A 3D CT Vessel Roadmap Over Live Fluoroscopy for Chronic Total Occlusion

What is the first step in using this technology?
It has two aspects. First of all, CTA is the foundation. We are building on an emerging treatment paradigm for total occlusions that uses cardiac CTA as a primary planning tool. The final step, that of using an investigational tool to superimpose CTA images, evolved from something we are using every day, which is cardiac CTA. At Columbia, we mainly use the GE 64-slice LightSpeed VCT, which has a new SnapShot Pulse mode that cuts the radiation by a significant amount. Dr. Andrew Einstein from our institution recently published an article in JAMA¹ showing that dosimetry calculation for CTA is actually obsolete. With GE’s new technology, the radiation dose is down about 75%, which is an important message.^2-4
Using CT as our planning tool in CTO cases, we can then visualize the path of the artery. Even though there is no dye penetration, we can see the vessel calcification and tortuosity, and whether there is any distortion from a previous bypass graft, which can sometimes tent the arteries. Many of these arteries have had previous bypasses. The collaterals are also visible if retrograde techniques are an option. Mainly, however, CT analysis allows the assessment of calcification and the tortuosity. When you look at the contemporary reasons for failure to cross a CTO, the most contemporary analyses cite calcification and tortuosity as the main predictors of failure. With CT technology, the complete vessel is visible, which is key.
Simply performing a regular cardiac CT isn’t enough to provide an accurate analysis. Some image tweaking is necessary. It is important to notify the person who is actually analyzing the CT so they know to specifically visualize the necessary segment, because there is work that will need to be done in optimization. In other words, all the information isn’t necessarily there just by sending the patient through the usual algorithm with any CT. CT is very popular, it’s very dispersed now, but still, some specialization is required and attention needs to be paid by the individual doing the exam to really tweak the CTA if possible. They need to optimize the views of the occluded segment, because it frequently has very little dye in it. So you can get a lot more information if you focus on that specifically and tell the operator who is acquiring the CT to do so. You can also pick your optimal view in which to work as well, off the CT. This means before you walk into the cath lab procedure room, you have a good idea of what views you will work in (generally at least two), and also that which best elongates the artery. This ability is actually accessible to anybody with a sophisticated coronary CT. Importantly, it all takes place before you get to the cath lab. We are doing investigational work with GE engineer Laurence Gavit, on an Advantage Windows workstation. The software CardIQ allows us to have a volume rendering of only the segment in which we are interested. Let’s say, for instance, that it is the right coronary artery (RCA). Then, with the modified workstation, we can take the CT analysis of the entire right coronary artery and overlay it over our fluoroscopy in the GE Innova room. The CT image of the RCA is displayed so that it is transparent over the fluoro. You can also get just an outline of the vessel, superimposed on the fluoroscopic vessel. If you iso-center it, just move the gantry and change your angle, and the CT angle will change also. The technology provides consistent overlay of the entire path of the vessel or any segment you wish. You may only want the occluded part of the vessel. Perhaps you may want what you can see on angio eliminated and only the overlaid CT segment visible. This is possible. You could also just have, for example, the angio-visible proximal vessel eliminated, as well as the entire distal vessel, even out to the part visualized by collaterals. You can visualize these in CT format as well. It gives you a lot of flexibility in guiding your wire, as well as other devices, through the total occlusion.

It seems that the CT overlay technology may allow for less use of contrast during fluoroscopy.

Yes. We always work with collateral visualization with CTOs. If there aren’t collaterals from the ipsilateral artery, we do a contralateral injection. This technology, by overlaying the entire vessel, may allow us to eliminate that second injection and minimize the dye for CTO patients, because we have the entire vessel already outlined on the fluoroscopy. CT overlay offers the potential of extending this technique, which can be very contrast-intense, and minimizing the contrast to a great degree. How much, we don’t know yet. We’ve only done a half-dozen cases so far, but I think that is a big part of its potential.

Could you describe the live case you did at TCT?
This case was a very complex right coronary artery with an occluded stent virtually all the way to the ostium. Since the stent actually extended somewhat into the aorta, it was even difficult to seat the guide catheter. Needless to say, because of the unusual angulation of the entry of the guide into the stented segment, it was difficult to get wires to penetrate. We used the retrograde technique, using transeptal collaterals from the left anterior descending artery. Advancing the wire retrograde into the stent and then dilating retrograde allowed us to clear a path that would permit us, finally, to enter antegrade with a wire and balloons. We used the overlay of the GE CT to guide our path and minimize the need for repeated injections of fluoroscopy. The outcome was superb.

Where does the retrograde technique stand in terms of how people are approaching CTOs today?
Well, if you go to Japan, it’s very commonly done and I would venture to guess that maybe a third of CTOs in the Japanese major centers are being done this way. In the United States, it is much less frequent. In my mind, it is a very important tool because it will increase your success rate an extra 10%. In other words, in order to get success rates in the mid-to-high 80’s to the low 90s, you have to have facility with the retrograde technique.

Are there any additional applications for the CT overlay technology beyond CTO usage?
If you think about it, you can apply it to anything. A very high percentage of patients coming into the lab already have CTs. So you could, for example, take the CTO overlay to any angioplasty you are doing, and just use the overlay to guide the wire, traversing the vessel, and not have to do repeated injections. The other applications where we are using this technology are not in coronaries. We are working on the ability to take these volume renderings and segment them out. We are then applying them to structural heart disease. There are two potential applications we are exploring: intramyocardial injections, i.e., stem cell work, and the potential for guiding transseptal catheterization by having the septum and the fossa ovale visualized and superimposed on the fluoroscopy.

How long have you been involved in the development of this technology?

We started this project about a year ago. We have the luxury and the good fortune of a pre-clinical laboratory at the Cardiovascular Research Foundation, the Skirball Center. We have the same Innova system at Skirball, so a great deal of the development work can be initiated there, and then we can move in to Columbia. This is a technology that I do believe will move forward rapidly. It is actually a hardware issue; there are no issues with patient safety or extra radiation exposure, so I think that after it is finalized in development, this technology will soon be dispersed widely in the cath labs.

References

1. Einstein AJ, Henzlova MJ, Rajagopalan S. Estimating risk of cancer associated with radiation exposure from 64-slice computed tomography coronary angiography. JAMA 2007 Jul 18;298(3):317-323.
2. Earls et al. Prospectively gated Axial Coronary CT Angiography: Improved Quality and Reduced Radiation Dose as Compared to Retrospectively Gated Helical Technique. Accepted in Oct 07 for publication in Radiology (pending publication).
3. Londt et al. Dose reduction of up to 89% while maintaining image quality in cardiovascular CT achieved with prospective ECG gating. Proceedings of the SPIE Mar 2007;6510:65102S.
4. Hsieh J, et al. Step-and-shoot data acquisition and reconstruction for cardiac x-ray computed tomography. J Med Phys Nov 2006:33(11):4236-4248.