Chapter 2 Thoracic imaging

Adults Susan J. Copley, Conor D. Collins, David M. Hansell, Paediatrics Christopher D. George

Chest radiography and other techniques 21

Different types of chest radiograph 21

Factors influencing the quality of a chest radiograph 22

Other techniques 23

Interventional procedures 27

The normal chest 28

Anatomy 28

Specific conditions 40

The postoperative and critically ill patient 40

Kyphoscoliosis 43

Bronchiectasis 43

Chronic airflow limitation 44

PAEDIATRICS ¹ 45

Introduction 45

Modalities in paediatric chest imaging 45

Basic signs on the plain chest radiograph 48

Common paediatric chest problems 50

Congenital abnormalities of the chest 50

Neonatal chest problems 53

Respiratory tract infections 55

Airway disease 56

ADULTS

CHEST RADIOGRAPHY AND OTHER TECHNIQUES

Different types of chest radiograph

Chest radiography has been used as the main radiological investigation of the chest since the discovery of X-rays by Röentgen in 1895 and chest radiographs constitute 25–40% of all radiological investigations. Chest radiographs are indicated in almost any condition in which a pulmonary abnormality is suspected.

The majority of chest radiographs are obtained in the main radiology department. The radiograph is obtained with the patient standing erect. Patients who are immobile or too ill to come to the main department have a chest radiograph performed using a mobile machine (portable film); the resulting radiograph differs from a departmental film in terms of projection, positioning, exposure and film used and is therefore not strictly comparable with a conventional posteroanterior (PA) film. Other types of chest radiograph are the lateral, lordotic, apical and decubitus views; these are generally taken in the main department.

Departmental films are referred to as ‘posteroanterior’ (or PA) chest radiographs and describe the direction in which the X-ray beam traverses the patient. The patient is positioned with his anterior chest wall against the film cassette and his back to the X-ray tube. The arms are abducted to rotate the scapulae away from the posterior chest and the radiograph is taken during full inspiration. The tube is centred at the spinous process of the fourth thoracic vertebra. For portable films taken in an anteroposterior (AP) projection, the patient’s back is against the film cassette and the X-ray tube is positioned at a variable distance from the patient. As the heart is placed anteriorly within the chest, it is further from the cassette and is therefore magnified in an AP radiograph. The degree of magnification depends on the distance between the patient and the X-ray tube.

For a lateral radiograph the patient is turned 90° and the side of interest placed against the film cassette. The arms are extended forwards and the radiograph is again taken in full inspiration.

Lateral decubitus views are sometimes useful for the demonstration of small pleural effusions. For this projection the patient lies horizontally with the side in question placed downwards. The film cassette is positioned at the back of the patient and the X-ray beam is horizontal centred at the midsternum. This provides a sensitive means of detecting small quantities of pleural fluid (50–100 ml) that cannot be identified on a frontal chest radiograph. However, ultrasonography is usually used as a reliable means of confirming the presence of small pleural effusions.

Lordotic films are sometimes used to confirm middle lobe collapse and for demonstrating a questionable apical opacity otherwise obscured by the clavicle and ribs. For this AP projection, the patient arches back so that the shoulders are touching the cassette with the centring point remaining the same. Linear tomography is another technique designed to reveal lesions otherwise hidden by the skeleton by blurring out everything overlying and underlying the lesion in question. This is achieved by having the X-ray tube and film cassette move at the same time but in opposite directions. These two techniques are less frequently used with the advent of computed tomography (CT).

Factors influencing the quality of a chest radiograph

The quality and thus diagnostic usefulness of a chest radiograph depend critically on the conditions under which it is obtained. Of particular importance are the radiographic exposure, the projection, the orientation of the patient relative to the film cassette, the X-ray tube to film distance, the depth of inspiration of the patient and the type of film–; combination used.

The ideal chest radiograph provides an image of structures within the chest while exposing the patient to the lowest possible dose of radiation. Most radio- logy departments have a policy of obtaining either high-kilovoltage (kVp) or low-kilovoltage chest radio- graphs. Radiographs performed at high kilovoltage (e.g. 140 kVp) have much to recommend them. Even at total lung capacity with the patient erect, nearly a third of the lungs is partially obscured by the mediastinal structures, diaphragm and ribs. With the low-kilovoltage technique (80 kVp or less) these areas are often poorly visualized. This problem is partially overcome by using films exposed at high kilovoltage. The normal vessel markings and subtle differences in soft tissue densities are better demonstrated and a further advantage is the better penetration of the mediastinum, which improves visualization of the trachea and main bronchi. The disadvantage of high-kilovoltage radiographs is the relatively poor demonstration of calcified structures so that rib fractures and calcified pulmonary nodules or pleural plaques are less conspicuous.

