The Clinical Role of Imaging Atherosclerosis
Technological advances have facilitated a rapid expansion in the use of noninvasive coronary artery imaging. There are now numerous options offering enhanced image quality and greater anatomic definition previously unavailable without catheterization. These less invasive imaging procedures are becoming important gatekeepers, helping to select patients for catheterization in anticipation of treatment.
In 10 years, for example, computed tomography (CT) has progressed from single-slice scanners to the current generation of 64-slice machines that can image the whole heart in seconds with submillimeter resolution. Consequently, multislice computed tomography (MSCT) scanning is rapidly gaining acceptance as an alternative to conventional X-ray coronary angiography.
Cardiac magnetic resonance (CMR) technology has improved, too, permitting noninvasive viability testing as well as improved diagnosis of patients with coronary artery disease (CAD). Although the ultimate ability of CT and CMR to visualize the coronary lumen and wall has yet to be determined, certainly major obstruction in the proximal coronary arteries can be recognized with good accuracy. The addition of physiologic information, such as that provided by positron emission tomography (PET) often obtained from the same gantry, will further refine the accuracy of noninvasive assessment of CAD.
Besides these new alternatives to conventional coronary angiography, even "traditional" imaging technologies have evolved. Advanced computer processing in echocardiography has increased the speed and resolution of imaging, permitting development of new modalities such as strain and synchrony imaging. In a recent editorial Steven E. Nissen, MD, FACC, president of the ACC, acknowledged the "near-annual leapfrogs in technology" that have been evident in nuclear cardiology. These advances are resulting in a new paradigm, he said, in which highly trained "imaging specialists" employ multiple diagnostic modalities to provide answers to clinical questions.
"There seems little doubt that access to multiple imaging modalities within a single practice environment can lead to more rapid and accurate diagnoses," said Dr. Nissen.
Anthony N. DeMaria, MD, MACC, editor-in-chief of the Journal of the American College of Cardiology, also sees "a blurring of the boundaries between disciplines, and perhaps the emergence of new types of cardiologists." For example, coupling ultrasound and radioisotope procedures with the new CMR and CT techniques could form the basis for a new cardiovascular imaging specialist.
There is also growing debate regarding how to use noninvasive imaging. Given the elevated risk of cardiovascular events and the higher prevalence of silent CAD in diabetic versus non-diabetic patients (Slide 1), for example, screening asymptomatic diabetic patients for CAD is an appealing concept. Two separate papers in the August 15, 2006, issue of JACC argue for and against such screening in moderate- to high-risk patients with diabetes.
(Enlarge Image)
Slide 1.
Risk of First Myocardial Infarction (MI) in a Diabetic Patient is Comparable to Risk of Reinfarction in Nondiabetics Description: Diabetes confers the same risk of myocardial infarction (MI) in a patient without a history of MI as a prior MI in a nondiabetic patient. Data are from an observational study of 1,373 nondiabetic subjects and 1,059 diabetic subjects in Finland after eight years of follow-up. On the basis of data presented by Haffner et al. (N Engl J Med 1998;339:229-34.) Citation: Reprinted with permission from Bax JJ, Bonow RO, Tschöpe D, Inzucchi SE, Barrett E, on behalf of the Global Dialogue Group for the Evaluation of Cardiovascular Risk in Patients With Diabetes. The potential of myocardial perfusion scintigraphy for risk stratification of asymptomatic patients with type 2 diabetes. J Am Coll Cardiol 2006;48:754-60. Copyright of the American College of Cardiology. Source: Cardiosource image provided by the American College of Cardiology Foundation
In considering the expansion of noninvasive imaging, recent studies have focused on concerns regarding the radiation dose received by patients who undergo noninvasive angiography. Coles and colleagues, for example, studied 91 patients referred for angiography and compared coronary MSCT scanning (12- or 16-slice) prior to conventional angiography in a real world, real patient fashion. The mean effective dose for MSCT coronary angiography was 14.7 mSv compared to 5.6 mSv for conventional angiography. (Slide 2 shows how these levels of radiation compare to other cardiac diagnostic procedures.) While this radiation dose is not prohibitive, strategies that can reduce the radiation exposure without limiting the quality of the study need to be identified.
