Course Authors
Stefano Sdringola, M.D., and Catalin Loghin, M.D.
Dr. Sdringola is Associate Professor of Cardiovascular Medicine and Dr. Loghin is Assistant Professor of Cardiovascular Medicine University of Texas Medical School, Houston, Texas.
Within the past 12 months, Drs. Sdringola and Loghin report no commercial conflicts of interest.
Estimated course time: 1 hour(s).
Albert Einstein College of Medicine – Montefiore Medical Center designates this enduring material activity for a maximum of 1.0 AMA PRA Category 1 Credit(s)™. Physicians should claim only the credit commensurate with the extent of their participation in the activity.
In support of improving patient care, this activity has been planned and implemented by Albert Einstein College of Medicine-Montefiore Medical Center and InterMDnet. Albert Einstein College of Medicine – Montefiore Medical Center is jointly accredited by the Accreditation Council for Continuing Medical Education (ACCME), the Accreditation Council for Pharmacy Education (ACPE), and the American Nurses Credentialing Center (ANCC), to provide continuing education for the healthcare team.
Upon completion of this Cyberounds®, you should be able to:
Describe the basic technology of Multi-Detector Computer Tomography (MDCT)
Discuss the clinical applications of MDCT angiography
Discuss the limitations, risks, potential use and misuse of this technology.
"The good doctor knows what to do and when to do it. But the very good doctor knows what not to do and when not to do it".
Dr. Herbert L. Fred
The medical community has a rapidly growing interest and enthusiasm in cardiac CT angiography for the non-invasive detection of coronary artery disease. But what are we really evaluating? Are our expectations justified and are we witnessing the birth of a technology capable of changing the way we practice?
In this Cyberounds®, we will try to address potential clinical applications and limitations of this emerging, fast developing technology.
The Technology
Fast acquisition of heart images first became possible about 30 years ago with the introduction of Electron Beam Computed Tomography (EBCT). This technology underwent subsequent rapid evolution and is currently also known as ultrafast CT.(1) The technique relies on powerful electron emitters to produce an electron beam which is then steered electromagnetically to encircle the heart without involving moving mechanical parts, thereby offering very rapid scanning speed (temporal resolution between 50 and 100 ms). This allows imaging of a beating heart; however, the overall area of coverage is limited with a relatively low spatial resolution.
The problem of maintaining a relatively high temporal resolution to minimize motion artifacts, while achieving a better spatial resolution to provide sufficient detail, has been addressed with the introduction of the helical CT, also referred to as spiral CT or Multidetector Computed Tomography (MDCT). This technology uses an X-ray source located in a rotating gantry that also contains an array of multiple X-ray detectors. The spatial resolution with MDCT is well below the millimeter (mm) and is approaching the 0.2 mm resolution typically seen by conventional radiographic angiography.(2) By increasing the gantry speed and the number of detector rows, the scan time shortens, allowing rapid image acquisition.
With current MDCT scanners, images are generated in a continuous spiral fashion while the person, lying on the scanner table, is automatically transported through a fast-rotating gantry (330 milliseconds per complete rotation). The total heart scan time with 64-slice MDCT is less than 10 seconds. However, while this methodology drastically reduces artifacts, the temporal resolution is not ideal to consistently capture the details of the beating heart. Electrocardiographic-gated scanning with heart rate ideally below 70 beats/min and rhythm control, as well as post acquisition processing, are two additional technological features of cardiac CT, which allow better images with least motion artifacts.(2)
Scanning Protocol
Several elements are necessary in order to achieve good image quality: sustained breath holding, a slow, regular heart rate and optimal contrast enhancement of the coronary vessels.
Depending on cardiac dimensions, scanning time is in the range of 15-25 seconds for a 16-slice MDCT scanner and below 10 seconds for a 64-slice scanner, which represents a reasonable breath hold time for most patients. Hoffman and colleagues demonstrated (with 16-slice CT scanners) that best image quality was achieved at heart rates of 75 or fewer beats per minute; occasional premature beats did not cause significant motion artifacts in most of the patients.(3)
The target is maintaining a heart rate below 70-75 bpm and a regular rhythm during scanning. Usually this can be best achieved with the use of oral or intravenous beta blockers, diltiazem or verapamil.
The two common methods used to achieve optimum contrast enhancement are the tracking bolus and test bolus. Tracking bolus is an automatic way to trigger scanning based on a preset Hounsfield Unit (HU) at a landmark such as the ascending aorta. Typically, CT scanning is triggered automatically as soon as the contrast enhancement at the landmark reaches 100 HU. With the test bolus method, the timing at which to start scanning is indicated for each patient based on the curve of the radiodensity reached over time from injection of contrast for a predetermined landmark. Typically. the test bolus consist of 20 ml of contrast.
Some authors also suggest the use of one dose of sublingual nitroglycerin 5-10 minutes before scanning to achieve maximum coronary vasodilatation at the time of the contrast study.(2) To study native coronaries, 85 ml of contrast, infused at a rate of 4-5ml/second, are usually enough.
