The Direct Anterior Approach to Hip Reconstruction

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SECTION III Intermediate Direct Anterior Approach Topics

of EMFs on the electrons being accelerated through the image intensifier between the input phosphor and the output phosphor. This type of distortion is most pronounced at the periphery of images where the elec trons are closer to the surface of the intensifier and thus more subject to EMF forces ( Figure 14.5 ). These EMFs may come from the earth’s magnetic field and may be generated more locally by pieces of electrical equipment within the operating room or nearby. Items such as the motor driving the operating table or nearby magnetic resonance imaging scanners, even a floor above or below, generate powerful EMFs. The proximity of the image intensifier to such equipment affects the degree of image distortion. Distortion is also pose dependent, meaning that the amount of image distortion varies with C-arm positioning (tilt, rollover, translation, etc.) because the position of the image intensifier changes the impact of the EMF vector on the electron path. Image-intensified units have shielding to help protect against EMF distur bances, but this does not remove the impact entirely. It has been stated that distortion in fluoroscopy is location dependent, pose dependent, and machine dependent and therefore is entirely unpredictable. This is distinctly separate from the phenomenon of parallax, which is not true image aberration but rather altered image interpre tation due to variations in the viewing angle. Parallax is an optical phenomenon in which the apparent displacement of an object changes based on the position of the observer and the relative depth of objects in the field. Parallax can be seen in a car when you look out the window and the objects nearby seem

Fluoroscopic machines are susceptible to two main types of image distortion due to the design of the image intensifier. The first is pincushion distortion arising from x-ray photons being projected onto a curved input phos phor and then transitioned to a flat output surface. The second is S-type distortion originating from the effect FIGURE 14.4 Imaging of the hip can be obtained to confirm size and positioning as well as rule out an obvious fracture or perfo ration. An AP image only is shown, but typically a lateral image is obtained as well by externally rotating the leg 90°.

A FIGURE 14.5 The effect of distortion on fluoroscopic images. A, A fluoroscopic image of a perpendicular grid in the setting of minimal distortion. Relatively straight lines are maintained out to the far margins of image. B, The same image of the grid in the setting of moderate distortion with the C-arm in a different room and positioned differently. As can be seen here, distortion becomes more pronounced at the periphery of the image. Copyright © Wolters Kluwer, Inc. Unauthorized reproduction of the content is prohibited. 2024 B