Chapter 10: Bony Thorax

Detailed Chapter Overview

Chapter 10 focuses on the radiography of the bony thorax, which primarily involves the sternum and the 12 pairs of ribs. This area presents unique technical challenges due to the superimposition of the thoracic spine, heart, and lung markings. A thorough understanding of the anatomy of the sternum and the classification and structure of the ribs is the foundation for producing diagnostic images. This chapter meticulously details the articulations of the bony thorax, including the sternoclavicular, costovertebral, and costotransverse joints, which are critical for understanding the mechanics of respiration and potential pathologies. A central theme of the chapter is the mastery of specialized radiographic techniques designed to overcome anatomical superimposition. It provides an in-depth explanation of the RAO position for the sternum, detailing why this specific oblique rotation is necessary to project the sternum over the homogenous heart shadow. Furthermore, it emphasizes the critical role of breathing techniques—differentiating between inspiration for upper ribs, expiration for lower ribs, and the orthostatic (shallow breathing) technique for the sternum—to blur out unwanted structures and enhance visibility. The chapter provides detailed, step-by-step instructions for all routine and special projections, including specific rules for oblique positioning to elongate the axillary portions of the ribs depending on whether the area of interest is anterior or posterior. For every projection, rigorous evaluation criteria are established, ensuring the radiographer can consistently produce high-quality, diagnostic images of this anatomically complex region.

In-Depth Study Guide

Anatomy of the Sternum

The sternum, or breastbone, is a flat, narrow bone located in the anterior median line of the chest. It consists of three main parts.

• Manubrium: The superior portion, which is the widest part of the sternum. Its superior border has a palpable depression known as the **jugular notch** (also called the suprasternal or manubrial notch), which lies at the level of the T2-T3 interspace.
• Body (Corpus or Gladiolus): The longest part of the sternum. The manubrium and body connect at an obtuse angle known as the **sternal angle** (angle of Louis), which is a palpable landmark located at the level of the T4-T5 interspace.
• Xiphoid Process: The most inferior and smallest part of the sternum. It is cartilaginous in youth and often ossifies in adulthood. It corresponds to the vertebral level of T9-T10.

Anatomy of the Ribs

The 12 pairs of ribs form the protective cage of the thorax. They are classified based on their anterior articulation.

• Classification:
  • True Ribs (1-7): These ribs connect directly to the sternum via their own individual costocartilage.
  • False Ribs (8-12): These ribs do not connect directly to the sternum. Ribs 8, 9, and 10 connect to the costocartilage of the 7th rib.
  • Floating Ribs (11-12): A subset of false ribs, they do not connect to any cartilage anteriorly and are embedded in the posterior abdominal wall muscles.
• Typical Rib Structure:
  • Head: The posterior end, which articulates with the vertebral bodies at the costovertebral joints.
  • Neck: A constricted portion just lateral to the head.
  • Tubercle: A small eminence lateral to the neck that articulates with the transverse process of a thoracic vertebra at the costotransverse joint.
  • Body (Shaft): The long, curved main portion of the rib. The sharp curve is known as the **angle** of the rib.
  • Costal Groove: A groove along the inferior and internal border of each rib that protects the intercostal artery, vein, and nerve.

Positioning Considerations and Technique

Radiography of the bony thorax requires careful manipulation of patient position and breathing to isolate the structures of interest.

Breathing Instructions:

This is one of the most critical aspects of bony thorax radiography.

• Sternum & Upper Ribs (Above Diaphragm): Projections are taken on **full inspiration**. This depresses the diaphragm to its lowest level, providing better visualization of the upper nine ribs.
• Lower Ribs (Below Diaphragm): Projections are taken on **full expiration**. This raises the diaphragm, providing a uniform density over the lower ribs and reducing the thickness of the abdomen.
• Orthostatic (Breathing) Technique for Sternum: For the RAO sternum projection, a specific technique is used where the patient breathes shallowly during a long exposure (2-3 seconds) with a low mA. This blurs the overlying lung markings and posterior ribs, making the sternum stand out clearly against the heart shadow.

Patient Position and Obliquity:

• Erect vs. Recumbent: Erect positions are generally preferred for patient comfort, especially with trauma, and to use gravity to help lower the diaphragm for upper rib studies.
• Rule for Rib Obliques: To visualize the axillary portion of the ribs and move them away from the spine, obliques are necessary. The rule is to rotate the patient **toward** the affected side for posterior rib pain, and **away** from the affected side for anterior rib pain.
  • Posterior Rib Injury (e.g., right posterior ribs): An RPO is performed. This places the posterior aspect of the right ribs closest to the IR.
  • Anterior Rib Injury (e.g., right anterior ribs): An LAO is performed. This rotates the spine away and elongates the profile of the right anterior ribs.

Routine and Special Projections

Sternum

• RAO Projection: This is the standard projection for the sternum, NOT an AP or PA. The patient is rotated 15 to 20 degrees into a Right Anterior Oblique position. This slight rotation shifts the sternum just enough to project it into the homogenous heart shadow, away from the dense thoracic vertebrae.
• Lateral Projection: The patient stands in a true lateral position with hands clasped behind their back to pull the shoulders posteriorly and out of the field of view. A 72-inch SID is used to reduce magnification and improve image detail.

Sternoclavicular (SC) Joints

• PA Projection: A baseline view of both SC joints.
• PA Oblique (RAO/LAO): A shallow 10-15 degree rotation is used to visualize the SC joint of interest open and projected next to the spine. The LAO position best demonstrates the left SC joint, and the RAO best demonstrates the right.

Ribs

• AP/PA Projections: The choice depends on the site of injury. For posterior rib pain, an AP projection is preferred as it places the area of interest closer to the IR. For anterior rib pain, a PA projection is used.
• Oblique Projections: A 45-degree oblique is standard. Follow the rules of obliquity described above to ensure the correct side is elongated and visualized. The projection is named for the side demonstrated (e.g., AP Oblique for Right Posterior Ribs).