Designed Synthesis Of Ultrasmall Iron Oxide Nanoparticles Based T1-T2 Dual Mode MRI Contrast Agent For High Performance Vascular Imaging

The assessment of vascular anatomy and functions is critical for medical diagnosis of tumor and cerebrovascular disorders.As one of the noninvasive medical imaging methods,magnetic resonance imaging?MRI?has great advantages in soft tissue imaging,and contrast-enhanced magnetic resonance angiography?CE-MRA?and dynamic contrast enhanced MRI?DCE-MRI?have been developed to study the vascular structure and function.Compared with the clinically applied low-field MRI??3 T?,ultra-high field MRI??7 T?has higher signal-to-noise ratio and resolution for vascular imaging,but the sensitivity still needs to be improved by MRI contrast agents.Commonly used contrast agents in CE-MRA and DCE-MRI are low-molecular-weight gadolinium-based contrast agents?GBCAs?,which are often used as T1 contrast agent and brighten the images in T1-weighted MRI.The short circulation time and the weakening of T1 contrast effect under UHF limit the applications of GBCAs for vascular imaging.Superparamagnetic iron oxide nanoparticles?SPIONs?-based T2contrast agents,which darken the images in T2-weighted MRI,usually suffer from susceptibility artifacts that become more prominent under UHF.Thus,SPIONs are not suitable for vascular imaging under UHF.T1-T2 dual-mode MRI can provide complementary T1-weighted images and T2-weighted images to obtain precise diagnosis information.Therefore,the application of T1-T2 dual-mode contrast agents can improve the accuracy and sensitivity for vascular imaging under UHF.In this paper,we designed and synthesized ultra-small iron oxide nanoparticles based T1-T2 dual-mode contrast agent?P-UDIOC?and studied the vascular imaging performance of P-UDIOC.The morphology and crystal structure were characterized by transmission electron microscopy?TEM?,dynamic light scattering analyzer,Zeta potential analyzer and X-ray diffraction?XRD?.The surface modification was characterized by the change of particle size,Zeta potential and Fourier transform infrared spectrometer?FTIR?.Results indicated that the uniformly-sized and monodispersed ultra-small iron oxide nanoparticles have been successfully synthesized.The crystal structure was?-Fe₂O₃.The surface of nanoparticles was successfully modified with polyethylene glycol?PEG?and still monodispersed in aqueous media with high stability.The in vitro magnetic resonance imaging performance of P-UDIOC at UHF was investigated using a magnetic resonance scanner.T1-weighted MR image and T2-weighted MR image showed that P-UDIOC has both T1 contrast effect and T2contrast effect under UHF.The longitudinal relaxivity?r1?and transverse relaxivity?r2?of P-UDIOC is 1.37 m M^-1s-¹ and 7.25 m M^-1s^-1 respectively,resulting in a r2/r1 ratio of5.50.The moderate r2/r1 ratio indicated that P-UDIOC could be used as T1-T2dual-mode MRI contrast agent under UHF.Mice subcutaneous tumor model and orthotopic tumor model were constructed to investigate the in vivo imaging and tumor diagnostic ability of P-UDIOC under UHF.MRI results indicated that for these two different tumor models,P-UDIOC could improve the signal-to-noise ratio of tumor tissues for a long period after injection,thereby realizing good in vivo MRI contrast effect.In vitro cellular and in vivo animal experiments verified that P-UDIOC had good biosafety.The laser scanning confocal microscopy was used to investigate the uptake of P-UDIOC by cells.CCK-8 assay indicated that P-UDIOC had no significant cellular cytotoxicity.The heart,kidney,liver,lung,spleen,and brain tissues of healthy rats injected with P-UDIOC after 24 hours and 15 days were examined by H&E staining to further investigate the in vivo biosafety of P-UDIOC.Results showed that P-UDIOC was biocompatible.Biochemical analysis of serum samples further confirmed that P-UDIOC caused no obvious hepatotoxicity or renal toxicity.Next,CE-MRA was used to evaluate the vascular structure assessing ability of P-UDIOC under UHF.Results showed that after the injection of P-UDIOC,the MR signal of cerebral vessels was significantly enhanced and the resolution of artery and microvascular structure,even as small as?140?m,was improved compared with that before injection.The signal-to-noise ratio was improved significantly by P-UDIOC compared with GBCA and SPION.The blood half-life of P-UDIOC was about 1 h,which prolonged the imaging time window and facilitated the CE-MRA.The ability of P-UDIOC as a T1-T2 dual-mode contrast agent for the evaluation of tumor vascular permeability by T1-T2 dual-mode DCE-MRI under UHF was investigated and compared with GBCA and SPION.The results of simulation before in vivo animal experiments predicted that P-UDIOC could significantly produce changes in T1-weighted magnetic resonance signals?represented by M0?and T2-weighted magnetic resonance signals?represented by R2?,indicating that P-UDIOC had the potential for T1-T2 dual-mode DCE-MRI.The results of in vivo animal experiments were basically consistent with the results of in vitro simulation results.The results of semi-quantitative analysis showed that after P-UDIOC injection,both T1-weighted DCE-MRI and T2-weighted DCE-MRI could characterize the vascular permeability of tumor tissue.The results of T1-weighted DCE-MRI and T2-weighted DCE-MRI are relatively consistent,which could be cross verified with high accuracy and sensitivity.And the MRI results were in consistent with the CD34 labeled immunohistochemical staining results.However,GBCA could only be used for T1-weighted DCE-MRI,when SPION could only be used for T2-weighted DCE-MRI.In conclusion,the ultra-small iron oxide nanoparticles?P-UDIOC?was designed and synthesized in this paper,which could realize high-resolution CE-MRA and T1-T2dual-mode DCE-MRI under UHF.P-UDIOC had good in vivo and in vitro magnetic resonance imaging performance under UHF,which could improve the signal-to-noise ratio,prolong the time window of vascular imaging,resulting in high-resolution imaging of rat brain vascular structures.And P-UDIOC could be used to evaluate tumor vascular permeability through T1-T2 dual-mode DCE-MRI.The results had a high contrast to noise ratio,and cross-validation can be performed to improve the accuracy and sensitivity of tumor vascular permeability evaluation.The ultra-small iron oxide nanoparticles,which can be used as T1-T2 dual-mode contrast agent under UHF,provide a new strategy for the accurate diagnosis of diseases by UHF MRI.