Shortly before the Coles paper was published, another team of investigators reported radiation dose estimates for 1,035 patients undergoing MSCT in daily practice and the impact of different scanning protocols on effective dose estimates. Radiation dose estimates were lower for patients undergoing CT angiography in this study (Slide 3) compared to the Coles study. Also, the two advanced scanning protocols evaluated reduced radiation exposure by about 40% to 60% (Slide 3). Importantly, measures of technical image quality were largely maintained with both dose-reduction algorithms.
To put these radiation levels in context, the International Commission on Radiological Protection (ICRP) has estimated that the additional lifetime risk of fatal cancer is approximately 1 in 20,000 per mSv for the whole population. However, this age-averaged value overestimates actual radiogenic risk in the older patients most likely to be investigated for CAD. For example, the ICRP estimates that the detriment per unit dose for people more than 50 years of age is one-fifth that of young adults.
Based on available estimates, a coronary CT angiogram with an effective dose of 14.7 mSv has a risk of inducing a fatal cancer of 1 in 1,400. Conventional coronary angiography (5.6 mSv) has a risk of 1 in 3,600, and a calcium-scoring scan (2.6 mSv) a risk of 1 in 7,700.
At least in terms of assessing research proposals, ICRP criteria indicate that the level of risk associated with the estimated radiation produced by 64-slice MSCT is "moderate" requiring "substantial societal benefit needed" to warrant the risk (Slide 4). However, with the use of dose-reduction scanning protocols, level of risk is lowered to an "intermediate" category requiring only "moderate" societal benefit to warrant the risk involved.
In an editorial accompanying the Coles paper, H. William Strauss, MD, and colleagues, noted that conventional coronary angiography is associated with nonradiogenic risks, too, including hemodynamic and neurologic complications as well as a very small risk of mortality. Consequently, after accounting for the known risks of conventional angiography and adjusting the risk of MSCT for patient age, MSCT angiography emerges as the safer of these two diagnostic procedures, despite its higher radiation dose.
Besides potential safety concerns, it's also important to carefully balance diagnostic yield with cost. The ACC has been very proactive in this regard, convening panels of experts to review the current state-of-the-art use and issue appropriateness guidelines indicating when to use particular imaging methods. The College has recently completed this process for nuclear perfusion imaging and will shortly release guidelines for CT and CMR. Echo guidelines are anticipated within the next year. According to Dr. Nissen, "Adherence to these guidelines by our membership is critical. If we do not police ourselves, others will undoubtedly do this for us, but with less expertise and patient focus."
In this interview, Zahi A. Fayad, PhD, FACC, reviews some of the issues and controversies surrounding the use of noninvasive imaging as a screening tool.
(Enlarge Image)
Slide 2.
Effective Radiation Dose from Various Coronary Diagnostic Imaging Studies Description: A study by Coles et al. quantified and compared effective doses from conventional angiography and multislice computed tomography (MSCT) coronary angiography using a 12- or 16-slice scanner. The study involved 180 patients with suspected coronary artery disease and analysis was conducted of a subset of 91 directly comparable patients. The mean effective dose was 5.6 mSv for conventional angiography. This increased significantly (p < 0.016, two-tailed) from a mean of 14.2 mSv when using 12 detectors (n = 51) to 15.3 mSv when using all 16 detectors or 14.7 mSv for MSCT overall. Comparison is made here to other diagnostic imaging studies. Data from: Coles DR, Smail MA, Negus IS, et al. Comparison of radiation doses from multislice computed tomography coronary angiography and conventional diagnostic angiography. J Am Coll Cardiol 2006;47:1840-5 and the international Administration of Radioactive Substances Advisory Committee. Citation: Reprinted with permission from Coles DR, Smail MA, Negus IS, et al. Comparison of radiation doses from multislice computed tomography coronary angiography and conventional diagnostic angiography. J Am Coll Cardiol 2006;47:1840-5. Copyright of the American College of Cardiology. Source: Cardiosource image provided by the American College of Cardiology Foundation
(Enlarge Image)
Slide 3.
