Spectrum Analysis of Relaxation Times in Pulsed Magnetic Particle Imaging

Relaxation mechanism of magnetic particles is crucial in differentiating particles and estimating temperature and viscosity for diagnosis and treatment. The magnetization recovery process in field flat phase of pulsed excitation generates decay signals that can be fitted by a bi-exponential model. The relaxation time spectrum can be generated by using inverse Laplace transform. The identification of Néel and Brownian relaxation times was implemented through the experiments of exciting solidified synomag-D samples with different gelatin concentrations in a trapezoid waveform relaxometer. The sensitivity of these relaxation times on predicting viscosity and temperature were evaluated. By combining field free point with a homogeneous pulsed excitation, a digital phantom was scanned for the generation of the relaxation time maps. The Néel and Brownian relaxation maps can be used for the detection of the catheter, plaque, and tumor hyperthermia regions. This work demonstrates a separate estimation of Néel and Brownian relaxation times through spectrum analysis, highlighting their potential usage in multi-color and multi-contrast magnetic particle imaging.