film imaging and processing Flashcards Preview

radiology BODC year 2 > film imaging and processing > Flashcards

Flashcards in film imaging and processing Deck (31)
Loading flashcards...
1

image receptor (film)

Protective coat
emulsion
adhesie coating
base
'repeat'

2

protective coat

transparent layer (usually gelatine) to protect emulsion

3

emulsion

sliver halide crystals held in gelatine suspension
-reacts to light

4

geletine

increases sensitivity of crystals
provides even suspension for crystals

5

adhesive layer

sticks emulsion to base

6

base

translucent blue plastic tp make the structure sturdy

7

image receptor screen

protective layer
reflective layer
phosphor layer
protective layer x ray film
...

8

protective layer

transparent layer to protect phosphor

9

reflective layer

reflects light back towards film
- increases sensitivity
- allows reduction of the dose due to scatter

10

image receptor screen interection steps

An x ray photon will likely interact in the phosphor layer above or below the film converting its energy to may light photons
- the photons are emitted in all directions (might hit reflective layer or not)
- those that head towards the film may activate the silver halide in the emulsion
- as the photons spread out from the interaction point they will cause a larger area of blackening
- this reduces the resolution of the system

11

what does speed depend on

thickness of phosphor layer in screen
crystal size in emulsion
conversation efficiency
light absorbing dyes within screen

12

image formation steps

When an x ray photon hits a crystal in the emulsion an electron from an Br ion gets enough energy to escape
- the electron moves to a defect in the crystal or to a Ag2S impurity known as a sensitivity speck
- this becomes – charged and attracts a mobile silver ion from within the crystal and to form an silver atom
- this Ag atom then acts as a trap for a second electron and the process is repeated
- by this small deposits of silver will have formed those crystals hit by an xray photon
- these crystals will then be more sensitive to the developer and the whole crystal will be blackened (not just the silver atoms)
can vary the crystal size – more dose by using smaller more crystals compared to larger crystals

13

developer characterisics

alkali
acts as reducing agent
converts remaining silber ions to silver atoms (adding e-)

14

what does washing do

stops developer working on film

15

fixing

acid
- removes untreated silverhalid crystals
also hardens the gelatine suspension
fixes crystals in position

16

latent image fading

Silver atoms may revert to silver ions
- can happen if not processed straight away
- optical density may fall by 10-20 percent in 24 hours

17

factors which affect radiographic image

1) contrast
- depends on subject and film
2) characteristics of the x ray beam
3) image geometry
4) image sharpness and resolution

18

factors which affect subject contrast

1) different target/filter combinations
2) reducing the beam penetration/tube voltage
- how easy to get through substance
- harder beam goes through everything lose contrast
- too soft a beam, give lots of radiation dose to soft tissues close, will need even bigger to reach deeper tissues
3) using contract media
- can be used
- not common
- something into soft tissue so it can be imaged
4) use of scatter rejection grids or air gap
- allows scatter from patient to spread out before interacting with the film
- should get less scatter of what you don’t want

19

film contrast

dependant on sensitivity on filter ray
narrow latitide, small change in dose big change in optical density

20

what is fog and what does it lead to

blackening of film from natural processes
fog leads to a smaller ratio, therefore reduces the contrast by having great layer over image

21

film speed

speed defined as change in optical density for change in exposure

22

ideal characteristics of x ray beam

Ideal characteristics
- parallel
- produced from a point source to reduce penumbra
- sufficiently penetrating to pass through to a varying degree

23

what is the beam held at and why

parallel to avoid distortion

24

focal spot size

can't have a small focal point as you need to dissipate the heat or a long time to do it slow (patient will move)

25

image geometry

Object and film should be
- parallel
- perpendicular to the beam
- close together

26

image sharpness types and resolution

1) Geometric unsharpness
- combination of penumbra and patient position
2) motion unsharpness
- blurred photo, worse with longer exposure photos more time to move
3) absorption unsharpness
- lots of tissue = lots of scatter
- reduces resolution on the film
4) screen unsharpness
- reduce resolution, good for dose
5) resolution of the film
- crystal size change to provide better or worse resolution at the cost of dose

27

issues affecting image quality

dark
pale films
opaque film
marks on film

28

how does computers radiography work

photo stimulate phosphors instead of filmq

29

how does compuersied radiograph work steps

- hits atom in image plate
- frees an e
- jumps into a crystal defect and stays there for a short while
- red laser makes it drop back down to the level
- blue light given off
- detected
- bright white light then erases it
- reusable

30

types of detectors that can be used in computerised

amorphous silicone
amorphous selenium