Figure 4.

Experimental results demonstrating the mechanism behind MPI gradient-based localization of heating.(a) Magnitude of the MPI gradient in the x-z plane (Lakeshore™Gaussmeter). The gradient field has zero magnetic field at the center and a high magnitude everywhere else. Due to the field-free-line geometry along the y-axis, each x-z slice along y has the same magnetic field profile. (b) SPION dynamic hysteresis loops was simulated at different positions in the gradient field. The hysteresis loops are most open at position A (|H| = 0) while the hysteresis loops are closed at other positions. Because heating depends on the area bounded by the hysteresis loop, the gradient field localizes heating to the field-free-line where |H| ≈ 0. Different drive frequencies have the same trend, showing that this localization method is flexible and works for a range of MPI drive fields. (c) Nanoparticles were put at different locations in the gradient field and heated with 354 kHz, 13 mT excitation. (see Methods: Heating Localization Characterization for details) The measured temperature rise and SAR (Neoptix™probes) is observed to be highest when the nanoparticle is located at position A (field-free-line), in line with simulations in (b). Heating was suppressed at other positions due to the large |H| away from the field-free-line. The 2.35 T/m gradient used here localizes heating to within a 7 mm radius region, but doubling the gradient to 7 T/m will improve localization to 2.3 mm radius.