Exposure Technique Factors Flashcards
mAs and Quantity of Radiation
Describe the relationship…
As mAs increases, the quantity of radiation reaching the IR increases.
As mAs decreases, the amount of radiation reaching the IR decreases.
Recall – mA & Time = mAs, increasing either factor will increase IR exposure
Doubling the mA (i.e. 200 to 400) OR time (i.e. 0.5 s to 1 s) will double the mAs
Milliamperage and Exposure Time
Describe the relationship…
mA and exposure time have an inversely proportional relationship when maintaining the same mAs.
Recall – in situations where you want to maintain IR exposure…
If mA (tube current) goes up then time should be decreased
If time is increased then mA would need to be decreased.
(Vice versa - time is decreased - mA must be increased)
mAs and Digital Image Brightness
Describe the relationship…
The level of mAs does not directly affect image brightness when using DIGITAL IRs.
During computer processing, image brightness is maintained when the mAs is too low or too high.
A lower-than-needed mAs produces an image with increased quantum noise, and a higher-than-needed mAs exposes a patient to unnecessary radiation.
Recall – digital detectors have a wide dynamic range (shades of gray) However, kVp influences the energy & penetration of the beam and mAs influences the number of photons produced – together they influence the exposure to the IR – look for the Exposure Index (EI)
Brightness is a function of the computer monitor and does not visually tell the radiographer that the image is acceptable or not – EI will tell you if the image is – underexposed - acceptable - overexposed.
We do not repeat an image that is overexposed
Exposure Indicator Value
Describe the relationship…
A numerical value or exposure indicator is displayed on the processed digital image to indicate the level of x-ray exposure received (incident exposure) on the IR. If the exposure indicator value falls outside the manufacturer’s suggested range, image quality, patient exposure, or both could be compromised.
mAs and Film-Screen Density
Describe the relationship….
The amount of mAs has a direct effect on the amount of radiographic density produced when using a film-screen IR.
The minimum change needed to correct a density error is determined by multiplying or dividing the mAs by 2
Kilovoltage and Image
Describe the relationship…
Increasing or decreasing the kVp changes the amount of radiation exposure to the IR and the contrast produced within the image.
Exposure Errors in Digital Imaging
Describe the relationship…
kVp and mAs exposure errors should be reflected in the exposure indicator value (EI); however, image brightness can be maintained during computer processing.
Radiation Protection Alert
Excessive Radiation Exposure and Digital Imaging…why is this a protection alert?
Although the computer can adjust image brightness for technique exposure errors, routinely using more radiation than required for the procedure in digital radiography unnecessarily increases patient exposure.
Even though the digital system can adjust overexposure, it is an unethical practice to knowingly overexpose a patient.
Exposure Errors and Film-screen imaging
Describe the relationship…
kVp directly affects the density produced on a film-screen image; however, its effect is not equal throughout the range of kVp (low, middle, and high).
KVp directly affects the contrast in the image – therefore it is not A PRIMARY way to control density in film-screen imaging
KVp and the 15% rule
Describe the relationship…
A 15% increase in kVp has the same effect on exposure to the IR as doubling the mAs.
A 15% decrease in kVp has the same effect on exposure to the IR as halving the mAs.
kVp and Radiographic Contrast
Important Relationship – describe…
A high kVp results in less absorption and more transmission in anatomic tissues, which results in less variation in the x-ray intensities exiting the patient (lower subject contrast), producing a low-contrast image.
A low kVp results in more absorption and less x-ray transmission but with more variation in the x-ray intensities exiting the patient (higher subject contrast), producing a high-contrast image.
Kilovoltage and Digital Image Quality
Describe the relationship…
Assuming that the body part has been adequately penetrated, changing the kVp affects the radiation exposure to the digital IR in a way similar to changing mAs; but unlike mAs, kVp also affects image contrast.
However, image brightness and contrast are primarily controlled during computer processing.
Kilovoltage, Scatter Radiation and Radiographic Contrast
At higher kVp, more x-rays are transmitted with fewer overall interactions; however, a greater proportion of the interactions are from Compton scattering than x-ray absorption (photoelectric effect), which decreases the radiographic contrast.
Decreasing the kVp will increase x-ray absorption and increase the number of interactions, but the proportion of Compton scattering will decrease compared to photoelectric interactions, increasing radiographic contrast.
Focal Spot Size and Spatial Resolution. (Part II)
Describe the relationship….
As focal spot size increases, unsharpness increases and spatial resolution decreases; as focal spot size decreases, unsharpness decreases and spatial resolution increases.
SID and X-ray Beam Intensity
Describe the relationship…
As SID increases, the x-ray beam intensity becomes spread over a larger area.
This decreases the overall intensity of the x-ray beam reaching the IR.
What is the inverse square law?
The intensity of the radiation is inversely proportional to the square of the distance from the source (to the object).
SID and mAs
Describe the relationship…
Increasing the SID requires the mAs to be increased to maintain exposure to the IR, and decreasing the SID requires a decrease in the mAs to maintain exposure to the IR.
When maintaining exposure – use the direct square law
SID, Size Distortion, and Spatial Resolution. (Part II)
Describe the relationship….
As SID increases, size distortion (magnification) decreases and spatial resolution increases; as SID decreases, size distortion (magnification) increases and spatial resolution decreases.
OID, Size Distortion, and Spatial Resolution (Part II)
Describe the relationship….
Increasing the OID increases magnification and decreases the spatial resolution, whereas decreasing the OID decreases magnification and increases the spatial resolution.
What is the formula for magnification factor? (Part II)
MF = SID / SOD
To find SOD = SID - OID
Percentage of Magnification (Part II)
A lesion on the radiographic image measures 1.16 cm and the lesion’s (object’s) true size measures 1.06 cm. What is the object % of magnification?
Object % of magnification = image size - object size/object size x 100
9.43% object magnification
Grids, Scatter, and Contrast (Part II)
Describe the relationship….
Placing a grid between the anatomic area of interest and the IR absorbs scatter radiation exiting the patient and increases radiographic contrast.
Grids and Image Receptor Exposure (Part II)
Explain the relationship….
Adding, removing, or changing a grid requires an adjustment in mAs to maintain radiation exposure to the IR.
Adjusting mAs for Changes in the Grid (Part II)
A quality radiograph is obtained using 5 mAs at 70 kVp without using a grid. What new mAs is needed when adding a 12:1 grid to maintain the same exposure to the IR?
25 mAs
