Radiology 4 Flashcards
Image receptor
the technical term for the item the x-ray image is formed on, so it could be a film, plate or digital sensor.
Fogging
Photons that don’t have enough energy to make a useful interaction with the receptor and this scatter can degrade the image.
2 ways to capture an x-ray image
film and digital
Two types of film
direct action film
indirect action film
Two types of digital
solid state sensors
photo-stimulable phosphor
Direct film
dental intra-oral. x-rays act directly on silver halide crystals in film emulsion. is an actual piece of film which is sensitive to x-rays that is wrapped in a packet. You still may come across direct action film in some general dentist practices.
indirect film
extra-oral. light from intensifying screens act on silver halide crystals in film emulsion. a film inside a cassette which contains intensifying screens. The x-ray photons react with the intensifying screens to produce light photons, which expose the film to produce the image.
digital solid state: two types
- solid state CCD (charge-coupled device)
- CMOS (complimentary metal-oxide semiconductor) sensors
digital solid state sensors: how do they work?
These sensors work by converting light into electrons. The information is then collected by a computer programme and transformed into the image.
digital PSP (photo-stimulable phosphor)
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.
Film image processing
processor with bottles of chemicals
Digital image processing: solid state
OPT machine
Digital image processing: PSP plates
vista scan towers
What is this?
Solid state sensors for intra-oral use (two silver plates on black cables).
What is this?
Large floor standing processor for the cassettes we use for extra-oral radiography
Main differences between PSP and solid state? (3)
- Solid state requires use of a specific holder, PSP can be used with existing film holders.
- Solid state is not tolerated well by patients (bulky), PSP is tolerated well by patients.
- Solid state shows better image resolution that phosphor plates.
What side is this of the phosphor plate?
phosphor coating (barium fluoride) - this is the side on which the image is formed.
What side is this of the phosphor plate?
Shows the size of the film and the orientation dot. This is the back of the plate.
What is this?
Protective bag for phosphor plate (you should be able to the see the black side of the plate through the window of the bag).
PSP image processing stages (3)
- Imaging plate + x-ray = stable excited state
- Stable excited state + red laser (vista scan unit) = unexcitable state.
- Unstable excited state + blue light emitted = converted to an image.
Stable excited state
When the imaging plate is placed into the patient’s mouth and the phosphor coating is exposed to x-rays which causes a reaction within the phosphor.
Latent image
The phosphor layer on the plate is exposed to x-ray photons. The phosphors then store the radiation in local electron energies.
What happens during the scanning with red laser?
the phosphor layer is scanned by a red laser which causes the electrons to drop to lower energy levels by emitting blue light.
How is the emitted blue light (after the red laser) processed to an image?
This light is then detected by a photodetector and sent to a signal digitiser. This is called an Analogue to digital convertor or ADC.
How does the ADC work?
The 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 (grey-scale).
The plate is erased by exposing the phosphor to bright light which releases any remaining trapped energy in the phosphor electrons, and the plate is then ready to reuse.
A (image of CCD)
Back cover incorporating cable to carry signal to PC
B (image of CCD)
Silicon wafer
(converts light to electrical signal)
C (image of CCD)
Scintillator layer (converts x-ray energy to light)
D (image of CCD)
Front Cover
How does the CCD work?
- X-rays hit the scintillator layer, which is either caesium iodide or gadolinium oxysulphide, and the reaction gives off light which hits the photosensitive cells within the silicon wafer.
- Electrons within each cell are released and these form a charge (voltage) which is converted to the image.
- Each cell is connected to its neighbour and the signal converted to the image is from a line of neighbouring cells.
What is the only difference between CCD and CMOS system?
The only difference being the photocells are electrically isolated therefore a signal is sent from each individual cell rather than a line of cells.
Why might it be more difficult to orientate a digital image?
There is no orientation dot present.
Key differences between CCD and CMOS (4)
- CCD better image quality than CMOS (CMOS = mobile phone camera vs CCD = high quality digital camera)
- CCD is more expensive to manufacture than CMOS.
- CCD possibly more reliable as has been around longer than CMOS.
- CCD uses more power than CMOS.
Why is digital better than film?
- No chemicals.
- Faster processing time (15 seconds compared with 90 seconds)
- Image stored onto computer (less chance of it getting damaged/lost, can easily email it etc).
- Lesser radiation dose.
Why is digital NOT as good as film?
- Expensive and easily damaged.
- Reduction in resolution due to pixel size.
- Relies on technology working (power cut/file deletes etc)
What is wrong with this PSP?
The PSP image has several scratches most likely from constant wear and tear from passing through the scanner, but it also has a white edge down the left hand side. You may initially think that the x-ray tube has not been lined up properly, but as the edge is wavy and not straight you can tell it is actually the phosphor layer starting to peel away from the base.
What is wrong with this solid state?
The solid state image has a dark area in the corner suggesting it has sustained damage, possibly from being dropped.
Exposure: too dark
Too much radiation has been given so high exposure.
Exposure: too light
Not enough radiation has been given so low exposure.
What’s wrong with this?
It’s flat, very few shades of grey, no contrast. This is because the high radiation has caused too much light to be produced in each pixel, which means there is little variation in the electrical signals which are converted to form the image.
What’s wrong with this?
Too low an exposure and you are left with white mottling on the image. If not enough radiation has hit the receptor, no reaction to produce light can occur, therefore no signal from that pixel will be produced and the computer cannot guess and make better what is just not there.
How are digital images stored?
Local server (PC)
Need a form of backup - CD, DVD, remote hard drive.
What is the archive system called used in DDH?
PACS (picture archiving and communication system).
Where is the national archive for images located?
Livingstone.
What does the unique accession number for images taken at DDH always start with?
T113H
