Echocardiography confronts era of cardiac CT and MRI

New advances in technology keep older modality in the picture for cost-effective cardiovascular diagnosis

The last decade has seen remarkable growth in applications of MR imaging and x-ray CT for the diagnosis of diseases affecting the cardiovascular system. Cardiovascular MR (CMR) provides extremely high-quality images of the cardiac chambers, their vascular connections, myocardial perfusion, myocardial scar, and myocardial tissue characteristics. New cardiac CT (CCT) scanners have the ability to generate high-resolution coronary artery images and provide chamber visualization.

Echocardiography, since its inception approximately 35 years ago, when the M-mode approach was developed, has also grown substantially, allowing imaging of cardiac morphology and function and assessment of hemodynamics through Doppler technology. This ubiquitous modality is versatile and portable. The place of echocardiography in the context of rapid growth in CCT and CMR is complex.

CMR imaging is performed at high magnetic field strengths (1.5T to 3T), requires no ionizing radiation, and uses mostly nontoxic gadolinium chelate-based contrast agents. The instrumentation is heavy and nonportable, however, and the intense magnetic fields preclude the use of CMR in patients with implantable ferromagnetic devices. The cost of a CMR scanner is about $1.5 to $2.5 million, depending on field strength, hardware, and software configuration.

CCT uses a substantial dose of ionizing radiation and requires the use of radiopaque contrast agents. The amount of radiation for a cardiac exam is in the neighborhood of 10 mSv, which is equivalent to twice the exposure received with diagnostic coronary angiography. Iodinated radiopaque contrast agents may cause allergic reactions and nephrotoxicity. The approximate cost of a CCT scanner is $1.5 million.

Echocardiography uses no ionizing radiation. Rather, it uses nonionizing ultrasound to image morphology, function, and flow. Cardiac ultrasound has no known harmful effects. The equipment is portable, and cost ranges between $100,000 and $250,000. Medicare global reimbursement for CMR and for CCT (if available) is two to three times more than for echocardiography.

NEW DEVELOPMENTS IN ECHO

With developments in computer processing speed and technology, the temporal resolution and image quality of echocardiography have markedly improved. Typical 2D frame rates are 50 to 100/sec, and frame rates up to 200/sec can be obtained by reducing either the field-of-view or line density. Myocardial velocity imaging allows evaluation of myocardial stress and strain rates at a temporal resolution of 5 to 6 msec.

These measurements can be used to assess myocardial synchrony and assist in the selection of appropriate candidates for biventricular pacing. Three-D echocardiography is an evolving clinical tool and should have the added benefit of allowing assessment of complex geometries and flow jets. The 3D images can be obtained in real-time or reconstructed from 2D data using temporal and spatial registration. Myocardial perfusion imaging is promising but still investigational, with a quality inferior to CMR and radionuclide techniques.

Echocardiography has several advantages over both MR and CT:

- Availability. Echocardiography is found in virtually all hospitals and most outpatient facilities. All cardiologists are trained to perform and interpret at least basic echo studies. Both CCT and CMR are becoming more widely available, but, while approved three-year cardiology fellowship programs require a modicum of training, that training is insufficient to allow credentialing in these technologies. Elective experience in CMR or CCT provides a means for a cardiology trainee to become credentialed in one of these two modalities within a three-year fellowship. A fourth year of training would provide a means for credentialing in both tomographic imaging modalities. Radiologists have general expertise in the technical aspects of CT and MR but require further training in the use of these specialized technologies for cardiac applications and in cardiac anatomy, physiology, and pathology.

- Portability. Only echo can be taken to the patient's bedside. This is important in critically ill patients in the ICU, CCU, and emergency room and for applications in the operating room. Some current systems that weigh only 10 pounds generate images of diagnostic quality.

- Cost. The initial and operating costs for echocardiography are considerably less than for CMR and CCT. The equipment cost for echocardiography is approximately 10% of the cost of the other techniques. Space and facility preparation costs are also substantially less for echo.

