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Huda > Chapter 12 CT II > Flashcards

Chapter 12 CT II Flashcards

1
Q

typical x-ray tube voltage in head CT

A

120 kV

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2
Q

why use higher voltage (140kV) in posterior fossa

A

minimize beam hardening artefacts

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3
Q

why use lower tube voltage (100 kV) in angiographic imaging

A

maximize visibility of iodinated contrast media

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4
Q

why are x-ray beam intensities in head CT relatively high?

A
  • reduce mottle
  • CTDIvol 60 mGy
  • can reduce with iterative reconstruction
  • if imaging airways and bony structures, don’t need such high intensity as noise is not as important
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5
Q

dose-length-product for routine head CT exam

A

1000 mGy- cm

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6
Q

typical tube voltage in abdo and pelvic CT

A

120 kV

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7
Q

typical CTDIvol for routine abdo or pelvic scan

A

15 mGy

20 mGy for detecting soft tissue lesions (ex liver)

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8
Q

dose-length product for abdominal or pelvic scan

A

300 mGy-cm

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9
Q

CTDIvol for chest

A

10 mGy

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10
Q

tube voltages for chest contrast studies

A

100 kV so you can see the iodinated material

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11
Q

dose-length product for typical chest CT

A

300 mGy-cm

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12
Q

head normal and failing CTDIvol

A

60 mGy

80 mGy

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13
Q

abdo normal and failing CTDIvol

A

15-20 mGy

30 mGy

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14
Q

ped abdo normal and failing CTDI vol

A

8 mGy
> 25 mGy
as measured in small phantom

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15
Q

what is used for cardiac imaging

A

> 64 slice multi0detector CT

-need good spatial and temporal resolution

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16
Q

kV for cardiac imaging

A

80 or 100 kV because of iodinated contrast

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17
Q

retrospective vs prospective gating

-cardiac imaging

A

prospective- only acquire images at certain parts of cardiac cycle

  • reduces patient dose
  • retrospective gating in cardiac imaging has pitch of 0.2-0.3
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18
Q

how many slices multi detector CT scanner can capture heart in single rotation

A

250-320 slice

-also dual-source CT operates at > 3 pitch and can catch whole cardiac cycle in single heart beat

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19
Q

sensitivity of cardiac CT

A

> 90%

negative result rules out coronary heart disease

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20
Q

diameter of a head size phantom

A

18 cm

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21
Q

diameter of adult abdomen phantom

A

28 cm

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22
Q

diameter of adult chest phantom

A

24 cm

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23
Q

what must be kept constant at CT detector?

A

Kair

-modify output to keep Kair constant

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24
Q

kV for non-contrast studies in infants

A

80 kV

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25
Q

kV for angio exams in kids

A

80 kV

100 kV if cannot penetrate

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26
Q

acceptable mottle in pediatric CT vs adult CT

A

pediatric is lower than adult

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27
Q

how is unecessary exposure limited in pediatric CT?

A

limit scan lenght

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28
Q

how is unecessary exposure limited in pediatric CT?

A

limit scan lenght

-single phase exam is sufficient for most clinical tasks

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29
Q

how are projection radiographs (localizer scans) obtained?

A
  • advance the patient couch through the gantry with the tube and detector in a fixed position using a fixed low mA
  • use same kV as in all subsequent scans
  • ensure patient is correctly centered
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30
Q

how is projection radiograph attenuation data used?

A

-used to select how the AEC will modulate the mA

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31
Q

how is projection radiograph attenuation data used?

A
  • used to select how the AEC will modulate the mA

- 2 projection radiographs will improve AEC peformance

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32
Q

when do we select scan length and type?

A

after projection radiography (localizer) scan

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33
Q

how much do the localizer scans increase patient dose by?

A

a few percent

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34
Q

what is dual source CT?

A
  • uses 2 x-ray tubes and 2 detector arrays
  • both angular systems are mounted on a rotating gantry with angular offset of 90 degrees
  • acquires twice as many CT projections
  • enables high pitch - can get faster scans for patients who can’t hold their breath or can’t stay still
  • image quality in central region where 2 FOVs overlap is superior to image quality in peripheral region
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35
Q

dual energy CT

A
  • need 2 x-ray tubes
  • uses 80 kV and 140 kV
  • can acqauire similar projections at 2 energies
  • improves delineation of different materials that have similar attenuation coefficients
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36
Q

applications of dual energy CT

A
  • subtraction of bone
  • assessment of pulmonary perfusion
  • kidney stone characterization
  • differentiating uric acid deposits from calcium in gout
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37
Q

how is dual energy CT used for liver scans?

