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Coronary artery disease (CAD) is the most common type of heart disease and is the leading cause death in both men and women in the United States. A 2004 census revealed that 24.7 million people in the United States are living with CAD which represents approximately 11.5 percent of American people (Chen, 2007). With these numbers still rapidly increasing, it is imperative that healthcare workers and patients are educated on the risks, diagnosis, and prognosis of CAD. Invasive coronary angiography is still considered the gold standard for diagnosing coronary artery stenosis. With the used of contrast media, this modality provides high spatial and temporal resolution with the option to perform percutaneous coronary intervention if felt needed by the doctor. However, 69.7 percent of coronary angiography exams conclude with the diagnosis of normal coronary arteries or non-obstructive disease which in turn does not need further revascularization treatment (Wijesekera, Duncan, & Simon, 2009). Therefore, these patients did not need to undergo an invasive coronary angiogram. However, in order to prevent patients' unnecessary invasive exams such as a coronary angiogram, a non-invasive study for the detection and grading of CAD would benefit patients. Noninvasive coronary angiography performed with multi-detector computed tomography (MDCT) has proven high diagnostic accuracy when evaluating the coronary artery morphology as well as the evaluation coronary artery plaque (Wijesekera, Duncan, & Simon, 2009).
CAD is a condition that affects the blood supply to the heart. The major vessels are stenosed or blocked due to fatty plaque buildup called atherosclerosis on the walls of the vessels. Furthermore, this reduces the oxygen supply and nutrients to the heart muscles which is vital for it to function properly. If the blockage or stenosis isn't fixed properly and promptly, it will lead to death of heart tissue which can lead to a myocardial infarction, or heart attack. The heart is the main organ that supplies the rest of the body with oxygenated blood. If the heart undergoes pumping failure, this affects the oxygen supply to other vital organs such as the brain and the kidneys. A perfusion defect could lead to the damage or death of tissue with in these organs and their ultimate failure (Chaothawee, 2008).
Most commonly, the first sign of CAD is chest pain also know as angina pectoris. Typically, the pain is felt over the central chest that sometimes radiates down the left arm which is usually aggravated by exertion. In other cases, patients feel the pain radiate to the jaw or the back. The pain is usually described as chest tightness in the middle of the chest cavity witch may last for five to twenty minutes. Angina pectoris is a key symptom of CAD and requires urgent medical care (Chaothawee, 2008).
Many factors raise the risk of developing CAD. The more risk factors one has, the more likely one may develop CAD. Unhealthy blood cholesterol levels such as having a high LDL (bad cholesterol), and having a low HDL (good cholesterol) are directly linked to atherosclerosis and eventually CAD. High blood pressure can also raise ones risk of developing CAD. A blood pressure of 140/90 mg/Hg for a period of time can damage the heart and the vessels supplying the heart with oxygen. Furthermore, smoking greatly increases ones risk for developing CAD. Smoking damages and constricts the blood vessels while also raising cholesterol levels and raising blood pressure. Smoking also doesn't allow enough oxygen to reach the bodies tissues. Other factors include diabetes, overweight or obesity, metabolic syndrome, lack of physical activity, poor eating habits, and age. As one ages, the risk of developing CAD greatly increases. Genetic or lifestyle changes cause plaque to build more rapidly in the arteries. For most men, the risk of developing CAD increases after the age of 45. For women, the risk of developing CAD increases after the age of 45. However, making lifestyle changes and taking medication to treat other risk factor can lessen the development of CAD even in older adults (American Heart Association, 2010).
