1.1 Physics of X-rays
X-rays are a type of electromagnetic radiation with wavelengths between 0.01 and 10 nm. On the electromagnetic spectrum, the wavelength of X-rays is shorter than that of ultraviolet radiation and longer than that of gamma radiation. Shorter wavelength X-rays (0.10-0.01 nm) are referred to as ‘hard’ because they can penetrate solid objects. It is these that are used in medical imaging. Since their discovery in 1895 by the German physicist Wilhelm Roentgen, X-rays have been used widely for medical imaging and remain key to diagnosing and treating patients.
X-ray production
X-rays are produced in an X-ray tube (Figure 1.1) by firing electrons at about half the speed of light from the cathode towards a metal target, the anode. The target is usually made from an alloy of tungsten. On impact with the target, the kinetic energy of the electrons is converted into X-rays (1%) and heat (99%). The target deflects and focuses the X-rays to form a beam.
X-ray attenuation
The X-ray beam is directed at the patient in a short pulse and is absorbed (attenuated) by the tissues of the body. Materials with a high electron density, such as bone, attenuate the beam to a greater extent than soft tissue, water or air.
Figure 1.1: An X-ray tube. Electrons are emitted by a cathode into the vacuum, emitting X-rays when hitting the anode at the right (i.e. current) speed.
Therefore the beam that emerges from the patient carries a pattern of intensity that reflects the physical anatomy through which it has passed.
Image production
The beam that emerges is directed onto either a photostimulated phosphor plate or a flat panel detector. These methods of image capture have superseded photographic film and allow rapid production of digital images.
Radiation risks and use
The risk to the patient from a chest X-ray is minimal. However, all health care professionals who request X-rays should be familiar with and keep in mind the potential dangers of radiation.
When X-rays are absorbed by tissue, they cause chemical, molecular and subsequently biological damage (in a timespan of seconds, minutes and decades, respectively). The direct consequences of radiation to the patient are categorised as deterministic or stochastic. Deterministic effects (e.g. skin damage, cataracts and sterility) are relevant to radiotherapy, and to a lesser extent interventional radiology, and occur once a threshold dose of radiation is administered. Stochastic effects (e.g. cancers) are relevant to diagnostic radiology and reflect the probability of harm, which is assumed to be proportional to dose (measured in mSv). Data to support estimates of radiation risk have been derived mostly from follow-up of survivors of the 1945 atomic bombing of Hiroshima and Nagasaki. The following facts help put the risks of chest X-rays in context.
- Background exposure to radiation for the whole population is 2.5 mSv/year.
- A 7-h airline flight exposes passengers to 0.02 mSv.
- The overall lifetime risk of cancer for the general population is about 40%.
- A chest X-ray has a dose of 0.1 mSv and increases a patient's lifetime risk of cancer by about 0.001%.
Use of radiation in medical practice is governed in the UK by the Ionising Radiation (Medical Exposure) Regulations 2000. These lay down the basic measures to protect persons from the dangers of medical radiation exposure. Legal responsibility 3for protecting those exposed to medical radiation lies with the person administering ionising radiation.
1.2 Positioning the patient and obtaining the image
Several techniques are used to visualise the structures of the thorax, but the chest X-ray remains the most common radiological examination. The difference in densities and resulting contrast between structures of the thorax make good visualisation and assessment of the lungs possible. This section deals with various techniques in plain chest radiography regarding patient positioning and obtaining the image.
Frontal chest X-ray
A frontal view of the chest is always obtained when a plain chest X-ray is requested. It is most often a posteroanterior (PA) view. Alternatively, an anteroposterior (AP) view is used if the patient is unable to stand or sit for a PA view. Lateral, apical, lordotic and decubitus views are occasionally obtained as adjuncts to the frontal (PA or AP) X-ray.
Posteroanterior X-rays
The PA view is obtained by positioning the patient facing the film cassette, with the shoulders rotated forwards to project the scapulae away from the lungs. With the chin raised, the chest and shoulders are in contact with the cassette. The view is centred on the midline at the level of the 5th thoracic vertebra and exposure obtained in arrested full inspiration (Figure 1.2). The following allow a PA film to be interpreted confidently.
