Thoracic Viscera: Chest & Upper Airway

Anatomy of the Body Habitus and Thoracic Cavity

The study of thoracic viscera begins with an understanding of body habitus, which refers to the common variations in the shape of the human body. These variations are classified into four primary categories: hypersthenic, sthenic, asthenic, and hyposthenic. Understanding these categories is essential for proper radiographic positioning and ensuring that the organs of the chest are captured accurately on the imaging receptor.

The thoracic cavity is defined as the space extending from the superior thoracic aperture to the inferior thoracic aperture. Critically, the diaphragm serves as the physical boundary that separates the thoracic cavity from the abdominal cavity. The cavity itself is not a single open space but contains three separate chambers: the pericardial cavity, which houses the heart, and the right and left pleural cavities, which contain the respective lungs. In addition to the lungs and heart, the thoracic cavity contains the organs of the respiratory, cardiovascular, and lymphatic systems, as well as the inferior portion of the esophagus and the thymus gland.

The mediastinum is the central space between the two pleural cavities. It is bounded anteriorly by the sternum, posteriorly by the spine, and laterally by the lungs. This space contains all the thoracic structures excluding the lungs, pleura, and the pleural cavities themselves. The specific contents of the mediastinum include the heart, the great vessels, the trachea, the esophagus, the thymus gland, lymphatics, nerves, fibrous tissue, and fat.

The Respiratory System and Airway Anatomy

The respiratory system functions as the passage of air into the body for gas exchange. The path of air begins at the nostrils and nasal cavity, where it is filtered, warmed, and moistened. From there, it moves to the pharynx, which is a common passage for both food and air. The pharynx is subdivided into three sections: the nasopharynx (located posterior to the nasal cavity), the oropharynx (located posterior to the mouth), and the laryngeal pharynx or laryngopharynx (located posterior to the larynx).

Following the pharynx is the larynx, or voice box, which is suspended from the hyoid bone. It is located between the superior margin of $C4$ and the junction of the trachea at the inferior margin of $C6$. The thyroid cartilage within the larynx forms the laryngeal prominence, commonly known as the Adam’s apple. The epiglottis guards the larynx superiorly and anteriorly, acting as a trap to prevent food or liquid from entering the airway during the act of swallowing.

The trachea, or windpipe, is a fibrous, muscular tube characterized by $16$ to $20$ C-shaped cartilaginous rings embedded in its walls to provide necessary rigidity and prevent collapse. It lies mostly in the midline and is positioned anterior to the esophagus. The trachea extends from the level of $C6$ down to $T4$ or $T5$. The final tracheal cartilage is elongated and features a hook-like process known as the carina. At the carina, the trachea bifurcates into the primary or main bronchi. As air moves inward, the primary bronchi branch into secondary (lobar) bronchi, then tertiary bronchi, followed by smaller bronchioles and terminal bronchioles. The terminal bronchioles communicate with alveolar ducts, which end in alveolar sacs lined with alveoli. It is within the alveoli that the exchange of oxygen ($O_2$) and carbon dioxide ($CO_2$) occurs between the lungs and the capillaries of the circulatory system.

Detailed Pulmonary Anatomy and Movement

The lungs are the primary organs of respiration and are responsible for the vital exchange of gases. They are composed of a light, spongy substance called parenchyma and are encased in a double-walled serous membrane sac known as the pleura. The inner layer, which adheres to the lung surface, is the visceral pleura, while the outer layer, which lines the thoracic cavity, is the parietal pleura. The potential space between these two layers is the pleural cavity, which contains serous fluid to reduce friction during movement.

Each lung is divided into lobes by deep fissures. The right lung is divided into three lobes (superior, middle, and inferior) by the oblique and horizontal fissures. The right lung is notably shorter and broader than the left lung due to the upward displacement caused by the liver. The left lung is divided into only two lobes (superior and inferior) by a single oblique fissure and features a cardiac notch on its medial border to accommodate the heart.

