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P2 PHYSICS > Computed Tomography: Main principles, CT Scanner Design > Flashcards

Computed Tomography: Main principles, CT Scanner Design Flashcards

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1
Q

How is the human body viewed in CT?

A

Built up of a finite number of discrete slices and volume elements (voxels)

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2
Q

In CT, what is the x/y plane?

A

Transverse plane

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3
Q

In CT, what is the z axis?

A

Orientated perpendicular to the scan and image plane

Approximately parallel to the body’s longitudinal axis

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4
Q

What is S?

A

Slice thickness

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5
Q

What has to be measured to calculate the attenuation value P along each ray from the X-ray source to detector?

A
  1. Intensity I of x-rays attenuated by object

2. Primary intensity Io of X-rays

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6
Q

What is the simplest case given in CT?

A

Measurement of a homogenous object with monochromatic radiation

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7
Q

What does the intensity of X-rays fall off exponentially with?

A

Absorber thickness d

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8
Q

What is the attenuation P defined as?

A

Natural logarithm of the radio of primary intensity Io to attenuated intensity I

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9
Q

For Inhomogenous object, what does the total attenuation resulting from each ray path interval depend on?

A

Local value of the attenuation coefficient μi

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10
Q

What does CT consist of?

A

Measuring many such line integrals exactly

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11
Q

How can a two-dimensional distribution of an object characteristic be determined?

A

Exactly if an infinite number of line integrals are given

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12
Q

What is sufficient to compute an image to a good approximation?

A

A finite number of measurements of the distribution of attenuation coefficient

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13
Q

What is used in real CT scanners?

A

Polychromatic X-ray radiation

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14
Q

What does Linear attenuation coeffucuent depend strongly on?

A

X-ray energy E

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15
Q

What are examples of dependence on energy may yield?

A
  1. Beam hardening effects that lead to image artefacts
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16
Q

How is an object measured in CT?

A

A sufficiently high number of attenuation integrals or projection values have to be recorded

Necessary to carry out measurements in all directions

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17
Q

What is the simplest measurement set up?

A
  1. A radiation source with adequate collimation emits a pencil beam
  2. The intensity, attenuated by the object, is registered by the detector placed outside
  3. For a given angular position, this set up of radiation source and detector is moved linearly (transition) and the intensity is measured
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18
Q

How is an attenuation profiled created which is termed as a projection?

A

Determining the ratios of primary intensity recorded in periphery and the attenuated intensities recorded behind the object and taking their logarihms

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19
Q

How are projections measured?

A

Successively for successive angular positions

20
Q

Where is the complete set of projections transferred to?

A

Data processing unit

21
Q

How is a CT image computed?

A

Back-projection is usually utilized

22
Q

How does back-projection work?

A

Each projection value is added to all the picture elements in the computer memory along the direction in which it was measured

23
Q

What does each detail in the object and that is represented in the attenuation profile contribute to?

A

Pixel value at the desired image point

Entire imge as well

24
Q

When will an unsharp image result from?

A

Considering only 2 or 3 projections

25
Q

How do you avoid the unsharpening of each projection?

A

Each projection has to be convoluted before back projection with a mathematical function, the convolution kernel

26
Q

What does the convolution kernel constitute?

A

A pointwise multiplication of the convolutional kernel and attenuation profile and addition of resulting values

27
Q

What does convolution represent?

A

A high-pass filtering procedure which generates overshoots and undershoots at object boundaries

28
Q

What does the convolution additionally offer the possibility to do?

A

Influence image characteristics by the choice and design of the convolution kernel
- from soft or smoothing to sharp or edge enhancing

29
Q

What does CT measure and compute?

A

spatial distribution of linear attenuation coefficient

30
Q

How is the computed attenuation coeficient displayed as?

A

CT number relative to the attenution of water

31
Q

What are CT numbers measured in?

A

Hounsfield units (HU)

32
Q

What is windowing?

A

Values above the chosen window will be displayed as white

Values below the window as black

33
Q

When is a narrow window chosen?

A

Display of very small attenuation differences as given in the brain

34
Q

What is an increase in CT number assigned to?

A

Increased density and/or an increase in effective atomic number

35
Q

What does Dual-energy CT make use of?

A

the energy dependence in μ due to the materials’ atomic numbers

two scans with different spectra are carried out and the attenuation values and the differences in attenuation for the two spectra have to be evaluated

36
Q

What is the aim of Dual energy CT?

A

Generate material-selective images and to determine material density as accurately as possible

37
Q

What do modern CT scanners mainly use?

A

Fan-shaped X-ray beam

38
Q

What are X-ray tubes capable of producing?

A

Tube currents up to 800mA and with significantly improved cooling systems

39
Q

Why is having an improved cooling system important?

A

Long continuous exposures can be made on a regular basis during the day

40
Q

Gantry tilt

A

The gantry has a capability to tilt, most CT scanners allow a tilt of the scan plane up to ±30% with respect to the axis of rotation. This allows the selection of scan planes directly through the structures of interest which is important for critical regions, such as the base of the skull or the lumbar spine.

41
Q

What do real X-ray tubes have?

A

Extended focus sizes

for example 0.8mm x 1.2mm

42
Q

What does Collimators in front of the detector serve?

A

to minimize signal contributions from scattered radiation

43
Q

What does the bow-tie filters reduce?

A

the demands on the dynamic range of the detector system and scattered radiation intensities arising from peripheral object zones

44
Q

What does the detector transform?

A

Incident X-ray intensity into a corresponding electrical sinal

45
Q

What can double the number of simultaneously measurable slices?

A

Flying focal spot

P2 PHYSICS (16 decks)

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