Introduction to imaging: The chest

In the fourth part of our series, John Frank outlines the techniques used to image the chest

A plain chest x ray will give useful information about the size of the heart and the state of the lungs, but further more detailed investigations are often needed to make a definitive diagnosis. Before requesting complicated procedures, please refer to the Royal College of Radiologists' guidelines, and discuss the case with the imaging department.¹

Examining a chest x ray

The main structures imaged in the chest are the heart and lungs, although useful information may be obtained about the bones and soft tissues as well. A chest x ray is the most commonly requested radiological examination, it is readily available and many patients present with some sort of chest symptoms such as cough or shortness of breath, or are being followed up for a malignancy. A lot of information can be got from chest x rays; they deserve careful study.

Each person has their own method for examining a chest x ray, but one way of doing it is to start centrally and work out. In this way, you will not neglect anything. In the middle is the heart, which usually lies to the left and measures up to half of the total thoracic measurement in adults (more in children). Look carefully at the heart, and if the patient has had previous heart surgery you may see prosthetic valves or coronary stents. Next, look at the hilar region which is the region where the main vessels from the upper lobe and those from the lower lobe meet. The left should be higher than the right by about 2 cm, and they should be of equal size and density. The lungs should be fairly evenly dark, and areas of increased opacity may well reflect consolidation, but do not be fooled by breast shadows. The breasts are quite dense, especially in younger women (fig 1). Next, look at the ribs and soft tissues, making sure that both breasts are present.

Fig 1 A normal chest x ray

Circular dense opacities indicate prostheses (fig 2). Finally, look carefully at the areas above the clavicles and below the diaphragm, as free gas below the diaphragm--either postoperative or indicating bowel perforation--is best seen on an erect chest film. Small pneumothoraces are often best seen at the apex of the lung or at the costophrenic angle, which is the junction between the diaphragm and the lateral chest wall. Do not forget to make sure that the film side marker is correct, as the commonest cause of diagnoses of dextrocardia is incorrect labelling.

Fig 2 Radiograph showing two circular dense breast implants. You may not see scars from surgery, and the patient may be reluctant to talk about them

The main use of plain film chest imaging is to detect relatively straightforward conditions, such as pneumonia, heart failure, pleural effusions, and pneumothorax. However, pleural effusions may be localised and marked for aspiration using ultrasound.

Computed tomography

As well as plain films, you can study the lungs using either conventional computed tomography or high resolution computed tomography with thinner slices, which is used to look for interstitial lung disease (figs 3 and 4). The most common use of computed tomography in the chest is to look for lung tumours (fig 5).

Fig 3 (left) Computed tomograph through a chest, viewed from the feet up, showing bone and soft tissue. You can also see the ascending and descending aorta and pulmonary arteries, which are white because the patient has had intravenous iodine containing contrast.

Fig 4 (centre) High resolution computed tomograph showing the anatomy of the lungs. Because the volume of tissue scanned is much less than on a normal computed tomograph, detail is much greater.

Fig 5 (right) Computed tomograph showing a solitary lesion in the right lung. From this image, it is impossible to say whether this is a cancer or not, so further tests are needed--either a fine needle biopsy if the lesion is accessible and large enough or positron emission tomography

Positron emission tomography

Positron emission tomography uses deoxyglucose labelled with fluorine-18; deoxyglucose is a glucose analogue, which is taken up avidly by malignant cells. Fluorine-18 emits positrons, positively charged electrons, which cannot exist independently and so immediately combine with electrons to produce two 511 keV photons. A special camera detects these (fig 6).

Fig 6 (left) Positron emission tomograph of a patient with a bronchial carcinoma showing increased uptake in the region of the right hilum. It also shows that there is no metabolic activity in the hilar or subcarinal lymph nodes, which would suggest malignancy has spread beyond the primary tumour or distant metastases

Ventilation-perfusion scans

Plain chest x ray images have no role in diagnosing pulmonary embolisms; for this, a nuclear medicine lung scan (a ventilation-perfusion scan) is best (figs 7 and 8). This technique studies pulmonary arterial circulation using macroaggregates labelled with technetium-99,m, which are injected intravenously with the patient supine. The macroaggregates fix in the distal pulmonary capillaries showing where blood is flowing. At the same time, the patient breathes a radioactive gas, which shows ventilation in the lungs. Some countries use krypton-81,m (with a half life of 13 seconds) or xenon-133 (half life 5.25 hours). An alternative agent for ventilation is diethylene triamine penta acetic acid labelled with ^99,mTc, which is vapourised and breathed in as an aerosol. The disadvantage of this is that the same isotope is used to show both perfusion and ventilation, and there may be contamination of the gamma camera and the room. The dose to operators from the aerosol or xenon is higher than the dose from krypton.

