Since the first arteriogram was successfully used to view an artery in 1929, the march toward finding the highest-quality-possible vascular image has proceeded rapidly. First done primarily through the use of traditional X-ray technology, vascular assessment is now done through ultrasound as well – primarily in vascular laboratories, as well as outside the lab, with emerging CT and MR technologies. Once, vascular imaging meant a simple iodine contrast-based view of an artery; however, newer modalities are advancing toward providing 3-D and even real-time vascular images.
The need for this visual accuracy is obvious. If more cases of atherosclerosis can be diagnosed earlier, a myriad of conditions – including coronary heart disease, stroke, and myocardial infarction – can be prevented more easily. As the technology improves, vascular imaging will have the capability to diagnose many other conditions as well as to assist with surgery and transplantation.
Interestingly, recent improvements can be found among all the major vascular imaging modalities – not just with MR and CT, but also with ultrasound and X-ray. Each modality has disadvantages and advantages as well as environments and uses for which they are ideally suited. However, as the modalities simultaneously advance, these areas are starting to blur.
According to Thomas Grist, MD, a professor of radiology at the University of Wisconsin-Madison, recent improvements in all of the vascular imaging modalities have been primarily in two areas: spatial and temporal resolution. MR and CT specifically are “providing us with 3-D volumetric data, which is very important,” he says. “In the case of MR [angiography], we now have the capability to provide time-resolved images, so you can see vessels fill – and how they fill – and look at the effects of the occlusions.”
Each modality also appears to have its own partisans, championing ultrasound’s low cost, CT’s ease of use, or MR’s ability to characterize soft tissue. However, as vascular imaging occurs, it’s clear that the technology – and with it, better diagnostics – will only improve.
Understanding Ultrasound
Ultrasound, the most commonly used imaging modality in vascular labs, often is seen as the least expensive. “It’s also very good for vessels near the surface, like the carotid arteries or the venous system of the lower extremities,” Grist says. “It doesn’t use ionizing radiation, and it can be performed at the bedside.”
Lars Shaw, director of marketing for the ultrasound division of Siemens Medical Solutions (Malvern, Pa) and an experienced ultrasound technologist, couldn’t agree more. “There’s no radiation with ultrasound, and it does a really good job on the vessels that are right there – carotid, legs, and the heart,” he says. “And ultrasound’s been doing that very well for quite a while.” He adds that the technology has vastly improved since he was trained to use it several years ago.
 |
| This color-flow, high-resolution ultrasound image of an internal carotid artery shows a previously blocked artery, now open with a stent. |
In the vascular laboratory at Albany Medical Center (New York), director Ann Marie Kupinski, PhD, says ultrasound is the only modality used for vascular imaging. Its primary benefit, she says, is that it’s noninvasive. “So there’s no risk to the patient, and there’s very little discomfort in the course of the examination. It’s also relatively portable. So if you’re in an in-patient setting, like our primary site is in a medical center, you can go to an emergency department, cardiac unit, or surgical/medical intensive care unit, and bring your equipment right to the bedside.”
Kupinski says ultrasound’s clarity and resolution have geometrically expanded in the time it’s been used in vascular imaging – for about the past 25 years. “We’ve really improved how discretely we can see tissue in terms of resolution, and we’ve refined things quite a bit so we can get better definition of disease. We can tell exactly what’s going on at a much more fine level,” she says, adding that color imaging refinement, as well as spectral Doppler analysis, also have improved. “These are basically all advances on the foundations that were laid back in the late 1970s with the technology.”
Shaw points out that a major disadvantage of ultrasound is its operator dependence. “Some people call ultrasound an art, and it really does take a while to learn,” he says, explaining that it doesn’t take nearly the amount of time to be trained in performing CT or MR scans.
Grist concurs, adding that ultrasound has a major disadvantage: “It is highly operator-dependent. For imaging renal arteries or mesenteric arteries, there’s more dependence on the patient’s size and the operator’s technique, so it might not be as widely applicable.”
 |
| Doppler spectral analysis on this ultrasound image was obtained from the mid-portion of a stented internal carotid artery at the Karmody Vascular Laboratory at the Vascular Group (Albany, NY). |
Shaw counters this shortcoming, however, by touting ultrasound’s ability to show a vessel actually pumping and pulsing. “You can look at the pressure, the artery, and the symptoms, and you can say, ‘This person isn’t compensating well, and they’re in more trouble than I thought,’ which is something different from what you get with MR or CT.” He also points out that ultrasound technology is increasingly becoming less operator-dependent.
