| 词条 | Automatic exposure control |
| 释义 |
OperationProjectional RadiographyIn projectional radiography an AEC system uses one or more physically thin radiation ionization chambers (the "AEC detector") which is positioned between the patient being x-rayed and the x-ray film cassette. Where low energy x-rays are used such as in mammography the AEC detector is placed behind the image receptor to avoid creating a shadow.[3]{{rp|106}} In a simple AEC system a weak ionization signal from the AEC detector is integrated as a ramp shaped voltage waveform. This ramp signal rises until it matches a pre-set threshold. At this point the x-ray exposure is terminated.[4] AEC devices are calibrated to ensure that similar exams have linearity in optical density.[5] Computed TomographyModern computed tomography (CT) scanners have AEC systems which aim to maintain image quality for patients of varying sizes, whilst keeping doses as low as reasonably practicable. The systems are also designed to maintain quality with the varying size and attenuation of an individual patient over their length. Implementations vary between manufacturers, some systems are based on a desired noise level in the image, while others are based on a specified reference output (milliampere second, mAs).[6][7] CT AEC systems use the initial "scanogram", a fixed angle planning view, to determine the relative size of the patient, and variation over their length. The tube output is then adjusted for overall size. The output is also typically modulated for each rotation in response to changes in attenuation over patient length. Some systems adjust output during each rotation, which is known as rotational modulation, based on measured attenuation in the previous rotation.[8] AdvantagesBecause patients vary in size and shape, an AEC device is very useful in achieving consistent x-ray film densities, which can be difficult when manually setting exposure factors without AEC.[3]{{rp|130}} DisadvantagesAEC devices are susceptible to operator error (usually due to mispositioned anatomy or having the incorrect AEC chamber selected).[9] Prosthetic devices such as total hip hardware can also cause the selected ionization chamber to overexpose the image receptor. This is due to the absorption of the x-ray beam into the metal of the hardware as opposed to exposing the ionization chamber.[10] References1. ^{{cite journal|last1=Sterling|first1=S|title=Automatic exposure control: a primer.|journal=Radiologic technology|date=1988|volume=59|issue=5|pages=421–7|pmid=3290991}} 2. ^{{cite web|title=Automatic exposure control devices|url=https://humanhealth.iaea.org/HHW/MedicalPhysics/DiagnosticRadiology/PerformanceTesting/Automaticexposurecontroldevices/|website=IAEA Human Health Campus|accessdate=16 December 2016}} 3. ^1 {{cite book|last1=Dance|first1=D R|last2=Christofides|first2=S|last3=Maidment|first3=A D A|last4=McLean|first4=I D|last5=Ng|first5=K H|title=Diagnostic radiology physics : a handbook for teachers and students.|date=2014|publisher=International Atomic Energy Agency|location=Vienna|isbn=978-92-0-131010-1|url=http://www-pub.iaea.org/books/IAEABooks/8841/Diagnostic-Radiology-Physics-A-Handbook-for-Teachers-and-Students}} 4. ^{{cite book|last1=Webb|first1=S|title=The physics of medical imaging|date=2009|publisher=Taylor & Francis|location=London|isbn=9780750305730|page=79|edition=2nd}} 5. ^{{cite journal|last1=Doyle|first1=P|last2=Martin|first2=C J|title=Calibrating automatic exposure control devices for digital radiography|journal=Physics in Medicine and Biology|date=7 November 2006|volume=51|issue=21|pages=5475–5485|doi=10.1088/0031-9155/51/21/006|pmid=17047264}} 6. ^{{cite journal|last1=Söderberg|first1=Marcus|last2=Gunnarsson|first2=Mikael|title=Automatic exposure control in computed tomography – an evaluation of systems from different manufacturers|journal=Acta Radiologica|date=July 2010|volume=51|issue=6|pages=625–634|doi=10.3109/02841851003698206|pmid=20429764}} 7. ^{{cite book|last1=Tack|first1=Denis|last2=Kalra|first2=Mannudeep K.|last3=Gevenois|first3=Pierre Alain|title=Radiation Dose from Multidetector CT|date=2012|publisher=Springer|isbn=9783642245350|page=261|url=https://books.google.com/books?id=Sf2fwu_xkV8C&pg=PA261|language=en}} 8. ^{{cite web|last1=Keat|first1=Nicholas|last2=ImPACT|title=CT scanner automatic exposure control systems|url=http://nhscep.useconnect.co.uk/ShowDocument.ashx?id=254|website=MHRA|publisher=Department of Health|accessdate=4 December 2017|format=pdf|date=2005}} 9. ^{{cite journal|last1=Walsh|first1=C|last2=Larkin|first2=A|last3=Dennan|first3=S|last4=O'Reilly|first4=G|title=Exposure variations under error conditions in automatic exposure controlled film–screen projection radiography|journal=The British Journal of Radiology|date=November 2004|volume=77|issue=923|pages=931–933|doi=10.1259/bjr/62185486|pmid=15507417}} 10. ^{{cite book|last1=Carroll|first1=Quinn B.|title=Radiography in the digital age: physics, exposure, radiation biology|date=2014|publisher=Charles C Thomas|location=Springfield, IL|isbn=9780398080976|page=415|edition=2nd}} Further reading
1 : Radiography |
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