A

enables iodine removal so you can generate unenhanced images
-DECT can be obtained without additional radiation dose, by acquiring high kV and low kV images with the summed dose sequal to the dose of single energy CT

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38
Q

describe CT fluoro

A

display of constantly updated images produced by continuous rotation of x-ray tube

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39
Q

rate of image update in CT fluoro

A

6 images/s

-fluoro and ultrasound update at 30 fps

40
Q

kV and mA for CT fluoro

A

120 kV

20-50 mA - helps to minimize radiation doses

41
Q

what makes up the dental cone beam CT

A

flat panel detectors combined with x-ray tube

42
Q

scan times for dental CBCT

A

20 s scan times

3 min to reconstruct an image

43
Q

tube voltages and currents for dental CBCT

A

90 kV

7 mA

44
Q

focal spot size dental CBCT

A

0.5 mm

45
Q

dental CBCT pixel size and spatial resolution

A

170 um pixels
uses small focal spot (0.5 mm)
spatial resoltuon 2.5 lp/mm

46
Q

patient doses in dental CBCT

A 
very low (< 0.1 mSv)
or low (0.1 - 1 mSv)
47
Q

what is major determinant of patient dose in dental CBCT?

A

irradiation geometry defined by the area of the 2D detector array
-doubling the detector array dimension will likely quadruple the patient dose as the exposed area is 4 times higher

48
Q

How do changes in CT contrast with kV change depending on properties of the tissue?

A

-changes are small for soft tissues (Z = 7.5) but large for high Z (iodine, Z= 53)

49
Q

definition of CT subject contrast

A

difference in HU of a lesion relative to background tissues

50
Q

contrast of blood vessel with dilute iodine (HU of 180) next to tissue (HU of 50)

A

contrast is 130 HU

diluted iodine attenuates 13 % more than soft tissue

51
Q

noise is +/- 3 HU- what are the pixel values

A

68 % have HU within +/- 3
95 % have HU within +/- 6
99% have HU within +/- 9

52
Q

what does CT mottle depend on?

A

number of x-ray photons used to make the image
-proportional to mA, x-ray tube rotation time, and reconstructed slice thickness

  • quadrupling mA will halve resultant mottle
  • quadrupling scan time or slice thickness also halves mottle
53
Q

how does reconstruction filter affect CT mottle?

A

filters with good resolution increase mottle

use of iterative reconstruction generally reduces the amount of mottle in CT images but also changes noise texture

54
Q

currently, what is shortest time that a standard CT imager can acquire 1,000 projections during a 360-degree x-ray tube rotation

A

0.3 s

55
Q

describe rebinning

A

after x-ray tube rotates 0-180 degrees, it is possible to synthesize projections pertainting to x-ray tube angles 180-360 degrees
-enales a CT image to be generated in half the x-ray tube rotation, but reduced spatial res due to reduced sampling and increased noise because fewer x-rays could create the image

56
Q

best achievable temporal resolution on CT scanner

A

half the tube rotation time
-150 ms

dual source CT scanners acquire image data at double the rate of single source scanners, offering better temporal resolution performance (best achievable is 75 ms)

57
Q

what determines spatial resolution in standard CT scanners

A
  • focal spot size and detector size determine in plane spatial resolution (~ 0.7 lp/mm)
  • detector thickness determines resolution in longitudinal direction

-inferior to digital x-ray radiogprahy (3 lp/mm) and mammo (7 lp/mm)

58
Q

what can help minimize motion blur?

A

reduce x-ray rotation time

59
Q

what reconstruction filters yield the best resolution?

A

detail (bone)

60
Q

what does zoom reconstruction do?

A

uses acquired projections to reconstruct a smaller FOV to improve resolution in central image region

61
Q

can smaller pixels improve CT resolution?

A

not if limits imposed by focal spot blur and detector blur have been reached

62
Q

what is partial volume artefact?

