of that from the total background relaxations, respectively. Compared to normal tissue, the ischemic brain tissue showed significantly lower ARR amplitude for APT but not for NOE, and there was minimal change in the ARR linewidth for both APT and NOE.

We demonstrated that APT and NOE signals can be obtained from the analysis of the power dispersion spectrum, which does not require a distinct signal peak in the Z-spectrum and can be a useful tool at lower magnetic fields.