- Sternal ends of the clavicles equidistant from the vertebral spinous process.
- Clavicles not obscuring the lung apices (Figure 1.2a).
- Lungs well inflated, allowing 10 posterior ribs to be seen above the diaphragm on each side (Figure 1.2a).
- Between 2.5 and 5 cm of lung fields visible above the clavicles
- Lateral borders of the ribs equidistant from the vertebral column.
Figure 1.2: (a) Posteroanterior (PA) chest X-ray in good inspiration with posterior tenth ribs Ⓐ visualised. Medial ends of clavicles Ⓑ equidistant from vertebral spinous process Ⓒ. (b) Positioning of the patient in a PA view, with the patient facing the cassette. The X-ray beam is behind the patient, centred on the 5th thoracic vertebra.
- Superior thoracic vertebrae visible through the heart.
- Well-defined costophrenic angles and margins of the mediastinum, heart and diaphragm.
Posteroanterior X-ray in expiration The PA X-ray in expiration is obtained to better visualise a small apical pneumothorax or to show the effects of an obstructing inhaled foreign body (i.e. air trapping). All other positioning factors remain the same as for an inspiratory PA X-ray.
Anteroposterior X-rays
The AP view (Figure 1.3) is used as an alternative to a PA view in patients who are very ill or unable to comply with the positioning requirements of a PA view. The AP view is occasionally used as an adjunct to further assess an opacity seen on a PA view.
Anteroposterior X-rays appear different from PA X-rays in multiple ways.
- The scapulae may obscure the lungs (Figure 1.3a), because AP X-rays are usually obtained in patients who are unable to project their shoulders downwards and forwards.
- The AP view magnifies the heart, because it is further away from the cassette compared with in a PA view, making assessment of cardiac enlargement difficult (Figure 1.3a).
- The clavicles are projected higher than in a PA view (Figure 1.3a).
Anteroposterior erect X-ray
The AP erect view is obtained with the patient standing or sitting with the back against the cassette and the upper edge of the cassette above the apex of the lungs. The ray is directed horizontally and centred on the suprasternal notch (Figure 1.3b).
Lateral chest X-ray
A lateral view of the chest is no longer routinely obtained. It is instead sometimes used as an additional view for localising a mass lesion (Figure 1.4a) or confirming a hiatus hernia if further assessment by computerised tomography is not possible.
The patient is positioned with the side of interest in contact with the cassette and the sagittal plane parallel to it (Figure 1.4b). The arms are raised above the head. The horizontal ray is centred on the midaxillary line. Infirm and very ill patients are unable to comply with this positioning.
Figure 1.3: (a) Anteroposterior (AP) chest X-ray with the scapulae overlapping more of the lung fields. The heart appears larger on an AP view. Ⓐ Medial margins of scapulae. Ⓑ Clavicles projected above the lung apices. (b) Positioning of the patient for an AP view. The patient faces the X-ray beam with the back against the cassette.
The sternum and vertebral column are better visualised on a lateral view (Figure 1.5). The lateral view can also often better show lesions obscured on the PA view (e.g. lobaratelectasis, posterior recess lesions, fluid in fissures and anterior mediastinal masses).
Figure 1.4: (a) Lateral chest X-ray showing a mass (arrowhead) next to the hilum. The mass is better localised with an additional posteroanterior view (Figure 1.5a). (b) Positioning of the patient for a lateral view. The patient has one side in contact with the cassette, while the X-ray beam is centred on the midaxillary line of the opposite side.
Conversely, lesions seen clearly on a PA view can be obscured by the mediastinum or overlapping lung fields. A lateral view involves a much higher radiation dose than a PA view.
Figure 1.5: (a) Posteroanterior chest X-ray showing a mass in the right upper zone (arrowhead) (same patient as in Figure 1.4). (b) Computerised tomography scan confirming the position of the mass (arrowhead).
