Maximizing dose reductions with cardiac CT

Multidetector computed tomography has come a long way in a short time, quickly becoming a standard tool in the cardiac imaging armamentarium. The promise of plaque imaging, combined with both anatomical visualization and stenosis detection, has made this a preferred first line test of many cardiologists and radiologists. This test is well suited to rule out coronary artery disease (obstruction) and still diagnosing subclinical plaque, with may be a good target for anti-atherosclerotic therapies. There has been recent criticism against CT imaging, and cardiac CT specifically, due to the high radiation doses that being employed. New advances have allowed for dramatic dose reductions. These include more routinely performed methods such as dose modulation, and newer methods such as prospective gating or minimizing the field of view. This paper will review the different applications to reduce cardiac CT radiation doses to nominal levels, potentially expanding the applications of cardiac CT by removing one of the biggest barriers.     

Techniques and parameters for estimating radiation exposure and dose in cardiac computed tomography

With increasing clinical use of cardiac CT imaging it is important that all health care providers referring for or administering such examinations are familiar with the concepts and values of radiation dosimetry in CT as well as with the basic principles of radiation protection. There are important technical differences pertinent to radiation dose between the CT scanner types that are currently being used for imaging of the heart and coronary arteries. As a result of these differences, the radiation dose typically is higher when a cardiac examination is performed with multidetector-row CT (MDCT) than when it is performed with electron beam CT. Several techniques have been described to reduce radiation dose of MDCT imaging by varying the X-ray tube current during a CT examination. The volume computed tomographic dose index (CTDI_vol), the dose length product (DLP), and the effective dose (E) are the most useful parameters to describe and compare radiation doses received from cardiac CT examinations. When comparing radiation doses between scanning protocols and scanner types, the degree of image noise must be considered. Diagnostic, rather than aesthetic, quality of images should be the most important factor guiding the development of scanning protocols for cardiac CT imaging. Cardiac CT examinations should be ordered only by qualified health care providers, and the ordering clinicians should be aware of their responsibility of weighing risks of the radiation exposure against the expected benefits.     

Dual Energy Imaging in Cardiovascular CT: Current Status and Impact on Radiation,Contrast and Accuracy

Dual-energy computed tomography (DECT) exploits the continuous energy distribution of x-rays to improve differentiation of tissues beyond what is possible with single-energy CT (SECT). DECT often uses smaller volumes of iodinated contrast agent and lower radiation doses than SECT. Clinical applications of DECT in cardiovascular imaging are emerging and include myocardial perfusion imaging, myocardial infarct and viability imaging, coronary plaque characterization, coronary stent assessment, and myocardial iron quantification. In this review, we discuss the available methods for acquiring and processing DECT data, the current status of DECT in cardiovascular imaging, and its impact on the dose of radiation and contrast agent.     

Review of radiation issues for computed tomography.

Over the past three decades, computed tomography (CT) has proven to be central in imaging evaluation. Multidetector technology continues to drive practice patterns by combining fast scanning with high quality data sets. This has resulted in new applications as well as improved use in traditional applications. With this recognition has also come the realization that there are potential costs of CT. One major cost is the radiation dose. Therefore, in order to begin to assess benefits (which are relatively familiar to radiologists) versus costs (which are less familiar), the issues related to CT radiation need to be addressed. Familiarity with measures of CT radiation and the actual doses delivered by CT are important issues as they provide a basis for understanding the potential cancer risks from CT radiation. Moreover, these justify development of strategies to minimize radiation dose. Strategies include obtaining only necessary CT examinations and adjusting the examinations based on scan indication, region examined, and patient size. These strategies must also be combined with efforts by manufacturers in development and implementation of technology aimed at radiation dose management, as well as efforts in research, education, and CT standards and regulation. This article reviews the subject of radiation dose with multidetector CT technology, including measures of CT radiation, the dose that can result from CT examinations, the risks of this amount of radiation, and strategies for minimizing CT radiation dose.     

