The tightest coronary stenosis is not necessarily the site of the infarction: 70% of myocardial infarctions occur in patients who have no stenoses greater than 50%. A simple radiograph of the chest does not reveal whether an area of myocardium is viable, stunned, or dead.
Depiction of cardiac anatomy with imaging is no longer enough: increasingly, imagers of the heart seek biochemical and functional information. One goal is to refine the selection of treatment for individual patients. This article looks at newer developments in cardiac imaging.
Echo Expands
Ultrasonography is one of the classic methods for imaging the heart and continues to improve and to be used in new applications. Today, contrast-enhanced studies, harmonic imaging, tissue tracking, and power Doppler studies are all possible. In utero studies allow pediatricians to be prepared to provide the best care for infants with congenital cardiac anomalies. Echocardiography can enable emergency department physicians to place most transvenous cardiac pacing devices themselves without sending for the cardiologist.1 According to a paper presented at the American Society of Echocardiography in June, ultrasonography may be superior to the treadmill stress test for evaluating women with chest pain.
In December 1999, the US Food and Drug Administration (FDA) approved a device that marks a new frontier in cardiac ultrasonography: a 3.3-mm catheter that can be passed into the heart from the femoral or internal jugular vein. The multifrequency tip can be steered and acquires images with various degrees of magnification. Spectral, color, pulsed-wave, and continuous-wave Doppler imaging are all possible. Commenting on the FDA approval, James Seward, MD, director, Echocardiography Laboratory, Mayo Clinic, Rochester, Minn, and one of the inventors of the device, says that it “enables us to see clearly areas of anatomy that govern heart function we have not been able to visualize previously.”
Single-photon emission computed tomography (SPECT) using one or two radioisotopes is already firmly established in cardiac imaging. It is superior to standard nuclear medicine studies for localizing coronary artery disease, and as many as 90% of myocardial perfusion studies are now done by SPECT.
The modality also has the potential to save a great deal of money, according to presentations at the Society for Nuclear Medicine meeting in 1998. A study by Emory University and the Miami Cardiac and Vascular Institute demonstrated that routine use of SPECT (approximate cost, $600) to measure perfusion reduced the need for contrast angiography (approximate cost, $3,450) by 70%. Over a 6-month period, using SPECT as the first study saved payors $759,000 for patients at both institutions. Another study, at St Luke’s-Roosevelt Hospital in New York City, found that stress perfusion scans with dipyridamole identified patients who could safely be released early from the hospital after myocardial infarction. Patients with normal stress SPECT scans realized a 4-day reduction in their hospital stay.
Jack A. Ziffer, MD, director of cardiac imaging, Miami Cardiac and Vascular Institute, says that “SPECT is superb, not only for defining whether patients have coronary artery disease, but also for determining whether they should undergo angiography and revascularization.
“As a society, we have to come to grips with the question of who needs to have the most expensive components of health care,” he continues. “Rather than rationing care, we are clearly better off if we can find out who will benefit most from a procedure. In coronary artery disease, SPECT provides the opportunity to do just that.”
Ziffer also is enthusiastic about an important technical advance. One drawback of SPECT has been the inevitable signal attenuation. That is, when the signal emitted by an isotope in the heart passes to the detector, some of it is lost through absorption (attenuation) by the intervening tissues. Different amounts and types of tissue intervene between the signal and the detector in different parts of the heart, reducing the accuracy of quantitative measurements. Various methods such as the line source technique have been used to correct for this signal loss.
“Instead of using a line source to obtain attenuation measurements, you can now use a small CT scanner that provides transmission images with good resolution and a much lower noise attenuation map to apply to the isotope images. Although there are as yet no data from prospective studies confirming that this is a better approach to attenuation correction, people who have worked with it are impressed,” he reports.
Ziffer believes that his institution was the first to use the instrumentation-a hybrid camera that combines CT and SPECT- for cardiac applications, but he suspects that hundreds of the scanners will be installed by the end of 2001.
“There is no CPT code for attenuation correction now,” he notes, “so we need to develop addendum codes. But I think that providing this technology, with the better service it yields, will distinguish a hospital from those that do not have it. Distinguishing yourself is important, given the wide availability of SPECT. Importantly, once you have the instrumentation, the technical costs of performing this attenuation correction are zero.” Other uses of SPECT are being explored. Leo Hofstra, MD, and his associates in the Department of Cardiology, University Hospital Maastricht, The Netherlands, injected a 99mTc-labeled protein, annexin-V, which binds to cells undergoing programmed death (apoptosis), into six patients who had undergone successful angioplasty after a myocardial infarction.2 Images were obtained at a mean of 3.4 and 20.5 hours later. In six of the seven patients, increased uptake of the labeled protein was seen in the area of the infarct, suggesting that early identification of the extent of myocardial death will be possible.
PET’s role
Positron emission tomography (PET) utilizes positron-emitting isotopes of elements that naturally occur in the body or of elements such as fluorine (a hydrogen substitute) that can take their places. Because it has higher resolution than SPECT, it has greater sensitivity.
