Atherosclerotic coronary heart disease (CHD) has become one of the most common diseases that seriously endanger human life and health. Early and accurate diagnosis of CHD and optimization of CHD treatment measures play an important role in the secondary prevention of CHD. This paper briefly reviews some progress in the application of imaging technology in the diagnosis of coronary heart disease.
1.Radiological techniques
1.1 X-ray plain film
Generally speaking, chest X-ray cannot be used to determine the presence or absence of coronary heart disease, but it can provide auxiliary clues for the diagnosis of coronary heart disease. For example, X-ray plain film can show the enlarged left ventricle and changes in pulmonary circulation in patients with coronary artery disease, the latter including pulmonary stasis, interstitial and alveolar pulmonary edema, etc., which are of great value in determining the condition and assessing the prognosis. In addition, X-ray plain film examination is also valuable for the diagnosis of some complications after myocardial infarction, such as septal rupture and ventricular wall tumor.
1.2 Radionuclide examination
Radionuclide myocardial imaging is one of the most effective and important non-invasive diagnostic techniques for the diagnosis, risk stratification and prognostic assessment of coronary artery disease. It includes nuclear ventriculography, myocardial perfusion imaging, and myocardial metabolic imaging. Among them, myocardial perfusion imaging and myocardial metabolic imaging are currently the most widely used.
1.2.1 Myocardial perfusion imaging
The principle of myocardial perfusion imaging is derived from the ability of myocardial cells to selectively take up certain cations, and the myocardium is visualized by radiolabeling, and the amount of local myocardial aggregation of radiopharmaceuticals is positively correlated with the blood flow of coronary artery perfusion in the region. 1974 Gould first elucidated the pathophysiological mechanism of applying exercise methods to diagnose coronary heart disease by nuclear myocardial perfusion imaging [1]. In myocardial ischemia, even if the degree of coronary artery stenosis reaches about 90%-95%, myocardial perfusion imaging can still be normal at rest; however, under exercise load or drug load, the hemodynamics of stenosed coronary arteries can change significantly, and myocardial perfusion imaging can become significantly abnormal. With the development of modern medicine, myocardial perfusion imaging has gone through a process from planar to tomographic to gated tomography, from the main application of radionuclide 201Ti for perfusion imaging to the widespread use of 99mTc-labeled myocardial perfusion imaging agents and emission positron imaging drugs, and from simple visual analysis to the development of local myocardial quantification and gated analysis. This series of developments has enabled modern myocardial perfusion imaging not only to more accurately detect the location of specific coronary stenosis and myocardial ischemia, but also to precisely quantify the extent and degree of myocardial ischemia and capture information on left ventricular function and myocardial motion, greatly enhancing the clinical value of myocardial perfusion imaging for the diagnosis of coronary artery disease and thus for its risk stratification and prognostic assessment.
1.2.2 Myocardial metabolic imaging
Under physiological conditions, myocardial metabolism obtains energy mainly through fatty acid oxidation (40% to 60% of the energy required by the heart). When myocardial ischemia occurs, the local blood oxygen level decreases, the oxidative metabolism of fatty acids decreases accordingly, and glucose becomes the main substrate of myocardial tissue metabolism. This change in metabolic pattern is an important basis for being able to apply posionuclide tomography (PET) methods to identify ischemic myocardium [2]. Myocardial metabolic imaging is commonly used to evaluate myocardial survival and cardiac function, to estimate prognosis, and to assist in the establishment of therapeutic regimens, including glucose metabolism, myocardial oxygen metabolism, and fatty acid metabolism. In 18F-FDG (fluorine 18-deoxyglucose), for example, the inconsistency between myocardial perfusion and FDG uptake (mismatch phenomenon) can be distinguished from non-surviving myocardium; necrotic myocardium mainly shows a decrease in both perfusion and FDG uptake, i.e., a decrease in flow-metabolism matching; and hibernating myocardium mainly shows a relative increase in FDG uptake.
