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SPE101: Ionising Radiation > DR: Instrumentation > Flashcards

DR: Instrumentation Flashcards

1
Q

What is the purpose of filtration of an X-Ray beam?

A

Removes lower keV photons that would only contribute to skin dose.

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2
Q

What photon-tissue interaction process is responsible for contrast in an image?

A

Photoelectric effect.

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3
Q

How does the probability of a photoelectric effect interaction vary with energy?

A

1/(E^3)

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4
Q

How does the probability of a photoelectric effect interaction vary with atomic number?

A

Z^3.

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5
Q

What is the equation to define contrast in DR?

A

C=(I1-I2)/I1 = 1-EXP[x(u1-u2)]
u1 is the attenuation coefficient for medium 1.
x is the thickness of the medium.

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6
Q

What does radiographic contrast depend on?

A

The thickness of the object.

The difference in attenuation coefficients.

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7
Q

How does radiation contrast change with energy?

A

Decreases with increasing energy

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8
Q

Why aren’t all DR exams done at lower energies to maximise contrast

A

It increases dose to the patient.

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9
Q

How does Compton scatter affect an image?

A

Compton scatter deflects photons, which reduces contrast if they are captured by the detector.

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10
Q

How does the probability of Compton scatter change with increasing atomic number?

A

It is independent of atomic number.

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11
Q

Where is Compton scatter dominant?

A

Higher energies and thicker sections.

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12
Q

Apart from a reduction in contrast, what other problems does Compton scatter cause?

A

Staff dose

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13
Q

How can scattered photons be removed from an image?

A

Use of an anti-scatter grid - parallel lead holes that only allow perpendicular photons to the image receptor.

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14
Q

How is noise measured on a DR image?

A

Noise is governed by random processes, so is described by Poisson statistics.
Standard deviation is used as a measure of noise.

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15
Q

How does the signal to noise ration (SNR) vary with an increase in the number of photons (N)?

A

Signal Increase with N
Noise increases with N^0.5
SNR = S/N = N/(N^0.5) = N^0.5

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16
Q

What are the three types of noise and where at what range are they significant?

A 
Quantum Noise (proportional to N^0.5) - dominates over the clinical range.
Electronic Noise - Fluctuations in electronics of the system - Can be significant at low doses, assumed to be constant.
Fixed pattern noise - Pixel to pixel variation - Should be low in DR due to flat fielding, like signal so proportional to N.
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17
Q

What can affect the spatial resolution of a system?

A 
Pixel size of the detector.
Spread of signal in the detector. 
Focussing of electrons (Image Intensifier). 
Focal Spot Size. 
Magnification.
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18
Q

Explain how Film-Screen Radiography produces an image.

A

Photons incident on a fluorescent screen the emits visible light.
Light recorded on double emulsion film, that is chemically processed to give an image.
Image display is fixed.
Requires precise exposure due to non-linearity of response.

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19
Q

Explain how Digital Radiography produces an image.

A

Photons captured by fluorescent screen, or directly.
Signal digitised and assigned to a pixel.
Image processed and display varied.
Has linear response over large dynamic window,
Flat-fielding can be used to increase uniformit

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20
Q

How does an indirect receptor form a signal from X-Rays, and what material is commonly used?

A

X-Rays -> Light -> Charge -> Signal

Amorphous SIlicon Flat Panel (also used in flurosocopy).

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21
Q

How does a direct receptor form a signal from X-Rays, and what material is commonly used?

A

X-Rays -> Charge -> Signal

Amorphous Selenium Flat panel

22
Q

How does a CR receptor form a signal from X-Rays?

A

X-Rays -> Delayed Light -> Signal

23
Q

What are the components of a direct or indirect detector?

A 
Material to absorb X-Rays.
Array of electronics to collect signal 
- Array of thin film transistors (TFTs) 
- Each Transistor forms a pixel 
- TFTs usually deposited in glass. 
- Signal from pixels transferred to a computer to create an image.
24
Q

What phosphor is commonly used for an indirect (A-Si) detector?

A

Caesium Iodide

GdOS also sometimes used

25
Q

Describe in detail how an indirect detector forms an image.

A

X-Rays absorbed in phosphor and produce light photons.
Photodiode array converts light to electric charge.
Signal captured by a-Si TFT array.
Each pixel 0.1-0.2mm square.
The image is read out in a progressive scan from top to bottom.
The charge stored is converted to a digital signal and the image is built up.

