Imaging the chest

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Chapter 35 Imaging the chest

Simon P G. Padley

RADIOLOGICAL TECHNIQUES

Of the imaging techniques available for investigating patients in the intensive care unit (ICU), the chest radiograph remains the most important, with ultrasound being utilised in a selected group of patients. High-resolution and spiral computed tomography (CT) allow further investigation of these patients in certain situations.

CONVENTIONAL CHEST RADIOGRAPHY

The views of the chest most frequently performed in the ambulant patient are the erect posteroanterior (PA) and lateral projections, taken with the patient breath-holding at total lung capacity. While portable or mobile chest radiography, as undertaken on an ICU, has the obvious advantage that the examination can be done without moving the patient from the ward, there are many disadvantages. These include:

• shorter focus–film distance causing magnification
• limited X-ray output necessitating long exposure times with resultant movement blurring
• difficulties with satisfactory patient positioning

DIGITAL CHEST RADIOLOGY

In the ICU, digital chest radiographs may be obtained utilising conventional X-ray generators, but the image is captured on a reusable photostimulable plate instead of conventional film. The digital information is then manipulated, displayed and stored in whatever format is desired. Traditional systems suffer greatly from the large day-to-day variations in the density of the radiograph due to small differences in exposure. Digital radiographic systems are able to capture and display a standard density image from a much wider range of exposures (Figure 35.1).

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Figure 35.1 Digital radiograph. The image has been edge-enhanced, which makes the multiple chest drains, Swan–Ganz catheter and endotracheal tube position more conspicuous. Note that two of the chest drain side holes lie within the soft tissues of the chest wall (arrows).

COMPUTED TOMOGRAPHY

CT relies on differing absorption of X-rays by tissues with constituents of differing atomic number, so slight differences in X-ray absorption can be interpreted to produce a cross-sectional image. The components of a CT scanner are an X-ray tube, which rotates around the patient, and an array of X-ray detectors opposite the tube. The speed with which a CT scanner acquires an image depends upon the time it takes to rotate the anode around the patient. Modern CT machines have tube rotation times of as little as 0.33 seconds.

Spiral (also known as volume or helical) scanning entails sustained patient exposure by the rotating X-ray tube during continuous movement of the examination couch through the CT gantry aperture. In this way a continuous data set or ‘spiral’ of information may be acquired in a single breath-hold. The information is reconstructed into axial sections, perpendicular to the long axis of the patient, identical to conventional CT sections. Three-dimensional reconstructions of complex anatomical areas can also be produced.

When short rotation times are coupled with the ability to acquire multiple spirals simultaneously (currently up to 64 slices), the speed of these systems is now so great that breath-holding or suspended ventilation is no longer necessary for high-quality imaging. Furthermore, the detector thickness that determines the minimum slice width of the reconstructed images is now commonly less than 1 mm. Thus a 64-channel scanner can acquire almost 200 submillimetre images per second.

INTRAVENOUS CONTRAST ENHANCEMENT

Because of the high contrast on CT between vessels and surrounding air in the lung, and vessels and surrounding fat within the mediastinum, intravenous contrast enhancement only needs to be given in specific instances, for example to aid the distinction between hilar vessels and a soft-tissue mass. The exact timing of the injection of contrast media depends most on the time the CT scanner takes to scan the thorax. Rapid scanning protocols with automated injectors tend to improve contrast enhancement of vascular structures at the expense of enhancement of solid lesions because of the rapidity of scanning. With spiral CT, it is possible to achieve good opacification of all the thoracic vascular structures with small volumes of contrast media. Optimal contrast enhancement of the pulmonary arteries occurs 10–15 seconds after the start of contrast injection, usually at 3–5 ml/s by a power injector. Timing of image acquisition is vital for accurate diagnosis of pulmonary embolism. However, when examining inflammatory lesions, such as the reaction around an empyema, it may be necessary to delay scanning by 30–40 seconds to allow contrast to diffuse into the extravascular space.

HIGH-RESOLUTION COMPUTED TOMOGRAPHY (HRCT)

Narrow collimation images of the lung correlate closely with the macroscopic appearances of pathological specimens. In diffuse lung disease, HRCT allows a substantial improvement in diagnostic accuracy compared with chest radiography. Narrow section images can be acquired either as part of a volumetric acquisition or as interspaced images, usually 1.5 mm thickness, obtained every 1 cm. The radiation burden to the patient of this interspace or sequential technique is considerably less than with volumetric scanning – typically by a factor of 10 times.

CLINICAL APPLICATIONS OF HRCT IN THE ICU PATIENT

HRCT is increasingly being used to confirm the impression of an abnormality seen on a chest radiograph. HRCT may also be used to achieve a histospecific diagnosis in some patients with obvious but non-specific radiographic abnormalities. Furthermore, HRCT has provided a number of useful insights into chest disease in the severely ill patient in an ICU setting.

