CS 
[123I]sodium iodide, [123I]NaI
Author of the review: Ing. Helena Švecová, Ph.D.
Radioactive gases have been used successfully for lung ventilation examinations since the 1950s. First, in 1953, Knipping et al. used a gas mixture with 131I, and two years later the same group introduced 133XeXe (half-life 5.27 h) into lung diagnostics. The greatest interest in 133XeXe arose in the 1960s, when a number of studies of lung diseases were carried out with this gas. As a result, 133XeXe became a routinely used tool for lung examinations.
In 1968, Yano and Anger outlined the possibility of using 81mKr for lung examinations. 81mKr has a half-life of only 13.1 seconds, but it can be obtained continuously from its parent nuclide 81Rb (4.576 h) in a radionuclide generator. The short half-life of 81mKr is, surprisingly, one of its advantages, because it significantly reduces the radiation burden on both the patient during the examination and the staff, and practically no radioactive waste is produced. Another advantage of 81mKr is the energy of the emitted γ radiation, 190 keV, which, in terms of resolution, is more favourable for SPECT cameras than the energy of the main γ line emitted by 133Xe (81 keV).
For many reasons, 81mKr is a very suitable radionuclide for lung ventilation examinations in cases of suspected pulmonary embolism, obstructive lung disease, pulmonary emphysema and other lung diseases.
Properties
The generator contains 81Rb (half-life 4.576 h) in equilibrium with the daughter radionuclide 81mKr (half-life 13.1 s), which emits γ radiation of energy 190.4 keV. In a mixture with air it is used for lung ventilation examinations.
Pharmacokinetics
Krypton is a noble gas that is completely chemically inert and, in the lungs, practically does not pass into the blood. When 81mKr is used for a lung examination, the mixture of krypton and air is inhaled and exhaled again in unchanged form. Given the physical half-life of 81mKr (13.1 s), it is not possible to determine the biological half-life or to study other pharmacokinetic data.
The pharmacodynamic effects of the medicinal product are also not manifested, because the amount of the agent taken in by inhalation is low and, given the half-life of the 81mKr nuclide (13.1 s) and the administered activity, the absorbed quantity of gas is negligible, as are any possible manifestations of interaction of the gas with the body.
Toxicity
The amount of radiopharmaceutical taken in by inhalation is far below the amount of krypton that we take in naturally by breathing air (on the order of 10,000× more). Considering the chemical toxicity of 81mKr is therefore irrelevant. The radiotoxicity of 81mKr is significantly lower than that of other radiopharmaceuticals, given its very short half-life and the absence of particulate radiation. The effective dose in an adult (70 kg) is 2.7×10-5 mSv/MBq, and in a routine examination it is in the range of tenths of a mSv. The equivalent dose in the lungs is between 60 and 140 μGy. The radiation burden of an examination using 81mKr is significantly lower than that of other commonly used nuclear medicine examinations.
Use
Lung ventilation examination
Pneumology
A lung ventilation examination using 81mKr is usually performed together with a lung perfusion examination using 99mTc-macroaggregates (99mTc-MAA). The γ energy of 99mTc is 140 keV, whereas Eγ of 81mKr is 190 keV. Thanks to the different energies, data acquisition on the camera can take place simultaneously. The radiation of each of the radionuclides is recorded in a different energy window.
Usually 4–6 projections are performed so that the number of counts per projection reaches 300–700 thousand. The time needed to acquire the necessary number of counts ranges from tens of seconds to minutes.
Pulmonary embolism
In pulmonary embolism, the vessels in the pulmonary bed become blocked and part of the lung is deprived of blood flow. A ventilation examination using 81mKr will in such a case be normal, without defects. By contrast, a perfusion examination using 99mTc-MAA will reveal absent or reduced lung perfusion. As the embolism develops further, defects also appear in ventilation. SPECT makes it possible to distinguish the stage of the disease and which course of treatment to choose.
COPD
In chronic obstructive pulmonary disease (COPD), the airways narrow due to chronic bronchitis or emphysema. The obstruction is irreversible or only partially reversible. At the onset of the disease, a change in lung volume may be observed, but without a change in gas distribution. Later, disturbances in lung ventilation begin to appear. They manifest as a series of heterogeneous defects in various places.
Bronchopulmonary neoplasm
In a tumour disease of the bronchi or lungs, a ventilation defect appears. The disorder may affect a lung segment, a lobe or the whole lung. A similar disorder also appears in lung perfusion. The extent of the damage appears greater on the scintigraphic examination than on the radiograph or bronchoscopy. The scintigraphy result is used to estimate how much lung function will be preserved after removal of the tumour.
Bullous emphysema
Bullous emphysema manifests as round defects on both ventilation and perfusion images. In diffuse emphysema, the defects are scattered throughout the lungs. Scintigraphic images of both lung perfusion and ventilation look roughly the same.
Asthma
During an asthma attack, a number of lesions can be observed on ventilation. Perfusion disturbances are less pronounced. When a bronchodilator is administered, the situation improves rapidly. During remission, scintigraphy is normal.
Sequelae of pulmonary tuberculosis
When examining the sequelae of tuberculosis in the lung parenchyma, a number of abnormalities appear on the radiograph. However, it is not possible to determine from radiographs how much lung function is impaired. On scintigraphy, some of these abnormalities no longer appear, while others manifest as extensive ventilation and perfusion defects.
Acute respiratory disease
Acute respiratory diseases result in severe restriction or complete absence of ventilation of the affected parts of the lungs. Perfusion is normal or altered. Blockage of a bronchus is associated with segmental ventilation disturbances. Corresponding perfusion defects are less significant.
Conclusion
The use of 81mKr for lung ventilation examinations makes it possible to study a range of diseases. Compared with 133XeXe, scintigraphy with 81mKr has better resolution, the patient and staff are exposed to a lower radiation burden, active cooperation of the patient is not required for the examination, and, in addition, the use of 81mKr allows several projections to be performed in quick succession. Given the half-life of the parent radionuclide (4.576 h), the 81Rb/81mKr radionuclide generator can also be distributed to relatively distant facilities, but on the other hand this half-life is so short that practically no radioactive waste is produced.
The 81Rb/81mKr radionuclide generator is used mainly for lung ventilation examinations in pulmonary embolism and COPD, and further for a number of other diseases of the respiratory tract.
References
Radioactive gases have been used successfully for lung ventilation examinations since the 1950s. First, in 1953, Knipping et al. used a gas mixture with 131I, and two years later the same group introduced 133XeXe (half-life 5.27 h) into lung diagnostics. The greatest interest in 133XeXe arose in the 1960s, when a number of studies of lung diseases were carried out with this gas. As a result, 133XeXe became a routinely used tool for lung examinations.
In 1968, Yano and Anger outlined the possibility of using 81mKr for lung examinations. 81mKr has a half-life of only 13.1 seconds, but it can be obtained continuously from its parent nuclide 81Rb (4.576 h) in a radionuclide generator. The short half-life of 81mKr is, surprisingly, one of its advantages, because it significantly reduces the radiation burden on both the patient during the examination and the staff, and practically no radioactive waste is produced. Another advantage of 81mKr is the energy of the emitted γ radiation, 190 keV, which, in terms of resolution, is more favourable for SPECT cameras than the energy of the main γ line emitted by 133XeXe (81 keV).
81mKr is, for many reasons, a very suitable radionuclide for lung ventilation examinations in cases of suspected pulmonary embolism, obstructive lung disease, pulmonary emphysema and other lung diseases.