A

result from averaging the linear attenuation coefficient in a voxel that is heterogeneous in composition

63
Q

what is photon starvation

A

metal implants and long pathlengths through highly attenuating material can result in the CT detectors recording no transmission
-photon starvation yields star and streak artifacts (positive and negative) along rays through high density structures

64
Q

what are ways to reduce streak artifacts

A

scan patient with arms above their head

iterative reconstruction

65
Q

what is beam hardening artifact

A

low-energy photons are preferentially absorbed, resulting in more penetrating beam

  • when the beam hardens, the computed HU values are lower, showing darker areas behind strong absorbers like bone
  • software algorithms that incorporate prior knowledge of the patient can reduce beam hardening artifacts
66
Q

single ring artifact

A

happens when single detector is faulty

67
Q

multiple ring artifact

A

happens when CT scaner not properly calibrated

68
Q

aliasing artifact

A

inadequate data sampling

edges are imaged into central FOV

69
Q

cone beam artifacts

A

occur with large detectors due to the inadequate anatomical coverage related to undersampling in the cone beam direction

70
Q

determinants of organ dose in CT

A
  • amount of radiation used to perform CT exam
  • scan length
  • patient size
71
Q

why is thyroid dose higher than brain dose for a head scan?

A

smaller size of neck and absence of attenuation by the skull

72
Q

at fixed CTDIvol, organ doses to smaller vs larger patients

A

smaller patient = doses 20% higher

larger patient = doses 20 % smaller

73
Q

is organ dose to organs that receive scattered radiation only concerning?

A

no

74
Q

average risks from body CT scans (effective dose 5 mSv)

A

0.06% in 25 yo female and 0.04% in 25 yo male (remember 10%/Sv)

75
Q

principal organs that contribute to cancer induction from CT scans

A 
colon
stomach
lung
bladder
prostate
76
Q

types of cancer that make up cancer risk from CT

A
  • lung cancer
  • leukemia
  • breast cancer
77
Q

what are k-factors

A

conversion factors to convert dose-length-products to effective doses in CT
-depend on anatomy, patient age and size, independent of kV

  • also depend on phantoms in which CTDIvol are measured
  • when a large phantom replaces a small phantom, values of CTDIvol and DLP are halved and k-factors are doubled
78
Q

effective dose for head scan

A

2 mSv

79
Q

effective dose for chest scan

A

5 mSv

80
Q

effective dose for abdo or pelvis scan

A

5 mSv

81
Q

why is k-factor smaller for head vs chest?

A

CTDI is larger for head than chest, but chest has more radiosensitive organs and thus has higher effective dose

82
Q

what happens to k-factors as age is reduced for children?

A

increase as effective dose increases

-increases from 5X that of adults to 1.6X that of adults over 0 to 10 years old

83
Q

k-factor for adult heaed, chest, abdo/pelvis/whole body (uSv/mGy-cm)

A

2.4, 20, 15

84
Q

infant effective doses for head CT vs adults

A

-twice those of adults, despite using half the radiation

85
Q

infant effective doses for chest CT vs adults

A

-a third of that of adults despite using 10X less radiation

86
Q

when are embryo/fetal doses important?

A

when x-ray beam directly irradiates the fetus

87
Q

what are embryo doses proportional to?

A

CTDIvol

-must always account for scan length and patient size

88
Q

what is dose to embryo for multi-phase body CT scan?

A

sum of doses associated with each phase

89
Q

embryo dose to pregnant patient during chest CT

A
  1. 1 mGy

- primarily due to internal scatter- placing lead apron wouldn’t have much effect on embryo dose

90
Q

tissue HVL in CT

A

4 cm

91
Q

will increasing kV decrease mottle?

A

yes because more penetration

92
Q

how to decrease photon starvation?

A

increase kV

93
Q

for a CAP CT scan with CTDIvol of 10 mGy, how much dose do we expect the organs within to get?

A

15 mGy

94
Q

for a body scan with CTDIvol of 20 mGy, how much dose do directly irradiated organs get?

A

30 mGy

95
Q

eye lens dose, thyroid dose, brain dose for head scan with CTDIvol 60 mGy

A

55, 65, 50 mGy

Huda (20 decks)

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