Imaging the chest

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Chapter 35 Imaging the chest

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.

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.

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.

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:

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).

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.

POSITIONING OF TUBES AND LINES4

INTRA-AORTIC BALLOON PUMP

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