PDF: CT Scan of the Heart Lecture Notes

Dijlah University College ‫كلية دجلة الجامعة‬ Department of Radiology technologies ‫قسم تقنيات االشعة‬ ‫المرحلة الرابعة‬ Topic: Computed tomography...

Dijlah University College ‫كلية دجلة الجامعة‬ Department of Radiology technologies ‫قسم تقنيات االشعة‬ ‫المرحلة الرابعة‬ Topic: Computed tomography ‫التصوير المقطعي‬ Lecture19 Subject: CT scan of the heart ‫ ريم عبد الجليل خليل‬.‫د‬ Cardiac Anatomy The human heart is a four-chambered muscular organ with a size and shape roughly equivalent to a person’s closed fist. The heart is lying in the mediastinum, approximately two-thirds of its mass is left of midline. The lower border of the heart lies on the diaphragm, pointing toward the left to form a blunt point known as the apex. The upper border of the heart, or base, lies just below the second rib Coverings  The heart is enclosed in a, double-layered sac called the pericardium.  The outer layer is made of a tough white fibrous tissue known as the parietal layer.  The inner, or visceral, layer of the pericardium is composed of a smooth, moist, serous membrane. This visceral layer closely envelops the heart and is also called the epicardium.  Between the two layers is the pericardial space, which contains a small amount of fluid that serves to lubricate the constantly rubbing surfaces.  The majority of the wall is composed of cardiac muscle, or myocardium the outer covering of the myocardium is the epicardium (which is the inner section of the pericardium); the inner lining of the myocardium is the endocardium Cavities  The interior of the heart is divided into four chambers, two upper and two lower.  The upper cavities are called atria; these collect blood as it comes into the heart.  The lower cavities are the ventricles, responsible for pumping blood out of the heart  Because of their heavy pumping burden, the ventricles are considerably larger and thicker walled than the atria. Furthermore, the left ventricle has particularly thick walls because of the higher force required to pump blood through all the vessels of the body, except to and from the lungs, whereas the right ventricle sends blood only to the lungs. Valves and Openings  Heart valves limit the flow of blood to only one direction. They lie at the exit of each of the four heart chambers.  When open, the four heart valves ensure that blood flows freely in a forward direction. When closed, the valves prevent blood from flowing backward to its previous location.  The two atrio-ventricular valves are the tricuspid and mitral valves.  The tricuspid valve has three folds and is in the right side of the heart, between the right atrium and the right ventricle.  The mitral valve has only two flaps and is therefore also called the bicuspid valve. It is in the left side of the heart, between the left atrium and left ventricle.  The pulmonary and aortic valves are semilunar valves, consisting of half-moon–shaped flaps.  The pulmonary valve is in the right side of the heart, between the right ventricle and the entrance to the pulmonary artery that carries blood to the lungs.  The aortic valve is in the left side of the heart, between the left ventricle and the entrance to the aorta. Coronary Circulation :  The myocardial cells that make up the heart tissues must also be supplied with nutrients and oxygen and be freed of waste products.  Coronary circulation refers to the movement of blood through the tissues of the heart.  Blood is carried to the heart by the two coronary arteries and their branches. Coronary Arteries: Right Coronary Artery (RCA): Supplies blood to the right atrium, right ventricle and part of the left ventricle. Left Coronary Artery (LCA): supplying the left atrium and left ventricle.  These major coronary vessels lie on the epicardial surface of the heart  Most of the blood supplied by the coronary arteries is returned to the right atrium via the coronary sinus. Physiology of the Heart: Cardiac Cycle: Systole: Ventricular contraction pumps blood into the pulmonary artery and aorta. Diastole: Ventricular relaxation allows filling from the atria. Blood Circulation: Pulmonary Circulation: Exchange of gases in the lungs. Systemic Circulation: Delivery of oxygen and nutrients to tissues. Coronary Artery Grafts and Stents The buildup of fat and cholesterol plaque is called atherosclerosis. When plaque builds up in the coronary arteries a partial or total blockage results and the heart muscle does not get an adequate blood supply. This is referred to as ischemic heart disease or coronary artery disease (CAD).  Coronary artery bypass graft surgery [CABG]: is typically recommended when there is disease of the left main coronary artery or in three or more vessels, or if nonsurgical management has failed, arteries or veins taken from elsewhere in the patient’s body are grafted from the aorta to the coronary arteries to bypass atherosclerotic narrowing and improve the blood supply to the coronary circulation supplying the myocardium.  Balloon angioplasty and coronary stenting : o Are less invasive than is CABG and are options for some patients, o Angioplasty is a technique that is used to dilate an area of arterial blockage using a catheter with a small, inflatable, sausage-shaped balloon at its tip. o Coronary artery stents were designed to overcome some of the shortcomings of angioplasty. o When the balloon is inflated, the stent expands and pushes against the inner wall of the coronary artery. o Compared with angioplasty alone, stents open the disease segment into a rounder, bigger, and smoother opening and reduce the chance of restenosis. Cardiac CT and coronary angiography:  For years conventional coronary angiography provided the only method of imaging the coronary arteries. Although it is a useful tool, it is invasive and is associated with a small risk of complications as stroke, bleeding severe enough to require transfusion, vascular access complications, myocardial infarction, and even death. In addition, the procedure is associated with significant costs. For these reasons, coronary angiography is typically reserved for patients with serious symptoms and a high likelihood of having significant coronary artery disease.  