During exposure the X-ray beam is modified according to the structures through which it passes. The photons that have passed through the patient carry information which then must be converted into a visual form. Some of the photons emerging from the patient are aligned in a virtually parallel direction and other photons are scattered. These scattered photons degrade the final image but can be absorbed by using lead strips embedded in an aluminium sheet positioned in front of the cassette. This device is known as a grid. Photons that are travelling in parallel pass through the grid to form the image on the film.

The sensitivity of film to direct X-ray exposure is very low and if film were used alone as the image receptor, this would result in a prohibitively large X-ray dose to the patient. Intensifying screens made of phosphorescent material are positioned on the inside of the cassettes and they convert the incident X-ray photons into visible light, which is recorded by the adjacent film. These phosphor screens are composed of either calcium tungstate or a rare-earth-containing compound. Rare- earth phosphors emit more light in response to X-ray photons and therefore less radiation is necessary to produce the image. Similarly, improvements in the quality of X-ray film have also occurred over the years. Standard film emulsions tend to lack detail in the relatively under- or overexposed areas of the radiograph and newer emulsions have been developed so that detail is similar in all areas of the chest radiograph. The choice of film–; combination has a crucial influence on the quality and ‘look’ of the radiograph produced. Further variations may result from film-processing problems.

In the intensive care setting, portable chest radiographs are often taken in less than ideal conditions. Multiple tubes, lines and dressings in conjunction with an immobile, supine patient and the use of a mobile low- kilovoltage machine often result in suboptimal radiographs. One approach to this is the development of phosphor plate technology, which is ultimately expected to replace conventional film–; radiography. The phosphor plate is placed inside a conventional cassette and stores some of the energy of the incident X-ray photons as a latent image (the image produced on a film or phosphor plate before development). The plate is scanned with a laser beam and the light emitted from the ‘excited’ latent image is detected by a photomultiplier. Thereafter this signal is processed in digital form. This digital image may be viewed either on a television monitor or on film (on which it has been laser printed). The great advantage of this system is that it can retrieve an image of diagnostic quality from a suboptimal exposure. Similar gross over- or underexposure would result in a non-diagnostic conventional radiograph. Manipulation of the digital image, particularly ‘edge enhancement’, aids the detection of linear structures such as the edge of a pneumothorax or central venous catheters (Fig. 2.1). With the advent of picture archiving and communication systems (PACS), which enable storage and transfer of digital images, many radiology departments are now ‘filmless’, with images available to view simultaneously on monitors throughout the hospital.

Figure 2.1 The same anteroposterior (AP) digital chest radiograph of a patient on the intensive care unit imaged on different settings. This technique has the advantage that the image can be made (A) darker or (B) lighter after it has been taken to allow better visualization of lines and tubes or lung parenchyma. Note the patient’s endotracheal tube, right internal jugular central line, pulmonary artery catheter and intra-aortic balloon pump.

Other techniques

Because of the cross-sectional nature of CT, it can accurately localize lesions seen on only one view on plain chest radiographs. The superior contrast resolution of CT allows superb demonstration of mediastinal anatomy (e.g. lymph nodes and vessels) (Fig. 2.3) as well as calcification within a pulmonary nodule. Highly detailed thin sections of the lung parenchyma can also be obtained, allowing the complex morphology of many interstitial lung diseases to be more accurately defined. The disadvantages of CT are its relatively high cost and increased radiation exposure to the patient compared with chest radiography.

Figure 2.3 Computed tomography (CT) image postintravenous contrast showing a lung tumour adjacent to the arch of the aorta (closed arrows). Lymph nodes are seen anterior to the trachea (open arrows), which were not visible on chest radiography.

Whereas conventional CT scanning involves alternating table movement through the gantry with exposure, helical or volumetric CT involves simultaneous table movement and X-ray exposure. The technique allows faster scan times and advantages are the elimination of respiratory artefacts, minimization of motion artefacts and production of overlapping images without additional radiation exposure. Helical (spiral) CT is so named because the X-ray can be thought of as tracing a helix or spiral curve on the patient’s surface. Multiple rows of detectors are used in the newer helical CTs, so-called multidetector CT (MDCT). The technique allows viewing of images in multiple planes (Fig. 2.4) and, due to the very fast acquisition times, is increasingly used to evaluate cardiac structures such as the coronary arteries. Helical CT is also now commonly used to demonstrate pulmonary emboli, as accurate timing of a bolus of intravenous contrast allows optimal enhancement of the pulmonary arteries (Fig. 2.5).

