Radiological interpretation

Elizabeth Stokell MA VetMB CertVR MRCVS
Department of Clinical Veterinary Medicine
University of Cambridge
Madingley Road

1. Introduction

Radiographic interpretation is based on the visualisation and analysis of opacities on a radiograph. These opacities are formed by the following processes:
  1. X-Ray photons have the potential to penetrate tissue
  2. X-Ray photons will be attenuated in part by the tissue, and in part will pass through the tissue to interact with and expose the radiographic film
  3. Absorption of X-Rays is a function of the atomic number and thickness of the tissues/objects
  4. The greater the amount of tissue absorption, the fewer X-Ray photons reach the film, and the whiter the image on the film
  5. The radiograph will display a range of densities from white, through various shades of grey, to black
  6. The resultant pattern of opacities forms an image on the radiograph, which is recognisable in form, and which can be interpreted

2. Radiopacity

The radiopacity of various objects and tissues results in radiographs showing different radiopacities, and hence they can be differentiated. Radiopaque tissues/objects result in a whiter image; less radiopaque objects result in a blacker image. The radiopacity depends on:
  1. Atomic number
  2. Physical opacity
  3. Thickness
Tissue/Object Effective Atomic Number Specific Gravity
1-2 0.001
fat 6-7 0.9
soft tissue/fluid 7-8 1
bone 14 1.8
metal (lead) 82 11.3

3. Basic tissue radiographic opacities

  1. Mineral opacity
  2. Soft tissue/fluid opacity
  3. Fat opacity
  4. Gas opacity
  5. Metal opacity
  6. Only these five radiographic opacities are visible on a radiograph

4. Radiologic interpretation

  1. Viewing the radiograph
  2. Three-dimensional concept
  3. Routine assessment of radiographs
  4. Every shadow visible must be evaluated to determine whether it is:

5. Evaluating the radiographs

  1. Determine whether an abnormality exists:
  2. Define the anatomic location of the abnormality
  3. Classify the abnormality according to its roentgen signs
  4. Make a list of differential diagnoses (gamuts) by considering what diseases could cause the observed roentgen signs
  5. If a number of abnormal roentgen signs are identified, then those gamuts common to all lists are more likely (assuming only one problem is present)

6. Description of radiologic abnormalities of tissues/organs/objects (roentgen signs)

  1. Changes in size of an organ or structure
  2. Variation in contour or shape
  3. Variation in number of organs
  4. Change in position of an organ or structure
  5. Alteration in opacity of an organ or structure
  6. Alteration in the architectural pattern of an organ or structure
  7. Alteration in the normal function of an organ

7. Other clues

  1. Summation shadows
  2. The silhouette effect
  3. Importance of a contrasting substance
  4. Perception

8. Pitfalls in interpretation

  1. The presence of an obvious abnormality that distracts the evaluator from systematic evaluation of the rest of the radiograph
  2. Discovery of a lesion that answers the clinical question that prompted the radiographic examination, thereby distracting the evaluator
  3. Tunnel vision, which is a preconception of what will be found, so that when the preconception is confirmed, viewing of the radiograph ends
  4. Failure to adopt a systematic approach, and using the error-prone 'Aunt Minny' approach;
  5. The 'Aunt Minny' approach has its devotees, and those who use it often appear to have a supernatural ability to make a diagnosis