Flashcards in Problems associated with scattered radiation Deck (25):
Define scattered radiation
X-rays that have been deflected from the primary beam by an interaction with matter, including the patient/tabletop/anything else the primary beam may have hit. Will travel in a variety of directions.
Why are scattered x-rays a nuisance?2
-A radiograph affected by excessive scatter looks indistinct and grey
-X-rays travelling in a variety of directions could hit animal handlers (safety hazard)
How to minimise scattered radiation affecting the image? 3
-Decrease volume of tissue irradiated by collimating (limiting) the size of the primary beam with a light beam diaphragm (to part of body interested in).
-put layer of lead-rubber between cassette and tabletop
-use high kVp exposures (increasing kVp makes beam more penetrating therefore easier to get x-ray through patient to film therefore can use fewer x-rays which minimises risk of scatter).
Structure - x-ray grids
Series of fine lead strips aligned parallel to the direction of the primary x-ray beam. typically 24 strips/cm. betwen lead strips are strips of material that lets most x-rays through (plastic or aluminium for example). Only x-rays in the primary beam tend to pass cleanly between the lead strips.
What is the grid factor?
The amount you increase the exposure, usually about 2-3 times (e..g if you are radiographing a fat animal and repeat it using a grid to improve the image, you need to increase the x-ray exposure to compensate for x-ray absorption in the grid).
Location of the grid when it is in use
The grid is being used when the casette is under the table.
What is radiation exposure?
Ability of radiation to ionise air
Define Roentgen (R)
A measure of radiation exposure
2.58 * 10^-4C/kg
Rate of radioactive decay is measured how?
Becquerels (Bq) (1sec^-1 = 1 disintegration per second, important for scintigraphy)
How do you measure absorbed dose of radiation?
Gray (Gy) = 1 joule/kg (of tissue)
What is the dose equivalent? How do you measure it?
Sievert (Sv) = Gray*q.f. (quality factor: 1 for xrays and gamma rays, higher for particular radiation)
How much radiation is a typical person exposed to in a year?
How do mammalian cells repair or neutralise the effects of damage to DNA caused by radiation? 3
radical detoxification, DNA repair, removal of damaged cells by immune system apoptosis.
Maximum Permissable Dose = 20mSv/pp/year, aged 18 years or more
As low as reasonably acceptable
Degin IRR 1999
Ionising radiation exposures
Detectors for occupational personnel monitoring 2
film badge and thermoluminescent detector (TLD)
How long are dosimeters worn for?
Either 4,8 or 13 weeks. Returned to supplier for dose readings. Readings for whole body and skin are given in mSv
3 basic ways to minimise occupational radiation exposure
Time, distance (remember the inverse square rule) and barriers
Define controlled area
the area around th eprimary beam within which the average radiation dose rate exceeds the permissible limit. Typically this is within a 2m radius of the x-ray tube or is defined as the radiography room. Warning signs must be in position
Define designated person
someone named in the Local Rules as permitted ot make x-ray exposures
Radiation Protection Advisor
Radiation Protection Supervisor
Distinguish RPA and RPS
-RPA = usually a DVR or hospital physicist, insepcts radiography facilities and provides a summary of facilities/problems/recommendations, provides 'Local Rules' = a guide to radiography procedures in the practice whcih states who takes radiapgraphs, how they are taken, define the controlled area. Read by anyone involved in radiography in the practice.
-RPS = usually a partner or CVR/DVR holder, notifies the Health and Safety executive that radiography is performed in the practice, principally responsible for administering the local rules, routinely monitors the dosimeter readings, person to whom problems should be first reported.