When measuring the entrance skin dose, where is the x-ray source typically located?

The entrance skin dose refers to the amount of radiation that reaches the skin of the patient at the point of entry when an x-ray is performed. In typical fluoroscopic procedures, the x-ray source is commonly located beneath the table. This orientation helps to ensure that the x-rays are directed upward toward the patient, allowing for effective imaging while minimizing the patient's radiation exposure, particularly to sensitive tissues.

Having the x-ray source under the table also aligns with the design of most fluoroscopy units, where the configuration allows for optimal geometry and reduces the distance that the x-ray photons must travel, thereby optimizing image quality and minimizing dose. This placement is crucial for accurately measuring the entrance skin dose as it directly impacts the intensity of radiation that can potentially reach the patient's skin during the examination.

The other positions, such as having the x-ray source over the table, would not be typical for standard fluoroscopic setups and might alter the dose distribution. Therefore, the positioning of the x-ray source under the table is essential for understanding how entrance skin dose is assessed in practice.