Cast Conversion Rule
A cast applied to upper or lower limbs (extremities) requires an increase in exposure. One suggested method for determining exposure compensation is to measure for the increased thickness of the part including the cast and adjust the exposure factors accordingly.
The above method can be used in general, but in addition to the added thickness of the cast, the different densities of cast materials also affect the required exposure adjustments. Therefore the following general cast conversion guide, which makes allowances for both the size and type of cast material, is suggested.
An upper or lower limb with a cast requires an increase in exposure. This increase depends on the thickness and type of cast, as outlined in the following table:
| Cast Conversion Chart |
| Type of Cast | Increase in Exposure* |
| Small to medium plaster cast | Increase 5–7 kV |
| Large plaster cast | Increase 8–10 kV |
| Fiberglass cast | Increase 3–4 kV |
Example: An AP and lateral ankle were taken at 66 kV and 6 mAs demonstrating a fracture. A medium-size plaster cast was applied, and postreduction projections were ordered. What exposure factors should be used?
*To reduce patient dose, it is recommended to increase kV rather than mAs.