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RT202 – PRINCIPLES OF IMAGING LESSON 1:
LESSON 2: Technical Factors in Radiography “MILLIAMPERES” |
Milliamperes (mA)
− | the current flow through the cathode filament during exposure. |
− | quantitative factor |
− | changes photon quantity |
− | factor of choice for controlling film blackening |
− | number |
↑ mA
= ↑ current flow
= ↑ temperature of the filament = ↑ number of electrons released (thermionic emission)
= ↑ number of x-ray photons produced at the anode target = ↑ patient absorption
Effect of ↑mA
− produces a greater number of electrons in the space charge for x-ray production
− purpose is to increase the number of electrons traveling from cathode to anode, thus
producing more x-ray photons
mA & Blackening
− film blackening is proportional to mA
mA doubled = blackening doubled
mA halved = reduce the blackening by half
− a change of at least 30-35% (minimum) is needed to produce a visible change in blackening
x2 & ÷2 (standard)
− can be adjusted without significant change in scatter radiation
Focal Spot “Blooming”
− ↑ tube current (mA) can ↑ actual focal spot size : focal spot blooming
− ↑ kVp = slightly ↓ actual focal spot size
Focal Spot Size and Radiographic Quality
− | focal spot size is crucial for producing highquality radiographs. |
− | blooming caused by high tube currents can adversely affect radiograph quality |
− | reducing focal spot blooming: |
ü lower mA
ü higher kVp
Exposure Time (s)
− sets the length of the time for an exposure.
− combined with mA, determines the exposure rate
formula: milliamperes x time = milliampereseconds
(mAs) mA x s = mAs
mA and Time
− | inversely proportional |
− | ↑mA requires a ↓exposure time to maintain the same exposure value and amount of blackening on the film |
− | mAs is considered a single quantitative factor |
− | ↑ mA = ↑ amount of x-ray photons exposing the film |
Reciprocity Law
− Arthur Fuchs
− states that blackening on the film remains constant as long as the total energy exposing the film is constant
− proper calibration of equipment is necessary for dependable results
− fast film-screen systems (e.g., rare earth) affect results
LIMITATIONS
§ screen exposure of less than 10 milliseconds and more than 6 or 7 seconds
§ considered a quantitative factor § short exposure times prevent body motion during exposure
§ longer exposures increase the risk of motion, detrimental to film quality
Distance
(Focal-Film Distance/FFD/SID)
− represents the length of space from the focal spot to the recording medium
General radiography: 40 to 42 inches
Thoracic cavity radiography: 72 inches
− | intensity of the x-ray beam is affected by changes in FFD |
− | similar to visible light, intensity decreases with distance as x-rays spread over a larger area |
− | x-rays diverge from the focal spot, so |
intensity↓ as distance ↑
SID (Source to Image Distance)
SOD (Source to Object Distance)
OID (Object to Image Distance)
Inverse Square Law − ¼ of its original − describes how the intensity of radiation decreases with increasing distance from the source − doubling the distance reduces intensity to ¼ because the same photons spread over four times the area
LESSON 3: Technical Factors in Radiography “RADIOGRAPHIC QUALITY” |
Quality
− characteristics of an object and indicates the degree of excellence an object may exhibit
High – Quality Radiograph
ü all factors are at optimum
ü contains maximum information for review
ü demonstrates specific anatomic part with a sharp image.
ü appropriate amount of blackening
(density) distributed over the film.
ü excellent subject contrast with sufficient distribution of film blackening
ü recorded with minimal magnification and distortion of actual size and shape of the anatomic part
Radiographer’s Role
− evaluate radiograph quality through visual inspection
− “art” of radiography involves working with concepts and characteristics on a finished radiograph to measure excellence
Quality Evaluation Factors o density o contrast o definition o distortion
– controlled by technique selection and use of radiographic accessories
Density
− | blackening on the finished radiograph |
− | produced by the development of exposed silver halide crystals in the film emulsion |
− | mAs (primary controlling factor) |
− | kVp (influencing factor) |
Contrast
− | represents variation in density levels across the film, making anatomic detail visible |
− | distribution of different density values |
− | functions to make structural detail visible |
− | refers to a range of density levels from very dark (black) to areas with little or no density (clear or white) |
− | kVp (primary controlling factor) |
↑ contrast = ↑ degree of blackening to the film = ↓scale
Definition
− refers to the clarity and distinctness of the fine structural lines or borders of anatomic
parts
− recorded detail
− resolution
FACTORS AFFECTING DEFINITION o selection of Focal-Film Distance (FFD) o focal spot size
o placement and alignment of tube, part, and
film
o amount of exposure to the film o use of adequate kVp
o use of accessories to control scatter radiation
Distortion
− misrepresentation or undesired change of the size and/or shape of the anatomic part of interest
− degree of distortion can vary from great to small
PRIMARY FACTORS
o placement of the tube o placement of the part
o placement of the film