Automatic exposure control systems designed to maintain constant image noise: effects on computed tomography dose and noise relative to clinically accepted technique chartsChristopher P Favazza et al. J Comput Assist Tomogr. 2015 May-Jun. Show
Free PMC article AbstractObjective: To compare computed tomography dose and noise arising from use of an automatic exposure control (AEC) system designed to maintain constant image noise as patient size varies with clinically accepted technique charts and AEC systems designed to vary image noise. Materials and methods: A model was developed to describe tube current modulation as a function of patient thickness. Relative dose and noise values were calculated as patient width varied for AEC settings designed to yield constant or variable noise levels and were compared to empirically derived values used by our clinical practice. Phantom experiments were performed in which tube current was measured as a function of thickness using a constant-noise-based AEC system and the results were compared with clinical technique charts. Results: For 12-, 20-, 28-, 44-, and 50-cm patient widths, the requirement of constant noise across patient size yielded relative doses of 5%, 14%, 38%, 260%, and 549% and relative noises of 435%, 267%, 163%, 61%, and 42%, respectively, as compared with our clinically used technique chart settings at each respective width. Experimental measurements showed that a constant noise-based AEC system yielded 175% relative noise for a 30-cm phantom and 206% relative dose for a 40-cm phantom compared with our clinical technique chart. Conclusions: Automatic exposure control systems that prescribe constant noise as patient size varies can yield excessive noise in small patients and excessive dose in obese patients compared with clinically accepted technique charts. Use of noise-level technique charts and tube current limits can mitigate these effects. Conflict of interest statementThe other authors declare no conflict of interest. Figures
 Examples of the tube current modulation schemes based on a 36-cm reference patient thickness (tref = 36 cm) and 225-mA reference tube current (mAref = 225 mA). The solid line represents the tube current required to achieve a constant noise across patient thickness variations. The scanner reaches a maximum tube current for patients at least 40 cm thick, which could lead to sacrifices in image quality through longer gantry rotation times and/or lower spiral pitch values. The dotted curve represents a less aggressive modulation of tube current, where the tube current is not increased as strongly for large patients or decreased as strongly for small patients. This maintains adequate image noise in small patients and avoids excessive radiation doses for large patients. The curves reflect the abdominal (HVLe = 10 cm; b = 0.36) and thoracic (HVLe = 13 cm; b = 0.28) technique charts used in our large clinical practice for more than 2 decades (see Eq. [7]). Photograph of the anthropomorphic thorax phantoms used for data measurement. The lateral width of the small phantom is 30 cm. The addition of the hollow, oval tissue–equivalent material results in a net lateral thickness of 45 cm (left) and 40 cm (right). Demonstration of the effect of varying b values to adapt the strength of the tube current modulation. Relative attenuation refers to attenuation compared with the attenuation for a reference patient thickness (ie, 36 cm). Relative tube current is compared with the tube current setting for the reference patient thickness. For b = 1, the tube current is linearly increased with patient attenuation, such that image noise is held constant. For b < 1 (0.3), the tube current is neither decreased nor increased as aggressively, resulting in less noise in small patients and more noise in large patients relative to that of the reference patient. For b > 1, the tube current is very aggressively decreased for small patients and increased for large patients relative to the reference patient. This scenario is not likely to be clinically relevant. Required tube current values for different phantom sizes using noise index of 10, 12, and 14, and our clinical technique chart. Similar articles
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When using AEC an increase in kVp will have what effect on the image?Using higher kVp with AEC decreases the exposure time and overall mAs needed to produce a diagnostic image, significantly reducing the patient's exposure. The radiographer must be sure to set the kVp value as needed to ensure adequate penetration and to produce the appropriate scale of contrast.
What exposure factor does AEC control?The intention of AEC is to provide consistent x-ray image exposure, whether to film, a digital detector or a CT scanner. AEC systems may also automatically set exposure factors such as the X-ray tube current and voltage in a CT.
When the automatic exposure control system AEC is being used for the PA projection which ionization chamber should be activated?Chapter 3. What is automatic exposure control AEC quizlet?This refers to the maximum length of time the x-ray exposure continues when using an AEC system. Backup timer. This allows the radiographer to adjust the amount of preset radiation detection values.
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Which of the following effects will occur when using an AEC system to image a Hypersthenic patient?
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