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CT pgc > CT Physics > Flashcards

CT Physics Flashcards

Bruce

1
Q

Name three important factors modern scanner have to balance between 3

A
  1. lower dose
  2. lower noise
  3. better resolution
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2
Q

Name 6 components of image productionr?

A

High voltage

generator

DAS data acquisition system computer - detector

data processing/reconstruction storage and recording - pacs

DAC digital analogue converter

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3
Q

Draw a CT scanning tube 6

A
  1. anode
  2. cathode
  3. high voltage
  4. rotor
  5. movable collimator
  6. thin window
  7. vacume tube
  8. tube housing
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4
Q

Explain how xrays are produced from a ct scanner? 5

A
  1. Production of free electrons - the large voltage delivers energy to the filment. The atoms begin to vibrate where energy is dissipated as heat and light. Energy is also lost as an outer electron gains enough to overcome its binding energy and is emmited from the atom and the filaments surface. This is thermonic emission.
  2. filament becomes momentarily positively charged allowing for reattraction of an electron hence causing a space cloud
  3. This happens in a vacume tube to ease movement of the flow of electrons from the cathode to anode and to prevent interaction with air particles.
  4. The electrons accelerate to the positive tungsten anode. This is aided with a focusing cup to narrow down the focal point. Tungsten has a high atomic number of 74 which creates the positive attractive force. 5 % of the electrons will pass near the atomic nucleus, which will contribute for the majority of xrays produced.
  5. The electrons slow down (Bremmstrahlung) as they also change direction from appraoching close to the tungsten nucleus (strong attractive forces) which causes energy to be emitted as an xray photon.
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5
Q

Discuss characteristic xrays that interact with the anode from the scanning tube 3

A
  1. involves a small minority of electrons which bombard the target.
  2. Some interactions with the anode are caused by cathode electrons that colide with an energy greater then the BE of a tungsten electron (69.5kev).
  3. This electron is ejected from the atom leaving a unbalanced positive charge in the shell it came from. Other electrons dissipate x-rays photon energy in order to fill in and balance the positive charge. When a valance electron is filled with an outer electron this is called an Auger electron.
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6
Q

Explain how the ct tube are desined to deal with excess energy? 3

A
  1. The anode rotates to distribute the heat load placed from the cathode
  2. the high rotation speed improves focal spot stability
  3. anode is oil cooled
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7
Q

provide a comming matrix size used 1

A
  1. 512 X 512- matrix resolution
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8
Q

What is the typical generator output and watage 2

A
  1. 50-250 kvp
  2. 100 kw
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9
Q

Why is beam filtration important? 3

A
  1. reduces beam hardening artefacts
  2. filtration absorbes low energy xray photons hence improves beam quality (average photon energy) which would otherwise contribute to patient dose
  3. uses typically 2.5mm of aluminium
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10
Q

explain how beam shaping plays a vital role in a modern scanner? explain the benefits 4

A
  1. Longer path through centre of patient
    greater attenuation, more beam
    hardening
  2. a bow tie filter is used see illustration
  3. this provides a uniform dose across the patient. hardening the beam means a more uniform signal at the detectors
  4. Evens out the noise distribution
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11
Q

Explain why an xray beam is polychromatic?

3

A
  1. Polychromatic means a typical xray beam will be composed of a spectrum of energy photons. This can be down to how the anode electrons interact with the target anode.
  2. The photoelectric effect by which bremsstrahlung produces the majority of the xray photons and
  3. characteristic xrays produced from electrons coliding with tungsten electrons where an imbalance with the charge of the atom causes other electrons to give off some their energies to replace the one lost including auger electrons from the outer shell.
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12
Q

Explain the importance of pre patient collimation? 3

A

Lowers the patient dose restricts the xray beam to anatomy of interest

by collimating the beam along the z axis

helps to define slice thickness

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13
Q

Draw a graph to explain the effect of beam hardening? 2

A
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14
Q

How much al is typically used in filtration? 1

A

2.5mm

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15
Q

What are the atomic numbers for the following: 3

al

pb

cu

A

al = 13

pb = 82

cu = 29

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16
Q

Explain what a focal spot is and why a flying focal spot would be used? 2

A
  1. the position of the focal spot is rapidly altered in the transaxial plane and/or the Z-axis.
  2. Each focal spot position increases the number of projections sampled and improves spatial resolution / allows for semi overlapping data to aid In accuracy of imaging
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17
Q

What defines the focal spot? 2

A
  1. dimensions of the filament tungsten coil
  2. construction of the focussing cup
18
Q

What is the advantage of a small focal spot?

and what are typical focal spot sized 2

A

Sharper image

0.5 to 1.2 mm

19
Q

Define the job of a detector?

