|Clinical image courtesy of Vital Images|
For years, conventional catheter-based angiograms have been the gold standard for imaging the coronary arteries and determining the presence and extent of cardiovascular disease. But now multislice CT technology is gaining a clinical foothold with users and earning believers as a new, noninvasive approach for studying the heart’s structure and its minute, vital arteries. The cardiovascular community is fighting a tough battle with America’s No. 1 killer, but the revolutionary capabilities of cardiac CT have the potential to offer earlier diagnosis and thus better cardiac patient management.
The cardiovascular community is committed to improving the diagnostic tools that can be used to evaluate and treat heart disease, utilizing imaging technologies such as nuclear medicine, stress echocardiography, catheter-based angiograms, cardiac MRI and cardiac CT. And in particular, volume acquisition, multislice technology, sub-second reconstruction of images and advanced visualization software is making CT an increasingly valuable tool in vessel assessment, particularly the coronary arteries.
“CT gives us that one more additional piece of information that we can feel comfortable in saying patients either do or do not have CVD,” says Peter Fail, MD, director of cardiac catheterization laboratories and interventional research for the Cardiovascular Institute of the South (CIS) in Louisiana. Advancements in CT technology are allowing cardiologists at CIS—and throughout the country—to reconstruct minute slices of the heart into three-dimensional images to better diagnose the presence and extent of disease.
CIS uses two Aquilion 64 CFX multislice systems from Toshiba America Medical Systems to image coronary and vascular arteries. “We do calcium scores, coronary artery angiography and peripheral angiography,” says Fail.
|A New Gold Standard Emerging|
In the past, ruling out—or determining the severity of arteriosclerosis in the coronary arteries—required interventional cardiologists to perform a diagnostic, catheter-based angiogram. CT angiography (CTA) is changing the diagnostic paradigm since it provides a means for contrast media to be administered intravenously and catheter insertion to be avoided completely. On the other hand, CTA images provide cardiologists a non-invasive look at the coronary arteries to determine whether either fatty deposits or calcium deposits exist.
According to Lenox Hill Hospital’s Harvey Hecht, MD, CTA could change the nature of the cardiac cath lab. “It could convert it from a diagnostic laboratory to one which is more and more devoted to interventions and those [patients] we already know by virtue of the CTA have significant [cardiac] disease,” explains Hecht.
Westside Medical Imaging’s Norman Lepor, MD, says, “I see CTA becoming the gold standard for coronary artery imaging. Therefore, doctors will only send their patients for conventional coronary angiographies either in the rare case of a non-diagnostic CT and there is an important clinical issue, or in anticipation of performing a percutaneous intervention.”
“We can look at physiology based on a perfusion scan or a stress echocardiogram, but we can not look at anatomy,” he continues. “The CT scan offers the opportunity to look at anatomy, which in correlation with physiology, helps put together the entire picture.”
The use of cardiac CT is evolving at Lenox Hill Heart & Vascular Institute, part of Lenox Hill Hospital, a 652-bed, acute-care hospital in Manhattan. “It is evolving fairly rapidly and we are using it for several purposes,” says Harvey Hecht, MD, director of cardiovascular CT. “We are using it to evaluate patients who have already had stress tests which are equivocal. Rather than take a patient directly to an invasive angiogram, we are doing coronary CT angiography [CTA]. Also, we are increasingly using it as a step before a stress test.”
The institute, an early adopter of multidetector CT, images cardiology patients on a Philips Medical Systems’ Brilliance CT 64-slice scanner. The system has 64 x 0.625 millimeter detector banks, giving total z-axis coverage of 40mm, and has a maximum gantry rotation speed of 0.4 seconds. “CT gets very high-quality pictures,” says Hecht. “With 64-slice scanners, the amount of artifact is much less and you can really see the entire coronary artery fairly well in most patients.”
The high sensitivity and specificity of CT imaging can assist in ruling out or determining the presence of coronary artery disease (CAD) in symptomatic and asymptomatic patients. The cardiology practice at Westside Medical Imaging in Beverly Hills, Calif., which utilizes a Siemens Medical Solutions’ Somatom Sensation Cardiac 64, images a number of patients who present with absolutely no symptoms but have risk factors, says Norman Lepor, MD, associate clinical professor of medicine at the UCLA School of Medicine and co-director of cardiovascular imaging at Westside.
