Functional and Molecular Imaging Using Nanoparticle Contrast Agents for Dual-Energy Computed Tomography
X-ray computed tomography (CT) is one of the most useful diagnostic tools for clinicians, with widespread availability, fast scan times, and low cost. CT imaging can reveal a patient’s anatomy in exquisite detail and is extremely useful in the diagnosis of a wide variety of diseases. However, CT is currently limited to anatomical imaging due to the lack of appropriate contrast agents and imaging protocols that would allow for molecular imaging, so clinicians must instead rely on other modalities which are more expensive and less readily available. Dual energy CT, a relatively new technique in which two x-ray energies are used for a single scan, can provide valuable information about tissue material composition. This information can potentially be used for molecular imaging if it is coupled with appropriately-designed contrast agents.
This work aims to extend the use of CT into the molecular imaging realm by developing and testing nanoparticle contrast agents for use with dual energy CT. Several studies were carried out, each of which focused on a different application of using nanoparticle contrast agents together with dual energy CT for molecular imaging.
A commercial blood pool iodine contrast agent for pre-clinical CT (Exia-160) has been shown to accumulate in the myocardium and continue to enhance the myocardium after the contrast agent has been cleared from the bloodstream. It was hypothesized that this agent would not accumulate in infarcted myocardium, which would allow for specific identification of myocardial infarction by CT. Mice were injected with the contrast agent following myocardial infarction, and dual energy CT was used to identify the iodine within the myocardium and separate the iodine from the calcium in the neighboring ribs. Regions of myocardial infarction showed no enhancement on CT, while the healthy myocardium was highly enhanced. Size and position of myocardial infarction determined by dual energy CT were compared with the standard molecular imaging technique for measuring myocardial viability (SPECT). It was found that dual energy CT using this nanoparticle contrast agent reliably agreed with the gold standard molecular imaging method.
Molecular imaging for the improved detection and characterization of lung tumors was also explored through two different studies. The first study used both gold nanoparticles and iodine-containing liposomes together with dual energy CT in order to measure tumor vascular functional parameters, including tumor fractional blood volume and vascular permeability. These dual energy CT measurements were confirmed with ex vivo tissue analysis to demonstrate the validity and accuracy of the in vivo dual energy CT method. The second study used antibody-targeted gold nanoparticles to image EGFR-positive tumors. Two different types of antibodies were tested: a clinically used humanized anti-EGFR antibody, and a small llama-derived single domain anti-EGFR antibody. The single domain antibody showed improved blood half-life and reduced immune clearance compared to the full-sized antibody when attached to gold nanoparticles, but the higher affinity of the full-sized antibody led to much higher overall tumor accumulation. This antibody significantly increased the accumulation of gold nanoparticles in tumors expressing high levels of EGFR. Together, these two studies showed that dual energy CT and nanoparticle contrast agents can be used to measure a wide variety of tumor functional parameters, including tumor vascular density, vascular permeability, and receptor expression. All these parameters can be combined with the anatomical CT imaging to better characterize lung tumors and differentiate between benign and malignant lesions.
The use of dual energy CT for measuring tumor vascular permeability changes after gold nanoparticle-augmented radiation therapy was also demonstrated. Liposomal iodine was injected into mice following radiation therapy in order to measure vascular permeability. Dual energy CT was used to differentiate the signal of the liposomal iodine from the CT signal of the gold nanoparticles already in the tumor. Tumor permeability was measured with CT using multiple combinations of gold nanoparticles and radiation doses to find the optimal conditions for enhancing the effect of radiation therapy on the vasculature. These conditions were then used to increase the delivery of a liposomal chemotherapy agent to tumors. Tumors treated with the gold-augmented radiation therapy and liposomal drug showed significant growth delay compared to the other groups, confirming the predictions made in the dual energy CT imaging.
Finally, a protease-responsive contrast agent was developed for use with dual energy CT imaging. Clusters of gold nanoparticles cross-linked together by protease-sensitive peptides were injected into mice along with liposomal iodine. In the presence of tumor proteases, the clusters degraded and the concentration of gold within the tumor decreased. Clusters without the protease-sensitive peptide did not degrade and did not leave the tumors. The ratio of iodine to gold in each tumor was measured, and it was found that the ratio was significantly higher in mice injected with the degradable gold clusters compared to mice injected with non-degradable control clusters. This demonstrated the ability to use multiple contrast agents with dual energy CT for enzyme-specific ratiometric molecular imaging.
This work confirms that dual energy CT can be used along with multiple nanoparticle contrast agents for molecular imaging applications. With continued contrast agent development and further application of dual energy CT, these methods can potentially be applied clinically to improve the power of CT imaging and improve diagnosis of a wide variety of pathologies.
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