The superior portion of each lung is called the apex, which reaches above the level of the clavicles. The inferior portion is known as the base, which rests obliquely on the diaphragm. The base of the lung is lower in the back and sides than it is in the front. The hilum is a depression on the medial border where the bronchi, blood vessels, lymph vessels, and nerves enter and exit the lung. The lateral-most inferior margin of the lung is referred to as the costophrenic angle.

Lung movement is dictated by the phases of respiration. During inspiration, the diaphragm moves downward and the ribs move outward, causing the lungs to move down and expand. During exhalation or expiration, the diaphragm moves upward and the ribs move inward, pushing the air out of the lungs.

Surface Landmarks and Anatomical Reference Points

Radiographic positioning relies on external surface landmarks to locate internal structures accurately. These include the hyoid bone at the level of $C3$ and $C4$, and the thyroid cartilage (Adam’s apple) at the level of $C5$. The vertebral prominens is located at the level of $C7$ and $T1$. Approximately $5\,\text{cm}$ or $2\,\text{inches}$ above the jugular notch lies the level of $T1$. The jugular notch itself is found at $T2$ and $T3$, while the sternal angle corresponds to $T4$ and $T5$.

The inferior angles of the scapulae are located at the level of $T7$. Moving lower, the xiphoid process is at $T9$ and $T10$. The inferior costal margin is used to locate $L2$ and $L3$, while the superior aspect of the iliac crests marks the level of $L4$ and $L5$. Further landmarks include the anterior superior iliac spine ($S1$) and the pubic symphysis, which is used to locate the coccyx and greater trochanters.

Radiographic Projections of the Upper Airway and Soft Tissue Neck

Essential projections for the upper airway and soft tissue neck include the Anteroposterior (AP) and the Lateral projections. These are primarily used to evaluate the airway from the superior oropharynx to the proximal trachea, the cervical spine, or the soft tissues of the anterior neck. Clinical indications for these views include the search for foreign bodies, assessment of swelling or masses, and identifying fractures of the larynx or hyoid bone.

For the AP projection, the patient is positioned either supine or upright with the midsagittal plane (MSP) centered perpendicular to the midline of the image receptor (IR). The neck is extended slightly, and the shoulders are adjusted to lie in the same transverse plane. The central ray (CR) is directed perpendicularly through the MSP at the level of the laryngeal prominence for the upper airway, or at the manubrium for the larynx and superior mediastinum. The collimated field should be $12\,\text{inches}$ long and extend $1\,\text{inch}$ beyond the skin on the sides, not exceeding $10\,\text{inches}$ in width.

For the Lateral projection, the patient is seated or standing in a right or left lateral position. The patient's hands are clasped behind the body with the shoulders rotated posteriorly. The MSP is centered parallel to the midline of the IR while the midcoronal plane (MCP) is perpendicular. The central ray is horizontal through the MCP at the level of the laryngeal prominence for the upper airway. For the trachea and superior mediastinum, the CR is directed at the level of the jugular notch midway between the notch and the MCP. For both projections, the exposure should be made during slow inspiration to ensure the airway is filled with air.

General Procedural Guidelines for Chest Radiography

Successful chest radiography requires careful patient preparation and adherence to technical standards. Patients must remove all artifacts from the anatomy of interest, including long earrings, necklaces, and clothing items such as bras with metal underwires, camisoles with adjustable straps, vinyl lettering, or rhinestones.

Ambulatory patients should be imaged in an upright position whenever possible. This position prevents the engorgement of pulmonary vessels, allows gravity to depress the diaphragm to its lowest position for full lung expansion, and demonstrates air-fluid levels. In cases where air-fluid levels are critical but the patient is nonambulatory, a decubitus position must be used. For a lateral projection, the left lateral is most common because it places the heart closer to the IR, thereby reducing cardiac magnification.

