Ingo Schmale

Fundamentals and applications of magnetic particle imaging

Magnetic particle imaging (MPI) is a new medical imaging technique which performs a direct measurement of magnetic nanoparticles, also known as superparamagnetic iron oxide. MPI can acquire quantitative images of the local distribution of the magnetic material with high spatial and temporal resolution. Its sensitivity is well above that of other methods used for the detection and quantification of magnetic materials, for example, magnetic resonance imaging. On the basis of an intravenous injection of magnetic particles, MPI has the potential to play an important role in medical application areas such as cardiovascular, oncology, and also in exploratory fields such as cell labeling and tracking. Here, we present an introduction to the basic function principle of MPI, together with an estimation of the spatial resolution and the detection limit. Furthermore, the above-mentioned medical applications are discussed with respect to an applicability of MPI.

Perspectives on clinical magnetic particle imaging

Abstract After realizing the worlds’ first preclinical magnetic particle imaging (MPI) demonstrator, Philips is now realizing the worlds’ first whole-body clinical prototype to prove the feasibility of MPI for clinical imaging. After a brief introduction of the basic MPI imaging process, this contribution presents an overview on the determining factors for key properties, i.e., spatial resolution, acquisition speed, sensitivity, and quantitativeness, and how these properties are influenced by scaling up from preclinical to clinical instrumentation. Furthermore, it is discussed how this scale up affects the physiological compatibility of the method as well as hardware parameters such as power requirements for drive field generation, selection and focus field generation, and the design of the receive chain of the MPI device.

Human PNS and SAR study in the frequency range from 24 to 162 kHz

In order to identify suitable operating conditions for future clinical Magnetic Particle Imaging, peripheral nerve stimulation (PNS) and specific absorption rate (SAR) experiments have been performed by exposing volunteers to sinusoidally time-varying magnetic fields along and transverse to the body axis at frequencies from 24 kHz to 162 kHz. The findings show that future clinical MPI can advantageously be performed at elevated drive-field frequencies, with PNS restriction actually relaxed at higher frequencies, and with still acceptable SAR exposure.

MPI Safety in the View of MRI Safety Standards

In assessing the safety aspect of future clinical magnetic particle imaging (MPI), this novel imaging technique can refer to expertise that is cumulated in the IEC standard for magnetic resonance imaging (MRI) safety. Both imaging techniques employ strong dynamic magnetic fields and therefore have to take caution to refrain from physiological effects such as peripheral nerve stimulation (PNS) or excessive tissue heating. This paper starts with an outline of the differences between MPI and MRI. Then, the basics of PNS and tissue heating are reviewed and applied to the specific MPI case. Finally, sequences for MPI are presented that will allow rapid MPI imaging at 150 kHz while being safe for the patient.

Continuous Focus Field Variation for Extending the Imaging Range in 3D MPI

The imaging volume that is rapidly encoded by drive fields in 3D magnetic particle imaging is limited by power dissipation and nerve stimulation thresholds. Additional coils have been implemented to generate so-called focus fields that operate at lower frequencies and extend the accessible imaging range. This contribution presents the possibility of sweeping the rapidly encoded imaging volume along an arbitrary 3D path using continuous focus field variations. This technique can be useful for following a tracer bolus, for tracking devices, or for dynamically moving the image focus to different regions of interest.

Point Spread Function Analysis of Magnetic Particles

Starting from the basic principles of a Magnetic Particle Spectrometer (MPS), this paper explains the benefits and limitations of conventional spectral representation of the magnetization behavior of magnetic particles. After motivating the advantages of direct m(H) representation for particle analysis, it is shown how this curve, or at least its derivative, which is related to the point spread function, can be derived from the measured data. To illustrate, MPS results for Resovist® are presented, that show experimental evidence of hysteresis in dynamically acquired m(H) curves.

Fast continuous motion of the field of view in magnetic particle imaging

When shifting the FOV during imaging, artifacts arise when the shift per volume encoding time is larger than the resolution. Up to shift velocities of about 1 m/s, these can be removed by compensating the system function for the rapid translation. Fast continuous FOV shifts may be used to rapidly steer a single imaging volume to a region of interest or to achieve large spatial coverage by repeatedly sweeping the FOV through a volume of interest.

3D line imaging on a clinical magnetic particle imaging demonstrator

A clinical MPI demonstrator system is being built [1] that will enable fast 3D imaging with 3D drive field excitation and rapid 3D focus fields. To date, all components have been realized only once, i.e., a 1D drive field coil (x direction), a 1D fast focus field (y direction), and a 1D selection and focus field (z direction). To test the respective components, a 3D image is acquired using a linear drive field trajectory.

Increased volume coverage in 3D magnetic particle imaging

Magnetic particle imaging (MPI) is a new tomographic imaging approach that detects and localizes magnetic nano-particles by their non-linear magnetization response to externally applied fields. MPI allows quantitative, sensitive, and rapid volumetric imaging of distributions of particles injected into the blood stream. Initial experiments showing 3D real-time in-vivo imaging of mice were conducted using small imaging volumes covering a single organ. In view of scaling up the hardware for future clinical imaging, the imaging volume has to be increased. This contribution describes the basics of particle detection and spatial encoding in MPI, limitations to the imaging volume, and one approach to circumvent these limitations. Experimental results with increased volume coverage are presented.

On the design of human-size MPI drive-field generators using RF Litz wires

This paper presents the design of the first human-size drive-field generator for Magnetic Particle Imaging. By deriving equations to calculate proximity losses, it motivates the usage of RF Litz wires.