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Flashcards in Problems associated with scattered radiation Deck (25):
1

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.

2

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)

3

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).

4

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.

5

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).

6

Location of the grid when it is in use

The grid is being used when the casette is under the table.

7

What is radiation exposure?

Ability of radiation to ionise air

8

Define Roentgen (R)

A measure of radiation exposure
2.58 * 10^-4C/kg

9

Rate of radioactive decay is measured how?

Becquerels (Bq) (1sec^-1 = 1 disintegration per second, important for scintigraphy)

10

How do you measure absorbed dose of radiation?

Gray (Gy) = 1 joule/kg (of tissue)

11

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)

12

How much radiation is a typical person exposed to in a year?

2.3mSv/year

13

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.

14

Define MPD

Maximum Permissable Dose = 20mSv/pp/year, aged 18 years or more

15

Define ALARA

As low as reasonably acceptable

16

Degin IRR 1999

Ionising radiation exposures

17

Detectors for occupational personnel monitoring 2

film badge and thermoluminescent detector (TLD)

18

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

19

3 basic ways to minimise occupational radiation exposure

Time, distance (remember the inverse square rule) and barriers

20

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

21

Define designated person

someone named in the Local Rules as permitted ot make x-ray exposures

22

Define RPA

Radiation Protection Advisor

23

Define RPS

Radiation Protection Supervisor

24

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.

25

How do you adjust mAs and kVp for decreasing scatter?

Low mAs adn high kVp reduces scatter (versus the opposite of this).

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