The Lungs: The lungs consist of the breathing tubes known as the bronchi and bronchioles, the blood vessels, the lymph system and respiratory portion of the lungs including the interstitial and supporting structures. Terms used to describe lung tissue include parenchyma, interstitium, interstitial or parenchymal, all of which mean the same thing. When asbestos-caused scarring occurs to the lung tissue, this is call asbestosis.
The Linings: There are linings to the chest cavity, which include the chest walls, diaphragms, spine adjacent areas and heart-adjacent areas known as the parietal pleura, linings of the lungs themselves known as the visceral pleura as well as a lining directly around the heart known as the pericardium. When asbestos-caused scarring occurs to the linings, this is called asbestos-related disease.
Pleural (Circumscribed) Plaques are seen in asbestos-related disease and involve the parietal pleura or lining of the chest cavity. They most commonly occur, either along the sides of the chest cavity known as in-profile plaquing; along the front or back of the chest cavity, known as face-on (en-face) plaquing; over the diaphragms, known as diaphragmatic plaquing; adjacent to the spine, known as paravertebral plaquing or adjacent to the heart, known as pericardial plaquing.
Q: Identify the structures between the heart and lungs.
A: The heart has a lining called the pericardium, but the chest cavity also has a lining called the parietal pleura that extends over the heart and the heart’s lining, the pericardium. The lungs have a lining called the visceral pleura. Thus, the order from inside to outside is, the heart, the pericardium, the pericardial portion of the parietal pleura (chest cavity lining), the visceral pleura (lung lining) and then the lung.
Further Knowledge: When somebody says that there is a pericardial calcified pleural plaque, this calcified plaque is on the pericardial portion of the parietal pleura or the cardiac portion of the chest cavity lining, overlying the pericardium or lining of the heart and underlying the visceral pleura or lining of the lung. The plaque does not actually involve the pericardium or lining of the heart, despite its name, but rather the pericardial portion of the parietal pleura overlying the pericardium.
Diffuse Pleural Thickening is a condition in which sticky fluid, most often, gets between the lung and the chest cavity. In most individuals, this fluid gets resorbed. However, in some individuals, the lining of the lung and the lining of the chest cavity stick together and scar. Thus, for an elongated distance, there is scarring and thickening of the visceral pleura (lung lining) stuck to the parietal pleura (chest wall lining). Although, some textbooks describe diffuse pleural thickening, as visceral pleural thickening, that is a misnomer. What is correct is that the visceral pleura is sticking to the parietal pleura and two combined, are resulting in scarred thickening, known as diffuse pleural thickening. There is an exception to this rule, where sticky fluid extends from between the chest cavity and the lungs into the separations between portions of the lungs known as the fissures. Each portion of the lung, known as a lobe, or its subdivision, known as a segment of the lobe, has visceral pleural lining and as such, sticking together and scarring across the fissure results in visceral pleura to visceral pleura scar thickening, a variant form of diffuse pleural thickening. Such causes of sticky fluid, includes asbestos-caused benign pleural effusions, bleeding (hemothorax) from rib fractures, chest trauma or chest surgery such as bypass or cardiac valve replacement surgery or large infections that break into the chest cavity from the lung, such as seen with tuberculosis, forming a giant pus pocket between the chest cavity and lung, known as an empyema. Note that pleural effusions in congestive heart failure are not sticky and thus, do not result in diffuse pleural thickening. Diffuse pleural thickening on x-rays causes blunting of the costophrenic angle – the triangular area of lung at the far peripheral aspect of the lung base. However, on CT and HRCT, it has been my experience, that this does not always have to be the circumstance and more focal scarring and fusion of the chest cavity lining to the lung lining or the lung lining to the lung lining across the fissures can occur. In addition, with diffuse pleural thickening there can be scars that extend into the lung itself, somewhat like fingers grabbing or pushing/extending into raw bread dough. These finger-like scars are known as parenchymal bands. If they are clustered together, they can have a radial or fan-like appearance called crow’s feet. If they are larger and wider, forming triangular or rectangular scars extending from the pleural surfaces, they are known as benign fibrotic/cicatricial masses. If the parenchymal band extensions grab the lung and fold it in, as if your fingers are folding into the palm of your hand, they will grab the lung and pull it towards the center of the scar mass, as the finger-like extensions bend downward into the center of the base of the mass. This becomes what is known as rounded atelectasis (atelectasis meaning collapse), which is most commonly a pleural based rounded mass of scar with collapsed lung centrally. The name, comet- tail sign, has been used to describe the whorled or spiral appearing blood vessels and the air tubes, known as the bronchioles, extending into the center of this mass of collapsed lung tissue. This finding can help one distinguish it from lung cancer. Still, sometimes biopsy or PET/CT is needed to exclude a carcinoma, if the comet-tail sign and pleural based location are not classic.
