Comprehension of the above principles.

Rationale for multiple bilateral cavities.

Bronchi and pulmonary artery characteristically branch many times in lungs. Obviously if there is bronchogenous or hematogenous process, you can encounter bilateral multiple lung lesions. Diseases of vessels, like vasculitis also can result in multiple bilateral lung lesions (Eg Wegners granulomatosis). Bronchogenous and inflammatory vascular processes have indistinct margins. Non-inflammatory interstitial vascular lesions like metastasis, the margins are sharp.

Why does reactivation Tb occurs in superior segments?

This is gravity dependant phenomenon. In upright position there is less blood flow to apices of lungs. Since Tb organism is aerobic it thrives better, where there is more alveolar oxygen. In Bats tuberculosis characteristically occurs in basal segments. I have seen basal segment Tb in diabeteics, reason not clear.

Why does aspiration lung abscess occur in the superior segment of lower lobes?

85% of aspiration lung abscesses occur in superior segment of lower lobes and axillary sub-segment of upper lobes. This is again a gravity dependant phenomenon. Location of aspiration depends on the position of patient at the time of aspiration. You also need to have an understanding of  three dimentional view of bronchial anatomy. If the patient is supine on back, the first dependant entry is into the superior segment of  lower lobe. If the patient is in right lateral decubitus position, it is the axillary sub-segments of RUL. If you are in supine positionwith face down, aspiration into lungs does not occur. Hence it is very unusual to encounter aspiration lung abscesees in anterior segemnets.

What is the criteria for thick and thin wall ?

This arbitrary. Thin wall is when it resembles a cavity drawn with a pencil. I suppose we can arbitrarily set up some rules.

Thin: < 1mm

Thick: > 5 mm

What is the pathogenesis of stalactites and stalagmites?

In necrotizing pneumonia and primary lung cancer you start off with a mass lesion. In cancer, when it outstrips its blood supply, necrosisoccurs and the contents eventually get evacuated through bronchi. You can visualize a cavity with irregular lumen, variable wall thicknessand possible fluid or necrotic tissue level as content.

What is crescentic sign?

How do you differentiate between aspergilloma and sub acute necrotizing aspergillosis?

Does the location of cavity in a density have diagnostic significance?

What is open negative Tb?

In metastatic disease when do you get thin walled cavities and when do you get thick walled cavities?