X-ray pulmonary function study

The functional respiratory system consists of many links, among which the pulmonary (external) respiration and circulatory systems are of particular importance. The efforts of the respiratory muscles cause changes in the volume of the chest and lungs, ensuring their ventilation. Due to this, the inhaled air spreads along the bronchial tree, reaching the alveoli. Naturally, violations of bronchial patency lead to a disorder of the external respiration mechanism. In the alveoli, diffusion of gases occurs through the alveolar-capillary membrane. The diffusion process is disrupted both when the alveolar walls are damaged and when capillary blood flow in the lungs is disrupted.

Conventional radiographs taken during the inhalation and exhalation phases and fluoroscopy can provide a rough idea of the mechanics of the respiratory act and pulmonary ventilation. During inhalation, the anterior ends and bodies of the ribs rise, the intercostal spaces widen, and the diaphragm descends (especially due to its powerful posterior slope). The pulmonary fields increase in size and their transparency increases. If necessary, all of these parameters can be measured. More accurate data can be obtained with CT. It allows one to determine the size of the chest cavity at any level, the ventilation function of the lungs as a whole and in any of their sections. Using CT scans, one can measure the absorption of X-ray radiation at all levels (perform densitometry) and thus obtain summary information on the ventilation and blood filling of the lungs.

Obstruction of the bronchi due to changes in their tone, accumulation of sputum, swelling of the mucous membrane, organic constrictions are clearly reflected in radiographs and CT scans. There are three degrees of bronchial obstruction - partial, valvular, complete and, accordingly, three states of the lung - hypoventilation, obstructive emphysema, atelectasis. A slight persistent narrowing of the bronchus is accompanied by a decrease in the air content in the part of the lung ventilated by this bronchus - hypoventilation. On radiographs and tomograms, this part of the lung is slightly reduced, becomes less transparent, the pattern in it is enhanced due to the convergence of vessels and plethora. The mediastinum on inhalation may shift slightly towards hypoventilation.

In obstructive emphysema, air enters the alveoli during inhalation, when the bronchus expands, but cannot immediately exit them during exhalation. The affected part of the lung increases in size and becomes lighter than the surrounding parts of the lung, especially during exhalation. Finally, when the lumen of the bronchus is completely closed, complete airlessness occurs - atelectasis. Air can no longer penetrate the alveoli. The air remaining in them is resorbed and partially replaced by edematous fluid. The airless area decreases and causes an intense homogeneous shadow on radiographs and CT scans.

When the main bronchus is blocked, atelectasis of the entire lung occurs. Blockage of a lobar bronchus leads to atelectasis of the lobe. Obstruction of a segmental bronchus results in atelectasis of the segment. Subsegmental atelectases usually have the form of narrow stripes in different parts of the lung fields, and lobular atelectases have the form of rounded compactions with a diameter of 1 - 1.5 cm.

However, the main radiation method for studying the physiology and identifying functional pathology of the lungs has become the radionuclide method - scintigraphy. It allows you to assess the state of ventilation, perfusion and pulmonary capillary blood flow, and to obtain both qualitative and quantitative indicators characterizing the entry of gases into the lungs and their removal, as well as the exchange of gases between the alveolar air and blood in the pulmonary capillaries.

In order to study the pulmonary cashilar blood flow, perfusion scintigraphy, venous and bronchial patency - inhalation scintigraphy are performed. Both studies produce a radionuclide image of the lungs. To perform perfusion scintigraphy, the patient is intravenously injected with ^99m Tc-labeled al6umin particles (microspheres or macroaggregates). Once in the bloodstream, they are carried away to the right atrium, right ventricle and then to the pulmonary artery system. The particle size is 20-40 μm, which prevents them from passing through the capillary bed. Almost 100% of the microspheres get stuck in the capillaries and emit gamma quanta, which are recorded using a gamma camera. The study does not affect the patient's well-being, since only an insignificant part of the capillaries is excluded from the bloodstream. A person has approximately 280 billion capillaries in their lungs, while only 100,000 to 500,000 particles are injected for the study. Several hours after the injection, the protein particles are destroyed by blood enzymes and macrophages.

In order to evaluate perfusion scintigrams, qualitative and quantitative analysis is performed. In qualitative analysis, the shape and size of the lungs are determined in 4 projections: anterior and posterior direct, right and left lateral. The distribution of the radiopharmaceutical over the lung fields should be uniform. In quantitative analysis, both lung fields on the display screen are divided into three equal parts: upper, middle and lower. The total accumulation of the radiopharmaceutical in both lungs is taken as 100%. The relative radioactivity is calculated on the computer, i.e. the accumulation of the radiopharmaceutical in each section of the lung field, separately left and right. Normally, a higher accumulation is recorded for the right lung field - by 5-10%, and the concentration of the radiopharmaceutical in the field increases from top to bottom. Capillary blood flow disorders are accompanied by a change in the above ratios in the accumulation of the radiopharmaceutical in the fields and sections of the lungs.

Inhalation scintigraphy is performed using inert gases - Xe or Kr. An air-xenon mixture is introduced into the closed system of the spirograph. Using a mouthpiece and a nose clip, a closed system of the spirograph - patient is created. After achieving dynamic equilibrium, a scintigraphic image of the lungs is recorded on a gamma camera and then its qualitative and quantitative processing is carried out in the same way as perfusion. Areas of impaired ventilation of the lungs correspond to places of reduced accumulation of the radiopharmaceutical. This is observed in obstructive lung lesions: bronchitis, bronchial asthma, local pneumosclerosis, bronchial cancer, etc.

^{Aerosols of 99m} Tc are also used for inhalation scintigraphy. In this case, 1 ml of the radiopharmaceutical with an activity of 74-185 MBq is introduced into the nebulizer of the inhaler. Dynamic recording is performed at a rate of 1 frame per 1 s for 15 min. An activity-time curve is plotted. At the first stage of the study, the state of bronchial patency and ventilation is determined, and the level and degree of obstruction can be established. At the second stage, when the radiopharmaceutical diffuses into the bloodstream through the alveolar-capillary membrane, the intensity of capillary blood flow and the state of the membrane are assessed. Measurement of regional pulmonary perfusion and ventilation can also be performed by intravenous administration of radioactive xenon dissolved in an isotonic solution of sodium chloride, followed by recording the clearance of xenon from the lungs on a gamma camera.