Magnetic particle imaging (MPI) is a radiation-free vessel- and target-imaging modality that can sensitively detect nanoparticles. A static magnetic gradient field, referred to as a selection field, is required in MPI to provide a field-free region (FFR) for spatial encoding. The image resolution of MPI is closely related to the size of the FFR, which is determined by the selection field gradient amplitude. Because of the limitations of existing gradient coil hardware, the image resolution of MPI cannot satisfy the clinical requirements of human in vivo imaging. Pulsed excitation has been confirmed to improve the image resolution of MPI by breaking down the ’relaxation wall.’ This work proposes the use of a pulsed waveform magnetic gradient from magnetic resonance imaging to further improve the image resolution of MPI. Through alignment of the gradient direction along the field-free line (FFL), each location on the FFL is able to have a unique excitation field strength that generates a specific relaxation-induced decay signal. Through excitation of nanoparticles on the FFL with many gradient profiles, a high-resolution, one-dimensional (1D) image can be reconstructed on the FFL. For larger magnetic nanoparticles, simulation results revealed that a pulsed excitation field with a greater flat portion generates a 1D bar pattern phantom image with a higher correlation and spatial resolution. With parallel FFL and gradient coil movements, high-resolution, two-dimensional (2D) Shepp–Logan phantom and brain vessel maps were reconstructed through repetition of the spatially resolved measurement of magnetic nanoparticles on the FFL.

Magnetic particle spectrometer (MPS) is the device for characterizing the properties of magnetic nanoparticle (MNP) suspensions, such as viscosity, temperature, and concentration. Specifically, in viscosity measurement, MPS usually works at one or several fixed frequencies. It is challenging to find the appropriate frequency for viscosity measurement of different MNP suspensions. The frequency parameter can be introduced into the measurement method to improve the measurement accuracy. Here, we developed an arbitrary frequency MPS to measure the harmonic amplitudes of the magnetization signals of MNPs, which were generated at various frequencies. A method of measuring the viscosity of MNP suspensions by harmonic ratios at different frequencies was proposed. The viscosity measurement time used in the experiment is 1 s, and the shortest measurement time available is 0.2 s. Experimental results show that the arbitrary frequency MPS can be used to measure viscosities ranging from 0.77 mPa·s to 3.87 mPa·s, and the average measurement deviation is 2.7%. The method was validated using plasma samples from normal rabbits and high-fat diet (HFD) rabbits. Normal rabbit measurements differed from the reference by less than 4.5%, and the plasma viscosity of normal and HFD rabbits could be clearly distinguished.

Magnetic particle imaging (MPI) is a tracer-based imaging modality known for its high sensitivity and temporal resolution. It makes the quantitative visualization of the concentration and spatial location of magnetic nanoparticles (MNPs) possible. The x-space method is widely used in MPI for high-speed image reconstruction. In this method, the received signal is mapped directly onto the corresponding spatial position. However, the anisotropic 2D point spread function (PSF) in the x-space method introduces artifacts and blur in the reconstructed images. In this study, an isotropic MPI method, transverse MNP signal-based isotropic imaging (TSI), was developed. It makes isotropic imaging possible in 2D unidirectional Cartesian MPI. Unlike conventional methods that rely on MNP signals registered from the same direction as the excitation, TSI only utilizes MNP signals received perpendicular to the excitation direction. In TSI, the velocity of the time domain signal is first normalized to obtain a “raw-image.” The raw-image is then integrated, differentiated, and added to generate an isotropic MPI image. Because all the operations in TSI are analytical, it preserves the real-time imaging characteristics of the x-space while isotropic imaging is performed. The PSF of TSI was calculated through analytical derivation and the resolution isotropy was validated through simulation and imaging experiments on an in-house-built MPI system. The simulation and experimental results demonstrate that, by improving the isotropy, TSI achieves an approximately 2.5-times increase in resolution in the nonexcitation direction compared with the conventional x-space method. The proposed TSI algorithm does not increase hardware complexity and requires only half the scanning time of the previously proposed multi-channel method. In addition, the transverse MNP signal used in TSI is geometrically decoupled from the excitation magnetic field, eliminating the need for a special compensation design for the receiving coils. This shows promise for future MPI designs with flexible surface coils.

Magnetic particle spectroscopy (MPS) allows for highly sensitive and real time characterization of magnetic nanoparticles (MNPs). When combined with surface-modified MNPs that can specifically bind to biomarkers, MPS can serve as a quantitative biosensing platform and have shown enormous potential in point-of-care diagnosis. To improve diagnostic efficiency and accuracy, the simultaneous detection of multiple biomarkers has been studied in MPS, which requires the simultaneous determination of the type and quantity of MNPs. Current methods generally rely on the higher order harmonics (e.g., third to fifth harmonic ratio) of the MNPs’ magnetic responses to the excitation fields, but facing challenges at low concentrations or with small-sized MNPs that are difficult to generate sufficient harmonics. In this paper, we propose a single-harmonic method for simultaneous determination of MNP types and quantities, named field-swept magnetic particle spectroscopy (FS-MPS). An adjustable static magnetic field perpendicular to the alternating magnetic field is introduced and MNPs’ field-dependent response is registered from the static field direction. By comparing simulation results of serval system configurations and the field-swept spectra of different harmonics, we found that the second harmonic component of the MNP signal combines good specificity and robustness with no direct feed-through interference issues. We built the first FS-MPS device and carried out experiments of binary MNP mixtures to verify the effectiveness of the proposed method. Our experimental results showed that the proposed method has up to 90% accuracy for the simultaneous determination of two types of MNPs with a maximum concentration ratio of 9:1. The proposed method can in principle determine more types of MNPs simultaneously, providing an effective tool for the detection of multiple biomolecular markers.