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Applied Medical Imaging > Fundamentals of CT > Flashcards

Fundamentals of CT Flashcards

1
Q

What are the main principles of CT?

A

X-ray tube
Patient
Fan of x-rays
X-ray detectors

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2
Q

What are the effects of a large focus spot?

A

Blurring

Geometric unsharpness

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3
Q

What focal spot should an X-ray have?

A

Small focal spot-high load/heat

Good cooling system

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4
Q

What does pre collimation determine?

A

Slice thickness

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5
Q

What does post collimation limit?

A

Amount of scatter “noise” reaching the detector.

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6
Q

What does filtration do?

A

Filters excess intensity at edges of projection

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7
Q

What are the essential components of CT?

A

X-ray tube
Collimation
Filtration

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8
Q

What do gas detectors do?

A

Ionise Xenon and electrons are attracted to the conductor producing an electrical signal.

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9
Q

What do solid state detectors do?

A

Crystals absorb radiation

Connected to a photomultiplier tube

Light is proportional to radiation

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10
Q

What are the advantages of SSD?

A

Fast
Stable
Small

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11
Q

How many detectors did a 1st Gen CT have?

A

Single

rotated round to fixed angles to acquire data

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12
Q

How many detectors dis the 2nd gen have?

A

Multiple (up to 30)

several projections at once

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13
Q

How many detectors dis the 3rd gen have?

A

Large bank (100S)
Continously rotating fan
Bowtie filter

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14
Q

What did the bowtie filter control?

A

Excessive variations in signal strength.

reduce skin dose/beam hardening

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15
Q

How many detectors dis the 4th gen have?

A

Ring of fixed detectors
X-ray tube rotates 360 degrees

Restrained to two rotations due to power cords

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16
Q

What does spiral/helical scanning allow?

A

Allows continuous rotation

Move patient in z direction allows large quantities of data.

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17
Q

What do multi-slice scanners allow

A

3rd or 4th gen
Multiple banks of detectors
Several slices of acquisiton per gantry rotation

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18
Q

What is the process of image acquisition?

A

Measure average LAC between X-ray and detector (generates a projection)

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19
Q

What is a projection?

A

Tube and detector are rotated a small amount and LAC is calculated (Voxel)
650-900 detectors
1-2000 projections

20
Q

What is spiral CT?

A

Gantry continuously rotating in 1 direction on slip ring

21
Q

What is the benefit of faster CT rotations?

A

Less patient motion artefacts.

22
Q

What is spiral pitch?

A

Ratio of patient movement through the gantry relative to beam collimation (slice thickness)

23
Q

Spiral pitch (equation)

A

Table movement per rotation/collimation

or slice thickness/collimation

24
Q

What is an increment?

A

Distance between slices

25
Q

What is the process of reconstructing images?

A

filtered back projection and adding information

26
Q

What is back projection?

A

overlaying multiple images over each other at different angles to improve quality

27
Q

What are some common CT artefacts?

A

Streaks
Shading
Rings
Distortion

28
Q

What causes streak artefacts?

A

-Patient motion
-High density materials
Noise
-Mechanical failure

29
Q

What causes ring artefacts?

A

Bad detector channels

30
Q

What is the partial volume effect?

A

Averaging a variety of CT densities causing blurring.

31
Q

What affect PVE?

A

Slice thickness

and spiral pitch

32
Q

What are some CT imaging Limitations?

A

2D representation of a 3D mobile object

Resolution limited to finite slice thickness and scatter processes

33
Q

What are the CT numbers for heart, brain, blood, liver, muscle and spleen?

A 
Heart 10-60
brain 20-40
blood 20
liver 20-80
Muscle 50
Spleen 40-60
CSF 15
white matter 45
Grey matter 40
34
Q

What are the CT numbers for Bone and metal?

A

Bone 150- 500
Bone 350-1000 dense
Metal >2000

35
Q

What is the CT number for water?

36
Q

What is the CT number for fat lung and Air?

A

fat -100
lung -200
Air -1000

37
Q

CT apertures for diagnostic and therapy?

A

70-90cm diagnostic

90-105cm therapy

38
Q

What is a scout image

A

Standard area projection image with relevant anatomical structures and localised relative to the couch

39
Q

What are the advantages of a scout?

A

Provides reference for slice data acquisition and display
Initial planning is performed on the scout
Scanning of desired anatomical section

40
Q

Production of scout?

A

Position patient
Set limits
Zero slice

41
Q

What are some characteristics of a scout image?

A

Tube and detectors are fixed or stationary
Couch moves at low speed
Low dose exam
Low spatial and contrast resolution

42
Q

What are two common image reconstruction algorithms?

A

Filtered back projection

Interpolation

43
Q

What are two types of image reconstruction?

A

Prospective and retrospective reconstruction

44
Q

Is lower pitch better for resolution in helical scanning?

45
Q

What are some types of image manipulation?

A

Multi-planar reformation
3D reconstruction
Volume rendered 3D
3D reconstruction

46
Q

What does PET provide?

A

Functional and metabolic information

Applied Medical Imaging (10 decks)

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