Chest radiography for radiologic technologists.

After completing this article, the reader should be able to:

* Identify the basic anatomy seen on a chest radiograph.

* Describe the anatomical relationships of various organs in the chest.

* Describe the basic positioning requirements for a chest exam.

* List the criteria used to critique a chest radiograph. * Identify radiologists' requirements for interpreting a chest radiograph.

* Discuss several common disease processes of the lungs and their radiographic appearances.

* Evaluate a chest radiograph for various devices such as endotracheal tubes, chest tubes and central venous catheters.

* Describe several pathologies of the chest.

Chest radiography is the most common radiographic procedure performed in medical imaging departments, and one of the most often repeated exams. (1-3) It is estimated that in the United States 68 million chest radiographs are performed each year. (4) Chest radiography is performed to evaluate the lungs, heart and thoracic viscera. Additionally, disease processes such as pneumonia, heart failure, pleurisy and lung cancer are common indications. The American College of Radiology (ACR) and others suggest that daily chest radiographs are indicated for critically ill patients. (5-7) This includes patients on ventilators, as well as those with acute cardiopulmonary problems. According to the ACR Practice Guidelines for the Performance of Pediatric and Adult Chest Radiography, there are several indications for a chest radiograph. (5) Some of these indications include:

* Evaluation of signs and symptoms potentially related to the respiratory, cardiovascular and upper gastrointestinal systems, as well as the musculoskeletal system of the thorax. The chest radiograph also can help to evaluate thoracic disease processes, including systemic and extrathoracic diseases that secondarily involve the chest. Because the lungs are a frequent site of metastases, chest radiography can be useful in staging extrathoracic, as well as thoracic, neoplasms.

* Follow-up of known thoracic disease processes to assess improvement, resolution or progression.

* Monitoring of patients with life-support devices and patients who have undergone cardiac or thoracic surgery or other interventional procedures.

* Compliance with government regulations that mandate chest radiography. Examples include surveillance posteroanterior chest radiographs for active tuberculosis or occupational lung disease or exposures and other surveillance studies required by public health law.

* Preoperative radiographic evaluation when cardiac or respiratory symptoms are present or when there is significant potential for thoracic pathology that could compromise the surgical result or lead to increased perioperative morbidity or mortality. (5)

The radiographer's role is to provide the physician with an image of the chest that is diagnostic and aids in the treatment of the patient. This cannot be accomplished satisfactorily without adequate knowledge of chest anatomy, pathology and consistent positioning in both the ambulatory and bedridden patient.

Normal Chest Anatomy

The Bony Thorax

The bony thorax of the chest is composed of the sternum anteriorly and 12 pairs of ribs that surround the lungs. Each pair of ribs connects to a corresponding thoracic vertebra posteriorly. The posterior rib attachments connect at the costovertebral and costotransverse joints. Each rib wraps around the lung and descends approximately 3 to 5 inches from its highest point posteriorly. (2) (See Figure 1.) The anterior portion of each rib connects by way of costocartilage to the sternum. The costocartilage usually does not show up on a radiograph unless it is calcified. The true ribs, numbers 1 though 7, connect anteriorly to the sternum by way of this costocartilage. (See Figure 2) The false ribs are numbers 8 through 12. Ribs 8 through 10 connect to the sternum by way of the costocartilages of the seventh ribs. False ribs 11 and 12 are short and do not wrap around the body; they also are called floating ribs. The ribs collectively provide a protective framework for the lungs.

