From sectra/medical/orthopaedics/about/image_magnification.html Due to the design of an X-ray modality, such as a CR or DR unit, all X-ray images are subject to a magnification mechanism of minor or larger magnitude. This means that all objects measured in an X-ray image will always appear larger on the image than in reality. The magnification factor depends on both the distance between the focal point of the X-ray tube and the image plate as well as the distance between the imaged object and the image plate. If the imaged object is moved toward the focal point of the X-ray tube the magnification factor is increased and if the imaged object is moved toward the image plate the magnification factor is reduced. Since all patients are different, they will all create different magnification factors on X-ray images. A larger distance between the focal point of the X-ray tube and the image plate reduces the effect of such movements. When acquiring a conventional X-ray image on which measurements of any type should be made, a calibration marker should always be present in the image. When acquiring the image, the following factors should be considered to improve accuracy: Use a spherical (round) calibration marker. Sectra offers 25-mm calibration markers for our customers. To order calibration markers, please contact your Sectra representative. Ensure that the marker is placed at the same level above the image plate as the anatomy for which planning or measurements is to be performed. Make sure the entire marker is fully visible in the image and that it is not covered by implants or other objects that could interfere with the image analysis.
Posted on: Sun, 05 Oct 2014 04:06:57 +0000
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