Catheter-based Techniques for Endovascular Local Drug Delivery

MRI in guiding and assessing intramyocardial therapy

Cardiovascular intervention, using MRI guidance, is challenging for clinical applications. Real-time imaging sequences with high spatial resolution are needed for monitoring intramyocardial delivery of drug, gene, or stem cell therapies. New generation MR scanners make local intramyocardial and vascular wall therapies feasible. Contrast-enhanced MRI is used for assessing myocardial ischemia, infarction, and scar tissue. Active (microcoils) and passive (T1 and T2* mechanisms) tracking methods have been used for visualization of endovascular catheters. Safety issues related to potential heating of endovascular devices is still a major obstacle for MRI-guided interventions. Fabrication of MRI-compatible interventional devices is limited. Noninvasive imaging strategies will be critical in defining spatial and temporal characteristics of angiogenesis and myocardial repair as well as in assessing the efficacy of new therapies in ischemic heart disease. MRI contrast media improve the capability of MRI by delineating the target and vascular tree. Labeling stem cells enables MRI to trace distribution, differentiation, and survival in myocardium and vascular wall. In the long term, MRI in guiding and assessing intramyocardial therapy may circumvent the limitations of peripherally administered cell therapy, X-ray angiography, and nuclear imaging. MRI represents a highly attractive discipline whose systematic development will foster the implementation of new cardiac and vascular therapies.

Imaging of vascular gene therapy

Gene therapy is an exciting frontier in medicine today. Many genes have been shown to be useful for treatment of various vascular diseases, including chronic cardiac and limb ischemia syndromes, vasculoproliferative disorder, hypercholesterolemia, atherosclerosis, thrombosis, and hypertension. Precise delivery of genes into target vessels, efficient transfer of genes into vascular cells of the target, and prompt assessment of gene expression over time are three challenging tasks for successful vascular gene therapy. Thus, in vivo imaging methods that can be used to monitor gene delivery and localize gene expression are needed. Modern imaging techniques provide an opportunity to monitor and direct vascular gene therapy. Radiologists play a key role not only in developing and mastering endovascular genetic interventions but also in assessing the success of vascular gene therapy and directing further refinement of vascular gene therapy technology. This article provides an overview of the current status of imaging of vascular gene therapy.

Development of an intravascular heating source using an MR imaging guidewire

PURPOSE

To develop a novel endovascular heating source using a magnetic resonance (MR) imaging guidewire (MRIG) to deliver controlled microwave energy into the target vessel for thermal enhancement of vascular gene transfection.

MATERIALS AND METHODS

A 0.032-inch MRIG was connected to a 2.45-GHz microwave generator. We 1) calculated the microwave power loss along the MRIG, 2) simulated the power distribution around the MRIG, 3) measured the temperature increase vs. input power with the MRIG, and 4) evaluated the thermal effect on the balloon-compressed/microwave-heated aorta of six living rabbits. In addition, during balloon inflation, we also simultaneously generated high-resolution MR images of the aortic wall.

RESULTS

The power loss was calculated to be 3.9 dB along the MRIG. The simulation-predicted power distribution pattern was cylindrically symmetric, analogous to the geometry of vessels. Under balloon compression, the vessel wall could be locally heated at 41 degrees C with no thermal damage apparent on histology.

CONCLUSION

This study demonstrates the possibility of using the MRIG as a multifunctional device, not only as a receiver antenna to generate intravascular high-resolution MR images of atherosclerotic plaques and as a conventional guidewire to guide endovascular interventions during MR imaging, but also as a potential intravascular heating source to produce local heat for thermal enhancement of vascular gene transfection.

Magnetic resonance imaging permits in vivo monitoring of catheter-based vascular gene delivery

BACKGROUND

Gene therapy is an exciting frontier in modern medicine. To date, most investigations about the imaging of gene therapy have primarily focused on noncardiovascular systems, and no in vivo imaging modalities are currently available for monitoring vascular gene therapy. The purpose of this study was to develop an in vivo imaging tool to monitor a catheter-based vascular gene delivery procedure.

METHODS AND RESULTS

We produced gadolinium/blue dye and gadolinium/gene-vector media by mixing Magnevist with a trypan-blue or a lentiviral vector carrying a green fluorescent protein (GFP) gene. The gadolinium was used as an imaging marker for magnetic resonance (MR) imaging to visualize vessel wall enhancement, and the blue dye/GFP was used as a tissue stain marker for histology/immunohistochemistry to confirm the success of the transfer. Using Remedy gene delivery catheters, we transferred the gadolinium/blue dye (n=8) or gadolinium/GFP lentivirus (n=4) into the arteries of 12 pigs, that were monitored under high-resolution MR imaging. The results showed, in all 12 pigs, the gadolinium enhancement of the target vessel walls on MR imaging and the blue/GFP staining of the target vessel tissues with histology/immunohistochemistry. This study shows the potential of using MR imaging to dynamically visualize (1) where the gadolinium/genes are delivered; (2) how the target portion is marked; and (3) whether the gene transfer procedure causes complications.

CONCLUSIONS

We present a technical development that uses high-resolution MR imaging as an in vivo imaging tool to monitor catheter-based vascular gene delivery.