AUTOMATIC EXPOSURE CONTROL
The AEC , also called phototimer AEC measures the actual amount of radiation exposure incident on IR and terminates the x ray production when required amount is obtained.
FACTORS AFFECTING EXPOSURE CONTROL
- kVp
- mAs
- Focal spot
- Filtration
- Focus-film distance
- Collimation
- Table-top attenuation
- Grid
kVp
kVp stands for kilovoltage peak.
It is a peak potential applied to the x ray tube which accelerates the electron from cathode to the anode resulting in the production of the x ray photons
The quality of x ray beam depends on the energy of x ray (kVp)
Higher the kVp greater the quality of x ray. It also effect the intensity of x ray beam.
mAs
mAs stands for milliampere second
The number of photons in the beam per unit energy is describe the quantity of x ray
Increase the current to the filament of x ray tube increase the number of photons and increase the quantity more electrons means more x ray production
Product of ma & second is called mAs
More mAs result in more radiation dose .
Focal spot
The area of target with in which the electrons are absorbed and x rays are generated is called focal spot
Focal spot is also called focal area
If the focal area is very small, penumbra will be lesser, and the picture sharpness will be good , but heat removal is difficult.
On the other hand
If the focal area is large , heat will be removed quickly , penumbra is larger and the picture sharpness is bad
Usually , focal spot is defined in two ways namely, actual and effective focal spots. The actual spot size is the area on the anode that is struck by electrons. The effective focal spot size is the length and width of the emitted x ray beam
FILTERATION
Filters are thin sheet of material (Al,Cu,Mo), which offer high attenuation for low energy photons.
The purpose of using filter is to reduce patient exposure at the skin level
Filters alter both the quality and quantity of x rays by selectively removing the low energy photons in the spectrum
Unit is mm AL equivalent
X rays are polychromatic ( with different energy)
Low energy photons get absorbed because it increase patient dose and reduce image quality
Focus-film distance(FFD)
The distance between the x-ray tube and the film. Increasing or decreasing this distance will have an effect on radiographic density
Increasing film-focus distance(FFD) from the traditional 100cm has been shown to be an effective method of reducing dose and image quality maintain
Also know as source-image distance(SID)
COLLIMATION
A collimator is a metallic barrier with an aperture in the middle used to reduce the size of the x-ray beam , thus, the volume of irradiated tissue within patient is also reduced.
Collimator is made from lead
Maintains the quality of image
Main Functions
Collimators adjust the size and shape of x-ray beam
Reduces the patient dose
THERE ARE FOUR TYPES OF COLLIMATOR
1.Parallel-hole collimator
2.Pin-hole collimator
3.Diverging
4.Converging
TABLE-TOP ATTENUATION
Various materials and thickness are used in the construction of the radiolucent top of the x-ray couch that supports the weight of the patient.
Since different designs vary in the amount of attenuation they impose on the x ray beam, radiographing a patient through different couch tops introduces an additional exposure variable which the radiographer may wish to consider.
Current thinking favours the use of carbon-fibre technology, which reduces beam attenuation and helps to minimize the radiation dose to the patient (Hufton & Russell,1986).
GRID
Grid are placed between the patient and the x-ray ray film to reduce the scattered radiation reaching the detector and thus improve image contrast .
The radiographic grid consists of lead foil strips. Invented by Dr.Gustave Bucky in 1913 Most effective way of removing scatter radiation from large radiographic fields The interspaces of grids are filled with aluminium or some organic compound
GRID RATIO-Ratio between the height of the lead strips and the distance between them Grid Ratio=t/d
Types of grid
1.Stationery grid:-
a. Parallel grid
b. Focussed grid
c.Crossed grid
d.Linear grid
2.Movable grid/potter bucky grid
Parallel grid
1.The lead strips are mounted parallel to each other
2.All lead strips are straight up and down
3.Less commonly grids only reduce scatter in the direction of the grid lines
4.Used for short fields size and long distances 2.Focused grid
focused grid
a.Focused grid is made up of the lead strips that are slightly towards the focal spot
b.A focused grid may be linear or crossed
c.The grid lines are angled to match the divergence of the beam
Crossed grid-
Crossed grid are very efficient in removing scatter radiation
Linear grid –
The lead strips are parallel to each other in their longitudinal axis
Movable grid/potter bucky grid
a It was invented by Dr. Hollis potter in 1920 so it is called a potter bucky grid
b.A moving grid is a device, in which the grid oscillates while the x ray is being taken
c.The moving grid moves continously by the electric motor during the exposure
CALCULATIONS FOR VARIOUS RADIOGRAPHIC PROCEDURES
FILM FACTOR(FF)
Amount of exposure(R) per unit area of the film required to produce a desired optical density
HALF VALUE THICKNESS(HVT)-
The thickness of a material, usually called an absorber, needed to reduce the intensity of radiation to half its initial value is known as half value thickness.
SOURCE TO FILM DISTANCE(SFD)
SFD-source-to-film distance –is the distance between the radiation source and the film in radiographic testing, as measured in the direction of the beam
SFD=SOD+OFD
a.OFD-object to film distance=is the distance between the radiation source and the radiation side of the test object, measured along the central axis of the radiation beam
b.SOD-source to object distance= is the distance between the radiation side of the test object and the film surface, measured along the cental axis of the radiation beam
HL(HALF-LIFE)
Each isotope has a half life- a length of time by which half (50%) of the radioactive isotope has decayed into a stable element –two half-lives mean the source has only 25% of its original strength, three half-lives 12.5%
RHM-Roentgen(unit of exposure) per hour at 1 meter for the source being used
