Magnetic Particle Imaging

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MPI signal generation.

Magnetic particle imaging ( magnetic particle imaging , MPI) is a method for determining the distribution of magnetic material in a volume. In contrast to MRI , where the influence of the material itself (magnetic resonance properties of protons) is measured, magnetic material is introduced and its magnetization is used to determine its location and concentration.

For the investigation of living beings, the method promises great sensitivity and high temporal resolution, but is still in the experimental stage. The material is brought into the bloodstream and its spread can be observed. The method is therefore just as suitable as z. B. PET to visualize biological processes, but is much faster and does not cause any radiation exposure.

The magnetic material typically consists of iron oxide particles with a diameter of a few tens of nanometers. To detect the particles, a spatially homogeneous and temporally sinusoidal magnetic field ( drive field ) is applied to the examination region . The magnetic material is then periodically driven into the non-linear part of the magnetization curve . As a result, the magnetization and the magnetic field generated by it have higher-frequency harmonics of the modulation field . These can be determined from the signal from a receiving coil.

The picture illustrates the generation of signals. The black curve is the magnetization curve of the iron oxide particles. Due to the small dimensions of these particles, no hysteresis occurs. This is also known as superparamagnetism . The red curve illustrates the modulation field as a function of time. This brings the iron oxide particles into magnetic saturation at a high frequency in order to demagnetize them again immediately. The green curve in the middle of the graph illustrates the magnetization of the iron oxide particles as a function of time. This function is no longer purely sinusoidal. However, this function must be periodic with the frequency of the modulation field. It can therefore be written as a Fourier series , with the fundamental frequency being that of the modulation field. The harmonics are a measure of the concentration of the iron oxide particles.

In order to achieve spatial resolution, a gradient field that is constant over time is used ( selection field ). This field has a zero point (field-free point) from which the amount of magnetization increases sharply in all spatial directions. The magnetization of particles that are sufficiently far away from this point is saturated by the selection field and does not produce any harmonics. The generation of the signal is therefore limited to a small volume.

To generate an image, the field-free region in which any particles that may be present there generate harmonics is moved over the object. The object can be moved mechanically for this purpose. However, it is more advantageous to move this region by additional homogeneous magnetic fields, which can also fulfill the function of modulation at the same time. In the experiment, a spatial resolution of around 1 mm was achieved with a temporal resolution of 42 volumes per second.

Magnetic particle imaging was developed by the German physicists Bernhard Gleich and Jürgen Weizenecker and their team from the Philips Research Laboratory in Hamburg.

Individual evidence

  1. ^ A b Bernhard Gleich, Jürgen Weizenecker: Tomographic imaging using the nonlinear response of magnetic particles . In: Nature . tape 435 , no. 7046 , June 30, 2005, p. 1214–1217 , doi : 10.1038 / nature03808 .
  2. J. Weizenecker, J. Borgert, B. DC: A simulation study on the resolution and sensitivity of magnetic particle imaging . In: Physics in Medicine and Biology . tape 52 , no. 21 , November 7, 2007, pp. 6363-6374 , doi : 10.1088 / 0031-9155 / 52/21/001 .
  3. Preclinical magnetic particle imaging (MPI) scanner. Bruker , accessed May 5, 2017 .
  4. ^ B. Gleich, J. Weizenecker, J. Borgert: Experimental results on fast 2D-encoded magnetic particle imaging . In: Physics in Medicine and Biology . tape 53 , no. 6 , March 21, 2008, p. N81-N84 , doi : 10.1088 / 0031-9155 / 53/6 / N01 .
  5. J. Weizenecker, B. Gleich, J. Rahmer, H. Dahnke, J. Borgert: Three-dimensional real-time magnetic particle imaging . In: Physics in Medicine and Biology . tape 54 , no. 5 , March 7, 2009, p. L1 – L10 , doi : 10.1088 / 0031-9155 / 54/5 / L01 .
  6. ^ Bernhard Gleich and Jürgen Weizenecker and team (Germany). Winner of the European Inventor Award (sic!) 2016. European Patent Office , March 27, 2017, accessed on May 5, 2017 .