13.2 Chest & Thorax
Key Takeaways
- The PA chest is performed erect at a 72-inch (180 cm) SID to minimize heart magnification, with the CR perpendicular to T7 (inferior scapular angle).
- Full inspiration is achieved by exposing on the SECOND deep breath; a properly inflated PA chest shows a minimum of 10 posterior ribs above the diaphragm.
- Rotation on a PA chest is detected by unequal distance between the vertebral column and the sternoclavicular joints; the scapulae are rolled off the lungs by rotating the shoulders forward.
- For a suspected pleural effusion (fluid), the lateral decubitus is taken with the affected side DOWN; for a suspected pneumothorax (air), the affected side is placed UP.
- High kVp (110-125) with a short exposure time produces the long-scale contrast and motion control needed to penetrate the mediastinum on a PA chest.
The Routine Chest: PA and Lateral
The chest is the most commonly performed radiographic examination, so ARRT tests it heavily. The routine consists of a PA and a left lateral, both performed erect whenever possible. Erect positioning lets gravity pull the diaphragm down (better lung aeration) and lets air-fluid levels settle so effusions become visible.
The 72-inch SID
The chest is imaged at a 72-inch (180 cm) source-to-image-receptor distance (SID) rather than the standard 40 inches. The reason is magnification control: the heart sits well anterior in the thorax, far from a PA-image receptor, so the increased object-to-image distance (OID) would magnify it. Increasing the SID to 72 inches reduces magnification of the heart and great vessels (keeping the cardiothoracic ratio accurate) and improves recorded detail.
PA Chest Positioning
For the PA projection, the patient stands facing the IR with the chin extended over the top and the midsagittal plane (MSP) perpendicular to the IR. The single most-tested positioning maneuver is rolling the shoulders forward and rotating them anteriorly; this draws the scapulae laterally off the lung fields so they do not superimpose the lungs. The CR is perpendicular to T7, which you locate at the inferior angle of the scapula, about 7-8 inches below the vertebra prominens.
Expose on full inspiration, taken on the second deep breath — the second inspiration aerates the lungs more fully and lowers the diaphragm, demonstrating maximum lung field.
Lateral Chest Positioning
The routine lateral is a left lateral: the patient's left side is against the IR because the heart is a left-sided structure, and placing it closer to the receptor reduces its magnification. The arms are raised above the head to move the humeri out of the field, the MCP is perpendicular, and the CR is directed to T7. A true lateral superimposes the posterior ribs; separation of more than about 1 cm indicates rotation.
Evaluation Criteria (High-Yield)
ARRT loves image-critique questions. Memorize the acceptance criteria:
| Criterion | Acceptable image shows |
|---|---|
| Inspiration | Minimum of 10 posterior ribs visible above the diaphragm |
| No rotation (PA) | Equal distance from the vertebral column (spinous processes) to each sternoclavicular (SC) joint |
| Scapulae | Projected outside the lung fields |
| Collimation | Apices to costophrenic angles included |
| No rotation (lateral) | Posterior ribs superimposed (<1 cm separation) |
| Penetration | Faint vertebrae and mediastinal detail visible through the heart shadow |
If fewer than 10 posterior ribs are seen, the exposure was made on incomplete inspiration and must be repeated. If the SC joints are unequal distances from the spine, the patient was rotated.
Technical Factors
A PA chest uses a high kVp technique (110-125 kVp) to create long-scale (low) contrast so that both the aerated lungs and the dense mediastinum are visualized on one image; the many shades of gray let you "see through" the heart. A short exposure time freezes involuntary cardiac and respiratory motion. A grid is used at 72 inches to control the substantial scatter produced by the high kVp. When automatic exposure control (AEC) is used, the two outer (lateral) chambers are activated so the sensors read lung tissue rather than the dense spine, preventing an underexposed image.
Common Mistake: The Supine (Mobile) AP Chest
When a patient cannot stand, the chest is taken AP and supine, often with a mobile (portable) unit at a shorter 40-48 inch SID. Understand what changes: the heart is now farther from the receptor and imaged at a shorter SID, so it appears magnified, and the diaphragm sits higher because the patient is recumbent, so fewer ribs are demonstrated and the lungs look more congested. These are expected artifacts of the AP supine technique — not disease and not a repeatable error. On the exam, do not "correct" a supine AP chest by comparing it to erect PA criteria; judge it by AP-supine expectations.
Special Chest Projections
Lateral Decubitus — Air vs. Fluid
When a patient cannot stand, or to confirm a small air or fluid collection, use a lateral decubitus with a horizontal CR. The rule is the heart of many exam items:
- Fluid (pleural effusion): affected side DOWN. Fluid gravitates and layers along the dependent lateral chest wall, where it is easily seen against air-filled lung.
- Air (pneumothorax): affected side UP. Free air rises to the uppermost point of the pleural space, best profiled when that side is elevated.
Allow several minutes for the air or fluid to settle before exposing, and center the horizontal CR to T7 (about 3 inches below the jugular notch).
Oblique Chest (RAO/LAO)
PA oblique chest projections (RAO and LAO) rotate the patient 45 degrees to visualize the lungs behind the heart and the great vessels. A 45-degree RAO demonstrates the maximum area of the right lung; the LAO better profiles the trachea and the aortic arch (often taken at 55-60 degrees for a cardiac series).
AP Axial Lordotic (Apical)
The AP axial lordotic projection, also called the apical lordotic or Lindblom method, is used to demonstrate the lung apices free of superimposing clavicles — valuable for detecting tuberculosis, a Pancoast tumor, or a calcified apical lesion. The erect patient leans back so the shoulders rest against the IR with the thorax lordotic, or the CR is angled 15-20 degrees cephalad if the patient cannot lean back. On a correct image, the clavicles are projected above the apices.
A final niche use: a PA chest on expiration is added to compare with the inspiration image when confirming a small pneumothorax or evaluating for a foreign body causing air trapping.
A PA chest radiograph is submitted for evaluation. Which finding confirms the patient took an adequate inspiration?
A physician suspects a small right-sided pleural effusion. To layer the free fluid where it can be visualized, which decubitus position should be used?
Why is the PA chest performed at a 72-inch SID rather than the standard 40 inches?