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November 28, 2005

3-D Rotational Angiography — Technological Developments Spur Growth in Versatile Imaging and Interventional Tool
By Dan Harvey
Radiology Today
Vol. 6 No. 24 P. 56 

Digital flat-panel detectors have driven significant improvement in three-dimensional rotational angiography (3D RA) in the past two years.

“The introduction of that technology raised 3-D rotational angiography to a higher level,” says Karl Kellar, CNMT, product manager of diagnostic imaging for GE Healthcare.

Richard van der Bruggen, director of the vascular business segment for Philips Medical Systems, says 3D RA has entered an exciting era—mainly due to these flat-panel advancements and the accompanying imaging capabilities. “The detectors’ dynamic behavior has enabled the imaging of soft tissue,” he points out. “Before, we could only see contrast volume in vessels. Now, the technology has made it possible to produce CT-like images on an angiography machine.”

This “remarkable” development, as van der Bruggen terms it, helps resolve a substantial problem interventionalists face. Previously, interventional radiologists and surgeons didn’t always have a CT system at their immediate disposal to do image-guided procedures or follow-up. Now, the new systems developed by major vendors not only enhance diagnosis of vascular diseases in 3-D, but they provide reconstruction so rapidly that real-time imaging is possible during interventional procedures. More rapid and accurate assessments help clinicians better choose the most appropriate treatments. Because of the continued development of 3D RA, the diagnosis and treatment of stroke, carotid artery disease, and other vascular disorders should be greatly enhanced. In addition, with the significantly greater number of views now provided in 3D RA, exam time, radiation dose, and contrast usage are greatly reduced.This 3D RA evolution has also brought significant benefits into the area of neurology. Now, aneurysms and lesions can be more rapidly and accurately assessed.

Beyond Image Intensifiers
As most people know it, 3D RA acquires a series of images by rotating an x-ray tube and image intensifier on a C-arm around the patient. The 3-D image volume is reconstructed via a “back projection” technique similar to what a CT scanner performs. But 3D RA is moving beyond image-intensifier systems.

Currently, two different 3D RA methodologies exist, says Kellar. In the first, which uses image-intensifier technology, users acquire a mask set of images and then acquire a contrast-injected set of images and subtract the first set from the latter. “You’re reconstructing a digital subtraction angiogram,” Kellar points out. “That’s how a lot of people gained their experience. In neuro-intervention, it’s virtually the standard of care.”

However, with the second, and newer, methodology—which became possible because of the advancement of flat-panel detectors—there is no need for a mask image set and subtraction. “Reconstruction is done in the same way as a CT scanner, providing the same full density range of tissues,” says Kellar. “You acquire one data set, do the reconstruction, and remove the soft tissue and bone.”

Essentially, as Kellar points out, 3-D flat-panel angiography is CT angiography accomplished with a different detector array. The approach offers advantages: Users can bring back as much or as little of the surrounding tissue as needed. If a vascular surgeon or neurologist wants to view the vasculature relative to surrounding anatomy, they can bring the anatomy back into the picture. “In the old method, with the mask subtraction done before reconstruction, you could never do that,” says Kellar.

In addition, flat-panel detectors can have nearly 10 times the dynamic range of the older image-intensifier systems. “That eliminates the problems that made 3-D imaging so challenging with image intensifier-based systems, such as off-body radiation and veiling glare,” explains Kellar.

Further, compared with the image intensifier-based systems, flat-panel detectors have functionally higher spatial resolution in detected quantum efficiency (DQE). Kellar points out that shape is even an advantage. “Flat panels are square. With image intensifiers, the image was round,” he says. “You had to scribe a square into that circle, because the volume of data acquired for reconstruction needed to be an even cylinder of data. Now, we already have a square field of view on a flat-panel detector, and we can acquire a cylindrical data set at the start.” Thus, a larger data set can be acquired more clearly at higher dynamic range.