“A third of our patients come in because they are concerned about whether or not they have CAD because of non-risk factors,” explains Lepor. “About two thirds of our patients come in because they have either had symptoms that are suggestive of disease or border line or mildly abnormal stress tests.”
No matter the reason for the exam, CT presents exquisite views of the heart’s anatomy in a relatively short amount of time so that cardiologists can definitively say whether or not a patient has CAD.
“The breakthrough in terms of the image quality, and certainly the big jump in the importance of this technology versus the older technology of coronary calcium imaging, is [doctors] also can see soft or non-calcified plaque,” explains Lepor. “Cardiologists can tell with an extremely high degree of sensitivity by looking at these images whether or not the patient has any disease at all. If the patient has disease, we can determine both the nature and severity of disease. This is having a huge impact on how we are taking care of patients.”
At the Baylor University Medical Center in Dallas, imaging on a GE Healthcare LightSpeed VCT 64-channel detector has become an in-between step in a large percentage of patients who have symptoms which are consistent with angina as well as for those who have risk factors for coronary disease, but in whom a stress test is likely to be a false positive or in whom the diagnosis of how much coronary disease is just as important as answering if they have coronary disease, says Jeffrey Schussler, MD, medical director, CCU.
“It has enabled us to reassure many patients on whom a stress test was performed, but who are still of a low to moderate likelihood of actually having the disease,” says Schussler.
How many slices are enough?
The clinical importance of cardiac CT has expanded with the advent of multidetector CT, particularly since the introduction of 16-slice CT systems. Technological and software improvements have allowed for the acquisition of thinner slices due to the introduction of 32-, 40- and 64-slice systems. But how many slices are enough when it pertains to cardiovascular imaging? A 64-slice CT scanner can cost anywhere between $1.4 million and $1.6 million depending on all the bells and whistles, which adds $200,000 to $300,000 over a 32-slice scanner and $400,000 to $600,000 to the price of a 16-slice system.
One advantage with more slice power is faster scanning times. For example, GE’s LightSpeed VCT 64-channel detector system captures an image of any organ in one second and can perform a whole body trauma scan in fewer than 10 seconds. In heart imaging, the scanner can capture images of the heart and coronary arteries in as few as five heartbeats. This correlates to shorter breath holds for patients. “Breath holds are an advantage of the 64-slice scanner,” says CIS’s Fail. “Breath holds are in the 5 to 10 second range. They are relatively short.” With 16-slice CT scanners, patients having their hearts scanned have to hold their breath for 20 seconds which can be difficult for sick or elderly patients.
In terms of resolution, Lenox Hill’s Hecht contends that there really is no difference when slices increase. “There is no difference in the resolution but the breath hold is shorter so there is less time for artifacts related to patient movement, respiration and arrhythmias,” explains Hecht.
According to Baylor’s Schussler, multislice CT outdoes electron beam CT (EBCT) with better spatial resolution, allowing physicians to see smaller arteries and smaller amounts of plaque. “The larger number of slices improves the test’s temporal resolution, enabling physicians to image the heart at faster heart rates and reducing the time it takes to obtain images,” Schussler explains. “Reducing the time it takes to acquire the scan reduces the amount of time the patient has to hold still and hold his or her breath. This, in turn, reduces the chance that the scan will be ruined by patient movement.”
Factor in the various medical conditions that cardiac patients present with and speed of acquisition is an even more attractive feature. For example, a patient with emphysema who has a difficult time holding his or her breath will experience an enhanced comfort level if the exam can be completed in five seconds. Patients with heart disease also may present with kidney disease and—although further investigation is required—there is potential for contrast dose to be reduced by 25 to 50 percent with a 64-slice system.
According to Claudio Smuclovisky, MD, director of South Florida Medical Imaging (SFMI) in Boca Raton, a minimum of 16 slices are needed to perform diagnostic coronary work. “All things being equal, a 40-slice and 64-slice would be better since they diminish the time of acquisition of the study, consequently decreasing the chance of motion artifact and improving temporal resolution,” Smuclovisky says. However, excellent cardiovascular imaging can be performed on 16-slice scanners today due to advanced reconstruction algorithms.
