Principles of Mobile Radiography

Mobile radiography using transportable radiographic equipment allows imaging services to be brought to the patient. In contrast to the large stationary machines found in radiographic rooms, compact mobile radiography units can produce diagnostic images in virtually any location (Fig. 28-1). Mobile radiography is commonly performed in patient rooms, emergency departments, intensive care units, surgery and recovery rooms, and nursery and neonatal units. Some machines are designed for transport by automobile or van to nursing homes, extended care facilities, or other off-site locations requiring radiographic imaging services.

Mobile radiography was first used by the military for treating battlefield injuries during World War I. Small portable units were designed to be carried by soldiers and set up in field locations. Although mobile equipment is no longer “carried” to the patient, the term portable has persisted and is often used in reference to mobile procedures.

This chapter focuses on the most common projections performed with mobile radiography machines. The basic principles of mobile radiography are described in detail, and helpful hints are provided for successful completion of the examinations. An understanding of common projections enables the radiographer to perform most mobile examinations ordered by the physician.

Mobile X-Ray Machines

Mobile x-ray machines are not as sophisticated as the larger stationary machines in the radiology department. Although mobile units are capable of producing images of most body parts, they vary in their exposure controls and power sources (or generators).

A typical mobile x-ray machine has controls for setting kilovolt (peak) (kVp) and milliampere-seconds (mAs). The mAs control automatically adjusts milliamperage (mA) and time to preset values. Maximum settings differ among manufacturers, but mAs typically range from 0.04 to 320 and kVp from 40 to 130. The total power of the unit ranges from 15 to 25 kilowatts (kW), which is adequate for most mobile projections. By comparison, the power of a stationary radiography unit can reach 150 kW (150 kVp, 1000 mA) or more.

Some mobile x-ray machines have preset anatomic programs (APRs) similar to stationary units. The anatomic programs use exposure techniques with predetermined values based on the selected examination. The radiographer can adjust these settings as needed to compensate for differences in the size or condition of a patient. Automatic exposure control (AEC) may be available for some mobile machines. A paddle containing an ionization chamber is placed behind the image receptor (IR) and is used to determine the exposure time. With the increasing use of computed radiography (CR), however, AEC may not be as useful. The much wider dynamic range available with CR and the ability to manipulate the final image with computer software results in images of proper density without the use of automatic systems.

Some mobile units have direct digital capability, where the image is acquired immediately on the unit. These machines have a flat panel detector, similar to those found in a DR table Bucky. The detector either is connected to the portable unit by a tethered cord or communicates through wireless technology (Fig. 28-2).

Technical Considerations

Mobile radiography presents the radiographer with challenges different from those associated with performing examinations with stationary equipment in the radiology department. Although the positioning of the patient and placement of the central ray are essentially the same, three important technical matters must be clearly understood to perform optimal mobile examinations: the grid, the anode heel effect, and the source–to–image receptor distance (SID). In addition, exposure technique charts must be available (see Fig. 28-5).


Because the phosphor material used in CR imaging plates has higher absorption in the scattered x-ray energy range compared with screen-film, image quality degradation from scatter is more pronounced when using CR. Grid use is crucial in portable CR imaging.

For optimal imaging, a grid must be level, centered to the central ray, and correctly used at the recommended focal distance, or radius. When a grid is placed on an unstable surface such as the mattress of a bed, the weight of the patient can cause the grid to tilt “off-level.” If the grid tilts transversely while using a longitudinal grid, the central ray forms an angle across the long axis. Image density is lost as a result of grid “cutoff” (Fig. 28-3). If the grid tilts longitudinally, the central ray angles through the long axis. In this case, grid cutoff is avoided, but the image may be distorted or elongated.

A grid positioned under a patient can be difficult to center. If the central ray is directed to a point transversely off the midline of a grid more than 1 to 1½ inches (2.5 to 3.8 cm), a cutoff effect similar to that produced by an off-level grid results. The central ray can be centered longitudinally to any point along the midline of a grid without cutoff. Depending on the procedure, beam-restriction problems may occur. If this happens, a portion of the image is “collimated off,” or patient exposure is excessive because of an oversized exposure field.

Grids used for mobile radiography are often of the focused type. Some radiology departments continue to use the older, parallel-type grids, however. All focused grids have a recommended focal range, or radius, that varies with the grid ratio. Projections taken at distances greater or less than the recommended focal range can produce cutoff in which image density is reduced on lateral margins. Grids with a lower ratio have a greater focal range, but they are less efficient for cleaning up scatter radiation. The radiographer must be aware of the exact focal range for the grid used. Most focused grids used for mobile radiography have a ratio of 6:1 or 8:1 and a focal range of about 36 to 44 inches (91 to 112 cm). This focal range allows mobile examinations to be performed efficiently. Inverting a focused grid causes a pronounced cutoff effect similar to that produced by improper distance.