Figure 2.4 Example of coronal reconstruction of multidetector helical CT (MDCT) in a patient with primary pulmonary hypertension. Note the enlarged central pulmonary arteries (arrows).

Figure 2.5 A CT pulmonary angiogram, showing bilateral filling defects of the pulmonary arteries (arrows) consistent with bilateral pulmonary emboli.

Common indications for CT of the chest

1. CT scanning is used to further evaluate hilar or mediastinal masses seen or suspected on a chest radiograph.

2. Within the lungs it can be used to further define the nature of a mass or cavitating lesion not clearly seen on the plain film.

3. In patients with normal chest radiographs but abnormal pulmonary function tests, thin high-resolution computed tomography (HRCT) sections of the lung may provide the first radiological evidence of parenchymal disease. This type of scanning is also very useful for assessing patients with suspected bronchiectasis.

4. CT is useful in patients with neoplasms, in assessing both their operability and their response to treatment.

5. Detection of pulmonary embolism (spiral CT).

6. For guiding the percutaneous needle biopsy of lung lesions, mediastinal masses or chest wall abnormalities.

Magnetic resonance imaging

The physical principles of magnetic resonance imaging (MRI) are more complex and very different from those of CT scanning. The equipment consists of a sliding table on which the patient lies within the bore of a large magnet. A combination of the intense magnetic field and a series of radiofrequency waves produces an alteration in the alignment of protons (mostly in water) resulting in the emission of different signals which are detected and subsequently analysed for their intensity and position by a computer. The major advantages of MRI are that images may be obtained in any plane without the use of ionizing radiation. The disadvantages are its inability to produce detailed images of the lung, cost and reduced acceptability to patients because of the claustrophobic bore of the magnet. There are also important contraindications such as permanent cardiac pacemaker devices. Its application to chest imaging is limited at present but the technique is good for imaging chest wall lesions, the great vessels (Fig. 2.6) and the heart.

Figure 2.6 Magnetic resonance image (MRI) of the thorax. The image shows a MR angiogram (MRA) of a coarctation of the aorta (arrow). Note the extensive arterial collaterals

Radionuclide imaging

Ventilation–perfusion () scanning is the commonest radionuclide study of the lungs. It is primarily used to investigate suspected pulmonary embolus. Perfusion is assessed by intravenous injection of minute particles labelled with technetium-99m, a radioactive tracer. These particles become temporarily lodged in a very small proportion of capillaries within the lung and the emitted radiation is detected by a so-called gamma camera. Ventilation is assessed by the inhalation of inert gases that have also been labelled with a radioactive tracer. The ventilation and perfusion images are then compared to see if there are any areas of mismatch (Fig. 2.7). Increasingly CT is used to investigate patients with possible pulmonary embolus, as scanning is more difficult to interpret in patients with coexisting lung disease such as asthma. In addition, the tracers have a short period of radioactivity after they are produced of a few hours and therefore scanning late at night and at weekends may be difficult.

Figure 2.7 Example of a radionuclide lung ventilation and perfusion scan. The examination shows a high probability for multiple bilateral pulmonary emboli as there are multiple perfusion abnormalities (top row of images) with no matched ventilation defects (bottom row of images), so-called ‘mismatched’ defects.

Positron emission tomography

Positron emission tomography (PET) is also a form of radionuclide imaging. A detector can pinpoint where there is uptake of tracer accurately within the body. PET highlights areas that are very metabolically active, such as cancers, but also infection and inflammation. Access to this type of costly scanner is becoming more widespread, and it is mainly used in the chest for assessment of disease spread of lung cancer to lymph nodes and sites outside the chest (Fig. 2.8). The images may be fused with CT images (PET/CT) to give a very precise location of tumour spread.

Figure 2.8 A PET scan showing metastatic spread of a lung cancer in the left lung (arrowhead) to hilar and mediastinal lymph nodes and the left adrenal (arrows). Note the normal increased metabolic activity in the brain, heart and gut.

Interventional procedures

Percutaneous needle biopsy

Percutaneous needle biopsy of a pulmonary or mediastinal mass to provide a histological specimen is usually performed in patients in whom a bronchoscopic biopsy has failed or a thoracotomy is inappropriate. Different types of needle are used and the complication rate (pneumothorax and haemoptysis) bears some relation to the site of the lesion, the size of the needle and the number of attempts to obtain tissue. Contraindications to the procedure include any patient with poor respiratory reserve unable to withstand a pneumothorax, pulmonary arterial hypertension and a previous contra- lateral pneumonectomy.

