Preclinical CT imaging is probably most commonly used in studies of bone anatomy and disease and as an anatomical reference for functional (i.e. PET and SPECT) modalities. Adipose tissue and lung tissue have innate contrast for CT imaging that can also be leveraged for preclinical studies of obesity and pulmonary disease (Sasser et al., 2012; Davison et al., 2013). Most soft tissue anatomy has limited innate CT contrast. However, some studies of soft tissue anatomy can be done using CT contrast agents (Lusic and Grinstaff, 2013).
There are a variety of commercial CT contrast agents. ExitronTM nano agents and VisipaqueTM are used in small animal studies for liver and kidney contrast, respectively. ExitronTM nano 6000 and 12000 are alkaline earth metal nanoparticle agents that provide both liver and spleen CT contrast (Boll et al., 2011). VisipaqueTM is an iodixanol based CT contrast agent that is approved for clinical arteriography/angiography, though in rodents VisipaqueTM readily provides contrast in the kidneys and bladder (Heglund et al., 1995).
Contrast imaging, including liver contrast imaging using ExitronTM nano agents and kidney contrast imaging using VisipaqueTM, was demonstrated in mice and employing the Albira CT by Wathen et al. (2013). While mouse imaging techniques are established with a variety of agents (Rothe et al., 2015), there is less information available on contrast imaging for rats, which are up to 10-fold larger by weight. Here, we have identified basic methods to image kidney and liver systems in rats using two (ExitronTM and VisipaqueTM) commercially available CT contrast agents. These methods may be applied in future studies of liver and kidney disease using rat models.
Contrast Agents and Imaging
Rats were anesthetized using a mixture of isoflurane gas (3 % in O2 (v/v)) at a flow rate of 2 L/min for animal preparation and imaging. For liver contrast, three rats were injected via tail vein with a bolus injection of 250 μL ExitronTM nano 12000 (Miltenyi Biotec, Auburn, CA, USA) and imaged immediately after injection and 24 and 48 hours post injection. Albira CT acquisitions were made using the Albira “Good setting” (400 projections, 125 μm voxels) at high dose (400 μA) and high voltage (45 kVp).
For kidney imaging three rats were injected via tail vein with a single bolus injection of 750 μL of VisipaqueTM (GE Healthcare, USA) 270 mg/ml and imaged immediately after injection, and at intervals for three hours thereafter. Albira CT acquisitions were made using the Albira “Standard setting” (200 projections, 125 μm voxels) setting at high dose (400 μA) and high voltage (45 kVp).
All images were reconstructed with the Albira Reconstructor Software Advanced option at high resolution with a FBP algorithm. Images were processed in the PMOD (PMOD Technologies Ltd., Zurich, Switzerland) software and VolView (Kitware Inc.) software. 3D image presentation was performed according to the methods detailed by Wathen et al. (2012).
Preliminary studies (not shown) were made to identify a general working volume range of ExitronTM nano 12000 and VisipaqueTM for contrast imaging in rats. Not surprisingly, a greater working volume is required for rat imaging than for mouse imaging. At 250 μL ExitronTM nano 12000 provided clear contrast for the liver and spleen for at least 48 hours after the initial bolus injection (Figure 1). This protocol provides a foundation for applied liver contrast imaging in rats. Previously ExitronTM agents have been used in μCT imaging mouse liver metastasis models (Boll et al., 2013), and the protocol described here could feasibly be translatable to imaging of rat liver tumor models.