Today most grids are mounted on a protective frame, and the IR is easily inserted behind the grid (Fig. 28-4). A final concern regarding grids relates to the use of “tape-on” grids. If a grid is not mounted on an IR holder frame but instead is manually fastened to the surface of the IR with tape, care must be taken to ensure that the tube side of the grid faces the x-ray tube. The examinations described in this chapter present methods of ensuring proper grid and IR placement for projections that require a grid.


Another consideration in mobile radiography is the anode heel effect. The heel effect causes a decrease of image density under the anode side of the x-ray tube. The heel effect is more pronounced with the following:

Short SIDs and large field sizes are common in mobile radiography. In mobile radiography, the radiographer has control of the anode-cathode axis of the x-ray tube relative to the body part. Correct placement of the anode-cathode axis with regard to the anatomy is essential. When performing a mobile examination, the radiographer may not always be able to orient the anode-cathode axis of the tube to the desired position because of limited space and maneuverability in the room. For optimal mobile radiography, the anode and cathode sides of the x-ray tube should be clearly marked to indicate where the high-tension cables enter the x-ray tube, and the radiographer should use the heel effect maximally (Table 28-1).

Radiation Safety

Radiation protection for the radiographer, others in the immediate area, and the patient is of paramount importance when mobile examinations are performed. Mobile radiography produces some of the highest occupational radiation exposures for radiographers. The radiographer should wear a lead apron and stand as far away from the patient, x-ray tube, and useful beam as the room and the exposure cable allow. The recommended minimal distance is 6 ft (2 m). For a horizontal (cross-table) x-ray beam or for an upright anteroposterior (AP) chest projection, the radiographer should stand at a right angle (90 degrees) to the primary beam and the object being radiographed. The least amount of scatter radiation occurs at this position (Fig. 28-7). Shielding and distance have a greater effect on exposure reduction, however, and should always be considered first.

The most effective means of radiation protection is distance. The radiographer should inform all persons in the immediate area that an x-ray exposure is about to occur so that they may leave to avoid exposure. Lead protection should be provided for any individuals who are unable to leave the room and for individuals who may have to hold a patient or IR.

The patient’s gonads should be shielded with appropriate radiation protection devices for any of the following situations:

In addition, the source-to-skin distance (SSD) cannot be less than 12 inches (30 cm), in accordance with federal safety regulations.1

Isolation Considerations

Two types of patients are often cared for in isolation units: (1) patients who have infectious microorganisms that could be spread to health care workers and visitors and (2) patients who need protection from potentially lethal microorganisms that may be carried by health care workers and visitors. Optimally, a radiographer entering an isolation room should have a full knowledge of the patient’s disease, the way it is transmitted, and the proper way to clean and disinfect equipment before and after use in the isolation unit. Because of the confidentiality of patient records, the radiographer may be unable to obtain information about a patient’s specific disease, however. All patients must be treated with universal precautions. If isolation is used to protect the patient from receiving microorganisms (reverse isolation), a different protocol may be required. Institutional policy regarding isolation procedures should be available and strictly followed.

When performing mobile procedures in an isolation unit, the radiographer should wear the required protective apparel for the specific situation—gown, cap, mask, shoe covers, and gloves. All of this apparel is not needed for every isolation patient. All persons entering a strict isolation unit wear a mask, a gown, and gloves, but only gloves are worn for drainage secretion precautions. Radiographers should always wash their hands with warm, soapy water before putting on gloves. The x-ray machine is taken into the room and moved into position. The IR is placed into a clean, protective cover. Pillowcases would not protect the IR or the patient if body fluids soak through them. A clean, impermeable cover should be used in situations in which body fluids may come into contact with the IR. For examinations of patients in strict isolation, two radiographers may be required to maintain a safe barrier (see Chapter 1).

After finishing the examination, the radiographer should remove and dispose of the mask, cap, gown, shoe covers, and gloves according to institutional policies. All equipment that touched the patient or the patient’s bed must be wiped with a disinfectant according to appropriate aseptic technique. The radiographer should wear new gloves, if necessary, while cleaning equipment. Handwashing is repeated before the radiographer leaves the room.

Performing Mobile Examinations


The radiographer should plan for the trip out of the radiology department. Ensuring that all of the necessary devices (e.g., IR, grid, tape, caliper, markers, blocks) are transported with the mobile x-ray machine provides greater efficiency in performing examinations. Many mobile x-ray machines are equipped with a storage area for transporting IRs and supplies. If a battery-operated machine is used, the radiographer should check the machine to ensure that it is acceptably charged. An inadequately charged machine can interfere with performance and affect the quality of the radiograph.

