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April 12, 2004

Future Echos
By Kate Jackson
Vol. 5 No. 8 p. 17

While slow to make inroads in clinical practice, three-dimensional (3-D) ultrasonography has been making waves in research centers, centers of excellence, and university medical centers, where scientists are exploring its promise. After approximately 15 years of development, 3-D ultrasound finally seems poised to become a widely used precision tool with far-reaching applications.

Obstetricians and gynecologists pioneered work with 3-D ultrasound, which was energized by the mildly controversial phenomenon of BabyFace ultrasound fetal portraits. That technology made it possible to view the coronal plane of the uterus and clearly visualize the exterior anatomy of the fetus. Outside of that field, clinicians have been somewhat reluctant to utilize this technology in clinical practice before it has been more widely proven. Nevertheless, clinicians over time have been finding new ways to implement 3-D ultrasound, particularly since the advent of four-dimensional (4-D), or real-time 3-D ultrasonography. At the same time, researchers are developing new applications.

Three-dimensional ultrasound, in clinical use for approximately eight years, and 4-D ultrasound, available in the past two years, expand the capabilities of two-dimensional (2-D) ultrasound, yet are used not in place of it but in addition to it. The third dimension and the introduction of real-time imaging (4-D) open the door for new ways of using ultrasound. The limitations of 2-D ultrasound are suggested by the name. The images are flat and, therefore, difficult to interpret. Furthermore, they offer only cross-sectional images, which render a restricted view of the anatomy. Three-dimensional ultrasound also creates cross-sectional slices of the anatomy, but it links them to other cross-sectional slices to create a 3-D image that can be viewed from different angles and measured.

Transducer Elements
Kevin Appareti, MBA, of Philips Medical Systems’ Ultrasound Business Unit, explains that in the past, there wasn’t sufficient computer power to build a transducer with the number of elements it can contain today. He describes the evolution of ultrasound in the cardiac arena. “You had to take a transducer and one 2-D image and then move the transducer position by 2° and take another 2-D plan, then move it another 2°, and so on,” says Appareti. After getting perhaps 15 or 20 of these planes, he explains, you would have to take the data set and go to an offline computer somewhere else in the lab that would render the 3-D image, which might take as long as 10 minutes or 15 minutes.

The limitation in cardiography, for example, he says, “was that you only ended up with one beat because you were only gating on the EKG for one beat.” It was possible to have a 3-D image, he says, “but it took 15 or 20 minutes to get it, was only one beat made up of many heartbeats, and it didn’t really represent the continuous beating of the heart over time.” Furthermore, he says, there was some spatial and timing artifact. Not only were clinicians impatient for the image, but many weren’t willing to “look at one that’s made up of so many loops that it might have some error in it.”

Ultrasound manufacturers hurdled these obstacles using what Appareti’s company calls Live 3-D, or what other manufacturers refer to as real-time 3-D imaging, or 4-D imaging. Today in cardiac imaging, multielement transducers produce images that can be rendered in real time so every single heartbeat can be viewed as the patient is being scanned. This ability to visualize complex anatomy without having to change the transducer position offers new opportunities to see views of the heart that were not possible with 2-D imaging—or in obstetrics, to see the exterior anatomy of a fetus.

“BabyFace got this started because of the emotional impact it had on the parents,” says Bill Carrano, vice president of marketing for Siemens Medical Solutions’ ultrasound division. “Everyone in the industry realized that the BabyFace is cute and interesting but that there had to be more clinical value beyond that, so they started to look into imaging nuchal fold thickening—looking at the back of the neck of the fetus—something that had been done with 2-D ultrasound for a long time.” With 4-D imaging, he says, being able to see the fetal head in three dimensions and in real time as the fetus moves “created some new spark of interest and clinical excitement in 3-D and 4-D imaging.”