26
Q

What property of CsI improves the spatial resolution of the receptor?

A

Needle-like structure allows for TIR of photons, reducing sideways spread.
Other phosphors have more advantageous k-edge but do not have needle-like structure, so it is counteracted by the spread of the light.

27
Q

What is the k-edge of CsI?

A

33keV

28
Q

What advantage does the k-edge of CsI offer?

A

Increased chance of photoelectric effect.

29
Q

How can the efficiency of the CsI phosphor be improved?

A

Increase thickness.

30
Q

Describe in detail how an image is formed in a direct detector?

A

X-Rays absorbed in a-Se - create electron/hole pair.
a-Se is a photoconductor - become more electrically conductive upon irradiation.
Voltage applied across a-Se layer.
Charge in a-Se is swept to the electrons and stored in pixels.
Image is read ou

31
Q

How is the spatial resolution of a director detector increased?

A

High electric field pulls electrons and holes in opposite directions minimising sideways spread.

32
Q

What is the k-edge of a-Se?

A

12.7keV - too low for diagnostic x-ray but good for mammography.

33
Q

How can the efficiency of the a-Se detector be improved?

A

Increase thickness.

34
Q

What types of phosphor does a CR cassette use?

A

Powder phosphor - individual crystals held together with a binder.

35
Q

How does the powder phosphor capture X-Rays?

A

Poor packing density (much less than 100%), phosphor must be thin to avoid light spread (increasing spatial resolution). Phosphor, therefore, relies on K-Edge to capture X-Rays.

36
Q

Needle plate phosphors are now available for CR cassettes, what material are they made from?

A

Caesium Bromide (CsBr)

37
Q

What advantages does a CsBr Phosphor have over a traditional powder phosphor fro a CR cassette?

A

Better image quality - less light spread

Higher packing density - no binder - greater x-ray absorption.

38
Q

Where area automatic exposure controls (AECs) situated in DR detectors?

A

Behind the grid but in front of the image plate.

39
Q

What is the function of an AEC?

A

Monitor the air kerma from the tube and terminate when a pre-determined limit is reached.
The must take into account patient thickness.

40
Q

Explain how an Image Intensifier works

A

X-Rays incident on input phosphor (CsI) and produce light photons.
Light incident on photocathode and produces electrons.
Electrons accelerated across a vacuum.
High energy electrons incident on output phosphor (ZnCdS) and produce more light (flux gain from acceleration of electrons -increased energy)
Area of input phosphor is larger than output leading to minification gain.
Light detected be camera and image is displayed.

41
Q

How can the mean energy of the X-ray beam be increased without increasing tube voltage?

A

Increasing the beam filtration and/or tube filtration.

42
Q

What is the normal operating range of tube voltages on a general diagnostic radiography unit?

A

The ‘normal’ range of tube voltages would be between around 50kVp – 125kVp

43
Q

Which photon interaction is responsible for producing contrast in the image?

A

Photoelectric effect

44
Q

What effect does increasing the tube voltage have on the image contrast of an object? and why?

A

Increasing the tube voltage will decrease the radiation contrast as the difference in attenuation coefficients of the object and the background become less.

45
Q

Considering only quantum noise, what is the effect on the signal-to-noise ratio of a two-fold increase in the number of photons?

A

SNR = N/sqrt(N) = sqrt(N)

sqrt(2) = 1.41

46
Q

Other than quantum noise, what are the two other main sources of noise in an image?

A

Electronic noise

Fixed pattern noise

47
Q

What are the three main components found in a radiographic ‘Bucky’ ?

A

Image receptor, anti-scatter grid and automatic exposure control device

48
Q

Why do intra-oral dental units not require a rotating anode X-ray tube?

A

The tube current is very low so the heat production in the anode is much less

49
Q

What are the two different imaging technologies utilised in fluoroscopy units?

A

Image intensifiers and flat panel detectors

50
Q

What material is generally used as the input phosphor for an image intensifier?

A

Csl (Caesium iodide)

51
Q

Describe how CR works

A

Image formed on phosphor cassette that is removed, read and then reset to be used again

  • X-ray photons absorbed by phosphor crystal
  • High energy photoelectron released which ionises atoms along its track releasing electrons → >100 electrons released per x-ray photon
  • Cassette removed and placed in machine for read-out
  • Red laser beam scans back and forth releasing energy from electrons which is released as blue light
  • Light collected by optical fibers to PMT
  • PMT produces electrical current

SPE101: Ionising Radiation (10 decks)

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