Darkening of the lung field or part of it

Most lung diseases are accompanied by compaction of the lung tissue, i.e. a decrease or absence of its airiness. Compacted tissue absorbs X-ray radiation more strongly. A shadow or, as they say, darkening appears against the background of a light lung field. The position, size and shape of the darkening depend, naturally, on the extent of the lesion. There are several typical types of darkening. If the pathological process has affected the entire lung, then the entire lung field is darkened to one degree or another on the radiograph. This syndrome is called "extensive darkening of the lung field." It is not difficult to detect - it is striking at first glance at the image. However, its substrate must be immediately determined. Darkening of the entire lung field is most often caused by blockage of the main bronchus and atelectasis of the corresponding lung.

An atelectatic lung is airless, so its shadow is uniform. In addition, it is reduced, so the mediastinal organs are shifted toward the darkening. These two signs are enough to recognize atelectasis of the lung and, using tomography and fibrobronchoscopy, accurately determine its origin (tumor of the main bronchus, its damage, foreign body). A similar picture can be obtained after lung removal (pneumonectomy), but this option is clear from the anamnesis.

Another pathological process in which the mediastinal organs are displaced toward extensive darkening is fibrothorax with cirrhosis of the lung. However, in this pathology, darkening is never uniform: against its background, areas of preserved lung tissue, swollen lobules, sometimes cavities, coarse fibrous cords, etc. are distinguishable.

Inflammatory infiltration very rarely spreads to the entire lung. If this does happen, then extensive darkening of the pulmonary field is also observed. It is distinguished from atelectasis not only by the clinical picture, but also by radiographic symptoms. The mediastinal organs remain in place during pneumonia, and against the background of darkening, the lumens of the bronchi filled with air can be detected.

Finally, it is very important to point out that darkening of the lung field can be caused not only by compaction of the lung tissue, but also by fluid accumulated in the pleural cavity. With a large effusion, the darkening becomes extensive and uniform, as with atelectasis, but the mediastinal organs are displaced to the opposite side.

Much more often, the pathological process affects not the entire lung, but only a lobe, part of a lobe, segment or even a subsegment. Radiographs reveal a shadow that matches the altered lobe, segment or subsegment in position, size and shape. This syndrome is called "limited darkening of the lung field". Its substrate is infiltration of the lung tissue (accumulation of any exudate in the alveoli), atelectasis or sclerosis of the lung tissue, tumor growth.

Having detected a limited darkening on radiographs, it is necessary first of all to establish its topography, i.e. to determine which lobe, segment or subsegment is compacted. The task is essentially simple if there are images in two projections, since each lobe and each segment occupies a certain place in the chest cavity. It is more difficult to establish the substrate of the darkening. Of course, anamnesis data, clinical and laboratory research results often shed light on the nature of the compaction of lung tissue. However, taking into account clinical information, the radiologist always forms his own opinion, guided by a number of considerations. They are conveniently listed using the example of damage to the upper lobe of the right lung.

In pneumonic infiltration, the darkening corresponds in size to the lobe, has a clear straight or convex downward border separating it from the middle lobe (interlobar pleura). The lumens of the bronchi may be visible against the darkening. The position of the mediastinum is unchanged. In atelectasis, the lobe is reduced, the lower border is drawn in, the shadow is uniform, and the mediastinum is slightly shifted toward the darkening. In pneumosclerosis, the lobe is also reduced, and the mediastinum is pulled toward it, but the darkening is non-uniform: against its background, clearings are visible corresponding to swollen areas of preserved lung tissue or cavities, as well as intertwined dark stripes of fibrous tissue. Unlike atelectasis, the patency of the bronchi is preserved, which is clearly displayed on tomograms.

The above considerations on differential diagnostics apply entirely to intralobar segmental pathological processes. However, the smaller the volume of the lesion, the more difficult it is to guess its nature. The most general considerations here are as follows. Pneumonic and tuberculous infiltration has the form of diffuse or focal darkening with unclear outlines (see below for more details). Tumor growth is indicated by a more or less delimited shadow with uneven contours. The lumens of the bronchi are not traced in it, enlarged lymph nodes in the root of the lung may be visible. Compaction caused by a large pulmonary infarction produces a triangular shadow, the base of which is adjacent to the chest wall or interlobar border. Of course, such facts as the presence of an obvious source of thromboembolism (for example, thrombophlebitis of the lower limb), chest pain, dyspnea, hemoptysis, overload of the right heart, revealed by electrocardiography, help in the diagnosis of infarction.

Darkening of a part of the lung field is not necessarily associated with compaction of the lung tissue: a tumor growing from a rib or pleura, pleural adhesions and pleural effusion will also cause darkening of the lung field, since they also absorb a large amount of X-ray radiation. However, with the help of X-rays in different projections and especially computer tomograms, it is always possible to establish the marginal localization of the lesion, outside the lung tissue.

Limited darkening of a part of the lung field may be caused by a diaphragmatic hernia, i.e. the exit of abdominal organs into the chest cavity through a defect in the diaphragm. In this case, the darkening is inseparable from the diaphragm contour and is sharply delimited from the lung tissue. If the hernia contains part of the stomach or intestinal loops, the darkening is non-uniform due to the presence of clearings caused by gas accumulations in these organs. All doubts are eliminated by a study conducted after the patient takes a barium suspension, which successively fills the stomach and intestines. In this case, the image shows what part of the digestive tract is part of the hernia, and the localization of the hernial orifice can be established.

