X-ray tube

Question 1

how are x-rays produced in the x-ray tube

Answer 1

by converting electrical energy into electromagnetic radiation

Question 2

what are the two main steps when producing an x-ray

Answer 2

achieved by

1) accelerating electrons from an electrically negative cathode towards a positive ‘target’ anode
2) when electrons collide with the anode they are rapidly decelerated

Question 3

what happens, and in what percentages, when the electrons are decelerated

Answer 3

99% of their kinetic energy is converted in heat…a major problem
1% of their kinetic energy is converted into x-rays photons, which we need for x-ray imaging

Question 4

purpose of an x-ray tube assembly (4 points)

Answer 4

1) to produce a useful beam of x-ray photons from a very small source
2) to cope with the heat generated during the process of x-ray production
3) to allow control of the quality and quantity of the radiation
4) the radiation should only emerge from the window or port of the housing of the x-ray tube

Question 5

an x-ray tube should be: (5 points)

Answer 5

1) electrically safe and provide protection from unwanted x-radiation
2) securely supported
3) capable of easy movement into any position and maintain that position
4) capable of precise angulation
5) easily cleaned and maintained

Question 6

definition of radiolucent

Answer 6

describes a structure that allows x-ray photons to pass through it with relative ease

Question 7

definition of radiopaque

Answer 7

a structure that offers more resistance to the passage of x-rays through it

Question 8

what two features does the x ray tube housing have to protect it from damage

Answer 8

1) filled with oil
2) a relatively large exterior surface
helps to dissipate the heat created within the tube

Question 9

what happens as the tube temperature increases

Answer 9

the oil expands. a flexible diaphragm within the housing moves in and out to allow the oil to safely expand and contract.

Question 10

what happens if the oil expands too much in the x ray tube housing

Answer 10

the temperature within the housing is dangerously high. a microswitch is triggered and all further exposures are prevented until the environment within the housing cools

Question 11

what is the x ray tube housed in

Answer 11

a protective steel, lead lined housing

Question 12

what is the function of the lead lined housing

Answer 12

acts as a shield and absorbs stray X-radiation

Question 13

what happens to the useful x ray beams

Answer 13

pass through a small radiolucent window/port in the tube housing

Question 14

what does the light beam diaphragm do

Answer 14

allows adjustment of the field size (collimation)

Question 15

what is collimation

Answer 15

allows the radiographer to adjust the size and shape of the x ray beam for each patient

Question 16

how does the light beam diaphragm work

Answer 16

uses a mirror and lamp system to illuminate the area of the patient which will be irradiated by x-rays, but it does this before the x-ray exposure commences

Question 17

what two things are achieved by limiting the area of the x ray beam on the patient

Answer 17

1) reduces the dose to the patient

2) reduces scattered radiation and so improves image quality

Question 18

before an exposure is made, the filament is heated and it glows, producing electrons by…

Answer 18

thermionic emission

Question 19

what happens to the anode and electrons during the x-ray exposure

Answer 19

the anode is made electrically very positive with respect to the cathode. The negatively charged electrons are attracted by the now positively charged anode

Question 20

when is the filament (cathode) heated

Answer 20

during ‘prep’ mode before an actual x-ray exposure is made by an electric current (12 volts, 4 amps)

Question 21

what controls the filament temperature

Answer 21

the tube current (mA) selector

Question 22

what is thermionic emission

Answer 22

when the heat of the glowing filament transfers energy to some of the electrons within the filament. the electrons are able to free themselves and leave the wire at its surface

Question 23

what does thermionic emission create

Answer 23

a ‘cloud’ of electrons that are held around the cathode in readiness to be attracted to the anode during the x ray exposure

Question 24

how many coal points do most diagnostic x ray tubes have

Answer 24

two separate filaments of different sizes i.e. are dual focus.
each filament has its own focusing cup. the electrons can be produced from a large ‘broad’ focus or a small ‘fine’ focus

Question 25

what is the benefit of having a small focal spot size

Answer 25

produces sharp images (very important in diagnostic radiography)

Question 26

what is the problem with using a fine focus

Answer 26

potential overheating of the x-ray tube

Question 27

the higher the x-ray exposure factors we use the…

Answer 27

more heat produced by the x-ray tube. Heat units produced in a given exposure= kV x mA x sec x 1.4

Question 28

the larger the source of x-rays the…

Answer 28

greater the unsharpness or blurring of the image

Question 29

when x-ray images are produced there is always a penumbra. what does this mean

Answer 29

that all images have inherent unsharpness and x-rays cannot be produced from a point source and cannot be focused like visible light

Question 30

what 3 things cause penumbra of images

Answer 30

1) focal spot size
2) source to detector distance (SDD)
3) Object to detector distance (ODD)

Question 31

what does the line focus principle aim to do (2 things)

Answer 31

1) having the x-rays generated from as large an area of the target as possible to cope with the heat produced
2) having the x-rays appear to come from as small an area as possible to produce a tolerable level of unsharpness in the image

Question 32

what happens if the anode is stationary during exposure

Answer 32

all electrons from filament fired at the same area. leads to a dangerous temperature rise and it will melt the anode

Question 33

what happens if the anode rotates during exposure

Answer 33

electrons from filament spread over a larger area. leads to lower temperature rise and will not melt the anode

Question 34

2 benefits of the line focus principle in the design of the x-ray tube (rotating anode)

Answer 34

1) represents a way of meeting (to a certain extent) the conflicting requirements of having a small apparent area of the x-rays and a large actual area of origin
2) it enables adequate heat dissipation properties whilst achieving a small focus to produce small amounts of geometric unsharpness due to penumbra

Question 35

what does the rotating anode assembly consist of

Answer 35

the anode disc, composed of tungsten/rhenium skin coated onto a molybdenum base. some anodes also have a carbon heat sink to further dissipate the heat produced.

Question 36

what is the anode disc supplied with and what is its purpose

Answer 36

supplied with a very high (50 to 150 kV) positive (+) voltage during exposure.
its purpose is to attract electrons from the cathode, decelerating them rapidly and convert their kinetic energy into x-rays

Question 37

main aim of the rotating anode assembly

Answer 37

has to be able to withstand high temperatures and be an efficient emitter of x-rays

Question 38

why is tungsten used (3 points)

Answer 38

1) has a high atomic number (74)- good for x-ray production
2) a high melting point (3,370 degrees Celsius
3) a high thermal conductivity

Question 39

why is rhenium used (3points)

Answer 39

1) increases thermal capacity and tensile strength
2) maintains a smoother surface- a rough surface would reabsorb a large proportion of the x rays produced
3) increases resistance to damage from overheating and continuous electron bombardment

Question 40

what is the purpose of the anode stem

Answer 40

also made of molybdenum- it connects the anode disc mechanically to the rotor and electrically to the high voltage terminal

Question 41

what is the purpose of the rotor & bearings

Answer 41

in conjunction with the stator windings, enables the anode to rotate freely at 3,000 rpm or 9,000 rpm

Question 42

what is the purpose of the stator windings

Answer 42

surround the neck of the x-ray tube, but still within the steel housing. when supplied with an alternating voltage, they induce rotation of the rotor and hence the anode