Calculation of magnifcation

The magnification on a radiograph is dependent on the distance of the x-ray source to the cassette, the focal film distance (FFD), and the distance of the x-ray source to the object, the focal object distance (FOD).

In formula Magnification :

Focal film distance can be determined either automatically with a digital counter on the x-ray tube, or with a tape measure. If the hip centre to cassette distance (HCC) is
measured, then the focal object distance can be calculated (FOD = FFD – HCC). Magnification can be calculated thus

Magnification :

If the FFD is set at a fixed value (120cm) and the distance from the table top to the digital cassette (TTC) is known, the only variable to change is the height of the patient’s hip joint above the table (HCT). Magnification can be calculated from the formula:

Magnification :

The calibrated ruler

From the above it is therefore possible to create a calibrated ruler where the resultant magnification is read directly from the ruler.
Important: This method does require the user to control the Film Focus distance and set it prior to any measurement to 120 cm. Not doing so will invalidate the measurement.