When trying to investigate and diagnose CAD, there are a variety of tools and tests the physician can utilize when a patient is experiencing CAD symptoms. An Electrocardiography (ECG) is usually the first test to be performed and is a great screening tool for a variety of cardiac abnormalities although it has its limitations. An ECG test reveals only the heart rate and rhythm during the specific time the ECG is taken. If sporadic cardiac rhythm abnormalities are present, the ECG is likely to miss them. Furthermore, the ECG test can often be normal in patients with undiagnosed CAD because of its one time testing approach which in turn produces false negative results. Also, many abnormalities that appear on the ECG results turn out to have no medical significance after thorough evaluation is done which produces false positive results. If a patient is experiencing CAD symptoms and the ECG test is normal, the patient usually undergoes an exercise stress test (EST). The traditional noninvasive stress test provides physiological evidence of flow-limiting stenoses through characteristic changes in the electrocardiogram, myocardial perfusion defects, or regional wall motion abnormalities (Chen, 2007). The EST often detects inadequate blood supply during physical exertion which can help the physician diagnose a defect. Although, the major limitation of an EST approach is that it requires a high-grade stenosis to indicate CAD (Chaothawee, 2008). A coronary angiogram (CAG) provides the most accurate information about the site and the extent of the stenosis of the coronary artery and is still considered the gold standard (Chen, 2007). CAG also helps in deciding the method of therapy required. In contrast, a coronary angiogram is an invasive procedure that requires the insertion of a catheter into the coronary arteries and the injection of large volumes of contrast media to highlight each of the arteries and their branches. CAG is associated with significant costs, inconvenience to patients, and risks of serious complications due to the innate invasive nature of the procedure. These risks include: Infection, blood clots, injury to an artery, stroke, kidney damage, complications from anesthesia, a reaction to the contrast media, erythema from large amounts of radiation, and death. From 1979 through 2003, the number of inpatient cardiac catheterizations in the United States increased 373% to total 1.4 million diagnostic procedures in 2003 (Chen, 2007). The overall success rates for a CAG can range from 50-90 percent. Furthermore, 20-50% of CAG cases do not need balloon dilation or stent replacement. In many of these patients, CAGs were done solely because the physician could not rule out coronary artery blockages based on results of other tests. In many heart centers, up to 70-80 percent of patients did not require and intervention during the CAG (Wijesekera, Duncan, & Simon, 2009). Therefore, these patients could have done with out a CAG and all the risks that this test carries with it. However, with the use of cardiac CT the Cardiologist would know beforehand whether or not the patient would benefit from a CAG. With the dawn of new Cardiac CT, the majority of CAD patients will get a safer and cheaper heart check-up. CAG will then only be used where intervention like balloon angioplasty with or without stenting would be beneficial (Chaothawee, 2008). In conclusion, Cardiac CT is a lot easier on the patient compared to a conventional CAG. Cardiac CT is a non-invasive procedure unlike CAG and does not require insertion of a catheter into the heart, so the procedure is almost risk free. Also, it is rather painless as there is no need to puncture the femoral or radial arteries and because of its simplicity, it is not necessary for the patient to get admitted to the hospital (Wijesekera, Duncan, & Simon, 2009).
With the help of Sir Godfrey Hounsfield in 1972, a new exciting diagnostic tool was invented called the computer axial tomography scanner. In 1979, Hounsfield shared the Nobel Prize for Medicine with Allan Cormack and in his acceptance speech he declared "â€¦â€¦ a further promising field may be the detection of the coronary arteries". Giving credit to Hounsfield, in recent year's health professionals have started experimenting with a newer sophisticated CT scanner called the multi-slice CT scanner (MDCT) which speeds up scan times and treatment. Furthermore, the multi-slice scanner provides the physician with a clearer and more detailed study than the conventional spiral CT. Conventional CT methods used the 'stop and shoot' method which required long scan times and repeated breath holds which only yielded a single volumetric data set per breath hold (Wijesekera, Duncan, & Simon, 2009). In contrast, using a MDCT, patients only have to hold there breath for a very short period of time. The MDCT is capable of producing up to 64 sub-millimeter slices simultaneously for fast, high quality image resolution of the coronary arteries. These images are then reconstructed by the computer software to create complete 3-D images of the coronary arteries, heart valves and other cardiac anatomy. New and successful generations of MDCT scanner have rapidly developed since the conventional dual-slice technology (Chen, 2007). These generations range from 4 to 8, 16, 32, 40, 64, 128, 256, and now 320 slice detectors are currently being used today (Wijesekera, Duncan, & Simon, 2009). In the early 1990's, electron beam CT was developed and made headway for the evolution of cardiac CT aiding in coronary calcium assessments and the initial work in coronary angiography. However, it fell short due its relative low spatial resolution (1.5-3 mm slice thickness), especially with contrast-enhanced coronary angiography (Wijesekera, Duncan, & Simon, 2009).
