Oliver Woywode

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.

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.

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.

Setup and Validation of an MPI Signal Chain for a Drive Field Frequency of 150 kHz

In this paper, an imaging system is considered, which is part of an activity to test the feasibility of clinical (whole-body) magnetic particle imaging (MPI). Recent studies on nerve stimulation in humans indicate that drive field amplitudes have to be limited to lower values compared with preclinical MPI. Since nerve stimulation thresholds increase with frequency, the drive field frequency for the clinical demonstrator is set to 150 kHz. To date, available MPI systems usually apply drive fields at frequencies ranging from 1 to 25 kHz. We report on the technical feasibility of a signal chain setup for a drive field frequency of 150 kHz.

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.

Control of a hybrid filter topology for the reduction of high frequency harmonics

The Digital Chain is realized as an adaptive finite impulse response filter (FIR). FIR filters with the order of N consist of (N-1) time delay elements, adder and filter coefficients w(n). Based on the measured harmonics in the voltage of ZLoad the filter coefficients are adjusted using the Filtered x Least Mean Square (FxLMS) algorithm [2]. The FxLMS is a stochastic gradient descent method producing the least mean squares of the detected harmonics by updating the filter coefficients with the given formula: w(n +1) = w(n) + μe(n)x(n).

Concept for a Modular Class-D Amplifier for MPI Drive Field Coils

This paper studies the performance of a modular class-D switching amplifier to supply the drive field coils of a magnetic particle imaging scanner with a high quality sinusoidal voltage. While a class-A or class-AB amplifier is capable of delivering such a sinusoidal voltage, its low efficiency does not qualify for an economic solution. Therefore, a class-D amplifier has been used. The modular amplifier consists of commercially available class-D amplifier chips, which are connected in parallel, to achieve the required output power. This paper describes the simulation model and a hardware setup of the modular amplifier which consists of 10 chips connected in parallel.