Question

What other data or indicator must the technologist use to assess whether or not an insufficient or excessive amount of radiation has been used to produce a diagnostic radiograph ? Explain.

(Radiologic question)

Answer 1

Radiographs are made using a specialized type of vacuum tube that produces x-rays. The tube current, measured in milliamperes (mA), and voltage, measured in kilovolts (kV). Kilovoltage potential (kVp) is the highest potential voltage achieved at any given kV setting.

Higher kV settings produce more penetrating beams in which a higher percentage of the x-rays produced penetrate the subject being radiographed. There is also a decrease in the percentage difference in absorption between tissue types. This results in decreased contrast (long-scale contrast) on the final image. High kVp techniques are most useful for studies of body regions with many different tissue densities (eg, thorax). Higher kVp techniques are appropriate for larger and thicker animals with limitations. Increasing kV is not a linear function, and small increases in kVp settings may substantially increase the number of x-rays penetrating the animal. However, for a number of reasons relating to the production and absorption of x-rays, this effect is much less dramatic above 85 kVp.

Increasing the mA setting on the machine increases the number of x-rays produced. The energy spectrum of the x-ray beam is essentially unchanged, as are the relative numbers of x-ray photons penetrating tissues of different densities such as bone, soft tissue, and fat. However, the amount of darkening on the image is related to the total number of photons reaching it. Therefore, increasing mA increases image contrast. Changes in mA settings are relatively linear; increased contrast is desirable when tissue densities are similar (eg, soft-tissue components of the musculoskeletal system). However, increasing mA generally results in more heat loading on x-ray tubes, thus limiting exposure times and reducing tube life.

The third major parameter in making a radiographic exposure is exposure time. Increasing the exposure time increases the number of photons produced and hence the darkness of the image. For exposures in the general diagnostic range, this is a linear function; as is the case with increasing mA, increasing exposure time generally results in greater heat loading of the x-ray tube than increasing kVp, once again potentially shortening tube life.

All three of the above parameters are interdependent, with exposure time and mA so much so that the term milliampere-seconds (mAs) is usually used to indicate the product of these two factors. Increasing the mA and decreasing the exposure time by a proportionate amount results in a radiograph less likely to be degraded by motion. As a rule, it is best to minimize the exposure time but maintain an appropriate mAs and scale of contrast. Increasing kVp increases the number of photons penetrating the subject and so darkens the image. This effect can be used within limits to correct an underexposure. The converse is likewise true.

When correcting a previously unsatisfactory image, underexposure or overexposure should be corrected for by adjusting the mAs when examining areas of high contrast (skeleton) or by adjusting the kVp when examining areas of low contrast (thorax). This will maintain the same relative contrast for that anatomic area while adjusting the film darkness.

Henceforth, the above 3 parameters are to be taken into account to understand and examine the over and under exposure of radiations.