There are two main roles for PET in cardiac imaging: detecting coronary artery disease and assessing myocardial viability.
“The reported accuracy of SPECT for detecting coronary artery disease is about 80%, whereas the accuracy of PET is about 90%,” says Johannes Czernin, MD, director of nuclear medicine, UCLA Ahmanson Biological Imaging Center, Los Angeles. “The problem is that this use of PET is not approved by the Health Care Financing Administration (HCFA) except when it is used with rubidium-82 as a blood flow tracer. Not many people do this study, as you have to have a generator to produce the radioisotope.”
A far more common use of cardiac PET is in the assessment of myocardial viability. Cells that are alive take up glucose, including that tagged with fluorine 18-labeled deoxyglucose (FDG). Thus, FDG-PET will show whether dysfunctional areas of the myocardium are viable. At UCLA, which has a large clinic for patients with heart failure and a cardiac transplant program, PET is part of the clinical algorithm.
“In patients with heart failure syndromes, the question is whether they should undergo bypass surgery or cardiac transplantation,” Czernin explains. “If PET shows extensive viability, and the patient undergoes bypass surgery, he or she has an 80% to 95% chance of obtaining significant improvement in cardiac function and will not need a transplant. Daily activity levels and symptoms improve, and the life expectancy seems to be extended.
“In about 25% of patients in whom all other tests indicate irreversible tissue damage, we find extensive viability by PET,” Czernin reports.
Use of PET in treatment selection is important, due to high demand for transplantation but a limited number of donor hearts.
“With PET, you can avoid using a donor heart in a patient who would be served at least as well with bypass surgery,” Czernin points out.
In December 2000, the Health Care Financing Administration made PET reimbursable for the evaluation of myocardial viability when the SPECT study is inconclusive. However, reimbursement is available only for studies performed on high-performance dedicated PET scanners. The HCFA explanation for this limit was that most of the data on which the approval was based had been obtained on such scanners and that the value of other instruments could not be assessed at this time.
Czernin and other PET specialists believe the study could be used for other purposes.
“If a patient has an occluded coronary artery, and the interventionalists or surgeons are considering opening the infarcted vessel, a PET scan might be reasonable to determine whether the myocardium in that region is viable,” he points out. “However, PET generally is not used for that purpose.”
Investigators from UCLA also demonstrated the value of two-isotope PET for evaluating heart disease in children as young as 2 weeks. In a presentation at the Society of Nuclear Medicine meeting in June 2000, Miguel Hernandez Pampaloni, MD, reported total agreement between the findings of angiography and the safer PET study performed with FDG and ammonia-13.
Update on Electron-Beam CT
The ability of electron-beam CT (EBCT) to depict calcium in the coronary arteries has created a great controversy: should it be used for screening people for coronary artery disease? Last summer, the American College of Cardiology and the American Heart Association (AHA) weighed in with a consensus document that may prove to be a setback to EBCT use in screening.3 The task force concluded that available data do not clearly show whether calcium scores add to the prognostic information already available such as the Framingham score. Moreover, the document states, the literature does not clearly define which asymptomatic persons require or will benefit from EBCT. The document recommends further trials but noted that, given the uncertainty, “EBCT screening should not be made available to the general public without a physician’s request.”3
While this debate continues, EBCT may be finding a role as a diagnostic tool in symptomatic patients. In an article published in July 2000, cardiologists from Harbor-UCLA and Long Beach Veterans Hospital reported that in patients with symptoms of coronary artery disease, EBCT had higher diagnostic ability than treadmill ECG or technetium stress testing in detecting obstructive lesions.4 The relative risk of obstructive disease was 4.53 for a positive EBCT test vs 1.72 for treadmill ECG and 1.96 for the nuclear medicine study.
Cardiac MRI: Are We There Yet?
For many years, advocates of MRI have described its value for cardiac imaging. However, the difficulties of imaging the moving organ held the modality back from routine clinical use. But the situation is changing. Equipment manufacturers are providing software and protocols specific for MRI evaluation of cardiac morphology, blood flow, perfusion, and function (eg, wall motion), as well as for MR angiography. A session at the Scientific Sessions 2000 of the American Heart Association, held in New Orleans, was titled “Cardiac Magnetic Resonance Imaging: The Future Is Now.” In that session, speakers described contemporary uses of MRI for evaluation of ventricular function, myocardial perfusion, myocardial viability, and coronary angiography, respectively.
Some centers are making use of many of these techniques. For example, Steven D. Wolff, MD, PhD, director of cardiovascular MRI, Integrated Cardiovascular Thera-
peutics in Woodbury, NY, has imaged more than 200 patients with suspected ischemic heart disease. In each patient, contrast-enhanced first-pass perfusion scans were performed at rest and during adenosine stress. Between these scans, a resting left ventricular function study was obtained. The entire set of scans required about a half hour. A team headed by Jürg Schwitter of University Hospital in Zürich, Switzerland, used phase-contrast MRI for comprehensive assessment of coronary hemodynamics and left ventricular function.5 The same technique also is being applied to evaluate patients with chest pain after percutaneous revascularization.6
Despite enthusiastic reports such as these, not everything that can be done by MRI is performed routinely at most medical centers.
