CARDIO.PDF - Cardiovascular System - PDF

# The Cardiovascular System ## Nuclear Imaging of the Cardiovascular System - Nuclear imaging of the cardiovascular system is an essential component of daily work in nuclear medicine departments. - Understanding all aspects of nuclear cardiology is crucial for preparing nuclear medicine technolog...

# The Cardiovascular System ## Nuclear Imaging of the Cardiovascular System - Nuclear imaging of the cardiovascular system is an essential component of daily work in nuclear medicine departments. - Understanding all aspects of nuclear cardiology is crucial for preparing nuclear medicine technologists. - Nuclear cardiology has become an independent modality within nuclear medicine. - This chapter covers the basics of nuclear cardiology; for more detailed information, consult *Advanced Nuclear Cardiology*. ## Cardiac Anatomy and Physiology - The **heart** is a muscular organ that rests in the center of the chest, within the **mediastinum**. - Two-thirds of the heart lies to the left of the midline, next to the base of the left lung. - The heart is angled away from the midline, with the apex being approximately 30-50 degrees from the sternum. - A sac called the **pericardium** surrounds the heart and holds it in place. - The wall of the heart is divided into three layers: - **Epicardium** (outer layer) - **Myocardium** (cardiac muscle) - **Endocardium** (inner layer) - The heart is divided into four chambers: - **Left atrium** - **Right atrium** - **Left ventricle** - **Right ventricle** - Deoxygenated blood enters the right atrium from the superior and inferior vena cavae. - The right atrium and ventricle are separated by the **tricuspid valve**. During ventricular diastole (relaxation or filling phase), this valve opens allowing the blood to flow from the right atrium into the right ventricle. - Blood exits the right ventricle during ventricular systole (contraction phase) and travels through the **pulmonary valve** into the pulmonary artery, which leads into the lungs. - **Carbon dioxide** and **oxygen** are exchanged in the lungs, resulting in oxygen-rich blood. - Newly oxygenated blood enters the left atrium by way of the **pulmonary veins**. - The blood then passes across the **mitral valve** into the left ventricle during ventricular diastole. - Oxygenated blood is introduced into the systematic circulation during ventricular systole by passing through the **aortic valve** and into the aorta. ## Coronary and Systemic Circulation - **Perfusion** of blood into the heart muscle itself is through **coronary circulation**. - Coronary circulation begins at the base of the aorta, where the right and left coronary arteries serve the heart with walls of varying thickness. - The **left coronary artery** (LCA) supplies the anterior and septal parts of the left ventricle. This includes the myocardium and the interventricular septum - The **right coronary artery** (RCA) supplies the right atrium and ventricle, as well as the posterior parts of the left ventricle. - Both the right and left coronary arteries supply the myocardium with nutrients and oxygen. ## Conduction System - A **cardiac cycle** is the period from the end of one heart contraction (systole) to the end of the next. - Each cardiac cycle is initiated by electrical stimulation in the heart, preparing it to contract. - This electric impulse starts in the **sinoatrial (SA) node**, a small mass of specialized tissue located in the right atrium, near the entrance of the superior vena cava. - The SA node serves as the pacemaker for the heart and generates regular impulses at the rate of 60-100 beats per minute. - Impulses from the SA node are directed to the **atrioventricular (AV) node**, where impulse transmission is slowed. It acts as a gatekeeper for the ventricles, passing on only 40-60 impulses per minute. - Impulses travel from the AV node through the "bundle of His" to the right and left bundle branches, which carry the impulses to the tips of both ventricles. - The bundle of His is also known as the AV bundle or atrioventricular bundle, a collection of heart muscle cells specialized for electrical conduction that transmits the electrical impulses from the AV node to the ventricles. - The terminal branches of the two bundle branches are called the **Purkinje fibers**, which carry impulses to the individual myocardial cells, resulting in the simultaneous contraction of both ventricles. ## Myocardial Ischemia - **Myocardial ischemia** is a term that indicates decreased blood flow to the myocardium resulting from narrowing or occlusion of a coronary artery. - **Angina pectoris** (chest pain) is the major symptom associated with myocardial ischemia. - **Stable angina** occurs during periods of stress and may not be apparent at rest. - **Unstable angina** is often a precursor to a heart attack. ## Myocardial Infarction - **Myocardial infarction (MI)** typically results from total occlusion of a narrow coronary artery by a blood clot. - Occlusion causes necrosis of the myocardium fed by the blocked vessel. - This necrotic tissue may be surrounded by an area of injured or ischemic myocardium. - This injured tissue is viable and can be improved via coronary artery bypass graft (CABG) surgery or percutaneous transluminal coronary angioplasty (PTCA). ## Myocardial Perfusion Imaging Myocardial imaging is used