“You do not acquire a cardiac CT scan in just one heart beat,” he continues. “You have to scan over multiple heart beats. Heart rate variability causes significant motion artifact because one segment of the heart is reconstructed in multiple heart beats. Therefore, the image may not be in the same space in time, resulting in a distorted and blurry picture.” Philips offers an algorithm that corrects motion, which is why more than 3,000 successful CTA studies have been performed using SFMI’s Philips’ MX 8000 16-slice CT scanner. Smuclovisky says the facility is planning on upgrading soon to a 64-slice system.
Pushing the envelope
University of Colorado Hospital’s new Fitzsimmons campus in Denver recently installed a Philips’ Brilliance 40-slice CT scanner that’s being used to perform CTA exams. However, interventional cardiologists pushing the envelope also are using the volumetric datasets acquired on the system to plan and execute cardiac interventions of different kinds.
“Our work is concentrating on using the CT images to plan percutaneous coronary interventions (PCI) so that when the patient comes to the cath lab, the surgeons do not have to repeat a full diagnostic catheter-based angiogram,” says John Carroll, MD, chief of cardiology, director of interventional cath labs and cardiac and vascular center at the University of Colorado Hospital.
“When I look at a CT [study], I look at a variety of things: lesion location, the curvature of the artery, extent of calcification and the amount of plaque,” explains Carroll. “That additional coronary data help in planning the PCI. Additionally, I look at the aorta and what guiding catheter fits best in that patient.”
Cardiac CT has the ability to capture vascular structures in cross-sectional images, which can then be post-processed into dynamic, 3D images. “This is different from the catheter-based coronary angiogram where they are 2D projection images,” says Carroll. “A major advantage of cardiac CT is its intrinsic 3D feature.
“I think at this stage, interventional cardiologists are learning how to use CT images to plan interventions,” he continues. “The vendors also are developing tools for processing and presenting the CT data in a way that makes sense for the interventionalists. We can export the CT images from workstations right into the interventional lab so we have those in front of us when we are doing our interventions.”
Software improves efficiency
Cardiologists agree that the key to cardiac CT angiography is high-quality, post-processed data sets. Depending upon the quality of the study and the complexity of the disease, it can take cardiologists anywhere from 5 minutes to more than an hour to post-process an exam.
The study is labor intensive, but the intensity is a challenge the medical imaging community is taking on due to CT’s many benefits. “Cardiac CT is really the most exciting thing in cardiology in the past decade,” says Lenox Hill’s Hecht. “The holy grail has always been to get a non-invasive cardiac angiogram. That is what we have now.”
At the Minneapolis Heart Institute based at Abbott Northwestern Hospital, cardiologists use a 64-slice CT scanner paired with Vital Images VitalCardia, an advanced visualization and analysis software package specifically designed for cardiac CT, to determine whether or not patients need an invasive angiogram after an equivocal stress test.
John Lesser, MD, director of cardiac CT and MRI at the institute, is proficient in using the software. “The software helps to reconstruct the acquired sliced images of the heart into a volumetric image so that physicians can look at the heart in 3D from any angle,” says Lesser. “It is really important to be able to look at the small arteries of the heart in different angles. This allows the software to help you know if there is a blocked artery or not. You can read the information with 2D type imaging, but [the software] puts it together in a way that helps doctors read and look around pieces of calcium that block your ability to see.
“To have a blockage that may be a little more than half the artery, you have to be able to discriminate a blockage of 30 percent versus one that is 70 percent and that can be fairly hard,” he continues. “You have to be able to look around things that get in your way so you can measure and get a sense of how blocked the artery is. The more software capabilities you have to look at the blockage in the 3D image, the better you are.”
The proof is in the training
According to Lenox Hill’s Hecht, cardiac CT is becoming more and more mainstream. “It is really the hottest thing in cardiology right now,” he says. “Cardiologists all over the country are scrambling to get trained so they can be certified to interpret these scans.”
