Ribs Positioning Series

AP ribs

Anteroposterior projection · bilateral, posterior ribs

Demonstrates

Both sides of the bony thorax, with the posterior ribs closest to the image receptor and therefore recorded with the least magnification and the sharpest detail. The vertebral ends, the necks and the angles of the ribs are the parts this projection is for. The thoracic spine lies in the midline with the ribs arching away from it on both sides, and the sternoclavicular joints sit equidistant from it when the thorax is square. The scapulae are drawn laterally and lie clear of the lung fields.

Part position

Erect for ribs above the diaphragm if the patient's condition allows, and supine for ribs below it. Align the midsagittal plane to the central ray and to the midline of the grid. Raise the chin so it does not superimpose the upper ribs and have the patient look straight ahead. Draw the scapulae laterally off the lung fields by resting the hands on the hips with the palms turned outward, which rolls the shoulders forward and carries the scapulae away from the rib cage. Allow no rotation of the thorax or the pelvis.

Central ray

Above the diaphragm: 90° to the image receptor, centered to the midsagittal plane at a level 3 to 4 inches (8 to 10 cm) below the jugular notch, which is the level of the seventh thoracic vertebra. Below the diaphragm: 90° to the image receptor, centered to the midsagittal plane midway between the xiphoid process and the lower margin of the ribs.

Why it is positioned this way

A rib injury is a question about one part of one rib, and the first thing to settle is whether that part lies above or below the diaphragm — because the two halves of the thorax have to be exposed as if they were different body parts. Above the diaphragm the ribs are surrounded by air-filled lung, so they are exposed with the patient holding a deep inspiration: the diaphragm drops, more ribs clear it, and the ribs are seen against a dark background. Below the diaphragm the ribs lie over the liver, the spleen and the stomach, so they need a heavier exposure and are taken on full expiration, which lets the diaphragm rise and carries those ribs up into the denser abdominal shadow where they can be matched to it. Choosing the wrong one is the classic way this examination is repeated: an above-diaphragm technique applied to the lower ribs leaves them far too light to read. The AP is chosen when the injury is posterior, because putting the posterior ribs nearest the receptor is what keeps them sharp. It is also the projection reached for when a patient cannot stand or cannot be turned, which is what the erect-or-supine choice turns on. The shoulder roll does not turn on it: both bilateral projections ask for the scapulae to be drawn off the lung fields, because a scapula lying over the upper ribs hides the very bone the examination is being taken to look at.