A special syndrome of a round shadow in the pulmonary field is a limited darkening of the pulmonary field, in which the shadow of the pathological formation on the images in all projections has the shape of a circle, semicircle or oval with a diameter of more than 1 cm. Such a shadow is caused by the focus of the lesion of a spherical or ovoid shape. The substrate can be an eosinophilic infiltrate, tuberculous infiltrate or tuberculoma, a rounded area of pneumonic infiltration, pulmonary infarction, a closed cyst (bronchial, retention, echinococcal, alveococcal), aneurysm, benign tumor, malignant tumor (primary or metastatic) and many other pathological conditions.

Differential diagnosis of single and multiple round shadows in the lungs is sometimes difficult. In these cases, anamnesis data and the clinical picture of the disease (for example, pneumonia, pulmonary infarction, metastatic tumors) play an important role. In addition, the fact that many diseases in which round shadows are visible on the radiograph are rare is of great help. "What is common is common, and what is rare is rare," old radiologists like to repeat. In practice, it is necessary to distinguish mainly between closed cysts, tuberculomas and lung tumors.

A closed cyst is defined as a round or ovoid shadow, sharply demarcated from the surrounding lung tissue. On CT, the cyst immediately gives itself away, since, according to densitometry data, its contents are fluid.

Differentiation between a tuberculoma, a benign tumor, and a cancerous nodule is facilitated by previously obtained radiographs, since the growth rate of the formation can be determined. Otherwise, a transthoracic puncture biopsy may be necessary, since the radiographic picture in these pathological conditions can be very similar. However, there are also reliable reference points for radiographic differential diagnostics. Of the benign tumors of the lung, hamartoma is the most common. Like a tuberculoma and cancer, it produces a round shadow on the radiograph with sharp and not quite even outlines, but it is easy to recognize if there are calcareous or bone inclusions deep in the node. To a certain extent, signs of a tuberculoma can be considered tuberculous foci around it or in other parts of the lungs, as well as the presence of a scallop-like cavity in the place where the draining bronchus enters the tuberculoma. Primary lung cancer is indicated by rapid growth, the appearance of narrow stripes of lymphangitis to the periphery of the node and in the direction of the root of the lung, and an increase in the lymph nodes at the root. When a single spherical formation is detected in the lung, the following diagnostic program is recommended.

A peculiar form of darkening is a ring-shaped shadow in the pulmonary field - a radiographic image of a cavity containing gas or gas and liquid. A mandatory requirement for identifying such a syndrome is the closure of the ring on radiographs in different projections. The fact is that in a picture in any one projection, intersecting shadows of vessels can resemble a ring. Sometimes ring-shaped figures in a picture in one projection can be formed by bone bridges between the ribs.

The abscess cavity contains gas and liquid; a characteristic horizontal fluid level is visible in it. The walls of the abscess are thick, and in the surrounding lung tissue there is an infiltration zone with fuzzy, blurry outlines. A fresh tuberculous cavity has the appearance of a ring-shaped shadow, around which tuberculous foci are scattered or a belt of compacted lung tissue is located. The internal contour of the cavity is initially uneven, bay-shaped, then becomes smooth. The size of the cavity varies from a few millimeters to several centimeters. Peripheral lung cancer often gives the symptom of a cavity. As a result of the disintegration of necrotic tumor tissue, one or more cavities with scalloped edges appear in it. As the necrotic masses are rejected, the cavity can become rounded with smooth outlines, but at least a limited area always leaves a lumpy mass on the wall of the cavity. The outer contours of the cavity are uneven and relatively sharply demarcated from the surrounding lung tissue.

The most frequently observed type of darkening is focal shadows. This term refers to round or irregular shadow formations, the sizes of which vary from 0.5 mm to 1 cm. Conventionally, foci up to 2 mm are considered miliary, from 2 to 4 mm small, from 4 to 8 mm medium, and from 8 to 12 mm large. Let us just note that a single round focus larger than 1 cm is usually referred to as the round shadow syndrome in the pulmonary field.

The number of focal shadows may vary. In some cases, it is a single formation, in others, a group of nearby foci. Sometimes there are many foci. If they cover a fairly large area, but not larger than the apex of the lung and two adjacent intercostal spaces on a direct radiograph, they speak of limited dissemination. A larger dispersion of foci over a larger area is called widespread dissemination. Finally, there are cases of diffuse dissemination, when foci densely dot both lungs.

When analyzing radiographs, the localization of foci should be taken into account first of all. Their location in the apices and outer sections of the subclavian zone in most cases indicates the tuberculous nature of the disease - focal pulmonary tuberculosis. The presence of foci in the middle and lower sections of the lungs is characteristic of focal pneumonia. It is necessary to analyze the contours and structure of the foci, as well as the pulmonary background around them, with particular care. Blurred outlines of foci are a sign of an active inflammatory process. This is also evidenced by an enhanced pattern in the same zone and the tendency of foci to merge. Dense, clearly defined foci are evidence of granulomatous or dormant inflammatory lesions. Some tuberculous foci calcify in the inactive phase of the disease.

Usually, diagnosis and establishment of the nature of focal lesions in the lungs do not cause great difficulties with due attention to clinical data. Difficulties arise mainly with diffuse disseminations. As a rule, the decision is made based on the results of the analysis of survey radiographs of the lungs, but in the presence of clinical signs of tuberculosis activity or closely grouped foci, it is advisable to perform tomography in order to identify cavities that are not visible on survey images.

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