Pulmonary and bronchial arteriography, superior vena cavography

Pulmonary arteriography.

This is usually undertaken in the investigation of suspected pulmonary arteriovenous malformations and, less commonly since the development of helical (spiral) CT, pulmonary embolism. It requires puncture of either the femoral vein in the groin or the antecubital vein in the elbow and the guiding of a catheter through the right side of the heart under fluoroscopy. The tip of the catheter is positioned in the main pulmonary artery or selectively placed in a smaller pulmonary artery. Contrast is then injected. It is currently the most appropriate technique for the demonstration of pulmonary arteriovenous malformations. These can be treated at the time of the arteriogram by the injection of occlusive materials (embolization).

Bronchial arteriography.

Demonstration of the bronchial arteries requires catheterization of the femoral artery and passage of a catheter into the midthoracic aorta from where the bronchial arteries are selectively catheterized. The major indication for this procedure is recurrent or life-threatening haemoptysis in patients with a chronic inflammatory disease, usually bronchiectasis. Accurate placement of the catheter not only allows demonstration of the bleeding vessel but also allows embolization to be performed simultaneously.

Superior vena cavography.

This is performed for the evaluation of superior vena caval (SVC) obstruction and the investigation of anatomical variants. More recently, patients with SVC compression due to tumour have been palliated by the insertion of an expandable metallic mesh wire stent at the site of the SVC narrowing, thus restoring flow and relieving symptoms.

THE NORMAL CHEST

Anatomy

On the normal posteroanterior radiograph (Fig. 2.9) the following structures can be identified:

Figure 2.9 (A) Normal posteroanterior (PA) chest radiograph. (B) Normal structures visible on a PA chest radiograph: 1 right atrium; 2 left ventricle; 3 right ventricle; 4 right pulmonary artery; 5 left pulmonary artery; 6 air within trachea; 7 clavicle; 8 first rib; 9 lateral border of hemithorax; 10 breast shadow; 11 right hemidiaphragm; 12 costophrenic angle; 13 gastric air bubble.

outline of the mediastinum and heart

the hila

pulmonary vessels and main bronchi

diaphragm

soft tissues and bones of the thoracic cage.

The heart and mediastinum

The mediastinum consists of the organs and soft tissues in the central part of the chest. These comprise the trachea, aortic arch and great vessels, superior vena cava and oesophagus. In children the thymus gland is a prominent component. On the two-dimensional chest radiograph these structures are superimposed and cannot be clearly distinguished from each other. The mediastinum is conventionally divided into superior, anterior, middle and posterior compartments. While the boundaries of the latter three are arbitrary, it is usual to divide the mediastinum into equal thirds. The superior mediastinum is that portion lying above the aortic arch and below the root of the neck.

The mediastinal border on the right is formed superiorly by the right brachiocephalic vein and superior vena cava. The mediastinal shadow to the left of the trachea above the aortic arch comprises the left carotid and left subclavian arteries together with the left brachiocephalic and jugular veins. On a correctly exposed chest radiograph, air in the trachea can be seen throughout its length as it descends downwards, deviating slightly to the right above the carina (where the trachea divides into the right and left main bronchi) due to displacement by the aortic arch.

The heart lies eccentrically in the chest, with one-third of the cardiac shadow to the right of the spine and two-thirds to the left. The density of the cardiac shadow on the left and right of the spine should be identical. The right cardiac border on a chest radiograph is formed by the right atrium. The left cardiac border is composed of the apex of the left ventricle and superiorly the left atrial appendage. The outline of the right ventricle, which is superimposed on the left ventricle, cannot be identified on a frontal radiograph. The maximum transverse diameter of the heart should be less than half the maximum transverse diameter of the thorax, as measured from the inside border of the ribs (the so-called ‘cardiothoracic ratio’).

The hila

Hilar shadows are a complex summation of the pulmonary arteries and veins with minor contributions from other components (the main bronchi and lymph nodes). In general, the hila are of equal density and are approximately the same size. Adjacent to the left hilum, the main pulmonary artery forms a localized bulge just above the left atrial appendage and just below the aortic arch. The area between the aortic arch and the main pulmonary artery is known as the ‘aortopulmonary window’.

The superior pulmonary veins run vertically and converge on the upper and mid-hilum on both sides. It is not possible to distinguish arteries from veins in the outer two-thirds of the lungs. The inferior pulmonary veins run obliquely in a near-horizontal plane below the lower lobe arteries to enter the left atrium beneath the carina. The hilar point is where the superior pulmonary vein on each side crosses the basal artery. This is more easily assessed on the right than on the left. Using this as an index point, the left hilum is normally 0.5–1.5 cm higher than the right one.