It is critical that you understand asbestosis begins most commonly at the posterior lung bases and then later extends to the middle lung zones and then, with more severe disease, into the upper lung zones. It appears as small, often horizontal linear densities called small irregular opacities on chest x-rays, often in the mid and lower lung zones and as intralobular interstitial thickening (primarily subpleural haziness, but also small dots and small vertical short lines) in the posterior lower lung zones, on prone HRCT in the mild state, progressing upward and laterally into the middle lung zones, to include bronchiolectasis, non-dependent subpleural line formation and parenchymal bands in the moderate state and later on, in advanced disease, also involving the upper lungs zones and more central lung tissue, with distortion, central ground glass opacities, bronchiectasis and honeycombing. Emphysema from smoking, on the other hand, begins in the upper lung zones, greater on the right than the left, extends into the middle lung zones and with severe disease extends into all six lung zones, which includes involvement of the lower lung zones. It commonly is not seen on chest x-rays or may have a non-specific finding of hyperaeration/overinflation. Occasionally, bullae are identified, most commonly in the upper lung zones. On CT and HRCT emphysema is most commonly seen as centrilobular (Swiss-cheese looking) or paraseptal (saw-toothed looking) holes. The centrilobular holes are in the center of the lungs and the paraseptal holes are along the perimeter of the lungs, adjacent the chest walls, including commonly in the very tops or the lungs called the apices and along fissures. Centrilobular and paraseptal emphysema can exist separately or can be both present simultaneously. Bullae (holes greater than 1 cm) can develop. Scarring is often not present except in smoking caused Respiratory Bronchiolitis – Interstitial Lung Disease (RB-ILD) or its variants.
Another common term to understand is traction bronchiolectasis. The bronchioles are visually not usually seen in the normal appearing peripheral subpleural lung. Lung tissue scarring pulls on the bronchiole walls, widening their diameter, known as traction bronchiectasis, which coupled with scar thickening of their walls, makes them visible. Thus, when there is moderate asbestosis, oftentimes, we see little black holes amongst the white scarring, which represents the distended and pulled open bronchioles with the scarring about them. In the mild form of asbestosis, this is often not seen.
Honeycombing is a condition, in which the lung tissue has been destroyed and all that is left is an empty cavity surrounded by scar material. Honeycombing commonly starts in the lower lung zones, but is often seen in all six lung zones with more advanced disease. Holes from emphysema are due to focal destruction of the respiratory portions of the lung tissue and most commonly begin in the upper lung zones, but can occur in other lung zones, as well, especially if there is advanced emphysema or if one has a subtype of paraseptal emphysema, involving the posterior lower lung zones. One method to distinguish honeycombing from emphysema, that is not fail safe, is that honeycombing is usually in the lower lung zones, is multi-layered and has scarring about it, whereas emphysematous paraseptal holes are usually in the upper lung zones, forming more often a single layer of holes and not frequently surrounded by scar material.
The most common cause for small nodules in the lungs (rounded densities) is the presence of granulomas, which are residual scars from prior tuberculosis or fungal infection. When one breathes in TB or fungus, this may result in small infections in the lungs, that are often asymptomatic. The body fights off this disease and walls off the tuberculous bacilli or fungal spores forming a spherical appearing scar, known as a granuloma. Small intrapulmonic lymph nodes, non-specific scars, early small lung cancer and metastatic disease (spread of cancer), however, can look similar. Because of this, when nodules are non-calcified or when the calcium deposit is only at the periphery, rather than rim-like, in the center or throughout the nodule, follow-up CT scans are required over time, to determine if the nodule(s) grows or changes in size or shape, as would be more likely to occur with either metastases or lung cancer.
This article is provided as a public service by Daniel Powers, M.D.: B-Reader and Board-Certified Diagnostic Radiologist, Certified by the American Board of Radiology.
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