Reduction of Radiation Doses in Cardiac Imaging, Part II: New Advances and Techniques in Nuclear Perfusion Imaging and Cardiac CT

Noninvasive cardiac imaging has become a critical pathway for diagnosis and risk assessment in patients with known or suspected ischemic heart disease. Low-level ionizing radiation is used in most of these procedures like radionuclide myocardial imaging and cardiac CT scanning. There is lack of any direct data which indicates the occurrence of any radiation-related adverse events with these diagnostic tests. However, recent concern has been raised due to the cumulative dose of radiation that patients receive during these procedures. Efforts are underway to help reduce the radiation exposure while preserving image quality for accurate interpretation and avoidance of repeat testing. This review will focus on the various advancements and methods to reduce the radiation dose in patients undergoing nuclear myocardial perfusion imaging and cardiac CT scanning.     

Prediction of coronary artery plaque progression and potential rupture from 320-detector row prospectively ECG-gated single heart beat CT angiography: Lattice Boltzmann evaluation of endothelial shear stress

Advances in MDCT will extend coronary CTA beyond the morphology data provided by systems that use 64 or fewer detector rows. Newer coronary CTA technology such as prospective ECG-gating will also enable lower dose examinations. Since the current standard of care for coronary diagnoses is catheterization, CT will continue to be benchmarked against catheterization reference points, in particular temporal resolution, spatial resolution, radiation dose, and volume coverage. This article focuses on single heart beat cardiac acquisitions enabled by 320-detector row CT. Imaging with this system can now be performed with patient radiation doses comparable to catheterization. The high image quality, excellent contrast opacification, and absence of stair-step artifact provide the potential to evaluate endothelial shear stress (ESS) noninvasively with CT. Low ESS is known to lead to the development and progression of atherosclerotic plaque culminating in high-risk vulnerable plaque likely to rupture and cause an acute coronary event. The magnitude of local low ESS, in combination with the local remodeling response and the severity of systemic risk factors, determines the natural history of each plaque. This paper describes the steps required to derive an ESS map from 320-detector row CT data using the Lattice Boltzmann method to include the complex geometry of the coronary arterial tree. This approach diminishes the limitations of other computational fluid dynamics methods to properly evaluate multiple coronary arteries, including the complex geometry of coronary bifurcations where lesions tend to develop.     

State of the Art Hybrid Technology: PET/CT

In the recent years, hybrid positron emission tomography (PET)/computed tomography (CT) scanners have been increasingly utilized in cardiac applications. PET imaging quality has been improved by the use of new scintillators, small detector element size, and fully 3D iterative reconstruction techniques with time-of-flight information and resolution recovery. Further quality enhancements for cardiac imaging can be obtained by tracking and correcting for cardiac and breathing motion with respiratory gating devices and advanced software techniques. The primary tracers used for PET/CT cardiac imaging are Rubidium-82 (⁸²Rb) and Nitrogen-13-ammonia (¹³N-ammonia) and 18-F fluorodeoxyglucose used for myocardial viability imaging. A new F-18 perfusion tracer (F-18 Flurpiridaz) is being evaluated. High-resolution multi-slice CT component of the hybrid scanner allows accurate attenuation correction for PET, measurement of CT calcium, and contrast CT angiography. Hybrid PET/CT protocols have demonstrated increased diagnostic accuracy for the detection of obstructive disease compared with standalone techniques. Radiation dose to the patient is a concern in hybrid imaging because multiple scans are performed in one scanning session. 3D PET acquisition combined with the new low-dose CT protocols can reduce the doses significantly. Hybrid PET/CT scanners have also been utilized for anatomically-guided molecular imaging of plaque biology in the carotid vessels, aorta, and coronary vessels. This review summarizes the state-of-the-art hybrid imaging PET/CT instrumentation and advances in the image quality related to cardiac imaging.     

Coronary CT angiography with low radiation dose

With the introduction of 64-slice CT and dual-source CT technology, coronary CT angiography (CCTA) has emerged as a useful diagnostic imaging modality for the noninvasive assessment of coronary heart disease. Recently, the risks associated with ionizing radiation on CT have raised serious concerns. The main concern of exposure to ionizing radiation is the potential risk of cancer. CCTA involves much higher radiation dose with the advances in the spatial and temporal resolution of cardiac CT. Currently, various dose-saving algorithms, such as ECG (electrocardiography)-based dose modulation, reduced tube voltage, and prospective ECG gating, high-pitch helical scanning are available to lower radiation exposure during cardiac CT. Therefore, careful selection of CT scanning protocols is needed to keep the radiation exposure ‘as low as reasonably achievable (ALARA)’. In this review we will discuss the radiation dose safety issues, the measurement of radiation dose and current use of dose-saving techniques in CCTA.     

