Basics of X-ray physics

The X-ray beam

X-rays travel in straight lines and a beam of X-rays diverges from its source. Structures the beam hits first will be magnified in relation to those which are nearer the detector. To reduce magnification the X-ray source can be moved further away from the subject. Structures that need to be measured accurately should be placed closer to the detector.

Key points

  • X-rays travel in straight lines
  • Body parts further away from the detector are magnified compared with those that are closer
  • Occasionally magnification can be helpful in localising abnormalities

Anterior-Posterior (AP) magnification

The X-ray beam for an anterior-posterior (AP) view of the chest exaggerates heart size as the heart is relatively near to the beam source.

Near beam magnification

A source that is too near the patient will further exaggerate the size of structures nearest to that source.

Posterior-Anterior (PA) projection

A posterior-anterior(PA) beam view of the chest allows more accurate representation of heart size as the heart is positioned closer to the detector and is therefore less magnified.

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Helpful magnification

Knowledge of the magnifying properties of an X-ray beam is sometimes helpful.

In this image there is a simulated abnormality (arrow) located in the posterior part of the chest, not visible on a PA view as it lies below the top of the diaphragm (green line).

This is a LEFT lateral image, that is, the LEFT side of the body is closest to the detector and the X-ray beam has passed from RIGHT to LEFT. Body structures such as the RIGHT ribs therefore appear bigger and are more posteriorly placed in relation to their counterparts on the LEFT which are less magnified.

It is possible to locate this lesion on this lateral view because it extends behind the line formed by the LEFT ribs. Assuming it lies within the rib cage at all, it must be located on the RIGHT.

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