Cardiac CT is less-invasive imaging modality for the diagnosis of coronary artery disease (CAD) and is often used to avoid coronary angiography in low- and intermediate risk patients.  Cardiac CT can provide not only anatomic information, but also functional information to aid in the diagnosis and treatment of CVD.  Cardiac CT acquisition techniques are needed to produce images free of motion artifact.  Care must be taken with the delivery of contrast medium to ensure optimal enhancement of the targeted structure and the surrounding tissues.  Visualization of the coronary arteries, a major application of cardiac CT, is difficult because the coronary arteries are of relatively small caliber, are often tortuous in shape, and are subject to constant, often rapid, heart motion. This can be overcome by advances in MDCT technology that have improved both spatial and temporal resolution.  Additionally, two other strategies are used to decrease cardiac motion artifacts. o First, the patient’s heart rate can be temporarily lowered by the administration of b-blockers. o Second, a technique called cardiac gating attempts to use only those images acquired during periods of lowest cardiac motion. Advantages of Cardiac CT High accuracy in detecting coronary vessel stenosis. High negative predictive value in ruling out significant disease. Capability to characterize plaque morphology. Indications for Cardiac CT 1. Patients with known or suspected chronic coronary artery disease. 2. Assessment of suspected anomalous coronary artery anatomy. 3. Screening in symptomatic patients with low probability of coronary artery disease. 4. Screening in asymptomatic high-risk patients or those with atypical chest pain. 5. Evaluation of coronary artery grafts, including left internal mammary artery (LIMA) grafts. 6. Alternative to diagnostic coronary angiography for planning percutaneous intervention or follow-up. Contraindications for Cardiac CT 1. Allergic reaction to iodinated contrast media, renal insufficiency. 2. Inability to remain still, follow breath-hold instructions. 3. Clinical instability: Cardiac failure, acute myocardial infarction, severe hypotension. Elevated heart rate (if β-blockers cannot be used). Cardiac arrhythmias (e.g., atrial fibrillation, frequent extrasystoles). Patient Preparation  Avoid caffeine for 24 hours (tea, coffee, energy drinks, Viagra-type medication).  Patients should take their normal cardiac medication as usual.  Obtain patient history, with particular attention to contraindications to iodinated contrast agents, b-blockers, or nitroglycerin.  Explain the procedure to the patient, including the importance of breath-hold.  Assess suitability for β-blockers and sublingual nitrates.  Insert an 18G cannula in the right ante-cubital fossa to minimize artifacts.  Connect ECG leads. Noise-free tracing requires a good contact between the lead and the patient’s skin.  Patients with cardiac arrhythmias require special consideration.  Beta-Blockers in Cardiac CT Used to reduce heart rate to 55-60 bpm for optimal imaging. Metoprolol is commonly administered orally (100 mg 1 hour before) or intravenously (5-30 mg in 5 mg increments if needed). Contraindications: Heart failure, aortic stenosis, heart block, asthma, use of other antiarrhythmic drugs.  Nitrate Administration Sublingual glyceryl trinitrate (400-800 μg) is given just before scanning to dilate coronary arteries. Contraindications: Hypotension, severe aortic stenosis, recent MI, severe anemia. CT Coronary Calcium Screening Atherosclerosis is a build-up of fat, plaque, and other substances, including calcium. Coronary artery calcification is a marker of coronary artery disease (CAD). Patients with CAD may exhibit no symptoms of the disease; in many patients myocardial infarction is the first sign of CAD. The goal of CT for calcium scoring is to determine the location and extent of calcified plaque in the coronary arteries. This is a helpful diagnostic tool; by measuring the amount of calcium that builds up in the coronary artery CT can be used to predict the likelihood of subsequent cardiovascular events in people with no symptoms. It is frequently performed as a screening study for patients with risk factors for CAD but no clinical symptoms. The amount of calcification on cardiac CT is expressed as a calcium score. A negative examination shows no calcification within the coronary arteries and suggests that atherosclerotic plaque is minimal and that the chance of CAD developing during the next 2 to 5 years is low. A positive test means that CAD is present, regardless of whether or not the patient is experiencing any symptoms Techniques and Protocols Cardiac CT can involve high radiation doses, so dose reduction measures are essential. ECG-gated scanning is crucial for image acquisition. The two techniques that attempt to minimize cardiac motion are called prospective ECG triggering and retrospective ECG gating. Retrospective ECG-Gated Helical Scan Acquires data throughout the cardiac cycle. Allows functional imaging but involves high radiation (~14 mSv). Used in patients with high heart rates or rhythm irregularities. Tube current modulation can reduce radiation dose by 50%. Prospective ECG-Gated Sequential Scan Limits x-ray exposure to specific cardiac cycle phases. Requires regular heart rhythm and low heart rate. Reduces radiation dose by up to 90% (2–4.5 mSv). No functional cardiac information is acquired. Contrast administration: Dual-injector heads are loaded with saline and iodinated contrast material. Transit (delay) time is determined either by the timing bolus method or bolus tracking method. Iodinated agent is injected: 70–150 mL at 4–6 mL/s, followed by 40 mL of saline solution. Scan data are acquired using MDCT with thin collimation, fast gantry rotation, and imaging that is synchronized to the patient’s heartbeat. Post Scan Patients who have received no b-blockers may leave. Patients who have received a b-blocker are observed for 30 minutes to ensure their heart rate returns to normal. Images are reconstructed per specific protocol.