Recent developments in wide-detector cardiac computed tomography

Multidetector computed tomography (MDCT) using 64 detectors is widely used for cardiac imaging in the clinical setting. Despite promising results, 64-slice MDCT has important limitations for cardiac applications related to detector coverage, which leads to longer scan times, image artifacts, increased radiation and the need for higher contrast doses. The advent of wide or full cardiac coverage with 256- or 320-slice MDCT provides important advantages that can potentially improve the status of these limitations and expand the utility of cardiac MDCT imaging beyond coronary imaging. Additionally, the combination of wide-detectors and multi-energy acquisitions offer interesting possibilities of improved coverage and temporal resolution that may improve plaque characterization as well as viability and perfusion imaging. In this review we will discuss the current status of wide-detector MDCT scanners and their advantages for clinical coronary and ventricular imaging. We will also review examples of wide detector coronary angiography imaging and discuss emerging complementary non-coronary applications that have been enabled by wide-detector MDCT imaging.     

Recent developments in wide-detector cardiac computed tomography

Multidetector computed tomography (MDCT) using 64 detectors is widely used for cardiac imaging in the clinical setting. Despite promising results, 64-slice MDCT has important limitations for cardiac applications related to detector coverage, which leads to longer scan times, image artifacts, increased radiation and the need for higher contrast doses. The advent of wide or full cardiac coverage with 256- or 320-slice MDCT provides important advantages that can potentially improve the status of these limitations and expand the utility of cardiac MDCT imaging beyond coronary imaging. Additionally, the combination of wide-detectors and multi-energy acquisitions offer interesting possibilities of improved coverage and temporal resolution that may improve plaque characterization as well as viability and perfusion imaging. In this review we will discuss the current status of wide-detector MDCT scanners and their advantages for clinical coronary and ventricular imaging. We will also review examples of wide detector coronary angiography imaging and discuss emerging complementary non-coronary applications that have been enabled by wide-detector MDCT imaging.     

Insights from CTA with Comparison to Modalities of Intravascular Ultrasound Imaging

Noninvasive imaging of atherosclerosis by cardiac CT continues to rapidly evolve. A large collection of data has emerged on detection and quantification of coronary plaque in vivo with cardiac CT with comparison to the gold standard of clinical plaque assessment, intravascular ultrasound. Given inherent spatial limitations, although the correlation is significant, the variability and limits of agreement of these measurements are wide. More recently, focus has shifted to detecting plaque stability, or rather high-risk features of plaque, and identifying those “vulnerable” to rupture. This is a concept originated in histopathology and translated clinically into invasive plaque characterization through virtual histology, or IVUS-VH. We will review the literature regarding methods of plaque assessment, as well as plaque progression and outcomes data, in cardiac CT with regard to its correlation with IVUS and IVUS-VH. The potential in cardiac CT lies within the noninvasive detection of coronary artery disease, its ability to help distinguish those plaques and thus, those patients most vulnerable, which ultimately may be utilized for risk stratification, direction of aggressive therapy, and even as a way to evaluate effects of medical therapies.     

Prospective ECG-gated 320 row detector computed tomography: implications for CT angiography and perfusion imaging

Cardiac multidetector computed tomography has evolved from early four detector systems that first demonstrated the feasibility of non-invasive angiography to today’s wide-area detector computed tomography systems, such as 320-row detector computed tomography. As detector arrays have widened, there have been great improvements in image quality that have improved test accuracy. In addition, wider detector arrays have allowed for the application of prospective ECG-gating for CT angiography, although the current 64-row detector systems have some limitations. 320-row detector computed tomography with full cardiac coverage allows for cardiac imaging in a single heart beat. This technology has realized some of the great advantages provided by full cardiac coverage in regards to image quality (elimination of step artifacts and variation in contrast enhancement), patient safety (reductions in overall radiation and contrast dose), and the prospects for combined CT angiography and myocardial perfusion imaging are very promising. We will review the technical aspects of 320-row detector computed tomography and their implications for coronary angiography and perfusion imaging.     

