Problems associated with scattered radiation

Question 1

Define scattered radiation

Answer 1

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.

Question 2

Why are scattered x-rays a nuisance?2

Answer 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)

Question 3

How to minimise scattered radiation affecting the image? 3

Answer 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).

Question 4

Structure - x-ray grids

Answer 4

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.

Question 5

What is the grid factor?

Answer 5

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).

Question 6

Location of the grid when it is in use

Answer 6

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

Question 7

What is radiation exposure?

Answer 7

Ability of radiation to ionise air

Question 8

Define Roentgen (R)

Answer 8

A measure of radiation exposure

2.58 * 10^-4C/kg

Question 9

Rate of radioactive decay is measured how?

Answer 9

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

Question 10

How do you measure absorbed dose of radiation?

Answer 10

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

Question 11

What is the dose equivalent? How do you measure it?

Answer 11

Sievert (Sv) = Gray*q.f. (quality factor: 1 for xrays and gamma rays, higher for particular radiation)

Question 12

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

Answer 12

2.3mSv/year

Question 13

How do mammalian cells repair or neutralise the effects of damage to DNA caused by radiation? 3

Answer 13

radical detoxification, DNA repair, removal of damaged cells by immune system apoptosis.

Question 14

Define MPD

Answer 14

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

Question 15

Define ALARA

Answer 15

As low as reasonably acceptable

Question 16

Degin IRR 1999

Answer 16

Ionising radiation exposures

Question 17

Detectors for occupational personnel monitoring 2

Answer 17

film badge and thermoluminescent detector (TLD)

Question 18

How long are dosimeters worn for?

Answer 18

Either 4,8 or 13 weeks. Returned to supplier for dose readings. Readings for whole body and skin are given in mSv

Question 19

3 basic ways to minimise occupational radiation exposure

Answer 19

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

Question 20

Define controlled area

Answer 20

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

Question 21

Define designated person

Answer 21

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

Question 22

Define RPA

Answer 22

Radiation Protection Advisor

Question 23

Define RPS

Answer 23

Radiation Protection Supervisor

Question 24

Distinguish RPA and RPS

Answer 24

• 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.

Question 25

How do you adjust mAs and kVp for decreasing scatter?

Answer 25

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