Neuromelanin Contrast Optimization and Improved Visualization of the Substantia Nigra in a 3D Gradient-Echo Sequence With Magnetization Transfer.

Neuromelanin (NM) magnetic resonance imaging has been used to evaluate the loss of melanized neurons in the substantia nigra, the main characteristic of Parkinson disease, typically by measuring the contrast ratio (CR) between the NM-rich areas and the cerebral peduncles in the midbrain. Neuromelanin contrast can be generated by employing a 3D gradient-echo (GRE) sequence with magnetization transfer (MT). In this work, we first evaluated the effect of the MT pulse frequency on the CR and contrast-to-noise ratio (CNR), analyzing a large frequency range from 500 to 100K Hz. Secondly, the impact of the 3D GRE sequence flip angle (FA) was evaluated using angles from 5° to 40°. Additionally, images were acquired both with and without MT for each FA, providing an opportunity to examine the effect of MT on tissue proton density, T1, and T2* values. Results showed that the highest CR and CNR were obtained for the lower MT frequencies (500-2000 Hz) and lower FAs (5°-10°). The lower FAs improved the visualization of the neuromelanin-rich area in the substantia nigra and facilitated delineating its volume. In addition, this work showed that the MT pulse decreased the T1 and PD of the tissue. Furthermore, simulations supported the obtained in vivo results.