Multidetector CT (MDCT) or 3T MRI? That is the question when it comes to cardiac imaging. But answering it isn’t easy, as opinions vary greatly. Radiologists and cardiologists, guided by results of early research, are exploring clinical applicability. Although much is yet to be discovered, it is clear that certain cardiac studies are uniquely suited for 16- to 64-channel MDCT and others for 3T MRI.
The use of MDCT systems has resulted in the tendency to acquire cardiac images using progressively thinner collimation and to routinely employ volumetric reconstructions. This has resulted in an exponential increase in the volume and complexity of these examinations, which, while improving diagnostic value to the study, has resulted in a data explosion and a few major challenges. These include increased image complexity, time required for image interpretation, and the need to rapidly communicate the results with referring physicians for patient planning and treatment.
All thin-section data acquired by the most advanced scanners is useless unless the images can be interactively presented in a way that’s understood by radiologists, cardiologists, and/or technologists, and then quickly interpreted so that reports can be generated. The results also need to be rapidly distributed for patient triage and management to surgeons and interventionalists. To meet “slice overload” challenges, advanced postprocessing workstations, as well as interactive clinical access to 3-D tools within the clinical workflow become critically important.
Finances are another important part of the equation. Until recently, the prohibitive cost of 3T MRI, and, to a lesser extent, the cost of 16-channel MDCT, relegated their installations and uses to the largest research and teaching institutions that could afford them.
A Focus on Usage
The initial clinical application driving the demand for 64-channel MDCT and 3T MRI is coronary CT angiography (CTA) and MR angiography (MRA), especially as it relates to the early detection and treatment of coronary artery disease. As cardiac applications are fully realized, hospitals, imaging clinics, and heart-only institutions are taking advantage of the new technology.
Currently, the Cleveland Clinic Heart Center is using a 16-channel MDCT and plans to receive a 3T MRI from Siemens Medical Solutions (Malvern, Pa) this summer. The Heart Center of Indiana (Indianapolis) is operating a 16-channel MDCT and installed a 64-channel last month, both from Siemens Medical. And the Oklahoma Heart Institute (Tulsa), which currently employs a 16-channel MDCT, is planning to install a 64-channel this fall, all from GE Healthcare (Waukesha, Wis).
|These contrast-enhanced cardiac CTA images highlight the right coronary artery with an aneurysm. All images are 3-D volume rendered via the Aquarius workstation from TeraRecon.|
“The advantages of the 16-channel MDCT were immediate,” says Richard White, MD, clinical director of the Center for Noninvasive Cardiovascular Imaging at the Cleveland Clinic. “The 16-channel MDCT represents a major breakthrough in cardiovascular imaging, demonstrating reliability and consistency. The faster speed and heightened resolution of the 64 channel are refining and providing additional meaning for imaging of the coronary arteries, evaluating plaque formation, evaluating changes in the arterial wall, diagnosing and treating pericardial disease, and performing function/dynamic studies. It also nicely complements echocardiography and serves as a reliable alternative for imaging patients with contraindications for MR angiography.”
Running close on the heels of high-channel MDCT is 3T MRI. Boasting twice the signal-to-noise ratio of a 1.5T, the 3T is a powerhouse with magnificent resolution. However, White cautions that many technical difficulties remain to be solved. “The move from a 1.5 Tesla to a 3 Tesla is not a lateral transition,” he says. “There are subtle and significant differences between the 1.5T and the 3T. Protocols are not interchangeable, and some things that work well at 1.5T do not work well at 3T-and vice versa. However, ultimately, in appropriate cases, 3T has the potential to replace invasive angiography.”
Enthusiasm for the 16-channel MDCT and the 3T MRI for noninvasive cardiovascular imaging is not limited to heart-only institutions. Cardiologist Andrew Hamilton, MD, is the director of cardiac imaging for the department of radiology at Evanston Northwestern Healthcare (Evanston, Ill) and is pleased by initial results received from both modalities.