HRCT in uncomplicated acute respiratory distress syndrome (ARDS):1

• reveals that the apparently homogeneous opacification apparent on the chest radiograph has an anterior-to-posterior graded increase in density – an appearance referred to as a gravitational gradient (Figure 35.2)
• demonstrates early dilatation of the smaller airways within areas of ground-glass density, an appearance that suggests development of fibrosis, and may prompt anti-inflammatory treatment
• demonstrates that a shift from the supine to the prone position results in redistribution of the previously dependent dense parenchymal opacification to the now dependent anterior lung, a phenomenon that may be accompanied by improvements in patient oxygenation
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Figure 35.2 Computed tomography of a patient with acute respiratory distress syndrome. Note the marked anterior-to-posterior density gradient. The anterior lung is of almost normal density, becoming of ground-glass attenuation in the mid-part of the lung before fading into consolidation posteriorly. There are also moderate-sized bilateral pleural effusions.

HRCT of complications of ARDS2,3

• Infection is common in ARDS. Diagnosis may be difficult. Radiographically there may be a lack of specific findings, often due to superimposition of changes attributable to ARDS. This is a particular problem in the critically ill patient when the usual indicators of pneumonia are also unreliable. Although the diagnosis may still be in question following HRCT, associated pneumonic changes such as abscess formation, empyema and mediastinal disease, as well as development of non-dependent areas of consolidation, are all useful pointers to superadded infection (see Figure 35.2).
• Barotrauma – mediastinal and interstitial emphysema, as well as pneumothorax, are all increasingly common at higher levels of peak end-expiratory pressure (PEEP). The incidence of barotrauma has been reported to be as high as 50%.
• HRCT delineates early change and allows exact location of loculated air collections to be defined.

NORMAL RADIOGRAPHIC ANATOMY

THE MEDIASTINUM, CENTRAL AIRWAYS AND HILAR STRUCTURES

Appreciation of abnormality requires a sound grasp of normal radiological anatomy. The mediastinum is delimited by the lungs on either side, the thoracic inlet above, the diaphragm below and the vertebral column posteriorly. Because the various structures that make up the mediastinum are superimposed on each other on the chest radiograph, they cannot be separately identified. Nevertheless, because a chest radiograph is usually the first imaging investigation, it is necessary to have an appreciation of the normal appearances of the mediastinum, together with variations due to the patient’s body habitus and age. Key points include:

• Only the outline of the mediastinum and the air-containing trachea and bronchi (and sometimes oesophagus) are clearly seen on a normal chest radiograph.
• The right superior mediastinal border is formed by the right brachiocephalic vein and superior vena cava, and becomes less distinct as it reaches the thoracic inlet. The right side of the superior mediastinum can appear to be considerably widened in patients with an abundance of mediastinal fat.
• The left mediastinal border above the aortic arch is the result of summation of the left carotid and left subclavian arteries together with the left brachiocephalic and jugular veins.
• The left cardiac border comprises the left atrial appendage which merges inferiorly with the left ventricle. The silhouette of the heart should always be sharply outlined. Any blurring of the border is due to loss of immediately adjacent aerated lung, usually by collapse or consolidation.
• The density of the heart shadow to the left and right of the vertebral column should be identical and any difference indicates pathology (for example, an area of consolidation or a mass in a lower lobe).
• The trachea and main bronchi should be visible through the upper and middle mediastinum.
• In older individuals, the trachea may be displaced by a dilated aortic arch. In approximately 60% of normal subjects, the right wall of the trachea (the right paratracheal stripe) can be identified as a line of uniform thickness (less than 4 mm in width); when it is visible it excludes the presence of an adjacent space-occupying lesion, most usually lymphadenopathy.
• The carinal angle is usually less than 80 °. Splaying of the carina is an insensitive sign of subcarinal disease, either in the form of massive subcarinal lymphadenopathy, or a markedly enlarged left atrium.
• The origins of the lobar bronchi, where they are projected over the mediastinal shadow, can usually be identified but the segmental bronchi within the lungs are not generally seen on plain radiography.
• Normal hilar shadows on a chest radiograph represent the summation of the pulmonary arteries and veins.
• The hila are approximately the same size and the left hilum normally lies between 0.5 cm and 1.5 cm above the level of the right hilum. The size and shape of the hila show remarkable variation in normal individuals, making subtle abnormalities difficult to identify.

THE PULMONARY FISSURES, VESSELS AND BRONCHI

The two lungs are separated by the four layers of pleura behind and in front of the mediastinum. The resulting posterior and anterior junction lines are often visible on chest radiographs as nearly vertical stripes, the posterior junction line lying higher than the anterior. The junction lines are not invariably seen and their presence or absence is not usually of significance (Figure 35.3).

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Figure 35.3 (a) Close-up of the mediastinum of a patient in whom both the anterior and posterior junction lines are evident. The anterior junction line is more inferior and slants from right to left (short arrows), whereas the posterior junction line is more vertical, superior and is delineated by the arrowheads. (b) Computed tomography scan through the upper mediastinum demonstrating the anterior (arrowheads) and posterior (arrows) junction lines in a different patient.

