Fig. 1. PbS quantum dots (QDs) as a contrast agent for intravascular OCT and second-near infrared (NIRF) dual-modal imaging^[23]. (a) Transmission electron microscope (TEM) image and size distribution of PbS QDs; (b) absorption and emission spectra of PbS QDs; (c) PbS QDs for OCT and NIRF dual-modal imaging of rabbit artery (Reproduced with permission, Copyright 2017, Wiley-VCH)

Fig. 2. In vivo cell migration tracking by CdS nanowire laser^[27]. (a) In vivo fluorescence images of macrophage migration projections through multiple observations; (b) top; (c) side, 3D OCT retinal layer reconstruction and the spatial locations of macrophages; (d) laser spectra of macrophages (correspond to position 1, 2, 3 in Fig. (a)) migration tracking over time; (e) 3D macrophages migration over 3 days. Scale bar: 100 μm (Reproduced with permission, Copyright 2020, The Optical Society of America)

Fig. 3. Chain-like gold nanoparticle clusters (CGNP) for OCT and PA dual-modal imaging of choroidal neovascularization in rabbit eyes^[34]. (a) TEM image of CGNP; (b) absorption spectra of CGNP and gold nanoparticles; PA images of vascular and neovascularization (c) before and (d) after injection of CGNP-RGD; OCT images of vascular and neovascularization (e) before and (f) after injection of CGNP-RGD (Reproduced under the terms of the CC-BY-NC license, Copyright 2021, Nature Publishing Group)

Fig. 4. Magnetic NPs (MNPs) for (a) MM-OCT and (b) OCT of mammary tumor model. I‒III correspond to rabbit mammary tumor after intravenous injecting with targeted-MNPs, MNPs and saline^[40]

Fig. 5. Multi-modal imaging of microspheres containing MNPs and dye^[41]. (a) TEM and scanning electron microscope (SEM) images of microspheres; (b) MRI images of microspheres in liver; (c) ultrasound images, (d) MM-OCT and (e) FI images of microspheres in tumor

Table 1. Summary of multi-modal contrast agents