“There still are few commonly accepted indications for MRI of the heart,” according to Leon Axel, PhD, MD, professor of radiology and medicine, University of Pennsylvania, Philadelphia. One of the more common studies done with MRI in his department is analysis of aortic dissections and dilations.
“The clinicians have been very happy with the information they can get from MRI in these patients,” he reports. He notes that MRI is valuable for delineating and characterizing masses in and around the heart, in depicting the geometry of congenital heart disease, and in assessing patients with complications after cardiac surgery.
An increasingly common indication is evaluation of patients with dysrhythmias. “People can acquire dysrhythmias as a result of abnormal muscle in the right ventricular wall,” he says. “This is a difficult area to examine because the ventricular wall is relatively thin.”
Most patients with heart disease have coronary atherosclerosis, and enormous effort has gone into developing coronary MR angiography. “Undoubtedly, MRI has the potential to provide valuable information in these patients; the question is whether MRI will be able to replace echocardiography or other established procedures,” Axel notes. He and his associates are using MRI to study regional myocardial contraction, searching for an efficient way to obtain quantitative information.
Many years ago, Valentin Fuster, MD, director, Cardiovascular Institute, Mt Sinai Medical Center, New York City, demonstrated that MRI can characterize atherosclerotic plaque. He and his associates have used MRI extensively in the laboratory to monitor the effects of cholesterol-lowering drugs and other interventions for atherosclerosis in laboratory animals. Such plaque characterization is moving beyond the laboratory. At the scientific sessions of the Radiological Society of North America in 1999, Zahi Fayad, PhD, director of cardiovascular imaging, Mt Sinai School of Medicine, described the use of fast double inversion recovery fast spin echo MRI at 1.5T in characterizing plaques. Among the features that could be seen were the fibrous cap and the lipid core, making it possible to identify vulnerable plaques at risk of rupture and thus patients at risk of a cardiac event. Another method of examining coronary vessel walls was described at the AHA meeting last year by Jeffrey H. Maki, MD, PhD, and his colleagues at the VA Puget Sound Medical Center-University of Washington, Seattle. They demonstrated that an experimental MRI contrast agent that binds to albumin can identify inflammation in the arterial wall, thought to be a sign of growing plaque.7
“When people talk about cardiac MRI, you hear the term one-stop shop,” Axel says. “With MRI, you can depict the anatomy very nicely, you can look at the coronary arteries, you can study perfusion by examining the dynamics of contrast enhancement, and you can characterize the tissue using relaxation times and contrast dynamics in a way you might not otherwise be able to. Moreover, all the information would be registered. Now, you must do several studies, and it is hard to correlate the results. However, comprehensive cardiac studies are not yet realized for turnkey use.”
Given that heart disease is the greatest health problem in the United States, enormous effort is being put into research on its diagnosis and treatment. Dramatic changes in the capabilities of the various imaging modalities are likely to continue, improving the selection of treatment.
Judith Gunn Bronson, MS, is a contributing writer for Decisions in Axis Imaging News.
References:
- Aguilera PA, Durham BA, Riley DA. Emergency transvenous cardiac pacing placement using ultrasound guidance. Ann Emerg Med. 2000;36:224–227.
- Hofstra L, Liem IH, Dumont EA, et al. Visualisation of cell death in vivo in patients with acute myocardial infarction. Lancet. 2000;356:209–212.
- O’Rourke RA, Brundage BH, Froehlicher VF, et al. American College of Cardiology/ American Heart Association Expert Consensus Document on Electron-Beam Computed Tomography for the Diagnosis and Prognosis of Coronary Artery Disease. Circulation. 2000;36:326–340.
- Shavelle DM, Budoff MJ, LaMont DH, Shavelle RM, Kennedy JM, Brundage BH. Exercise testing and electron beam computed tomography in the evaluation of coronary artery disease. J Am Coll Cardiol. 2000; 36:32-38.
- Schwitter J, DeMarco T, Kneifel S, et al. Magnetic resonance-based assessment of global coronary flow and flow reserve and its relation to left ventricular functional parameters: a comparison with positron emission tomography. Circulation. 2000;101:2696–2702.
- Hundley WG, Hillis LD, Hamilton CA, et al. Assessment of coronary arterial restenosis with phase-contrast magnetic resonance imaging measurements of coronary flow reserve. Circulation. 2000;101:2375–2381.
- Maki JH, Wilson GJ, Lauffer RB, Weisskoff RM, Yuan C. Vessel wall enhancement with MS-325 facilitates plaque detection and characterization [abstract 1832]. Circulation. 2000;102(suppl):II-375.
