This technique is carried out by the rotation of an X-ray tube around the patient in a series of complete circles. The signals are detected by an array of scintillation crystals and are processed quantitatively by a computer to produce a two-dimensional image of high resolution for each axial scan or cut. CT gives a numerical indication of the relative densities of particular tissues, but only fat is different enough to be reliably diagnosed. Normal lung is of sufficiently low density (90% air, 10% soft tissue) to allow the trachea and main bronchus to show up on cuts, particularly when the latter are running at right angles or at the same plane rather than in’ an oblique direction. This is also true for the large pulmonary vessels and for other mediastinal structures to the extent that CT is the radiographic procedure of choice for investigating the mediastinum and hilar regions.
CT is essential in staging of carcinoma of the bronchus and CT scanning should be extended to include the liver, adrenal glands and brain. Although CT will identify enlarged mediastinal lymph nodes, these may not be malignant and require biopsy. Nevertheless, the absence of any lymph node enlargement on CT scanning is a useful indication favouring operability. High-resolution CT in which the slices are between 1 and 2 mm thick compared to the conventional CT slice of 10 mm has improved the value of the technique for the assessment of interstitial lung disease and can produce diagnostic information. It is of particular value in:
1 Detection of pulmonary involvement and its extent in sarcoidosis, lymphoma, cryptogenic and extrinsic alveolitis, occupational lung disease.
2 Bronchiectasis. High-resolution CT has a sensitivity and specificity of greater than 90% compared with bronchograms
3 Distinguishing emphysema from interstitial lung disease or pulmonary vascular disease as a cause of a low gas transfer factor with otherwise normal lung function 4 Diagnosis of lymphangitis carcinomatosa.
Magnetic resonance imaging (MRI)
This technique has proved less useful than CT because of its poorer imaging of the lung parenchyma. It can produce images in sagittal and coronal as well as transverse planes and for this reason is of value in assessing disease near the lung apex, the spine and the thoraco-abdominal regions. Flowing blood does not provide a signal in MRI. Vascular structures appear as hollow tubes making this technique useful in the differentiation of masses around the aorta or in the hilar regions.
Radioisotope lung scanning
This technique has been widely used for the detection of pulmonary emboli.
Macro-aggregated human albumin labelled with technetium- 99m is injected intravenously. The particles are of such a size that they impact in pulmonary capillaries, where they remain for a few hours. A gamma camera is then used to detect the position of the macro-aggregated human albumin. The resultant pattern indicates the distribution of pulmonary blood flow; cold areas occur where there is defective blood flow (e.g. in pulmonary emboli). DISADV ANTAGES. Areas of lung with diminished perfusion caused by pulmonary emboli cannot be distinguished from those in which pulmonary blood flow has been closed down by poor ventilation of the adjacent lung. For example, mild asthma can cause a patchy appearance. (NB Peak flow should always be measured before a scan.)
In addition, in patients with an obvious radiological abnormality, a scan cannot differentiate pulmonary embolism from other causes of defective perfusion. VALUE. Radioisotope scanning is particularly of value in patients whose chest X-ray is normal and in whom pulmonary embolism is suspected. Multiple cold areas on the scan with only a single abnormality on the X-ray support the diagnosis of pulmonary embolism.
Xenon-133 gas is inhaled into the lung and its distribution is detected at the same time as a perfusion scan is carried out. Using the two scans, a pulmonary embolus causes a striking diminution of perfusion relative to ventilation Other lung diseases (e.g. asthma or pneumonia) impair both ventilation and perfusion. Unfortunately, however, a pulmonary embolus often produces substantial changes in the lung substance (e.g. atelectasis) so that such a clear distinction is not always obvious. Nevertheless, this is a better technique than perfusion scan alone.
Respiratory function tests
In practice, airflow limitation can be assessed by use of relatively simple tests that have good intrasubject repeatability. Normal values are required for their interpretation since these tests vary considerably, not only with sex, age and height, but also within individuals of the same age, sex and height. The standard deviation about the mean for a group of individuals is therefore very high; for example, the standard deviation for the peak expiratory flow rate is approximately SO litre min-‘ and for the FEV, it is approximately 0.4 litres. Repeated measurements oflung function are required for assessing the progression of disease in an individual patient.