B 0 Correction for 3T Amide Proton Transfer (APT) MRI Using a Simplified Two-Pool Lorentzian Model of Symmetric Water and Asymmetric Solutes.

Amide proton transfer (APT)-weighted MRI is a promising molecular imaging technique that has been employed in clinic for detection and grading of brain tumors. MTR asym , the quantification method of APT, is easily influenced by B 0 inhomogeneity and causes artifacts. Current model-free interpolation methods have enabled moderate B 0 correction for middle offsets, but have performed poorly at limbic offsets. To address this shortcoming, we proposed a practical B 0 correction approach that is suitable under time-limited sparse acquisition scenarios and for B 1 ≥ 1 μT under 3T. In this study, this approach employed a simplified Lorentzian model containing only two pools of symmetric water and asymmetric solutes, to describe the Z-spectral shape with wide and 'invisible' CEST peaks. The B 0 correction was then performed on the basis of the fitted two-pool Lorentzian lines, instead of using conventional model-free interpolation. The approach was firstly evaluated on densely sampled Z-spectra data by using the spline interpolation of all acquired 16 offsets as the gold standard. When only six offsets were available for B 0 correction, our method outperformed conventional methods. In particular, the errors at limbic offsets were significantly reduced ( n = 8, p < 0.01). Secondly, our method was assessed on the six-offset APT data of nine brain tumor patients. Our MTR asym (3.5 ppm), using the two-pool model, displayed a similar contrast to the vendor-provided B 0 -orrected MTR asym (3.5 ppm). While the vendor failed in correcting B 0 at 4.3 and 2.7 ppm for a large portion of voxels, our method enabled well differentiation of B 0 artifacts from tumors. In conclusion, the proposed approach could alleviate analysis errors caused by B 0 inhomogeneity, which is useful for facilitating the comprehensive metabolic analysis of brain tumors.