Projectional radiography

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Author: Mikael Häggström [notes 1]


By location


Geometric magnification

Source-object distance (SOD) and source-detector distance (SDD) and geometric unsharpness.[1]
This image also shows geometric unsharpness, which is a decrease in the spatial resolution of an image because the X-rays from the generator comes from an area rather than a single dot, and is decreased by having the object closer to the detector.

Because of geometric magnification, distances measured on a projection become larger than the actual size of the imaged object:

  • SizeObject = SizeProjection * Source-object distance (SOD) / Source-detector distance (SDD).[2]

Alternative names for source-detector distance(SDD) are source[3]/focus to detector/image-receptor[3]/film distance (SID[3], FID or FRD). On lumbar and chest radiographs, it is anticipated that the magnification factor is between 1.05 and 1.40.[4] Because of the uncertainty of the true size of objects seen on projectional radiography, their sizes are often compared to other structures within the body, such as dimensions of the vertebrae, or empirically by clinical experience.[5]



  1. For a full list of contributors, see article history. Creators of images are attributed at the image description pages, seen by clicking on the images. See Radlines:Authorship for details.


  1. Bruce Blakeley, Konstantinos Spartiotis (2006). "Digital radiography for the inspection of small defects ". Insight 48 (2). Archived from the original. . 
  2. DICOM (2016-11-21). DICOM PS3.3 - Information Object Definitions - Table C.8-30. XA Positioner Module Attributes. Retrieved on 2017-01-23.
  3. 3.0 3.1 3.2 . source-to-image-receptor distance. Farlex medical dictionary, in turn citing Mosby's Medical Dictionary, 9th edition.. Retrieved on 2018-01-28.
  4. M Sandborg, D R Dance, and G Alm Carlsson. Implementation of unsharpness and noise into the model of the imaging system: Applications to chest and lumbar spine screen-film radiography. Faculty of Health Sciences, Linköping University. Report 90. Jan. 1999. ISRN: LIU-RAD-R-090
  5. Page 788 in: Joseph C. Masdeu, R. Gilberto Gonzalez (2016). Neuroimaging, Part 2 Volume 136 of Handbook of Clinical Neurology . Newnes. ISBN 9780702045387.