Aim To determine the variability of magnification with digital radiographs and thereby improve the accuracy of templating

Materials and methods We have previously described a method of taping a ten pence coin in the region of the greater trochanter, on the basis that it is cheap, easily available and of constant diameter and radio-opacity. The magnification may then be deduced by dividing the coin’s maximal projected diameter by its actual diameter of 24.5 millimetres, as per the Royal Mint’s specifications. The perceived maximal diameter of the coin’s projected image was measured, to the nearest 0.5mm on each illuminated hard film using a standard 30 centimetre ruler with millimetres scale. Coin diameters were subsequently measured to the nearest hundredth millimetres on the digital radiographs.

Results 20 AP digital pelvic radiographs (hard copies and digital images) were analysed. The ranges were as follows: 27.17 to 29.40 mms actual, equating to magnification range 10.9 to 20.00 per cent

Conclusions This study has demonstrated that magnification on digital radiographs is not as constant as previously assumed. Prior to this study a magnification factor of 10% was assumed to be standard on all pelvic X-rays and was the basis of templating.

We now recommend using a set magnification of 15% in our department, or to be more accurate using radio-opaque markers for templating.