A handheld SPIO-based sentinel lymph node mapping device using differential magnetometry.

Sentinel lymph node biopsy has become a staple tool in the diagnosis of breast cancer. By replacing the morbidity-plagued axillary node clearance with removing only those nodes most likely to contain metastases, it has greatly improved the quality of life of many breast cancer patients. However, due to the use of ionizing radiation emitted by the technetium-based tracer material, the current sentinel lymph node biopsy has serious drawbacks. Most urgently, the reliance on radioisotopes limits the application of this procedure to small parts of the developed world, and it imposes restrictions on patient planning and hospital logistics. Magnetic alternatives have been tested in recent years, but all have their own drawbacks, mostly related to interference from metallic instruments and electromagnetic noise coming from the human body. In this paper, we demonstrate an alternative approach that utilizes the unique nonlinear magnetic properties of superparamagnetic iron oxide nanoparticles to eliminate the drawbacks of both the traditional gamma-radiation centered approach and the novel magnetic techniques pioneered by others. Contrary to many other nonlinear magnetic approaches however, field amplitudes are limited to 5 mT, which enables handheld operation without additional cooling. We show that excellent mass sensitivity can be obtained without the need for external re-balancing of the probe to negate any influences from the human body. Additionally, we show how this approach can be used to suppress artefacts resulting from the presence of metallic instruments, which are a significant dealbreaker when using conventional magnetometry-based approaches.