Advances in technology continue to expand the role of medical imaging beyond the walls of the radiology department. One technique that has seen particular growth in recent years is intraoperative imaging. Two new systems have recently been added to the surgical imaging armamentarium: one using infared technology to assess graft patency during coronary bypass surgery and the other using volume rendering to track surgical instruments within the surgical field.

Thermal coronary angiography

JAG Medical, Inc. (New York, NY) recently introduced the IRIS-III, a third-generation thermal coronary angiography (TCA) imaging system for use during open heart surgery. The system, which uses state-of-the-art infared technology, provides real-time images of the temperature differences between the blood vessels of the heart and the myocardium. It was designed to provide the surgeon with a noninvasive means of evaluating graft patency, anastomosis patency, and blood flow through native vessels following completion of coronary bypass graft (CABG) surgery.

"The system consists of a small mobile tower with a high-resolution camera, monochrome and color video monitors, and a separate control console," said Kym Secrist, vice president and director of marketing at JAG Medical. "The camera is designed to be positioned above the operative field during surgery. The control console contains an S-VHS videocassette recorder, controller, and a video printer."

"The IRIS-III contains both the thermal sensing array, as well as a charge-coupled device (CCD) panel for capturing visible images," he continued. "The thermal sensing array provides real-time images of the temperature differences between the blood vessels of the heart and the myocardium. These thermal images show the flow of blood through the exposed arteries without the need for ionizing radiation, such as an x-ray or angiography, or the use of contrast media."

According to the company, the infared-sensing array can sense temperature differences of <0.1šC.

With the present system, images can be saved and reviewed via the video recorder, and the frame can be printed on the video printer. The company is developing a digital version that will allow the images to be stored in a digital file and archived in a DICOM-3.0­compliant format.

Daniel Swistel, MD, chief of cardiothoracic surgery at St. Luke's/ Roosevelt Hospital Center in New York City, was the first U.S. surgeon to use this system. In an interview with Applied Radiology , he explained that this technology takes on added importance with the advent of "off-pump" coronary revascularization procedures, noting that questions have been raised about the accuracy of anastomotic construction performed on a beating heart.

"It is not possible with flow meters to get an absolute accurate assessment of flow," he said. "Depending on the vascular bed, flow through a vessel has a wide variation. Therefore, the surgeon can't really know if a graft is open just by measuring its flow. The infared camera gives you a real-time picture, not only of the graft, but of the anastomosis as well."

Swistel, who began using the system in February 2001, estimates that he now uses the IRIS-III in approximately 75% of his off-pump CABG procedures. "That is where I find it most useful," he said. "However, it is actually easier to use in 'on-pump' procedures due to how the system works. The camera works by measuring changes in temperature. In situations where the heart is being perfused by its normal blood supply constantly, it is hard to isolate the exact area you want to study from the surrounding blood supply. In an on-pump situation, there is no blood supply to the heart in general, so it is very easy to introduce any kind of flow into a graft and then see the contrast to the surrounding areas much more easily."

Swistel explained that the use of the IRIS-III does not negate the need for standard preoperative imaging studies. "It cannot replace a preoperative angiogram or the new high-speed computed tomography (CT) scan," he said. He did note that, in theory however, use of this technology could replace postoperative angiography. "This would probably function very well to document graft patency in the immediate postoperative period, which otherwise we cannot do unless we do a postoperative angiogram," he said. "But we don't do postoperative angiography routinely unless there are postoperative complications or if the procedure is part of a research protocol."

Although the IRIS-III is only indicated for use in CABG surgery currently, the manufacturer believes it has potential for other applications. "Right now, the indications for use are strictly for open heart surgery," said Secrist. "But, it is our opinion, and it will certainly be a focus of future research, that hospitals will also benefit from using this technology in transplant procedures, neurosurgery, and many other surgical applications in which differences in temperature exist among the various surfaces of the exposed anatomy. For example, we know that there is a distinct difference in temperature between a brain tumor and the brain tissue surrounding it. You may soon see an application using the IRIS-III whereby a neurosurgeon will be able to remove a brain tumor without destroying as much of the surrounding tissue."

Image Guidance

A second recently introduced surgical imaging system is the CBYON Suite by CBYON, Inc. (Palo Alto, CA). Ramin Shahidi, PhD, the company's founder and chief technology officer and director of the Image Guidance Laboratory of the Department of Neurosurgery at Stanford University (Palo Alto, CA) explained how the volumetric navigational system works.

"In a nutshell," he said, "we take the patient's imaging data, whether it is preoperative or intraoperative, and we correlate all the data. We then construct the data into a 3-dimensional format and superimpose it on the patient's physical anatomy. Then, during surgery, we can track surgical tools in surgical space, in a manner similar to the global positioning systems used in cars. Two cameras track indicators attached to the surgical tools, telling the surgeon exactly where the surgical tool is with respect to that patient's imaging data."

According to the company, the tracking capability of this system is accurate to approximately 1 mm at best and 6 mm at worst.

What distinguishes the CBYON Suite from other available image guidance systems, said Shahidi, is that in addition to orienting surgeons, this system shows them what lies ahead. "We are giving the surgeon the ability to look through the tissue and see where the tumor is ahead of the surgical tools," he said. "The principle is that the surgeons already know where they are. They want to know what is ahead of them."

"Three-dimensional imaging has been around for a long time," he said, "but people don't really use it effectively because there is no standard for image acquisition, visualization, and interpretation of this technology. In order for this technology to be used effectively, there should be a 3-dimensional imaging protocol from the acquisition, through the visualization, and during the navigation of how you will utilize it." The CBYON Suite, he said, has customized the software to show the surgeon only the structures that are important for that particular phase of the surgery. CBYON has developed imaging protocols for the applications to which they believe 3-dimensionality adds the most value, in particular endoscopy, vascular, and tumor resection, sinus, and spinal applications.

Shahidi believes the future of image guidance and intraoperative imaging includes the development of:

* Smart tools, such as scalpels and suction tools, that contain embedded miniaturized nanno-sensors designed to perform intraoperative imaging and analysis;

* Robotic automation as an extension of surgeons' hands to help surgeons perform surgery more accurately and confidently;

* Increased visualization in the form of enhanced endoscopy; and

* Correlation of all the information in a user-friendly format that will not overwhelm the surgeon during surgery.

Conclusion

Currently, surgeons control these intraoperative imaging techniques. But that might not remain the case, and radiologists could use these devices in the future. As Secrist points out, "Radiologists need to understand all the possible imaging modalities in the hospital, because this is new technology and they may find additional applications for it."