Radiation Dose Estimates from MSCT in Daily Practice and Impact of Dose-Saving Algorithms Description: Recently, Hausleiter et al. (Circulation 2006;113:1305-10) compared the estimated dose received during 16- and 64-slice computed tomography angiography (CTA) in daily practice and the impact of different scan protocols on dose and image quality. Radiation dose was estimated for 1,035 patients. The increase in spatial and temporal resolution with 64-slice CTA was associated with an increased radiation dose for coronary CTA. However, dose-saving algorithms were very effective in reducing radiation exposure without reducing diagnostic image quality. Source: Cardiosource image provided by the American College of Cardiology Foundation
(Enlarge Image)
Slide 4.
General Guidelines of the International Commission on Radiological Protection (ICRP) for Assessing Research Proposals Citation: ICRP Publication 62: Radiological Protection in Biomedical Research. A Report of Committee 3 adopted by the International Commission on Radiological Protection. Edited by ICRP ISBN: 0-08-042203-9, Year: 1993 Source: Cardiosource image provided by the American College of Cardiology Foundation
Brindis RG, Douglas PS, Hendel RC, et al. ACCF/ASNC appropriateness criteria for single-photon emission computed tomography myocardial perfusion imaging (SPECT MPI): a report of the American College of Cardiology Foundation Quality Strategic Directions Committee Appropriateness Criteria Working Group and the American Society of Nuclear Cardiology endorsed by the American Heart Association. J Am Coll Cardiol 2005;46:1587-605.
Budoff MJ, Cohen MC, Garcia MJ, et al. ACCF/AHA clinical competence statement on cardiac imaging with computed tomography and magnetic resonance: a report of the American College of Cardiology Foundation/American Heart Association/American College of Physicians Task Force on Clinical Competence and Training. J Am Coll Cardiol 2005;46:383-402.
Technological advances have facilitated a rapid expansion in the use of noninvasive coronary artery imaging. There are now numerous options offering enhanced image quality and greater anatomic definition previously unavailable without catheterization. These less invasive imaging procedures are becoming important gatekeepers, helping to select patients for catheterization in anticipation of treatment.
In 10 years, for example, computed tomography (CT) has progressed from single-slice scanners to the current generation of 64-slice machines that can image the whole heart in seconds with submillimeter resolution. Consequently, multislice computed tomography (MSCT) scanning is rapidly gaining acceptance as an alternative to conventional X-ray coronary angiography.
Cardiac magnetic resonance (CMR) technology has improved, too, permitting noninvasive viability testing as well as improved diagnosis of patients with coronary artery disease (CAD). Although the ultimate ability of CT and CMR to visualize the coronary lumen and wall has yet to be determined, certainly major obstruction in the proximal coronary arteries can be recognized with good accuracy. The addition of physiologic information, such as that provided by positron emission tomography (PET) often obtained from the same gantry, will further refine the accuracy of noninvasive assessment of CAD.
Besides these new alternatives to conventional coronary angiography, even "traditional" imaging technologies have evolved. Advanced computer processing in echocardiography has increased the speed and resolution of imaging, permitting development of new modalities such as strain and synchrony imaging. In a recent editorial Steven E. Nissen, MD, FACC, president of the ACC, acknowledged the "near-annual leapfrogs in technology" that have been evident in nuclear cardiology. These advances are resulting in a new paradigm, he said, in which highly trained "imaging specialists" employ multiple diagnostic modalities to provide answers to clinical questions.
"There seems little doubt that access to multiple imaging modalities within a single practice environment can lead to more rapid and accurate diagnoses," said Dr. Nissen.
Anthony N. DeMaria, MD, MACC, editor-in-chief of the Journal of the American College of Cardiology, also sees "a blurring of the boundaries between disciplines, and perhaps the emergence of new types of cardiologists." For example, coupling ultrasound and radioisotope procedures with the new CMR and CT techniques could form the basis for a new cardiovascular imaging specialist.
