In this study, a novel human-size handheld magnetic particle imaging (MPI) system was developed for the high-precision detection of sentinel lymph nodes for breast cancer. The system consisted of a highly sensitive home-made MPI detection probe, a set of concentric coils pair for spatialization, a solenoid coil for uniform excitation at 8 kHz@1.5 mT, and a full mirrored coil set positioned far away from the scanning area. The mirrored coils formed an extremely effective differential pickup structure which suppressed the system noise as high as 100 dB. The different combination of the inner and outer gradient current made the field free point (FFP) move in the Z direction with a uniform intensity of 0.54T/m, while the scanning in the XY direction was implemented mechanically. The third-harmonic signal of the Superparamagnetic Iron Oxide Nanoparticles (SPIONs) at the FFP was detected and then reconstructed synchronously with the current changes. Experiment results showed that the tomographic detection limit was 30 mm in the Z direction, and the sensitivity was about 10 μg Fe SPIONs at 40 mm distance with a spatial resolution of about 5 mm. In the rat experiment, 54 μg intramuscular injected SPIONs were detected successfully in the sentinel lymph node, in which the tracer content was about 1.2% total injected Fe. Additionally, the effective detection time window was confirmed from 4 to 6 min after injection. Relevant clinical ethics are already in the application process. Large mammalian SLNB MPI experiments and 3D preoperative SLNB imaging will be performed in the future.

We developed a novel high resolution open-bore narrowband magnetic particle imaging (MPI) system for big animal and future clinical usage. Optimized strength and direction of excitation field was invested for enlarged field of view (FOV), and a 4T/m gradient field as well as its linear scanning trajectory were formed by 16 square coils without coupling to excitation, forming a maximum 120mm×120mm open bore FOV. Further, a new differential pick-up structure consisted of multi Helmholtz excitation and pickup coils away from the FOV was proposed in order to suppress the system noises, which was very important for the limited coil setting space in open-bore MPI. The new differential structure contributed significantly to improve the signal-to-noise ratio as high as about 100dB without lose of harmonic signals. Several home-made resonant circuits, filters and lock-in amplifier was used to constitute the high precision signal acquisition system. Experimental results showed that the detection limit of the system was 1ug Fe of Resovist sample with a spatial resolution of 1mm after reconstruction, satisfying many clinical needs such as drug delivery and bedside-monitoring.

Magnetic particle imaging (MPI) and other medical magnetic detection technologies often employ coil sensors to detect signals. Although high-temperature superconducting (HTS) pickup coils offer excellent near-zero-resistance background interference at low frequencies, enabling very high signal-to-noise ratio (SNR), the internal losses and equivalent resistance of HTS pickup coils increase significantly above 1kHz which is necessary for MPI. This article establishes a theoretical calculation model for the losses of HTS tape materials under MPI environment (i.e., combined external driving field and induced secondary magnetic field with high frequency) as a pickup coil, including the generation mechanisms and loss calculations of superconducting loss, eddy current loss, and hysteresis loss produced by various thin films of the superconducting tape, as well as the analysis of their correlation with coil sensitivity. Based on the model analysis and mechanical conditions of HTS tapes, a series of HTS pickup coils were fabricated and their performance was verified. The numerical and experiment results show that eddy current loss of the HTS pickup coil is the dominant loss under various loss mechanisms in MPI conditions. HTS materials with thinner copper stabilizers can greatly reduce internal coil losses near MPI frequencies, significantly improving the sensitivity limit. Furthermore, this paper also presents the development of a mechanical MPI system optimized for the HTS pickup coil and an ultra-high sensitivity MPI imaging experiment. A small 32mm-diameter HTS pickup coil made of copper-free HTS tape was used in the MPI system, which successfully imaged 100μg of superparamagnetic iron oxide nanoparticles (SPIONs) from 100 mm away.