# The Applications of Magnetic Particle Imaging: From Cell to Body
## 1 Introduction  
Medical imaging has been increasingly involved in decision making in every aspect of disease treatment, including diagnosis, treatment, and prognosis [1]. Assistance from medical imaging techniques has enhanced the understanding of disease and helped clinicians to achieve precise treatment and, therefore, improve the quality of human life and probability of survival. For instance, cancer is among the most important causes of human death. The poor performance of treatment for cancer diagnosed at an advanced stage has urged the development of detection techniques such as multiple types of medical imaging methods [2]. Imaging methods play an essential role in the diagnosis and treatment of different stages of cancer, such as the tracing, analysis, and treatment of tumors. Commonly used imaging methods include radionuclide imaging, optical imaging, magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound (US). These imaging methods are capable of visualizing lesions and, therefore, play a crucial role in guiding the protocols and processes of cancer treatment. The imaging methods can generally be divided into categories according to their imaging principles: techniques that directly image tissues and methods that locate and image regions of interest by detecting and imaging tracers. The commonly used medical imaging techniques have advantages and disadvantages in different scenarios. In terms of imaging methods that use markers to label the region of interest, MRI using contrast agents offers strong depth penetration and high spatial resolution. However, in T\({}_{1}\)-weighted MRI, the contrast agents show poor sensitivity, and in T\({}_{2}\)-weighted MRI, the agents are difficult to distinguish from biological tissue [3; 4; 5; 6; 7]. Radionuclide imaging has high sensitivity and can achievelong-term tracing, but the long half-life of the tracers may involve the greater net dose to the patient or weaker signal to noise ratio [8].Additionally, this imaging method has low spatial resolution and requires the injection of radioactive tracers [9; 10; 11]. Optical imaging has remarkably high sensitivity, but its penetration depth is low because photons are easily absorbed and scattered when penetrating biological tissues; therefore, it is difficult to image deep biological tissues with this technique [12; 13]. The contrast-enhanced CT has high resolution but relatively low sensitivity [14]. Furthermore, CT scanning exposes patients to ionizing radiation [14]. The contrast-enhanced US can achieve real time imaging, has no ionizing radiation, and is cost efficient for patients [15]. However, it has relatively low sensitivity and low resolution [16]. Magnetic particle imaging (MPI) is a cutting-edge medical imaging technique. The principle of this technique is to locate and quantify the tracers at the region of interest by directly scanning superparamagnetic iron oxide nanoparticles (SPIONs) as an imaging tracer. MPI scans the electronic moments of SPIONs, which is 22 million times more intense than the nuclear moments that detected by MRI [8]. In addition, MPI has the characteristics of linear quantitatively, high sensitivity, and high spatial resolution. The spatial resolution of MPI is approximately 1 mm, and has the potential to reach more than 300 \(\upmu\)m with the improvement with SPIONs, hardware, and pulse sequences [8]. MPI has a voxel size of approximately 1 mm3, which is less than pre-clinical 1 HMRI but close to that of 19 FMRI and Positron Emission Computed Tomography (PET) [17].Additionally, because the device directly scans the imaging tracer, it also has high tissue penetration and almost no background signal from biological tissue [8]. In addition, MPI uses SPIONs as imaging tracers, which enable long-term tracking and are metabolizable and nonradioactive.