18. X-RAY DETECTORS Flashcards
1
Q
- What are various Radiation Detectors designed for?
A
- different purposes within Medical Imaging
2
Q
- List the 5 types of Radiation Detectors.
A
- Film-Based Detectors
- Computed Tomography Detectors
- Detectors used in Fluoroscopic Imaging
- Automatic Exposure Devices
- Detectors used within General Radiography
3
Q
- What is the function of the Image Receptor used in forming a radiograph?
A
IT PLAYS AN ESSENTIAL ROLE IN DETERMINING:
- contrast
- noise level of the image
- spatial resolution
- the patient’s radiation dose
4
Q
- What was the standard choice for recording X-Ray images for decades after the early days of Radiography?
A
- Film
5
Q
- List the 5 advantages of Film Based Radiography?
A
- Extremely good spatial resolution
- Good sensitivity
- Low Price
- Widely available
- Permanency for record keeping
6
Q
- What parts does the Film used in Radiography consist of?
A
- A FILM BASE
- this can be made of transparent polyester
- or it can be made from an acetate sheet
- EMULSION COATINGS
- these capture the images
- the emulsion is a suspension of tiny silver bromide
grains in gelatine
7
Q
- How does Film Based Radiography Work?
A
WHEN THE FILM IS EXPOSED:
- the photons will interact with the grains
- this sensitises the grains
- these grains form a latent image
- this means the image exists but has not yet been
developed
8
Q
- What happens to the sensitised grains?
A
- they are retained
- they are blackened during the process of development
- this leaves those regions to be opaque
9
Q
- What happens to the Unsensitised Grains?
A
- they are removed
- this leaves behind a transparent base
10
Q
- How is the Radiographic image formed when it comes to Film Based Radiography?
A
- it is formed by the resulting pattern of the transparent
and opaque regions of film
11
Q
- Is photographic film sensitive to X-Rays by itself?
A
- no
- even though it has basic processes that allow it to
create a radiograph
NB:
- this sensitivity can be increased
12
Q
- How can the sensitivity of photographic film to
X-Rays be increased?
A
- the film can be placed close to one or two of the thin
intensifying screens of Fluorescent Material - this forms a Film-Screen combination
13
Q
- What do the Intensifying Screens do?
A
- they convert the X-Ray photons to visible photons
- these interact with the motion of the film
- this helps to form the image
NB:
- an increase in the intensifying screens leads to an
increase in the details shown in the photo
- it also leads to an increase in the sensitivity of the
photographic film to the X-Rays
14
Q
- How does the Intensifying Screen system work?
A
- the X-Ray photons hit the intensifying screen
- this emits visible photons
- these photons hit and expose the film emulsion
- Phosphors in the screens will absorb the X-Rays
- they then re-emit their energy as visible light
- the entire system will then be encased in a light-tight
cassette for handling
15
Q
- How can we improve the detection efficiency of Film further?
A
- the X-Ray film can be made with an emulsion on both
sides
16
Q
- Do these images make sense?
A
- yes
17
Q
- Can X-Ray film images be manipulated?
A
- not really
- there are very few manipulations that can enhance the
quality of the images - other than magnification
18
Q
- What are some issues of Film Based Radiography?
A
- FILM IS LIMITED
- in its ability to distinguish between subtle variations
in the absorption of X-Rays
- this restricts its ability to have contrast in the image
19
Q
- Who typically views the X-Ray films?
A
- Radiologists
- they are viewed in transmission
- they are illuminated from behind with a light box
20
Q
- What does Optical Density measure?
A
- it measures the perceived extent of film transparency
and opaqueness
21
Q
- What does Optical Density do?
A
- it measures the extent of film transparency and
blackness in the image
AN INCREASE IN OPTICAL DENSITY:
- leads to the film being more black
- the film is more exposed to the x-Ray
- the film is more transparent
22
Q
- What do the Dynodes in this image allow for?
A
- they allow for the attraction of negative electrons
23
Q
- What happens in the Photomultiplier tube?
A
- the photon is converted to a photo electron
- this photo electron needs to be multiplied
- this will convert it into an electrical signal
- this will form the digital image
24
Q
- What does the Scintillator detector do?
A
- it uses a crystal
- this crystal absorbs X-Rays
- it then re-emits their energy as Visible Light Photons
25
Q
- What happens to these Visible Light Photons?
A
- they enter the Photomultiplier Tube
26
Q
- What happens to these Visible light photons inside the Photomultiplier Tube?
A
- they hit a Photocathode
- this is a device that converts light into photoelectrons
- it does this via the photoelectric effect
27
Q
- What can be said about these Photoelectrons in the Photomultiplier Tube?
A
- they are too few to produce an appreciable electrical
signal
THE OVERCOME THIS PROBLEM BY:
- accelerating at a large voltage towards a positive
electrode
- this is known as the Dynode
28
Q
- What happens when the energetic incoming electrons and the Dynode metal collide?
A
- they free many more electrons
- these newly freed electrons are accelerated to the next
dynode - this further multiplies the signal
- this causes these electrons to collide again
- this then frees more electrons
29
Q
- What happens after many such multiplications between the electrons and the Dynodes?
A
- a very large electrical signal is produced from each
original X-Ray
