CT of the lungs

A CT of the lung provides results about the smallest changes in the lung and this within a few seconds in which the entire lung can be displayed. Both the blood vessels of the lung and the lung tissue itself can be better assessed by computed tomography than by almost all other common examinations. A frequent reason for CT of the lungs is the examination for chronic respiratory diseases, especially COPD, which also leads to changes in the supporting framework.

Here, the result of the examination can significantly influence the course of therapy. A further field is the examination of changes in the X-ray image which could be a tumour. Computed tomography then makes it possible to differentiate between various causes of changes in the X-ray image, as these all look similar on conventional X-ray images.

Since during a computed tomography many pictures of every small section of the lung can be taken, even changes in the millimeter range can be assessed and if it is a tumor, it can be detected in very early stages. As with computed tomography of the abdomen, CT of the lungs can also be performed with the administration of contrast medium. This is necessary to be able to display the small and smallest structures well.

If an examination with contrast medium is to be carried out, it is important to take blood to check the functionality of the kidney on the basis of some values, since the contrast medium is excreted via the kidneys and the kidney must be in tact, or in patients with limited kidney function, the dose must be adjusted. Patients with thyroid dysfunction should definitely inform us about this, as the contrast medium contains iodine and this can also lead to thyroid dysfunction, especially if its function is already disturbed. As indispensable as computed tomography has become nowadays, its harmfulness due to radiation exposure is controversial, especially in patients who have to undergo such examinations more often.

The word radiation dose is a somewhat vague term in radiology. It is called absorbed dose and describes how much of the X-ray radiation is absorbed by the tissue as energy. It is expressed in Gray (Gy), where 1 Gy = J/kg, i.e. energy absorbed by the tissue per kilogram.

Another important parameter is the equivalent dose. In addition to the amount of energy absorbed, it takes into account the type of radiation. This is important because there are different types of radiation, which differ significantly in their effects (and harmfulness to the human organism).

Therefore, for the equivalent dose, the absorbed dose is multiplied by a radiation weighting or quality factor. It is expressed in sievert (Sv). The effective dose is also derived from this, which also takes into account the fact that different organs react differently to radiation.

For example, the gonads such as the testicles and ovaries and the red (haematopoietic) bone marrow are very sensitive to radiation, whereas the skin and bone surface are less so. This is taken into account by multiplying the equivalent factor by an organ weighting factor; the unit remains the same, namely sievert (Sv). These values can now be used to describe the radiation exposure associated with a radiological examination such as computer tomography.

A distinction is made here as to which part of the body is examined by CT. A computed tomography examination of the abdomen (abdomen CT) means an effective dose of about 7 mSv for the body. That of the chest (thorax CT) is about 10 mSv and that of the skull about 2mSv. For better comparability, these values are compared with those of a normal X-ray examination.

An x-ray of the abdominal cavity (x-ray abdomen) means an effective dose of about 1 mSv, an x-ray of the chest cavity (x-ray thorax) in 2 planes about 0.1 mSv and an x-ray of the head about 0.07 mSv. These values can be put in approximate relation to the natural radiation exposure. Thus, the effective dose of an x-ray thorax examination – in clinical everyday life of the common – corresponds to a natural radiation exposure that one would achieve in about 15 days of normal everyday life.

A thoracic CT means a natural radiation exposure of about 3.5 years. It is therefore clear that computed tomography is associated with a significantly higher radiation exposure than the conventional X-ray examination. From this it becomes clear why magnetic resonance imaging, which like CT enables cross-sectional imaging of body structures, is so important. It works with magnetic fields, so there is no exposure to radiation at all – in contrast to CT.