# Calibration-Free Relaxation-Based Multi-Color Magnetic Particle Imaging

## TL;DR

This paper introduces a calibration-free method for multi-color magnetic particle imaging that estimates nanoparticle relaxation times directly from the signal, enabling distinction of different nanoparticles without prior calibration.

## Contribution

The paper presents a novel relaxation time estimation technique based on mirror symmetry, eliminating the need for extensive calibration in multi-color MPI.

## Key findings

- Successfully distinguishes nanoparticles without calibration.
- Validated method through simulations and experiments.
- Effective at multiple frequencies (550 Hz and 9.7 kHz).

## Abstract

Magnetic Particle Imaging (MPI) is a novel imaging modality with important applications such as angiography, stem cell tracking, and cancer imaging. Recently, there have been efforts to increase the functionality of MPI via multi-color imaging methods that can distinguish the responses of different nanoparticles, or nanoparticles in different environmental conditions. The proposed techniques typically rely on extensive calibrations that capture the differences in the harmonic responses of the nanoparticles. In this work, we propose a method to directly estimate the relaxation time constant of the nanoparticles from the MPI signal, which is then used to generate a multi-color relaxation map. The technique is based on the underlying mirror symmetry of the adiabatic MPI signal when the same region is scanned back and forth. We validate the proposed method via extensive simulations, and via experiments on our in-house Magnetic Particle Spectrometer (MPS) setup at 550 Hz and our in-house MPI scanner at 9.7 kHz. Our results show that nanoparticles can be successfully distinguished with the proposed technique, without any calibration or prior knowledge about the nanoparticles.