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physics spring semester 2015 > Computed Tomography (CT) > Flashcards

Computed Tomography (CT) Flashcards

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1
Q

what is the goal of CT

A

to overcome the limitations of radiography and tomography by achieving
minimal superimposition
improved image contrast
the recording of very small differences in tissue contrast

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2
Q

early CT units were only capable of

A

transverse images

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3
Q

how does CT work

A

xray beam directed to pt.
attenuated radiation is picked up by detector (IR)
computer analyze signal from detector
reconstructs an image
displays image on monitor
reconstruction of cross sectional anatomy with the aid of algorithms

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4
Q

scanogram

A

AKA scout film

a localization image displayed as the first image of an exam

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5
Q

the tube and detector are mounted

A

opposite each other

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6
Q

beam geometry

A

size and shape of xray beam

CT beam is pencil size

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7
Q

Early CT

A

had a parallel beam that rotated around the pt. and scanned across the head from one side to the other

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8
Q

current CT

A

has a fan shaped beam
30 detectors
reduced time

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9
Q

helical/ spiral CT

A

permits scanning of entire body in a helical pattern without stopping the tube

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10
Q

CT components

A 
gantry, table
xray tube
detectors 
computer
display console 
image storage units
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11
Q

gantry

A

movable frame of the CT unit

contains xray tube and detectors

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12
Q

table

A

motor driven

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13
Q

what is the CT table made out of?

A

carbon graphite

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14
Q

what is the main cause of CT malfunction

A

xray tube failure

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15
Q

focal spot size

A

0.6 and larger

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16
Q

T or F

CT uses a pulsed beam to reduce heat load on the small focal spot

A

true

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17
Q

modern pulsed scanner tubes use

A

120 kvp
1-5 msec pulses
1000 mA

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18
Q

T or F

the radiation beam is double collimated

A

true

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19
Q

what are the 2 collimators

A

pre patient collimator

pre detector collimator

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20
Q

pre patient collimator

A

at tube exit
limits the area of pt. that receives useful beam
determines pt. dose

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21
Q

pre detector collimator

A

at detector entrance right before it hits detector

reduces scatter and improves image contrast

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22
Q

how much generator power is needed to run CT

A

50 kW power

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23
Q

generator

A

has a high voltage generator

step up transformer is mounted on the rotating gantry

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24
Q

Ct filtration serves a dual purpose :

A
  1. filtration removes long wavelength xrays

2. filtration shapes energy distribution across the beam to produce uniform beam hardening

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25
Q

what is the disadvantage to a dual filtration

A

may cause artifacts

can be solved by adding additional filtration to the beam

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26
Q

bow tie filter

A

matches the beam divergence and shape of the pts. body

filters out low energy xrays and creates a better pic

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27
Q

CT detectors should have

A

high capture efficiency
high absorption efficiency
high conversion efficiency

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28
Q

detector dose efficiency

A

how well the detectors work

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29
Q

high capture efficiency

A

how well detectors receive photons from the pt. f

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30
Q

high absorption efficiency

A

the number of photons absorbed

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31
Q

high conversion efficiency

A

how well detectors convert absorbed photon info to digital signal for computer

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32
Q

high stability

A

how often detectors must be recalibrated to meet QC standards

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33
Q

fast response time

A

speed with which detector can react to recognize incoming photon and recover for next input

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34
Q

wide dynamic range

A

ratio of largest signal that can be measured to the smallest

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35
Q

typical modern scanners are capable of a dynamic range of

A

1,000,000 to 1

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36
Q

early CT had how many detectors

A

1

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37
Q

modern CT has how many detectors

A

multiple in an array

up to tens of thousands

38
Q

the scintillator is mad of

A

cadmium tungstate

39
Q

scintillator

A

converts xray energy into light

40
Q

photodiode/ photomultiplier tube

A

converts light into electrical energy

41
Q

data acquisition system DAS

A

a computer controlled electronic device
works with the detectors
located between detectors and computer

42
Q

functions of the DAS

A

measures transmitted radiation
encodes these measurements into binary data
logarithmic conversion of data
transmits data to computer

