MPI performance of magnetic nanoparticles depends on matrix composition and temperature: implications for in vivo MPI signal amplitude, spatial resolution, and tracer quantification.

One of the hallmark advantages of magnetic particle imaging (MPI) is the linear relationship between MPI signal and the concentration of magnetic nanoparticles (MNPs), allowing absolute tracer quantification. However, intrinsic tissue matrix parameters may affect the MPI signal, often unknown a priori, presenting a challenge for accurate in vivo MNP quantification in living subjects when using standard calibration curves obtained from simple aqueous MNP suspensions. We investigated the effects of matrix composition and temperature on the MPI signal amplitude and full width at half maximum (FWHM, metric for spatial resolution) for three different MNPs in gelatin and bovine serum albumin (BSA) phantoms and five different ex vivo tissues. Decreasing matrix compressibility (increasing viscosity) led to decreased MPI signal amplitude and increased FWHM. For 8% w/v gelatin (compressibility = 3.5 × 10(-10) m(2) N(-1), viscosity = 363.4 × 10(3) mPa s), the MPI signal amplitude of MNPs was ∼50% of that in aqueous solutions (compressibility = 8.4 × 10(-10) m(2) N(-1), viscosity = 1 mPa s), while the FWHM increased by an average of 115%. For 5% w/v BSA samples (compressibility = 1.2 × 10(-10) m(2) N(-1), viscosity = 198.6 × 10(3) mPa s), a 44% MPI signal reduction and 98 and 90% increase of FWHM was observed for gold-iron oxide nanoflowers and ferucarbotran, respectively, compared to water (0% w/v). MNPs injected in ex vivo tissues also showed lower MPI signal amplitudes compared to aqueous solutions. Temperature also played a small role in MPI quantification, with the MPI signal amplitude of ferucarbotran decreasing by nearly 10% from 55 to 10 °C. The current results suggest that accurate in vivo MNP quantification will require reference/calibration samples with matching tissue matrix composition and temperature.