LECTURE 3

Question 1

what is the xray tube?

Answer 1

a vacuum tube that converts electrical input into x-rays

Question 2

what are the 2 elements of an xray tube?

Answer 2

cathode & anode

Question 3

what is the role of the cathode?

Answer 3

creates free electrons through thermionic emission- negatively charged

Question 4

what is the role is the role of the anode?

Answer 4

absorbs electrons & creates xrays- positvely charge & attracts electrons

Question 5

what surrounds the xray tube?

Answer 5

a glass envelope & tube housing

Question 6

what is the function of the glass envelope?

Answer 6

creates an airfree vacuum around cathode & anode & protects tube from oxidation & corrosion

Question 7

what is the function of the tube housing?

Answer 7

prevents xray from being emiited throughout exam room- contains oil- absorbs xray photons

Question 8

what is inside the cathode?

Answer 8

tungsten filament- when it get hot it creates electrodes

Question 9

what is thermionic emission?

Answer 9

heating the cathode to create electrons- limit to 1000ma (Space charge effect)

Question 10

what surrounds the filament?

Answer 10

focusing cup that is negatively charged to focus the electron beam and gives sharpness

Question 11

what are the 2 types of anodes?

Answer 11

rotating & stationary

Question 12

what are stationary anodes?

Answer 12

simple design but low heat capacity- only use in low exposure- only used in dental machines

Question 13

what are rotating anodes?

Answer 13

rotates to spread out electrons and heat capacity- high heat capacity & used at high exposures- heat is spread around circumference of the anode

Question 14

what turns the anode?

Answer 14

induction motor- turns anode without touching the anode

Question 15

what are the components of the induction motor?

Answer 15

stator (electromagnets surrounding rotor)- rotor (rotates-tube shaft attached to anode disc)- bearing (between rotor & stator- bearings)

Question 16

what did rontgen discover in 1895?

Answer 16

first discovered xrays

Question 17

what did crookes create?

Answer 17

created the crookes tube- realised that having a glass envelope & created a vacuum- remove all the air

Question 18

what is ionisation?

Answer 18

creation of a positive & negative ion- if xray hits neutral air, the xray will kick out one of its electron and you create a positve ion

Question 19

what is the coolidge tube?

Answer 19

produce many more electrons by heating the filament and release more electrons

Question 20

what are the challenges of xray tube design?

Answer 20

efficient xray production, heat, geometry, reliability, loading

Question 21

what is geometric unsharpness?

Answer 21

refers to the loss of definition that is the result of geometric factors of the radiographic equipment and setup. It occurs because the radiation does not originate from a single point but rather over an area.

Question 22

what occurs when the sharpness is small?

Answer 22

better definition & sharpness but too hot

Question 23

why do we do PA chest xray?

Answer 23

less magnification of the heart- heart is closer to the receptor- sharpness of heart is reduced in AP position but enlarged

Question 24

what is a diode?

Answer 24

2 electrodes

Question 25

what is the glass envelope made of?

Answer 25

borosilicate- strong, heat resistant, xray/heat transparent (got to get heat out of xray tube)- you want to stop the xray escaping and only allow 1 section to allow heat through

Question 26

what is vaporisation?

Answer 26

as you heat filamet, particles of filament can be scattered and lie on glass envelope- creating a filter and stopping xray but sometimes filtering is good but not necessarily good if there is vaporisation

Question 27

what is a method to get around vapourisation?

Answer 27

metal tubes- tougher than glass- mass attenuation as it affected absorption of xray and scattered the xrays

Question 28

what is around the glass envelope?

Answer 28

housing- helps with heat & radiation detection- contains air or oil

Question 29

what is the role of oil in the housing?

Answer 29

absorbs heat and disperse it

Question 30

what are mechanisms in place to avoid breaking xray tube?

Answer 30

microswitch- if oil heats up too much it expands and push a switch and prevent further exposures

Question 31

what surrounds the housing of xray tube?

Answer 31

lead shielding except where xrays come out to prevent unnecessary xrays dispersing & only contacting the patient

Question 32

what is amount of leakage radiation allowed?

Answer 32

1mGy/h at 1 meter (1 milligrey per hour)- usually caps at 0.3mGy/h

Question 33

what is a mechanism to confine the xrays to only the body part wanting to be xrayed?

Answer 33

fixed lead diaphragm at port

Question 34

why does a lead shield not reduce radiation to 0?

Answer 34

due to exponenital relationship- only fractional changes e.g. 2mm lead= 50% reduction- you'll never get to zero

Question 35

why do we need filtration of xrays?

Answer 35

removes lower energy xrays that would not add to the image but rather absorb by patient or scatter

Question 36

what are filters usually made up of?

Answer 36

aluminium or copper as leads atomic number is too high and would absorb the xrays

Question 37

by addind filtration, what is the average energy of xray beam?

Answer 37

by removing lower xray beams, avergae energy goes up which is not good for image contrast (image contrast is best at lower energy)- increase energy reduces image contrast (but we need image contrast)

Question 38

what is inherent beam filtration?

Answer 38

inherent filtration from components in the x-ray tube, i.e. window, housing, cooling oil- 0.8mm aluminium equivalent but we need it up to 2.5 (add filters)

Question 39

what is added filtration?

Answer 39

added to beam to bring it up to 2.5mm e.g. aluminium or copper plates

Question 40

what does the light beam diaphragm allow?

Answer 40

allows you to shape the area around what you want to xray

Question 41

what allows the light beam diaphragm to move in multiple directions?

Answer 41

lead leaves

Question 42

what is the cathode filament made of?

Answer 42

tungsten (atomic number: 74- high)- need high number that has lots of electrons can be released

Question 43

what is the nickel cathode hood?

Answer 43

wraps around filament that is negatively charged & rounded to allow electrons come together in a focused way, allowing precise geometry

Question 44

what is focal spot size? .

Answer 44

0.3mm and 0.6mm, usually for mammography. 1.0mm and 1.2mm, usually for general radiography

Question 45

what is filament suppy?

Answer 45

heat given to heat filament by thermionic emission- heating filament to get over binding energy and can escape and form xray

Question 46

what happens to the filament when it releases electron?

Answer 46

more positive

Question 47

why do xrays not use alternating current (AC)?

Answer 47

we only want electrons to flow in one direction- every xrays electrical supply is rectified

Question 48

what does three emission types?

Answer 48

3 different focal spot sizes- size of filament may determine size of focal spot size

Question 49

what degrees does cathode produce its electrons?

Answer 49

2300 degrees