Other views of the chest
Additional views can be helpful, including:
- decubitus views, which show air-fluid levels and can show subpulmonic effusions that are not loculated
- supine views, which are obtained for very ill and debilitated patients, for babies and, rarely, for subpulmonic effusions.
Three other views are used less frequently:
- apical and lordotic views to better visualise the apices and middle lobe atelectasis, respectively (now rarely used)
- oblique views, no longer routinely used to show rib fractures.
1.3 Radiographic densities
The principle of imaging using X-rays is to differentiate body parts through differences in their constituents. Body tissues vary in density and therefore the amount of radiation they absorb.
The densest abnormalities visible on an X-ray are metallic and appear white. They include inhaled or swallowed foreign bodies or surgical artefacts (e.g. pacemakers).
Bone and calcific lesions (e.g. calcified nodules from previous tuberculosis) have the highest density of the body tissues of the chest. They will absorb most radiation and therefore appear whiter than surrounding tissue.
The soft tissues (e.g. body wall, heart and abdominal organs) are similar in density so appear mid-grey. Layers of fat in the chest wall may appear slightly darker.
Air has the lowest density and appears black. Lungs contain mostly air and are therefore grey or black.
Nodule assessment depends on the ability to identify the higher density of the nodules compared with surrounding soft tissue. Calcified nodules resulting from previous tuberculosis or chickenpox pneumonia (Figure 1.6) are benign.
The silhouette sign
The edges of structures such as the heart, diaphragm and masses are visible only if a difference in density exists between them and the adjacent tissue. This is the silhouette sign.
Figure 1.6: Chest X-ray showing calcified lung nodules caused by old chickenpox pneumonia. Multiple nodules (arrowheads) have a whiteness or density similar to or greater than that of bone. They are calcific and therefore almost certainly benign.
It is helpful because if an edge of a structure that is usually visible is no longer visible, it means that the adjacent lung is of the same density, i.e. more solid than usual (Figure 1.7).
Most lung diseases increase density, e.g. pneumonia or tumour fills air spaces with material. Lower density (darker) pathological changes such as a pneumothorax or emphysematous bullae are seen when there is increased air in the region.
Description of abnormalities
The cause of abnormalities seen on a chest X-ray is not always immediately apparent. Using descriptions such as size, density and clarity of margins helps to categorise lesions, and making a diagnosis becomes easier when these findings are put together. Thus the interpretation of X-rays is similar to a clinical examination; a diagnosis is more likely when all the signs are considered together rather than when a single aspect is focused on. For example, if an opacity is described as an ill-defined area of shadowing without discrete edges and with an air bronchogram, it is almost certainly infective consolidation. However, if it is a well-defined opacity with clear margins, it is most likely a tumour.
Figure 1.7: Mediastinal mass causing loss of normal silhouette. (a) Chest X-ray. The edge of the mass is clear because of the sharp demarcation with the adjacent lung (arrowheads). Ⓐ The left heart border is obscured by a large mediastinal mass (thymoma) lying next to it; with no difference in density, the structures appear continuous. (b) Computed tomography scan. Ⓐ The left border of the radiographic opacity is created by the lateral aspect of the mass. Ⓑ Normal hilar structures are outlined by the aerated lung and so are visible on the chest X-ray.
1.4 Picture archiving and communication systems: image optimisation and pitfalls
In most countries, a picture archiving and communication system (PACS) is used to deliver imaging to clinicians. With previous film-based systems, images were made singly and the image could not be manipulated once produced.
Picture archiving and communication systems allow easy electronic storage with instant simultaneous access at many distant sites. The PACS may be integrated with other patient data and interface with patient medical records. Reports can be made available electronically without the need to post paper copies. The images can also be manipulated to improve analysis.
Disadvantages of picture archiving and communication systems
It has always been necessary to view X-rays in good viewing conditions because excess background light impairs visualisation, hence the darkening of radiologists' viewing rooms. This is even more important with PACS workstations. An old-fashioned light box emits much more light than a computer screen, so if a monitor is used any outside glare from lights or the sun will decrease visibility. If films are viewed in wards, it is essential to move monitors away from direct sunlight and ward lights, preferably into a side room.
