Lecture 4: x-ray equipment & specifications Flashcards
What 3 things must intra-oral equipment be, to get it as close to the patient?
- small
- light
- manoeuvrable
What kind of electrical supply do x-ray machines require?
A dual electrical supply (for high voltage and low voltage components)
What is the electrical supply of the filament (cathode)?
- low voltage (12v)
- high current supply (2-3mm thick)
What is the electrical supply of the tube circuit (cathode to anode)?
- high voltage (60-70kV)
- low current (very thick HT cables)
Where are the high and low voltage supplies provided from?
From the mains supply
How is the available mains supply converted to the required voltage?
Using transformers
What does a transformer consist of?
How does voltage change work?
- It consists of two sets of coils (windings)
- Voltage change through a transformer will depend on the ratio between the number of turns on the primary winding to the number of turns on the secondary winding
How are the windings arranged on a step-down transformer?
What is the ratio of coils?
- There are less turns on the secondary winding than there are on the primary winding
- The ratio is 4:1
How are the windings arranged on a step-up transformer?
What is the ratio of coils?
- There are more turns on the secondary winding than there are on the primary winding
- The ratio is 1:4
What does the single tank construction model mean?
Why is this method used?
- All the electrical components are contained within the tube head itself
- It is used to overcome the flexibility and versatility problem dental x-ray tubes have.
What is total filtration made from?
- Inherent (3 components)
- added (1 component)
What are the inherent filtration components of an x-ray tube?
- mineral oil
- boro-silicate glass envelope
- berryllium exit port
What are the added filtration components of an x-ray tube?
-aluminium filter outside exit port
What are the 3 functions of the mineral oil?
1- provide electrical insulation
2-removes heat from the glass envelope to the tube casing through convection
3- filters off low energy photons
What are the legal minimum requirements for filtration?
- Below 70kV there must be 1.5mm aluminium
- At of above 70kV there must be 2.5 mm aluminium
What is mA?
milliamps. It is the tube current.
There is a direct relationship between mA and intensity of the beam.
Double the mA gives double the number of photons, this decreases exposure time and reduces the possibility of patient movement
What will higher mA cause to the image?
It will produce higher contrast images due to a higher intensity of the x-ray beam
What is the relationship between the xray beam and kV?
The intensity of the x-ray beam is proportional to kV²
Small changes in kV will therefore cause large changes in the intensity of the beam
What happens to penetrating power of beam if kV increases?
What type of scatter is produced and how does it affect the image quality?
-Increasing kV will increase penetrating power of the beam so all areas allow the passage of an increased number of photons.
-The scatter produced by xray interactions will be in a forward direction
This forward scatter reduces both contrast and sharpness of the image
How does high and low kV scatter differ?
They both have the same amount of scatter, but their angles differ.
What happens to low kV scatter (60kV)?
It scatters a long way from its original path
it produces wide angle scatter
What happens to high kV scatter (70kV)?
It scatters close to its original path
it produces narrow angle scatter
What are 2 advantages of high kV (70kV)?
- narrow scatter produced, this means there is lower radiation dose for patient
- less exposure time, so less patient movement
What are the 2 disadvantages of high kV (60kV)?
- low contrast
- low sharpness
Why is high kV used to produce x-rays?
Although it reduces contrast and sharpness, the dose reduction is so much less it is worth the low sharpness and contrast
How much less is the x-ray dose from high kV (70kV) compared to low kV (60kV)?
It is 60% less
What does the external collimator of an xray machine do?
It strictly limits the beam to the required area
What does the cone of the xray machine do?
It maintains the correct FFD
It shows the extent of the beam at the patient’s skin surface
What does the rectangular collimator of the xray machine do?
- It reduces the dose to the patient by up to 50% by limiting the total amount of radiation that impacts the patient
- improves contrast and sharpness of image
The ionising radiation regulations require a record is kept of the radiation doses delivered to patients.
What information is required?
- kV
- mA
- filtration
- FFD
- total area exposed
- exposure time
What is the purpose of an OPG machine?
To produce an image of the whole mandible and maxilla with a single exposure in one image
What are the advantages of an OPG?
- it allows easy assessment of the relationship between all of the teeth and other anatomy
- it is less time consuming for the operators
- it is less invasive than whole mouth PAs
What are the disadvantages of an OPG?
- The exposure takes around 15 seconds, this can be a long time for a patient to sit still and can result in movement
- The level of fine detail is much lower than a PA
What is the focal trough set at?
2-3
What is the purpose of the focal trough?
To ensure the anterior teeth 3-3 are in focus
How can the focal trough be adjusted?
- moving the patient in and out of the machine by moving the chin rest
- changing the position of the central pivot/fulcrum point
What is the field of focus like on an OPG machine that has an adjustable focal trough?
they have a narrow field of focus
What is the field of focus like on an OPG machine that does not have an adjustable focal trough?
they have a wide field of focus, to accommodate all bite types
