11.1 Anatomical Terminology, Body Planes & Chest Radiography (PA, Lateral, Decubitus)

11.1 Anatomical Terminology, Body Planes & Chest Radiography (PA, Lateral, Decubitus)

Precision in radiographic positioning requires a strict, standardized vocabulary of body planes, anatomical directions, and projection terminology. Radiologic technologists utilize these anatomical concepts to align the central ray (CR), anatomical structures, and image receptor (IR) accurately. Mastery of chest positioning criteria—including source-to-image distance (SID), respiration control, rotation assessment, and decubitus protocols—is fundamental to diagnostic image acquisition.

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Anatomical Terminology & Body Planes

Radiographic positioning relies on reference planes that divide the body in anatomical position (standing erect, arms at sides, palms facing forward):

• Sagittal Plane: A vertical plane passing longitudinally from anterior to posterior, dividing the body into right and left portions. The Midsagittal Plane (MSP) divides the body into equal right and left halves.
• Coronal Plane: A vertical plane passing longitudinally from side to side at right angles to the sagittal plane, dividing the body into anterior and posterior portions. The Midcoronal Plane (MCP) divides the body into equal anterior and posterior halves.
• Transverse (Axial / Horizontal) Plane: A horizontal plane passing through the body at right angles to both sagittal and coronal planes, dividing the body into superior and inferior portions.
• Oblique Plane: Any plane passing through the body at an angle that is not parallel to the sagittal, coronal, or transverse planes.

Positional & Directional Terms

• Anteroposterior (AP): Central ray enters the anterior surface and exits the posterior surface.
• Posteroanterior (PA): Central ray enters the posterior surface and exits the anterior surface.
• Decubitus: Patient lying down (supine, prone, or recumbent on side) with a horizontal central ray directed parallel to the floor. Named by the body surface on which the patient rests (e.g., Left Lateral Decubitus = patient lying on left side).
• Axial Projection: Any projection where the central ray is angled 10 degrees or more along the long axis of the body or body part.
• Oblique Position: Body rotated so that the coronal plane forms an angle (typically 45 degrees) with the IR.
• Medial vs. Lateral: Medial indicates toward the midsagittal plane; Lateral indicates away from the midsagittal plane toward the outer boundary.
• Proximal vs. Distal: Proximal refers to a location closer to the trunk or point of origin; Distal refers to a location farther from the trunk or point of origin.

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Routine Chest Radiography: PA & Lateral Projections

Chest radiography is the most frequently performed radiographic examination. Obtaining diagnostic chest images requires adherence to specific technical parameters and anatomical criteria.

Posteroanterior (PA) Chest Radiograph

1. Source-to-Image Distance (SID): Standardized at 72 inches (183 cm). A longer SID minimizes cardiac magnification and geometric blur caused by beam divergence, maximizing spatial resolution.
2. Patient & IR Placement: Patient stands erect facing the chest bucky/IR, shoulders rolled forward (to move the scapulae laterally out of the lung fields), chin elevated, and anterior chest pressed against the IR.
3. Central Ray (CR) Centering: CR is directed perpendicular to the IR, centered to the midsagittal plane at the level of T7 (which corresponds to the inferior border of the scapula, or approximately 7 to 8 inches below the spinous process of C7).
4. Respiration & Inspiration Criteria: Exposure is taken on full inspiration (specifically at the end of the second full deep inspiration) to maximize thoracic volume. A fully inspired adult PA chest radiograph demonstrates at least 10 posterior ribs above the diaphragmatic contour.
5. Rotation Criteria: Absence of rotation is evaluated by ensuring that the sternoclavicular (SC) joints are equidistant from the vertebral column and that the distance from the lateral rib margin to the spine is symmetric on both sides.

Lateral Chest Radiograph

1. Routine Left Lateral: Performed as a left lateral position (left side against the IR) to place the heart closer to the IR, thereby minimizing cardiac magnification.
2. Patient Alignment: Patient stands erect with arms raised above the head, left side flush against the IR, and midcoronal plane perpendicular to the IR.
3. CR Centering: CR is directed perpendicular to the midcoronal plane at the level of T7 (3 to 4 inches below the level of the jugular notch).
4. Rotation Criteria: Evaluated by the superimposition of the posterior ribs. True lateral positioning exhibits superimposition of the posterior rib margins, with no more than 0.5 inch (1.25 cm) of separation between the right and left posterior ribs.

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AP Supine & Bedside Portable Chest Radiography

When patients are critically ill, comatose, or immobilized, chest radiography must be performed AP supine or bedside portable.

Magnification & Geometric Alterations

• In an AP projection, the heart sits anteriorly within the mediastinum, further from the IR. Combined with a shorter SID (typically 40 to 48 inches / 100 to 122 cm due to space constraints), cardiac magnification is significantly increased compared to standard PA 72-inch chest radiographs.
• The clavicles appear higher (superimposing the lung apices), and fewer posterior ribs (typically 8 to 9) are demonstrated above the diaphragm due to limited patient inspiration.

Fluid Dispersion & Air-Fluid Dynamics

• In a supine position, pleural effusions spread evenly across the posterior chest wall, producing a diffuse, hazy opacification across the affected hemithorax rather than a crisp fluid level in the costophrenic angle.
• Free intrapleural air (pneumothorax) moves anteriorly and superiorly, appearing as a subtle subpulmonary or apical clear zone that can easily be missed without upright or decubitus technique.

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Lateral Decubitus Chest Radiography

Lateral decubitus chest projections are indicated when a patient cannot assume an erect position but evaluation of free intrapleural air (pneumothorax) or liquid pleural effusion is required.

The 5-Minute Positioning Rule

• The patient MUST remain in the lateral decubitus position for at least 5 minutes prior to exposure. This waiting period allows fluid to settle to the lowest point of the pleural cavity or free air to rise to the highest lateral chest wall margin.

Decubitus Positioning Rules for Air vs. Fluid

• Suspected Pneumothorax (Air): Position the patient with the AFFECTED SIDE UP. Free air rises to the highest point along the elevated lateral chest wall, contrasting sharply against the lateral rib cage.
• Suspected Pleural Effusion (Fluid): Position the patient with the AFFECTED SIDE DOWN. Fluid gravitates to the lowest point along the dependent lateral chest wall, layering out in the costophrenic gutter.
• Technical Note: A horizontal central ray (parallel to the floor) is mandatory; angling the beam vertically destroys the ability to demonstrate air-fluid layering.

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Chest Radiography Positioning Summary