There is also growing debate regarding how to use noninvasive imaging. Given the elevated risk of cardiovascular events and the higher prevalence of silent CAD in diabetic versus non-diabetic patients (Slide 1), for example, screening asymptomatic diabetic patients for CAD is an appealing concept. Two separate papers in the August 15, 2006, issue of JACC argue for and against such screening in moderate- to high-risk patients with diabetes.
(Enlarge Image)
Slide 1.
Risk of First Myocardial Infarction (MI) in a Diabetic Patient is Comparable to Risk of Reinfarction in Nondiabetics Description: Diabetes confers the same risk of myocardial infarction (MI) in a patient without a history of MI as a prior MI in a nondiabetic patient. Data are from an observational study of 1,373 nondiabetic subjects and 1,059 diabetic subjects in Finland after eight years of follow-up. On the basis of data presented by Haffner et al. (N Engl J Med 1998;339:229-34.) Citation: Reprinted with permission from Bax JJ, Bonow RO, Tschöpe D, Inzucchi SE, Barrett E, on behalf of the Global Dialogue Group for the Evaluation of Cardiovascular Risk in Patients With Diabetes. The potential of myocardial perfusion scintigraphy for risk stratification of asymptomatic patients with type 2 diabetes. J Am Coll Cardiol 2006;48:754-60. Copyright of the American College of Cardiology. Source: Cardiosource image provided by the American College of Cardiology Foundation
Radiation Exposure
In considering the expansion of noninvasive imaging, recent studies have focused on concerns regarding the radiation dose received by patients who undergo noninvasive angiography. Coles and colleagues, for example, studied 91 patients referred for angiography and compared coronary MSCT scanning (12- or 16-slice) prior to conventional angiography in a real world, real patient fashion. The mean effective dose for MSCT coronary angiography was 14.7 mSv compared to 5.6 mSv for conventional angiography. (Slide 2 shows how these levels of radiation compare to other cardiac diagnostic procedures.) While this radiation dose is not prohibitive, strategies that can reduce the radiation exposure without limiting the quality of the study need to be identified.
Shortly before the Coles paper was published, another team of investigators reported radiation dose estimates for 1,035 patients undergoing MSCT in daily practice and the impact of different scanning protocols on effective dose estimates. Radiation dose estimates were lower for patients undergoing CT angiography in this study (Slide 3) compared to the Coles study. Also, the two advanced scanning protocols evaluated reduced radiation exposure by about 40% to 60% (Slide 3). Importantly, measures of technical image quality were largely maintained with both dose-reduction algorithms.
To put these radiation levels in context, the International Commission on Radiological Protection (ICRP) has estimated that the additional lifetime risk of fatal cancer is approximately 1 in 20,000 per mSv for the whole population. However, this age-averaged value overestimates actual radiogenic risk in the older patients most likely to be investigated for CAD. For example, the ICRP estimates that the detriment per unit dose for people more than 50 years of age is one-fifth that of young adults.
Based on available estimates, a coronary CT angiogram with an effective dose of 14.7 mSv has a risk of inducing a fatal cancer of 1 in 1,400. Conventional coronary angiography (5.6 mSv) has a risk of 1 in 3,600, and a calcium-scoring scan (2.6 mSv) a risk of 1 in 7,700.
At least in terms of assessing research proposals, ICRP criteria indicate that the level of risk associated with the estimated radiation produced by 64-slice MSCT is "moderate" requiring "substantial societal benefit needed" to warrant the risk (Slide 4). However, with the use of dose-reduction scanning protocols, level of risk is lowered to an "intermediate" category requiring only "moderate" societal benefit to warrant the risk involved.
In an editorial accompanying the Coles paper, H. William Strauss, MD, and colleagues, noted that conventional coronary angiography is associated with nonradiogenic risks, too, including hemodynamic and neurologic complications as well as a very small risk of mortality. Consequently, after accounting for the known risks of conventional angiography and adjusting the risk of MSCT for patient age, MSCT angiography emerges as the safer of these two diagnostic procedures, despite its higher radiation dose.
