Magnetic Particle Imaging

Magnetic Particle Imaging ( magnetic particle imaging, MPI ) is a method for determining the distribution of magnetic material in a volume. Unlike MRI, where the influence of the material is measured in the magnetic properties of the protons, while the magnetization of the magnetic material is directly detected. For the study of living things, this approach promises a high sensitivity and a high temporal resolution.

The magnetic material is typically composed of iron oxide particles having a diameter of few tens of nanometers. For the detection of the particles is applied to the examination region with a spatially homogeneous and temporally sinusoidal oscillating magnetic field ( Aussteuerungsfeld, engl. Drive field). The magnetic material is then driven periodically in the non- linear part of the magnetisation curve. This points to the magnetization and the higher frequency of their generated magnetic field harmonics of the Aussteuerungsfeldes. These can be determined from the signal of a receiver coil.

The picture illustrates the signal generation. The black curve here describes the magnetization of the iron oxide particles. Due to the small size of these particles is no hysteresis. This is also known as superparamagnetism. The red curve illustrates the Aussteuerungsfeld as a function of time. This brings the iron oxide particles with high frequency in magnetic saturation to make them equal to demagnetize again. The green curve in the center 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 Aussteuerungsfeldes. It can be thus written as a Fourier series with the fundamental frequency with the frequency of Aussteuerungsfeldes is identical. It follows that in view of the measuring signal can be used as a Fourier series, the higher harmonics, as a measure for the concentration of the iron oxide particles.

In order to achieve a spatial resolution is a time-constant gradient used (selection box, Eng. Selection field). This field has a zero ( field-free point) in all directions increases sharply from which the magnitude of the magnetization. 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 origin of the signal is thus limited to a small volume.

For forming an image, the region in which the oscillating field-free point over the object. To the object can be mechanically displaced. Advantageously, however, the region to move through additional homogeneous magnetic fields, which can also perform the function of modulation simultaneously. In the experiment, a spatial resolution of about 1 mm and a temporal resolution of 42 volumes per second could be achieved.

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