Basic Concepts of Image Formation

Basic Concepts of Image Formation

The radiographic image is a two-dimensional representation of a three-dimensional body part. This is not a concept that can be grasped easily. Individuals vary in their ability to apply spatial relationships to radiography. To some it comes easily; others need to develop an appreciation for it.

An understanding of certain fundamental principles is necessary before attempting to interpret radiographs. They are as follows: any substance will have a characteristic radiographic appearance that is dependent upon its thickness, form, and atomic number; and, the image is a summation of anatomic shadows.

Let's first discuss how the radiographic image is formed. The useful x-ray beam comes into contact with the body part (in this case, the foot). These x-rays either travel through the foot and exit or are absorbed by it. Those that exit the foot will come into contact with the x-ray film and screen (if utilized). The x-rays photons (and light photons emitted from a screen) convert the silver halide crystals that are embedded in the film emulsion to atomic silver. This is known as the latent image. Processing converts the latent image into the radiographic image.

The x-ray beam is composed of individual x-ray photons with varying degrees of energy. Higher energy x-ray photons have shorter wavelengths. They are able to penetrate matter more readily than lower energy photons. Therefore, higher energy photons stand a greater chance of reaching the film; the lower energy photons will be absorbed by the foot.

X-rays interact with matter in different ways. X-rays may be scattered in different directions (classical scatter and Compton effect), or, they can be absorbed by an atom (photoelectric effect). Scattered x-rays can fog the film and are of no diagnostic value. Whether an x-ray photon is absorbed or not is dependent on two factors: the energy of the photon (discussed above) and the atomic number of the substance. The difference between those x-rays absorbed by the foot and those that penetrate it is known as differential absorption. Objects with greater atomic numbers will absorb x-rays more readily than those with lower atomic numbers. Therefore, lead, with an atomic number of 82, will absorb more x-ray photons than bone, made of calcium which has an atomic number of 20.

Another factor to consider is the thickness of the material. The thicker any particular substance is, all other factors remaining unchanged, the more x-rays it will absorb. This process is known as attenuation.

The term radiopaque applies to those substances that absorb x-rays; representative areas appear white on the exposed x-ray film. Radiolucent refers to substances that x-rays penetrate more readily; representative areas appear dark or black on the film . When dealing with shades of gray, as in radiography, radiodensities are discussed in relative terms. Bone is radiopaque relative to muscle, but radiolucent compared to a heavy metal such as lead. How does all this relate to formation of the radiographic image? Since radiopaque objects absorb more x-ray photons than those that are radiolucent, less x-rays will reach the film. Therefore, fewer silver halide crystals are ionized, and a radiopaque object will appear whiter. The table below lists matter found in the foot and their relative radiodensities.

Air

Fat

Connective Tissue

Bone

Surgical Pin

Radiolucent <–––––––––––––––––––––––––––––––> Radiopaque

A thicker foot will appear more radiodense than a thinner foot, assuming all other factors, such as radiographic technique, remain the same when exposing the two feet. Also, areas containing fluid (edema, for example) will add density to that part of the image.

Next, we need to consider the form of the object being viewed radiographically. Interpretation of radiographs requires imagination and logical analysis. This is especially true if the object in question is not parallel to the x-ray film or has a complex form. (You are encouraged to read the introductory chapters of Fundamentals of Radiography by Lucy Frank Squire . He presents an excellent discourse on this subject matter.)

If a solid, flat object such as a quarter lays parallel to the x-ray film, and the x-ray beam is perpendicular to the object and film, the shape of the quarter will be recognized true to form. However, if this same object is obliqued or turned vertically, its circular shape will not be distinguished:

A B C

Radiographs of a familiar object (a quarter) laying (A) flat, (B) oblique, and (C) on its edge.

Let's consider complex objects. As an exercise, close your eyes and try to imagine what an infertile chicken egg would look like. Next imagine a conch shell with and without spines. Check your imaginative pictures with the egg and shell images. An x-ray image is not a picture per se, but a collection of shadows. Think of these shadows being laid upon one another, layer by layer. The egg, for example, is not a homogeneous density. The outer shell is made of calcium; the inner fluids and pocket of air are less radiodense. The resultant image is a summation of the inner substances superimposed on the outer shell, which is viewed, roughly speaking, as a flat sheet. Note the radiopaque periphery or margin of the egg. As a rule, a curved object will appear radiopaque if it is perpendicular to the film and parallel to the x-ray beam. It will appear relatively radiolucent if parallel to the film and perpendicular to the x-ray beam, as in the center of the egg. Did you imagine the conch shell to appear the way it does in the figure? The shell is physically the same density throughout, yet many shadows of differing densities are appreciated radiographically. Apply the concept described above regarding the curved object and its position relative to the film and x-ray beam to the conch shell.

Tubular objects are commonly encountered in a foot radiograph. Observe the characteristic image of a tubular object, such as the metatarsal.

The periphery of the metatarsal shaft is radiopaque. This is because the curved margin of the shaft is perpendicular to the film and parallel to the x-ray beam. The center consists of a less dense material, the bone marrow; cortical bone is superimposed on the marrow but is nearly paralllel to the film and perpendicular to the x-ray beam. Therefore, the center of the tube appears relatively lucent.

An object with complex form can look different in many ways simply by the way it is positioned relative to the x-ray film and beam.

A B

Radiographs of an ink bottle with applicator brush in two views perpendicular to one another.

Interpreting foot radiographs is particularly challenging because multiple bones are superimposed upon one another, especially in the tarsal region. You will eventually become familiar with the radiographic appearance of each bone. Always think in layers; shadows are layers of objects superimposed on one another. The boundary of an object generally appears as a well-defined shadow on the radiograph. Separate each shadow mentally, subtracting everything else. This concept is extremely important and will be emphasized again in the following chapter dealing with normal radiographic anatomy.

Finally, think three-dimensionally. In order to mentally formulate a three-dimensional picture in your mind, two views that are perpendicular to one another are necessary. The most common examples would be anteroposterior (dorsoplantar) and lateral views of the part in question. This is best illustrated by the following distal fibular fracture:

A B

(A) Anteroposterior and (B) lateral views of a distal fibular fracture. Because the fracture is displaced only in the sagittal plane, it is nearly imperceptible in the anteroposterior view but fairly obvious in the lateral view.

In summary, interpreting a radiograph is an exercise of imagination and reasoning. The formation of the image is dependent on the object's atomic number and thickness. The image is a summation of shadows superimposed upon one another. So, think in layers when viewing a radiograph. An object can be recognized by its form. However, a familiar object may look quite unfamiliar if it is positioned differently relative to the film and x-ray beam. Finally, think three-dimensionally, always correlating the two-dimensional radiographic shadows to the three-dimensional object.

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This page was updated May 5, 1998.