Scanography for Leg-Length Measurement: An Easy Satisfactory Method

Abstract

Scanography is “a method of making radiographs by the use of a narrow slit beneath the tube in such a manner that only a line or sheet of x-rays is employed and the x-ray tube moves over the object so that all the rays of the central beam pass through the part being radiographed at the same angle” (1). Sante (2) cites Millwee (3) as the first to use this technic to accurately determine the length of bones of the extremities. Other methods of measuring bone length have been employed and sometimes called “scanography” although they may not have the x-ray tube moving during the exposure but depend on multiple exposures with the tube perpendicular to the ends of the bones during each exposure. In these methods a radiopaque ruler or grid is placed over the film during exposures and utilized to measure the length of the bones. When the slit-like beam is employed, an image of a body part is recorded only if the beam is centered on it: thus in the direction of the tube motion, the angle of image projection to the film is the same for all parts shown in the roentgenogram. For this reason, magnification with respect to length is eliminated when tube motion is along the long axis of the examined part. Magnification across the width of the examined part occurs as usual, since the roentgen-ray beam employed is wide enough to expose the entire film in that direction, but this is of no significance (Fig. 1). Recently, we began using the slit-like beam technic of scanography because of its simplicity of application and accuracy of results. Previous similar methods were modified, and the purpose of this presentation is to describe a technic we have had in continuous use for about two years and found most satisfactory. Our Method In our department, scanography is used chiefly in roentgenography of the bones of the legs to determine, by comparative measurement, differences in length. In the scanographic roentgenogram the slit-like beam is moved down the length of the patient's legs from hips to ankles. Tube motion and exposure are continuous and simultaneous. An image of the patient's legs, recorded on two 14 by 17-in. films contained in a cassette 36 in. long, can be accurately measured with any ruler. Thus, leg length and differences in length are quickly determined (Fig. 4). The width of the x-ray beam at film level should be as narrow as practical for scanography, because widening increases on the roentgenogram the loss of bone detail due to x-ray tube motion during exposure. Unfortunately, narrowing the x-ray beam requires increasing the exposure. For practical reasons, one must employ a beam wide enough to keep the required exposure within the capacity of the x-ray tube despite the resulting loss of detail.