João Pedro Ribeiro
Why is X-ray calibration a relevant subject? It is viable to do most orthopedic procedures without pre-operative planning. However templating prevents surprises and reduces intraoperative complications. Conventional acetate templates provided by manufacturers have a fixed magnification and will only allow adequate implant sizing if the X-ray image magnification is equivalent. It is well known that X-rays used for planning vary in magnification (110%-130%). This scope of variation exists in part because of the diversity of techniques used by radiographers but is also due to patient positioning and size. Even with a strict imaging protocol, the magnification of the X-ray image will vary. A thin patient lying on an X-ray cassette is only a few centimeters above the plate, giving little distance for the X-rays to diverge. In a larger patient, the distance may be twice as far from the plate, allowing the divergent beam to spread more before it hits the plate.
As radiology becomes ‘filmless’, current planning methods with acetate sheets are becoming outdated. For that reason, a surgical planning software that uses digital templates is today, a “must-have” tool. One of the great advantages of digital templating is the possibility to correct any magnification with mechanisms for scaling the image.
Correct X-Ray Calibration
In 1976, Clarke et al. recognized the importance of knowing the magnification to improve the validity of pre-operative planning. He identified several sources of error in the procedure. The most important one was the distance between the X-ray source and the film, which resulted in a magnification factor of the hip anatomy, in the normal situation amounting to approximately 20%.
Lack of knowledge of the magnification factor leads to mismatching of the implant size. Preoperative estimates of magnification are frequently incorrect. The magnification factor for pelvic images normally ranges from 109% to 128% (Knight and Atwater 1992; Pickard et al. 2006). That affected the choice of implant size in 17 % of the cases. (Knight and Atwater 1992).
For instance, an oversized femoral component may cause a fracture of the femur if it has to be forced in place. On the other hand, a stem that is too small may not be stable and can increase the risk of loosening. Other complications would be significant differences in leg length or disturbance of the biomechanical parameters of the hip joint, leading to excessive joint contact forces and limping (Bono 2004). Accurate preoperative planning is also of high importance in cases where costly, custom-designed implants have to be used.
As such, any radiology service that is performing pre-operative X-rays for digital templating must provide a method to measure image magnification.
X-Ray Calibration: Magnification and Scaling
Image magnification is measured, to accurately scale the anatomy, by placing a calibration object, or a radiopaque object of known dimension, in a precise position in the X-ray’s field of view. There are several alternative objects, usually called scaling markers, available for image calibration.
Type of markers
Validated markers of different shapes and sizes are available and may be used.
Radio-opaque rulers have been used (The et al. 2005), but they need to be placed in the plane of the joint or area of interest and also held perpendicular to the X-ray beam, which may be tricky. The main challenge is to ensure the ruler stays fixed in the correct position while the image is acquired. The advantage of rulers is that a scale of 50, 100, or even 150mm can be used which reduces the end error.
A disc (or a coin) makes a very good marker (Pickard et al. 2006; Wimsey et al. 2006), creating an elliptical shadow on the x-ray. The maximum diameter of the ellipse is always the diameter of the disc if it is laid down with the beam at 90º. The main advantage of a disc is that it is light and quite easy to manipulate, so it can be introduced into the correct plane effortlessly.
Metal spheres, usually with a 25mm diameter, can be used. The main advantage is that orientation is not important as they are entirely symmetrical. The disadvantage is that spheres can be quite heavy and difficult to hold in the correct plane, even using adhesive strips to hold them. These calibration spheres might be mounted on an adjustable flexible arm. The arm allows the object to be correctly positioned in the same plane as the anatomy of interest quite easily. This method is, eventually, the most recommendable.
X-Ray Calibration: How to position the markers
Palpate the most lateral point and position the marker at the same plane as the greater trochanter. Optimal placement is medical, midline to femurs and proximally towards the symphysis pubis. Lateral placement is acceptable if the patient does not have a significant amount of adipose tissue.
In LAT view, position the marker on a plane equivalent to the midline of the femur or just behind and below the patella. Place the marker at the level medially or laterally of the knee joint assuring that the entire item is located within the field of view.
In AP view, position the calibration marker on the lateral side of the knee, superior or inferior to the joint, and midway between the anterior and posterior surfaces of the knee. The patient should be standing.
Position the calibration marker at the level of the medial or lateral malleolus, if the ankle is the bony landmark of interest.
Position the calibration marker at the level of the first metatarsal.
Position the calibration marker at the plane of the acromion process in the LAT and AP views. The patient should be fitted or standing with his arms internally rotated and comfortably resting on the waist.