Image receptors 1 Flashcards
digital receptors are split into 2 types?
- Solid state sensors
- photo stimulable phosphor
solid state sensors fall into 2 categories
CCD which stands for charge-coupled device, and CMOS, which stands complimentary metal-oxide semiconductor, these sensors work by converting light into electrons
Describe PSP or photostimulable phosphor plates
are the type of receptor we use in the department. Images are produced by the interaction of x-rays with the phosphor layer on the plate, which is then scanned by a laser to produce the image. The extra-oral cassettes in the department are also PSP plates
describe phosphor plates
The plate has 2 very different sides, a pale side which has the phosphor coating. The phosphor can be various compounds for example barium fluoride. This is the side on which the image is formed
the other side is black and shows the size of the film and the orientation dot. this is the back of the plate.
the plate is then placed in a protective bag before being used. You should be able to see the black side of the plate through the window of the bag
describe stable excited state
the imaging plate is then placed into the patient’s mouth and the phosphor coating is exposed to x-rays which causes a reaction within the phosphor
describe unstable excited state
the plate is then dropped into the vista scan unit where it is scanned by a red laser. The blue light which is then emitted is collected and then converted to an image
describe latent image
So the phosphor layer on the plate is exposed to x-ray photons.The phosphors then store the radiation in local electron energies.
describe Analogue to digital convertor or ADC
The plate is then inserted into the scanner and the phosphor layer is scanned by a red laser which causes the electrons to drop to lower energy levels by emitting blue light. This light is then detected by a photodetector and sent to a signal digitiser.
describe grey-scale
he ADC converts the light to a digital signal, which is basically a numerical value assigned to each pixel according to the intensity of the light detected. Each pixel’s numerical value corresponds to a different shade of grey which forms the image on the monitor.
describe charge couple device systems
The detector is made up of4 layers, a front cover, a scintillator layer which converts x-ray energy to light, a silicon wafer which converts the light into an electrical signal, and in this particular sensor, a back cover incorporating a cable to carry the signal to a PC
describe the most recent digital detectors
wireless, so do not have to connect directly to the computer. This makes it easier for the patient and the image appears within seconds
describe CMOS system (complimentary metal oxide semiconductor)
is very similar to CCD, the only difference being the photocells are electrically isolated therefore a signal is sent from each individual cell rather than a line of cells
What is the issue with digital images ?
there is no orientation dot. This means when you have many images to look at it could be very easy to turn an image the wrong way and get a bit muddled up
So which system is better?
CCD possibly better quality image ( low noise and more pixels) but CMOS catching up. CMOS cheaper to manufacture. CCD have been around longer therefore possibly more reliable. CMOS uses less power. CMOS - mobile phone camera CCD - high quality digital camera.
How to tell if too much or too little exposure is given to patient from radiographic image?
If the image is too dark this is because too much radiation has been given (high exposure)
If the image is too pale this is because not enough radiation has been given (low exposure)
