What is the minimum safe distance from a source of ionizing radiation during radiography?

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Radiation Exposure Protection

External exposure is radiation that comes from somewhere outside the body and interacts with us. The source of radiation can be a piece of equipment that produces the radiation, like an x-ray machine, or it can be from radioactive materials in a container. The amount of external radiation exposure received is related to the distance from the source, the energy of the emitted radiation, the total amount of radioactive material present or the machine setting, and the time of exposure. Radiation workers can control and limit their exposure to penetrating radiation by taking advantage of time, distance, and shielding.

Reduce Time: By reducing the time of exposure to a radiation source, the dose to the worker is reduced in direct proportion with that time. Time directly influences the dose received: if you minimize the time spent near the source, the dose received is minimized. For example, if possible, interview a nuclear medicine patient before drug administration not after.

Increase distance: When appropriate, increase the distance between you and the radiation source (e.g., sealed source, x-ray tube). The exposure rate from a radiation source drops off by the inverse of the distance squared. For example, if a problem arises during a fluoroscopy procedure, stand on the image intensifier side of the C-arm if possible, or, when not assisting, step away from the patient if feasible.

Use shielding: The third exposure control is based on the proper radiation shields, automatic interlock devices, and in-place radiation monitoring instruments. Except for temporary or portable shields, protective drapes, lead or lead equivalent aprons, this type of control is usually built into the particular facility, such as concrete walls next to a radiation oncology accelerator. For portable x-ray devices, follow the vendor instructions.

In general, alpha, beta, gamma and x-ray radiation can be stopped by:

• Keeping the time of exposure to a minimum,
• Maintaining distance from the source,
• When appropriate, placing a shield between yourself and the source, and
• Protecting yourself against radioactive contamination by using proper protective clothing.

5.0 Diagnostic X-ray Equipment

The following rules are to ensure the safe use of human-use and veterinary-use diagnostic X-ray equipment at Virginia Tech. These rules are in conformity with the radiation safety standards recommended by the Commonwealth of Virginia.

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5.1 Personnel Monitoring

All individuals working with diagnostic X-ray equipment shall wear a whole-body personnel monitoring device.

Operators of fluoroscopic units shall wear a ring badge and a whole-body badge.

When a lead apron or thyroid shield is worn, the whole-body monitoring device shall be worn at the collar outside the apron or shield.

Declared pregnant radiation workers shall wear a second whole-body monitoring device under the lead apron at waist level.

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5.2 Patient Protection

The following rules are to protect patients from exposure to ionizing radiation, except that which is intended for diagnostic purposes.

1. The useful beam shall be collimated to cover only the area of clinical interest.
2. All exposures shall be specifically and individually ordered by a licensed Medical Doctor or a Doctor of Veterinary Medicine.
3. Humans shall not be exposed for training, demonstration, or other non-healing art purposes.
4. Exposure of individuals for healing arts screening is prohibited unless approved by the Radiation Safety Committee and the Virginia Bureau of Radiological Health.
5. Procedures shall be used to keep patient exposure at a minimum, while still obtaining the necessary diagnostic information.
   1. The film or screen must be the fastest speed that can be used, yet be consistent with the diagnostic objective.
   2. The radiation exposure to the patient must be the minimum required to produce good diagnostic images.
6. For human-use units, other than fluoroscopy, the X-ray tube must be at least 30 cm (approximately 1 ft.) from the patient.
7. The source-to-patient distance must be at least 38 cm for image-intensified fluoroscopic units.
8. Gonad shielding of at least 0.25 mm lead equivalency must be used on patients of reproductive age, if the gonads are in the primary beam and the shielding does not interfere with the diagnostic procedure.
9. Aluminum filtration shall be placed in the primary beam to reduce the quantity of soft X-rays to the patient.

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5.3 Personnel Protection

The following rules are to protect operators, holders, and other people from exposure to ionizing radiation.

5.3.1 Stationary Units

1. The operators of human-use units must stand behind the protective barrier at the controls during the exposure. The operators of veterinary-use units should stand behind the protective barriers at the controls during the exposure.
2. An operator who is required to be in the X-ray room to take an exposure of an animal must stand at least 6 feet from the useful beam and the animal.
3. Only individuals required for the radiographic procedure are to be in the room during the exposure.
4. All individuals present in the X-ray room during an exposure must be protected from the primary beam by at least 0.5 mm lead equivalency and from scatter radiation by at least 0.25 mm lead equivalency.
5. Access to the X-ray room must be secured during the exposure.

5.3.2 Portable and Mobile Units

1. Operators shall stand at least 6 feet from the X-ray tube head and wear a lead apron of at least 0.25 mm lead equivalency.
2. The area or room where the equipment is being used shall be temporarily posted with a "Caution -- X-ray Equipment" sign.
3. Bystanders must stand at least 12 feet from the X-ray tube head and the patient being X-rayed.
4. Mobile X-ray units shall not be hand held.
5. The primary beam shall not be directed at bystanders.