In the realm of 3D RA, Kellar reports that GE offers the Innova platform of products (the 2100, 3100, and 4100). The Innova systems feature GE’s Revolution digital flat-panel detector technology for cardiac, angiographic, interventional, and neurological diagnosis and treatment. The detector converts x-ray signals into digital images at the point of acquisition, capturing more information over the full range of exposures. Artifacts and distortions common to conventional image-intensifier chains are eliminated. The detailed, real-time images enable physicians to see the smallest vessels and to better maneuver small devices such as catheters and stents. In this way, various diseases can be treated without major surgery.

GE followed these advancements with the Innova 3D and Innova CT, which provide significant enhancements to the 3100 and 4100 systems. The Innova 3D, which is primarily a vascular product, has 150 axial angles and can better visualize low-contrast structures such as soft tissue and bone detail. The Innova CT, which received FDA clearance last September, offers even more effective real-time guidance by providing 3-D imaging of bone and soft tissue with better accuracy. It is especially helpful when targeting lesions, guiding devices, and assessing the impact of interventional procedures. “Simply by improving the statistics of the image, which helps improve the dynamic contrast range and the spatial resolution, you can see more dynamic range in soft tissue,” says Kellar.

Consulting a Road Map

Besides flat-panel technology, another significant development in 3D RA—developed by Philips Medical Systems—is called 3-D roadmapping. According to the company, the technology ensures that the 3-D image is registered within the system and overlaid with live 2-D fluoroscopy, which provides a sustainable “roadmap.”

The technique integrates 2-D imaging with 3-D angiographic data. 3-D vessel views are superimposed on live 2-D data sets, so catheter movements appear in registration with 3-D images, which enhances navigation. Van der Bruggen describes it as a key development in 3D RA. “Instead of having a normal 2-D roadmap that’s used for navigational purposes, we now can use 3D RA fornavigation.” This sustainable 3-D “roadmap,” van der Bruggen indicates, provides more freedom of positioning. “Because it’s 3-D, when you rotate or angle your gantry, or if you change your field of view, this road map remains intact,” he says.

This is particularly important in neurointerventional treatment, when physicians have to find their way through tiny vessels and then need to adjust the gantry to get the best view of the vasculature. “Normally, with 2-D, if you wanted to reposition your gantry, you’d lose your road map,” explains van der Bruggen. “With a 3-D roadmap, you can always find the exact position navigations.” This has tremendous clinical potential for applications such as real-time catheter navigation and monitoring coil delivery into aneurysms, says van der Bruggen. He adds that the durable roadmap improves workflow and further reduces contrast media and x-ray doses.

Built for Speed
A third considerable development that has boosted 3D RA, van der Bruggen reports, is the continuously increasing speed of computer technology, which helped make possible the capability for real-time 3D RA. “We strongly believe that the future for 3-D will be in interventional applications, which means that it as to be available on the spot,” he says.

To increase speed, Philips has integrated the 3-D imaging chain into the x-ray system, producing real-time 3D RA. When the acquisition scan is completed, the image is immediately reconstructed. “That reduces time from about one minute down to real time,” says van der Bruggen.

The integration of 3-D into the x-ray system also enhances ease of use. “The integration creates a bidirectional link between the x-ray system and the 3-D unit. When you rotate the C-arm, your 3-D volume follows,” explains van der Bruggen. “Before you press your x-ray panel, you can already see in 3-D what the image will look like and the position you’re about to radiate.”

Neuro Applications
One of Philips’ latest products is the Allura Xper FD20/10 x-ray system for neuroradiology. It enables physicians to capture and view detailed 3-D images of a patient’s vascular structure, leading to faster and more accurate diagnosis and treatment of vascular disease through less radiation. Also, the “biplane” configuration system allows physicians to simultaneously view anatomy in two planes, which provides a big advantage in complex brain procedures.

The system offers an array of applications in neurovascular interventions by providing extensive 3-D insight into neurovascular pathologies from a single rotational angiographic x-ray acquisition. This helps physicians navigate small and tortuous vessels and avoid surrounding structures. The system’s 3-D visualization assists with better treatment strategy and it provides 3-D, real-time monitoring of treatment progress and enables treatment of more difficult pathologies.

