The influence of Holmium-166 loaded microspheres on ADC measurements using DWI

Introduction: Diffusion-weighted imaging (DWI) is a powerful tool for the evaluation of radiation therapy since the apparent diffusion coefficient (ADC) provides information about tumor viability [1]. Problems may arise, however, when DWI is used in the presence of paramagnetic particles, e.g., to evaluate the effect of internal radiation therapy of liver tumors using Holmium-166 loaded microspheres (HoMS) [2]. In this case, the presence of HoMS invokes local non-linear gradients since the paramagnetic microspheres act like small dipoles. These local gradients add to the applied diffusion gradients but are not taken into account in the calculation of the ADC. For nanometer sized iron oxide particles, for which T2 signal decay is known to be strongly influenced by diffusion through local gradients, interference between local and applied gradients has been shown to result in an underestimation of ADC values, dependent on the concentration of the paramagnetic agent [3]. In previous work, we have shown that diffusion through local gradients is not only effective for nanometer particles, but also affects T2 (spin echo) decay for micrometer-sized particles [4]. Therefore it may be expected that ADC values will also be sensitive to the presence of micronsized particles. In this work we aim to test this expectation and provide a basis to the interpretation of DWI in the presence of micrometer particles. This will be done by measuring the ADC of agarose gels containing various concentrations of HoMS, using a pulsed-gradient spin echo (PGSE) sequence with b-values ranging from 0–600ms/mm .