43
Q

pixel

A

each cell of info
picture element
2D

44
Q

voxel

A

the volume of tissue
volume element
3D

45
Q

hounsfield unit HU

A

numeric information contained in each pixel

related to the xray attenuation of the tissue contained in the voxel

46
Q

the xray attenuation is a combination of

A

the energy of the xray beam
the atomic number of the absorbing tissue
expressed as the attenuation coefficient or the Linear attenuation coefficient

47
Q

the hounsfield unit for water is

A

0

48
Q

the hounsfield unit for bone is

A

+ 1000

49
Q

the hounsfield unit for air is

A
  • 1000
50
Q

the higher the number,

A

the more dense the tissue

51
Q

matrix

A

a grid made up of rows of pixels

52
Q

the more pixels, the better

A

the resolution

53
Q

the bigger the matrix, the better

A

the resolution

54
Q

field of view FOV

A

the diameter of image reconstruction

set to accomodate the size of the part under examination

55
Q

scan field of view

A

a circular region from which the measurements are recorded during scanning

56
Q

display field of view

A

smaller than the scan FOV because we are pinpointing a certain area to be focused on

57
Q

pixel size =

A

FOV/matrix size

58
Q

voxel size =

A

pixel size * slice thickness

59
Q

region of interest ROI

A

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

60
Q

ROI is necessary for

A

image magnification, obtaining an average hounsfield unit measurement, etc

61
Q

good CT magnification is achieved by

A

choosing the correct field of view

62
Q

the image will be distorted when magnifying more than

A

3x decreasing image quality

63
Q

what produces streak artifacts

A

motion

64
Q

what can be used to fix motion

A

a filter

65
Q

metal or star artifact

A

occurs when there are metallic materials in the patient

braces, prothesis

66
Q

how to fix a star artifact

A

some CT units have software algorithms to reduce and partially correct for these artifacts

67
Q

beam hardening / cupping

A

a result of the attenuation of the beam as it passes through the patient
as a result broad dark band or streak, known as cupping occur

68
Q

ring artifacts

A

occurs when a single detector is out of calibration and does not properly record incoming attenuation data

69
Q

partial volume effect

A

occurs when data from an entire section thickness is averaged together to form the image
creates artifacts that appear as streaks

70
Q

how to reduce partial volume effect

A

by using thinner slices

71
Q

T or F

xray beam collimation is so tight in CT that it makes contrast resolution superior

A

True

72
Q

window width

A

the range of numbers (contrast)

73
Q

window level

A

the center of the range (density)

74
Q

the wider the width = less contrast = more shades of gray

A

true

75
Q

the smaller the width = more contrast = black and white

A

true

76
Q

lung windows/soft tissue

A

looks just at lungs or looks just at soft tissue

77
Q

bone windows

A

looks just at bone

78
Q

the quality of CT image is determined by

A 
spatial resolution 
contrast resolution 
noise 
radiation dose
image artifacts
79
Q

the smaller the pixel size the better the

A

spatial resolution

80
Q

noise is directly related to the amount of data collected by the detector

A

true

81
Q

most CT image noise is a result of

A

fluctuation in the information recorded by the detector

82
Q

a standard CT quality assurance measurement is done daily to evaluate noise. this is done how?

A

by scanning a water phantom

a large variation of pixel values shows there is high image noise

83
Q

various digital filters or algorithms are available to suppress noise and improve detail the 3 that are used frequently are

A

standard
smoothing
edge enhancement

84
Q

smoothing

A

reduce image noise and shows good soft tissue anatomy

85
Q

edge enhancement

A

emphasize the edges of structures and improve detail but create image noise in exams in which fine detail is important

86
Q

minimum amount of filtration

A

2.5 mm aluminum equivalent

87
Q

measurement units in CT

A

CTDI
MSAD
DLP

88
Q

CTDI

A

CT dose index
measures the radiation dose to the pt. measured within the primary beam of CT scanner
provides measurement or exposure per slice
uses pencil ionization chamber method

89
Q

MSAD

A

multiple scan average dose

the average dose a pt. receives during an exam

90
Q

DLP

A

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

91
Q

power injectors

A

help to improve the control of the rate of contrast injection and keep tech out of room

physics spring semester 2015 (5 decks)

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