Detection and quantification of Holmium-166 loaded microspheres in patients with liver metastases: initial experience within the framework of a phase I study

Introduction: Internal radiation therapy using radioactive Holmium-166 loaded microspheres (HoMS) has shown to be a promising technique for treatment of liver metastases [1]. Unlike Yttrium-90 microspheres, which lack imaging capabilities, HoMS can be detected by Single Photon Emission Computed Tomography (SPECT) using the Holmium-166 photopeak and by MRI using the paramagnetic properties of Holmium. Due to their paramagnetic character, the microspheres induce local field inhomogeneities which cause MR signal dephasing and therefore additional T2 decay [2]. The T2 relaxation time or its inverse, the R2 relaxation rate, can be used to quantify the amount of HoMS [3, 4]. In this paper we report the first results of the detection and quantification of HoMS in patients with liver metastases included in a phase I clinical study [5]. We will demonstrate a) that accurate HoMS concentration maps can be estimated from T2 weighted data, b) that the resulting concentrations maps show good correspondence to activity maps acquired with SPECT and c) that the total amount of HoMS determined using the MR-based concentration maps agrees well with the total amount of HoMS that was delivered.