THE THORAX
Echocardiography
Computed tomography (see Fig. 4.24)
Two-dimensional echocardiography uses ultrasound to image the heart. A subcostal or intracostal window may be used and images may be obtained in any plane. Longitudinal images through the outflow tracts are usually obtained, as well as cross-sectional images through the valves and chambers. Ultrasound is probably the best modality for imaging the internal anatomy of the heart, the walls, chambers and valves. The movement of the walls and valves may also be assessed dynamically throughout the cardiac cycle. Transoesophageal echocardiography allows much closer inspection of the heart because of the close apposition of the left atrium to the anterior wall of the distal oesophagus, without intervening air or lung.
CT scanning shows the heart and vessels in cross-section. The pericardium may be identified between epicardial and mediastinal fat. Dynamic scans obtained during intravenous infusion of contrast demonstrate the cardiac chambers and vessels to greater advantage. ECG gating allows images to be acquired during the same part of the cardiac cycle, thus reducing motion artefacts and providing better images.
Angiocardiography This technique involves the injection of contrast directly into the heart chambers via a pigtail catheter, which is usually introduced through the femoral artery or vein for the left and right chambers, respectively. The chambers are recognized by their position and characteristic configuration.
Magnetic resonance imaging The applications for MRI in cardiac radiology are steadily increasing. Acquisition of images is gated to the ECG to overcome motion artefact, and faster scan times have improved image quality. The cardiac chambers, valves and major vessels may be imaged in any plane to give information previously only obtainable with cardioangiography, and with the added advantage of demonstrating the soft tissues. The pericardium is shown as a dark line 1-2 mm thick. THE GREAT VESSELS
Coronary angiography Coronary angiography involves selective catheterization of the coronary arteries. A small volume of contrast is injected and images may be obtained in lateral, anterior oblique and AP projections. There is individual variation in the branches of the coronary arteries, which are demonstrated from case to case. This is due both to anatomical variation and technical factors. The major arteries are demonstrated in Figures 4.26 and 4.27. Nuclear medicine Nuclear medicine studies are used mainly for functional assessment of the heart, which in clinical practice is often more important than the demonstration of the anatomy. Thallium-201 ( Th) and technetium-99m ( Tc)-labelled MIBI (2-methoxy isobutyl isonitrile) are taken up by normally perfused myocardium, and images obtained by gamma camera show the heart. The use of SPECT (single photon emission CT) allows images to be constructed in any plane - usually with three sets of images - along the short cardiac axis (at right-angles to the long axis of the heart), and along the vertical and horizontal long axes. It also improves the target:background ratio, as neither radiopharmaceutical agent is taken up exclusively by the myocardium. Other functional information on ventricular filling, ejection fraction and so on may be obtained by blood-pool imaging using Tc-labelled red blood cells and electrocardiogram (ECG)-gated acquisition of data. 201
99m
99m
(see Figs 4.10, 4.20, 4.22, 4.40 and 4.42-4.44) The aorta (see Figs 4.10, 4.31 and 4.32) The aortic arch The ascending aorta begins at the aortic valve at the level of the lower border of the third costal cartilage. It ascends to the right, arching over the pulmonary trunk to lie behind the upper border of the second right costal cartilage. The first few centimetres of the ascending aorta and the pulmonary trunk are enclosed in a common sheath of pericardium. At its origin it lies behind the outflow tract of the right ventricle and the pulmonary trunk, and the right atrial appendage overlaps it. It ascends anteriorly and to the right, passing over the right pulmonary artery and right main bronchus. The right lung and sternum are anterior. The coronary arteries arise from aortic sinuses - three localized dilatations above the cusps of the aortic valve. The arch of the aorta passes posteriorly and from right to left. It passes anterior to the trachea and arches over the left mainstem bronchus and pulmonary artery to come to lie to the left of the body of T4. Anterior and to the left of the arch are the pleura and left lung. On its right side, from front to back, are the trachea, oesophagus, thoracic duct and body of T4. Its inferior aspect is connected to the ligamentum arteriosum, the fibrous remnant of the ductus arteriosus. Superiorly are the three branches of the arch that are crossed anteriorly by the left brachiocephalic vein. The left
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