Ch 11 "Hard"

• Equipment design affects radiation protection in medical radiography

Radiation Safety Features

• Ensure measures are taken to protect patients and personnel during radiographic equipment operation.
• Every diagnostic imaging system must have a protective tube housing and a correctly functioning control panel.
• Radiographic examination tables and accessories should be designed to reduce the patient's radiation dose and be radiolucent, carbon fiber is preferrable.

Diagnostic-Type Protective Tube Housing

• These are lead-lined and metal to protect the patient and imaging personnel from off-focus or leakage radiation.
• Emission of x-rays is being restricted to the area of the useful or primary beam.
• Radiation leakage through any part of the housing away from the useful beam, must not exceed 0.88 mGya/hr, measured at 1 m from the x-ray source.
• The limit remains when the tube operates at its highest voltage and current for continuous operation.
• Dielectric oil helps with cooling and inherent filtration

Control Panel or Console

• Located behind a suitable protective barrier with a radiation-absorbent window.
• It must provide a positive indication when the x-ray tube is energized.
• Visible mA and kVp digital readouts permit the operator to assess exposure conditions.
• A tone is emitted when the exposure starts and stops at the end.
• An audible sound indicates that the x-ray tube is energized, emitting ionizing radiation.

Radiographic Examination Table

• Must be strong enough to support patients whose weight exceeds 300 pounds.
• Floating tabletop to easily maneuver the patient during imaging procedure
• Tables must have uniform thickness .
• Tables must be radiolucent as possible to minimize radiation absorption, reducing patient dose.
• Carbon fiber material is frequently used.

Source-to-Image Receptor Distance (SID) Indicator

• Indicators measure the distance from the anode focal spot to the image receptor to maintain correct SID .
• A tape measure is attached to the collimator or tube housing for manual SID measurement.
• Lasers are also used to measure SID.
• Distance indicators must be accurate to within 2% of the SID.
• Centering indicators must be accurate to 1% of the SID.

X-Ray Beam Limitation Devices

• The primary x-ray beam needs to be adequately collimated so that it is no larger than the size of the image receptor.
• Light-localizing variable-aperture rectangular collimators adjust the x-ray beam size and shape, and is used automatically or manually.
• The collimator is currently the predominant x-ray beam limitation device.
• Other types of x-ray beam limitation devices include cones and cylinders.
• Benefits of restricting x-ray field size: there is a significant reduction in patient dose because less scatter is produced, and it will improve the overall image quality.
• Positive beam limitation collimates the radiographic beam to the IR size, and should not exceed it.
• Cones are used mostly with dental x-ray machines.
• Examinations of the head sometimes use these.
• Cones consist of flared metal tubes and straight cylinders.

Filtration

• Beam filtration reduces the absorbed dose to the patient by removing low-energy photons.
• The types of filtration are inherent + added = total filtration.
• Aluminum is the most widely selected metal.
• It effectively removes low-energy/soft x-rays from a polyenergetic/heterogeneous x-ray beam without severely decreasing the beam intensity.
• Diagnostic x-ray beams must always be adequately filtered.
• The minimum total filtration for a radiographic beam is 2.5 mm of aluminum equivalent

Half-Value Layer (HVL)

• HVL measures the beam quality.
• It uses the thickness of a designated absorber (e.g., aluminum) required to decrease the intensity of the primary beam by 50% of its initial value.
• It must be measured to verify adequate filtration of the x-ray beam.
• A radiologic physicist should obtain measurements at least once a year.
• Measurement should also be taken after an x-ray tube is replaced or when repairs have been made on the diagnostic x-ray tube housing or collimation system.
• HVL is expressed in millimeters of aluminum.
• A minimal HVL is required at a given kVp.

Compensating Filters

• Filters can be made of aluminum, lead-acrylic, or other suitable materials
• They accomplish uniform imaging of body parts with varying thickness or tissue composition.
• Compensating filters partially attenuate x-rays directed toward the thinner areas and permit more x-radiation to strike the thicker areas.
• Types include wedge filters, trough filters (bilateral wedge filter), and boomerangs.

Exposure Reproducibility/Exposure Linearity

• Are used to measure of exposure value when using the same exposure settings.
• Exposure reproducibility indicates consistency in output radiation intensity for identical generator settings from one individual exposure to subsequent exposures.
• The same technical exposure factors can be used to make a series of repeated exposures and observe how radiation intensity varies using a calibrated ion chamber.
• A variance of 5% or less during tests is acceptable.
• Exposure linearity indicates consistency in output radiation intensity at selected kVp settings when generator settings change from one mA and time combination to another.
• A variance of 10% or less is acceptable.

Mobile Radiography

• This is only for patients who cannot be transported to a fixed radiographic installation.
• The SID for mobile radiography is 100 cm (40 inches) or even 120 cm (48 inches).
• The minimum SSD must be at least 30 cm (12").
• The minimum SSD for stationary machines must be at least 38 cm (15").
• Increased SSD, such as 100 cm or 120 cm, results in a more uniform exposure distribution throughout the patient.
• Portable machines and C-arm machines are examples of mobile radiography.