X-rays Flashcards
Describe briefly how x-rays are produced in an X-ray tube (3 marks)
- Electrons are accelerated through a high voltage
- (High speed) electrons hit a metal / an anode
- Kinetic energy of the electrons produced / is transferred into x-rays / photons
Which terminal/electrode produces the electrons and by which technique are electrons produced
The cathode is a heater, which produces electrons by thermionic emission
What material is the anode usually made of and why?
Tungsten, has a high melting point
What are the 4 attenuation mechanisms? What are their ranges?
- Simple/Elastic/Thompson scattering: 1-20 keV
- Photoelectric effect: Below 100 keV
- Compton/Inelastic scattering: 0.5-5 MeV
- Pair production: greater than greater than 1.02 MeV
Describe Simple/Elastic/Thompson scattering
The X-ray photons interacts with an electron in the atom, but has less energy than the required to remove the electron.
So the X-ray photon simply bounces off (is scattered) without any change to its energy
Describe Photoelectric effect using x-rays
The X-ray photon is absorbed by one of the electrons in the atom.
The electron uses this energy to escape from the atom.
1 to 1 relationship
Describe Compton/Inelastic scattering
The incoming X-ray photon interacts with an electron within the atom.
The electron is ejected from the atom, but the X-ray photon doesn’t disappear completely.
Instead it is scattered with reduced energy
In the interaction both energy and momentum are conserved
Describe pair production
An X-ray photon interacts with the nucleus of the atom.
It disappears and the electromagnetic energy of the photon is used to create an electron and it’s antiparticle, a positron
Why are contrast mediums used
Soft issues have low absorption coefficient therefore it can be hard to differentiation between different internal structures
Mark scheme :
- Barium / iodine
- (Contrast medium absorbs X-rays because it) has large attenuation coefficient / has large absorption coefficient / has large Z values
- Ideal for imaging the outline (of soft tissues
What can be used as a contrast medium and why
Barium and iodine both have large atomic numbers, Z
Attenuation coefficient, μ is proportional to depends on Z^3 (μ ∝ Z^3). Average in soft tissue Z=7
This means that iodine and barium have a much large attenuation coefficient.
Iodine is used as a contrast medium in liquid E.g blood flow
Barium Sulfate is often used to to image digestive systems. It is given to a patient in the form of a white liquid mixture
Apart from imagery, what can X-rays be used for?
X-rays are also used for therapy. Specialised X-ray machines are used to create high energy X-ray photons which are used to kill off cancerous cells.
What does a CAT scan stand for
Computerised axial tomography
Describe how the components of a CAT scanner are used to produce a three-dimensional
image of a patient
- X-ray tube/source and X-ray detector rotate around (the patient) / X-ray beam passes through the patient at different angles
- A thin X-ray beam is used
- Image(s) of slice(s) / (cross) section(s) through the patient are taken
- Idea of absorption / less gets through / more is absorbed …
- by dense material / bone / material of high Z / High Z related to materials such as bone / Low Z to materials such as soft tissue
- attenuation is by the photo-electric effect
- the possibility of using a contrast medium.
- X-ray tube moves / spirals along (the patient)
- a computer (analyses the data) / identifies the position of organ/bone and forms a 3-D image.
What are the advantages and disadvantages if CAT scans
Advantages:
- Can create 3D images of the patients, therefore helping doctors assessing the shape, size, and position of disorders (such as tumors)
- CAT scans can distinguish between soft tissues of similar attenuation coefficients.
Disadvantages:
- Single traditional X-ray scans are quicker and cheaper
- X-rays are ionising radiation and as such are harmful. CAT scans can be prolonged and so expose the patients to a radiation dose equivalent to several years of background radiation, much more than a simple X-ray
- Patients have to remain very still during the scanning process otherwise the slice will blur
In order to take an X-ray photograph, the X-ray beam is passed through an aluminium filter to safely remove low energy X-ray photons before reaching the patient.
Suggest why it is necessary to remove these low energy X-rays.
Low energy X-rays are absorbed by the skin / undesirable as can cause damage /greater ionising
