Ch 36 Radiation Protection Design RTMR 284 Winter 2013

Ch 36-Designing for Protection…….Features: Protective X-Ray tube housing Protective housing to reduce leakage radiation Must be less than 100 mR/hr at a distance of 1m from protective housing. Control Panel Must show exp. Conditions and when tube is energized Beam ON must be clear to techs. SID Indicator Indicator must be present (tape measure or readout) Must be accurate within 2% of the indicated SID

Protection Features cont. Collimation Light field, variable aperture X-ray beam and light filed must coincide w/in 2% of SID PBL-Positive Beam Limitation Auto collimation circa 1974-1994 Must be accurate w/in 2% of SID Beam Alignment How do we know the tube is aligned with the image receptor ???

Protection Features cont. Filtration Inherent plus added Total must be at least 2.5 mm above 70 kVp Reproducibility Constant output radiation intensity Should not exceed 5% through same technique Linearity Constant output for varied mA settings while time is adjusted to keep mAs the same. Max variation is 10% from one mA to adj. mA station

Portable / Fluoro Protection Operator Shield It must not be possible to expose in a room outside of the operator booth Portable x-ray must have >2m tether for exposure Fluoroscopic Protection SSD-source to skin distance Divergence of x-ray beam means the ESE or entrance skin exp. is lessened for the required exit exposure as SSD is increased SSD must be not less than 38cm on stationary fluoroscopes and not less than 30cm on mobile fluoroscopes.

Portable / Fluoro Protection

Fluoro Protection cont. Additional Fluoro Protection Primary Protective Barrier IR assembly must be 2 mm Pb eq. w/ interlock Filtration Total filtration at least 2.5 mm Al eq. Collimation Visible unexposed boarder w/ II 35 cm above TT Exposure Control Dead man… Bucky Slot Cover / Protection Curtain At least 0.25 mm Pb eq. Cumulative Timer Audible signal if time > 5 min. Intensity At tabletop must not exceed 2.1 R/min for ea. mA @ 80kVp

DAP Dose Area Product The intensity of the x-ray beam at the tabletop of a fluoroscope should not exceed 2.1 R/min per mA @ 80kVp. If there is no optional high-level control, the intensity must not exceed 10R/min (20 R/min with high level control) Quantity that reflects not only the dose, but also the volume of tissue irradiated. R-cm^2 units DAP increases with increase field size Used for monitoring equipment

Protective Barriers 3 types of radiation to consider when designing barriers primary radiation the useful beam secondary radiation scatter radiation the patient is the most important scattering object Intensity of SR 1m from the pt. ~0.1% of intensity of the useful beam at the pt. leakage radiation limit of 100mR/hour (not recommended)

Protective Barriers Primary Protective Barrier protects against the primary beam floor wall behind an upright bucky Secondary Barriers protects against secondary radiation control booth walls without upright bucky

Factors Affecting Barriers Distance distance between source of radiation and barrier Use what is the area used for? Controlled area Uncontrolled area Use factor Workload the greater the # of exams/week the thicker the shielding

Definitions Controlled area area occupied mostly by Radiology Personnel and patients. Exposure rate must be less than 100 mR/week Uncontrolled area occupied by anyone maximum exposure rate is 2 mR/weekUse Factor Use Factor The percentage of time that the x-ray beam is on and directed toward a particular wall

Radiation Detection & Measurement Instruments are Designed to: Detect radiation Measure radiation Both detect and measure radiation

4 Types of Radiation Detection Devices Gas-filled ionization chambers proportional counters Geiger-Muller detectors Thermoluminescence dosimeters (TLD) Scintillation detectors Optically Stimulated Luminescence (OSL)

Gas-filled 3 types of gas-filled radiation detectors ionization chambers proportional counters Geiger-Muller detectors

Gas Filled All 3 types of Gas filled Radiation Detection Devices are based on the same principle Radiation passes through gas and ionizes atoms of gas Electrons released in ionization are detected as an electrical signal Electrical signal is proportional to radiation intensity Usually, the larger the chamber, the more sensitive the instrument Because there are more gas molecules for ionization A pressurized chamber has more molecules available as well

Scintillation detectors Some materials emit a flash of light in response to the absorption of ionizing radiation The light emitted is proportional to amount of energy absorbed most often used to indicate individual ionizing events used in fixed or portable radiation detection devices Used in detector arrays of many CT scanners Basis for the gamma camera in Nuclear Med

Thermoluminescence dosimeters (TLD) Some materials glow when heated After this material is exposed to ionizing radiation, it is heated The light let off is proportional to the ionizing radiation it was exposed to

Optically Stimulated Luminescence (OSL) Developed in the late 1990s Uses aluminum oxide to detect radiation Irradiation of aluminum oxide stimulates electrons into an excited state During processing, laser light stimulates these electrons returning them to normal state with the emission of visible light. The intensity of visible light emission is proportional to the radiation dose received.

Optically Stimulated Luminescence (OSL) The OSL process is similar to the TLD Both are based on stimulated luminescence. Advantages of OSL over TLD are: More sensitive than TLD More precise Reanalysis for confirmation of dose is available Qualitative information about exposure conditions A wide dynamic range Excellent long-term stability

The End Projects Due in Class next week….