Still, he says, the other techniques are catching up with ultrasound. “Other modalities do have radiation, and that’s bad. They do have contrast agents – that’s not so good. But if they give you the same answer in a lot less time, well, maybe that’s something good.”
Advances in MR
MR testing is much less operator-dependent than ultrasound; it also provides the best view of soft tissue. It’s a modality with many advantages, Grist says, adding, “It’s been shown to be effective for the carotid [artery], for the abdominal and thoracic aorta, the branch vessels, and the lower-extremity vessels.”
Jeffrey Bundy, Siemens Medical’s US director of MR research and development, says the information provided through MR is primarily different from ultrasound because of its 3-D view. He explains that a 3-D image can be acquired through one of two ways: Conduct a timing run to see how fast the contrast agent travels through the vessels; or acquire data in real time and, as soon as the user sees the contrast “light up,” switch to a high-level 3-D model.
The latest MR technology, time-dissolve MR angiography (MRA), actually allows the technologist to acquire multiple, 3-D data sets that allow him or her to pick the exact phase he or she wants to examine. “Usually, there’s a specific clinical question or a specific region of the body where you want to do your imaging,” Bundy says. “So you then can try to either time it correctly – or, with today’s scanners, move fast enough so that you can do some time-resolve imaging and then retrospectively pick out the data set of the phase you’re most interested in.” He says that this technology is sometimes referred to as “4-D,” because it’s 3-D using multiple time points.
He explains that there are some new contrast agents, not yet approved by the FDA, that stay in blood vessels longer, allowing for more time to conduct an MRI. Other advancements not yet available include a technology for multiple arrays providing whole-body imaging.
In short, he says, MR is an advantageous modality because there is no radiation – it’s noninvasive – and because it can inherently characterize tissue. The research behind this modality is growing rapidly; the number of MR studies in this country doubled last year. “Five or 6 years ago, it used to be a brilliant-academic-center type of application, but it is not that way anymore,” Bundy says.
As for the other modalities, he thinks there might be a time in the future when a combination of vascular imaging techniques will be used. An example: “An initial scan to see if there’s any blockage, and then a follow-up study [with a different modality] to see what type of blockage it is – and that will determine the treatment,” Bundy notes.
Grist suggests that although MR could be considered more difficult to perform and attain information from other modalities, such as the very portable ultrasound, it will become easier to use once the technology advances. “MR offers some advantages, such as in soft tissue imaging, and the fact that it doesn’t use an ionizing radiation. But the disadvantage now is, it’s a little more difficult to acquire the data.”
The Benefits of CT
CT scanners, which can now provide as many as 64-slice views, are being seen as faster, easier-to-use methods of conducting vascular imaging. “CT [angiography (CTA)] has been very effective for the aorta and the carotid arteries, as well as for the lower-extremity arteries,” Grist says, adding that although CTA has not been researched as frequently as other modalities, that will likely change in the not-so-distant future.
With the latest scanners, he says, “CTA is relatively easy to perform. It can be done by a large number of people. For many centers, there are fewer pitfalls associated with CT, especially with the larger vessels.”
Praveen Nadkarni, MD, manager of cardiac CT and preventive care for Siemens Medical, says the modality is known for its speed and resolution. “In the ER, where there are multiple traumas, using CT is the best scenario. Because of its speed, it enables trauma doctors to see an image within 20 to 30 seconds. It’s also helpful if someone comes in with chest pain – you can rule out several different conditions very quickly.”
 |
| This volume-rendered CT image demonstrates visualization of three bypass grafts as well as the native vessels of a 77-year-old female patient. The high spatial resolution yields to a very low amount of artifact from the sternal wires. This high-resolution image was acquired in just 15 seconds with full 0.4-mm spatial resolution. |
Like MR, CT can provide 3-D images, which can be very helpful for surgeons, he says. “As we look at the future of vascular imaging, people are trying to look at the smallest vessels possible. MR has its benefits here as well. But in cardiac imaging, CT really comes into play. It can look at very small vessels, and for surgeons, they can see real-time 3-D images right on the screen as they’re working.”
