Magnetic particle imaging utilizing orthogonal gradient field and third-harmonic signal detection

We have developed a 2-D magnetic nanoparticle (MNP) imaging system using an orthogonal gradient field and third-harmonic signal detection. An ac excitation field was applied to generate third-harmonic signals from MNP on the basis of their nonlinear magnetization curves, and a dc gradient field was applied perpendicular to the excitation field. The dependence of the third-harmonic signal on the dc field was very different from that in the conventional parallel case. We obtained a contour map of the signal field from the MNP located at (boldsymbol z ) = 35 mm below the pickup coil using an excitation field of 1 mT and gradient field of 0.4 T/m. We could obtain spatial resolution of 10 mm even in this small gradient field. Then, we analyzed the contour map using singular value decomposition (SVD) to improve the spatial resolution of the position estimation of MNP. Using SVD, we converted the field map to the MNP distribution. The spatial resolution was improved from 10 to 6.7 mm. We could clearly distinguish three MNP samples separated with 10 mm spacing located at (boldsymbol z) = 35 mm.