Valve Problems
In cardiac care, real-time 3-D imaging is a boon to the millions of Americans with mitral valve or aortic valve disease. According to Appareti, it allows more accurate assessment of valvular disease by increasing the amount of data that can be obtained about the mitral and aortic valves. Two-dimensional imaging of the heart, he explains, relied upon many assumptions about the size and shape of the left ventricle. “Now you’re able to take those out of the variable set because you actually can get a 3-D image of the left ventricle, so you know when you calculate the volume—either a global cardiac function parameter or regional parameter—that you’re getting the accurate data.”

That, he believes, is going to be the most valuable benefit of real-time 3-D imaging within cardiac imaging. “When you ask cardiac surgeons, echocardiographers, and internists what they want to know about the heart,” he says, “they want to know the left ventricular volume and how the ventricle is working.” The fact that you can assess the left and right ventricles and any chambers that have a 3-D shape and get more accurate volume measurements, he suggests, will vastly change the way that heart disease is detected and treated.

Real-time 3-D ultrasound can now be immediately applied to a large population of patients with coronary artery disease, including mitral stenosis, mitral regurgitation, aortic stenosis, and aortic insufficiency. It’s also helping clinicians better understand and treat congestive heart failure because it provides more information about left ventricular function.

Other patients with cardiac conditions who can benefit from 3-D imaging are those with rhythm abnormalities. Philips’ researchers have been looking for opportunities for real-time 3-D imaging to be useful in the electrophysiology lab, where they’re assessing these abnormalities and trying to plan treatment. Because real-time 3-D imaging permits a volume image, it improves the accuracy of calculations in the electrophysiology lab, which can have a significant impact on patients with rhythm abnormalities. Carrano says this ultrasound technology also gives clinicians a window on the valves of the heart, which can be invaluable for the planning of valve replacement surgery and postsurgical assessment. Surgeons, he explains, can give a patient an artificial valve and then look at the valve function after surgery with 3-D and 4-D imaging to assess the success of the procedure.

Three-dimensional ultrasound is also proving valuable in pediatric cardiology, where it permits the visualization of complex congenital anomalies. Pediatric cardiologists, notes Appareti, favor the technology because not only does it make the job of diagnosis easier, but it’s an excellent communication tool. It’s easier, he explains, for clinicians to explain conditions to parents because they can see an image that mirrors their expectations of what the heart looks like—that is, that it’s a 3-D structure. It’s easier, too, for referring clinicians to appreciate 3-D imaging and gain a better sense of patients’ conditions.

Outside of cardiology, 3-D ultrasound is currently used most commonly in obstetrics. “Aside from the BabyFace, people are now looking at the whole fetus—the legs, hands, fingers, and torso,” says Carrano. “It’s being used to look at early gestations and assess the mobility of the fetus in real time.” In gynecology, it’s also used to visualize the ovaries and other anatomical structures.

Perfusion to Tumors
Three-dimensional and 4-D ultrasound are also being used to look at the organs of the abdomen, such as the liver or kidneys, and can aid both in the diagnosis of disease and the measurement and treatment of tumors and cysts. It doesn’t merely indicate a baseline of a tumor’s size, Appareti explains, but permits a better assessment of its volume. “Because we also have 3-D color flow, you can also look at the perfusion to those tumors,” he observes. In addition, its ability to delineate the size and location of tumors will improve radiographers’ ability to plan more accurately targeted radiation therapy.

Another broad application of 3-D imaging, both within and outside of echocardiography, is in guiding biopsies and percutaneous drainage of cysts on the ovaries, the liver, and other organs. “In the past, because you only had a two-dimensional image, there was often confusion about where the needle actually was. Now, with a three-dimensional image, you can be much more accurate because there is a whole volume of space that you’re imaging. You can see the needle much better and for a longer period of time,” says Appareti.

There’s a wealth of research that promises to extend the use of this technology into much broader reaches of clinical practice. “The next frontiers will involve looking at fetal cardiac anomalies and defects,” says Carrano. “We’ll be looking at everything from heart chamber sizes to—as the resolution gets better—3-D and 4-D valve motion of the fetus.” As the technology improves, he suggests, it will become possible to quantify 3-D and 4-D images for fetal cardiac function.

