Figure 5

Reconstructed|∇B _z | images using the fixed background field inhomogeneity with${\mathcal{N}}_{\delta }=5$ and${\mathcal{N}}_{\delta }=30$.a) reconstructed |∇B _z | images using the fixed background field inhomogeneity with${\mathcal{N}}_{\delta }=5$. b) reconstructed |∇B _z | images using the fixed background field inhomogeneity with${\mathcal{N}}_{\delta }=30$. From top-left to bottom-right, each image corresponds to the j-th iterative updated |∇B _z |, j=0,1,⋯,9.