Question. Outline the technical factors in radiography techniques and their
importance or usefulness.
Introduction
Technical factors in radiography are variables used to determine the technical quality of a
processed radiographic film. These factors affect the amount of radiation exposure to the patient
and the image receptor(IR). Various image receptors respond differently to radiation exiting the
patient. Therefore, changes in the quantity and the quality of radiation exposure to a film-screen
image receptor affect the amount of density and contrast visible on the processed radiograph.
The Technical factors in radiography
They are categorized into primary and secondary factors.
Primary factors
The primary exposure technique factors are selected on the control panel and these include
milliamperage (mA), time of exposure, and kilovoltage peak (kVp).
a. Milliamperage(mA) and Exposure time: Adjusting the mA and the time of exposure,
affect the number of X-ray photons produced by the X-ray tube and has a subsequent effect
on the number of photons reaching the image receptor. Therefore, as the quantity of x-rays
is increased, the exposure to the IR proportionally increases. Similarly, when the quantity
of x-rays is decreased, the exposure to the IR decreases. The product of mA and exposure
time is the milli-ampere-second (mAs). mAs is adjusted according to the size and tissue
type of the body part being examined. mAs can be changed by altering either the amount
of time used in making the exposure or the milliamperage (mA) setting. Therefore, to have
the same mAs, the mA and exposure time have to be inversely adjusted. The mAs is
responsible for controlling the density of the radiographic image.
b. Kilovoltage peak(kVp): This determines the power and strength of the x-ray beam. The
kVp affects the exposure to the IR because it alters the amount and penetrating ability of
the x-ray beam. Therefore, increasing or decreasing the kVp changes the amount of
radiation exposure to the IR and the contrast produced within the image. Higher kVp
decreases image contrast. This can be adjusted depending on the thickness of the part of
the body being imaged. Thicker body parts will require higher kVp compared to the
thin/small body part. Higher kVp also contributes to the overall density of the image.
Secondary factors
1. Focal Spot Size: This controls the sharpness of the image. The smaller the focal spot size
the sharper the image and hence controlling the spatial resolution of the image.
2. Source-to-Image Receptor Distance(SID): This refers to the distance from the anode to
the image receptor. It controls radiographic distortion. The greater the SID the lower the
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intensity reaching the film. Therefore, to obtain the same film blackening, if the SID is
increased the mAs must also be increased.
Object-to-Image Receptor Distance(OID): is the distance between the object to the
image receptor. This also affects the magnification of the body part on the film.
Central Ray Alignment: Improper central ray alignment distorts the radiographic image
Grids: They are placed between the patient and the x-ray film to reduce the scattered
radiation reaching the image receptor and thus improve image contrast.
Collimation: Increasing collimation means decreasing field size, and decreasing
collimation means increasing field size. This serves two purposes; limiting patient
exposure and reducing the amount of scatter radiation produced within the patient.
Filters: These are metal sheets placed in the x-ray beam between the window and the
patient that are used to attenuate the low-energy (soft) x-ray photons from the spectrum.
They help to reduce scatter and patient dose.
Compensating Filters: These are special filters used to limit the primary beam at an
anatomical region during a specific examination that requires fine radiographic detail at
both the 'denser' and 'less dense' regions.
Other factors
Patient Factors
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Body Habitus
Part Thickness
Pediatric Patients
References
1. https://radiologykey.com/exposure-technique-factors/
2. https://www.glaciermedicaled.com/courses/301/07_Technique_Factors.htm
3. Radiopedia