**3. Nanomedicine in Liver Fibrosis Diagnosis**

For liver diseases, early detection of liver fibrosis would be helpful for treatment. Unfortunately, most cases of liver disease are diagnosed late because of no symptoms. Current strategies for the diagnosis of liver fibrosis rely on an invasive biopsy which would cause damage for the patients [53]. Recently, magnetic resonance imaging (MRI) has been developed as a method with high diagnostic accuracy for the detection of fibrosis [54]. Magnetic NPs play an important role in the diagnosis and imaging of liver fibrosis [55]. For example, dextran stabilized superparamagnetic iron oxide NPs (D-SPIONs) with high blood compatibility and low cytotoxicity was used as an MRI contrast agent for liver fibrosis detection [56]. D-SPIONs enhanced image contrast of tissue and led to a 55% decrease in the pixel intensity, and therefore improved the contrast difference between the fibrotic tissue and the rest of the extracellular matrix rich hepatic parenchyma at the fibrosis stage significantly. Citrate-coated ultrasmall iron oxide NPs were also shown to provide a good MRI of liver fibrosis [57]. In one study, Fe3O4 NPs coated with SiO2 and then coupled with indocyanine green (ICG) and arginine–glycine–aspartic acid (SPIO@SiO2–ICG–RGD) were constructed for HSC targeting and early detection of liver fibrosis (Figure 2) [18]. Fe3O4 NPs and ICG as the photographic developers for T2 MRI and near-infrared (NIR) imaging, respectively. NIR fluorescence (NIR) and MRI revealed that SPIO@SiO2–ICG–RGD could elicit accurate identification of fibrotic regions in the liver. These NIR hybrid NPs combined imaging and MRI and provided higher sensitivity and spatial resolution for liver fibrosis detection, compared with MRI alone.

**Figure 2.** In vivo optical imaging and MRI of liver fibrosis using SPIO@SiO2–ICG–RGD. (**A**,**C**) A model of hepatic fibrosis in mice. (**B**,**D**) Healthy mouse model (control). Adapted with permission from [18]. Copyright RSC publishing, 2018.

Furthermore, zero-valent iron (ZVI)-based NPs were also fabricated as novel contrast agents for MRI. After functionalized with liver specific polysaccharide pullulan and fluorescent carbon dots, a dual imaging contrast agent (P@ZVI-Cdts) was obtained. The efficiency of the developed systems for targeted liver imaging and optical imaging has been successfully demonstrated in vivo. The high r1 relaxivity enables ZVI NPs to be a competent T1 MRI contrast agent for various clinical applications including diagnosis of liver fibrosis [58].

In addition to MRI, the combination of an ultrasound agent with a targeting peptide has been reported for the early and non-invasive diagnosis of liver fibrosis. One study found that core–shell perfluorooctyl bromide (PFOB) coated with poly(lactic-co-glycolic acid) (PLGA) polymers and modified with a cyclic RGD (cRGD–PLGA–PFOB NPs) exhibited powerful ultrasound molecular imaging features, including high-contrast imaging among liver fibrotic stages and adjacent tissues [59].
