Image artifacts in very low magnetic field MRI: The role of concomitant gradients

While MRI at very low magnetic fields has certain potential advantages, it may also face problems that are not typical for MRI at conventional and high field (0.1-10 T). Major differences arise due to the presence of concomitant components of inhomogeneous magnetic field (gradients) that are transverse to the major B_z field, B₀. These concomitant transverse field components are inevitably generated by the same gradient coils that generate desired B_z imaging gradients as routinely used in MRI for spatial encoding. In the hypothetical case (linear spatial variation of B_z field amplitude due to the imaging gradients, no concomitant transverse field components, no B₀ and B₁ field inhomogeneities, etc.), Fourier transform MRI preserves the shape of the real object being examined. It is demonstrated herein that unavoidable concomitant transverse field gradients, G, result in an image deformation of the object's actual shape by bending straight lines and planes with a characteristic curvature radius R_c = B₀/G. For imaging gradients on the order of 10 mT/m and B ₀ of 1 T, the radius R_c is about 100 m and image distortions are generally negligible. However, for B₀ of 1 mT, R _c is 10 cm, which is less than a typical FOV in human studies. This manuscript derives expressions describing geometrical relationships between the imaged object and the obtained MR data. In addition to geometrical distortions, image intensity will be modulated in a complex, spatially dependent manner. Hence, if unaccounted for, corresponding image distortions-geometry and intensity-will create substantial difficulties in very low field image interpretation.