X-ray Flashcards
glass tube
It is sealed and evacuated. It allows electrons to travel unimpeded
rotating anode
Rotation of the anode spreads out the heated area on the target It is made of metal (tugsten or molybdenum) and emits X-rays when hit by (energetic) electrons
filament [or cathode]
It is the heat source necessary for the release of electrons from surface of cathode by thermionic emission
lead housing
It prevents X-rays from escaping in unwanted directions
Explain why the anode is rotated
The anode is rotated so that the heat produced is spread over a greater volume/area/section This allows more energetic X-rays to be produced [or allows X-rays to be generated for longer] without risk of melting the anode.
Explain why the anode has a bevelled edge.
The bevelled edge gives larger target area for the electrons
but a smaller source area from which the X-rays are produced. This gives a sharper image beam of X-rays and produces a sharper image.
Define for a material, the linear attenuation coefficient
The linear attenuation coefficient, is the fraction of X-rays removed per unit thickness of the material.
Define for a material, the half-value thickness.
The thickness of the material which will reduce the intensity to half its original level for a specified energy of the X-rays
State the purpose of the fluorescent screen,
It converts X rays to visible photons
state the purpose of photocathode
It converts photons to emission of electrons
State the purpose the anodes,
They increases the kinetic energy of electrons travelling from cathode to anode and they also focus the rays of electrons to produce a faithful image
state the purpose he fluorescent screen
It converts (increased) electron energy into light photons
Give one example of a medical application for which an image intensifier might be used. Explain why the use of an image intensifier is required.
It might be used for a dynamic process - such as fluid flow It cuts the radiation dose whilst still providing good image [or allows multiple or continuous use of X rays]
State two methods which can be used to increase the intensity of the X-ray beam produced by the tube.
Method 1: increasing the potential difference (voltage) across the tube
Method 2: increasing the tube current or increasing the filament temperature
For each method of increasing intensity, state the effect on the maximum X-ray photon energy.
(Method 1: increasing the potential difference (voltage) across the tube)
Method 1: will increase the maximum photon energy
(Method 2: increasing the tube current or increasing the filament temperature )
Method 2: will not change the maximum photon energy
Before taking an X-ray photograph, the X-ray beam emerging from the tube is passed through an aluminium filter. State and explain the reason for filtering the X-rays
The aluminium filter reduces the intensity of low energy photons but hardly changes intensity of high energy photons at all. We need high energy for the X-ray picture production [low energy tend to be attenuated by the body] Therefore reducing low energy photons reduces the dose received by patient without affacting picture quality.
When an X-ray image is obtained of certain organs, image contrast enhancement is necessary. Explain why image contrast enhancement is needed and describe how this might be achieved.
For a clear image you need a large difference in densities between part being investigated and parts around it. When this is not natural, you can add material to the part under investigation which has high density to provide good attenuation of X-rays An example of a suitable contrast material for a barium meal is barium sulfate
In the course of diagnosis and treatment of a child’s broken arm, several images of the arm are required.
In each case, state which imaging technique would probably be used and give two reasons for the choice.
(a) Broken arm
You would use an X-ray because an x-ray image gives:
a sharp image
with good contrast
and good resolution.
Similarly, to check the progress of a woman’s pregnancy, several images of the foetus are required.
(b) Foetus
You would use an ultrasound scan because ultrasound:
is non-ionising and therefore safe to use on the unborn foetus,
detects change in tissue type and so allows the development to be monitored and
allows real-time imaging to check on heart development and movement.
Explain why the spectrum has spikes at specific photon energies.
The spikes are specific to the anode target element. The energy level transitions of electrons within the atoms are fixed. That is why they are called characteristic spikes.
scintillator crystal(s)/fluorescent screen
It converts X-ray photons into light photons
photocathode
Incident light energy on the photocathose releases electrons. The number of electrons released is proportional to the X-ray intensity
anodes
They increase the kinetic energy of the electrons by accelerating them towards their positive charge. The are also positioned to focus the electrons to form an image
fluorescent screen
This converts the kinetic energy of the incident electrons into light photons
Explain what is meant by the half-value thickness of lead for X-rays.
The half-value thickness is the thickness of lead needed to reduce intensity of the X-rays of a specific energy by half
The X-ray spectrum for a certain X-ray tube target is shown below. Explain the process which gives rise to spikes at certain photon energies.
Electrons strike the metal anode and ionise/excite the target atoms. This causes vacancies in inner electron levels.
Excited/higher electrons fall to inner/lower energy level states and the change in energy is released as a electromagnetic photon. Most of these photons are in the infra-red energy range but some are of X-Ray energy.
The gaps between the energy levels in a particular atom are fixed. Therefore the transitions that produce X-rays will be fixed for a particular target element. This is why the X-rays produced will be of a certain energy - these X-rays form the characteristic peaks.
Explain how the intensifying screens in the film cassette achieve their purpose and state their benefit to the patient.
Most X-rays pass through the cassette and do not interact with it at all. Including the two intensifying screen layers as well as the film increases the chance of X-ray interaction being detected. Therefore the use of such screens reduces the radiation dose to the patient by making the exposure time shorter.
X-rays that interact with the film cause exposure. The intensifying screens have a fluorescent coating; this converts any X-ray photons that interact with it into light photons. The light photons produced, that are emitted in the direction of the film, expose the film in the correct place due to closeness of the screens to the film. You therefore do not need such a big dose of X-rays to achieve the required exposure.
X-rays are used in a CT scanner. Describe briefly how a CT scanner produces an image
Narrow fan-shaped beam of X-rays
X ray generator rotated (in circular path) around patient on a ring
Detectors are positioned on the ring opposite the generators in an arc long enough to receive the fan of X-rays.
The detectors register the transmitted intensity from a single blast of X-rays
The transmitter and detectors are then rotated so that the next section can be scanned. This is repeated while the body slowly moves on a motorised platform. In this way a spiral scan pattern is obtained.
Detector output is sent to a computer which (over time) produces a series of cross sectional images that can be analysed
Explain the term contrast enhancement when applied to X-ray photographic imaging and explain how such enhancement is achieved
The clearest images are obtained when there is a large difference in density/proton number between the part of the body being investigated and the surrounding parts.
You can improve contrast by introducing a high proton number material into a bady cavity, such as barium sulphate - this is done as a ‘barium meal’ or ‘barium enema’.
What is meant by half-value thickness?
Half-value thickness is the thickness of material needed to reduce the transmitted intensity of the X-rays to half of the intensity incident on the material.