Prospective ECG-gated 320 row detector computed tomography: implications for CT angiography and perfusion imaging

Cardiac multidetector computed tomography has evolved from early four detector systems that first demonstrated the feasibility of non-invasive angiography to today’s wide-area detector computed tomography systems, such as 320-row detector computed tomography. As detector arrays have widened, there have been great improvements in image quality that have improved test accuracy. In addition, wider detector arrays have allowed for the application of prospective ECG-gating for CT angiography, although the current 64-row detector systems have some limitations. 320-row detector computed tomography with full cardiac coverage allows for cardiac imaging in a single heart beat. This technology has realized some of the great advantages provided by full cardiac coverage in regards to image quality (elimination of step artifacts and variation in contrast enhancement), patient safety (reductions in overall radiation and contrast dose), and the prospects for combined CT angiography and myocardial perfusion imaging are very promising. We will review the technical aspects of 320-row detector computed tomography and their implications for coronary angiography and perfusion imaging.     

Responsible Use of Computed Tomography in the Evaluation of Coronary Artery Disease and Chest Pain

Many options are available to clinicians for the noninvasive evaluation of the cardiovascular system and patient concerns about chest discomfort. Cardiac computed tomography (CT) is a rapidly advancing field of noninvasive imaging. Computed tomography incorporates coronary artery calcium scoring, coronary angiography, ventricular functional analysis, and information about noncardiac thoracic anatomy. We searched the PubMed database and Google from inception to September 2009 for resources on the accuracy, risk, and predictive capacity of coronary artery calcium scoring and CT coronary angiography and have reviewed them herein. Cardiac CT provides diagnostic information comparable to echocardiography, nuclear myocardial perfusion imaging, positron emission tomography, and magnetic resonance imaging. A cardiac CT study can be completed in minutes. In patients with a nondiagnostic stress test result, cardiac CT can preclude the need for invasive angiography. Prognostic information portends excellent outcomes in patients with normal study results. Use of cardiac CT can reduce health care costs and length of emergency department stays for patients with chest pain. Cardiac CT examination provides clinically relevant information at a radiation dose similar to well-established technologies, such as nuclear myocardial perfusion imaging. Advances in technique can reduce radiation dose by 90%. With appropriate patient selection, cardiac CT can accurately diagnose heart disease, markedly decrease health care costs, and reliably predict clinical outcomes.CAC = coronary artery calcium; CAD = coronary artery disease; CT = computed tomography; CTA = coronary computed tomographic angiography; EBCT = electron beam CT; ED = emergency department; ICA = invasive coronary angiography; MDCT = multidetector helical CT; MI = myocardial infarction; MPI = myocardial perfusion imaging; NPV = negative predictive value; PPV = positive predictive valueCardiac computed tomography (CT) is a rapidly evolving technology for the noninvasive evaluation of the cardiovascular system. Numerous potential roles for cardiac CT have been developed recently, such as investigating anomalous coronary arteries, evaluating for pulmonary vein stenoses, and preparing for repeated coronary artery bypass grafting. However, the indication of most interest to the public and physicians is evaluating patients for native vessel coronary artery disease (CAD) using coronary artery calcium (CAC) scoring and coronary computed tomographic angiography (CTA).We searched the PubMed database and Google, from inception to September 2009, for keywords coronary artery calcium, coronary CT angiography, and radiation risk to identify information sources of interest. We also searched references in other review articles. From Google, we selected publications from trusted sources, such as the Food and Drug Administration and the National Academy of Sciences. From PubMed, we selected articles about test performance characteristics based on the quality of their methods, preferentially using randomized controlled trial data. We selected articles about clinical outcomes from randomized trials when available and from large cohorts as secondary sources. The purpose of this review is to summarize the recent data regarding accuracy, sensitivity, and specificity of CTA and the responsible use of cardiac CT.     