Radiation Exposure
How much radiation does a patient receive from a CT examination? Renston and colleagues(4) surveyed 1,000 board certified, non-radiologist physicians nationwide regarding the risk of chest CT. More than 90% either did not know or significantly underestimated the degree of radiation exposure. Many of the technical factors that have improved image quality, particularly temporal and spatial resolution, also affect the radiation dose received during a CT examination of the heart.(5)
To increase spatial resolution it has been necessary to decrease collimation and the respective slice width. To maintain comparable image quality with a reduced slice width (e.g., going from 16- to 64-slice CT), the number of photons received by the detection array is usually increased by raising the tube current, which results in greater radiation exposure and effective dose estimates.
Similarly, in order to increase the gantry rotation speed (temporal resolution) without decreasing the rate of photons received by the detector array and thus maintaining the image quality, the tube current needs to be further increased, resulting in a higher dose estimate.
Hausleiter and colleagues in 1,035 patients undergoing cardiac multislice computed tomography angiography (CTA) found a radiation dose estimated at 6.4 + 1.9 mSv and 11.0 + 4.1 mSv for 16-slice and 64-slice CTA, respectively.(6) For comparison, this is approximately 250-500 times the typical effective dose for a single chest x-ray study (0.02 mSv). The natural background radiation dose is about 3 mSv/year.
The International Commission on Radiological Protection has estimated the additional lifetime risk of fatal cancer as approximately 1 in 20,000 per mSv for the whole population(7) and likely less so in the geriatric population. Based on this information, a coronary CT angiogram with an effective dose of 14.7 mSv has a risk of inducing fatal cancer in 1 of 1,400. Conventional diagnostic coronary angiography (5.6 + 3.6 mSv) has a risk of 1 in 3600 and a calcium-scoring scan (2.6 mSv) a risk of 1 in 7,700.(8) To put these calculations in the same context, a typical effective dose delivered during a rest-stress nuclear myocardial single photon emission computed tomography (SPECT) using technetium-99m is 8 mSv. The effective dose for a positron emission tomography (PET) rest-stress using N-13 ammonia is usually 2.2 mSV, with Rubidium-82 5.0 mSv and for a viability metabolic study using F-18-FDG approximately 7 mSv.
Recently the application of new dose-saving algorithms during CTA, such as ECG-dependent dose modulation and reduced tube voltage, has been shown to be very effective in reducing radiation dose by 30-40%. However, further studies are needed to investigate the balance between dose saving and maintained diagnostic image quality for coronary CT angiographic investigations.(6)
Clinical Applications
Assessment of Luminal Stenosis of Native Coronary Arteries
Since at least 25% of all radiographic angiograms performed do not lead to further coronary intervention,(9) CTA may decrease the number of unnecessary invasive procedures. If performed at a high level of expertise, MDCT angiography can reliably rule out the presence of significant coronary artery luminal stenosis in patients with low to intermediate probability.(10),(11),(12) The high negative predictive value of a normal scan(13) has the potential to rapidly exclude significant epicardial artery stenosis at least in the proximal segments of the coronary tree and can reduce the number of "negative" diagnostic catheterizations. The finding of moderate to severe coronary narrowing can, however, actually increase the number of patients unnecessarily referred for invasive angiography and possible revascularization because of the frequent CT overestimation of luminal stenosis, particularly in patients with calcific deposits.(14)
Assessment of Luminal Stenosis of Coronary Bypass Grafts
Cardiac CTA has a high sensitivity and specificity for the evaluation of coronary bypass graft patency. Three prospective studies(15),(16),(17) showed that the sensitivity and specificity for graft occlusion were 100% and 98-100%, respectively, and for graft stenosis 60-90% and 88-100%, respectively.
The maximum proportion of non-evaluable grafts was as high as 12%. The relatively low accuracy in evaluation of stenosis severity, rather than occlusion-patency for both bypass grafts and native coronary arteries, may be explained by the difficult overall interpretability of the CTA in the presence of extensive coronary calcifications (more common in patients having disease advanced enough to require bypass surgery) and surgical clips.
