How to identify normal lung anatomy on chest CT

You can't identify lung pathology if you can't tell it apart from normal tissue. In this video, we'll cover lung anatomy inside-out, key things to look out for that indicate disease, and how a sharp pencil can help you figure out if the bronchial wall is compromised.

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In normal lungs, we see background lung tissue, the larger airways in vessels, and the fissures. The background lung is 20 percent tissue and 80 percent air, and has a certain uniform greyness that you'll learn to recognize. It is lighter than the black color of the airways, but darker than the light color the subcutaneous fat.

The attenuation is generally homogeneous, although when the patient is lying supine, as they do in a CT scanner, the weight of the non-dependent lungs can cause a little atelectasis in the dependent lung, increase its density and making it look whiter. If you're concerned about pathology, rescan the patient in a prone position.

If it goes away, it was simply atelectasis. A key anatomic feature of the lung that determines how lung pathology appears on CT, is the secondary pulmonary lobule. As the airways move from the Hila out to the periphery, they change from conducting airways to respiratory airways and ultimately to asinine and alveoli.

To support these smaller structures, the lung is organized into secondary pulmonary lobules, as shown in this close up diagram. These are one to 2.5 centimeter polyhedral structures, each is supplied by a small bronchial and pulmonary arterial, and is bounded by connective tissue forming interlobular septa. The septa, contain draining venules and lymphatics.

Bronchi and pulmonary arteries run together and branch in parallel. Arteries divide into two branches of similar size, each smaller than the parent vessel. The airways and arteries smoothly taper as they move from the center to the periphery. Veins run independently of the arteries in the airways.

They also taper as they move peripherally, but rather than dividing into just two branches like the arteries, veins have multiple branches, generally at right angles to the parent vessel. The diameter of the artery, and the accompanying bronchus should be about the same. This is called the arterial bronchial ratio.

It is very useful in evaluating the pulmonary vasculature Since the bronchus made of cartilage doesn't change size with increased pulmonary vascular pressures, while the artery does. Normally the ratio is one to one, with the arteries being a bit larger in the dependent lung, and a bit smaller in the nondependent lung due to gravity.

If the artery is larger than the bronchus, it's a sign of increased vascular pressure. If the bronchus is larger than the artery, assuming normal vascular pressures, then may indicate bronchiectasis. Assessing the bronchial wall is always problematic. The wall should appear thin, about 10 percent of the diameter of the bronchus. But, this evaluation is very subjective with high interobserver variability.

One of the best descriptions I've heard is that if the wall looks like it was drawn with a sharp pencil, then it's normal. But, if it looks like was drawn with a crayon, then it's too thick. For me, since a wall thickening rarely indicates anything in the absence of other radiologic or clinical findings, I tend to be conservative and not report it unless the walls are markedly thickened, or there are associated findings. No long markings can be seen within one centimeter of the pleura.

This means that in this area are no visible airways, vasculature or interstitial in the subpleural space. The pleura is found at the lung edges and should be smooth, uniform and sharply defined. Since normal pleura is less than one millimeter thick, it's usually not visible unless you're looking along a fissure. Only about 20 percent of people have all three complete fissures, which include the right oblique and horizontal and the left oblique. Commonly fissures are incomplete or accessory fissures exist.