7.2 Exposure Factors & Dose

How Technique Choices Change Patient Dose

Every prime exposure factor has a predictable effect on patient dose, and the ARRT expects you to convert a technique change into a dose change. This section applies the physics from Chapters 5 and 6 to the patient rather than re-deriving it.

mAs: The Direct Dose Lever

Milliampere-seconds (mAs) is the product of tube current and exposure time and controls the quantity of x-ray photons. mAs is directly proportional to both receptor exposure and patient dose: double the mAs and you double the dose; halve it and you halve the dose. Because mAs adds photons without changing beam energy, it is the cleanest single dose lever — but it is also the factor most often inflated by "dose creep," where technologists push mAs to guarantee a low-noise image. Under reciprocity, any mA and time combination that yields the same mAs yields the same exposure and the same dose, so a shorter time at higher mA holds dose constant while reducing motion.

kVp and the 15% Rule Trade-off

Kilovoltage peak (kVp) controls beam energy (penetration) and image contrast. Raising kVp makes the beam more penetrating, so a larger fraction of photons pass through the patient rather than being absorbed by the photoelectric effect — which means higher kVp lowers entrance skin exposure. This is the basis of high-kVp, low-mAs (optimized) technique. The 15% rule quantifies the swap: raising kVp by 15% roughly doubles receptor exposure, so mAs is halved to keep the image the same.

Worked example: An abdomen is imaged at 80 kVp and 40 mAs. To reduce patient dose while holding receptor exposure constant, raise kVp 15% (80 × 1.15 = 92 kVp) and halve mAs (40 → 20 mAs). The patient's skin dose falls because the more penetrating beam is absorbed less. The trade-off is reduced radiographic contrast (a longer gray scale) and more scatter, which is why high-kVp technique is paired with a grid and tight collimation. When the exam is bone-detail sensitive, the contrast loss may not be acceptable — ALARA is balanced against diagnostic need.

SID, Grids, and AEC

Source-to-image distance (SID). Increasing SID requires more mAs (the direct square law, mAs₁/mAs₂ = SID₁²/SID₂²), yet the entrance skin exposure still falls because the patient's skin sits relatively farther from the source and the beam is less intense at the surface. This is why regulations set a minimum source-to-skin distance (about 30 cm / 12 in for fixed fluoroscopy, 38 cm / 15 in for mobile fluoroscopy) and a minimum 40-inch SID for mobile radiography: short distances concentrate skin dose.

Grids. A grid removes scatter to improve contrast but demands a large mAs increase described by the grid conversion (Bucky) factor, typically 2–6×. Because that added mAs raises dose, a higher grid ratio means higher patient dose. For thin parts (< 10 cm) or pediatric imaging, removing the grid — or using the air-gap technique (increased OID so scatter misses the receptor) — lowers dose.

Automatic exposure control (AEC). AEC uses ionization chambers that terminate the exposure once a preset receptor exposure is reached, optimizing dose when used correctly. Three requirements protect the patient: (1) select the chamber(s) under the anatomy of interest; (2) ensure the anatomy actually covers the chamber — small or off-center parts, or a prosthesis over the chamber, cause premature or delayed termination and a repeat, so switch to manual technique; and (3) rely on the backup timer (about 600 mAs or 150% of the expected value) to cut off exposure if the chamber never satisfies, protecting both patient and tube. Density (exposure-adjustment) controls shift the target in small steps and should be returned to zero afterward.

Worked SID Example and the Skin-Dose Paradox

Suppose a technique of 40 mAs is correct at a 40-inch SID and you must move to a 48-inch SID. The direct square law gives the new mAs: 40 × (48/40)² = 40 × 1.44 = 57.6 mAs. At first glance more mAs sounds like more dose — and receptor exposure is indeed held constant — yet the patient's entrance skin exposure falls. The reason is geometry: the skin now sits relatively farther from the focal spot, so beam intensity at the skin surface is lower even though total output rose to fill the longer distance. This is the physics behind minimum source-to-skin distances and behind the guidance to use the longest practical SID in mobile radiography.

Putting the Levers Together

The practical dose-reduction hierarchy at the console is: choose the highest kVp that preserves needed contrast, pair it with the lowest mAs that avoids quantum mottle, use a grid only when the part thickness or kVp requires it, keep the SID long, and collimate tightly. Each lever is individually small, but stacked together they can cut entrance skin exposure by half or more without sacrificing a diagnostic image — the everyday meaning of optimization.