Density–Collimation Field Size Conversions
Accurate collimation of the primary x-ray beam to the area of interest reduces the area and volume of tissue irradiated. This not only reduces patient dose but also improves image quality by reducing the amount of undesirable scatter radiation reaching the image receptor (IR). Therefore reducing the collimation field size reduces the amount of scatter reaching the IR, resulting in less image density. This requires an adjustment in mAs or kV to maintain adequate image density when the collimation field size is significantly reduced.
The tissue density and part thickness, as well as screen type and speed, affect these factors for film-screen systems, but for general purposes the following conversion factors can be used as a suggested starting guide for exposure adjustments.
| Field Size–Exposure Conversion Chart (with 400 Speed Screens) |
| Exposure Field Size Change | Increase in mAs Required | Multiplication Factors |
| Abdomen | ||
| 35 × 43 cm (14 × 17″) → 24 × 30 cm (10 × 12″) | 25%–35% | 1.25–1.35× |
| 35 × 43 cm (14 × 17″) → 18 × 24 cm (8 × 10″) | 50%–75% | 1.5–1.75× |
| 35 × 43 cm (14 × 17″) → 10 × 10 cm (4 ×4″) | 100%–120% | 2.0–2.2× |
| Skull | ||
| 24 × 30 cm (10 × 12″) → 8×8 cm (3 × 3″) | 30%–40% | 1.3–1.4× |
Example: Calculate the new mAs range required for an abdomen when collimation field size is decreased from 35 × 43 cm (14″ × 17″) to 18 × 24 cm (8″ × 10″) (collimated gallbladder). Original mAs = 65 @ 80 kV.
Answer: Increase mAs 50%-75%. (1.5 × 65 = 98, 1.75 × 65 = 114) New mAs = 98-114.