MPI

Introduction

Magnetic Particle Imaging (MPI) is a new noninvasive medical imaging and therapeutic (theranostic) modality that could answer clinical and research needs for a safe diagnostic technique without ionizing radiation or toxic tracers in such applications as angiography; cancer detection and staging; therapeutic stem cell tracking to sites of pathology; cancer therapy by heating nanoparticles at the site (hyperthermia); real-time tracking of interventional instruments. 
My research is focused on the instrumentation for the new generation MPI scanner. In general, an MPI scanner consists of a static magnetic gradient field, called the "selection field", which has a field-free region: a field-free point (FFP) or a field-free line (FFL) at the center of the scanner. By superimposing an additional oscillating magnetic field to the static selection field, the field-free region may be rastered throughout an imaging volume. This time-dependent magnetic field could be generated by an AC source or physical motion in the selection coils themselves. The scanner also has receive coils that detect the magnetization response of the particles to the excitation field using the Faraday induction in a similar fashion as done in MRI. To be imaged by MPI, the object must contain magnetic nanoparticles, which are called tracers. A contrast agent containing such a tracer is injected into the animal or patient prior to the scan. During the data acquisition, the MPI scanner moves the field-free region in space along a predefined trajectory that covers the volume within the field of view of the scanner. The magnetic nanoparticles within the object experience a changing magnetic field and respond by changing their magnetization in a nonlinear fashion. The changing magnetization of the nanoparticles induces a time-dependent voltage in the receive coils. This voltage is sampled in a receiver associated with the receive coil. The samples output by the receivers are recorded and constitute the acquired data. In the second step of image generation, referred to as image reconstruction, the image is computed, or reconstructed, from the data acquired in the first step. 
The FFL-based device could potentially produce better image quality than the FFP-based one at the same nanoparticle concentration. However, from a technical point of view, the creation of a required high-strength magnetic gradient with FFL, which is capable of encoding 3D volume, is challenging thus limiting the expected resolution of such devices. 

MPI Major Advantages

The tracer IronOxide is safer than Iodine (used in CT and fluoroscopy) or Gadolinum (used in MRI); 

No ionizing radiation => reduced long-term medical costs for the patient.

MPI Developments

Cylindrical bore – closed geometry (similar to MRI/CT):

Head scanners: Hamburg, MGH, Magnetic Insight, S. Korea, Mitsubishi (Japan).

Leg (interventional) scanner: Würzburg (Germany)

Single sided devices – open geometry (external probe).

Two selection field types are:

FFP – field free point;

FFL – field free line – up to 10x higher signal to noise. 

My research

Funded by NIH, my current research focuses on the instrumentation, applications, and imaging methods development for a sensitive and power-efficient single-sided MPI scanner based on FFL. The two types of such scanners have either all electromagnetic coils or a combination of a permanent magnet structure and electromagnetic coils. 
My other interests are experimental and theoretical studies of the dynamics of super-paramagnetic nanoparticles.

Relevant Publications


Patent: US 10,261,141 B2 (issued 2019), EP 3374779 B1 (issued 2021)  - A. Tonyushkin. “Apparatus and Methods for Spatial Encoding of FFL-Based MPI Devices”