1.3 Computed tomography (CT) scan
Early applications of image-enhanced fluoroscopy or radiographs (including cine films) were used to detect coronary artery calcification. the utility and efficiency of CT, especially electron beam CT (EBCT) and spiral CT, has further improved the detection of coronary artery calcification, especially the quantitative analysis performed with the application of an integration system. A study included a sample of 568 cases [3], including 376 patients with coronary artery disease and 142 without coronary artery disease (all confirmed by coronary angiography), with the aim of verifying the diagnostic value of EBCT for coronary artery disease. The results showed that the sensitivity, specificity and accuracy of EBCT examination of coronary artery calcification and integral for the diagnosis of coronary heart disease were 83%, 66.8% and 77.5%, respectively, and the coronary artery calcification integral in patients with coronary heart disease was significantly higher than that in those without coronary heart disease. Because EBCT is a mechanical scan with an electron beam rotating to produce X-rays instead of the X-ray tube ball and detector rotating in conventional CT machines, it has a fast scanning speed (50ms/100ms), high temporal resolution, density resolution and spatial resolution, and can clearly display the anatomical structure and pathological changes of the heart and coronary arteries. Cine scan and flow scan can also evaluate ventricular wall motion, quantitatively assess ventricular function, and understand myocardial and coronary perfusion status, which are valuable in the prediction and diagnosis of coronary heart disease, coronary artery bypass grafting and follow-up after PTCA treatment.
1.4 Magnetic resonance imaging (MRI)
With the advancement of spin echo (SE) and fast/ultrafast pulse sequence containing EPI (echo planar imaging), magnetic resonance imaging (MRI) has become one of the main imaging techniques to observe the morphology and function of the cardiovascular system. In particular, MR myocardial perfusion imaging (MRMPI) can be used to evaluate myocardial microcirculatory perfusion that cannot be revealed by coronary angiography, and is valuable for the detection of infarcted and surviving myocardium [4]. The current imaging sequence used in MRMPI is Turbo FLASH sequence, which can acquire three to four levels in one cardiac cycle, covering a large part of the ventricle with high temporal and spatial resolution, and can truly reflect myocardial perfusion and its degree of wall penetration, especially for subendocardial myocardial lesions that are most sensitive to ischemia and necrosis.
2. Ultrasound technology
Two-dimensional echocardiography and Doppler echocardiography are now commonly used for the diagnosis of coronary artery disease. The observation of overall and segmental left ventricular wall motion function by short/long axis position of the left ventricle, combined with loading tests (mostly drugs) showing segmental abnormalities of left ventricular wall function is often helpful in the determination of myocardial ischemia or myocardial infarction. Areas of chronic myocardial infarction show thinning of the ventricular wall with enhanced fibrotic echogenicity and reduced or absent systolic motion. If the ventricular wall is locally dilated on top of this, it is a sign of a ventricular wall tumor. Post-myocardial infarction septal rupture is seen as interrupted septal echogenicity in the myocardium, and color Doppler examination may show a short-circuit shunt. In recent years, with the rapid development of ultrasound medicine, the application of ultrasound technology in the diagnosis of coronary heart disease has developed greatly.
2.1 Myocardial contrast echocardiography (MCE)
Usually, coronary angiography shows only the flow status of the subepicardial coronary arteries and does not reflect myocardial perfusion at the capillary level because there is no exact relationship between coronary artery stenosis and myocardial perfusion. It is myocardial perfusion at the microcirculatory level that determines whether myocardial cells are adequately supplied with blood and oxygen. MCE is a new technique developed in recent years, which is based on the injection of a special microbubble contrast agent through the coronary artery or peripheral vein and the observation of the backscattered signal of the microbubbles using two-dimensional or Doppler ultrasound techniques [5]. The scope of application of MCE is: (1) early diagnosis of acute myocardial infarction and establishment of treatment plan; (2) bedside evaluation of thrombolytic efficacy; (3) identification of non-recurrent flow with reperfusion therapy; (4) identification of coronary artery and peripheral vein. (4) assessment of coronary collateral circulation and myocardial viability; (5) evaluation of the efficacy of PTCA and coronary artery bypass surgery; (6) assessment of microvascular flow reserve in coronary artery disease and early non-invasive diagnosis of chronic coronary artery disease.
