X-Rays

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Introduction

X-Rays, are often used to determine the type and extent of a fracture as well as for detecting pathological changes in the lungs. With the use of radio-opaque contrast media, such as barium, they can also be used to visualize the structure of the stomach and intestines - this can help diagnose ulcers or certain types of colon cancer.  Radiographic interpretation is based on the visualisation and analysis of opacities on a radiograph.

Formation of Radiographs

X-Ray photons have the potential to penetrate tissue and will be attenuated in part by the tissue, and in part will pass through the tissue to interact with and expose the radiographic film.  Absorption of X-Rays is a function of the atomic number and thickness of the tissues/objects.  Tissues/objects with a higher atomic number will absorb more radiation than tissues with a lower atomic number. Thicker tissue/objects will absorb more X-Rays than thinner tissue of similar composition.  The greater the amount of tissue absorption, the fewer X-Ray photons reach the film, and the whiter the image on the film. The radiograph will display a range of densities from white, through various shades of grey, to black.  Radiopaque tissues/objects appear more white and radiolucent tissues/objects appear more black. The resultant pattern of opacities forms an image on the radiograph, which is recognisable in form, and which can be interpreted.

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 the atomic number (the higher the atomic number, the more radiopaque the tissue/object), physical opacity (air, fluid and soft tissue have approximately the same atomic number, but the specific gravity of air is only 0.001, whereas that of fluid and soft tissue is 1, therefore air will appear black on a radiograph, compared with fluid and soft tissue, which appear more grey), and thickness (the thicker the tissue/object, the greater the attenuation of X-Rays and the more white the image will be.

Basic tissue radiographic opacities

Mineral.  Bone is composed primarily of calcium and phosphorus.  There is a normal variation in radiopacity within the same bone and between bones because of the difference in radiopacity of compact vs spongy bone, trabecular bone vs intertrabecular spaces and cortical bone vs medullary canal.  Diseased bone may be more (sclerotic) or less (porotic) opaque than normal bone.
Soft tissue/fluid.  Both soft tissues and fluids have the same radiopacity.  This is the radiopacity of normal soft tissue and fluid-filled organs (heart, liver, spleen, urinary bladder).  Variation in volume, thickness and degree of compactness of soft tissue creates a pattern of various densities on the radiograph
Fat.  Fat is more lucent than bone or soft tissue, but is more opaque than gas.  Fat produces radiographic contrast for differentiation and visualisation of many organs and structures, in that fat surrounding an organ or structure will allow it to be delineated.  In immature and thin animals, the lack of fat results in poorer contrast in the radiograph
Gas.  Gas is the most radiolucent material visible on a film.  This lucency provides contrast to allow visualisation of various structures, e.g. the heart and great vessels outlined against the air-filled lungs in the chest.
Metal.  This is the most opaque shadow seen on radiographs, and may be seen as contrast media (barium, water-soluble iodine), orthopaedic implants, metallic foreign bodies.

Only these five radiographic opacities are visible on a radiograph, however, there is some variation in opacity within each group.

X-Ray Interpretation

1. Ensure that the radiograph is the one of the patient being examined, and check the date. 
   
2. The position of the patient during exposure should be known, and left/right markers should be identified. 
   
3. Every shadow visible must be evaluated to determine whether it is: a feature of normal anatomy, a composite structure formed by superimposition of structures, an artefact produced by inaccurate positioning, a pathologic lesion (the first three should be ruled out first).
   
4. In evaluating the radiographs determine whether an abnormality exists.  This is often the most difficult part as there is a wide range of normal anatomic variants.  Reference should be made to textbooks, normal radiographs, tissue specimens or the contralateral limb.
   
5. Define the anatomic location of the abnormality and classify the abnormality according to its roentgen signs.
   
6. Make a list of differential diagnoses by considering what diseases could cause the observed roentgen signs.  For example, if the roentgen sign is the presence of an fluid on the lung, then possible differentials are: pneumonia, pleural oedema, abscess etc.  If a number of abnormal roentgen signs are identified, then those differentials common to all are more likely (assuming only one problem is present.
   

Roentgen signs

Roentgen signs are the description of radiologic abnormalities of tissues/organs/objects such as:

• changes in size of an organ or structure
  
• variation in contour or shape
  
• variation in number of organs. Many organs or structures may be present in increased or decreased number or absent completely e.g. supernumerary vertebra and ribs, absence of a kidney
  
• change in position of an organ or structure such as presence of abdominal organs in chest in diaphragmatic rupture, mediastinal shift in pneumothorax
  
• alteration in opacity of an organ or structure such as Altered radiopacity. Increased opacity in air-filled space, calcification within soft tissues, radiopaque foreign body and increased lucency sucha as gas in abnormal sites e.g. subcutaneous emphysema.  Bone may appear more lucent with osteoporosis, osteomyelitis and neoplasia
  
• alteration in the architectural pattern of an organ or structure such as change in normal bone trabeculation, or bronchovascular markings in the lungs
  
• alteration in the normal function of an organ following secretory contrast studies, transit contrast studies, physiologic phases and moving picture image intensification contrast studies.

Other things to look out for

Summation shadows.  This results when parts of a patient or an object in different planes are superimposed.  The result is a summation image representing the degree of X-Ray absorption by all the superimposed objects.  Radiolucent summation shadows are formed in the 'Swiss cheese ' effect.  Radiopaque summation shadows are involved in the 'bunch of grapes' effect.

The silhouette effect.  This principle is based on the fact that when two structures of the same radiopacity
are in contact, their individual margins at the point of contact cannot be distinguished.  One is said to silhouette with the other, or to form a positive silhouette sign. This terminology is confusing, and the term 'border effacement' has been suggested when their is a loss of the clear margins of a structure.

Importance of a contrasting substance
Just as the lack of a contrasting material prevents distinguishing between two structures of the same radiopacity, the presence of a contrasting substance allows some structures to become exquisitely visible.  This principle is particularly important when the contrasting substance is air, and the object in question is on the surface of the body

Perception
When evaluating radiographs, the eyes are used to detect abnormalities which are interpreted by the brain.   However, the eyes and brain do not always perceive appearances accurately, and optical illusions may occur.  What appears as concrete visual evidence is not always such, and perception is an important part of radiographic interpretation.  What appears to be an obvious finding to inexperienced radiologists may be an incorrect assessment because of perception

Pitfalls in interpretation

The presence of an obvious abnormality that distracts the evaluator from systematic evaluation of the rest of the radiograph
Discovery of a lesion that answers the clinical question that prompted the radiographic examination, thereby distracting the evaluator
Tunnel vision, which is a preconception of what will be found, so that when the preconception is confirmed, viewing of the radiograph ends
Failure to adopt a systematic approach, and using an the error-prone approach such as looking for an abnormality which the evaluator has seen before, or been told about, identifying a boring abnormality which is often present or looking for an abnormality which the evaluator has not seen for a long time, but would like to see.

Resources

The Norwich Image Interpretation Course. This is a great free online course on x-ray interpretation by Heidi Gable DCR(R) PgCert from the Norfolk & Norwich University Hospital NHS Trust.

References

Elizabeth Stokell (2007), "Radiological interpretation", Available from the Vet On-line site, http://priory.com/vet/intrad.htm [accessed 16 April 2009]

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