Problems Associated with Scattered Radiation
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What is scattered radiation?
Xrays that have been deflected from the primary beam by an interaction with matter (patient/table top etc.)
Why is scattered radiation a problem?
Darkens image and decreases contrast. Health and safety - could affect vets/nurses.
Give 3 ways to minimise scattered radiation affecting the image
• Minimise volume of tissue being irradiated - collimate with light beam diaphragm • Lead rubber between casette and table top • Use ^kVp exposure with vmAS (use less xrays to get the same result)
What are xray grids useful for?
Animals thicker than 10cm
What are xray grids composed of?
Lead strips (24/cm) then aluminium or plastic inbetween. Scattered radiation comes in at an angle and is absorbed, only radiation from the primary beam fits smoothly through.
How do you prevent the lead lines showing up on a radiograph?
Wobble the grid
How else must the exposure be compensated when using a grid?
Increased the amount of radiation (mA) to compensate for xrays absorbed - called the grid factor Usually 2-3x normal
Where is the grid usually located?
Within the table. Move cassette under table to make use of grid/
WHat are the units of radiation exposure?
Roentgen
What are the units of rate of radioactive decay?
Becquerel
What are the units of absorbed dose of radiation?
Gray
What are the units of dose-equivalent measure of effect of radiation
Sievert
How are sieverts calculated?
Grays x Quality Factor (q.f). Quality factor = 1 for gamma and xrays, for more ionising radiation is higher
What is the MPD (maximum permissable dose) for persons at work 18 years +
20mSv/year
What two types of detector exist?
Film badge Thermoluminescent detector
What three ways can you minimise your exposure to radiation?
• Time: minimum time spent in contact with xrays by only taking them when clinically relevent and minimising retakes • Distance: keep away from patient (source of scatter) • Barriers: Personal - lead apron/gloves Mobile - Lead plywood doors Walls - Brick/leadsheet/barium plaster
Define the controlled area
Area around the primary beam within which the average radiation dose rate exceeds the permissable limit Typically 2mx2m OR the radiography room Warning signs necessary
Define the designated radiation protection advisor
Someone named in the local rules as permitted to inspect and advise on safe radiographic practices, provide local rules and define controlled area.
Define the designated radiation protection supervisor
Monitors dosimeter readings and problems.
What does ALARA stand for?
As low as reasonably acheivable