Medical Imaging Flashcards
How are x-rays produced in a modern x-ray tube?
Electrons are emitted from a filament, heated by an electric current and enter a vacuum with an anode (a metal e.g. tungsten) and a cathode (filament) with a high potential difference. When the electrons hit the anode, x-rays are produced.
What are the two ways that x-rays can be produced?
- When fast moving electrons smash into a metal target. As the electron interacts with the electric field around the nucleus, an x-ray photon is produced.
- When a fast moving electron ejects an inner electron in an atom of the target metal. An electron from a higher energy level moves to occupy the vacancy and an x-ray photon is produced.
What is the maximum energy of a photon given by through a potential difference?
E (max) = eV
What is Compton scattering?
The effect whereby an x-ray deflected by interaction with an orbital electron has a longer wavelength than its initial wavelength. The electron is emitted at high speed.
What is attenuation?
The gradual decrease in intensity?
What are the four ways x-rays can interact with matter?
- Simple scattering
- Photo electric effect
- Compton scattering
- Pair production
Explain simple scattering.
When low energy x-rays encounter the electrons in an atom, the energy of the x-ray photon is not sufficient to cause ionisation so the photon is scattered (deflected so its direction changes), but there is no change in energy or absorption of the photon.
Explain the photoelectric effect (x-rays).
x-rays can cause the emission of photo-electrons if the energy of the photon equates or exceeds the work function of the metal surface, meaning the photon is absorbed and the resultant photo-electron has a kinetic energy equal to the photon energy minus the work function.
Explain the Compton scattering (x-rays).
When the x-ray comes in contact with an electron, a scattered x-ray photon is produced as well as a photo electron. Due to conservation of momentum the photon deflected can have a longer wavelength due to it being ejected at a large angle meaning it will have lost energy. For mass and energy to be conserved the electron must have a low kinetic energy.
Explain pair production (x-rays).
In a beam of high frequency electrons, an x-ray photon can interact with the nucleus of an atom producing an electron and positron pair.
What is intensity?
Power per unit area.
What is Computerised Axial Tomography (CAT)?
A process using multiple x-ray scans to produce images of “slices” through the body in one plane, to produce a 3D image.
What is the problem with 2D x-rays?
Traditional x-rays show a shadow image if the part of the body through which the x-rays have passed. Inevitably there will be overlap of images of different organs or bones above or below each other.
How is a CAT scan image produced?
A thin fan-shaped x-ray beam is produced and rotated around the patient and by the time it has completed one complete revolution, bot it and the detectors have moved up one centimetre, this continues and gets fed into a computer, which processes the information to create an image.
What are the advantages of a CAT scan over an x-ray image?
It provides the doctor with a very accurate image of the positions of internal organs without being obscured by other structures in the body. They are particularly sensitive to changes in density and give much better contrast for different soft tissues. The images can be looked at from many angles and the computer technology allows the doctor to remove areas with the density of bone or air by making them transparent.
