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Coronary Heart Disease
Coronary artery disease is by far the leading cause of mortality and morbidity in the western world. The impact of coronary heart diseases and related atherosclerotic conditions like stroke, in terms of reduced quality of life, life-years lost, and direct and indirect medical costs, remains enormous.
Clinical Manifestations of Coronary Artery Disease
Stable Coronary Artery Disease
In stable coronary artery disease, atherosclerotic plaques accumulate in the coronary arteries leading to significant narrowing of the coronary lumen, with subsequent obstruction of the bloodstream. This causes deficient oxygenation of the downstream myocardium during situations of increased demand, for instance during physical exercise. There is no direct correlation between the anatomic degree of luminal obstruction and the extent of downstream ischemia during exercise; it depends upon several factors. These include length of obstruction, presence of collateral flow, and the amount of dependent myocardium.
Acute Coronary Syndromes
In acute coronary syndromes, the index event is the rupture or erosion of the fibrous cap of an atherosclerotic plaque. Material from the cap is exposed to the bloodstream and immediately incites a thrombocytic aggregation, so that a thrombus forms on the surface of the ruptured plaque. The thrombus can obstruct coronary blood flow, and depending upon the degree of obstruction and the related myocardial damage, the resulting clinical manifestation is either completely silent or may be symptomatic in the form of unstable angina, non–ST-elevation myocardial infarction, or ST-elevation myocardial infarction (STEMI).
Heart Failure
Acute coronary syndromes, including myocardial infarction, may remain completely silent clinically, and therefore substantial myocardial damage may occur without the patient noticing any chest pain symptoms. Therefore, heart failure with severely impaired ventricular function may be the first manifestation of disease. Any patient with new-onset heart failure needs to be worked up for coronary artery disease.
Sudden Cardiac Death
Sudden cardiac death is a possible first manifestation of coronary artery disease. The underlying event is almost uniformly arrhythmia such as ventricular fibrillation or, rarely, myocardial rupture secondary to an acute myocardial infarction.
Imaging Goals for Evaluation of Coronary Heart Disease
There are two different pathways for evaluation of coronary heart disease. One pathway focuses on detecting coronary artery stenosis via direct visualization of the coronary artery, utilizing invasive coronary angiography or coronary computed tomography angiography (CTA). Recently, it has been possible to use volumetric magnetic resonance imaging (MRI) for visualization of coronary arteries; however, the spatial resolution of the technique is low compared to both coronary CTA and invasive catheter angiography and its use is limited to evaluation of coronary artery anomalies in the pediatric population and evaluation of gross stenosis in patients with contraindication to iodinated contrast. However, because not all areas of coronary artery stenosis cause ischemia, an alternate mechanism is visualization of ischemic myocardium using stress-induced ischemia.
Imaging goals for coronary heart disease are:
- 1.
Detection of coronary artery stenosis;
- 2.
Physiological imaging of ischemic myocardium;
- 3.
Detection of complications of myocardial infarction;
- 4.
Risk analysis and role of imaging in prevention of future events; and
- 5.
Viability or evaluation of potential of recovery of function.
Detection of Coronary Artery Stenosis
Coronary Computed Tomography Angiography: Acquisition of Images
Patient Selection
The ideal patient for coronary CTA is a thin person who is not pregnant, not too young (radiation issue), and not too old (prevalence of heavy calcification), with a low and steady heart rate, with normal renal function, and otherwise no contraindication to iodinated contrast material, beta-blockers, or nitroglycerine. Clinical indications currently considered valid for coronary CTA will be reviewed later in the chapter. Please note that “currently” is the most important word of the last sentence; cardiac computed tomography (CT) is a rapidly developing field (as are many others in medicine) and much of what may be accepted as standard of care today may be considered obsolete within a decade or sometimes even sooner.
Before discussing the clinical applications of cardiac CT, the technical limitations and pitfalls of this technique must be reviewed.
Technical Development of Scanners
Recent technical developments in mechanical cardiac CT have dramatically improved the ability of CT to visualize the heart and coronary arteries. The major improvements that have made this possible are fast gantry rotation speed and image reconstruction algorithms that allow us to use only a subset of projections from one (or possibly multiple) rotation of the gantry. Generally, half a gantry rotation is necessary to acquire all projections that generate an axial image. The temporal resolution is the time it takes to collect these projections and is calculated as one-half the gantry rotation speed. Thus, if the gantry rotation speed were 330 ms (gantry spins around the patient 3 times per second), then the temporal resolution is 1/2 330 ms equaling 165 ms. The temporal resolution is comparable to the shutter speed of a camera; the shorter (or faster) the shutter speed is, the more likely you are to generate motion-free images of a rapidly moving object. Although 165 ms represents one of the fastest temporal resolutions of current 64-slice multidetector computed tomography (MDCT) systems, it is not fast enough to obtain motion-free images of the coronary arteries in all phases of the cardiac cycle. Therefore, image reconstruction is typically performed in mid to late diastole, where there is the least cardiac motion. Additionally, beta-blockers are administered before the scanning to reduce the patient’s heart rate to 60 beats per minute or below. This results in a longer diastolic rest period, less motion of the coronary arteries, and reduces the risk of motion artifact.
The major step in the development of cardiac CT was the development of 64-slice MDCT. All major vendors have offered a 64-slice MDCT system, even though the number of slices is calculated in different ways. Some vendors actually have 64 equally sized detector rows within the gantry and have x-rays emitted from one focal spot in the x-ray tube. One vendor, however, uses 32 equally spaced detectors and two focal spots on the x-ray tube that alternate in emitting x-rays. Thus, they acquire two different projections for each of the 32 detector rows, resulting in 64 individual projections.
Up to 64-slice technology, all vendors were going along the same route with their technical innovations. However, from here on, there are substantial differences in the newer generation of scanners. One vendor developed a two-x-ray tube and two-detector array system, that is, a dual-source system. This allows collecting 180 degrees worth of projections in only one quarter of an actual gantry rotation. Thus, the temporal resolution is a little more than one fourth the gantry rotation speed (not one half). At a gantry rotation speed of 250 ms, this system has a temporal resolution of 66 ms, currently the fastest temporal resolution available.
Other vendors have widened their detector arrays to 256 and 320 detector rows, which cover up to 16 cm of the chest with only one rotation. This eliminates some arrhythmia issues. Although gantry rotation speeds have improved to 280 ms and temporal resolution has decreased to 140 ms with whole heart coverage systems, the temporal resolution remains lower than that for dual-source CT, and therefore motion artifact remains an issue at higher heart rates. Another recent advancement is improving the axial step-and-shoot acquisition method to allow substantial reduction in radiation dose in patients with low heart rates. However, patients with higher heart rates may not benefit from this acquisition method.
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