Low radiation dose imaging of myocardial perfusion and coronary angiography with a hybrid PET/CT scanner

Objectives: To test the image quality and feasibility of a sequential low radiation dose protocol for hybrid cardiac PET/CT angiography (CTA). Background: Multidetector computed tomography (MDCT) is a non‐invasive method for coronary angiography. The negative predictive value of MDCT is high but perfusion imaging has a role in detecting functional significance of coronary lesions. This has encouraged combining these techniques. However, radiation dose is of concern. We report our first experiences with a low dose sequential CTA mode applicable to hybrid imaging. Methods: In the first phase, 10 consecutive cardiac MDCT angiographies were performed with spiral acquisition and compared in terms of image quality and dose with the following 10 patients performed with a new sequential mode. In the second phase, feasibility and radiation dose of a combined ¹⁵O‐water rest‐stress PET perfusion/sequential CTA protocol were assessed in another group of 61 consecutive patients. Results: Mean effective radiation dose was 60% lower in the sequential group than in the spiral group (19·3 versus 7·6 mSv, P<0·001). In the second phase, the new sequential hybrid protocol proved possible in 87% of the patients given the preconditions determined by the manufacturer. Mean effective dose of the CT acquisition was 7·6 mSv and total dose from the PET/CTA hybrid study 9·5 mSv. Conclusion: Low dose PET/CT allows cardiac hybrid studies with <10 mSv. The protocol can be applied to almost nine out of 10 patients with CT image quality comparable to spiral acquisition.     

Low kVp imaging for dose reduction in dual-source cardiac CT

With the widespread application of cardiac CT has come increasing concern over the effects of radiation dose associated with this exam. Dual source CT provides a number of methods for dose reduction in helical ECG-gated cardiac CT studies. This article discusses several of these methods, with a particular emphasis on low kVp scanning, which can be applied in a large percentage of patients.     

Low kVp imaging for dose reduction in dual-source cardiac CT

With the widespread application of cardiac CT has come increasing concern over the effects of radiation dose associated with this exam. Dual source CT provides a number of methods for dose reduction in helical ECG-gated cardiac CT studies. This article discusses several of these methods, with a particular emphasis on low kVp scanning, which can be applied in a large percentage of patients.     

Myocardial perfusion imaging by computed tomography: today and tomorrow

Cardiac perfusion along with imaging of coronary artery stenosis is an important tool in assessing the degree of coronary artery disease (CAD) and decision-making regarding further treatment. SPECT, PET, echocardiography and cardiac magnetic resonance imaging are clinically established techniques to evaluate myocardial perfusion and viability with a high diagnostic accuracy and relatively few unwanted side effects. However, none of these modalities Glose can reliably assess the extent and morphology of CAD, features which also have implications as well as for patient management. In contrast, cardiac CT has emerged over the last years as a reliable tool to visualise coronary atherosclerotic plaque and stenosis, nearly unaffected by heart rate and carrying a relatively low radiation exposure; however, without allowing an adequate assessment of myocardial perfusion. Given the great promise of a combined cardiac CT examination to assess morphology and function, much research has recently been focused on the development of CT-based myocardial perfusion imaging techniques. In this article, we review recent developments in cardiac CT with respect to myocardial perfusion imaging, especially the two main techniques, first-pass and dynamic CT acquisitions.     

Reducing Radiation Exposure from Cardiovascular Imaging Through Implementation of Appropriate Use Criteria

Cardiac imaging is a key instrument in the evaluation of patients with known or suspected coronary artery disease. Although clear benefits accompany the use of nuclear cardiology and cardiac CT techniques, is well-documented in the medical literature, there is growing concern about the risk related to exposure to ionizing radiation. Although the true impact of low-level ionizing radiation is often poorly characterized, clinicians and medical organizations encourage minimization of exposure, with a focus on a balance between benefits and risks of cardiac imaging procedures. The appropriate use criteria (AUC), developed by the American College of Cardiology Foundation, American Heart Association, and multiple other societies, provide guidance regarding test utilization and assist in optimizing an approach involving the right test for the right patient at the right time. By reducing inappropriate use of cardiac CT and radionuclide imaging, exposure to unnecessary ionizing radiation may be minimized. Evaluation of appropriateness allows for practitioners to monitor their performance and serves to provide focus for educational efforts related to inappropriate test indications. Several key areas, including test layering and the use of serial imaging in asymptomatic patients likely contribute to inappropriate use and increased radiation exposure. Therefore, the use of AUC, in conjunction with other radiation dose reduction efforts, promotes significant improvement in patient safety.