“With regard to high-channel MDCT,” Hamilton says, “the improved speed and resolution gives me more choices, and choices are always nice to have. Now, I can choose CT angiography in some instances as an alternative to catheter angiography. I first ask myself, ‘What information do I need back to best diagnose and treat this particular patient?’
Then, I must consider the risks and benefits of receiving a bit less information noninvasively over receiving a bit more information invasively.”
Although catheter angiography is still the gold standard for cardiac angiography, Hamilton thinks change is on the horizon. “As technological difficulties are resolved in hardware, software, and postprocessing, the utilization of 16- to 64-channel MDCT will, in many instances, replace catheter angiography,” he says. “One of the greatest strengths of CT angiography could be its negative predictive value, and that value will save a lot of patients from going to the cath lab. Right now, 16- to 64-channel MDCT is potentially of tremendous value in the ER. When a patient walks in with chest pain of unknown origin, the scan and its reformation is so fast, it can give us a quick read to rule out significant coronary artery blockages as a cause of chest pain.”
On MDCT’s Side
|Here, various views of the three great vessels, segmented from the heart, are shown via contrast-enhanced cardiac CTA. Using TeraRecon’s Aquarius workstation, the software can automatically segment the coronary arteries.|
The hum of the MDCT is a familiar sound to John Rumberger, PhD, MD, FACC, medical director of HealthWISE Wellness Diagnostic Center (Dublin, Ohio), clinical professor of medicine in the division of cardiology at the Ohio State University (Columbus), and a Medical Imaging ?Editorial Advisory Board member. He describes himself as a preventive cardiologist who has been developing, validating, and applying cardiovascular CT for 20 years, several of them with electron beam CT.
“I think of myself as a ‘radiac-cardiologist,'” he says. “I am chiefly a preventive cardiologist who uses cardiovascular CT as my stethoscope.” Rumberger is hopeful that the new generation of MDCT scanners will eliminate a significant number of invasive cardiac studies, and, when used sequentially with echocardiography and/or MRI, will refine the use of catheterization labs.
Rumberger says that he finds 16- to 64-channel MDCT most useful and applicable for coronary CTA. Specifically, he cites a number of applications for both contrast-enhanced and noncontrast cardiac CT examinations, including:
- looking for significant coronary stenoses in patients with atypical-type chest pain;
- assessing emergency-department patients who present with a variety of chest pain syndromes;
- avoiding direct cardiac catheterization in patients with equivocal stress tests;
- defining coronary bypass graft status and stent patency;
- noting early atherosclerosis;
- performing calcium scoring to identify and characterize plaque;
- picking up mildly stenotic or “soft” plaque;
- seeing arterial wall remolding; and
- establishing calcium scoring as part of a preventive cardiac program.
|The pulmonary vessels are shown here in 3-D volume rendering.|
Additional applications allow one to use cardiac CT to examine the heart and its chambers specifically as well as to define left ventricular and right ventricular function, including:
- assessing and treating hypertrophic cardiomyopathies;
- defining the right ventricle in patients suspected of arrhythmogenic right ventricular dysplasia – potentially going to MRI if an equivocal reading is found;
- looking at the heart chambers as a whole, such as in patients with atrial fibrillation or other rhythm problems;
- evaluating patients before and after radiofrequency ablation to define any changes to the pulmonary veins; and
- identifying left atrial clots.
Furthermore, newer CT scanners also can help clarify findings of other noninvasive and more traditional cardiology tools, such as nuclear perfusion scans and echocardiography.
Pro 3T MRI
MDCT is the current star of noninvasive cardiac imaging. MRI has taken a slower course; however, it is a far more complex and sophisticated modality. And 3T has given MRI a big boost by immediately doubling the signal-to-noise ratio. “Add improvements in contrast agents to the 3T platform, and you potentially have a modality that, in 10 years, will rival or surpass clinical use of MDCT for noninvasive cardiovascular imaging,” contends radiologist Robert Edelman, MD, chairman of the department of radiology at Evanston as well as author of a number of current articles detailing cardiac applications for both MRI and CT. “As the technology develops and our understanding of its application expands, other advantages will be further realized.”
