Magnetic Resonance Imaging (MRI) is a medical imaging modality that provides structural and functional information. To improve the contrast of MR images, contrast agents, such as Ultrasmall SuperParamagnetic Iron Oxide particles (USPIO), are more commonly being used. Quantifying and locating these nanoparticles is of high interest. An efficient technique consists in images analyzing of the default field inhomogeneities induced by USPIOs. In a previous work, we have introduced such a quantification framework. Here, we improve our approach by deriving from two methods: Wiener and CLS filtering. Both methods have been evaluated on realistic data demonstrating that CLS filtering gives very promising quantification results.