Radiographic Technique

Exposure Factors

As a radiographer you must know at least the average exposure factors for all standard projections in radiography. In the era of digital radiography with kV and mAs exposure factors preset automatically, it may seem unnecessary. Moreover, many universities offering diagnostic radiography courses don’t include these factors in direct teaching and often don’t test this knowledge in exams. However, clinical practice relies on this knowledge to adjust exposures to patient habitus or in the fairly common cases of equipment failure when you are forced to set your own exposures.

Differences and role of kV vs. mAs

kV and mAs are critical values that radiographers adjust to influence X-ray image quality. Understanding their roles is essential: kV affects the image’s contrast, while mAs determines its brightness. To explain each further:

  • kV (Kilovoltage): controls the penetration power of the X-rays (their QUALITY), affecting image contrast. Higher kV produces X-rays with more energy, allowing them to penetrate through denser body parts. kV is about the quality of the X-ray beam in terms of its ability to differentiate between different tissues.
  • mAs (Milliampere-Seconds): controls the density that is the number of photons hitting the detector (QUANTITY). Higher mAs means more photons, which increases the overall density (brightness) of the image.
    Quantum NOISE (mottle) visible especially within soft tissue is a classic sign of TOO LOW mAs. The noisy, grainy area indicates a low density or sparse photon coverage. Hence we stress in radiography that exposure factors must provide an adequate signal-to-noise ratio.

    Controlling the quantity of photons, mAs is also directly responsible for radiation exposure to the patient. A higher mAs setting increases the total amount of radiation the patient receives, as it involves both the current (number of electrons) and the time the X-ray is being produced.

    One of the most challenging aspects to understand about exposure factors is that increasing kV enhances penetration but reduces image contrast. Lower kV results in higher contrast, making darker and lighter areas more distinct. In contrast, higher kV leads to lower contrast and more uniform image tones. This occurs because the overall differences in attenuation are reduced (while black remains black, even dense areas like cortical bone are more penetrated, diminishing the contrast).
  • kV Increases Penetration but Reduces Contrast: Higher kV settings produce X-rays with more energy, which can penetrate denser tissues more effectively. This means that structures like bones and soft tissues will absorb similar amounts of X-rays, leading to more uniform shades of gray in the image and thus lower contrast.
  • Lower kV Results in Higher Contrast: At lower kV settings, there’s a greater difference in the X-ray absorption between dense (like bone) and less dense (like soft tissue) structures. Bones absorb more X-rays and appear whiter, while soft tissues absorb fewer X-rays and appear darker, resulting in a higher contrast image with more distinct differentiation between these areas.
  • Effect of Higher kV on Contrast: As you described, with higher kV, even areas of cortical bone become more penetrated, making them appear less white (or more gray) than they would at lower kV settings. This reduces the difference in appearance between dense and less dense areas (less attenuation difference), leading to an overall lower contrast in the image.

Different values in different hospitals?

These are average indicative exposure factors, arranged from highest to lowest exposure (and radiation dose), to facilitate easier memorization and assist with clinical practice mindset. Many trusts, departments, and clinical sites will employ different kV and mAs values for the projections mentioned below. Often, these are dependent on the brand of the X-ray machine used. However, the relative values between body parts, as well as the inverse relationship between kV and mAs, will remain consistent. For instance, the denser body parts such as the abdomen and lumbar spine views naturally require higher values to achieve a diagnostic image, whereas fingers, hands, and feet are the least demanding. Some departments may operate with less sensitive old CR equipment using higher mAs values. Please note the exceptional chest examination, which employs a high kV technique due to the specific diagnostic requirements of the lung area, which is predominantly filled with air.

Average kV and mAs values for standard X-ray views

Body Part/ProjectionApprox. kVApprox. mAs
Pelvis/Abdomen (AP)80-9020-25
Lumbar Spine (AP/LAT)80-9020-25
Thoracic Spine (AP/LAT)70-8015-20
Hip (AP/LAT)80-9010-16
Femur (AP/LAT)70-8010-16
Knee (AP/LAT)60-705-10
Tib/Fib (AP/LAT)60-705-10
Chest (PA/LAT)110-1253-5
Shoulder (AP/LAT)70-8010-16
Cervical Spine (AP/LAT)70-8010-15
OM/OM30 (facial bones)65-8010-15
Elbow (AP/LAT)60-702.5-5
Forearm (AP/LAT)60-702.5-5
Wrist (PA/LAT)55-651-3
Hand (PA/OBL/LAT)50-601-3
Ankle (AP/LAT)55-654-6
Foot (AP/OBL/LAT)50-603-5
Xray Exposure Factors Kv Mas
Cheat sheet average exposure factors for Radiographers