PENETRAMETER

Typical Number in Hospital: 1

Cost Bands: 3,4

References:

It is possible to estimate the kilovoltage applied to an X-ray tube without making a direct electrical connection to the X-ray set. The penetrameter is a test cassette in which an X-ray film is exposed with a pattern caused by the X-rays being attenuated by passing through layers of copper sheeting of different thickness.

The relationship between X-ray attenuation in copper and kilovoltage is well documented, and thus it is possible to calculate kilovoltage from densitometer readings of the exposure patterns using a table of values provided with the penetrameter.

The construction of the penetrameter is normally in seven layers:

1. The top cover

2. A copper sheet

3. A plastic sheet with alternate rows of copper discs set into it, and matching holes.

4. The intensifying screen

5. An optical attenuator

6. The X-ray film

7. The bottom cover.

In use it would be normal to expose the cassette several times at different kilovoltage settings, but masking different areas on the cassette with lead sheets. It would also be usual to expose a number of films in this way but at different mAs settings.

There are variations on this theme including the Wisconsin test cassette, and the Ardran-Crooks cassette.