Though the Allura was originally targeted for neuroradiologists, its user base has broadened. “A lot of radiologists have started using it for different radiology applications, so we’re seeing a tremendous spread,” says van der Bruggen. The versatile system has been used in procedures such as diagnostic catheterizations, stenting, balloon angioplasty, vertebroplasty, and embolizations.

Philips also offers a version optimized for cardiac procedures. Featuring a real-time imaging algorithm, Xper enables clinicians to see and work in the smallest vessels of the heart. The Xper CT boasts high-speed reconstruction. “It only takes three minutes from acquisition to visualization, which is extremely important,” says van der Bruggen. “For instance, in a situation where you suspect a bleeding, you need to be able to make a rapid assessment.”

At last year’s RSNA, Siemens preintroduced its DynaCT product, which incorporates digital flat-panel technology advantages to provide enhancements to C-arm angiography, enabling soft-tissue imaging. The system rotates the gantry in a 10-second sweep while acquiring images, allowing for up to 600 projections.

Contrast Resolution
“Contrast resolution is much higher on flat-panel detectors, so you are able to see differences in densities a lot better, but we wanted to go a step farther,” says Tom Treusdell, product manager of Siemens Medical Solutions Angiography & X-ray Division. “Generally, you are only able to see fairly high-density objects on 3D RA. With DynaCT, we can do a rotational angiography run, without subtraction, of course, and that allows us to see things such as soft tissue, tumors and cerebral hemorrhaging. So we get CT-like images in the vascular suite.”

An enhancement to Siemens’ Axiom Artis systems, DynaCT enables physicians to perform angiographic CT with the Axiom Artis flat-panel detector technology systems, thus increasing diagnostic capabilities and, in turn, immediate treatment planning. DynaCT eliminates the need to transfer patients to other imaging equipment for follow-up procedures. “You don’t have to remove the patient from the vascular lab and run them over to the CAT scanner, just to make sure you didn’t puncture the vessel wall,” comments Treusdell.

Siemens also offers the Axiom Artis dTA, its original digital ceiling-mounted C-arm angiography system with flat-detector technology. “It can do 60°-per-second rotation, which is the fastest out there right now,” says Treusdell, “and we don’t reduce our rotation speed when the gantry is pivoted to either side of the table, which is not the case on other systems. We are able to control the complete system and 3-D workstation right at tableside.”

Siemens also provides roadmapping capabilities with its new feature called iPilot. “It’s like a 3-D roadmap,” comments Treusdell. “It allows us to merge 3-D images from 3-D angiography and DynaCT right onto live fluoroscopy. Users can move the C-Arm, change zoom sizes, and move the patient around and still have the 3-D overlay fused to the fluoroscopy image.”

Future Directions
With the new capabilities that 3D RA technology provides, vendors expect increased interest and adoption. “It’s been around for a while, but people have been hindered by its functionality,” says Treusdell. “Once, you needed to be a computer scientist. Now, everything has become so automated that all you need to do is touch a button and it’s done.” Kellar says the next three to five years should prove very interesting, as people figure out how to incorporate the new capabilities into interventional practice. “For instance, with vertebroplasty, at the end of the procedure, you need to know exactly where the glue went,” he says. Currently, he adds, when patients leave the recovery room, they’re taken to a CT scanner. “But with tools like the Innova CT or 3-D, you can take an image both during and after injecting the glue, to document the procedure and potentially save the patient a trip to the CT scanner,” he says. Any correction can be made in the same session of care.

“The same thing is true with stenting and the use of stent grafts,” he adds. “These are the kinds of procedures where the technology would be valuable.” The 3D RA technology has already proven itself in neurology and cardiology, but Kellar also sees potential in oncology. “For oncology applications, physicians want to see the vascularity of a tumor in three dimensions as well as image the soft tissue,” he says. “This is just beginning to fall into place, but we think 3-D will assume a valuable position with oncology intervention.

“I think it’s going to take a couple of years for the user base to plumb all of the benefits they can get from this,” concludes Kellar. “So I think things will get very interesting.”

— Dan Harvey is a freelance writer based in Wilmington, Del., and frequent contributor to Radiology Today.

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