# The Applications of Magnetic Particle Imaging: From Cell to Body
## 4 Applications of MPI
### Tumor Detection  
As cancer remains among the leading causes of human death, its diagnosis and treatment, especially early-stage diagnosis, have become among the most crucial research areas [8]. The cure rate of cancer strongly correlates with the stage of cancer that is diagnosed. If cancer can be diagnosed and provided with proper treatment at an early stage, the cure rate will increase significantly [2]. Research on devices with high sensitivity and high resolution is critical to the diagnosis of early-stage cancer. The high sensitivity of MPI is suitable for tumor detection because its characteristics satisfy the requirements of early-stage cancer diagnosis [8]. In practice, MPI takes advantage of the permeability and retention (EPR) effect to detect tumors. The EPR effect is caused by leaky vessels with large pores that enable the tumor to grow rapidly. This effect is commonly used to target tumors in the application of magnetic nanoparticles [38] and other nano contrast agents [8; 65] in medical imaging. Therefore, MPI has great clinical application value in the early diagnosis of cancer. Moreover, experiments show that MPI can track tumors in vivo. Therefore, scientists from different countries have conducted various experiments and made significant progress in the application of MPI in tumor detection. For example, the [PERSON] group of the University of California, Berkeley, explores the application of MPI to tumor detection (Figure 2). In their research, they used magnetic nanoparticles as imaging tracers because the particles accumulated in the tumor by the EPR effect [8]. The tracer was intravenously injected into the mice through the tail vein, and then a series of scanning time points was selected for tumor tracking. According to the MPI imaging results, the magnetic nanoparticles were first distributed to organs with larger blood volumes, such as the heart and lungs. As the intravascular signal decreased with time, the signal in the liver and kidney increased. In terms of tumor detection, the tumor rim enhancement caused by the initial vascular wash-in could be seen in detail through MPI scanning. Then, the scanning results showed that the magnetic nanoparticles began to accumulate  
Figure 1: In vivo experiments that used MPI to track different numbers of stem cells injected subcutaneously (s.c.) into mice. The cells were labeled with the MPI-tailored tracer CIONs-22 or VivoTrax. The images are MPI images of a mouse (**a**) injected with 100 K stem cells labeled with CIONs-22, (**b**) injected with 100 K or 5 K stem cells labeled with VivoTrax, (**c**) injected with 5 K or 2.5 K stem cells labeled with CIONs-22, (**d**) 3 h after intravenous (i.v.) injection of 100 K CIONs-22-labeled stem cells and postmortem CIONs-22-labeled stem cell biodistribution 20 min, 3 h, or 7 days after i.v. administration, and (**e**) 3 h after the i.v. injection of 300 K or 100 K VivoTrax-labeled stem cells. (**f**) MRI images of a representative mouse after i.v. injection of 300 K CIONs-22-labeled stem cells. (**g**) Longitudinal 2D projection MPI images of a representative mouse within 1 week after i.v. injection of 100 K CIONs-22-labeled stem cells. Image reproduced, with permission, from Reference [40]. Copyright 2020 American Chemical Society. inside the tumor due to the EPR effect. By 96 h after injection, the magnetic nanoparticles had cleared through metabolization. In addition, recent studies have concentrated on the multimodality of medical imaging using MPI [PERSON] et al. of the University of Washington prepared a glioma-targeted MPI tracer by modifying iron oxide nanoparticles with lactoferrin-targeting brain tumors. They conducted in vivo imaging on tumor-bearing mice using a modified tracer [22]. According to the imaging results, the MPI signals of the tumor-bearing mice have high sensitivity and contrast. The [PERSON] group of Stanford University designed a multimodality nanoparticle tracer that enabled multimodal imaging to detect tumor xenografts in living mice [65].