The great vessels

Echocardiography 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.

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.

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 (201Th) and technetium-99m (99mTc)-labelled M I BI (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 w i th 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 99mTc-labelled red blood cells and electrocardiogram (ECG)-gated acquisition of data. Computed tomography (see Fig. 4.24) 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.

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 w i th cardioangiography, and w i th the added advantage of demonstrating the soft tissues. The pericardium is shown as a dark line 1-2 mm thick.

THE GREAT VESSELS

(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

134 ANATOMY FOR DIAGNOSTIC IMAGING

Fig. 4.31 Aortograms: (a) DSA; (b) MR angiogram.

superior intercostal vein runs down to the brachiocephalic vein, passing anterior to the aorta and occasionally causing a small bulge on the arch, which is visible on the chest radiograph. The branches of the arch of the aorta are the brachiocephalic, the left common carotid and the left subclavian arteries. The brachiocephalic and left common carotid arteries ascend on either side of the trachea in a V shape to come to lie behind the sternoclavicular joints, at which point the brachiocephalic bifurcates into the right common carotid and subclavian arteries. From this point the common carotids ascend symmetrically into the neck.

Variation is common in the branches of the aortic arch, such that the 'normal' pattern as described above is only seen in 65% of subjects. • In 5% of subjects the left vertebral artery arises directly from the arch of the aorta, between the origins of the left common carotid and left subclavian arteries. • 2.7% have a common origin of the left common carotid and subclavian as a left brachiocephalic artery. • In 2.5% the left common carotid arises from the brachiocephalic artery. • In 0.5% an aberrant right subclavian artery arises distal to the left subclavian artery and passes to the right, posterior to the oesophagus. • Rare: right common carotid and subclavian arteries arising separately. • Very rarely the common carotid is absent so that the internal and external carotid arteries arise separately from the aortic arch on one or both sides.

• The following other arteries may also arise from the aortic arch: — One or both bronchial arteries — Thyroidea ima artery — Inferior thyroid artery — Internal thoracic artery • The origins of the brachiocephalic and the left common carotid arteries may also vary. If the vessels arise earlier than normal the left common carotid has to pass anterior to the trachea to assume its position left of the aorta. If the vessels arise more distally the brachiocephalic artery passes anterior to the trachea to get to its right side. Either arrangement may lead to anterior compression of the trachea, which may cause symptoms especially in the infant, whose anterior mediastinum is crowded by the thymus.

The aortic isthmus is the junction of the arch of the aorta and the descending aorta. This area is relatively fixed and is thus prone to injury w i th the shearing forces of blunt trauma.

The descending aorta The descending aorta passes inferiorly through the posterior mediastinum to the left of the spinal column. It passes posterior to the diaphragm at the level of T12. On its left side are the pleura and left lung. Posteriorly are the vertebral column and the hemiazygos veins. At its highest point the oesophagus lies to its right. The descending aorta then lies behind the oesophagus as the latter passes anteriorly, and the right lung and pleura lie to its right in contact w i th it. As it descends it passes behind the left main bronchus and pulmonary artery, the left atrium, the oesophagus and the posterior part of the diaphragm. To its right are the thoracic duct and the azygos vein.

The branches of the descending aorta are as follows: • Nine pairs of posterior intercostal arteries and a pair of subcostal arteries arise from the posterior aspect of the descending aorta to run in the neurovascular grooves of the third to twelfth ribs. These anastomose w i th the anterior intercostal arteries, which are branches of the internal thoracic aorta. The intercostal arteries give rise to radicular and radiculomedullary arteries to the spinal cord and its nerve roots; • Two to three bronchial arteries, the origins of which are variable. The right bronchial artery usually arises from the third posterior intercostal artery and the two left bronchial arteries from the aorta itself. The upper left usually arises opposite T5 and the lower left bronchial artery below the left main bronchus; • Four to five oesophageal branches arise from the front of the aorta. These ramify on the oesophagus, forming a network w i th oesophageal branches of the inferior

thyroid artery and ascending branches of the left phrenic and left gastric arteries; • Mediastinal branches; • Phrenic branches to the upper part of the posterior diaphragm (which receives most of its blood supply by inferior phrenic arteries that arise from the abdominal artery; and • Pericardial branches to the posterior pericardium.

Subclavian artery (see Fig. 1.42) The right subclavian artery arises from the bifurcation of the brachiocephalic trunk behind the right sternoclavicular joint. The left subclavian artery arises from the arch of the aorta in front of the trachea at the level of the T3/T4 disc space. It ascends on the left of the trachea to behind the left sternoclavicular joint. From this point, both arteries have a similar course. The scalenus anterior muscle (which passes from the transverse processes of the upper cervical vertebrae to the inner border of the first rib) divides the artery into three parts. The first part arches over the apex of the lung and lies deeply in the neck. The second part passes laterally behind the scalenus anterior muscle, which separates it from the subclavian vein. The third part passes to the lateral border of the first rib, from where it becomes the axillary artery (see Chapter 1 for branches).

Pulmonary arteries (see Figs 4.15 and 4.20) (see also pulmonary arteries in the lung) The pulmonary trunk begins at the pulmonary valve and is approximately 5 cm long. At first it lies anterior to the aorta; it then passes posteriorly and to its left to lie in the concavity of the arch, where it bifurcates into right and left main pulmonary arteries. The entire pulmonary trunk is covered by the pericardium in a common sheath w i th the ascending aorta.

The right and left atrial appendages and the right and left coronary arteries surround the base of the pulmonary trunk. Anterior and to the left it is in contact w i th the left lung and pleura.

The right pulmonary artery runs horizontally and to the right, passing behind the ascending aorta and SVC in front of the right main bronchus. It is crossed anteriorly by the right superior pulmonary vein as this drains to the left atrium.

The left pulmonary artery runs to the left, anterior to the left main bronchus and arching over this structure as it gives off the upper-lobe bronchus. It has a slightly higher position in the chest and is slightly shorter and smaller than the right pulmonary artery. It is crossed anteriorly by the left superior pulmonary vein. It is attached to the concavity of the aortic arch by the ligamentum arteriosum.