APPLICATIONS

The use of echocardiography in various aspects of cardiac imaging demonstrates its strengths in comparison with CCT and CMR.

- Assessment of hemodynamics. An optimal 30 to 60-minute echo study allows visualization and functional assessment of all four cardiac chambers, all four valves, and all the venous and arterial connections of the heart. Echocardiography is the gold standard for evaluation of valvular anatomy and function. The hemodynamic assessments are well validated, and these are a major strength compared with CCT and CMR. Approximations of quantitative pulmonary artery pressure and ventricular filling pressures can be provided. LV diastolic performance can be evaluated in a semiquantitative fashion through combinations of tricuspid and pulmonary regurgitant velocities, mitral flow, pulmonary vein flow, and myocardial velocities using Doppler techniques.

- Valvular anatomy. An example of this application is the assessment of the anatomy of the mitral apparatus in a patient with mitral regurgitation to define cause, mechanism, and surgical reparability. Individual segments of the mitral leaflets, leaflet mobility, leaflet size, annulus dimension, and subvalvular apparatus can be imaged and quantified within a matter of minutes during a routine comprehensive imaging protocol. In patients with suboptimal images, transesophageal echocardiography can be used. Echocardiography is a reliable technique to determine the appropriate operative approach, such as repair versus prosthetic valvular replacement.

- Valvular function. Echo can be used in the assessment of valvular function in a patient with mitral regurgitation. Using the color flow jet size, vena contracta, and proximal isovelocity surface area, the examiner can assess severity of regurgitation and also obtain regurgitant volume, regurgitant fraction, and effective regurgitant orifice area. Left atrial pressure can be assessed using a combination of mitral inflow and myocardial velocity. The size of the left atrial V wave can be estimated from the mitral regurgitant continuous velocity profile and the duration of LV isovolumetric relaxation time. Pulmonary artery pressure related to mitral regurgitation can be assessed by examining tricuspid and pulmonary regurgitant velocities. None of these assessments is easily obtained with CCT or CMR.

- Congenital heart disease. Because of echocardiography's ability to assess cardiac anatomy and abnormal flows, it is the initial imaging study used for assessment of congenital heart disease. It allows ready quantitation of flow and comprehensive dynamic assessment. The addition of CMR is especially valuable for visualization of venous and arterial connections, their origins and course, abnormal connections, narrowings, and anatomic characterization of coronary anomalies. Echocardiography and CMR play complementary roles in the assessment of patients with complex congenital heart disease, generally precluding the need for cardiac catheterization.

- Aortic dissection. Acute aortic dissection is a potentially life-threatening condition, and rapid diagnosis and triaging to medical versus surgical therapy are critical. All three imaging modalities are equivalent in terms of diagnostic accuracy. In a hemodynamically unstable patient, transesophageal echocardiography's portability and ability to rapidly assess aortic valve involvement and ventricular dynamics give it an advantage. CMR or CCT, with their 3D imaging capability and greater comfort level, are superior in a stable patient (not forgetting the effect of the contrast agent on kidney function). CMR or CCT of an unstable patient cannot be performed in the operating room, where transesophageal echocardiography helps not only in diagnosing dissection and associated lesions but also in the assessment of successful repair postoperatively.

- Endocarditis. Transesophageal echocardiography is the best available modality to visualize vegetations, because of its high spatial and temporal resolution and the ability to easily maneuver the imaging plane in real-time to optimize that visualization as well as the system's attachments, mobility, and size. It can also very accurately assess valvular perforations, perivalvular abscesses, fistulae, and pseudoaneurysms. These are easier to obtain with transesophageal echo because of its unlimited ability to adjust the tomographic plane for optimal visualization of the pathological process.

