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IMAG 211- Complimentary Imaging Systems > CT- the basic principles section 1 > Flashcards

CT- the basic principles section 1 Flashcards

1
Q

in Ct attenuated beam is detected by

A

specialised detectors

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2
Q

what do these detectors measure

A

the attenuated beam and these measurements are sent to a computer system and converted to create a ‘cross sectional’ image which is displayed on the monitor

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3
Q

what is a CT protocol

A

the protocol consists of scan parameters e.g. exposure factors. differs for each body part

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4
Q

what are the 7 main components of a CT scanner

A

1) the CT gantry
2) the high voltage generator
3) the x-ray tube
4) collimators and filters
5) the detector array
6) the data acquisition system (DAS) and Ct computer
7) the Ct table

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5
Q

what is the CT gantry

A

the vertical frame or ‘doughnut’ that surrounds the patient

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6
Q

what is the bore or aperture called in the gantry housing

A

the opening where the patient is positioned, usually 70cm in diameter

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7
Q

what is the mylar window

A

it is a narrow, transparent window that surrounds the bore and is where the x-ray beam will pass

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8
Q

what 3 things does the gantry housing contain

A

1) detectors
2) collimators
3) data Acquisition system (DAS)

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9
Q

what controls can be found on the gantry housing

A

gantry angulation, table movement, laser lights and emergency stop

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10
Q

why is it important to warn patient about laser lights for positioning

A

as can cause damage to the eye

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11
Q

what does the emergency stop button do

A

removes power to the table drives, gantry drives and x-ray system

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12
Q

where and when should the system emergency off (SEO) be used

A

located on the wall and in the event such as a fire or smoke

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13
Q

what 3 things are mounted on the rotating gantry

A

1) x-ray tube
2) detectors
3) CT generator

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14
Q

what does the high rotational speeds have the potential to do

A

create a massive centrifugal force

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15
Q

what are slip-rings

A

electro-mechanical devices consisting of parallel, electrically conductive rings and brushes attached to the rotating gantry

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16
Q

what is axial scanning also referred to as

A

rotate/translate

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17
Q

what happens during axial scanning

A

the x-ray tube and detectors rotate 360 degrees around the patient. The CT table is stationary during this. After each 360 degree rotation the gantry halts and the xray table moves or ‘translates’ into its next scanning position. the gantry then rotates 360 degrees in the opposite direction

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18
Q

what type of images does axial acquisition produce

A

discrete ‘packages’ of image data

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19
Q

what is the disadvantage of packages of data in axial scanning

A

slice thickness, position and orientation of each image are locked. the resultant images can only be manipulated in a very limited way

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20
Q

what happens in helical/spiral scanning

A

the table moves at a constant speed through the gantry as the x-ray beam rotates continuously around the patient

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21
Q

what is helical scanning also referred to as

A

rotate-rotate scanning

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22
Q

What are the three advantages of helical scanning

A

1) faster scan times and minimal interscan delay
2) data is acquired as a volume
3) increased temporal resolution

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23
Q

a major benefit of helical scanning is

A

that images in alternative planes can be produced, known as multiplanar reformation (MPR)

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24
Q

what type of generators are used by helical CT scanners

A

high-frequency, solid-state generators. These are small,compact and more efficient and provide a stable,high power output to the x-ray tube

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25
Q

what is the rotating anode x-ray tube mounted onto

A

the rotating gantry

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26
Q

what is the disadvantage of lengthy exposure times for CT

A

X-ray tube is subjected to far greater thermal load

27
Q

what is heat capacity

A

the x-ray tube’s ability to withstand the heat production

28
Q

what is the heat dissipation rate

A

the rate at which the x-ray tube is able to rid itself of the heat produced

29
Q

what is the reason behind replacing a glass envelope by a metal envelope

A

this eliminates the risk of electrical arcing caused by tungsten deposits, due to vaporisation, on the glass envelope

30
Q

what do ceramic insulators provide

A

superior high voltage insulation and isolate the metal envelope from the anode and cathode voltage

31
Q

what is the effect of using large, rotating anode disks (typically 12-13cms or more in diameter)

A

this increases the heat storage capacity and have an improved heat dissipation rate allowing higher tube currents

32
Q

what do brazed graphite anode discs consist of

A

tungsten-rhenium focal track brazed onto graphite based bodies

33
Q

what is the purpose/benefit of graphite

A

increases the heat storage capacity because of its high heat capacity

34
Q

if scans are requested that exceed the current capacity, what happens?

