Top 10 takeaways from cardiac imaging: ECHO and Nuclear imaging
The "ASPC Top Ten Imaging" summarizes ten things to know about ten important CVD-related imaging studies. In our last section, we discussed about the application aspects of coronary calcium estimation and coronary artery CT angiography. In the second section of this series, we bring to you the most important aspects of how, when and for whom to use echocardiography and nuclear imaging in cardiovascular diseases.
1. Cardiac ultrasound (echocardiography):
Echocardiography utilizes ultrasound waves to provide hemodynamic information about heart function. When accompanied by stress testing, echocardiography is often used to assess myocardial ischemia (i.e., coronary artery atherosclerosis), left ventricular function (i.e. heart failure, cardiomyopathy) and structural heart disease (i.e., valvulopathy, congenital heart disease, aneurysm, cardiac tumor, pericarditis, endocarditis, aortic dissection, heart chamber thrombosis).
The advent of 3D echocardiography beyond the routinely used conventional parameters like M-mode, Doppler etc. aids in characterization of mitral valve structure, atrial septal defect size estimation, etc.
Stress echocardiography is reasonably sensitive and specific for diagnosing coronary artery disease in symptomatic patients. Being a non-invasive procedure without any radiation exposure, its remarkable safety makes it applicable in almost all patient scenarios, including pregnancy. Following points are our top 10 nuggets for echocardiography:
| 1. Transthoracic echocardiography is the most common approach, with transesophageal echocardiography preferred in patients with conditions that compromise transthoracic imaging quality (e.g., obesity, certain lung conditions). Contrast options include agitated saline or commercial ultrasound contrast agents |
| 2. Transesophageal echocardiography may provide better resolution images of the left heart, evidence of potential endocarditis, mitral and aortic valves, and aorta (i.e., aortic dissection). |
| 3. Doppler echocardiography can assess stroke volume, heart chamber pressure gradients, valvular regurgitations, and intracardiac shunts. |
| 4. echocardiography can help diagnose alternative cardiac etiologies of chest pain, identify regional wall-motion abnormalities, determine left ventricular ejection fraction, and evaluate diastolic dysfunction for stratification purposes. |
| 5. stress echocardiography provides cardiac functional assessment. |
| 6. Echocardiography is commonly used to assess left ventricular ejection fraction. |
| 7. While heart failure can occur with reduced ejection fraction, symptomatic heart failure can also occur with preserved ejection fraction (HFpEF) (i.e., ejection fraction ≥ 50%). |
| 8. Echocardiography may provide helpful information regarding microcirculatory dysfunction that may contribute to angina without obstructive lesions in major coronary arteries |
| 9. echocardiogram assessment of heart function and anatomy can provide peri-operative risk stratification. |
| 10. Echocardiogram assessment can provide cross-sectional and longitudinal cardiac assessment in patients undergoing chemotherapy, helping to monitor for potential adverse effects of chemotherapy on cardiac structure and function. in this aspect strain imaging is emerging as an important tool to detect subclinical dysfunction. |
2. Nuclear myocardial perfusion imaging (MPI)
Nuclear myocardial perfusion imaging through Single Photon Emission Computed Tomography (SPECT) utilizes small amounts of nuclear tracer (i.e., the isotope technetium-99 or thallium-201) injected into the blood to assess myocardial segments that do not take up the tracer (i.e., damaged myocardium) or areas with delayed uptake of the tracer (i.e., ischemic myocardium).
The radiotracers are generally injected into the blood with imaging occurring at rest, or with exercise, or both. For patients unable to physically exercise, then an A2A adenosine receptor agonist (i.e., regadenoson) can be injected as an alternative to exercise.
A positron emission tomography (PET) scan of the heart utilizes a radiotracer (i.e., often 82rubidium or 13ammonia for rest and stress perfusion). Uptake of the radiotracer by the myocardium is proportional to myocardial blood flow.
Strengths of PET MPI include high diagnostic accuracy, safety with low radiation exposure (lower than SPECT), efficient with 5-min image acquisition times (may take only 30 minutes to perform), ability to accommodate ill or higherrisk patients, ability to assess patients with large body habitus, and ability to assess non-obstructive coronary microvascular dysfunction.
The most significant facets off nuclear imaging are summarised below:
| 1. SPECT is a perfusion imaging study that typically uses technetium-99 (99mTc). 99mTc produces less radiation than thallium-201 (201TI), ~6 mSVversus ~17 mSV respectively. Thus technetium is most commonly used. |
| 2. MPI may utilize different tracers, depending upon the imaging device, and purpose of the imaging (e.g., perfusion imaging, atherosclerosis imaging, metabolic imaging, inflammation imaging, and/or innervation/sympathetic imaging) |
| 3. . Appropriate use of MPI can help stratify CVD risk; inappropriate use of MPI may not help stratify CVD risk |
| 4. MPI may help augment CAC CVD risk stratification. |
| 5. MPI imaging may help identify obstructive coronary artery disease as the etiology of chest pain. |
| 6. MPI can be used in patients with immobility, cardiac rhythm disorders, impaired kidney function, or presence of cardiac devices. |
| 7. Over 50% of patients may be unable to adequately exercise during MPI, with an inability to achieve 85% of maximum predicted heart rate and 5 metabolic equivalents (METS). This often prompts the alternative of pharmacologic stress testing. |
| 8. If stress MPI is normal, resting MPI may be redundant and not necessary. Employing stress MPI results alone may reduce radiation and cost. |
| 9. PET has a high sensitivity and specificity to detect anatomic and functional atherosclerotic lesions useful for CVD risk stratification. |
| 10. PET may help identify functional abnormalities suggestive of microvascular CAD. |
Stress echocardiogram (Echo) has a sensitivity of 85% and specificity of 82% to detect significant CAD. Positron emission tomography (PET) has sensitivity of 90% and specificity of 85% in this regard.
Source: American Journal of Preventive Cardiology: https://doi.org/10.1016/j.ajpc.2021.100176
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