CLINICAL EXAMPLE

A 73-year-old patient with three-vessel coronary artery bypass surgery three years earlier presented with dyspnea and lower limb edema. The apical long-axis view showed akinesis of the LV inferolateral wall with preserved wall thickness, indicating possible viable ischemic wall (Figure 1: click here to see the video). The mitral leaflets were apically tented, posterior more than anterior, due to this wall motion abnormality, allowing mitral regurgitation. Color flow imaging showed mitral regurgitation with an effective regurgitant orifice area of 27 mm2, indicating moderate regurgitation (Figure 2).

Mitral flow pattern (Figure 3) showed an E/A (early to atrial, or late) ratio of 2.5 and E wave deceleration time of 100 msec, indicating high left atrial pressure. Figure 4 shows a continuous wave velocity profile of aortic regurgitation with rapid deceleration (pressure half-time, 250 msec) associated with a prominent holodiastolic flow reversal in the aortic arch (Figure 5), suggesting volumetrically and hemodynamically severe aortic regurgitation. The inferior vena cava was dilated, suggesting elevated right atrial pressure (Figure 6: click here to see the video).

Color flow imaging from the subcostal view showed patent foramen ovale due to atrial septal stretching (Figure 7: click here to see the video) with a peak velocity 2.5 m/sec in late systole corresponding to a pressure gradient of 25 mm Hg (Figure 8). Assuming an estimated right atrial pressure of 15 mm Hg, this corresponds to a left atrial V wave amplitude of 40 mm Hg.

FUTURE roles

Applications of echocardiography, CCT, and CMR continue to increase. Healthcare costs associated with diagnostic cardiac imaging have increased in the past decade and are likely to continue to rise because of the rapid progress in each of these three areas. The appropriateness criteria for CMR and CCT recently developed by the American College of Cardiology provides guidelines for the use of these technologies.

Echocardiography has become the first line of diagnostic evaluation for the cardiologist. It is the diagnostic approach used after the stethoscope, even, unfortunately, in place of the stethoscope by the poorly trained cardiovascular specialist. One condition in which the echocardiogram might not be the first approach for diagnosis is coronary artery disease in one of the most common scenarios seen by the cardiologist: chest pain where a stress perfusion study, adenosine stress CMR, or even a CT coronary angiogram provide equally useful information for the diagnosis and staging of the severity of coronary artery disease.

The most effective noninvasive imaging approach to detect the presence of significant coronary artery disease and the need to progress to a coronary angiogram in anticipation of revascularization (percutaneous coronary intervention or coronary artery bypass graft surgery) depends on the physician's expertise and the availability, risk/benefit, and effectiveness of the technology in the diagnostic facility. Stress echocardiography has been used widely, but it is not necessarily the most effective approach for diagnosis of coronary artery disease in patients unable to exercise or those with a poor acoustic window. The cardiologist still uses the treadmill stress test as an important, easily performed, and readily available approach.

If CMR is available in a laboratory with a high level of expertise, a study to evaluate ventricular function, myocardial perfusion (with adenosine), and the presence of myocardial scar would provide the most information without ionizing radiation. This would thus be the best first approach. While CCT provides better images of the coronary arteries and can be used to diagnose significant coronary artery disease, it cannot address microvascular disease and introduces both a greater radiation dose than that of diagnostic coronary angiography and the potential for renal toxicity from iodinated contrast.

Myocardial perfusion imaging using a radionuclide approach is another important alternative. It is widely available, and there is reasonable expertise among many cardiovascular specialists. But the need for a radionuclide with attendant ionizing radiation is considered by most to be a negative feature.

Rapid improvements in CMR and CCT have made choosing the optimal diagnostic imaging approach a moving target. In 2006, echocardiography remains the most cost-effective first-line imaging modality for many cardiac diagnoses. We believe that this will continue to be the case for the foreseeable future.

Dr. Pai is a professor of medicine, Dr. Varadarajan is an assistant professor of medicine, and Dr. Pohost is a professor of medicine, all in the division of cardiovascular medicine at the University of Southern California-Keck School of Medicine in Los Angeles.