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Chest x-Ray Interpretation

Essentials

  • When examining the picture, the background lighting should be dim.
  • Always compare present abnormalities with findings in earlier x-rays if they are available.
    • Was the abnormality present already in the earlier x-ray?
    • Has it remained the same, become larger or declined in size?
  • Interpretation of the chest x-ray is not simple. Ask for interpretation by a radiologist if necessary.
  • A chest x-ray that appears normal at first sight may hide significant pathology, e.g. behind the heart, mediastinum or diaphragm. Erroneus interpretation of an x-ray image is often associated with problems of perception. Many normal structures (e.g. cross-sections of blood vessels) and/or their superimposed projections may also cause false positive interpretations.
  • The radiation dose from a chest x-ray is rather low (postero-anterior x-ray about 0.03 mSv, corresponding to 3 days' natural background radiation, and lateral x-ray about twice as much).

Technical quality of the x-ray image

  • The lung parenchyma is difficult to see in an overexposed (too dark) x-ray.
  • Abnormalities behind the heart are not visible in an underexposed (too light) x-ray.
  • The posteroanterior radiograph must be straight (vertebral spinous processes are located in the middle of the medial ends of the collarbones) because, for example, the hiluses cannot be assessed properly if the x-ray has been taken in an oblique position.
  • The costophrenic sulcuses should be visible.
  • A supine x-ray looks different from an erect x-ray. The heart looks larger, the pulmonary vascular pattern more prominent and the mediastinum wider. E.g. pleural fluid or pneumothorax may be difficult or impossible to discern.

Examples of eventual findings

Inflammatory changes

  • In acute bronchitis the chest x-ray is normal.
  • In pneumonia the chest x-ray shows one or more ill-demarcated opacities (picture 1), or sometimes an increased streaky pattern e.g. in viral pneumonia.
  • Tuberculotic changes are usually seen in the apex of the lungs (picture 2). Caverns may also be visible (picture 3). A cavern, possibly with associated fluid level, is highly infectious.
  • Pneumonia may be caused by a specific abnormality (lung tumour, bronchiectasis, aspiration, foreign body or immunological disorder).

Atelectasis

  • A local infiltrate confined to one lobe that is caused by the occlusion of a branch of the bronchus because of a tumour, foreign body or viscous mucus. The aetiology should always be determined.
    • Platelike atelectases are a much more common finding than ordinary atelectases. Their clinical significance is unclear (cancer may sometimes be found even behind these changes).

Lung cancer

  • The radiographic findings are varying. The most common abnormalities are opacity (picture 4; from 1-2 cm to 10 cm in diameter and either well or poorly demarcated) atelectasis, unilateral hilar enlargement or widening of the mediastinum, pleural fluid, bone destruction (usually looked for in the ribs).
  • If the tumour is small or situated intrabronchially or behind the mediastinum or the diaphragm the x-ray may be normal.
  • A patient with pneumonia may have lung cancer as an underlying disease. The healing of pneumonia should be confirmed by a follow-up x-ray after about 6 weeks, except in non-smoking persons under 50 years of age.

Pulmonary metastases

  • One or more round opacities of varying size. As a rule of thumb, the probability of malignancy in a solitary rounded shadow detected by chest x-ray is about 50%. Sometimes plenty of small densities or linear enhancement are visible all over the lungs. However, all of these changes are highly non-specific.

Sarcoidosis

  • The hilar lymph nodes are symmetrically enlarged (picture 5) especially in the initial stage. The lung parenchyma may show increased streaky or spotted pattern accentuated in the upper and/or middle lung fields (picture 6).