Fig 7 (centre) A normal lung scan shows the same appearances in both perfusion and ventilation images

With pulmonary embolism, the embolus obstructs distal blood flow and so there are defects in perfusion. This is clearly seen on a ventilation-perfusion scan as a perfusion mismatch. This is because there is no obstruction to ventilation, which is normal.

When a nuclear medicine scan is unavailable, you can see large central emboli using contrast enhanced helical computed tomography. This is an examination of the pulmonary circulation using computed tomography with intravenous contrast given during the study to opacify the pulmonary arterial tree (fig 9).

Fig 9 Contrast enhanced helical computed tomograph. The filling defects in the contrast can be seen in the right main pulmonary artery and the left lower lobe artery

This technique has advantages over nuclear medicine because it is available all the time and may also give information about other diseases within the chest and the chest wall. The disadvantages are that it can only show central emboli and gives a larger dose of radiation to the patient. However, treatment should not be withheld if a lung scan is not available--rather, begin treatment, and ask for an image within 24 hours of admission.

Bone scans

Trauma to the chest may well result in rib fractures, and you may see these with plain x rays. However, if the trauma is not severe, rib fractures, although obviously important to the patient, may not be visible in radiographs. Rib fractures will be visible on a bone scan (fig 10).

Fig 10 The punctate areas of increased uptake in the ribs are typical of fractures, as they lie linearly. The linear plate-like uptake in the body of L3 is due to osteoporotic collapse

Cardiac isotope scans

Plain chest x rays give information about cardiac enlargement, heart failure, and previous cardiac surgery but not much else. Cardiologists can carry out more detailed studies using ultrasound (echocardio- graphy). You can also use nuclear medicine techniques to label the myocardium with a technetium-containing agent, which shows myocardial perfusion (figs 11 and 12). In a normal scan, no defects should be under stress. The scan also gives an idea of the left ventricular volume. In contrast, an abnormal scan shows defects in perfusion under stress, and if these fill in at rest, it indicates reversible ischaemia. Defects both on stress and at rest indicate an infarcted area of myocardium. The use of colour scales allows a degree of quantification of the amount of perfusion--in fig 11, the more red and white in the image, the greater the blood flow.

Fig 11 Normal isotope scan showing perfusion in three planes across the left ventricle (transverse, vertical long axis, and horizontal long axis). The upper row of the pair shows the myocardium under stress and the lower at rest.

Fig 12 A grossly dilated left ventricle with underperfusion of the anterior wall under stress and filling in at rest indicating reversible ischaemia. However, matched underperfusion of the inferior wall on both stress and rest indicate an old infarct

Multiple gated acquisition scans

The cardiac isotope studies in figs 11 and 12 show myocardial perfusion. But you can also label and image red blood cells to get an idea of how the left ventricle contracts and get an ejection fraction. This type of scan is called a multiple gated acquisition (fig 13). These scans are used to assess cardiac function preoperatively and during cardiotoxic cancer chemotherapy.

Fig 13 Two multiple gated acquisition scans. The first shows the region of interest drawn around the left ventricle at end diastole and end systole; the second superimposes them--from this image the ejection fraction is calculated. As well as activity in the left ventricle, the right ventricle, pulmonary trunk, and aorta are also visible

Angiography

The other major structure within the thorax is the aorta, and this can be imaged either by contrast computed tomography or by direct angiography, in which a catheter is introduced into the arch of the aorta by direct puncture of the femoral artery (fig 14). This is usually done to show the arch of the aorta and the great vessels in cases of neurological problems which may arise from atheroma of the vessel wall. Atheroma may have occurred in patients presenting with a stroke; small pieces of atheroma may have broken free from the wall of the aorta or carotid arteries, and the pieces may have travelled to the brain.

Fig 14 A catheter in the arch of the aorta with contrast filling the great vessels of the neck

1. Making the best use of a department of clinical radiology: guidelines for doctors. 5th ed. London: Royal College of Radiologists, 2003.