In the cardiac realm, says Appareti, researchers are exploring the ways in which 3-D and 4-D imaging may play a role in some areas where echo has not traditionally played a significant role. Among the possibilities he suggests are in cardiac resynchronization therapy, in the cath lab when working with closure devices, and in the operating room for complex valve repairs or replacements. In echocardiography, he says, one of the bigger areas of research is to combine 3-D imaging with contrast so you can try to look at the myocardium and see how well it is being perfused in each region. “In the past, you were only looking at one slice of the heart with two-dimensional imaging. With 3-D scanning, you can see all of the heart, and that will give you an opportunity to detect with more accuracy areas where there’s a defect in perfusion and assess its size and determine the mass of the myocardium that is being impacted,” he explains.

Vascular Disease
Another area in which 4-D ultrasound can have a significant impact down the road, says Carrano, is in the assessment of vascular disease. Clinicians will be able to use 4-D ultrasound to obtain a 3-D view of the carotid arteries as they pulse and beat. Because the image can be rotated with a track ball, it can be rotated in three dimensions—an advance, he says, that will have great potential in aiding diagnoses of arterial disease.

As traditional surgical procedures have given way in many cases to minimally invasive procedures, 2-D ultrasound has been employed to guide such interventional surgeries. Three-dimensional ultrasonography, however, promises increased precision for image-guided surgeries and therapies and a more reliable tool with which to monitor patient response. Says Appareti, surgeons and interventionists are turning to ultrasound companies to see how the new technologies can help them with these port-hole and minimally invasive catheterization procedures.

“We’re looking at how live 3-D imaging can first of all help visualize those instruments,” Appareti says, “and also how we might be able to better interpret how the procedures have performed—for example, whether they really did get rid of all the mitroregurgitation.” In addition, he says, researchers are exploring the use of 3-D imaging in electrophysiology to determine the results of cardiac resynchronization therapy, or biventricular pacing. “The basic concept is that you want to get the most blood coming out of the heart every time the heart contracts. Researchers are using 3-D imaging to try to assess the best setting with which to get the most blood out and also to predict who is going to be a responder of that kind of therapy,” says Appareti.

Drug Delivery
One of the more distant yet intriguing uses of 3-D imaging that researchers are exploring is in therapeutics to help deliver either a drug or gene therapy in a bubble—for example, in the heart or liver, says Appareti. “You can visualize a certain part of the heart or the liver, inject some contrast that has a certain drug in the bubble, wait until that contrast gets to the area within the organ where you want to deliver the drug, and then you send out a kind of power burst—you break the bubbles—and the drug is delivered in the area in which it’s needed.” It’s a highly promising area of study because it may allow drugs that might be extremely toxic to the whole body to be delivered directly to the organ where they’re needed.

Carrano believes that the greatest number of future applications of 3-D and 4-D imaging, however, will reside in radiology. Ultrasound, he says, will become more and more like MR and CT imaging in that it will increasingly do more volume acquisitions—for example, of the kidneys, pancreas, liver, and other abdominal organs—giving radiologists views that they could not normally obtain today. “Looking at the uterus today, you can only get certain acoustic windows when you put the transducer on the patient. But, if you acquire a three-dimensional volume of the uterus—a volume data set—then let the computer slice it in any plane that you want to view it in, you can generate, for instance, a coronal view of the uterus.” The goal, he says, would be to be able to reconstruct a 2-D image from a 3-D volume of data, but the technology will have to advance to the point at which that 2-D image would be of equal or better quality than that which is available now.

Research in radiology applications today is largely at the university level, he explains, and the technology has not yet reached the point at which it can be used effectively in clinical practice. “We’re just scratching the tip of the iceberg with 3-D and 4-D radiology. There’s a lot more to be learned over the next year or two, and there’s likely to be a proliferation of 3-D and 4-D radiology applications over the next two to four years,” says Carrano.

— Kate Jackson is a staff writer for Radiology Today.

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