Edelman adds that another benefit of MRI is that it’s radiation free. “This means that we can do risk-free, repeated studies throughout the course of a patient’s diagnosis and treatment, including postoperatively,” he says. Other MRI advantages are that it has no known biological hazards; can perform a number of studies without contrast agents; and, as a noninvasive modality, is unrivaled in its ability to provide a high degree of contrast and temporal resolution for the assessment of cardiovascular anatomy and pathology. “It also excels at evaluating cardiac function, myocardial viability, and masses and thrombi,” he adds.
When comparing MDCT to 3T MRI, Hamilton has a prediction: “For the next five years, MDCT will probably enjoy far greater utilization than 3T MRI for the heart. But in ten years, it is quite possible that 3T will take over.”
As with his colleague Hamilton, Edelman is quick to point out the significance that 3T MRI will play in the future of cardiac imaging. “It is important to understand that MRI has had a slower and longer evolution than CT. MRI has intricately complex capabilities; and because it’s complex and can do so much more, it is going to take longer to fully come into routine clinical practice.”
In addition to adding sheer power, 3T MRI potentially can provide images with higher spatial resolution and show exquisite details of both anatomy and function. Edelman says, “Once a number of technical challenges of 3T are overcome-such as bumping up against the specific absorption rate, encountering increased artifacts due to the higher magnetic field, adjusting to different tissue-relaxation times, RF homogeneity issues, and the continued need for cardiac gating-the full clinical potential of the higher field strength can be realized.”
|Shown here is the posterior descending artery in 3-D (left) and the aortic arch with calcified plaque (right).|
Also, although 1.5T MRI currently remains the preferred field strength for cardiac imaging, Edelman believes 3T has many potential applications, including:
- evaluating cardiac function;
- imaging coronary artery anatomy;
- depicting cardiac structure via black blood and bright blood studies;
- depicting cardiac motion with dynamic views;
- evaluating changes in the pericardium;
- performing myocardial viability studies in the assessment of coronary artery disease, including use of delayed-enhancement technique; and
- evaluating morphology and hemodynamics as an alternative to invasive cardiac cauterization in appropriate cases for patients with congenital heart disease.
CT and MRI: Synergistic Use
Imagine a cardiac imaging center designed for the synergistic use of high-speed MDCT, 3T MRI, echocardiography, and any other noninvasive imaging modality one’s heart might desire-all in one room. Users merely press a button, and the combined data acquired from multiple modalities is instantly displayed. While this is considered the cardiac lab of the future, some pieces of the puzzle needed to create such a design already exist; others are in development.
Since the early 1970s, the Biomedical Imaging Resource Center at the Mayo Clinic Foundation (Rochester, Minn), for example, has been involved in the design and implementation of computer-based techniques for the display and analysis of multidimensional biomedical images displayed in large, 3-D formats. Today, a number of research institutions continue to experiment and explore the concept of synergistic utilization of imaging modalities from multiple perspectives.
Also, White of the Cleveland Clinic Heart Center is exploring the idea of a cardiac imaging suite. “It’s a lot easier for patients and operators to move between stations than to move between rooms,” he notes. “The idea is to create an imaging suite in which patients feel like they are in a cardiac imaging center, not in a room for a CT scan and another for an MRI. Techniques already are being developed to combine the data acquired by cardiac CT angiography of the coronary arteries and the data regarding myocardial viability acquired by cardiac MRI.”
As a perfect example, White cites the synergistic use of MDCT and MRI to meet the need of increased demand for high-quality prerevascularization planning. “Based on the co-registration of the CT angiography and MRI data, a spatial relationship can be established directly between the diseased coronary artery distribution and the myocardium at risk. The combined data produces a noninvasive road map of the coronary arteries, while MRI identifies areas of the myocardium needing revascularization.”