Advantages of PACS systems
Radiologists usually have expensive high-quality screens in specialised viewing rooms. Therefore review images that are insufficiently clear with a radiologist in the radiology department.
Radiology systems have many software manipulations to facilitate image assessment, but many of these will also be available on the ward or clinic-based computers.
- Window level (i.e. contrast and brightness): adjusting the whiteness and contrast makes some areas more visible, but take care to avoid losing detail elsewhere.
- Magnification: magnifying part of an image may help characterise its nature (Figure 1.8).Figure 1.8: Magnification. (a) Chest X-ray showing a large opacity (arrowhead) in the left upper zone. (b) Detail can be improved by magnifying part of the image. III-defined edges Ⓐ suggest consolidation. Branching lucencies in the lesion Ⓑ are now visible, consistent with consolidation (air bronchogram).
- Size measurements: many measuring tools are available on PACS software, most commonly for measuring sizes (e.g. of the heart in Figure 1.9).
1.5 Errors of perception and interpretation
Image analysis and interpretation is a complex multistep process with anatomical, physiological, neuropsychological and psychoemotional components. Errors are therefore common and interpretation of chest X-rays is notoriously difficult, with false negative rates of 20-30% and false positive rates of 2-5%.
Figure 1.9: Measurements. Chest X-ray showing cardiac size Ⓐ (15.5 cm) and thoracic diameter Ⓑ (34 cm). The cardiothoracic ratio is 15.5:34.0, meaning that the heart size is normal. Incidentally, in this case there has been a left mastectomy and breast implant: the breast border is asymmetrical and the soft tissues overlying the left breast area are of increased density.
The mechanics of vision
The eye is better suited to daytime hunting and identifying predators than to detecting small pulmonary nodules. Only the fovea, which measures 1.5 mm across, contains cones, the receptors used to analyse fine detail. To compensate for this small population of cones, the eye executes rapid jerky movements (saccades) to maximise exposure of the fovea to the scene it is surveying. However, during motion the fovea is blind, acquiring data only at rest. Therefore vision is a non-continuous process with noise and image blur during movement interspersed with high-resolution static vision.
Lesion identification
Identification of lesions relies on the abnormality lying in the field of active search, its physical characteristics and recognition that it is abnormal. Therefore the first step towards reducing misses is to follow a systematic pattern of review (see section 2.4). The size, density, contour and location of a lesion all affect the ability to recognise its presence; a small fatty lesion lying behind the heart will probably be undetected.
Recognition of the abnormal requires familiarity with the normal and comes only with practice. Until you have seen thousands of chest X-rays, it is worth asking a senior colleague or a radiologist for help when you feel unsure.
Cognitive errors
Cognitive errors are essentially human errors and come in many guises. Understanding the nature of these errors will help you to reduce them; some examples follow.
- Satisfaction of search: having spotted an error, you stop looking and miss further potentially more significant findings.
- Availability bias: having recently seen a patient with, or more importantly missed the diagnosis of, a particular disease, you assume that the next patient has the same disease. Conversely, if you have not seen a particular disease for a long time you will not think about it for the current patient.16
- Capture: you get interrupted in the middle of systematically reviewing a patient's chest X-ray and do not complete your analysis; you may thereby miss a pneumothorax.
- Gambler's fallacy: having recently seen several patients with the same problem, you assume that the next patient cannot have the same diagnosis. The same assumption is made by gamblers, who after flipping heads on a coin 10 times in a row falsely assume that the likelihood of heads again is lower than before.
- Anchoring: you form an opinion early in the analytic process and then seek evidence to support your diagnosis, prematurely discarding contrary evidence. The first diagnosis gains its own momentum, making consideration of an alternative diagnosis more difficult.
- Alliterative errors: previous reports or opinions from others, which may be incorrect, influence your thinking, making it more difficult for you to think of other possible diagnoses.
- Overconfidence: the tendency to believe that you know more than you do!