Design of a non-linear human breast diffusion coil

Diffusion-weighted imaging (DWI) in the female breast is a magnetic resonance imaging (MRI) technique, which may potentially replace or effectively complement both x-ray mammography and contrast-enhanced MR mammography. To further improve specificity of DWI in the breast, stronger and faster diffusion weighting is advantageous. With the target application in mind, the gradient coil is designed on an irregularly shaped semi-open current-carrying surface. Due to the coil former closely fitting the non-spherical target region, non-linear encoding fields become particularly advantageous for achieving locally exceptionally high gradient strengths. As the breast tissue has a predominantly isotropic cellular microstructure, the direction of the diffusion-weighting gradient may be allowed to vary within the target volume. However, due to the quadratic dependency of the b-factor on the gradient strength, variation of the gradient magnitude needs to be carefully controlled. To achieve the above design goals the corresponding multi-objective optimization problem is reformulated as a constrained optimization, allowing for flexible and precise control of the coil properties. A novel constraint is proposed limiting the gradient magnitude variation within every slice while allowing for variations both the direction of the gradient within the slice and the magnitude across the slices. The above innovations enable the design of a unilateral coil for diffusion weighting in the female breast with the local gradient strengths exceeding 1~T/m with highly homogeneous diffusion weighting for imaging in the coronal slice orientation.

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