Besides potential safety concerns, it's also important to carefully balance diagnostic yield with cost. The ACC has been very proactive in this regard, convening panels of experts to review the current state-of-the-art use and issue appropriateness guidelines indicating when to use particular imaging methods. The College has recently completed this process for nuclear perfusion imaging and will shortly release guidelines for CT and CMR. Echo guidelines are anticipated within the next year. According to Dr. Nissen, "Adherence to these guidelines by our membership is critical. If we do not police ourselves, others will undoubtedly do this for us, but with less expertise and patient focus."
In this interview, Zahi A. Fayad, PhD, FACC, reviews some of the issues and controversies surrounding the use of noninvasive imaging as a screening tool.
(Enlarge Image)
Slide 2.
Effective Radiation Dose from Various Coronary Diagnostic Imaging Studies Description: A study by Coles et al. quantified and compared effective doses from conventional angiography and multislice computed tomography (MSCT) coronary angiography using a 12- or 16-slice scanner. The study involved 180 patients with suspected coronary artery disease and analysis was conducted of a subset of 91 directly comparable patients. The mean effective dose was 5.6 mSv for conventional angiography. This increased significantly (p < 0.016, two-tailed) from a mean of 14.2 mSv when using 12 detectors (n = 51) to 15.3 mSv when using all 16 detectors or 14.7 mSv for MSCT overall. Comparison is made here to other diagnostic imaging studies. Data from: Coles DR, Smail MA, Negus IS, et al. Comparison of radiation doses from multislice computed tomography coronary angiography and conventional diagnostic angiography. J Am Coll Cardiol 2006;47:1840-5 and the international Administration of Radioactive Substances Advisory Committee. Citation: Reprinted with permission from Coles DR, Smail MA, Negus IS, et al. Comparison of radiation doses from multislice computed tomography coronary angiography and conventional diagnostic angiography. J Am Coll Cardiol 2006;47:1840-5. Copyright of the American College of Cardiology. Source: Cardiosource image provided by the American College of Cardiology Foundation
(Enlarge Image)
Slide 3.
Radiation Dose Estimates from MSCT in Daily Practice and Impact of Dose-Saving Algorithms Description: Recently, Hausleiter et al. (Circulation 2006;113:1305-10) compared the estimated dose received during 16- and 64-slice computed tomography angiography (CTA) in daily practice and the impact of different scan protocols on dose and image quality. Radiation dose was estimated for 1,035 patients. The increase in spatial and temporal resolution with 64-slice CTA was associated with an increased radiation dose for coronary CTA. However, dose-saving algorithms were very effective in reducing radiation exposure without reducing diagnostic image quality. Source: Cardiosource image provided by the American College of Cardiology Foundation
(Enlarge Image)
Slide 4.
General Guidelines of the International Commission on Radiological Protection (ICRP) for Assessing Research Proposals Citation: ICRP Publication 62: Radiological Protection in Biomedical Research. A Report of Committee 3 adopted by the International Commission on Radiological Protection. Edited by ICRP ISBN: 0-08-042203-9, Year: 1993 Source: Cardiosource image provided by the American College of Cardiology Foundation
Guidelines
Brindis RG, Douglas PS, Hendel RC, et al. ACCF/ASNC appropriateness criteria for single-photon emission computed tomography myocardial perfusion imaging (SPECT MPI): a report of the American College of Cardiology Foundation Quality Strategic Directions Committee Appropriateness Criteria Working Group and the American Society of Nuclear Cardiology endorsed by the American Heart Association. J Am Coll Cardiol 2005;46:1587-605.
Budoff MJ, Cohen MC, Garcia MJ, et al. ACCF/AHA clinical competence statement on cardiac imaging with computed tomography and magnetic resonance: a report of the American College of Cardiology Foundation/American Heart Association/American College of Physicians Task Force on Clinical Competence and Training. J Am Coll Cardiol 2005;46:383-402.
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