Chest Radiology Hariqbal Singh
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Anatomychapter 1

Parvez Sheik
 
ANATOMY OF CHEST AND MEDIASTINUM
Embryologically, airway starts developing by fifth week of gestational age in the form of lung buds which grow from ventral aspect of primitive foregut. Trachea and esophagus are also separated by fifth week. Hereafter tracheobronchial tree is formed from fifth to fifteenth week. There are 23–25 airway generations from trachea to bronchiole. Bronchus has cartilage in the wall, whereas bronchiole is devoid of cartilage.
Interstitium of lung is divided into axial interstitium, parenchymal interstitium and peripheral interstitium. Axial interstitium is made of bronchovascular sheaths and lymphatics. Parenchymal interstitium includes interalveolar septum along alveolar walls. Peripheral interstitium includes sub-pleural connective tissue and interlobular septa which encloses the pulmonary veins and lymphatics.
Pulmonary circulation includes primary pulmonary circulation, bronchial circulation and the anastomoses between the two. Primary pulmonary circulation consists of pulmonary arteries and veins that travel down to sub-segmental bronchial level and has a diameter same as that of the accompanying airway. Main pulmonary artery arises from the right ventricle. Bronchial circulation originates from thoracic aorta and supplies through the intercostals arteries which are two in number for each lung.2
Mediastinum is the space between the lungs. It is divided into a superior and an inferior compartment. Superior compartment consists of the thoracic inlet. Inferior compartment has anterior, middle and posterior sub compartments. Retrosternal region is included in the anterior compartment, heart lies in the middle compartment and descending aorta with esophagus and paraspinal region is located in the posterior mediastinal compartment. Thymus is located in the anterior part of superior as well as inferior compartment of mediastinum.
The application of chest CT has greatly increased over the years, however, chest radiography remains the most frequently requisitioned and performed imaging examination. A good understanding of normal anatomy and variations is essential for the interpretation of chest radiographs.
On posteroanterior (PA) view (Figs 1.1 and 1.2), the X-ray beam first enters the patient from the back and then passes through the patient to the film that is placed anterior to the patient's chest.
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Fig. 1.1: X-ray chest PA view
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Fig. 1.2: X-ray chest PA view shows mediastinal borders
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Fig. 1.3: X-ray chest PA view shows the zones
It uses 60–80 kV and 10 mAs keeping the focus film distance of 6 feet. On a PA film, lung is divided radiologically into three zones (Fig. 1.3):
  1. Upper zone extends from apices to lower border of 2nd rib anteriorly.
  2. Middle zone extends from the lower border of 2nd rib anteriorly to lower border of 4th rib anteriorly.
  3. Lower zone extends from the lower border of 4th rib anteriorly to lung bases.
Radiological division does not depict anatomical lobes of the lung.
 
Anatomically Segmental Division of Lungs
 
Right lung has three lobes
  1. Upper lobe which has an apical, anterior and a posterior segment.
  2. Middle lobe has a lateral and a medial segment.
  3. Lower lobe has superior segment, medial basal segment, anterior basal segment, lateral basal segment and a posterior basal segment.
 
Left lung has two lobes
  1. Upper lobe which has an apicoposterior, anterior, superior lingular and an inferior lingular segment.
  2. Lower lobe has superior segment, anterior basal segment, lateral basal segment and a posterior basal segment.
 
