Spectral CT, an emerging technique that images using X-rays of more than one energy, can differentiate multiple tissue types or contrast agents within the body. At the SPIE Medical Imaging conference, held last week in Lake Buena Vista, FL, Cristian Badea from Duke University Medical Center (Durham, NC) described the application of dual-energy microCT to simultaneously image iodine and gold probes in vivo.
Badea and colleagues examined two nanoprobes: liposomal iodine (Lip-I) and gold nanoparticles. The Lip-I probes comprised an aqueous iohexol core surrounded by a lipid bilayer and PEGylated to ensure retention in the blood. The gold probe was AuroVist, a commercial contrast agent comprising 15 nm nanoparticles. Both probes are cleared from the body via the liver and spleen.
As gold and iodine exhibit different attenuation coefficient profiles, with K-edges at different energies (81 and 33 keV, respectively), it should be possible to distinguish the two by comparing X-ray images at different energies. To determine the optimal imaging parameters, Badea's team built a dual-energy microCT system, containing two X-ray tubes and two detectors.
First, the researchers used the Spektr computational tool to model the microCT system's performance, using a phantom containing varying volumes of the two probes and imaging at voltages from 40 to 150 kVp. Plots of contrast versus voltage revealed that at 40 kVp, signals from regions containing gold nanoparticles were brightest, while iodine provided maximum enhancement at around 70 and 80 kVp.
They then performed phantom experiments to measure CT enhancement for the probes across the same voltage range. The measured images agreed with the simulated results, showing maximum CT enhancement at 40 kVp for the gold nanoparticles and 80 kVp for the Lip-I. On a mass-concentration basis, the relative average enhancement of gold to iodine was 2.75 at 40 kVp and 1.58 at 80 kVp.
The team also used the phantom images to determine the limits of detectability of the two probes. Calculating the contrast-to-noise ratio at different concentrations revealed a limit of 20 mg/ml for the Lip-I probes and 10 mg/ml for gold nanoparticles. The use of averaging to reduce the noise improved these values to 15 and 6 mg/ml for Lip-I, and gold respectively.
In vivo assessment
The researchers also used the dual-energy microCT system to perform in vivo imaging of mice with colon cancer tumours. Mice were injected with 0.1 ml gold nanoparticles (with a gold concentration of 200 mg/ml) and then imaged 72 hours later at 40 and 80 kVp. The mice were then injected with 0.4 ml of the Lip-I probe and rescanned.
Maximum intensity projection images showed that Lip-I probes visualized the blood vessels, while gold probes revealed the liver, spleen and some enhancement into the tumour due to extravasation of the nanoparticles.
"This dual-energy microCT provides a way to image 3D vasculature and permeability in tumours," concluded Badea, associate professor of radiology at Duke's Center for In Vivo Microscopy. "These are both important biomarkers in cancer studies."