Space-Specific Mixed Excitation Technique for High Signal-to-Noise Ratio Spatial Encoding in Magnetic Particle Imaging

Background Introduction

Magnetic Particle Imaging (MPI) is an emerging, radiation-free tracer imaging technology that visualizes the spatial distribution of superparamagnetic iron oxide nanoparticles (SPIOs) to achieve high-sensitivity quantitative imaging. Unlike optical imaging, MPI has no limitations in imaging depth and can quantitatively measure SPIOs without interference from tissue background signals. However, conventional MPI spatial encoding methods rely on gradient magnetic fields with fixed gradient strength to generate Field-Free Points (FFP) or Field-Free Lines (FFL) for spatial scanning. Increasing the gradient strength can enhance theoretical spatial resolution but also causes a decline in the imaging system’s Signal-to-Noise Ratio (SNR) and sensitivity.

MPI shows great potential in preclinical applications such as cancer detection and vascular anomaly monitoring, but balancing spatial resolution and sensitivity remains a technical challenge. To address this challenge without increasing hardware complexity, researchers Yanjun Liu, Guanghui Li, Jiaqian Li, Zhenchao Tang, Yu An, and Jie Tian from Beihang University have proposed a novel Space-Specific Mixing Excitation (SSME) technique.

Source of the Paper

The research paper was authored by Yanjun Liu, Guanghui Li, and others. The research team is affiliated with the School of Biological Science and Medical Engineering and the School of Instrumentation Science and Opto-electronics Engineering at Beihang University, in collaboration with the Key Laboratory of Big Data Precision Medicine (Beihang University) of the Ministry of Industry and Information Technology of China. The paper has been accepted by IEEE Transactions on Biomedical Engineering and will be officially published in 2024.

Detailed Research Process

Workflow

Research Subjects and Experimental Equipment

The research team used a custom-built MPI scanner for validation experiments. The mouse-sized MPI scanner was equipped with a dual-channel receiver, with an aperture of 30 mm and a two-dimensional Field of View (FOV) of 20×20 mm² in the X-Y plane. To achieve rapid multidimensional spatial encoding, the device uses the SSME technique, generating a dynamically scaled Elastic Field-Free Region (EFFR) through an Oscillating Gradient Field (OGF).

Space-Specific Mixing Excitation (SSME)

The SSME technique introduces dual-frequency excitation magnetic fields and non-uniform field strength, causing magnetic particles at each position to generate unique intermodulation responses. This excitation strategy relies mainly on the contraction and expansion of the EFFR in space, independent of FFP’s trajectory. Through multi-channel acquisition and system matrix-based reconstruction, rapid multidimensional high SNR imaging can be achieved.

Research Methods and Steps

The study first introduced the theory and concept of SSME and performed a simulation analysis of the two-dimensional system function of SSME-MPI. The experimental process included tests with human models, particles, and phantom imaging experiments. After the system matrix calibration was completed, images were generated using a Kaczmarz algorithm-based reconstruction method. The researchers then compared the system functions of SSME mode and conventional MPI, evaluating their spatial resolution, sensitivity, and imaging performance.

Main Results

1. System Function Comparative Analysis:

   • In the SSME mode, the typical one-dimensional system function showed the energy distribution and spatial structure of each frequency component. Compared to the SGF mode, the harmonic SNR in the SSME mode significantly improved, especially in high-frequency components, with a harmonic SNR gain up to 20 dB.

2. Two-Dimensional Phantom Imaging Experiment:

   • Using double-point source phantoms with varying distances, the SSME mode effectively distinguished at a 1 mm distance, while the SGF mode couldn’t. The SSME mode also effectively distinguished two targets in the Y direction.

3. Sensitivity Evaluation:

   • Through tests with particle samples of different iron contents, the Limit of Detection (LOD) for SSME-MPI was determined to be 64 ng, significantly improved compared to the SGF mode (231 ng). Imaging results showed that samples with iron contents up to 850 ng could be accurately reconstructed.

4. In Vivo Imaging Experiment:

   • Mouse in vivo experiments verified the high sensitivity and feasibility of SSME-MPI in detecting tumor location and quantity. Especially in detecting tumors at the edge of the FOV, traditional MPI faced limitations, while SSME-MPI successfully achieved high-quality imaging.

Summary and Significance

By deeply analyzing the results of in vivo and phantom experiments, this study demonstrates that the proposed SSME technique significantly enhances MPI’s SNR and sensitivity and achieves a spatial resolution of 1 mm without increasing hardware complexity. This provides an effective solution to the balance between spatial resolution and sensitivity in MPI, with potential for broader medical applications. One future research direction is to integrate SSME with existing MPI hardware configurations to further optimize overall imaging performance and reduce system complexity and cost.

These research results indicate that the SSME technique not only achieves innovative breakthroughs technologically but also provides new approaches and methods for the further development of biomedical imaging technology.