week 7 Flashcards
what is detective quantum efficiency (DQE)?
measure of how well x-rays are transferred into an image
DQE = 1.0
means 100% of the incident x-rays are used to produce an image
what are two types of gas detectors and how do they work?
ionisation chambers, xenon gas detectors
in a cavity of gas (air/xenon), interactions occur with photons and gas particles to produce electrons and thus a charge, voltage is applied and charged particles move to their respective electrodes, electrons attract to the anode and this forms the electrical signal proportional to the amount of ionisation in the chamber
how do scintillator detectors work?
solid detector, scintillation crystal absorbs x-rays and emits a light, which is then converted into an electrical signal by the photocathode, amplified by photo-multiplier tube or photodiode
describe the electronic band theory of solids
electrons have an energy set by band number, this causes a band gap between energy levels, there is a band gap between valence shell and conduction band (free electron), called forbidden band
conductors have very little band gap, semi-conductors have a little band gap, insulators have a large band gap
describe computed radiography (CR)
x-rays interact in a phosphor screen, electrons are raised into the conduction band energy level and try to de-excite to valence energy but become trapped in the forbidden band
red light is scanned over the screen allowing electrons to drop down, thus causing them to emit a blue light which is used to create an image
what is digital fluoroscopy?
creating a real-time image, CCD cameras and image intensifiers were used but replaced by flat panel technologies, early flat panel technologies had lag but improvements were made with recent systems
describe indirect and direct digital radiography
indirect radiography
- x-ray hits a scintillator
- electron - hole pairs produced
- fluorescent light produced (excess energy- electrons gain energy to rise to conduction band then drop to valence band)
- light detected by thin film transistor (TFT) or charged coupled device (CCD)
- light converted to electric signal
- processed into digital image
- good for fine detail, less efficient due to light scatter, slight delay
direct radiography
- x-ray interacts with amorphous selenium/silicon semiconductor
- high voltage applied, electrons -> electrodes
- charge is stored in TFT and read out
- some areas will be taken up with electronics
what is automatic exposure control (AEC) in ion chambers?
receptor behind patient measured exposure (number, energy of x-rays) and when desired level is reached, feeds back to x-ray tube and it switches off