[FIGURES 1-2 OMITTED]

The Respiratory System

The respiratory system is composed of the larynx, trachea, bronchi and lungs. The larynx, commonly referred to as the voice box, is the most superior structure in the respiratory system and houses the vocal cords. In close proximity to the larynx are the thyroid cartilage, laryngeal prominence or Adam's apple, and the cricoid cartilage. The epiglottis also is located nearby and acts as a covering for the trachea when food is swallowed. The trachea descends inferiorly beginning at about the level of C5 to approximately T5 or T6, where it bifurcates at the carina into the right and left primary bronchi. The bronchi then subdivide into several branches. Three secondary branches feed the right lung and 2 secondary branches feed the left lung. These branches divide into tertiary levels and smaller segments, eventually ending in the terminal bronchioles where the alveoli exchange oxygen and carbon dioxide. (2)

The Lungs

The lungs are composed of a spongy material called the parenchyma. The parenchymal tissue contains the fine structures of the bronchial trees and pulmonary circulation. The exchange of oxygen and carbon dioxide takes place at the alveolar level within the parenchyma. There are millions of alveolar sacs within each lung. Daniels and Orgeig stated that "in humans there are ~25 branches and 300 million alveoli. This structure allows for the generation of an enormous respiratory surface area (up to 70 m2 in adult humans)." (8)

The alveoli are composed of 2 types of cells, identified as Type I and Type II cells. Daniels and Orgeig defined the purpose of each of these cell types as follows:

* Type I cells are the main constituent of the walls of each alveolus.

* Type II cells secrete surfactant, (8) which reduces surface tension, thus reducing the tendency of the alveolar sacs to collapse. (9)

The pulmonary arteries and veins supply blood to all portions of the lungs. This network surrounds the alveoli, where oxygen and carbon dioxide are exchanged with the blood. (2) (See Figure 3.)

[FIGURE 3 OMITTED]

Divisions of the Lungs

Structurally, the right lung is composed of 3 lobes. They are named according to location as the upper, middle and lower lobes. The upper and middle lobes are separated by a fissure called the horizontal fissure. Occasionally, this fissure shows as a lucent line on a radiograph. An additional oblique fissure separates the middle and lower lobes.

The left lung is composed of 2 lobes--a superior and inferior lobe divided by an oblique fissure. The lung parenchyma superior to each clavicle is called the apical portion of the lung. This area is often the hiding place for pulmonary nodules and can be hard to evaluate because of the overlying anatomy of the clavicles. Radiographers use the lordotic position to visualize this area.

Inferiorly, the lateral lung angles are in close proximity to the ribs. These angles are named after their location: hence the term costophrenic angles. (See Figure 4.) The right and left costophrenic angles are important radiographically because they can be used to detect effusions and other abnormalities. When this happens, they appear flattened or blunted as a result of fluid buildup or retention.

[FIGURE 4 OMITTED]

Diaphragm

The diaphragm is a muscular structure located immediately below the lung bases. Though it is a single organ, it is divided into 2 sections called the right and left hemidiaphragms. The right hemidiaphragm is higher on a chest radiograph because of the location of the liver, which is immediately inferior to it. The term cardiophrenic angles is sometimes used to describe the area where the heart's border comes in contact with the diaphragm. There are both right and left cardiophrenic angles, which should be visualized on a normal chest radiograph. (See Figure 4.)

Pleura

Each lung is surrounded by a thin-walled sac called the pleura. The pleura completely encases the lung with an inner layer called the pulmonary or visceral layer and an outer layer called the parietal layer. The potential space between these 2 layers is called the pleural space. Radiographically, this space is important because it can fill with air (pneumothorax) or blood (hemothorax), which can be seen on a chest radiograph. A chest tube can be placed within the pleural space to drain accumulated fluid or air.

The Mediastinum

The mediastium is the space between the lungs that houses the heart and great vessels, including the proximal pulmonary arteries and aortic root. Additionally, the proximal bronchial trees, pulmonary veins, a portion of the esophagus and lymphatic vessels are important structures found in the mediastinum. The hilum "is the central area of each lung, where the bronchi, blood vessels, lymph vessels and nerves enter and leave the lungs." (2) (See Figure 4.) Furthermore, the thymus gland is located above the heart in the superior mediastinal compartment.

Patient Preparation for the Chest Exam

All Patients