Radiology year 4 lecture 4 Flashcards
(41 cards)
What is the predominant method in veterinary medicine to record a radiographic image?
conventional film-screen system
Why can the radiographic image be described as a shadowgraph?
The image is fomed when x-rays pass through a patient and are selectively absorbed by structures within the patient. Any remaining x-rays fall on the film.
What is the film held in and what else does it normally contain and why?
cassette. Intensifying screens- convert incident xray photons to light, which is mainly responsible for exposing the film
Where does the film sit compared to the intensifying screens?
Film sits between them.
Why should the cassette back be more radio-opaque than the front?
The front needs to let xrays pass through whilst the back should be radio-opaque to prevent rays scattered from the table top reaching the film.
What are the back and the front of the cassette normally made of?
fron- aluminium, plastic or carbon fibre. Back- steel with a lead foil lining. The frame is usually steel.
Where do the patient details go on the film?
a “window” in one corner, where a piece of lead prevents radiographic exposure of that area.
What are the standard cassette sizes?
18x24cm 24x30cm 30x40cm 35x35cm 35x43cm
what can happen is a cassette is bent/ dented?
close contact between the film and the intensifying screens can be lost, resulting in unsharpness of the image
What are the layers of the intensifying screens (from closest to film to closest to entering xrays) and what are they made of?>
Supercoat- protective cellulose acetate layer
phosphor layer- contains either: calcium tungstate (emits blue light- use a monochromatic-blue sensitive-film)
barium lead sulphate
Gadolinium oxysulphide (emits green light- use a orthochromatic film)
lanthanum oxybromide (clue)
yttrium tantalate (UV)
First two aren’t used as much, last three are “rare earths” and are more efficient (4x more) and allow increased contrast as less scattered light is produced.
Reflecting layer- layer of whit pigment which reflects light from the phosphor towards the film (only present in high speed screens)
Base- polyester (inert, stong, stable and radiolucent). `
What is the phosphor on the intensifying screen for? and why?
absorbs electromagnetic radiation of a short wavelength and re-emits it at a longer wavelength (converts xrays to light). The amount of xrays needed to produce a visible image is vastly reduced in comparison with exposing a film whithoug screens and therefore the xray doses and exposure times are greatly reduced.
what deterimines the “speed” of the intensifying screen?
type of phosphor used and size of crystals. Increased speed with increasing phosphor size but reduced resolution.
what is the intensification factor?
The ratio of exposure without screens to exposure with screens required to produce the same density radiograph.
how long should well cared for intensifying screens last?
10 years
What layers are a double sided emulsion film made up of and what are they made of?
Base (in the middle)- usually polyester, provides support for the emulsion layers
substratum- adhesive layer made from a mixture of gelatine and film base material- sticks emulsion film to base
emulsion- silver halide (mostly silver bromide) suspended in gelatine.
Supercoat- thin layer of pure hardened gelatine to protect the emulsion
What additives to the film emulsion may there be and what are they for?
Colour sensitizers: extend the spectral sensitivity of the silver halide.
hardeners: make the emulsion more resistant to damage
bacteriocides and fungicides
wetting agents: overcomes the natural repulsion of gelatine for water, allowing the processing chemicals to penetrate the emulsion.
What is a single-sided emulsion film used for and why and what extra layer does it have?
mammography cassettes, duplicating film, multi-format cameras. Allow image of greater resolution. have anti-halation layer between the base and the emulsion to prevent light being reflected back into the emulsion from the base.
how does a non-screen film work and what can it be used for?
Use without intensifying screens. The image is formed solely be xrays. Emulsion layers are thicker as xrays penetrate deeper than light but unsuitable for automatic processing. used in dosimetry.
what type of image does a polaroid film produce and how is this different from conventional radiographs? What are the advantages and disadvantages?
produces a positive image viewed with reflected light. (normal produce a negative image with transmitted light). Adv: processingfacilities are not needed Disadv: reversed image difficult to view unless used to it and resolution limited.
What is the film speed determined by?
size and amount of silver halide grains. larger grains give greater speed but less resolution. a fast film will become blacker for a given exposure than a slower film.
How should an xray film be stored?
10-18 degrees celcius, no more than 50% humidity, no sudden changes in either. Protected from fumes of processing chemicals and ionising radiations. boxes should be stored on edge (not on top of each other). xray film deteriorates with age so small amount stored at once.
What are silver halides naturally ssensitive to?
xrays, UV, visible light up to blue.
what do colour sensitisers do to films?
extend the sensitivity of the film (eg up to green light). safe lights must be not too close to the sensitive region.
what is a monochromatic film sensitive up to?
blue in visible spectrum- safelight yellow, orange, red