Nadkarni explains that another ideal use of vascular imaging is to view dynamic flow – to actually see the blood flowing in real time. “In the future of vascular imaging, we’ll see a wider coverage area in a faster time with dynamic flow,” he says.
As technology advances, he predicts that using newer contrast media with PET and SPECT in conjunction with CT also will greatly improve the modality’s capabilities. “You will find a scenario where PET or SPECT will be used to highlight vessels, and then CT will be used to provide more sharp detail of the anatomy. Then, CT will become the road map.”
Shaw agrees that CT has many advantages, especially with 64-slice scanners, but says he doesn’t believe it is as practical as ultrasound. “CT also has the issue of radiation. The 64-slice is great, but how much radiation is that? And is it going to be a problem?” Still, he admits, “For my money, the images you get out of it far outweigh any risk analysis.”
Possible radiation is definitely a drawback to be considered with CT, Grist says. “The fact that it still does use ionizing radiation is a factor, and that the radiation doses are not inconsequential, that is a factor,” he says. “Although, with careful protocol selection, the doses can be minimized.”
X-Ray Improvements
In all of this talk about the newest modalities for use in vascular imaging, it shouldn’t be forgotten that traditional angiography – through X-ray – still exists, and it too has experienced recent technological advancements.
“There are some remarkable new developments in flat-panel X-ray detection technology in conventional angiography,” Grist continues. “It provides very high-level resolution images and lower doses with capabilities for digital processing that are quite remarkable.”
In other words, the invasive procedures are by no means “dead” for diagnosing arterial disease, he explains. “It still can be used on any artery and is considered the gold standard, though it is invasive,” he says. “But it is important to remember that there are still developments in the flat-panel detector technology.”
This is an area upon which Toshiba America Medical Systems (TAMS of Tustin, Calif) has focused great attention. Rick Stouffer, MD, director of the cardiac catheterization laboratories and of interventional cardiology at the University of North Carolina Hospitals (Chapel Hill), says the company’s flat-panel detectors that he uses within the cardiac cath lab have been extremely helpful.
“One major improvement I can see is that the imaging is better, so you can see the edges of the arteries more clearly for placing stents,” he says. “Of course, when we place a stent, we want to know the exact size and the exact length [to use]. And since the imaging is better now, our sizing is better.”
Stouffer also says that radiation exposure is lower with these flat-panel detector products. “That’s obviously important to us,” Stouffer says, “as we absorb so much radiation during a lot of procedures. Any decrease in radiation makes a procedure safer for us, and also safer for the patients.”
He predicts that as this technology continues to improve, “We’ll be able to see irregularities better and see atherosclerosis more clearly. So I think as the equipment gets better, our interventions will improve as well.”
Into the Future
As for the direction in which vascular imaging is heading, all of these specialists – no matter which modality they favor – only see positive changes.
Shaw says the major recent improvements in vascular imaging have been in clarifying images (that is, being able to see the vessel better) through the use of technology, including 3-D viewing capabilities. He also says that ultrasound is starting to be used for screening. “It is being used to measure the intima of the artery, and I think there have been big advances in the measurement area, looking at the disease process, and being able to track it better.”
Ultrasound also will be improved through patient-specific imaging, he continues. “Here, based on the echoes that respond to the ultrasound, the machine will adjust itself to give you an optimized image. Then, people who know where to scan will not have to interact with the machine as much, and the exam will go faster, the images you get will be better optimized, and the studies will be more diagnostic.”
Kupinski of Albany Medical agrees that ultrasound equipment has improved-that it’s becoming faster and more accurate. She adds that, for her, “there isn’t a major thing that [ultrasound equipment] can’t do now.”
Within ultrasound, MR, and CT, Bundy says, advancements are being seen in vulnerable plaque imaging. “You now can see whether there are any places where the vessels are closed. The next wave is looking at what is blocking the vessels, what is causing the blockage, and what type of plaque it is. Is it a stable plaque, or one that could break into smaller pieces, which can cause stroke and heart attack? That’s an area of research for all three of those modalities.”
Grist of the University of Wisconsin-Madison says, “In the future, each [modality] is going to have certain indications. As far as weighing the disadvantages and advantages of each, they’re all balancing acts.”
Nadkarni notes, “We all want vascular imaging to be faster, with higher resolution – so you can see a bigger area in a shorter amount of time. [These modalities] are making strides toward doing that.”
Sarah Schmelling is a contributing writer for Medical Imaging.
