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Physics 2 > CT > Flashcards

CT Flashcards

1
Q

Goal of CT

A

minimial superimposition and improve image contrast

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2
Q

what is the IR in CT

A

detectors

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3
Q

what is a scanogram

A

localization image is often displayed as the first image on an examination “scout image”

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4
Q

what is beam geometry

A

size and shape of the x-ray beam

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5
Q

how does beam geometry work

A

exits from the x-ray tube, passes through the patient and strikes a set of detectors

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6
Q

what is the parallel beam

A

rotated around the patient and scanned across head from one side to other

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7
Q

what is the arc of the parallel beam

A

180 degrees

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8
Q

what kind of beam do we use in CT now

A

fan shaped beam

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9
Q

what is the disadvantage of the fan beam

A

increased scatter

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10
Q

how long does it take to scan a patient with fan shaped

A

10-90s

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11
Q

Helical or spiral CT uses what that consits of brushed that fit into grooves

A

slip rings

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12
Q

what does the helical or spiral CT permit

A

scanning of entire body in a helical pattern without stopping tube

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13
Q

what is the gantry

A

moveable from of the CT unit

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14
Q

what is the gantry made of

A

Carbon graphite

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15
Q

what does the gantry contain

A

x-ray tube and detectors

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16
Q

anode heat capacity for CT

A

several million heat units (MHU)

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17
Q

what is the biggest cause of CT malfunction

A

x-ray tube failed from excessive heat

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18
Q

how does CT disperse heat

A

high speed rotors are best for heat dissipation

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19
Q

what is the focal spot size

A

0.6mm or larger

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20
Q

what does the beam do to reduce heat load on small focal spot

A

pulses

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21
Q

what is the modern pulsed scanner tube rates

A

120 kVp, 1000mA 1-5ms

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22
Q

what is the pre patient collimator and where is it located

A

(at tube exit): limits the area of patient that receives useful beam (determines patient dose)

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23
Q

what is the pre detector collimator

A

(at detector entrance): reduces scatter radiation incident on detector array thereby improving image contrast

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24
Q

how much power does the generator use

A

50kW

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25
Q

what is the purpose of filtration (2)

A

removes long wanelengths x-rays (pt dose) and shapes energy distribution across the radiation beam to produce uniform beam

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26
Q

what is the disadvantage of filtration

A

may cause artifacts

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27
Q

where are the filters

A

between tube and patient

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28
Q

what is the shape of the filter in Ct

A

bowtie-matches divergence and patine body in transverse section

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29
Q

what are the 3 parts of detector dose efficiency

A

High capture efficiency: how well detectors receive photon from pt
high absorption efficiency: the number of photons absorbed
high conversion efficiency : how well detectors convert absorbed photon info digital signal for computer

30
Q

what is the dynamic range for CT

A

1,000,000:1

31
Q

modern systems have detectors in an array of

A

tens of thousands

32
Q

what is the 3 types of detectors

A

scintillator, photodiode, gas (not used anymore)

33
Q

what is the scintillator made of

A

cadmium tungstate

34
Q

what does the scintillator do

A

converting x-ray energy to light

35
Q

what does the photodiode (photomultiplier tube) do

A

converts light to electrical signal

36
Q

what is the approx, detection efficiency for scintillor

A

90%

37
Q

what does DAS stand for

A

data acquisition system

38
Q

where is DAS located

A

between detectors and computer

39
Q

what does DAS do

A

measures transmitted radiation
encodes there measurement into binary data
logarithmic conversion to data
transmits data to computer

40
Q

each cell of info is A

A

pixel

41
Q

the volume of tissue is known as

A

voxel

42
Q

what is the 2D representation

A

pixel

43
Q

what is the 3D representation

A

voxel

44
Q

what is the HU or CT number

A

numeric information contained in each pixel-related to the x-ray attenuation of the tissue contained in the voxel

45
Q

what is linear attenuation coefficient

A

energy of the x-ray beam atomic number of absorbing tissue

46
Q

what is the HU of air

A

-1000

47
Q

what is the HU of water

A

0

48
Q

what is the HU of fat

A

-100

49
Q

what is the HU of bone

A

+1000

50
Q

what is the matrix

A

grid made up of rows of pixels

51
Q

what is the FOV

A

the diamter of image reconstruction

52
Q

equation for pixel size=

A

FOV/Matirx

53
Q

equation for voxel size

A

pixel size/ slice thickness

54
Q

what is the ROI

A

a display function available on all scanners is that of defining an area on the image

55
Q

two types of artifacts and from what

A

Streak=motion

star=metal

56
Q

what is windowing

A

range of CT numbers

57
Q

what is WW

A

range of numbers is window width

58
Q

what is WL

A

center of the range

59
Q

what can WW and WL alter

A

image contrast

60
Q

what is image reformation

A

process of using data to create a view in a different body plane

61
Q

what are the 3 algorithsms that are used

A

standard, smoothing, edge enhancement

62
Q

what is smoothing

A

reduce image noise and shows good soft tissue anatomy

used in exams where soft tissue discrimination is important to visualize very low contrast structures

63
Q

what is edge enhancement

A

algorithms emphasize the edges of structures and improve detail but create image noise in exams in which fine detail is important (inner ear, bone structures, thine slice and fine pulmonary exams)

64
Q

minimum amount of filtration -that operate over what kVP

A

2.5mm Al at 90kVp

65
Q

what is CTDI

A

CT dose index

66
Q

what does CTDI represent

A

measures radiation dose to patient measures within the primary beam of CT scanner
provides measurement of exposure per slice
determined by using pencil ionization chamber method

67
Q

what is MSAD

A

multiple scan average dose

68
Q

what does MSAD represent

A

represents the average dose a patient receives during an examination–actual tissue dose

69
Q

what is DLP

A

dose length product

70
Q

what does DLP represent

A

provides measurement of total amount of exposure for a series of scans
product of CTDI and slice thickness

Physics 2 (3 decks)

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