The “Absent Bowtie Sign” typically refers to a radiographic appearance in the lower lobes of the lungs, where there is a lack of normal vascular markings that resemble a bowtie shape. In a normal chest X-ray, the blood vessels in the lower lobes of the lungs create a pattern that resembles a bowtie or butterfly shape. When this pattern is absent or diminished, it can suggest a loss of blood flow or vascular markings in that area, which could be due to various conditions, including:

1. Pulmonary embolism: A blood clot in the pulmonary arteries can block blood flow to a portion of the lung, leading to decreased vascular markings in that area.
2. Atelectasis: Atelectasis is the collapse or closure of a part of the lung, which can reduce blood flow and result in an absent bowtie sign.
3. Other lung pathology: Various lung diseases or conditions can affect the normal vascular markings on a chest X-ray, leading to an absent bowtie sign.