# The Applications of Magnetic Particle Imaging: From Cell to Body
## 4 Applications of MPI
### Hyperthermia  
Magnetic hyperthermia therapy kills cancer cells by heating tumors tissue. The approach includes delivering superparamagnetic nanoparticles to tumors and then adding a high frequency AMF that has no limitation on penetration depth. The particles respond to the AMF generate and release heat to the surrounding environment. However, if all particles within the AMF generate heat, surrounding healthy tissues may be damaged. Therefore, the particles that respond to the AMF need to be restricted  
Figure 3: The imaging results of MPI coregistered with projection X-ray capture whole-body tracer biodistribution. (**a**) MPI shows the dynamics of tracer extravasation into the gut of an ApcMin/+ mouse over time. (**b**) No tracer extravasation into the gut is captured in the MPI images of a wild-type mouse over time. Image reproduced, with permission, from Reference [51]. Copyright 2017 American Chemical Society. in the targeting region. In addition, accurate dosage is needed to ensure that the tumor cells are killed without damaging the surrounding healthy tissues. Therefore, the precise targeting and dose control of magnetic hyperthermia are critical problems to be solved in clinical practice [67]. Because SPIONs can generate MPI signals and heat simultaneously, MPI can form a platform for precise real-time visualized magnetic hyperthermia based on those particles [58]. Precise heating location can be achieved if the nanoparticles are transferred to the tumor area and the hyperthermia treatment is limited to the treatment area. Therefore, MPI can achieve precision in magnetic hyperthermia because according to its imaging principle, only the particles within the FFR respond to the high frequency AMF and generate energy to heat surrounding tissue. In contrast, particles covered by the gradient field but outside the FFR are in a state of saturation and will not generate heat even within the AMF. Then, if the frequency of the AMF is intense enough, the magnetic nanoparticles within the FFR will be excited by the field and generate adequate thermal energy, thereby killing the cancer cells. The location of the hyperthermia treatment can be accurately manipulated by controlling the position of the FFR [58]. MPI can enable dose planning for hyperthermia. The MPI signal is linearly quantitative for the mass of the magnetic nanoparticles, and the specific absorption rate (SAR) is proportional to the mass of the magnetic nanoparticles. Therefore, the SAR image can be obtained from the MPI image. The MPI gradient field is used as an SAR filter to locate the spatial position of the treatment area accurately. Therefore, the SAR dose after gradient localization can be predicted [58]. [PERSON] et al. of the University of California, Berkeley, used different coils to change the frequency of the excitation field to achieve various purposes, such as diagnosis or treatment (Figure 4) [58]. In their research, the [PERSON] group demonstrated the application of MPI in hyperthermia and achieved dose planning and heat location. This idea was proven through a series of experiments, including in vitro experiments, in vivo experiments, and real-time temperature monitoring. [PERSON] from the Chinese Academy of Sciences and co-authors also works on applications of MPI in hyperthermia [59]. They designed a multimodal MPI/MRI tracer with high imaging performance and active biological targeting for magnetic hyperthermia therapy. This tracer distributes uniformly in the tumor when injected intratammally and, therefore, provides superior performance in hyperthermia because it achieves uniform heating of the tumor. Figure 4: Theranostic workflow on a xenograft mouse model with SPIONs in its liver and tumor. Step 1: With an MPI scan at 20 kHz and 20 mT, the SPION present in the tumor and in healthy clearance organs (liver) can be seen clearly. The imaging parameters do not cause the SPIONs to generate heat. Step 2: The user selects a region, such as the tumor, to localize the magnetic hyperthermia.