Radiological Technology: Image Formation, Exposure Factors, and Quality Control
Core concepts
Radiological technology questions test whether you can trace a single exposure decision through the imaging chain. Know how the primary beam is shaped (collimation, beam-limiting devices), how differential attenuation produces subject contrast, and how the image receptor converts transmitted photons into signal. Understand the exposure factor relationships: mAs controls quantity of photons and therefore receptor signal, while kVp controls beam penetrability and, through the photoelectric/Compton balance, both contrast and patient dose. Learn the inverse square law, the 15% rule (a 15% kVp increase roughly doubles receptor exposure, so halve mAs), and grid ratio/contrast trade-offs. For digital systems, distinguish exposure index from deviation index and remember that windowing can mask, but not repair, an exposure error.
Traps to expect
Questions often conflate spatial resolution with contrast resolution, or ask which factor changes patient dose versus image noise. Watch for distractors that increase mAs to "fix" motion when the correct answer is shorter exposure time. Another common trap is assuming higher kVp always improves image quality; it lowers contrast but reduces dose. Also separate quantum mottle (too few photons) from electronic noise (detector problem).
Worked example
A lateral lumbar spine image is underexposed with visible mottle at 70 kVp and 20 mAs. Increasing kVp by 15% to about 80 kVp doubles receptor exposure, so the technologist can reduce mAs to 10 and maintain density while lowering patient dose. If motion is the real problem, shorten time instead: at 80 kVp use 400 mA and 25 ms rather than adding mAs. Always state which variable is being held constant before calculating.