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MRAD 251 - Computed Tomography > Week 4 Lecture 2 > Flashcards

Week 4 Lecture 2 Flashcards

1
Q

scatter is absorbed by?

A
  • collimators and/or detectors
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2
Q

scatter detection is affected by?

A
  • placement of detectors
  • shape of the detectors
  • size of the detector aperture
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3
Q

do spacing bars play a role in distance measurements?

A

yes. they contribute to distance measurements

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4
Q

*what size of detector aperture absorbs more scatter?
short, wide? long, narrow?

A

short and wide

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5
Q

↑ aperture size = (↓/↑) spatial resolution

A

↑ aperture size = ↓ spatial resolution
*due to the decrease in volume averaging of the pixels

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6
Q

what are the four characteristics that determine detector quality?

A
  1. quantum detector efficiency (QDE)
  2. high stability
  3. fast response time
  4. wide dynamic range
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7
Q

what determines a detector’s stability?

A

calibration requirements

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8
Q

QDE

A

capture, absorption and conversion of radiation energy (into electrical energy)

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9
Q

what describes stability?

A

consistency in response to radiation

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10
Q

what is afterglow?

A

it creates lag time which is undesirable for a detector’s response time

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11
Q

wide dynamic range

A

detector’s ability to measure different signals

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12
Q

what type of detectors are found in conventional scanners?

A

*ionization (xenon gas) detectors

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13
Q

xenon gas detectors are found in SDCT, as well as MDCT. t/f

A

false - it is not used in MDCT

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14
Q

what are the 3 advantages to ionization detectors?

A
  1. lower cost
  2. high stability
  3. response time (no lag/afterglow)
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15
Q

what are the disadvantages to xenon gas detectors?

A
  1. requires constant pressurization
  2. ↓ QDE (~50-87% QDE)
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16
Q

why is QDE only 50-87% in xenon gas detectors?

A

due to the aluminum casing and spacing between the plates

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17
Q

what type of detectors are used in modern systems?

A

solid state/scintillation detectors

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18
Q

what is the flow of energy conversion in a scintillation detector?

A

x-ray energy to light energy to electrical energy

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19
Q

what is used in scintillation detectors to convert the light into electrical energy?

A

photocathodes or photodiodes

20
Q

what are the advantages to solid state detectors?

A

increases absorption efficiencies (94-100% QDE)

21
Q

what are the disadvantages to the scintillation detectors?

A
  • can exhibit afterglow
  • temperature and moisture sensitive
  • requires spectral matching
22
Q

what are the characteristics of a scintillation detector?

A
  1. high light output
  2. high x-ray stopping power
  3. good spectral match
  4. fast response time
23
Q

what is an amplifier needed for in a solid state detector?

A

to ensure that the x-ray energy captured will be equal to the light energy produced

24
Q

what is spectral matching?

A

when energy conversion results in the proper colour of light emitted

25
Q

detector arrangements play a role in determining how many image slices can be recorded with a single gantry rotation. t/f

A

true

26
Q

SDCT = ____ beam
MDCT = _____ beam

A

SDCT - fan beam
MDCT - cone beam

27
Q

detectors in a SDCT vs. MDCT.

A

SDCT: side by side and in a single row
MDCT: an array forming rows and columns

28
Q

z axis in SDCT vs. MDCT

A

SDCT: wide z-axis
MDCY: narrow z-axis

29
Q

what is the largest allowable slice thickness in a SDCT?

A

LESS THAN the full detector’s width

30
Q

***MDCT:
collimation + # of detectors correlates to?

A

slice thickness + # of slices

31
Q

what are the two main configurations of the MDCT?

A
  1. matrix array
  2. adaptive array
32
Q

matrix array aka?

A

fixed or uniform array

33
Q

adaptive array aka?

A

non-uniform or hybrid array

34
Q

matrix array

A

goal is to improve spatial resolution

35
Q

matrix array’s voxels?

A

isotropic resulting in symmetry in slice thickness

36
Q

adaptive array’s voxels

A

anisotropic resulting in slice thickness variation
(thinner in the center and wider at the periphery)

37
Q

***image slice thickness can’t be smaller than the slice thickness selected during acquisition

A
38
Q

what are the advantages to MDCT?

A
  • more slices per 360 degree rotation which reduces the scan time, which can help with imaging motion
  • can combine detector elements
39
Q

*retrospective slice thickness

A

*MDCT obtained slices can have their slice thickness manipulated after acquisition

40
Q

are thicker or thinner slices better to improve the appearance of 3d reformatted images?

A

thinner slices

41
Q

*acquisition slice thickness must be selected before scan acquisition

A
42
Q

what are some considerations to selecting slice thickness?

A
  • size of scanned area
  • tube heat limits
  • reformatting capabilities
  • what are you looking for in an image? (spatial resolution)
43
Q

image slice thickness (=/≠/≥/≤) scan slice thickness

A

image slice thickness ≥ scan slice thickness

44
Q

*acquisition slices (can/not) be combined to create larger image slices

A

they CAN be combined
acq. slices combined to = larger image slices

45
Q

retrospective slice incrementation

A

ability to change the starting point of an image slice

46
Q

retrospective slice incrementation applies to both axial and helical scans. t/f

A

false.
helical scans only

MRAD 251 - Computed Tomography (13 decks)

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