Multi-echo spin-echo (MESE) signal behavior of paramagnetic Holmium-166 loaded microspheres for radiotherapy: experiment and simulation

Introduction: Internal radiation therapy using Holmium-166 loaded microspheres (HoMS) has shown to be a promising technique for the treatment of liver metastases [1]. Due to their paramagnetic character, HoMS cause additional T2* decay which can be used to quantify the amount of microspheres in the liver using gradient echo sequences [2]. However, for large concentrations of microspheres the signal decays too fast to be accurately sampled on a clinical MR scanner. An alternative approach can be provided by using a Carr-Purcel-Meiboom-Gill multi-echo spin echo (MESE) sequence to recover the signal for longer times. In this work we aim to explore the potential of MESE for quantification of HoMS, especially for high concentrations. To achieve this we will investigate the signal decay due to HoMS by experiments using HoMS suspended in aqueous solutions and by Monte Carlo simulations. The results will be compared with the model developed by Jensen and Chandra [3] in which they argue that additional signal decay due to the diffusion of protons through magnetic field inhomogeneities induced by paramagnetic objects in a MESE experiment can be modeled as S(t) = exp[-αt] assuming Strong Field Behavior. Here, α depends on particle size r, volume fraction f, susceptibility difference Δχ, diffusion coefficient D, field strength B0 and echo spacing Δt. Since in this model α is directly proportional to the volume fraction of the particles, it would provide a tool for the quantification of HoMS.