Chest Imaging

Ventilation perfusion scintigraphy

radionuclide imaging study of pulmonary circulation and ventilation. It is mainly indicated in:

  • the detection of pulmonary embolism (Fig.1);

     

  • monitoring the natural history or treatment of thromboembolic disease;

     

  • quantitative evaluation of distribution of obstructive pulmonary disease; and

     

  • preoperative evaluation of patients with emphysema, lung cancer; and bronchiectasis.

    Techniques for assessing pulmonary ventilation involve the inhalation of a radioactive gas (Xe-133 or Kr-81m) or a nebulized aerosol of a radioactive material (albumin labelled with I-131 or Tc-99m).

    For studies of the pulmonary circulation with particulates, the commonest vehicle for the radionuclide is macroaggregated albumin prepared by heating human serum albumin. The resultant particle sizes range from 10 to 100 in diameter. The albumin may be labelled with I-131, Tc-99m, or I-113m. Other radiopharmaceuticals include Tc-99 sulphur colloid macroaggregated albumin, radioactive albumin microspheres. The procedure of perfusion scanning is based on trapping particles within the small pulmonary arteries of the lung. A particle size of 10 - 50 in diameter is optimal. The optimum number of particles is about 100,0000. Disappearance time from the lungs is nearly exponential, biological half life ranging from 4 to 20 hours. There is no significant toxicity or morbidity related to the injection of human serum albumin. The radiation dose to the lungs is quite acceptable.

    The gaseous pharmaceutical most commonly used for studying pulmonary circulation is Xe-133. The gas is dissolved in saline and injected intravenously. The gamma camera records perfusion while the patient holds his breath. The xenon passes almost immediately into the alveoli. As a result, during subsequent respiration the distribution of xenon in all areas of ventilation can be recorded. Xenon is cleared from the lungs in 3 - 4 minutes in normally ventilated areas, but is delayed in regions poorly ventilated (air trapping). For further description of the technique, see radionuclide lung imaging.

     

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    Fig.1

    Ventilation (A) and perfusion (B) scans in a patient with pulmonary embolism. There are several bilateral segmental perfusion defects without matched ventilation defects in the same territories (mismatch).
    Ventilation perfusion scintigraphy, Fig.1 (a)
    Ventilation perfusion scintigraphy, Fig.1 (b)