[Radiologic Physics] Ep 11 – Timers

This episode covers exposure timers — the circuit components that determine how long the x-ray tube emits radiation and directly govern dose, image quality, and patient safety. Prof. Arya traces the evolution from spring-wound mechanical timers and synchronous motor timers through electronic and mAs timers, comparing their accuracy, circuit location, and clinical applications, including the mAs timer's role in falling-load generators and pediatric imaging. The second half focuses on Automatic Exposure Control — how ionization chamber and photodiode detectors terminate exposure based on radiation reaching the image receptor, and the factors that influence AEC performance: detector cell selection, patient positioning, collimation, body thickness, tissue composition, pathology, and IR type. The backup timer is examined as a regulatory safety requirement, and the episode closes with a practical summary of how each technical factor affects AEC exposure time. This episode aligns with the Safety content category — Radiation Physics and Radiobiology subcategory — of the ARRT Radiography Examination Content Specifications. Audio content is adapted from original instructional material developed by Professor Sanjay Arya, M.S., R.T.(R)(MR) for radiologic technology education. Part of the Radiologic Physics series — Quantum Imaging Lab. © 2026 Quantum Imaging Lab. All rights reserved.