Abnormalities of the hilar shadows in the form of increased density or abnormal configuration are usually the result of lymph node or pulmonary artery enlargement. The detection of subtle hilar abnormalities is difficult and requires experience and knowledge of the many outlines that the hila may assume in normal individuals.

Fissures, vessels and segmental bronchi within the lungs

Each lung is divided into lobes surrounded by visceral pleura. There are two lobes on the left (the upper and lower, separated by the major (oblique) fissure) and three on the right (the upper, middle and lower lobes which are separated by the major (oblique) and minor (horizontal or transverse) fissures). In the majority of normal subjects some or all of the minor fissure is seen on a frontal radiograph. The major fissures are only identifiable on the lateral projection. Each lobe of the lung contains a number of segments, which have their own segmental bronchi. The walls of the segmental bronchi are invisible on the chest radiograph, except when seen end-on as ring shadows measuring up to 7 mm in diameter.

The pulmonary blood vessels are responsible for the branching and linear structures within the lungs. The diameter of the blood vessels beyond the hilum varies with the position of the patient and with haemodynamic factors. In the erect position there is a gradual increase in the diameter of the vessels, travelling from apex to base. This increase in size is seen in both the arteries and veins and is abolished if the patient lies supine.

The diaphragm

The interface between the lung and diaphragm should be sharp and, in general, the diaphragm is dome shaped with its highest point medial to the midclavicular line. The margin of the right hemidiaphragm at its highest point lies between the anterior ends of the fifth and seventh ribs. The right hemidiaphragm is usually higher than the left by up to 2 cm in the erect position. Laterally, the diaphragm dips downwards, forming a sharp angle with the chest wall known as the ‘costophrenic angle’. Filling in or blunting of these angles reflects pleural disease, either fluid or thickening.

Thoracic cage

On a high-kilovoltage chest radiograph it should be possible to identify the edges of the vertebral bodies of the dorsal spine through the heart shadow. However, a high-kilovoltage radiograph may ‘burn out’ the ribs, particularly the posterior portions. Because of this the chest radiograph may be an insensitive means of demonstrating rib abnormalities, particularly fractures.

Common anatomical variants

The trachea lies centrally, but in the elderly may deviate markedly to the right in its lower portion due to unfolding and dilatation of the aortic arch. A small ovoid soft tissue shadow just above the origin of the right main bronchus represents the azygos vein. This may be enlarged as a result of posture (supine position) or haemodynamic factors. It may be indistinguishable from an azygos lymph node.

Occasionally, extra fissures are seen in the lungs. The most common of these is the azygos lobe fissure; this is seen as a fine white line running obliquely from the apex of the right lung to the azygos vein. Other accessory fissures are the superior and inferior accessory fissures, both of which are in the right lower lobe.

The surfaces of the two lungs abut each other anteriorly and posteriorly and give rise to two white lines projected over the vertebral column, known as the ‘anterior and posterior junction lines’, respectively. Both of these may be seen overlying the trachea – the anterior line extending from the clavicles to the left main bronchus and the posterior line lying more medially and extending above the clavicles. The azygo-oesophageal recess line is a curved line projected over the vertebral column and extending from the azygos vein to the diaphragm. It represents the interface between the right lung and right oesophageal wall.

A small ‘nipple’ may occasionally be seen projecting laterally from the aortic knuckle due to the left superior intercostal vein. The term ‘paraspinal line’ refers to the line that parallels the left and right margin of the thoracic spine. The left is thicker than the right because of the adjacent aorta.

It is conventional to read the lateral film (Fig. 2.10) with the heart to the viewer’s left and the dorsal spine to the right, irrespective of whether the film is labelled ‘right’ or ‘left’. The chamber of the heart that touches the sternum is the right ventricle. Behind and above the heart lies lung, the density of which should be the same both behind the heart and behind the sternum. As the eye travels down the spine, the vertebral column should appear increasingly transradiant or ‘dark’ (Fig. 2.10A); the loss of this phenomenon suggests the presence of disease in the posterobasal segments of the lower lobes. In the middle of the lateral film lie the hilar structures with the main pulmonary artery anteriorly. The aortic arch should be easily identified but only a variable proportion of the great vessels is visible depending on the degree of aortic unfolding. The brachiocephalic artery is most frequently identified arising anterior to the tracheal air column. The left and right brachiocephalic veins form an extrapleural bulge behind the upper sternum in about a third of individuals.