Radiography and CT Flashcards
How was the X-ray discovered?
- By accident
- Trying to improve the cathode ray tube by making stronger electric fields to cause electron beams
- Accidently exposed a photographic film that was in the line of the cathode
- Some non-visable radiation more penetrating more than visable light
- Wilhelm Conrad Rontgen (1901)
What did the discovers of X-rays find out that the new radiation could do?
- Penetrate tissues but the body absorbs to varying degrees
- Different levels of exposure on film revealed the internal structure of the body
Where do X-rays sit on the electromagnetic spectrum?
- Very small wavelengths- the shorter the wavelength, the higher the energy it carries
- The ionising radiation has enough energy to harm tissue
What are the components of electromagnetic waves?
Partly electric and partly magnetic
What is the range of a radio wave?
M
What is the range of a microwave?
cm
Outline the components of the cathode ray tube
- Electron beam
- Cathode
- X-rays
- Tungsten anode
How does the cathode ray tube work?
- Pump out the internal air to create a vacuum
- Apply a voltage between the negative anode and positive cathode
- The voltage (when high enough) causes electrons to be dislodged from the cathode and then accelerates toward the anode
- As it accelerates, it suddenly stops as it hits the high density metal material
- All the energy gathered over the path of acceleration is released in an instant
- This energy dissipates as heat and x-rays
What determines the different strengths of the x-ray generated?
The voltage between the anode and the cathode
What are the two ways that x-rays can be produced?
First
- Electron collides with a tungsten atom
- Tungsten atom loses electron from lower from lower orbit
- Electron from higher orbit falls to lower orbit
- Excess energy released as an X-ray
Second
- Nucleus may attract speeding electron
- Electron slows and changes course
- Braking action slows electron which releases excess energy
- Excess energy released as an X-ray
How is the x-ray beam focused?
Tungsten anode geometry is such that the angle creates the x-ray going out the window
The light determines the field of exposure
How is heat produced by the x-ray dealt with?
- Is normally suspended in an oil bath (high density medium to dissipate the heat)
- The tungsten filament is a plate which slowly rotates presenting a different area and preventing overheating
What is penetration in terms of x-rays?
The higher the energy, the easier penetration of the tissue is
There is less interaction with the tissue
What is attenuation in terms of x-rays?
Inverse to penetration
Good penetration means harder to interact with the tissue
Whereas interaction with the tissue means it is absorbed
Attenuation increases with atomic number of exposed material
- Attenuation falls with increasing energy of photons
What happens to penetration and attenuation when there is low energy (soft)?
Allow the visualisation of fatty tissue
What happens to penetration and attenuation when there is high energy (hard)?
Can visualise bones and not tissue
How does analogue imaging work?
- Through film
- The light exposes crystals on the film
- Gets developed and can see areas where crystals have been exposed
How does digital imaging work?
- Have individual pixels
- Sharper image is smaller pixels
- Independent of the size of the pixels (within the pixel is only a single grey (or colour) value)
How does film radiography work?
- X-rays are converted to visible light by an intensifier
- Visible light oxidises radio-sensitive crystals producing a latent
Why did origional film x-rays need longer exposure?
- The images that are generated to visualise film are high energy, high penetration and low attenuation
- X-rays are poor at interacting with thin film so need longer exposure- however this increases an individual’s radiation dose
What was invented to reduce exposure time and radiation in film X-rays?
An intensifier- converts the x-rays into visable light which is good for film exposure and safer
How is film radiography visualised?
- Exposed radiosensitive crystals are converted to black metalic silver
- Unexposed crystals are washed out of the emulsion
- Uses a developer, a fixer and then a washer
What do the light and dark areas represent in film radiography?
- Dark indicate low tissue density (muscle)
- Light indicate high tissue density (bone)
How does digital radiography work?
- A layer of phospher (encased in plastic) is exposed to X-rays
- The phosphor absorbs energy and releases it over time (latent image)
- The intensity is read by a light detector of certain locations and the computer puts the image back together again