A Spectrometer Using Oscillating and Static Fields to Measure the Suitability of Super-Paramagnetic Nanoparticles for Magnetic Particle Imaging

Magnetic particle imaging (MPI) is a new tomographic imaging modality to measure the spatial distribution of super-paramagnetic iron oxide nanoparticles (SPIOs). MPI applies a sinusoidal oscillating magnetic field to the SPIOs. Due to their nonlinear magnetization behaviour, the magnetization response contains harmonics of the excitation field frequency. The temporal changes of the magnetization response can be measured by receive coils. By superimposing the excitation field with a static gradient field, spatial encoding is achieved. The SPIOs currently used in MPI have been originally designed for magnetic resonance imaging (MRI) and not for MPI. Thus, there was no aim during particle syntheses to fulfil particular requirements for MPI. It is assumed that optimal SPIOs could increase the sensitivity of MPI by more than a factor of 100. To measure the SPIO performance during the syntheses process, a magnetic particle spectrometer (MPS) is an essential monitoring tool. MPS so far presented, consist of a homogeneous oscillating excitation field. This allows only for analysing the magnetization response of particles at the so-named field-free-point (FFP) of the field-of-view (FOV), where the gradient field is zero. In this contribution, an MPS is presented that additionally applies a static offset field, such that the magnetization response of particles outside the FFP can be measured as well. This enables a new dimension of particle analysis. The presented MPS consists of a Helmholtz coil pair for generating the oscillating and the static field. Both can be set to a maximum field amplitude of 40 mT allowing for covering a large range of the SPIO magnetization curve. In figure 1, the measured spectral magnetic moment of the MRI contrast agent Resovist® is shown for an oscillating field amplitude sweep between 1 and 20 mT. At the left, no offset field and at the right an offset field of 5 mT is applied. It can be seen that without offset, only odd harmonics are intensified, while when using an offset field, also the even harmonics rise, which improves the sensitivity of the spectrometer for tracer analysis.