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CT & MRI > CT Principle 2 > Flashcards

CT Principle 2 Flashcards

1
Q

factors involved in data acquisition

A
  • pitch
  • kVp
  • mAs
  • time
  • beam width
  • scan type
  • SFOV
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2
Q

factors involved in image reconstruction

A
  • slice thickness
  • FOV
  • matrix
  • filter
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3
Q

factors involved in display control

A
  • window width
  • window length
  • zoom
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4
Q

SFOV

A

scan field of view

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5
Q

SFOV is the area where ___

A

raw data is collected within gantry

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6
Q

DFOV is the area which ___

A

determines how much of SFOV is reconstructed into an image

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7
Q

How does manipulating the pitch in CT scanning affect the quality of the images and the radiation
dose to the patient?

A

Lower pitch results in better image quality and higher radiation dose; Higher pitch results in lower image quality and lower radiation dose

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8
Q

Which of the following scenarios would benefit from using a pitch value of less than 1? Select all that apply

a) Rapid whole-body scans.
b) Low-dose lung cancer screening.
c) High-detail imaging requirements.
d) Imaging small structures like the internal auditory canal

A

c & d

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9
Q

What are the possible reasons for adjusting the display field of view (DFOV) post-acquisition? Select all that apply.

a) To exclude unnecessary areas from the image.
b) To focus on a specific anatomical region.
c) To reduce the radiation dose.
d) To include more anatomical information in the image.

A

a & b

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10
Q

In which clinical situations would a high pitch value be preferred? Select all that apply

a) Imaging of small detailed structures.
b) Low-dose lung cancer screening.
c) CT angiography.
d) Imaging of the internal auditory canal

A

b & c

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11
Q

What is the significance of scan field of view (SFOV) in CT imaging? Select all that apply

a) It determines the total area from which data is acquired..
b) It limits the maximum possible display field of view (DFOV).
c) It can be adjusted after the scan to display a different area.
d) It affects the initial data acquisition during scanning.

A

a, b & d

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12
Q

kVp used in CT

A

constant at 120 / 130 kVp

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13
Q

what does a lower kVp result in

A
  • more contrast by exploiting k-edge
  • more noise (countered by higher mAs)
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14
Q

CT dose is proportional ____ of kVp

A

to square

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15
Q

kVp

A

The peak voltage applied to the X-ray tube, influencing the maximum energy of the X-ray photons produced.

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16
Q

keV

A

The energy of individual X-ray photons

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17
Q

average keV

A

The mean energy of the X-ray photons in the beam, impacting overall image contrast and quality

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18
Q

max keV

A

The highest energy level of X-ray photons, determined by the kVp setting, indicating the most penetrating photons

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19
Q

iodine attenuation ___ at ___ kVp due to decreased compton scatter & greater photoelectric effect as photon energy approaches k-edge of iodine

A

increases; decreased

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20
Q

What is the primary role of kVp in a CT scan?

A

To measure the beam intensity or ability to penetrate anatomy

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21
Q

what is the r/s between slice thickness and noise & contrast & spatial resolution

A

thicker slices = less noise & better contrast & spatial res for soft tissues

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22
Q

what happens during image reconstruction

A

3D raw / helical data converted to 2D image data

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23
Q

what happens before reconstruction

A

interpolation; raw data is interpolated into series of 2D images before FBP

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24
Q

how is interpolation achieved

A

weighted average of helical data from either side of reconstruction plane

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25
Q

what is simple back projection

A

emulates acquisition process in reverse where each view represents an individual measurement of attenuation factor

26
Q

assumptions of SBP

A
  • infinitely small FSS
  • monochromatic spectrum
  • small DELs
27
Q

what are sinograms

A

data acquired for 1 CT slice displayed before reconstruction; not for clinical use

28
Q

horizontal & vertical axis of SBP

A

horizontal = diff rays in each projection
vertical = projection angle

29
Q

what happens when a vertical line on sinogram is seen

A

bad detector

30
Q

main issue of SBP

A

lots of artifacts / blurring since assumptions are not met

31
Q

how are the main issues of SBP countered

A

Filtered back projection (FBP)

32
Q

what do the diff filters offer

A

tradeoffs between spatial resolution & noise

33
Q

what can FBP be affected by

A
  • high image noise
  • artifacts
  • poor low contrast
34
Q

IR

A

iterative reconstructions

35
Q

how does IR work

A
  1. uses image assumption based on vendor
  2. compares it to real time measured values while making constant adjustments
  3. adjust till both assumption & measured value are in agreement
36
Q

advantages of IR

A
  1. reduced noise
  2. improved CNR & spatial resolution
37
Q

disadvantages of IR

A
  1. time consuming due to slow processing
  2. less predictive behavior
38
Q

Dose modulation in CT scans adjusts tube current to ___

A

maintain image quality while minimizing patient radiation dose.

39
Q

which CT dose index measures absorbed dose

A

volume CT dose index (CTDIvol)
Dose-Length Product (DLP)

40
Q

DLP formula

A

CTDIvol x CT length of scan range

41
Q

Effective Dose formula

A

DLP x conversion factor

42
Q

absorbed dose unit

43
Q

effective dose unit

44
Q

CTDI weighted formula

A

1/3 CTDI center + 2/3 CTDI peripheral

45
Q

CTDIvol formula

A

CTDI weighted / pitch

46
Q

effective dose is used for

A

risk estimations

47
Q

what does DLP represent

A

total radiation energy deposited in the patient’s body

48
Q

CT dose reduction strategies

A

DIMAR

  1. automatic tube current modulation
  2. reduce kVp
  3. DRLs
  4. minimize scan range
  5. iterative reconstruction
49
Q

what are DRLs

A

dose estimates to monitor practice to promote improvements in patient protection

50
Q

DRLs highlight dose variations between ___

A

healthcare facilities for same exam and similar patient group

51
Q

DRLs are ____

A

median values observed for representative samples of patients of a particular group

52
Q

what does a good CT image possess

A
  1. high spatial res
  2. high low contrast detail res
  3. acceptable temporal res
  4. minimal noise
  5. no artifacts
53
Q

DFOV formula

A

pixel size x matrix size

54
Q

what does detector array thickness affect

A

axial res when reconstructed into sagittal/coronal views; impt for 3D/MPR reconstruction

55
Q

how is spatial resolution measured

A
  1. axial resolution (matrix size, DFOV, pixel size)
  2. resolution bar patterns
  3. lp/mm or lp/cm
56
Q

how does thinner slice thickness affected spatial resolution

A

(+) better spatial res and less partial volume effect

(-) lower contrast res and more noise

57
Q

what happens when the slice thickness becomes too thick

A

partial volume effect; object does not fill entire depth of scan plane leading to underestimated CT numbers

58
Q

how to improve contrast in CT images

A
  1. increase dose, mAs, slice thickness
  2. reduce tube voltage
  3. reconstruction algorithms
59
Q

how does mAs & slice thickness improve contrast

A

by reducing noise

60
Q

CT noise sources

A
  1. reconstruction kernels esp if used to preserve high frequency contents
  2. electronic noise
  3. quantum noise
61
Q

what affects quantum noise

A

scanning technique, detector efficiency, patient habitus

CT & MRI (12 decks)

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