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2.2 Echocardiographic stress test
Echocardiographic stress test is a different loading method to increase myocardial oxygen consumption, resulting in insufficient coronary blood flow reserve to meet its needs, thus inducing myocardial ischemia and abnormal myocardial contractility. The more commonly used clinical test is the dobutamine loading test. The method is to record four images of parasternal left ventricular long-axis views, left ventricular short-axis views at the level of the papillary muscle, apical four-chamber view, and apical two-chamber view in sequence for 4 min after load cessation at rest, 3 min after pumping different doses of dobutamine, and to score ventricular wall motion abnormalities and calculate the wall motion scoring index and systolic wall thickening rate. Echocardiographic stress tests are valuable for early diagnosis of coronary artery disease, monitoring of surviving myocardium, evaluation of recanalized patients [6], and prediction of cardiac events.
2.3 Intravascular ultrasound (IVUS)
Intravascular ultrasound is a new ultrasound technique that has been developed and applied to clinical practice in recent years. Its miniature ultrasound probe can follow the finger-guided filaments of the coronary arteries and reach the main branches of the coronary arteries. It not only provides the morphology of the vessel lumen at the location of the probe in real time, but also shows the morphology, structure and function of the vessel wall, which is by far the most ideal method for diagnosing vascular diseases from the morphological aspect, and is known as “in vivo histology”. In addition, intravascular ultrasound can observe changes in vascular systolic and diastolic functions, and perform qualitative and quantitative analysis of atherosclerotic plaques, such as soft plaques, fibrous plaques, calcified plaques and plaque volume (load) [7, 8], and identify arterial entrapment, thrombosis, coronary artery spasm, aneurysmal dilatation, as well as intravascular cysts and intravascular hemorrhage. Intravascular ultrasound has an important guiding value for coronary interventions. It is particularly important in the intervention of left main lesions and open lesions because of its ability to accurately evaluate plaque loading, presence or absence of calcification, lumen diameter, and complete wall apposition of the implanted stent [9]. However, it is invasive and expensive, which limits its common application in clinical practice.
3.Coronary angiography (coronary angiogram)
Coronary angiography is an invasive technique widely used in recent years to diagnose coronary artery disease, and is recognized as the “gold standard” for the diagnosis of coronary artery disease. In combination with left ventriculography, coronary angiography can reveal the extent and distribution of coronary stenosis or obstructive lesions, the characteristics of certain atherosclerotic lesions [10], the status of collateral circulation, and the overall and segmental motion function of the left ventricle, providing a definite diagnostic basis for confirming the diagnosis of coronary artery disease and difficult cases of coronary artery lesions, selecting the indications for interventional and/or bypass surgery, and verifying the efficacy. However, coronary angiography is an invasive technique and may cause certain complications, which may lead to death in serious cases.
4.Summary
The requirement for the diagnosis of coronary artery disease imaging is that it should be able to provide a basis for reasonable treatment decisions. Different types of coronary artery disease, different periods of development and related pathological and pathophysiological changes have different requirements for treatment. Medical, surgical and interventional treatment of coronary artery disease and their interactions require that imaging must provide exact and comprehensive diagnostic information. In turn, certain important information provided by imaging can guide and modify the treatment plan and facilitate the selection of different or more effective treatments. For example, for the diagnosis and verification of the efficacy of myocardial ischemia or myocardial infarction, radionuclide examination and echocardiography (both including stress test) should be used, the former being more accurate and the latter simple and easy to perform. The detection of myocardial survival is best performed by PET and myocardial sonography. MRI and CT, for example, are suitable to show the site and extent of old myocardial infarction and ventricular wall tumors and attached wall thrombi. In contrast, MR and CT are not usually preferred in patients with acute myocardial infarction. MR and CT coronary angiography (including 3D reconstruction techniques), which can show proximal and middle coronary artery and severe stenosis, are useful for screening for coronary artery disease. Coronary angiography remains the “gold standard” for confirming the diagnosis of coronary artery disease and coronary lesion patterns, especially for the selection of indications for surgical and/or interventional treatment. However, this “gold standard” also has its limitations. For example, it is not possible to evaluate myocardial perfusion at the microcirculatory level (which can be compensated by myocardial sonography), and it is not possible to determine the condition of the coronary vessel wall and the volume of plaque (which can be compensated by intravascular ultrasound). Different treatment options dictate the choice of different imaging techniques. In addition, the patient’s condition, the cost and invasiveness of the examination, and safety should be considered. In conclusion, the selection of imaging techniques should be combined with the specific situation of the unit and the patient, and the specific problem should be analyzed, with the maximum benefit to the patient as the primary principle.