Oblique ribs

The oblique projection is designed to turn the lateral portions of the ribs away from their profiled position as seen on the PA chest radiograph. Of course, this means that other aspects of the ribs will not be well demonstrated on the oblique projection. For this reason, oblique ribs projections must always be accompanied by a PA chest radiograph.

It is more than obvious that exposures should be made on arrested respiration, but the phase varies according to the ribs under examination owing to the position of the diaphragm in relation to individual rib height. Because the diaphragm effectively splits the area covered by ribs into two different densities, this has implications for adequate demonstration of ribs on the radiograph. As a result, exposure for oblique projections of the upper ribs (1–6) is made on arrested inspiration to facilitate their demonstration over the air-filled lung tissue, and ribs 7–12 on expiration to demonstrate them over abdominal tissue below the diaphragm.

In addition to the phase of respiration, angulation can be used to maximise the number of ribs shown above or below the diaphragm. Caudal angulation will project the image of the diaphragm lower in the case of the upper ribs, as can cranial angulation to project it higher and maximise the number of lower ribs shown below the diaphragm.

Posterior oblique for upper ribs (Fig. 15.1A,B)

Image receptor (IR) is vertical for projections of the ribs unless otherwise stated; antiscatter grid is often required for lower ribs.

Figure 15.1 Posterior oblique – upper ribs.

(B) Reproduced with permission from Bryan GJ. Skeletal anatomy. 3rd ed. Edinburgh: Churchill Livingstone; 1996 and Gunn C. Bones and joints. 4th ed. Edinburgh: Churchill Livingstone, 2002.

Traditionally the upper rib oblique projection has been described using antiscatter grid,^5,⁶ but as ribs 1–6 lie superior to the diaphragm in a low-density area, direct exposure is possible without an antiscatter device. This allows reduction in exposure factors and therefore affords less radiation dose to the patient. In the case of larger patients it is possible that provision of adequate contrast may be compromised by an increase in tissue density. This, coupled with the higher density over the mediastinum, may require the use of a grid.

Expose on arrested inspiration

Criteria for assessing image quality

• The entire length of ribs 1–6 are demonstrated above diaphragm

• Arm is cleared from thorax

• Heart shadow may overlie medial aspect of sixth rib in the case of right-sided ribs, or most of the sixth rib in the case of left-sided ribs

• Oblique appearance of thoracic vertebrae

• Image of anterior ribs moved laterally in comparison to their position as seen on the PA chest image

• Sharp image demonstrating ribs in contrast to air-filled lung tissue and viscera of the mediastinum and heart

Common errors	Possible reasons
High-contrast image which does not demonstrate ribs in contrast to viscera and heart	kVp selected is too low. It will be necessary to compensate for kVp increase by reducing mAs proportionally to avoid over-blackening over the lung area
Over-blackened image generally	If automatic exposure chamber used, the central ray and chamber selected may be lying over the dense mediastinum
Pale shadow overlying lateral ribs	Arm not cleared from area
Lower ribs (4–6) pale and shown below diaphragm	Exposed on expiration