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

Computed Tomography (CT) Flashcards

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
what is the disadvantage to a dual filtration
may cause artifacts | can be solved by adding additional filtration to the beam
26
bow tie filter
matches the beam divergence and shape of the pts. body | filters out low energy xrays and creates a better pic
27
CT detectors should have
high capture efficiency high absorption efficiency high conversion efficiency
28
detector dose efficiency
how well the detectors work
29
high capture efficiency
how well detectors receive photons from the pt. f
30
high absorption efficiency
the number of photons absorbed
31
high conversion efficiency
how well detectors convert absorbed photon info to digital signal for computer
32
high stability
how often detectors must be recalibrated to meet QC standards
33
fast response time
speed with which detector can react to recognize incoming photon and recover for next input
34
wide dynamic range
ratio of largest signal that can be measured to the smallest
35
typical modern scanners are capable of a dynamic range of
1,000,000 to 1
36
early CT had how many detectors
1
37
modern CT has how many detectors
multiple in an array | up to tens of thousands
38
the scintillator is mad of
cadmium tungstate
39
scintillator
converts xray energy into light
40
photodiode/ photomultiplier tube
converts light into electrical energy
41
data acquisition system DAS
a computer controlled electronic device works with the detectors located between detectors and computer
42
functions of the DAS
measures transmitted radiation encodes these measurements into binary data logarithmic conversion of data transmits data to computer
43
pixel
each cell of info picture element 2D
44
voxel
the volume of tissue volume element 3D
45
hounsfield unit HU
numeric information contained in each pixel | related to the xray attenuation of the tissue contained in the voxel
46
the xray attenuation is a combination of
the energy of the xray beam the atomic number of the absorbing tissue expressed as the attenuation coefficient or the Linear attenuation coefficient
47
the hounsfield unit for water is
0
48
the hounsfield unit for bone is
+ 1000
49
the hounsfield unit for air is
- 1000
50
the higher the number,
the more dense the tissue
51
matrix
a grid made up of rows of pixels
52
the more pixels, the better
the resolution
53
the bigger the matrix, the better
the resolution
54
field of view FOV
the diameter of image reconstruction | set to accomodate the size of the part under examination
55
scan field of view
a circular region from which the measurements are recorded during scanning
56
display field of view
smaller than the scan FOV because we are pinpointing a certain area to be focused on
57
pixel size =
FOV/matrix size
58
voxel size =
pixel size * slice thickness
59
region of interest ROI
a display function available on all scanners is that of defining an area on the image most often circular
60
ROI is necessary for
image magnification, obtaining an average hounsfield unit measurement, etc
61
good CT magnification is achieved by
choosing the correct field of view
62
the image will be distorted when magnifying more than
3x decreasing image quality
63
what produces streak artifacts
motion
64
what can be used to fix motion
a filter
65
metal or star artifact
occurs when there are metallic materials in the patient | braces, prothesis
66
how to fix a star artifact
some CT units have software algorithms to reduce and partially correct for these artifacts
67
beam hardening / cupping
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
ring artifacts
occurs when a single detector is out of calibration and does not properly record incoming attenuation data
69
partial volume effect
occurs when data from an entire section thickness is averaged together to form the image creates artifacts that appear as streaks
70
how to reduce partial volume effect
by using thinner slices
71
T or F | xray beam collimation is so tight in CT that it makes contrast resolution superior
True
72
window width
the range of numbers (contrast)
73
window level
the center of the range (density)
74
the wider the width = less contrast = more shades of gray
true
75
the smaller the width = more contrast = black and white
true
76
lung windows/soft tissue
looks just at lungs or looks just at soft tissue
77
bone windows
looks just at bone
78
the quality of CT image is determined by
``` spatial resolution contrast resolution noise radiation dose image artifacts ```
79
the smaller the pixel size the better the
spatial resolution
80
noise is directly related to the amount of data collected by the detector
true
81
most CT image noise is a result of
fluctuation in the information recorded by the detector
82
a standard CT quality assurance measurement is done daily to evaluate noise. this is done how?
by scanning a water phantom | a large variation of pixel values shows there is high image noise
83
various digital filters or algorithms are available to suppress noise and improve detail the 3 that are used frequently are
standard smoothing edge enhancement
84
smoothing
reduce image noise and shows good soft tissue anatomy
85
edge enhancement
emphasize the edges of structures and improve detail but create image noise in exams in which fine detail is important
86
minimum amount of filtration
2.5 mm aluminum equivalent
87
measurement units in CT
CTDI MSAD DLP
88
CTDI
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
MSAD
multiple scan average dose | the average dose a pt. receives during an exam
90
DLP
dose length product provides measurement of total amount of exposure for a series of scans product of CTDI and slice thickness
91
power injectors
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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