Projection Terminology
A projection is described by the direction of the central ray relative to aspects and planes of the body.
Antero-posterior
Postero-anterior
Lateral
In the case of a limb, the central ray either is incident on the lateral aspect and emerges from the medial aspect (latero-medial), or is incident on the medial aspect and emerges from the lateral aspect of the limb (medio-lateral). The terms ‘latero-medial’ and ‘medio-lateral’ are used where necessary to differentiate between the two projections.
Beam angulation
for example, a lateral 20-degree caudad or a lateral 15-degree cephalad.
Oblique
The central ray passes through the body along a transverse plane at some angle between the median sagittal and coronal planes. For this projection, the patient is usually positioned with the median sagittal plane at some angle between zero and 90 degrees to the cassette, with the central ray at right-angles to the cassette. If the patient is positioned with the median sagittal plane at right-angles to or parallel to the cassette, then the projection is obtained by directing the central ray at some angle to the median sagittal plane.
Anterior oblique
The central ray enters the posterior aspect, passes along a transverse plane at some angle to the median sagittal plane, and emerges from the anterior aspect. The projection is also described by the side of the torso closest to the cassette. In the diagram
below, the left side is closest to the cassette, and therefore the projection is a described as a left anterior oblique.
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Posterior oblique
The central ray enters the anterior aspect, passes along a transverse plane at some angle to the median sagittal plane, and emerges from the posterior aspect. Again, the projection is described by the side of the torso closest to the cassette. The diagram below shows a left posterior oblique.
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Oblique using beam angulation
When the median sagittal plane is at right-angles to the cassette, right and left anterior or posterior oblique projections may be obtained by angling the central ray to the median sagittal plane.
(NB: this cannot be done if using a grid, unless the grid lines are parallel to the central ray.)
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Lateral oblique
The central ray enters one lateral aspect, passes along a transverse plane at an angle to the coronal plane, and emerges from the opposite lateral aspect. With the coronal plane at right-angles to the cassette, lateral
oblique projections can also be obtained by angling the central ray to the coronal plane.
(NB: this cannot be done if using a grid, unless the grid lines are parallel to the central ray.)
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The chapters that follow describe radiographic projections by reference to the following criteria:
• The position of the patient relative to the cassette.
• The direction and centring of the X-ray beam: this is given by reference to an imaginary central ray of the X-ray beam.
• Beam angulation relative to horizontal or vertical.
Examples of these are given below:
Position: erect; anterior aspect facing the cassette and median sagittal plane at right-angles to the film
Direction and centring of X-ray beam: the central ray is directed
horizontally (along the median sagittal plane) and centred to the spinous process of the sixth thoracic vertebra
Position: erect; left side against the cassette and median sagittal plane
parallel to the cassette
Direction and centring of X-ray beam: the central ray is directed
horizontally (at right-angles to the median sagittal plane) to a point 5 cm anterior to the spinous process of the seventh thoracic vertebra
Position: left lateral decubitus; median sagittal plane parallel to the cassette
Direction and centring of X-ray beam: from the vertical, the central ray is angled 10 degrees caudally and directed (along a coronal plane) to a point in the mid-axillary line 7.5 cm anterior to the posterior aspect of the patient, at the level of the lower costal margin
Position: supine; median sagittal plane at right-angles to the table
Direction and centring of X-ray beam: from the vertical, the central ray is angled 30 degrees caudally and directed to a point 2.5 cm superior to the
symphysis pubis
Position: supine; median sagittal plane parallel to the cassette placed in a vertical Bucky against the patient’s left side
Direction and centring of X-ray beam: the horizontal central ray is directed (at right-angles to the median sagittal plane) to the lower costal margin in the mid-axillary line
Position: supine and then rotated so that the left side is moved away from the table to bring the median sagittal plane at 45 degrees to the table
Direction and centring of X-ray beam: the vertical central ray is directed to a point 2.5 cm to the right of the midline at the level of the third lumbar vertebra
Position: left lateral decubitus; median sagittal plane parallel to the table and at right-angles to a cassette supported vertically against the patient’s anterior aspect
Direction and centring of X-ray beam: the horizontal central ray is at right-angles to the posterior aspect of the patient in the midline (and passes along the median sagittal plane) at the level of the third lumbar vertebra
Position: semi-recumbent or supine; leg extended fully; posterior aspect of the ankle in contact with cassette; lateral and medial malleoli
equidistant from the cassette
Direction and centring of X-ray beam: vertical central ray directed to a point midway between the malleoli