Editorial Feature

Novel Clinical Uses of Magnetic Particle Tracking

What is magnetic particle tracking, and what are the latest developments related to this technology? This article considers principles, limitations, and recent applications.

MPT, magnetic particle tracking, particle tracking, imaging

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Overview of MPT

MPT is a recently developed technology that can measure the translation and rotation motion of a particle in an opaque environment. This technique has potential applications in studying the accurate position and three-dimensional (3D) orientation of organs in the human body, such as in capsule endoscopes.

Using this technique, the organs can be functionally examined without any radiation exposure. In pharmaceutical research, this method offers a great opportunity to study remote drug release at any position of the gut. In addition, MPT also has significant applications in diagnostics, imaging, targeting, and therapy owing to its suitable magnetic properties and biocompatibility.

Principles of a MPT System

The MPT technique tracks the magnetic field of a single magnetic tracer, which is measured using anisotropic magnetic resonance (AMR) sensors, and detects the translation and rotation motion of the particle in a completely 3D cylindrical fluidized bed. In MPT, a magnetic source is located according to its field, which is modeled as a dipole; MPT uses a single tracer particle that has a dipole. This allows for the detection of the particle position and orientation.

MPT overcomes the challenges in studying collision detection for particles of arbitrary shape and size. In this technique, magnetic markers are monitored by employing a specially designed permanent magnet aligned by a vertically oriented pulsed magnetic field. This alignment allows monitoring of marker position from the stray field components.  

Researchers have monitored marker position as a 3D plot in real-time. The method provides a spatial resolution of better than 10 mm and a temporal resolution of about 1 s, and thus can distinguish between adjacent loops of the gut. This allows for viewing tracers in cells (cell tracking), blood (perfusion), and other functional systems such as drug targeting and drug delivery systems within a living organism. It also offers a wide range of applications in the field of engineering, physics, chemistry, medicine, mathematics, and computer science.

 Advantages and Limitations

MPT has multiple advantages - it is a safe technique and does not involve any radioactive material or X-rays. It is a highly sensitive method in granular dynamics as it provides translation and rotational information of a particle, which is difficult to obtain using other techniques. It offers high temporal resolution to measure high-speed flow, and it is cost-effective as the magnetometers and sensors are less expensive in comparison to the equipment used in non-optical approaches.

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The accuracy and precision of this method lie in the reconstruction of the position and orientation of a magnetic dipole. Thus, the reconstruction process of MPT using typical optimization approaches limits its applications as it is highly time-consuming. In addition, the magnetic field decays rapidly. This is inversely proportional to the cubic of the distance, and so the tracking range is limited.

Recent Developments

Existing analytical reconstruction algorithms have certain limitations, and they usually depend on the gradient of the magnetic field. This is often not easy to measure accurately in different applications. Researchers from the University of Kansas, writing in the journal Plos One, recently developed and examined new MPT reconstruction algorithms and compared the results with an optimization approach.

The researchers have developed a semi-analytical solution and the related reconstruction algorithm. To reduce the measurement uncertainty in practical applications, deep neural network (DNN)-based models have also been developed to denoise the reconstructed trajectory. These are more accurate in position reconstruction.

Recent approaches have combined the traditional wavelet model with DNN, which provides a more accurate velocity reconstruction and performs well in the orientation reconstruction. Further, to improve the reconstruction accuracy, convolutional neural networks (CNN), gated recurrent units (GRU), and Wavelet transform (WT)-based methods were combined to denoise the position, velocity, and orientation signals. All these methods improved the reconstruction accuracy.  

The extended Kalman filter (EKF) algorithm has an accuracy of the same amount as the optimization method, however it is orders of magnitude faster. This is explored in the journal AIP. The accuracy of the position obtained using EKF is about 0.6%, whilst the uncertainty of orientation is less than 1.5°. MPT was applied in order to measure a dense granular shear flow, and this was used to investigate the spatial distribution of a tracer particle.

Conclusion

MPT is one of the most promising imaging technologies to emerge in the last two decades, and is expected to change the landscape of modern medical imaging and in vivo translational research. Magnetic particle imaging (MPI) is an emerging radiation-free imaging modality that utilizes sensitive, safe, and biocompatible superparamagnetic iron oxide nanoparticles (SPIONs) as its tracing material. With multiple promising approaches for clinical translation and sustained research in biomedical applications, MPT will be a crucial, complementary clinical diagnostic and therapeutic tool in the near future.

References and Further Reading

Buist KA, Jayaprakash P, Kuipers JAM, Deen NG, Padding JT. Magnetic particle tracking for nonspherical particles in a cylindrical fluidized bed. AIChE J. 2017;63(12):5335-5342. https://pubmed.ncbi.nlm.nih.gov/29213144/

Tao X, Tu X, Wu H. A new development in magnetic particle tracking technology and its application in a sheared dense granular flow. Rev Sci Instrum. 2019 Jun;90(6):065116. https://pubmed.ncbi.nlm.nih.gov/31255032/

Huixuan Wu, Pan Du, Rohan Kokate, Jian-Xun Wang. A semi-analytical solution and AI-based reconstruction algorithms for magnetic particle tracking. PloS July,  2021. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0254051

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