The coronary arteries are complex structures with small diameters, measuring from 4-5 millimeters in the left main stem and 1 millimeter in the distal left anterior descending artery. Therefore, it is imperative that sub-millimeter spatial resolution with equal resolution in all three planes is demonstrated to ensure visualization of the coronary artery branches. In order to differentiate a 10-20 percent stenosis, spatial resolution of .3 millimeters is necessary. Spatial resolution of a CT scanner solely depends on the size of the three-dimensional pixels (voxel) which make up the image on the monitor. Each voxel displays a shade of grey depending on the average attenuation of that specific tissue. An artifact known as partial voluming occurs when the voxel sizes are too large. Partial voluming occurs when the a voxel contains both high attenuation structures such as plaque which is demonstrates as white, and low attenuation structures such as fatty plaque which is displayed as a dark grey. If the voxel size is too large, then the voxel will be displayed as an average density and useful information is lost. As a rule, the smaller the voxel size, the less partial volume effects will be seen a better spatial resolution will be resolved. Voxel size is solely depended on the resolution of the x-ray sensors and the focal spot size, rather than the number of acquired slices (Wijesekera, Duncan, & Simon, 2009).
Cardiac CT is a rapidly emerging technique for the noninvasive visualization of coronary arteries. It is very useful in evaluating the blocked or stenosed coronary artery, both interiorly and exteriorly. This scan is also unique in performing a 'calcium score' in the heart chamber and coronary vessels whereas a CAG cannot. Furthermore, cardiac CT gives the physician the ability to distinguish between different types of atherosclerotic plaque. This new feature helps identify patients at elevated risk of future coronary episodes using different mean Hounsfield densities to distinguish between soft plaques and calcified plaques. Furthermore, this is important because calcified plaque is associated with plaque stability, whereas non-calcified plaque may be more unstable and prone to rupture leading to an acute coronary syndrome or myocardial infarction. Cardiac CT accurately detects calcified plaques up to 90%, but is less accurate for the detection of non-calcified plaques with sensitivities ranging from 60-80% (Wijesekera, Duncan, & Simon, 2009).
Cardiac CT allows three-dimensional anatomical views and two-dimension multi-planar views of the heart to detect abnormal heart muscles and valves. Furthermore, it is used to calculate parameters indicating heart function, such as stroke volume, ejection fraction, cardiac output and right and left ventricular fractions. Cardiac CT demands high temporal resolution, high spatial resolution, and true volume data sets (Wijesekera, Duncan, & Simon, 2009). When evaluating different multi-detector CT scanners, it is imperative to use at least a 64-slice scanner because of its very short image acquisition times (<12 seconds), gantry rotation times of 220 milliseconds, and spatial resolution of .4 millimeters. In contrast, the 4-slice and 16-slice scanner requires breath holds up to 45 seconds, poor resolution and a substantial number of non-evaluable segments (up to 43%). Although the spatial and temporal resolution of cardiac CT is less that conventional invasive coronary angiography, (spatial resolution of .25 mm and temporal resolution of 5 msec), cardiac CT still has a sensitivity values up to 97% and specificities as high as 94% for the detection of a relevant coronary artery stenoses (Chen, 2007).
The coronary arteries are imaged during continuous cardiac motion. In order to reduce the motion artifact, the image acquisition must take place when the heart is relatively stationary, typically during late diastole of the cardiac rhythm. Temporal resolution is determined by the speed of rotation of the gantry around the patient. To increase temporal resolution, it is possible to accurately reconstruct images using data from 180 degrees rather that the full 360 degree rotation which makes the temporal resolution equal to half of the gantry rotation speed and overall better temporal resolution. Over recent years, the gantry speed has been increased to 250 milliseconds per rotation. In order to get a diagnostic temporal resolution, a gantry speed of less than 250 milliseconds is need for coronary CT angiography to cover all the heart rates. With the advent of new reconstruction algorithms and dual source scanners, temporal resolution of 65 milliseconds is achievable with motion-free images up to a heart rate of 100 beats per minute. This is one method of reducing motion, but there is another imaging technique than can be used that involves two different methods of ECG gating (Wijesekera, Duncan, & Simon, 2009).