to demonstrate: - Myocardial ischemia - MI - Hibernating myocardium - Stunned myocardium Myocardial imaging can be performed with: - Thallium-201 (201Tl)-thallous chloride - Technetium-99m (99mTc)-sestamibi - 99mTc-tetrofosmin - Rubidium-82 (82Rb)-chloride - Nitrogen-13 (13N)-ammonia - Fluorine-18 (18F)-fluorodeoxyglucose (FDG) ## Treadmill Exercise Testing - Treadmill exercise testing is used to determine the presence of CAD by recreating symptoms experienced on exertion. - The test is performed with a 12-lead ECG, and symptoms are correlated with electrical data to determine whether ischemic changes occur. - Nuclear imaging can be added by administering a radiopharmaceutical during peak exercise to visualize ischemic areas. - The patient must continue to exercise for 1-2 min after tracer administration to maintain the maximum myocardial oxygen demand while the tracer is being taken up by the myocardium. ### **Contraindications to Exercise Stress Testing** - Unstable angina - MI within 2-4 days - Uncontrolled hypertension - Pulmonary hypertension - Untreated life-threatening arrhythmias - Uncompensated congestive heart failure - Advanced atrioventricular heart block - Acute myocarditis - Acute pericarditis - Severe mitral or aortic stenosis - Severe obstructive cardiomyopathy - Acute systemic illness ### **Patient History and Preparation** - Patients should be hemodynamically stable at least for 48 hours before undergoing an exercise test. - The patient is instructed to fast for at least 3 hours before the exercise test. - A physician must determine the optimal diet for the day of the study for patients with diabetes. - The patient should wear comfortable clothing and footwear. - Avoid any unusual physical exertion for at least 12 hours before the stress test. - Provide pertinent patient information including family history, lifestyle (smoking, alcohol use, exercise), recent symptoms, previous history of heart disease or hypertension, and current medications. ### **Pharmacologic Stress Agents** - Patients who are unable to perform a stress test due to physical limitations, poor motivation, or medications can be given pharmacologic stress agents. - Pharmacologic agents such as **dipyridamole**, **adenosine**, and **regadenoson** mimic the effects of exercise on the myocardium. #### Dipyridamole - **Intravenous administration** of **dipyridamole** increases adenosine levels in the blood via deactivating adenosine deaminase. - The increased adenosine results in vasodilation of the coronary arteries. - Adenosine activates A2 receptors on smooth muscle, creating a "myocardial steal" phenomenon that diverts blood away from stenotic coronary arteries, resulting in increased uptake in healthy myocardium and decreased or absent uptake in areas supplied by stenotic arteries. **Contraindications for Dipyridamole:** - History of bronchospasm - Pulmonary disease - Active wheezing - Hypotension - Severe mitral valve disease - MI within 2 days - Unstable angina within 48 hours - Severe aortic stenosis - Severe obstructive hypertrophic cardiomyopathy - Severe orthostatic hypotension **Adverse Effects:** - Chest pain - Headache - Dizziness - Nausea - Flushing - Tachycardia - Dyspnea - Hypotension **Aminophylline** can reverse adverse effects, typically in doses of 125-250 mg intravenously. #### **Adenosine** - **Intravenous infusion** of **adenosine** directly increases adenosine levels in the blood and causes coronary vasodilation. **Contraindications:** - Conditions as listed for dipyridamole - Second- or third-degree AV block (when a pacemaker is not present) - Taking oral dipyridamole **Adverse Effects:** - Flushing - Chest pain - Dyspnea - ST segment depression - Dizziness - Nausea - Hypotension **Aminophylline** can be used if adverse effects do not dissipate as quickly as expected. #### **Regadenoson** - **Regadenoson** is an A2A adenosine receptor that activates the A2A adenosine receptor and produces coronary vasodilation. **Contraindications:** - Second- or third-degree AV block - Sinus node dysfunction (unless patient has a functioning artificial pacemaker) - Bronchospasm - History of hypersensitivity to regadenoson - Systolic blood pressure less than 90 mmHg **Adverse Effects:** - Dyspnea - Headache - Flushing - Chest discomfort - Angina pectoris - Dizziness - Chest pain - Nausea - Abdominal discomfort - Dysgeusia (distortion of the sense of taste) **Aminophylline** can be used to alleviate or reverse these effects, typically at doses of 50–250 mg by slow intravenous injection (50 to 100 mg over 30–60 sec). #### **Dobutamine** - **Dobutamine** is a positive inotropic and chronotropic pharmaceutical that enhances the force of heart contractions and increases the heart rate, thereby increasing myocardial oxygen demand. - It stimulates the B-1 receptors in the myocardium, resulting in an increase in the force and frequency of contractions. - It is indicated for patients who are unable to exercise and cannot undergo pharmacologic vasodilator stress infusion. **Contraindications:** - Beta-blockers should be discontinued for 24 hours before testing - Recent MI (<1 week) - Unstable angina - Hemodynamically significant left ventricular outflow tract obstruction - Critical aortic stenosis - Atrial tachyarrhythmias with uncontrolled ventricular response - Ventricular tachycardia - Uncontrolled hypertension - Aortic dissection - Large aortic aneurysm **Adverse Effects:** - Chest pain - Palpitations - Headache - Flushing - Dyspnea - Paresthesia (burning or prickling sensation) - Ischemic ST segment depression **Beta-blockers** (esmolol) can be used to reverse adverse effects. ## Planar Myocardial Perfusion Imaging - 3 standard views are obtained during planar myocardial perfusion imaging. The **left anterior oblique (LAO)** view is usually obtained at 45°, the **anterior** view at 45° back from the LAO angle, and the **left lateral** view is obtained with the patient lying on the right side (right lateral decubitis). - The patient should be placed in the **decubitis** position to decrease diaphragmatic attenuation which can cause a false-positive defect in the inferior wall of the left ventricle. - **Breast tissue** can be another source of attenuation. ## Image Processing - **Image processing** of planar or gated SPECT images usually consists of side-by-side displays of the background-subtracted and smoothed stress and rest or redistribution images. - **Circumferential profile analysis** is performed to provide quantitative myocardial perfusion information. - The percentage washout of 201Tl and the **heart-to-lung ratio** can also be calculated. ## SPECT Myocardial Perfusion Imaging ### Image Acquisition - SPECT images are acquired over 180° or 360° using a circular or noncircular orbit with continuous or step-and-shoot motion. - The step-and-shoot method consists of 32 or 64 stops for a 180° rotation or 64 or 128 stops for a 360° rotation. - The recommended pixel size is 6.4 ± 0.2 mm for a 64 x 64 matrix acquisition. - The patient is positioned with both arms or the left arm up over the head. - The patient should be as comfortable as possible to minimize motion. - Devices such as armrests, wedges, and Velcro straps can provide stability and comfort. - SPECT studies can also be acquired in the prone or decubitus position for patients who are likely to move if imaged in the supine position. - The technologist must explain the importance of remaining still to the patient before starting the acquisition. - The patient needs to be closely monitored for movement. If a significant amount of movement occurs, the study should be terminated and re-done or planar images should be acquired instead. ## Dual-Radionuclide Myocardial Perfusion SPECT - This protocol involves the use of 2.5 mCi (92.5 MBq) of 201Tl injected at rest and 22-25 mCi (814-925 MBq) of a 99mTc-based myocardial perfusion agent administered during stress. - Rest/stress imaging can be accomplished on the same day. The acquisition parameters for the SPECT studies are the same as those for 201Tl- and 99mTc-labeled perfusion agents, respectively. - This method combines the unique characteristics of 201Tl for determining myocardial viability with 99mTc-sestamibi imaging at stress. ## Cardiac PET Imaging - Myocardial perfusion and viability can be assessed using positron-emitting radionuclides such as 82Rb, 13N-ammonia, and 18F-FDG. - The 82Rb infusion system eliminates the need for a cyclotron. - 82Rb is a potassium analog that is cleared from the bloodstream and taken up by the myocardium via the sodium-potassium adenosine triphosphatase pump. - 13N-ammonia has a short half-life and is typically used in facilities with a cyclotron. - 18F-FDG is a glucose analog with a relatively long half-life. - These three agents are administered during both rest and stress. - Imaging is typically acquired between 30-60 minutes after administration and can be reconstructed into standard slices of the heart. ## Equilibrium-Gated Blood Pool Imaging - Equilibrium-gated blood pool imaging (or multigated blood pool acquisition [MUGA], radionuclide ventriculography [RVG], and equilibrium radionuclide angiography [ERNA or RNA]) involves labeling red blood cells (RBC) with 99mTc-pertechnetate and imaging the blood pool in the heart using a gated acquisition technique. - This test can be performed at rest or during stress; however, stress blood pool imaging is seldom performed. **Clinical Indications for ERNA:** - Detection or assessment of CAD - MI (acute or remote) - Detection or assessment of congestive heart failure - Assessment of cardiac function in chemotherapy patients - Evaluation of cardiac function in patients with valvular disease ## First-Pass Imaging - First-pass radionuclide angiography is a useful tool in the evaluation of patients with: - Right ventricular dysfunction - Interventricular shunts - Myocardial ischemia - Infarction - This technique records the initial passage of the radiopharmaceutical through the cardiac chambers. - An advantage of first-pass analysis is that the tracer activity is limited to a single chamber, making it easier to define the region of interest. - Background interference is decreased. - The procedure is also completed rapidly and does not require the patient to remain still on an imaging table. ## Dual Isotope Myocardial Perfusion Imaging - Dual Isotope MPI is used to combine the characteristics of 201Tl for myocardial viability testing, with 99mTc-sestamibi or 99mTc tetrofosmin for stress imaging. - The patient receives 2-4 mCi (201Tl) for cardiac cycle viability and 25-30 mCi of 99mTc-sestamibi or 99mTc-tetrofosmin for stress imaging on the same day. The 201Tl portion is performed first to avoid backscatter radiation.