Left lung has no middle lobe and left lower lobe has no medial basal segment
In a well-centered chest X-ray, medial ends of clavicles are equidistant from vertebral spinous process. Lung fields are of equal transradiance.
Horizontal fissure might be seen on the right side as a thin white line that runs from right hilum to sixth rib laterally.5
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Fig. 1.4: X-ray chest-apicogram
For a fissure to be seen on a radiograph, the X-ray beam has to be tangential to it. The most frequently observed accessory fissure is the azygos lobe fissure which is seen in 1 percent of people. Apices are visualized free of ribs and clavicles on apicogram (Fig. 1.4).
Both hila are concave outwards. The pulmonary arteries, upper lobe veins and bronchi contribute to the making of hilar shadows. The left hilum is slightly higher than right hilum.
The normal length of trachea is 10 cm, it is central in position and bifurcates at T4–T5 vertebral level. Left atrial enlargement increases the tracheal bifurcation angle (normal is 60°). An inhaled foreign body is likely to lodge in the right lung due to the fact that the right main bronchus is shorter, straighter and wider than left main bronchus.
Normal heart shadow is uniformly white with maximum transverse diameter less than half of the maximum transthoracic diameter. Cardiothoracic ratio is estimated from the PA view of chest. It is the ratio between the maximum transverse diameter of the heart and the maximum width of thorax above the 6costophrenic angles: a = right heart border to midline, b = left heart border to midline, c = maximum thoracic diameter above costophrenic angles from inner borders of ribs. Cardiothoracic ratio = a+b:c. Normal cardiothoracic ratio is 1:2 (Fig. 1.5). In children, this cardiothoracic ratio may be increased.
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Fig. 1.5: X-ray chest PA view shows measurement of cardiothoracic ratio
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Fig. 1.6: X-ray chest lateral view
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Fig. 1.7: X-ray chest PA view (negative) to visualize bony thorax
Borders of the mediastinum are sharp and distinct (Figs 1.2, 1.5 to 1.7). The right heart border is formed by superior vena cava superiorly and right atrium inferiorly, the left heart border is formed by the aortic knuckle superiorly, left atrial appendage and left ventricle inferiorly.
Right hemi diaphragm is higher than left. Costophrenic angles are acute angles.
To detect any pulmonary pathology it is important to remember the normal thoracic architecture, both lungs are compared for areas of abnormal opacities, translucency or uneven bronchovascular distribution in the lungs.
An abnormal opacity should be closely studied to ensure that it is not amalgamated, opacity formed by superimposed normal structures such as bones, costal cartilages, vessels, muscles or nipple. Any opacity is evaluated by its extent, margins and location with presence or absence of calcification or cavitation.8
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Fig. 1.8:
A general assessment survey is made to look for any other lesion or displacement of adjacent structures.
On CT chest the sections are made in axial or transverse plane 8 to 10 mm in thicknesses, a miniature topogram should accompany each section or image to show the level of the sections relative to the anatomic structures at that level.
It is important to evaluate CT chest not only in soft-tissue and lung windows settings but also in intermediate windows by playing with window width and window center when considered essential specially when the lesions have intermediate densities.
The evaluation of CT chest should start with the soft tissues of the thoracic wall, the breasts and fat in the axilla (Figs 1.8 to 1.16), followed by assessment of mediastinum in soft-tissue windows. It is good to start with orientation to aortic arch (Fig. 1.10), and moving superiorly looking for any mass or node in region of the major branches of aorta, the brachiocephalic trunk, the left common carotid artery and the subclavian artery (Fig. 1.9) The brachiocephalic veins, superior vena cava, esophagus and trachea are also evaluated to exclude any abnormal mass lesion or deviation from normal.
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Fig. 1.9:
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Fig. 1.10:
10Moving inferiorly from the aortic arch assessing aortopulmonary window (Figs 1.11 and 1.12), the tracheal bifurcation (Figs 1.10 to 1.16), the hilar and perihilar tissues (Figs 1.12 to 1.14), carefully looking for lymph nodes. The presence of less than 3 small nodes or single node measuring less than 10 mm in diameter in the aortopulmonary window can be considered normal. Heart is examined for any ventricular aneurysm or coronary calcification (Figs 1.14 to 1.16).
The right ventricle lies anteriorly, posterior to the sternum and the right atrium lies on the right lateral side (Figs 1.14 and 1.15). The left ventricle lies on the entire left side (Figs 1.14 to 1.16), the outlet of the left ventricle and the ascending aorta lie in the center of the heart. The left atrium is the most posterior chamber of the heart. The pulmonary veins join the left atrium posteriorly (Fig. 1.14). The inferior vena cava is seen further caudally just at the section the diaphragm appears together with the upper part of liver (Fig. 1.16).
The azygos vein lies dorsal to the trachea adjacent to esophagus; it arches as azygos arch above the right main bronchus and drains anteriorly into the superior vena cava.
Just caudal to aortic arch lies the pulmonary trunk, which divides into the right and left pulmonary arteries, at the level lies the aortopulmonary window. Inferior to the level of aorta the tracheal bifurcation takes place into right and left main bronchus. The aortopulmonary window and subcarinal region have predilection for mediastinal lymph nodes or malignant masses.
Now the lung parenchyma, ribs and other bony structures are assessed. The pattern of the pulmonary vasculature is scrutinized on the lung windows (Figs 1.17 to 1.28). The lungs show negative density values in the Hounsfield range. The pulmonary vasculature continues from the hilum to the periphery with steady decrease in their thickness with relative oligemia in the periphery and along the margins of the lobes.
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Fig. 1.11:
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Fig. 1.12:
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Fig. 1.13:
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Fig. 1.14:
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Fig. 1.15:
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Figs 1.8 to 1.16: Axial CT sections of chest in mediastinum window
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Fig. 1.17:
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Fig. 1.18:
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Fig. 1.19:
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Fig. 1.20:
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Fig. 1.21:
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Fig. 1.22:
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Fig. 1.23:
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Fig. 1.24:
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Fig. 1.25:
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Fig. 1.26:
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Fig. 1.27:
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Figs 1.17 to 1.28: Axial CT sections of chest in lung window
Just caudal to aortic arch lies the pulmonary trunk, which divides into the right and left pulmonary arteries, at the level lies the aortopulmonary window. Inferior to the level of aorta the tracheal bifurcation takes place into right and left main bronchus. 20The aortopulmonary window and subcarinal region have predilection for mediastinal lymph nodes or malignant masses.
The pattern of the pulmonary vasculature is scrutinized on the lung windows. The lungs show negative density values in the Hounsfield range.
Application of magnetic resonance imaging (MRI) in intrinsic lung disease is limited by signal loss from lung motion, paucity of protons, and magnetic field inhomogeneities because of air and tissue interfaces in lung. These problems will be overcome in future with improvements in imaging hardware and pulse sequences. However, MRI is an important tool in assessment of diseases of the heart, mediastinum, pleura, and chest wall (Fig. 1.29). Strengths of MRI lies in excellent tissue contrast, multiplanar capability, sensitivity to blood flow and lack of ionizing radiation.
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Fig. 1.29: T2W coronal MR section at the level of arch of aorta
21Clinical indications for chest MRI include chest wall tumors, infections, chest wall or diaphragmatic extension of intrathoracic masses, pleural effusions, pleural wall lesions and vascular pathologies.
 