Assessment of Atherosclerotic Plaque Burden
Conventional angiography is regarded as a "luminogram." It is an excellent way to provide detailed data regarding the luminal size in different views but does not give any data concerning wall thickness or plaque composition. Due to the phenomenon of positive wall remodeling and the diffuse nature of coronary atherosclerosis, a "luminogram" may be inadequate for precise assessment of the degree of luminal reduction and the effective functional severity of stenosis. With contrast enhanced MDCT, the overall thickness of the atherosclerotic plaque can be directly determined. The newest scanners allow imaging of the entire epicardial coronary tree during one breath hold, a distinct advantage over magnetic resonance imaging. Although MDCT can grossly assess the thickness of the atherosclerotic wall with acceptable reliability and can readily identify calcific deposits, further plaque characterization, such as lipid pools and fibrous tissue, a prerequisite for the identification of most vulnerable lesions, is not yet possible with the current technology.(18)
Assessment of Coronary and Great Vessels Congenital Anomalies
Diagnosis of these fairly rare conditions has been traditionally established by cardiac catheterization and is often an incidental finding. To determine the clinical significance of a coronary anomaly in the cardiac catheterization laboratory is often time- consuming and may be technically challenging. Furthermore, it is not unusual to be uncertain about the relative position of an anomalous coronary artery to other adjacent structures. This relationship is particularly relevant clinically in those cases where the anomalous vessel courses between the aorta and main pulmonary artery. Cardiac CTA can routinely provide a three-dimensional view and an infinite viewing angle for the accurate assessment of the anatomic anomaly of interest.
Assessment of Cardiac Morphology and Function
Electron-beam CT with temporal resolution close to that of cardiac MRI is one of the most accurate ways to measure ventricular ejection fraction. Sufficient data is now available supporting the use of 4-slice and 16-slice MDCT for evaluation of cardiac chamber morphology, wall motion, systolic thickening and ejection fraction.(19) With advanced electrocardiographic gating and improved temporal resolution of MDCT scanners, cardiac CTA can be a valuable non-invasive method for the assessment of cardiac structural and morphologic abnormalities. Finally, cardiac CTA has already become an important part of the evaluation of the pulmonary veins and left atrium before and after radiofrequency ablation in atrial fibrillation.(2),(20)
Limitations
MDCT has the potential to become a valuable complement of invasive diagnostic angiography; however, it must ideally provide an accurate, reproducible and detailed visualization of the complete coronary tree. Even with multi-detector technology, the completeness of CT coronary evaluation (which is highly dependent on cardiac motion) does not match the high resolution of conventional invasive angiography.
Earlier studies compared CT and invasive angiography, limiting their analysis to segments that could be visualized by CT. Non- diagnostic image quality was identified in 6.4% segments in one recent study using 16-slice MDCT.(21) In addition, even when the segment can be visualized, particularly in presence of heavy calcification, it is usually not possible to perform an accurate evaluation of the severity of a potentially flow limiting stenosis.
Clinical decisions on an individual patient must account for "missed segments" of the coronary tree. Ultimately, with current technology, the diagnosis of presence and severity of coronary artery disease and subsequent therapeutic decisions remain purely clinical. Such decisions involve an estimate of the prevalence of disease in the non-imaged coronary segments, based on the pre-test probability of significant coronary artery stenosis.
The initial experience with 64-slice scanners, which can image coronaries with a diameter >1.5 mm, suggests that all coronary segments of clinical relevance can now be evaluated. Leschka et al. reported that none of the coronary segments of 67 consecutive patients needed to be excluded from image analysis.(22) The fact that studies thus far have been performed at single centers on selected patient subsets represents a significant limitation and multicenter trials in large populational groups are necessary to validate these findings.
Future Directions
There is little question that non-invasive coronary angiography will become a clinical reality and change the workup of patients suspected of having obstructive coronary artery disease, in particular for those in the low to intermediary risk group. Further development will involve improvement in spatial and temporal resolution, while decreasing the exposure to ionizing radiation. New techniques to detect myocardial viability and perfusion using first-pass CT angiography will expand the potential clinical utility of this imaging technology.(23)
Hybrid technology represented by PET-CT scanners has the potential to offer combined anatomical and functional information on the whole coronary tree (Table 1) once limitations related to image registration are overcome. Development of cineangiographic CT scanning techniques versus the current helical scans has the potential to eliminate misregistration of CT and PET images and may, potentially, result in the development of the most powerful diagnostic tool of clinically significant coronary artery disease.
Table 1. Comparison of Diagnostic Information Provided by PET and Multislice CT
| Parameter | PET | Multislice CT |
|---|---|---|
| Left Ventricular Function | ++ | +++ |
| Coronary Calcification | - | ++ |
| Coronary Angiography | - | ++ |
| Perfusion | +++ | + |
| Metabolism | +++ | - |
| Viability | +++ | + |
| Plaque Morphology | + | - |
Conclusions
MDCT angiography is a powerful imaging technology with significant potential in screening for coronary artery disease and selection of patients who need coronary revascularization.
While the role of MDCT in evaluation of bypass grafts and coronary anomalies is already established, the ability of this technology to correctly assess the severity of regional coronary stenosis and provide functional information on coronary perfusion, is still limited.
The indiscriminate use of MDCT can be potentially harmful because of substantial radiation exposure and the generated "need" for additional confirmatory invasive testing in those cases where the findings are of unclear significance. Such a trend could ultimately further contribute to the already skyrocketing cost of health care.
Specific guidelines on strict patient selection criteria and test indication appropriateness are needed as soon as the results of carefully planned multicenter clinical trials are available.