CT- the basic principles section 1

Question 1

in Ct attenuated beam is detected by

Answer 1

specialised detectors

Question 2

what do these detectors measure

Answer 2

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

Question 3

what is a CT protocol

Answer 3

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

Question 4

what are the 7 main components of a CT scanner

Answer 4

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

Question 5

what is the CT gantry

Answer 5

the vertical frame or ‘doughnut’ that surrounds the patient

Question 6

what is the bore or aperture called in the gantry housing

Answer 6

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

Question 7

what is the mylar window

Answer 7

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

Question 8

what 3 things does the gantry housing contain

Answer 8

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

Question 9

what controls can be found on the gantry housing

Answer 9

gantry angulation, table movement, laser lights and emergency stop

Question 10

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

Answer 10

as can cause damage to the eye

Question 11

what does the emergency stop button do

Answer 11

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

Question 12

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

Answer 12

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

Question 13

what 3 things are mounted on the rotating gantry

Answer 13

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

Question 14

what does the high rotational speeds have the potential to do

Answer 14

create a massive centrifugal force

Question 15

what are slip-rings

Answer 15

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

Question 16

what is axial scanning also referred to as

Answer 16

rotate/translate

Question 17

what happens during axial scanning

Answer 17

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

Question 18

what type of images does axial acquisition produce

Answer 18

discrete ‘packages’ of image data

Question 19

what is the disadvantage of packages of data in axial scanning

Answer 19

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

Question 20

what happens in helical/spiral scanning

Answer 20

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

Question 21

what is helical scanning also referred to as

Answer 21

rotate-rotate scanning

Question 22

What are the three advantages of helical scanning

Answer 22

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

Question 23

a major benefit of helical scanning is

Answer 23

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

Question 24

what type of generators are used by helical CT scanners

Answer 24

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

Question 25

what is the rotating anode x-ray tube mounted onto

Answer 25

the rotating gantry

Question 26

what is the disadvantage of lengthy exposure times for CT

Answer 26

X-ray tube is subjected to far greater thermal load

Question 27

what is heat capacity

Answer 27

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

Question 28

what is the heat dissipation rate

Answer 28

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

Question 29

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

Answer 29

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

Question 30

what do ceramic insulators provide

Answer 30

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

Question 31

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

Answer 31

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

Question 32

what do brazed graphite anode discs consist of

Answer 32

tungsten-rhenium focal track brazed onto graphite based bodies

Question 33

what is the purpose/benefit of graphite

Answer 33

increases the heat storage capacity because of its high heat capacity

Question 34

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

Answer 34

the system will not operate in order to protect the tube

Question 35

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

Answer 35

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

Question 36

what do collimators and filters do

Answer 36

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

Question 37

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

Answer 37

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

Question 38

what is contrast resolution

Answer 38

the ability to differentiate between small differences in tissue density

Question 39

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

Answer 39

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

Question 40

what do flat filters (often copper sheets) do

Answer 40

help to reduce the artefact known as beam hardening artefact

Question 41

why are bow tie filters used

Answer 41

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

Question 42

what are bow tie filters usually made of

Answer 42

Teflon

Question 43

where are the CT detectors located

Answer 43

on the rotating gantry opposite the x-ray tube

Question 44

what happens when the attenuated beam enters the detector chamber

Answer 44

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

Question 45

what are the optimal detector characteristics (6 points)

Answer 45

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

Question 46

what are the two types of detectors used in CT

Answer 46

1) ionisation chambers | 2) scintillation detectors (also known as solid state detectors SSD)

Question 47

what are ionisation chambers made out of

Answer 47

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

Question 48

what happens in an ionisation chamber

Answer 48

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

Question 49

3 advantages of ionisation chambers

Answer 49

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

Question 50

2 disadvantages of ionisation chambers

Answer 50

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

Question 51

what is the scintillating material used in the form of crystals

Answer 51

caesium iodide and ceramic materials

Question 52

what happens to the scintillation/SSD

Answer 52

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

Question 53

what is desirable to have for the SSD and why

Answer 53

as little afterglow as possible as this will reduce contrast resolution

Question 54

what is cross talk

Answer 54

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

Question 55

how is cross talk minimised

Answer 55

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

Question 56

advantage of SSD

Answer 56

absorb 100% of photons

Question 57

disadvantage of SSD

Answer 57

more sensitive to temperature and moisture fluctuations

Question 58

what are multi slice CT (MSCT) scanners

Answer 58

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

Question 59

what does multiple rows of detectors mean (6 points)

Answer 59

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

Question 60

where is the DAS located

Answer 60

close to the detectors

Question 61

what does the DAS do

Answer 61

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

Question 62

where does the CT table sit and what does it allow

Answer 62

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

Question 63

what is referencing and what does it allow

Answer 63

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