Relaxation Effects in MPI

Relaxation Delays USPIO Magnetization

Ultra-small superparamagnetic iron oxide nanoparticle (USPIO) dipole moments align with the applied field in the scanner due to the magnetic torque created by the field. Under the adiabatic assumption, we assume that the USPIO magnetization instantaneously follows applied field. However, the orientation of a USPIO magnetic moment is also affected by thermal fluctuations and other factors such as viscous resistance. These competing forces cause relaxation effects - delays in USPIO magnetization in response to applied field.

Relaxation Blurs Image in Scanning Direction

Adiabatic x-space scanning blurs the SPIO density input according to the Langevin magnetization of the SPIOs. Relaxation effects further blur the image and create an asymmetrical shape to the PSF. This blurring effect occurs in the scanning direction, which results in non-identical PSFs for the two scanning directions.

Relaxation Effects Essential for X-Space Theory to Predict PSFs of Larger USPIOs

(a) The adiabatic x-space theory predicts a point spread function (PSF) which closely resembles an experimentally measured PSF for Chemicell particles, which have low relaxation times (<1 /mus). (b) For particles such as Resovist which have longer relaxation times (>2 /mus), the adiabatic x-space theory underestimates the FWHM of the PSF as compared to experimental data. (c) The non-adiabatic x-space theory predicts two different PSFs for Resovist particles, one for each scanning direction. The non-adiabatic theoretical PSFs are blurred in the scanning direction by relaxation, which agree well with experimentally measured PSFs of Resovist.