A

the system will not operate in order to protect the tube

35
Q

If the scanner is left idle, what must you do and why

A

a tube warm up to prevent the cold x-ray tube having to deal with the heat generated. this maintains image quality and extends the x-ray tube life

36
Q

what do collimators and filters do

A

modify the x-ray beam, filters the x-ray photons and guard the detectors from scattered radiation

37
Q

what is the purpose of the additional collimators in front of the detectors

A

reduce signal generation from scattered radiation, thus improving contrast resolution and reducing dose

38
Q

what is contrast resolution

A

the ability to differentiate between small differences in tissue density

39
Q

what do the filters attached to the x-ray tube housing do

A

reduce radiation dose and improve image resolution by removing low energy photons from the x-ray spectrum to produce a more uniform beam

40
Q

what do flat filters (often copper sheets) do

A

help to reduce the artefact known as beam hardening artefact

41
Q

why are bow tie filters used

A

as they are shaped to reduce beam intensity at the periphery of the beam, to correspond with the thinner parts of the patients anatomy

42
Q

what are bow tie filters usually made of

A

Teflon

43
Q

where are the CT detectors located

A

on the rotating gantry opposite the x-ray tube

44
Q

what happens when the attenuated beam enters the detector chamber

A

its energy is amplified and converted to an electrical signal which is then processed by the CT computer

45
Q

what are the optimal detector characteristics (6 points)

A

1) high detector efficiency
2) high scatter suppression
3) high stability
4) low afterglow
5) low cross talk between detector elements
6) low cost and easy to service

46
Q

what are the two types of detectors used in CT

A

1) ionisation chambers

2) scintillation detectors (also known as solid state detectors SSD)

47
Q

what are ionisation chambers made out of

A

thin tungsten plates that sit within a hollow chamber that is filled with pressurised Xenon gas

48
Q

what happens in an ionisation chamber

A

as the x-ray beam enters the chamber, the gas becomes ionised and the ions are collected by the electric field that exists between the plates, producing a signal that is amplified and processed by the computer as raw data

49
Q

3 advantages of ionisation chambers

A

1) less expensive than SSD
2) easier to calibrate
3) stable

50
Q

2 disadvantages of ionisation chambers

A

1) have to be kept under pressure in an aluminium casing but this filters the beam reducing the efficiency of the system
2) bulky

51
Q

what is the scintillating material used in the form of crystals

A

caesium iodide and ceramic materials

52
Q

what happens to the scintillation/SSD

A

these convert the energy absorbed from the x-ray beam to a light pulse. photo diodes detect this and convert it into a corresponding electrical signal which will be processed as raw data

53
Q

what is desirable to have for the SSD and why

A

as little afterglow as possible as this will reduce contrast resolution

54
Q

what is cross talk

A

this occurs when a photon strikes a detector and is partially absorbed. the same photon then enters the detector adjacent to it and is detected again. this produces two weak signals coming from 2 detectors and reduces image resolution

55
Q

how is cross talk minimised

A

by using a crystal that is highly efficient at absorbing x-ray photons

56
Q

advantage of SSD

A

absorb 100% of photons

57
Q

disadvantage of SSD

A

more sensitive to temperature and moisture fluctuations

58
Q

what are multi slice CT (MSCT) scanners

A

have multiple, parallel rows of detectors- known as detector array

59
Q

what does multiple rows of detectors mean (6 points)

A

1) rapid scan speed
2) better coverage in single breath hold
3) reduced patient moving artefact
4) the ability to scan at peak contrast enhancement due to quicker scan speed
5) less requirement to sedate paed/agitated patients
6) good temporal resolution

60
Q

where is the DAS located

A

close to the detectors

61
Q

what does the DAS do

A

it samples each of the detector elements, amplifies the signal and converts it from an analogue signal to a digital signal. This sampled data is processed by the CT computer and is now known as image data. this image data is displayed on a screen monitor where it can be manipulated

62
Q

where does the CT table sit and what does it allow

A

within a cradle and allows for vertical movement of the table for patient positioning and is to be motored in and out of the gantry bore

63
Q

what is referencing and what does it allow

A

where the table position is set to zero when it has been centred over an anatomical landmark. allows for consistency between examinations

IMAG 211- Complimentary Imaging Systems (11 decks)

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