Thorax Anatomy and Chest/Rib Positioning

Thoracic Skeletal Anatomy

The thorax is a bony cage formed by the sternum anteriorly, twelve pairs of ribs laterally, and twelve thoracic vertebrae posteriorly. The ribs are numbered 1–12 from superior to inferior. Ribs 1–7 are true ribs, each attaching directly to the sternum via costal cartilage. Ribs 8–10 are false ribs, whose cartilage joins the cartilage of rib 7 before reaching the sternum. Ribs 11 and 12 are floating ribs with no anterior attachment. On a PA chest radiograph you can count posterior rib arcs to assess lung expansion: ten posterior ribs visible above the diaphragm indicates adequate full inspiration.

The sternum has three segments: the manubrium (superior, articulates with the clavicles and first ribs), the body, and the xiphoid process (inferior). The sternum is best demonstrated on a lateral or oblique projection because its midline position overlaps the spine on a PA view.

Soft-Tissue Thoracic Anatomy on Radiographs

On a properly exposed PA chest radiograph you should identify:

• Trachea: midline radiolucent column descending to approximately the level of T4–T5, where it bifurcates.
• Carina: the point of tracheal bifurcation into the right and left main bronchi; the carina angle widens with left atrial enlargement.
• Lung fields: the right lung has three lobes; the left lung has two lobes and a cardiac notch. The right hilum is slightly lower than the left on most individuals.
• Mediastinum: the central compartment containing the heart, great vessels, trachea, esophagus, and lymph nodes. It divides into superior, anterior, middle, and posterior regions.
• Heart silhouette: occupies roughly two-thirds of the thoracic width or less on a PA projection. On AP projections the heart appears magnified because it is farther from the image receptor.
• Diaphragmatic domes: the right dome is normally slightly higher than the left (liver beneath it). Both domes should be visible at full inspiration.
• Costophrenic angles: the acute angles where the diaphragm meets the lateral chest wall. Blunting indicates fluid accumulation.

PA Chest Projection

The PA chest is the standard chest projection. The patient stands erect, anterior chest wall against the upright Bucky, chin raised, hands on hips with elbows forward to rotate the scapulae away from the lung fields. The central ray (CR) is directed horizontally to T7 (inferior angle of the scapula). Source-to-image distance (SID) is 72 inches (180 cm) — the extended SID minimizes cardiac magnification. Breathing instruction: "Take a deep breath in and hold." Expose at full inspiration. On the image, ten posterior ribs above the diaphragm confirms adequate inspiration; fewer than nine suggests the patient did not inhale fully.

Lateral Chest Projection

The patient stands with the left side against the upright Bucky (left lateral is standard; the heart is closer to the receptor, reducing magnification). Arms are raised overhead or held forward. CR is horizontal to T7 at the midcoronal plane. SID is 72 inches. Breathing instruction: same — full inspiration and hold. The lateral demonstrates the retrosternal and retrocardiac spaces, the posterior costophrenic sulci, and superimposition of the posterior ribs.

AP Portable Chest

When a patient cannot stand or sit upright at a wall Bucky, a portable (mobile) unit is used. The image receptor is placed behind the patient, the CR is directed perpendicular to the receptor at T7. SID is typically 40–72 inches depending on patient condition and equipment reach — document the SID used because it affects magnification. Because the patient is supine or semi-upright and the heart is farther from the receptor, cardiac silhouette appears larger than on a PA. Mark all portable images with the patient position (AP, supine, or semi-erect). Breathing instruction: "Take a breath in and hold." Collimate closely to reduce scatter and radiation dose to surrounding patients and staff in the room.

Rib Projections

Rib studies require separating the area of clinical interest relative to the diaphragm.

• Ribs above the diaphragm (posterior, anterior, or oblique): expose on full inspiration and hold to elevate the diaphragm and uncover the lower ribs.
• Ribs below the diaphragm: expose on full expiration and hold to elevate abdominal contents downward and push the diaphragm up, moving it out of the area of interest.

Oblique rib projections (AP or PA oblique, 45°) separate the axillary ribs from the spine and sternum. The affected side is placed closest to the receptor.

Radiation Safety and Patient Safety

Always place lead markers (R or L) within the primary beam before exposure — never add a marker digitally after the fact on initial positioning. Use gonadal shielding when the gonads fall within or near the primary field and shielding will not obscure diagnostic information. Collimate the beam to the area of interest to reduce patient dose and improve image contrast. Confirm patient identity and imaging order before positioning. Communicate breathing instructions clearly; a patient who moves on exposure produces a blurred image and may require a repeat, increasing dose. Minimize repeat exposures — each exposure carries risk, however small.