Chapter 20: Mobile Radiography

Detailed Chapter Overview

Chapter 20 delves into the challenging and dynamic specialty of mobile radiography, where imaging is brought directly to patients who cannot be transported to the main radiology department. This chapter is essential for developing the adaptability, critical thinking, and advanced technical skills required to perform diagnostic imaging in suboptimal and unpredictable environments like patient rooms, the emergency department (ED), and the intensive care unit (ICU). The central theme of the chapter is the principle of adaptation—modifying standard radiographic procedures to accommodate patient limitations while adhering to fundamental imaging rules. It provides a detailed look at the types of mobile x-ray equipment and their capabilities. A significant portion of the chapter is dedicated to overcoming the three main challenges of mobile radiography: manipulating equipment in confined spaces, accurately positioning patients who are ill or in pain, and maintaining rigorous radiation safety protocols in uncontrolled areas. The chapter thoroughly explores the technical variables unique to mobile imaging, such as estimating SID, contending with grid cutoff due to improper alignment, and correctly applying the anode heel effect. For every common mobile examination—from the supine chest to the cross-table hip—the text provides detailed procedural guidelines and adaptive positioning strategies, ensuring the radiographer can consistently produce high-quality, diagnostic images regardless of the clinical scenario.

In-Depth Study Guide

The Essence and Challenges of Mobile Radiography

Mobile radiography, often called portable radiography, is performed on patients in their rooms, in the ED, ICU, recovery rooms, or any other space outside the radiology department. The primary challenge is to produce department-quality images under less-than-ideal conditions.

Three Key Challenges:

1. Equipment Manipulation: Navigating a large mobile unit through hallways, around hospital beds, IV poles, ventilators, and other medical equipment in a confined space requires skill and situational awareness.
2. Patient Condition: Mobile exams are ordered because the patient is too sick, injured, or unstable to move. This requires the radiographer to be compassionate, efficient, and highly adaptive in their positioning techniques.
3. Maintaining Imaging Standards: It is difficult to achieve perfect part-IR-CR alignment, accurate SIDs, and correct grid use, all of which are essential for diagnostic quality and are easily controlled in a standard radiographic room.

Mobile X-ray Equipment

• Types of Generators: Modern mobile units are typically battery-powered. They use a bank of batteries to power the x-ray tube, providing consistent, high-power output similar to a stationary unit. Older capacitor-discharge units exist but are less common.
• Drive System: Most units are motor-assisted, making them easier to maneuver. They have controls for forward, reverse, and speed.

Fundamental Principles in a Mobile Environment

Despite the challenges, the core principles of radiography must be maintained.

• Two Projections at 90 Degrees: This remains the cardinal rule. If the patient cannot be rotated for a lateral view, a horizontal beam (cross-table) projection must be performed.
• Correct Part-IR-CR Alignment: The central ray must be centered to the image receptor and properly aligned with the anatomy of interest to avoid distortion and grid cutoff.
• Radiation Safety: This is arguably the most important consideration in a mobile setting due to the uncontrolled environment.

Radiation Safety: The Mobile Mandate

Protecting the patient, staff, and the public is a primary responsibility of the mobile radiographer.

• Announce Your Presence: Before making an exposure, the radiographer must loudly and clearly announce "X-RAY!" or "X-RAYING!" This gives any other individuals in the room (staff, family) the opportunity to step out.
• The 6-Foot Rule: The exposure cord on a mobile unit must allow the radiographer to stand at least 6 feet (2 meters) away from the patient and x-ray tube during the exposure.
• Position of Least Scatter: The safest place for the radiographer to stand is at a **90-degree angle** to the primary beam and the object being radiographed. This is the position of least scatter radiation. Standing directly behind or in front of the patient should be avoided.
• Patient Shielding: A lead apron must be placed over the patient's gonadal region and any other radiosensitive areas not in the primary field of view.
• Shielding Others: If other patients are in the same room, ask them to step out if possible. If they cannot, provide them with a lead apron or use a mobile lead shield as a barrier between the x-ray tube and the other patient.

Technical Considerations for Mobile Imaging

• Source-to-Image Distance (SID): It is difficult to achieve a precise 40- or 72-inch SID in mobile radiography. The SID must be estimated and measured as accurately as possible. Any significant deviation from the standard SID must be compensated for with an adjustment in mAs to maintain proper image density.
  • **40 inches (100 cm):** Standard for most mobile exams (abdomen, pelvis, extremities).
  • **72 inches (180 cm):** Should be used for mobile chest x-rays whenever possible to minimize heart magnification. If a 72-inch SID cannot be achieved, the actual SID used must be noted.
• Grids: Grids are essential for any body part measuring more than 10-13 cm in thickness (e.g., abdomen, pelvis, thoracic spine) to reduce scatter and improve contrast. However, they are highly susceptible to errors in a mobile setting.
  • Grid Cutoff: This occurs when the primary beam is angled against the lead strips of the grid. The most common causes in mobile radiography are **off-level** (the grid is not parallel to the CR) and **off-center** (the CR is not centered to the grid) errors. Careful alignment is crucial to prevent grid cutoff.
• Anode Heel Effect: This effect should be consciously used to advantage. For anatomy with varying thickness, like the femur or a supine chest, the **cathode end** of the x-ray tube (which has a more intense beam) should be placed over the thicker body part to achieve a more uniform image density.

Common Mobile Radiographic Procedures

• Chest (AP Supine/Semi-Erect): The most common mobile exam. The patient is propped up as much as possible to demonstrate air-fluid levels. The CR should be angled caudad so it is perpendicular to the long axis of the sternum to prevent the clavicles from obscuring the apices.
• Abdomen (AP Supine - KUB): Requires a grid. The entire abdomen must be included from the diaphragm to the pubic symphysis.
• Decubitus Views (Chest and Abdomen): Performed to demonstrate air-fluid levels. A horizontal beam is used with the patient in a lateral recumbent position.
• Pelvis and Hip: The initial AP view is taken without rotating the legs. The critical second view for a hip fracture is the **trauma cross-table lateral (Danelius-Miller method)**, which provides a lateral view without moving the patient's injured leg.
• Extremities: The guiding principle is to move the tube and IR around the injured part, rather than moving the part itself. Two projections, 90 degrees apart, are always required.