Quantification of holmium‐166 loaded microspheres: Estimating high local concentrations using a conventional multiple gradient echo sequence with S0‐fitting

Abstract

To provide a best estimate of the R 2* value and hence of the local concentration of highly paramagnetic holmium-166 loaded microspheres (HoMS) in voxels for which R 2* cannot be characterized by conventional fitting of multigradient echo (MGE) data because of fast signal decay due to high local concentrations. A postprocessing method, S(0)-fitting, was implemented in a conventional R 2* fitting method that is used for quantification of HoMS. S(0)-fitting incorporates the estimated initial amplitude of the free induction decay (FID) curve, S(0), of neighboring voxels into the fitting procedure for voxels for which the conventional algorithm failed. The method was applied to HoMS in vitro and ex vivo in a rabbit liver. The performance of the S(0)-fitting method was evaluated by comparing results qualitatively and quantitatively with results obtained with quantitative ultrashort TE imaging (qUTE). Applying S(0)-fitting provided a best estimate for R 2* up to a value of about 2300 s(-1) compared with a maximum value of about 1000 s(-1) that could be characterized using conventional fitting. A good agreement was observed both qualitatively and quantitatively for in vitro experiments as well as for ex vivo rabbit liver experiments between results obtained with S(0)-fitting and results obtained with qUTE imaging. S(0)-fitting is a postprocessing method that can provide a best estimate of high R 2* values that cannot be characterized by conventional relaxometry. The method can be applied to conventional MGE datasets and was shown to be beneficial for quantification of high local concentrations of holmium-loaded microspheres.