The AHA and the American College of Cardiology addressed the growing use of cardiac CT in clinical settings by developing a competency statement for clinicians performing cardiovascular CT and cardiovascular MRI (www.acc.org/clinical/statements.htm). The organizations say the goals of the standards ensure that clinicians have appropriate education, training, experience and cognitive skills to provide the best possible care for patients.
“These images are not trivial to create,” says Westside’s Lepor. “From a technical point of view, the 3D reconstructions are actually quite complex. You must make sure that the team that is performing coronary imaging is experienced.”
Guy Weigold, MD, director of cardiac CT at Washington Hospital Center in Washington, D.C., says the procedure definitely requires a significant investment in time and energy to learn how to properly do cardiac CT. “There really are no short-cuts to becoming proficient at this,” says Weigold. “This is a way of looking at the heart that is really quite new and different, no matter which side you are coming from. If you are a radiologist, you basically have to learn all of your cardiac and coronary anatomy. For the cardiologist, the post-processing and 3D reconstruction is relatively second nature. However, trying to look at the heart in radiological planes and trying to do this type of 3D analysis is something that is going to be new for cardiologists.”
Weigold says cardiac CT is most commonly performed at the hospital when trying to establish a first time diagnosis or exclusion of CAD in patients who have chest pain symptoms of some sort and either have or have not had another type of antecedent non-invasive imaging stress test.
“I think that especially with the research that has been coming out in the last couple of years, people are starting to see the utility of using CTA to exclude significant coronary disease in those just types of patients—and being able to exclude it with a high degree of accuracy,” says Weigold. “And we are talking about greater than 90 percent accuracy.”
|Expert View: Cardiac CT of the Future|
Cardiac CT is becoming a mainstream application in the cardiology community, and the future looks increasingly bright. Experts in the field of cardiac CT provide their insight on where this modality may be heading in the future.
Peter Fail, MD, director of cardiac catheterization laboratories and interventional research for the Cardiovascular Institute of the South (CIS) in Louisiana: “I think the future is going to be in algorithms and software issues. There are going to be better tools to determine a vulnerable or stable plaque, how to determine better degrees of stenosis, and how to take stents out of the equation.”
Harvey Hecht, MD, director of cardiovascular CT at Lenox Hill Heart & Vascular Institute: “Once the resolution improves significantly, it will hopefully replace diagnostic angiography across the board and dramatically decrease nuclear stress testing and stress echocardiography. I think cardiac CT will give us information about plaque that can be used in drug trials to evaluate the effects of cholesterol medication. Cardiac CT also has enormous applications in the world of electrophysiology. EP is currently heavily evolved in ablation of the pulmonary veins for atrial fibrillation. By defining the coronary venous anatomy in the pulmonary veins, CTA will tremendously facilitate EP procedures.”
Norman Lepor, MD, associate clinical professor of medicine at the UCLA School of Medicine and co-director of cardiovascular imaging at Westside Medical Imaging: “I see CTA becoming the gold standard for coronary artery imaging. Therefore, doctors will only send their patients for conventional coronary angiograms either in the rare case that patients have a non-diagnostic CT and there is an important clinical issue, or in anticipation of performing a PCI.”
Jeffrey Schussler, MD, medical director, CCU, at Baylor University Medical Center: “The future of cardiac CT is bright. I think that it will eliminate the need for some of our stress testing, reduce the need for invasive cardiac cath, and ultimately will be incorporated into an algorithm for screening high-risk patients—such as hypertensive, diabetic, strong family history of coronary disease—for the presence of early plaque formation. However, I think that in the future it will be extremely cost effective (in a similar fashion to screening colonoscopy) in defining who needs to be on aggressive, early therapy for hypercholesterolemia.
Guy Weigold, MD, director of cardiac CT at Washington Hospital Center in Washington, D.C.: “We can advance cardiac CT in terms of coronary imaging to more and more different types of patients: those who have stents, known coronary disease, patients who do not have any symptoms but are going to the OR and need to have their coronary arteries evaluated and patients with new diagnosis of heart failure who need to have their coronary status evaluated. Trauma is another potential application.
“Outside the coronary tree, everyone is really looking to see if cardiac CT can take on the role of being able to evaluate myocardial perfusion. That is an area that is mostly addressed by nuclear cardiology, echo and in some cases cardiac MRI.”