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Key principles


Introduction

Patients with traumatic injuries can be placed into one of three major groups. The imaging approach will differ between these groups.

Polytrauma (in which one injury may be life threatening)

  • ▪

    Imaging:

    • Strict local protocols and algorithms utilising early ultrasound (US) and/or multidetector computed tomography (CT). The use of plain film radiology in the Emergency Department (ED) is generally limited .

Multiple injuries (none of which is life threatening)

  • ▪

    Imaging:

    • Plain film radiology is utilised in the ED.

Single injury (not life threatening)

  • ▪

    Imaging:

    • Plain film radiology is the principal imaging investigation.

This book describes the assessment and interpretation of the plain radiographs that are customarily obtained in patients who have not sustained a life threatening injury.

Basic radiology

The radiographic image

The tissues that lie in the path of the X-ray beam absorb (ie attenuate) X-rays to differing degrees. These differences account for the radiographic image.

Radiograph of a chicken leg (bone) partially submerged in a layer of vegetable oil (fat) floating on water (soft tissue). Note the difference in the blackening of the X-ray film due to absorption by the different tissues.

Fracture lines: usually black, but sometimes white

When a fracture results in separation of bone fragments, the X-ray beam that passes through the gap is not absorbed by bone. This results in a black (ie lucent) line on the radiograph.

On the other hand, bone fragments may overlap or impact into each other. The resultant increased thickness of bone absorbs more of the X-ray beam and so results in a white (ie sclerotic or denser) area on the radiograph.

Three fractures. On the left the fragments are distracted and the fracture shows as a dark black line. In the centre the fragments overlap resulting in a dense region on the radiograph. On the right the fragments are impacted, also producing a dense region.

Fat pads and fluid levels

There are radiological soft tissue signs which can provide a clue that a fracture is likely. These include displacement of the elbow fat pads (see pp. 97 and 102 ), or the presence of a fat–fluid level at the knee joint (see pp. 248–249 ).

The principle of two views

‘One view only is one view too few’

Many fractures and dislocations are not detectable on a single view. Consequently, it is normal practice to obtain two standard projections, usually at right angles to each other. The example below shows two views of an injured finger.

At sites where fractures are known to be exceptionally difficult to detect (for example a suspected scaphoid fracture), it is routine practice to obtain more than two views.

Injured finger.

The true extent of the injury is only evident on the lateral view.

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