The recommended Source-to-Image Receptor Distance (SID) for chest radiography is at least $72\,\text{inches}$. This large distance is used to decrease magnification of the heart and increase the spatial resolution or sharpness of the image. A grid is often used to eliminate excess scatter radiation reach the IR, which enhances contrast resolution.

Technical factors typically involve a high Kilovoltage Peak (kVp) of approximately $110$ to $120$ to ensure penetration of the mediastinal structures. Radiation protection involves proper beam restriction and lead gonadal shielding when appropriate. Standard breathing instructions involve making the exposure after the second full inspiration to ensure maximum lung expansion. However, separate radiographs on inspiration and expiration may be taken to diagnose conditions such as pneumothorax, atelectasis, foreign body presence, or to assess diaphragm movement.

Specific Chest Projections: PA, Lateral, and Oblique

The Posteroanterior (PA) projection is a staple of chest imaging. The patient stands upright facing the vertical grid device with the light field adjusted $1.5$ to $2\,\text{inches}$ above the shoulders. The MSP is centered perpendicular to the IR, the chin is extended, and the arms are placed down with the shoulders rotated forward to touch the grid; this rotation pulls the scapulae laterally out of the lung fields. The central ray enters perpendicularly at the level of $T7$. A successful PA image displays the air-filled trachea, lung apices, heart, and costophrenic angles. The ribs should be counted to ensure at least $10$ posterior ribs are visible above the diaphragm.

The Lateral projection involves placing the patient's left side against the IR with the MSP parallel and the MCP perpendicular. The arms are raised above the head. The central ray enters perpendicularly at the MCP at the level of $T7$. This view provides a profile of the sternum, the heart shadow, and the posterior ribs, while also demonstrating the hilar region and the costophrenic angles.

Oblique projections (PA or AP) are used to visualize specific lung areas or the heart and great vessels. For a PA Oblique projection in the Left Anterior Oblique (LAO) position, the patient is rotated $45$ degrees toward the left side. This position demonstrates the maximum amount of the right lung, the trachea, the carina, the aortic arch, and the heart. The Right Anterior Oblique (RAO) position, where the patient is rotated $45$ degrees toward the right, highlights the left lung and the left atrium. AP Oblique projections (LPO and RPO) are used when patients are too ill for prone positioning. An LPO position provides the same anatomical visualization as an RAO position, while an RPO position corresponds to the LAO view.

Advanced and Specialized Chest Projections

The Anteroposterior (AP) Chest projection is typically utilized for patients who are too ill to stand or sit. The patient is supine or upright in a wheelchair or stretcher. The MSP is centered to the IR, and the central ray is directed perpendicularly to the MSP at a level $3\,\text{inches}$ below the jugular notch. If possible, the patient’s hands are placed on the hips to draw the scapulae laterally. AP chest images often show more horizontal clavicles and some magnification of the heart compared to PA images.

To visualize the pulmonary apices specifically, the AP Axial projection can be performed using the Lindblom method. The patient is placed in a lordotic position, standing about $1\,\text{foot}$ in front of the grid and leaning backward until the shoulders rest on the IR. The MCP is angled $15$ to $20$ degrees from the vertical. The central ray enters the midsternum, approximately $3$ to $4\,\text{inches}$ below the jugular notch. If the patient cannot achieve the lordotic position, an alternative axial projection uses a $15$ to $20$ degree cephalic angle on the central ray while the patient is supine or upright.

Decubitus positions are used to identify pleural effusions or pneumothorax when the patient cannot stand. For a suspected pleural effusion, the patient lies on the affected side so the fluid will settle and not be obscured by the heart. For a suspected pneumothorax, the patient lies on the unaffected side so the air will rise and be easily seen. It is essential for the patient to remain in the lateral decubitus position for at least $5\,\text{minutes}$ before the exposure to allow air to rise or fluid to settle. These projections use a horizontal central ray to capture air-fluid levels.