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Radiation protection in medicine
Published in Alan Martin, Sam Harbison, Karen Beach, Peter Cole, An Introduction to Radiation Protection, 2018
Alan Martin, Sam Harbison, Karen Beach, Peter Cole
Recent years have seen the development and implementation of many techniques aimed at optimizing patient dose in CT imaging. These include improved beam filters, better beam collimation systems, improved detector efficiency, and noise reduction algorithms such as iterative image reconstruction. Noteworthy amongst these techniques are automated ‘z’ and ‘xy’ mA modulation based on patient habitus in helical CT scanning. These can be combined into a system that is guided by a user-selected indication of image quality where the CT X-ray tube current is automatically varied both during gantry rotation and along the length of the patient dependent upon the patient's X-ray attenuation characteristics at any particular position in three dimensions. Such techniques are analogous to automatic exposure control (AEC) devices on conventional radiographic X-ray systems.
Clinical Radiographic Units
Published in Paolo Russo, Handbook of X-ray Imaging, 2017
An automatic exposure control (AEC) is a device that is used to automatically terminate the X-ray exposure when a specific pre-set dose to the detector has been reached. AECs are placed between the patient and the image receptor device. The most common types of AEC is a radiolucent, flat ionization chamber, or solid-state semiconductor. For the ionization chambers, ionization occurs when radiation hits a gas chamber, generating charge. The charge generated is proportional to the number of X-ray photons passing through the chamber. The charge is very small, so it needs to be amplified; as different techniques require different exposures, resistance is adjusted per technique. When a pre-set appropriate amount of charge has been reached, a feedback system terminates the X-ray exposure. This is done by means of either a pulse counting circuit or crystal oscillation. With pulse counting, the exposure will cease when a binary counter determines that the pre-selected number of current pulses have been generated. Likewise, with a quartz crystal, current causes oscillations in the crystal, these oscillations are again counted and, when the required number have been reached, exposures are terminated (Stockley 1986).
Analysis of phantom centering positioning on image noise and radiation dose in axial scan type of brain CT
Published in Radiation Effects and Defects in Solids, 2020
Dae Cheol Kweon, Beomjong Seo, Jiwon Choi, Kyung-Rae Dong, Woon-Kwan Chung
The AEC function is used widely in the posterior thoracic direction and the abdominal anteroposterior direction as a method for the reducing dose exposure in a CT examination. First, the AEC is an automatic exposure control system that can adjust the radiation dose automatically considering the patient size and shape during a CT scan. Before the CT scan, the patient’s position (topogram or surview or scout) image is used to obtain information regarding the patient’s size and attenuation and combined with the reference image. The analysis is performed if the size of the patient is larger than the subject of the reference image. The AEC method is an angular method, in which an X-ray tube rotates a patient and adjusts a tube current according to a damping coefficient per projection based on the patient’s x-axis and y-axis. In the longitudinal method, the tube current is controlled by applying the two methods simultaneously. When the AEC is used, the fluctuations of the noise value are small and uniform images can be obtained, and the exposure dose can be reduced (7).