Methods of calculating magnifcation

Current methods of scaling x-rays involve placing an object of known size within the field of view at the same height as the area of interest, the hip joint. Since the hip joint is an internal structure the greater trochanter is used as an indirect method of identifying the height of the hip joint.

As part of a standard AP pelvis x-ray the patient is positioned with the hips internally rotated 15-20 degrees. This places the femoral neck perpendicular to the x-ray cassette and positions the greater trochanter at the same height as the hip joint.

A radio-opaque object can then be placed on the greater trochanter. The projected image is measured on the x-ray and the magnification calculated. Numerous radio- opaque objects have been used. A coin is simple, cheap and effective.

In order to avoid tape and adhesive, calibration devices have been developed. These consist of a ball of known size mounted on a telescopic arm. Because of the size of the digital cassettes and the increasing incidence of obesity, scaling devices placed on the outside of the hips are often missed from the field of view. This problem has been solved by placing the calibration device between the patient’s legs (e.g. Hipscaler). This is unpopular with patients and radiographers as it raises concern about hygiene, placement of the device causes embarrassment and time is added to the procedure.