T₁-weighted imaging in ultra-high magnetic field suffers from the convergence of T₁ relaxation times with an increase of field strength¹, which makes difficult or even impossible generation of sufficient positive contrast between white (WM) and gray matter (GM) for structural neuroanatomical applications. Macromolecular proton fraction (MPF) mapping^2,3 is a method potentially capable to mitigate this problem due to strong myelin-based WM-GM contrast^4-6 and independence of this parameter of magnetic field strength. MPF is a key parameter of the two-pool model of magnetization transfer (MT) defined as a relative amount of immobile macromolecular protons involved into magnetization exchange with water protons⁷. A recent time-efficient single-point 3D MPF mapping method³ allows reconstruction of MPF maps from three source images with high spatial resolution and excellent tissue contrast. However, this approach is based on constraining certain two-pool model parameters and their combinations, which are field-dependent and need to be determined for specific field strength. The objective of this study was to adopt fast 3D MPF mapping as anatomical and quantitative neuroimaging modality for small animal applications in ultra-high magnetic fields based on the characterization of the two-pool model parameters in brain tissues.

Image Acquisition. Four adult male Wistar rats were imaged under isoflurane anesthesia on a 11.7T animal MRI scanner (Bruker BioSpec, Germany). Brain Z-spectroscopic images were obtained using a 3D MT-prepared spoiled gradient echo (GRE) sequence with TR/TE=25/2.7 ms and excitation flip angle α=9°. For off-resonance saturation Gaussian pulse was used with duration 10 ms, 12 offset frequencies (Δ) in a range 0.75-48 kHz and 3 effective flip angles FA_MT = 500, 1000, and 1500º. Complementary R₁=1/T₁ maps were obtained using the variable flip angle (VFA) method with a 3D GRE sequence (TR/TE=25/2.7 ms, α=3, 12, 20, 25, 30°). All images were acquired with resolution of 200x200x400 µm³ and whole-brain coverage. Scan time for each Z-spectral and VFA data point was 1 min 41 s. To correct for field heterogeneities, 3D B₀ and B₁ maps were acquired using the dual-TE⁸ and AFI⁹ methods, respectively. Additionally, 2D T₂ mapping was performed using multiple spin-echo sequence with TR=5 s and 16 echoes with 10 ms echo spacing. To demonstrate the feasibility of high-resolution whole-brain MPF mapping, 3D MPF maps were obtained from three source images (MT-, PD-, and T₁-weighted) with isotropic 170 µm resolution using the single-point method with the synthetic reference image³.

Image Reconstruction and Analysis. To comprehensively characterize tissue contrast, the following parametric maps were compared: MPF, k, T₂^F, and T₂^B maps obtained by fitting Z-spectroscopic data to the pulsed MT model²; MPF map reconstructed using the single-point method with synthetic reference image³ from a data point with Δ= 5 kHz and FA_MT = 500º; maps of combinations of the two-pool model parameters used as constraints in the single-point MPF reconstruction algorithm, R=k(1-MPF)/MPF and R₁T₂^{F 2}; R₁ and proton-density (PD) maps reconstructed from VFA images; and T₂ and PD maps reconstructed from multi-echo images. Parameter measurements were performed in regions-of-interest (ROIs) for a series of WM and GM structures. These data were used to determine parameter constraints in the algorithm², compare MPF obtained from Z-spectra fit and single-point reconstruction, and compare WM-GM contrast between parameters.

Example experimental and fitted Z-spectra from ROIs corresponding to the corpus callosum and cortex are presented in Figure 1. Parameter constraints for the single-point algorithm were determined as follows: R = 23 s^-1, R₁T₂^F = 0.013, and T₂^B =10 µs. There was an excellent agreement between MPF maps reconstructed from Z-spectra fit and single-point synthetic reference method (Figure 2). MPF values obtained with both methods correlated with r=0.99 and no significant bias (Figure 3). MPF showed notably superior tissue contrast compared to other parametric maps, as indicated by percentage differences between WM and GM (Figure 4). Illustration of the rat brain anatomy on a high-resolution 3D MPF map is presented in Figure 5.This study demonstrates the first application of fast 3D MPF mapping for high-resolution quantitative neuroimaging in ultra-high magnetic fields and indicates that MPF provides the most effective source of brain tissue contrast. MPF values in WM and GM structures at 11.7T (Figure 3) were similar to those at lower field strengths^4-6, thus confirming field independence of MPF. Among constrained parameter combinations, only the product R₁T₂^F exhibited apparent field dependence, being about two-fold smaller compared to 3T ². Unique contrast features, high spatial resolution, and quantitative information about myelination make MPF mapping an ultimate tool for neuroimaging in high magnetic fields.