5.3.3 Holders

When a patient, animal, or film cassette must be provided with auxiliary support during an X-ray exposure:

1. Mechanical holding devices must be used whenever possible.
2. No individual shall be used routinely as a holder, to the exclusion of others who could be used.
3. Personnel used as holders must be protected from the primary beam by at least 0.5 mm of lead equivalency, and from scatter radiation by at least 0.25 mm of lead equivalency.
4. Every effort should be made to position the holder so that the primary beam will strike no part of the body.
5. Pregnant workers will not be required to be used as holders.

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5.5 Equipment Requirements

The control panel shall contain the following legible and accessible warning statements:

1. "WARNING: This X-ray unit may be dangerous to patient and operator unless safe exposure factors and operating conditions are observed."
2. "CAUTION: This equipment produces radiation when energized, to be operated only by qualified personnel."

The total filtration permanently mounted in the useful beam shall not be less than:

• 0.5 mm aluminum equivalent for machines operating up to 50 kilovolts peak (kVp),
• 1.5mm aluminum equivalent for machines operating between 50-70 kVp, and
• 2.5 mm aluminum equivalent for machines operating above 70 kVp.

The tube housing assembly support shall ensure that the tube housing remains stable during the X-ray exposure.

The technique factors to be used during an exposure shall be visible before the exposure begins.

On battery-powered equipment, visual means shall be provided on the control panel to show the charge of the battery.

A source-to-image distance (SID) indicator must be provided and be accurate to within 2% of the indicated SID.

5.5.1 Stationary, Portable, and Mobile Units

A means for stepless adjustment (e.g. variable aperture collimator) of the size of the X-ray field shall be provided.

Means shall be provided to visually define the perimeter of the X-ray field.

The X-ray field shall not exceed the visually defined field by greater than 2%.

A method shall be provided to show when the axis of the X-ray beam is perpendicular to the plane of the image receptor.

The exposure shall be ended at a preset time interval, product of current and time, number of pulses, or radiation exposure to the image receptor.

The X-ray control shall provide a visual indication of X-ray production and an audible signal when the exposure is finished.

The X-ray control for stationary systems shall be permanently mounted in a protected area.

5.5.2 Fluoroscopic Systems

X-ray production shall be controlled by a dead-man switch.

The on-time of the fluoroscopic tube shall be controlled by a timing device, which ends the exposure after 5 minutes.

An audible signal shall signal the completion of the preset on-time. This signal will remain on until the timing device is reset.

Protective barriers of at least 0.25 mm lead equivalency shall be used to attenuate scatter radiation from above the table top (e.g. drapes, bucky-slot covers). This shielding is in addition to the lead apron worn by personnel.

Scattered radiation from under the table shall be attenuated by at least 0.25 mm lead equivalency.

The fluoroscopic imaging assembly shall be provided with a primary protective barrier, which intercepts the entire cross section of the useful beam.

The X-ray tube used for fluoroscopy shall not produce X-rays unless the barrier is in position to intercept the entire useful beam.

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5.6 Operator's Booth

The operator's booth shall have at least 7.5 square feet of unobstructed floor space in the booth. The booth may be of any shape with no dimension less than 2 feet. The booth is to be located or constructed so that the direct beam and unattenuated direct scatter radiation cannot reach the operator.

5.6.1 Structural Requirements

The booth walls shall be at least 78 inches high and permanently fixed. A door or panel that is permanently part of the booth must be interlocked. Sufficient shielding shall be provided to prevent occupational limits from being exceeded.

5.6.2 Control Placement

The X-ray control shall be fixed within the booth at least 40 inches from the edge of the booth wall closest to the examining table.

The placement of the control shall allow the operator to use most of the viewing window.

5.6.3 Viewing requirements

The booth must have a window that will allow the operator to view any occupant in the room and any entry into the room. Access doors that cannot be viewed by the operator must be interlocked.

The window shall have an area of at least 1 square foot with the lower edge at least 4.5 feet from the floor. The edge of the window shall be at least 18 inches from the edge of the booth. The glass shall have the same lead equivalency as the walls of the booth.

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5.7 Records

The Laboratory Authority shall maintain the following records and information:

1. An X-ray log containing the patient's name, type of examination, and the date of the examination.
2. Maximum ratings and technique factors of the equipment.
3. Model and serial number of all components.
4. Tube rating charts and cooling curves.
5. Assembler report for certifiable units.
6. Records of calibrations, maintenance, and modifications.
7. Aluminum equivalent filtration of the useful beam, including any routine variations.
8. Virginia Tech license, amendments, surveys and inspections.

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5.8 Surveys and Inspections

Radiation safety and equipment performance surveys shall be performed by a Qualified Expert annually on human-use units and at least every 3 years on bone densitometers or veterinary-use units.

A survey for leakage radiation shall be performed following any maintenance, modification or relocation of the system.

Radiation surveys of areas adjacent to the X-ray producing facility and in the booth will be performed after installation of new equipment or the relocation of a unit.

1. The survey shall include a scale drawing of the areas adjacent to the X-ray room and an estimate of their occupancy.
2. The drawing shall include the type and thickness of the walls or their lead equivalency.

Reports of all surveys and inspections will be maintained in the Radiation Safety Office.

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