Hodgkin's disease and other lymphomas

  • Mediastinal widening, hilar enlargement

Heart failure

  • Heart size is increased. The heart width in the PA view is more than half of the inner width of the thorax (cardiothoracic index).
    • The variation of the heart width in the PA view does not usually exceed 1.5 cm in an individual. The old rule of thumb tells that an increase in the heart width of more than 1.5-2 cm compared to previous situation is abnormal and may suggest heart failure or pericardial fluid.
    • If the heart volume measured from PA and lateral films and proportioned to the body surface area exceeds 500 ml/m2 in men and 450 ml/m2 in women, the finding is interpreted to be pathological. These limits do not, however, necessarily apply e.g. in athletes.
      • To determine the heart volume, multiply the three axes of the heart (length, width, thickness; do not include pericardial fat!) by each other and then multiply the result further by 0.42 if the imaging distance is 1.5 m and by 0.44 if the distance is 2 m. The body surface area can be roughly estimated by the following formula: patient height in metres minus 0.60 (1.xx-0.60) + weight in kilograms divided by 100 (0.xx). E.g. for a person with a height of 1.78 m and a weight of 75 kg the result is 1.18 + 0.75 = 1.93 m2 .
      • Body surface area can also be determined using the Mosteller formula: see BSA calculator Body Surface Area (Bsa) Calculator.
  • In mild left ventricular failure the upper zone veins are enlarged. Assessment of mild failure is difficult.
  • In interstitial oedema (picture 7) the vascular pattern becomes dizzy, the lobar interspaces become clearly visible, horizontal 1-2 cm lines (Kerley B lines), and pleural effusion (usually first on the right side) are observed.
  • Alveolar oedema is seen as poorly demarcated patchy infiltrates.
  • In patients with emphysema the findings may be atypical and resemble pneumonia.

Pleural effusion

  • The costophrenic recesses are usually, but not always, rounded. The posterior costophrenic recess is the first to be rounded if the amount of fluid is small. If there is a clinical suspicion of pleural effusion, the fluid can best be diagnosed by ultrasonography. If ultrasonography is not available, freely moving fluid can be demonstrated by an x-ray taken in the lateral recumbent position (the patient lying on the side of suspected fluid) with horizontal beams: the fluid is seen as a layer between the lung and the chest wall (the translateral view). Adhesions in the recess need to be considered in the differential diagnosis.

Benign pleural lesions

  • May be associated with the inner pleural leaf (fibrosis of the visceral pleura) or with the outer pleural leaf (plaques of the parietal pleura).
    • The former are often associated with adhesive, rounded pleural recesses and lengthy adhesion streaks. The latter are more local, often calcified prominences associated with the pleura.
    • Both are found in several per cent of individuals in the general population. Prevalence is higher in men than in women and increases with age.
  • Particularly the plaques of the parietal pleura are often a result of occupational exposure to asbestos Asbestos-Related Diseases and, if bilateral, should in such a case be registered as an occupational disease.
  • In differential diagnostics, insertion points of costal muscles, extrapleural fat and malignant lesions of the pleura (metastates; mesothelioma, see picture 8) should be taken into account.

Spontaneous pneumothorax

  • Air (a dark area without lung structures) is visible between the lung and the chest wall. The lung may be totally collapsed. In an unclear situation an expiratory chest x-ray may provide additional information.
  • Sometimes the pressure in the pleural cavity exceeds atmospheric pressure (tension pneumothorax). The mediastinum shifts to the contralateral size. Tension pneumothorax must be treated immediately by puncture or pleural suction.

Other pulmonary opacities

  • Pulmonary opacities are seen in many diseases (eosinophilic pneumonia, allergic alveolitis such as farmer's lung, and fibrotizing alveolitis).
  • A solitary rounded opacity should be considered malignant until proven otherwise. If a rounded opacity remains unchanged for two years, it can in practice be considered benign.

Pulmonary embolism

  • Even large pulmonary emboli may not cause abnormalities in the chest x-ray, and the eventual findings are often atypical. Clinical presentation is essential in primary diagnosis.
  • The primary further radiological investigation is contrast-enhanced CT scan, in special cases (contrast media contraindicated), radioisotope scanning of the lungs or (magnetic resonance) pulmonary angiography.

Arterial calcifications

  • Arterial calcifications possibly detected in chest x-ray films should be noticed. E.g. aortic calcification clearly increases the risk of cardiovascular death independent of other risk factors.

References

  • Zitting AJ. Prevalence of radiographic small lung opacities and pleural abnormalities in a representative adult population sample. Chest 1995 Jan;107(1):126-31. [PubMed]
  • Witteman JC, Kok FJ, van Saase JL, Valkenburg HA. Aortic calcification as a predictor of cardiovascular mortality. Lancet 1986 Nov 15;2(8516):1120-2. [PubMed]