Fig. 3.

Obtaining Swiss axial images and calculating the alpha angle. a The Swiss axial (axial oblique) images are obtained along the long axis of the femoral neck. b To calculate the alpha angle, the axial oblique image through the midportion of the femoral neck (red line) is chosen. A circle is drawn over the femoral head cortex (blue circle). A line is drawn through the long axis of the femoral neck at its narrowest point (purple line), through the center of the femoral head. Another line is drawn from the center of the femoral neck to the point where the femoral head–neck junction meets the circle (green line). The alpha angle is the angle between the two lines. An alpha angle of >55° is considered a risk factor for FAI