CT CORONARY ANGIOGRAPHY (NORMAL ANATOMY)
Heart imaging methods such as cardiac CT are allowing physicians to take a closer look at the heart and great vessels at little risk to the patient. A traditional CT scan is an X-ray procedure which combines many X-ray images with the aid of a computer to generate cross-sectional views of the body. Cardiac CT uses advanced CT technology with or without intravenous iodine-based contrast to visualize cardiac anatomy, including the coronary arteries and great arteries and veins. With multi-detector scanning, it is possible to acquire high-resolution three-dimensional images of the heart and great vessels.
Cardiac CT is especially useful in evaluating the myocardium, coronary arteries, pulmonary veins, thoracic aorta, pericardium, and cardiac masses, such as thrombus of the left atrial appendage.
 
Coronary Arteries
The four main coronary arteries evaluated by CT are the right coronary artery (RCA), the left main coronary artery (LCA), the left anterior descending (LAD) artery, and the left circumflex (LCx) artery (Figs 1.30 and 1.31).
 
Dominant Coronary Artery
Whichever artery crosses the crux of the heart and gives off the posterior descending branches is considered to be the dominant 22coronary artery.
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Figs 1.30 and 1.31: Coronary arteries and their branches
In approximately 85% of individuals, the RCA crosses the posterior interventricular groove and gives rise to the posterior descending branches (right dominance); in 7–8 percent, the LCx artery crosses the interventricular groove and gives rise to branches to the posterior right ventricular surface (left dominance); and in the remaining 7–8 percent, the inferior interventricular septum is perfused by branches from both the distal RCA and the distal LCx artery (co-dominance).
Right Coronary Artery: The RCA arises from the anterior right coronary sinus somewhat inferior to the origin of the LCA (Figs 1.32 to 1.35). The RCA passes to the right of and posterior to the pulmonary artery and then downward in the right atrioventricular groove toward the posterior interventricular septum.
In more than 50 percent of individuals, the first branch of the RCA is the conus artery, unless it (the RCA) has a separate origin directly from the right coronary sinus.
The second branches usually consist of the sinoatrial node/nodal artery and several anterior branches that supply the free wall of the right ventricle. 23
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Fig. 1.32: 3D coronary CT in axial plane
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Fig. 1.33: 3D coronary CT in coronal plane
The branch to the right ventricle at the junction of the middle and distal RCA is called the acute marginal branch.
The distal RCA divides into posterior descending artery (PDA) and posterior left ventricular branches (PLV) in a right dominant anatomy.24
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Fig. 1.34: 3D coronary CT in posterior coronal plane
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Fig. 1.35: 3D coronary CT in posterior oblique coronal plane
Left Coronary Artery: The LCA arises from the left posterior coronary sinus, is 5–10 mm long. The LCA passes to the left of and posterior to the pulmonary trunk and bifurcates into the LAD and LCx arteries. Occasionally, the LCA trifurcates into the LAD and LCx arteries and the ramus intermedius. The ramus intermedius has a course similar to that of the first diagonal branch of the LAD artery to the anterior left ventricle (Figs 1.36 to 1.38). 25
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Fig. 1.36: Coronal plane (MIP image)
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Fig. 1.37: Axial plane (MIP image)
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Fig. 1.38: Oblique coronal plane (MIP image)
The LAD artery passes to the left of the pulmonary trunk and turns anteriorly to course in the anterior interventricular groove toward the apex. It provides the diagonal branches (D) to the anterior free wall of the left ventricle and the septal branches to the anterior interventricular septum.
The Left Circumflex Artery (LCx) courses in the left atrioventricular groove (Figs 1.36 to 1.38) and gives off obtuse marginal branches (OM) to the lateral left ventricle. In a left dominant or codominant anatomy, the LCx artery gives rise to the PDA or posterior left ventricular branches.