A Hearty Focus in Indiana
The Heart Center of Indiana (THCI of Indianapolis) is a 210,000-sq-ft facility that received delivery of a 64-slice MDCT scanner, the Sensation 64, from Siemens Medical Solutions (Malvern, Pa) at the end of January. One week later, the unit was up, running, and scanning patients.
Installation of the MDCT at THCI was a carefully planned event. The hospital opened its doors February 2003 with the intent of exemplifying the best in cardiovascular care. Designed as a three-floor facility with the structural capability of growing to six floors, THCI currently includes 56 beds, 32 day/outpatient beds, six cardiac catheterization labs, and four surgical suites. Plus, the facility’s emergency department is operational 24/7.
Shane Aaron, lead technologist and PACS administrator, sees the new MDCT scanner as an exciting evolution in CT technology. “It amazes me how much the technology has changed since I started in 1996,” he says. “The first job I had in CT was using a single-acquisition helical scanner; and now, 9 years later, I’m using a 64-slice scanner. It’s exciting for me, and I think most technologists feel the same way.”
Aaron anticipates that the scanner will be an important asset for both clinicians and patients. “The newer, faster scan is really great for our patients, because exams only last a few minutes; then the patient is on his or her way,” he notes. “For the hospital, the new scanner, with its advanced capabilities, will serve us very well, particularly in conjunction with our surgery suites and catheterization labs.”
MRI use at THCI is not nearly as active as CT is. “At the moment, we are using a Siemens 1.5T MRI scanner,” Aaron explains. “There seems to be more applications for MRI in neuro imaging than cardiac imaging right now, but I suspect that as newer applications become available, 3T MRI will be an excellent tool for examining the coronary arteries and also for doing flow quantifications.”
A well-designed facility and state-of-the-art imaging modalities make a nice package for THCI – both for patients and employees. “I actually like coming to work in the morning,” Aaron admits. “The quality of care we can deliver here makes it an appealing place for technologists, radiologists, cardiologists, staff, and for all of the many patients we see each year.”
3-D and 4-D Workstations: A Critical Role
“As CT and MR angiography enter routine clinical practice, modern 3-D and 4-D workstations play an increasingly critical role in high-tech, noninvasive cardiovascular imaging,” explains Steve Sandy, VP of marketing at TeraRecon (San Mateo, Calif).
Advanced 3-D workstations for MRI and CT can render accurate, high-resolution images for assessing large and medium-size segments of coronary arteries. They also can reveal coronary artery origins and course; the presence, extent, or absence of coronary artery plaque; anatomic severity of the plaque at each location; integrity of bypass grafts; and the patency of intracoronary stents. Images on a 3-D workstation present a complete visualization of the pericardium and pericardial space, the cardiac valves, and the sizes and connections of the great vessels and cardiac chambers.
Today’s 3-D workstations also make possible visualization of the left arterial anatomy, pulmonary venous anatomy, and myocardial venous anatomy. Many forms of congenital heart disease, including coronary artery anomalies, can be characterized. The 3-D ability is especially significant for symptomatic or high-risk patients who can be studied noninvasively before performing a diagnostic angiogram to determine the suitability for interventional versus medical therapy.
Using 4-D multiphase cardiac visualization, ventricular wall motion abnormalities can be identified, and ejection fraction can be calculated. “Most significantly,” Sandy explains, “imaging provides for functional analysis of multiphase studies for interactive image review between the various phases of the coronary cycle.” With 64-slice MDCT, it is important that an advanced workstation be capable of fast-loading all phases of the cardiac cycle and be able to interactively switch between each phase to depict the best pathological view.
Lawrence M. Boxt, MD, chief of cardiovascular medicine at Beth Israel Deaconess Medical Center (Boston), says, “CT angiography and MR angiography have largely replaced conventional catheter angiography for diagnosing vascular disease.” And if Lawrence’s findings are accurate, 3-D and 4-D workstations will necessitate a paradigm shift from the 3-D lab into routine clinical review in many physician practices.
Elizabeth Morgan is a contributing writer for Medical Imaging.