Micro-CT based experimental liver imaging using a nanoparticulate contrast agent: a longitudinal study in mice

Abstract

Background: Micro-CT imaging of liver disease in mice relies on high soft tissue contrast to detect small lesions like liver metastases. Purpose of this study was to characterize the localization and time course of contrast enhancement of a nanoparticular alkaline earth metal-based contrast agent (VISCOVER ExiTron nano) developed for small animal liver CT imaging.

Methodology: ExiTron nano 6000 and ExiTron nano 12000, formulated for liver/spleen imaging and angiography, respectively, were intravenously injected in C57BL/6J-mice. The distribution and time course of contrast enhancement were analysed by repeated micro-CT up to 6 months. Finally, mice developing liver metastases after intrasplenic injection of colon carcinoma cells underwent longitudinal micro-CT imaging after a single injection of ExiTron nano.

Principal findings: After a single injection of ExiTron nano the contrast of liver and spleen peaked after 4-8 hours, lasted up to several months and was tolerated well by all mice. In addition, strong contrast enhancement of abdominal and mediastinal lymph nodes and the adrenal glands was observed. Within the first two hours after injection, particularly ExiTron nano 12000 provided pronounced contrast for imaging of vascular structures. ExiTron nano facilitated detection of liver metastases and provided sufficient contrast for longitudinal observation of tumor development over weeks.

Conclusions: The nanoparticulate contrast agents ExiTron nano 6000 and 12000 provide strong contrast of the liver, spleen, lymph nodes and adrenal glands up to weeks, hereby allowing longitudinal monitoring of pathological processes of these organs in small animals, with ExiTron nano 12000 being particularly optimized for angiography due to its very high initial vessel contrast.

Figure 1. Time course of contrast enhancement within the vascular system and the liver of C57BL/6J mice (n = 3 per group) after a single i.v. injection of 100 µl ExiTron nano 6000 or ExiTron nano 12000.

Measurements were performed by placing a ROI within the left ventricle (vessel contrast) and within the liver avoiding large intrahepatic vessels. The baseline level ( = 100%) refers to measurement of the relative density of the liver and the vascular system prior to administration of contrast agent.

Figure 2. A shows a volume rendering of a mouse 30 minutes after i.v. injection of ExiTron nano 12000.

B is a curved maximum intensity projection in coronal orientation of the same scan. A and B demonstrate the feasibility to perform CT angiography during the early intravascular phase of the tested contrast agent. Additionally, A and B show the early contrast agent uptake by the RES with increasing contrast of liver and spleen. C is a coronally oriented curved maximum intensity projection of a mouse that did not receive contrast agent.

Figure 3. A and B show intrasplenic (*) and intrahepatic (LMet) growing tumors 26 days after intrasplenic injection of C15A3 colon tumor cells.

A and B were acquired 4 hours after i.v. injection of 100 µl ExiTron nano 12000. B, C, and D illustrate contrast enhancement of the abdominal and mediastinal lymph nodes (LN) and of the adrenal glands (AdrG). C was acquired 4 hours after i.v. injection of 100 µl ExiTron nano 12000; D was acquired 22 days after i.v. injection of 100 µl ExiTron nano 12000. Micro-CT scanning parameters: 40 sec scan time; 190° rotation; 1200 projections; voxel size 41×41×55 µm³.

Figure 4. A and B show a partial diaphragmatic herniation of the left upper liver lobe in coronally (A) and sagittally (B) reconstructed maximum intensity projections of a C57BL/6J mouse 22 hours after i.v. injection of 100 µl ExiTron nano 12000.

The herniated liver tissue can be easily delimited from the adjacent heart due to the positive liver contrast. C and D are micro-CT scans of the murine liver before (C) and 24 hours after (D) intravenous administration of 100 µl ExiTron nano 6000.

Figure 5. Repeated micro-CT of a mouse illustrates development of liver metastases 9, 12, 14, and 19 days after intrasplenic injection of MC38 colon tumor cells.

Smallest detectable liver metastases (arrows) measured 300 µm in diameter.