Motion artifacts of the heart can be greatly reduced by reconstructing images taken over consecutive cardiac cycles at times when cardiac motion is least usually during late diastole. These heart cycles can be determined by an ECG monitor. The most commonly used ECG techniques are prospective ECG triggering and retrospective ECG gating. Using the prospective gating method, the scan is triggered at a specified point along the R interval of the cardiac cycle. It then stops after a certain period of time and resumes again close to the same time during the next cardiac cycle. This technique reduces patient radiation as it only scans during a chosen phase rather than scanning during the whole cardiac cycle. In contrast, prospective gating greatly depends on a normal heart rate. If a single ectopic heart beat or dysrythmia occurs, image data will not be accurate and image quality will suffer. In retrospective gating, data is acquired continuously throughout the cardiac cycle allowing images to later be reconstructed at the appropriate R interval. Several different phase reconstructions may be used to obtain optimal images for all vessels. Therefore, retrospective ECG gating is the method of choice for evaluating contrast aided images of the coronary arteries. Consequently, patient dose will be higher when using this method (Wijesekera, Duncan, & Simon, 2009).
A low heart rate is preferred even when using the ECG gating methods. Studies show that a low heart rate of <65 beats per minute can substantially improve the image quality. To help aid, oral or intravenous beta-blockers can be administered if the patient's heart rate is > 65 beats per minute. For patients who are intolerant of beta-blockers, a calcium channel antagonist can be administered. In some facilities, it is protocol to give patients sublingual nitrates immediately before the cardiac CT exam to dilate the coronary arteries (Lazoura, Vlychou, Vassiou, Rountas, & Ioannis, 2010).
Further exam protocols may include introducing a contrast agent intravenously if calcium scoring is not preferred. For optimal opacification of the coronary arteries, it is recommended that a high concentration contrast agent is injected at a high flow rate (4-5ml/s) followed by a 40-50 cc saline chaser to washout contrast from the right ventricle. The start of the scan needs to coincide with the arrival of contrast in the ascending aorta. Typically, 120 cc of contrast is injected for a high-resolution volume acquisition. The patient will need to perform a breath hold for approximately 5-20 seconds depending on the scanner generation and the dimensions of the heart (Wijesekera, Duncan, & Simon, 2009).
After the test is complete, the reconstruction of data from all cardiac phases may generate 2000-3000 individual axial images. Slice thickness is usually selected between 0.5 and 1.0 millimeters. The reconstructed sections are spaced so that consecutive images overlap by approximately 50%. Analysis and interpretation of these large data sets can take up to an hour to dictate. Three-dimensional volume reconstructions can be visually impressive, but are rarely helpful in a diagnosis. Four-dimensional functional analysis of wall motion and dynamic cardiac cine loops can be performed using a gated data set. Furthermore, wide field of view images are also taken into account as they may reveal unexpected cardiac pathology (Wijesekera, Duncan, & Simon, 2009).
Although the radiation dose is higher during a cardiac CT than a CAG (10mSv vs. 2-5 mSv), new exciting technology is on the horizon that can reduce patient radiation exposure to less than 1 mSv. This can be achieved by using a dual-source CT scanner and a very high pitch value (3.0 or more). The system uses two x-ray tubes and two detectors arranged at an angle of 90 degrees. In other words, only one quarter rotation of the gantry is necessary to acquire the x-ray data set for one cross sectional image. In doing so, this doubles the temporal resolution when compared with a single source CT at the same rotation speed. The high pitch and the fast couch speed allow image acquisition for the entire data set of the heart within a single cardiac cycle even in heart rate > 75 beats per minute. Therefore, radiation exposure is low (<1mSv) since no slice overlap is used (Achenbach, Marwan, Ropers, Schepis, Pflederer, Anders, Kuettner, Werner, Uder, & Lell, 2010).
In conclusion, CT is still rapidly advancing as a medical imaging modality. Cardiac CT has proven to be a safe effective alternative to the invasive CAG. Furthermore, there is growing evidence that cardiac CT may also improve the management of patients presenting with acute chest pain who have a low likelihood of CAD (Wijesekera, Duncan, & Simon, 2009). Although questions and concerns still remain about radiation dose, new dual source CT scanners are promising in reducing radiation dose. As cardiac CT continually becomes clinically validated, it may prove to be the new golden standard in diagnosing coronary artery disease.
The New Golden Standard?
Jacob David Swenson
Saturday, April 17, 2010