The upper and lower lobes of the left lung are separated by the major (or oblique) fissure. The upper, middle and lower lobes of the right lung are separated by the major fissure and the minor (horizontal or transverse) fissure. The minor fissure is visible in over half of normal PA chest radiographs. The major fissures are not visible on a frontal radiograph and are inconstantly identifiable on lateral radiographs. In a few individuals, fissures are incompletely developed; a point familiar to thoracic surgeons performing a lobectomy, because of incomplete cleavage between lobes. Accessory fissures are occasionally seen.

All of the branching structures seen within normal lungs on a chest radiograph represent pulmonary arteries or veins. It is often impossible to distinguish arteries from veins in the lung periphery. On a chest radiograph taken in the erect position, there is a gradual increase in the diameter of the vessels, at equidistant points from the hilum, travelling from lung apex to base; this gravity-dependent effect disappears if the patient is supine or in cardiac failure.

THE DIAPHRAGM AND THORACIC CAGE

The interface between aerated lung and the hemidiaphragms is sharp and the highest point of each dome is normally medial to the mid-clavicular line. The right dome of the diaphragm is higher than the left by up to 2 cm in the erect position unless the left dome is elevated by air in the stomach (Figure 35.4).

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Figure 35.4 Erect chest radiograph of a patient with a pneumoperitoneum demonstrating the normal thickness in position of the hemidiaphragms. The right lies slightly higher than the left.

Filling-in or blunting of these costophrenic angles usually represents pleural disease, either pleural thickening or an effusion.

POSITIONING OF TUBES AND LINES4

CENTRAL VENOUS CATHETERS (CVC)

The end of a CVC needs to be intrathoracic, and is ideally in the superior vena cava. CVCs may be introduced via an antecubital, subclavian or jugular vein. Subclavian venous puncture carries a risk of pneumothorax and mediastinal hematoma. Rarely, perforation of the subclavian vein leads to fluid collecting in the mediastinum or pleura. All catheters have a potential risk of coiling, misplacement, knotting and fracture (Figure 35.5). The tip should not abut the vessel wall at an obtuse angle.

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Figure 35.5 Intensive care unit patient with multiple tubes and lines in place. The endotracheal tube tip position is satisfactory, and there are sternotomy wires, an intra-aortic balloon pump (radiopaque tip) and a prosthetic heart valve. The central line inserted into the left internal jugular vein passes into the right internal jugular vein. The Swan–Ganz catheter, which has been inserted via the right internal jugular vein, loops into the left brachiocephalic vein, before taking a satisfactory course through the cardiac chambers (black arrows).

PULMONARY ARTERY FLOTATION CATHETERS

Ideally the end of the catheter should be maintained 5–8 cm (2–3 inches) beyond the bifurcation of the main pulmonary artery in either the right or left pulmonary artery (see Figure 35.5). When the pulmonary artery occlusion pressure is measured the balloon is inflated, and the flow of blood carries the catheter tip peripherally, to an occluded position. After the measurement has been made the balloon is deflated and the catheter returns to a central position; otherwise there is a risk of pulmonary infarction. The inflation balloon is radiolucent. The balloon should normally be kept deflated.

NASOGASTRIC TUBES

These should reach the stomach but may coil in the oesophagus or occasionally are inserted into the tracheobronchial tree (Figure 35.6).

image image image

Figure 35.6 Misplaced nasogastric tube. (a) The tip of the tube is in the bronchus intermedius. (b) The tube is looped in the oesophagus (black arrowheads) before passing into the trachea, as demonstrated on this lateral spine view. Note the anterior wedge fracture of C5. (c) The tube is coiled in the oesophagus without passing into the stomach. Note the bullet projected over the left upper zone and the adequately positioned Swan–Ganz catheter (white arrow).

ENDOTRACHEAL TUBES

Extension and flexion of the neck may make the tip of an endotracheal tube move by as much as 5 cm. With the neck in neutral position the tip of the tube should ideally be about 5–6 cm above the carina. A tube that is inserted too far usually passes into the right bronchus, with the risk of non-ventilation or collapse of the left lung (Figure 35.7).

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Figure 35.7 Endotracheal tube inserted into the right main bronchus as demonstrated on computed tomography. This image, obtained in expiration, demonstrates air trapping in the left lung.

TRACHEOSTOMY TUBES

The tube tip should be situated centrally in the airway at the level of T3. Acute complications of tracheostomy include pneumothorax, pneumomediastinum and subcutaneous emphysema. Long-term complications include tracheal ulceration, stenosis and perforation.

PLEURAL TUBES

These are used to treat pleural effusions and pneumothoraces. A radiopaque line usually runs along pleural tubes, and is interrupted where there are side holes. It is important to check that all the side holes are within the thorax. Tracks may remain on the chest X-ray following removal of chest tubes, causing tubular or ring shadows. When doubt remains about tube position, then CT scanning should be considered (Figure 35.1 and 35.8).

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Figure 35.8 Computed tomography scan demonstrating loculated pneumothoraces in a patient with acute lung injury. The posteromedial pneumothorax on the left is not being drained by the multiple chest tubes.

MEDIASTINAL DRAINS

These are usually present following sternotomy. Apart from their position, they look like pleural tubes.

INTRA-AORTIC BALLOON PUMP

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