Which type of AEC is considered an entrance type device?

• A. Photo timer
• B. Solid-state detector
• C. Ionization chamber
• D. Optical sensor

Answer: (C)

An entrance type AEC (Automatic Exposure Control) device is designed to measure the radiation that enters the patient or is absorbed by the patient. The ionization chamber functions based on this principle; it uses a gas-filled chamber to detect ionizing radiation. When radiation passes through the chamber, it ionizes the gas, creating an electrical charge that signals the end of the exposure once a predetermined amount of ionization has occurred. This characteristic makes ionization chambers particularly suitable for measuring the radiation exposure right at the point of entry, thus effectively controlling the radiation dose according to patient size and density.

In contrast, other types of AEC devices, such as photo timers and solid-state detectors, have different configurations and operational principles. Photo timers, for example, typically detect the radiation that has already passed through the patient by measuring optical density on the image receptor, making them more suited to exit or exit-type devices. Solid-state detectors detect radiation but do not specifically function as entrance-type devices. Optical sensors, while useful for various applications, do not specifically fit the definition of an entrance type AEC either. Therefore, the ionization chamber is the most accurate answer to the question regarding entrance type AEC devices.