3

A
  1. Capture
  2. absorb
  3. convert xray photon into an electrical signa
  4. or
  5. –
    Quantitative recording of incident ionising radiation
20
Q

List two types of detectors?

A
  1. Ionisation detectors (gas)
  2. Scintillation detectors (solid)
21
Q

List the characteristics of CT detectors. 4

A

•
Efficiency
•
Response time
•
Stability/Uniformity
•
Dynamic range
•
Cost & size

22
Q

Define the term geometric efficientcy? 2

A

GE = Area under dose profile within total imaged slice width

Total area under dose profile

or

Active detector area

Irradiated area

23
Q

What two factors could influence GE of detectors?

2

A
  1. Avoid gaps between detector elements
  2. Reduce irradiation outside active detector area
24
Q

Define quantum efficiency 2

formula

A

This is the absorption efficiency.

Hence the number the photons absorbed by the detector

25
Q

Define the typical QE of the different types of CT detectors 2

A
  1. scintillating = diagnostic energy range is ~100%
  2. QE of gas detectors is ~60%
26
Q

Describe how a gas detector works 3

Explain the advantage and disadvantages of gas detectors.

picture illustration too

5

A

Gas detectors contains pressurised large atomic xenon gas, which is ionised with the xray photon. The resulting in the formation of iones which are collected by the protruding electrodes as an electrical signal.

Disadvantages

  1. poor QE 60%
  2. pressurised Xenon gas
  3. difficult to manufacture in a multirow format

Advantages

  1. fast response
  2. high precision
  3. no afterglow
  4. simple construction
27
Q

Describe how a Scintillating detector works 3

A

X-rays are converted into light photons then an recorded as an analogue signal.

A photodiode or photomultiplier could be used to pick up the photons presence.

Usually, the scintillating (Crystal or ceramic) material would vary from such cesium iodide or cadmium tungstate, which had a long afterglow to newer detectors, which are gadolinium based and tend to have a quicker decay.

28
Q
A
29
Q

Define response time 1

A

How quick a detector can discard the evidence of a previous xray in readines for the next event.

30
Q

fill in the blanks

A
31
Q

Define the job os a DAS 5

A
  1. measures data from the detectors which firstly amplified
  2. the log amplifier converts transmission data into attenuation and thickness
  3. encondes into digital with the Analogue digital converter
  4. data typically in 16 bit formate meaning data can be stores with 0 to 65000 different binary values
  5. the data is then transmitted to a computer via optic technology.
32
Q

Name two interactions which are mainly detected? and why? 3

A
  1. Photoelectric effect
  2. Compton scatter
  3. remember elastic scattering is composed of low energy photons possibly from characteristic radiation which is hardened by the al filter.
33
Q

How many projections do there tend to be in one rotation? and what effect does this have on the image 2

A

1000

improved spatial resolution

34
Q

In CT how many sampes are there in one projection? 1

A

This is the same as the number of detectors

35
Q
A
36
Q

define complementary projection? 1

A

we assume that attenuation measurements in the opposite direction are equal. hence only a minimum of 180 degrees of data is required

37
Q

Explain how the first generation scanner worked? 3

A
  1. Pencil beam with single tube and two rows of one detector each
  2. worked through translate and rotate along the x and y axis, which took about 4.5 mins per slice at 160 samples.
  3. The tube would then rotate 1 degree and repeat the process.
38
Q

Describe a secon generation ct scanner

A

Change from pencil beam to fan beam at 10 degrees with multiple detectors.

Still had to translate along the x-y axis and 20 secs per slice

39
Q

Describe how a 3rd generation scanner works? 4

A
  1. more detectos with a wider fan beam
  2. no translation was required
  3. would take less than 1 sec per rotation
  4. septa of the detector elements are aligned to the xray focal spot to prevent misregistration of scattered radiation hence a form of intrinsic collimation
40
Q

describe a 4th gen ct scanner 3

A
  1. Fan beam
  2. 360 ring of detectors
  3. avoids ring artefact
  4. poor scatter descrimination and expensive to produce
41
Q

State the advantage of a third generation ct system 4

A
  1. no translation required and faster data acquisition compared to 1st and second gen
  2. rotating detectos reduce the number of detectors. Hence reduce manufacturing costs.
  3. Detector collimation reduces scatter
  4. Adaptable to multirow of detectors

CT pgc (8 decks)

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