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Applied Medical Imaging > Fundamentals of CT > Flashcards

Fundamentals of CT Flashcards

1
Q

What are the basic principles of a CT scanner?

A

An x-ray tube produces a beam which passes through a patient. Underneath the patient is the detectors arranged in a circle and move around the patient. Irradiating the patient from a large variety of angles.

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2
Q

What is under the hood of a CT scanner? (no matter how old)

A
  1. The x-ray tube
  2. X-ray detectors
  3. Cooling system
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3
Q

What are the essential imaging components of the X-ray Tube?

A
  • Small focal spot
  • High tube current
  • Anode (high heat capacity)
  • Tube needs sophisticated cooling system
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4
Q

Why is a small focal important for good images?

A

Smaller focal spot means that all x-rays appear to have come from the same point source.
A large focal spot means lots of x-rays come from slightly different angles creating a blurry image.

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5
Q

What are the essential imaging components of the collimation?

A
  • Pre collimation determines the slice thickness

- Post collimation limits the amount of scatter reaching the detectors which would add ‘Noise’ to the image

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6
Q

What is an essential component of filtration?

A

As there is higher intensity at the edges of the fan beam compared to the centre. A bow-tie shaped filter is used. (bow tie smaller in the centre and thicker on the edges). This equalises the intensity across the fan beam creating a clearer image.

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7
Q

How does a gas detector (xenon) work?

A

Radiation ionises the xenon molecules and the electron produced are attracted towards a conductor to produce an electrical signal

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8
Q

How does a solid state detector (based on scintillation ) work?

A

Crystals absorb radiation and excess energy emitted as light.

Connected to a multiplier tube creating an electrical signal proportional to the amount of light which is in turn proportional to the amount of radiation.

(Lots of x-rays through crystals = Lots of light coming out)

The scintillator is backed by a photo-diode which changes the light to an electrical signal

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9
Q

Advantage of using a solid state detector compared to gas?

A
  • Very fast (not depended on build up of ionisation)
  • Extremely stable
  • very small (just need a crystal and a photo diode)
  • Smaller detectors = better image quality
  • Lots of fans and detectors able to take multiple images
  • Faster image= less patient movement
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10
Q

What does the CT 1st generation involve?

A

Single detector opposite the source exposed then rotated fractionally round to acquire data again

  • Several minutes per slice
  • require the patient to stay still for a long period of time
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11
Q

What does the CT 2nd involve?

A
  • Multiple detectors (up to 30)
  • Several projections per slice
  • 20 seconds per slice
  • Gantry would take an image, rotate 30 degrees.
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12
Q

What does the CT 3rd generation involve?

A
  • Large bank of detectors (usually 1000)
  • Continuously rotating fan beam?
  • 1 second per slice
  • Bowtie filter controls excessive variations in signal strength
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13
Q

What does the bowtie filter do?

A
  • Attenuate peripheral aspect of fan beam
  • overcome effects of beam hardening
  • minimise skin dose
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14
Q

What does the CT 4th generation involve?

A
  • Ring of fixed detectors surrounds the patient
  • X-ray tube rotates inside detector ring through 360 degrees with a wide fan beam

-Limited by only being able to do 2 rotations before going in reverse to unwind the power cables

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15
Q

How was the power cable limitation of the 4th generation fixed?

A

Instead of power cables supplying power to the x -ray tube a slip ring is now used.

  • Allows for continuous rotation
  • Moves patient in Z direction on couch allowing for rapid gathering of data
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16
Q

How is image acquisition affected by the linear attenuation coefficient (u)?

A

As the x-ray beam passes through the patient, the change in output is completely dependent on the change in intensity.
(u) represents the extent to which the x-ray intensity is reduced by a material.

Bone (high LAC) = big reduction in intensity
Air (Low LAC)= Minimal Reduction of intensity

Each (u) is sampled at each detector to generate a projection.

17
Q

How does Spiral CT work?

A
  • Gantry rotates continuously in one direction on the slip ring
  • At the same time the patient table moves at the same speed through the gantry
  • faster CT means less chance of patient movement
18
Q

What is spiral pitch?

A

The ratio of the patients movement through the gantry during one rotation relative to the beam collimation (slice thickness)

19
Q

What happens if we have large pitch?

A

Pitch greater than 1mm creates interspaces and some portion of the body may be missed.

20
Q

What does the ‘Increment’ refer to?

A

-The distance between the slices

21
Q

How is image reconstruction achieved?

A

Through filtered back projection and adding information together.
A CT number is then assigned

22
Q

What is filtered back projection?

A

As back projection produces blurred axial images. Filtering is used to improve the image quality.

23
Q

What are CT numbers and how are they computed?

A

They are the pixel values assigned in the image.

-Computed by calculating the relative difference between the LAC of tissue and water

24
Q

What is the CT number for water air and bone?

A 
W= 0
A= -1000
B= 1000
25
Q

How do artefacts affect image quality?

A

-Degrade image quality by causing streaks, shading, rings and distortion.

26
Q

What can cause streak artefacts?

A

Patient motion, organ motion, high density material, noise, mechanical failure

Rings on the image can be produced by 3rd generation scanners.

27
Q

What is partial volume effect?

A

Assume a voxel comprises of a number of different substances. The calculated CT number represents an average of their properties.

Therefore material in any one voxel can affect values of surrounding voxels.

28
Q

What does the partial volume effect result in?

A
  • Difficult to differential between tissue boundaries

- slice thickness and spiral pitch affect PVE

29
Q

What are the limitations of CT imaging?

A
  • 2D representation of a mobile 3D object
  • Only a small snapshot of the overall picture
  • Resolution is limited due to finite slice thickness and scatter processes
  • Discrimination between tissue types is difficult

Applied Medical Imaging (3 decks)

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