Before entering the patient’s room with the machine, the radiographer should follow several important steps (Box 28-1). The radiographer begins by checking that the correct patient is going to be examined. After confirming the identity of the patient, the radiographer enters, makes an introduction as a radiographer, and informs the patient about the x-ray examinations to be performed. While in the room, the radiographer observes any medical appliances, such as chest tube boxes, catheter bags, and intravenous (IV) poles, that may be positioned next to or hanging on the sides of the patient’s bed. The radiographer should ask family members or visitors to step out of the room until the examination is finished. If necessary, the nursing staff should be alerted that assistance is required.

Communication and cooperation between the radiographer and nursing staff members are essential for proper patient care during mobile radiography. In addition, communication with the patient is imperative, even if the patient is or appears to be unconscious or unresponsive.


Chairs, stands, IV poles, wastebaskets, and other obstacles should be moved from the path of the mobile machine. Lighting should be adjusted if necessary. If the patient is to be examined in the supine position, the base of the mobile machine should be positioned toward the middle of the bed. If a seated patient position is used, the base of the machine should be toward the foot of the bed.

For lateral and decubitus radiographs, positioning the base of the mobile machine parallel to or directly perpendicular to the bed allows the greatest ease in positioning the x-ray tube. Room size can also influence the base position used.

The radiographer sometimes may have difficulty accurately aligning the x-ray tube parallel to the IR while standing at the side of the bed. When positioning the tube above the patient, the radiographer may need to check the x-ray tube and IR alignment from the foot of the bed to ensure that the tube is not tilted.

For all projections, the primary x-ray beam must be collimated no larger than the size of the IR. When the central ray is correctly centered to the IR, the light field coincides with or fits within the borders of the IR.

A routine and consistent system for labeling and separating exposed and unexposed IRs should be developed and maintained. It is easy to “double expose” IRs during mobile radiography, particularly if many examinations are performed at one time. Most institutions require additional identification markers for mobile examinations. Typically the time of examination (especially for chest radiographs) and technical notes such as the position of the patient are indicated. A log may be maintained for each patient and kept in the patient’s room. The log should contain the exposure factors used for the projections and other notes regarding the performance of the examination.


Patients requiring mobile radiography often are in extended care facilities or are immobile and among the most sick. They may be awake and lying in bed in traction because of a broken limb, or they may be critically ill and unconscious. A brief but total assessment of the patient must be conducted before and during the examination. Some specific considerations to keep in mind are described in the following sections.

Patient mobility

The radiographer must never move a patient or part of the patient’s body without assessing the patient’s ability to move or tolerate movement. Gentleness and caution must prevail at all times. If unsure, the radiographer should always check with the nursing staff or physician. Many patients who undergo total joint replacement may be unable to move the affected joint for many days or weeks, but this may not be evident to the radiographer. Some patients may be able to indicate verbally their ability to move or their tolerance for movement. The radiographer should never move a limb that has been operated on or is broken, unless the nurse, the physician, or sometimes the patient grants permission. Inappropriate movement of the patient by the radiographer during the examination may harm the patient.

Interfering devices

Patients who are in intensive care units or orthopedic beds because of fractures may be attached to various devices, wires, and tubing. These objects may be in the direct path of the x-ray beam and consequently produce artifacts on the image. Experienced radiographers know which of these objects can be moved out of the x-ray beam. When devices such as fracture frames cannot be moved, it may be necessary to angle the central ray or adjust the IR to obtain the best radiograph possible. In many instances, the objects have to be radiographed along with the body part (Fig. 28-9). The radiographer must exercise caution when handling any of these devices and should never remove traction devices without the assistance of a physician.

Positioning and asepsis

During positioning, the IR (with or without a grid) often is perceived by the patient as cold, hard, and uncomfortable. Before the IR is put in place, the patient should be warned of possible discomfort and assured that the examination will be for as short a time as possible. The patient appreciates the radiographer’s concern and efficiency in completing the examination as quickly as possible.

If the surface of the IR touches bare skin, it can stick, making positioning adjustments difficult. The skin of older patients may be thin and dry and can be torn by manipulation of the IR if care is not taken. A cloth or paper cover over the IR can protect the patient’s skin and alleviate some of the discomfort by making it feel less cold. The cover also helps to keep the IR clean. IRs that contact the patient directly should be wiped off with a disinfectant for asepsis and infection control.

The IR must be enclosed in an appropriate, impermeable barrier in any situation in which it may come in contact with blood, body fluids, and other potentially infectious material. A contaminated IR can be difficult and sometimes impossible to clean. Approved procedures for disposing of used barriers must be followed.

Mar 4, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on MOBILE RADIOGRAPHY
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