Three-dimensional fast single-point macromolecular proton fraction mapping of the human brain at 0.5 Tesla

Abstract

Fast single-point macromolecular proton fraction (MPF) mapping is a recent magnetic resonance imaging (MRI) method enabling quantitative assessment of myelin content in neural tissues. To date, the reported technical implementations of MPF mapping utilized high-field MRI equipment (1.5 T or higher), while low-field applications might pose challenges due to signal-to-noise ratio (SNR) limitations and short T₁ . This study aimed to evaluate the feasibility of MPF mapping of the human brain at 0.5 T. The three-dimensional MPF mapping protocol was implemented according to the single-point synthetic-reference method, which includes three spoiled gradient-echo sequences providing proton density, T₁ , and magnetization transfer contrast weightings. Whole-brain MPF maps were obtained from three healthy volunteers with spatial resolution of 1.5×1.5×2 mm³ and the total scan time of 19 minutes. MPF values were measured in a series of white and gray matter structures and compared with literature data for 3 T magnetic field. MPF maps enabled high contrast between white and gray matter with notable insensitivity to paramagnetic effects in iron-rich structures, such as globus pallidus, substantia nigra, and dentate nucleus. MPF values at 0.5 T appeared in close agreement with those at 3 T. This study demonstrates the feasibility of fast MPF mapping with low-field MRI equipment and the independence of brain MPF values of magnetic field. The presented results confirm the utility of MPF as an absolute scale for MRI-based myelin content measurements across a wide range of magnetic field strengths and extend the applicability of fast MPF mapping to inexpensive low-field MRI hardware.

Keywords: Low-field MRI; cross-relaxation; macromolecular proton fraction; magnetization transfer; myelin.

Figure 1

Simulated dependences of the MPF-to-noise ratio (MNR) on the offset frequency Δ (A) and flip angle FA_MT (B) of the off-resonance saturation pulse computed for the two-pool model parameters of WM and GM at 0.5 T with SNR_ref =30 and the sequence parameters used in the actual imaging protocol. The dependences of MNR on Δ (A) were simulated at FA_MT =700°. The dependences of MNR on FA_MT (B) were simulated at Δ =1.5 kHz.

Figure 2

Simulated dependences of the MPF-to-noise ratio (MNR) on the offset frequency Δ (A) and flip angle FA_MT (B) of the off-resonance saturation pulse computed for the two-pool model parameters of WM at magnetic field strengths of 0.5 T (solid lines), 1.5 T (dot lines), and 3 T (dash lines). The SNR_ref values were set as 30, 90, and 180 for 0.5, 1.5, and 3 T magnetic fields, respectively. The dependences of MNR on Δ (A) were simulated at FA_MT =700°. The dependences of MNR on FA_MT (B) were simulated at Δ =1.5 kHz. Other sequence parameters were the same as those detailed in the experimental imaging protocol.

Figure 3

MPF map acquisition scheme illustrated by actual images obtained from a 22-year-old female volunteer: source PD-, T₁- and MT-weighted images (A,B,C), synthetic reference image (D), T₁ map (E), and MPF map (F).

Figure 4

Representative sagittal (A,E), coronal (B), and axial (C,D,F,G,H) reformatted cross-sections of a 3D MPF map obtained at 0.5 T demonstrating the appearance of anatomic structures. Images (A,B,C,D) show the anatomic regions where MPF values were measured for quantitative analysis. Images (E,F,G,H) illustrate contrast features of MPF maps including clear GM contrast of iron-rich structures, such as SN, DN, and GP (E,G,H) and fine WM structural distinctions delineating compact fiber tracts within the brain stem (F). The MPF map is presented with grayscale range corresponding to MPF values 0–18%. FWM, frontal WM; OWM, occipital WM; CCG, genu of the corpus callosum; CCS, splenium of the corpus callosum; CR, corona radiata; P, pons; Pu, putamen; CN, caudate nucleus; Th, thalamus; GP, globus pallidus; SN, substantia nigra; DN, dentate nucleus; CST, corticospinal tract; ML&CTT, medial lemniscus and central tegmental tract.

Figure 5

Correlation between structure-averaged MPF values measured in this study at 0.5 T and literature data (5) obtained at 3 T for the brain anatomic structures listed in Table 1.