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Physics > Imaging with X-rays > Flashcards

Imaging with X-rays Flashcards

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1
Q

Compton Scatter

A

Increases with:

  • increasing denisty
  • increasing electron density
  • lower x-ray beam energy

Increasing beam energy results in lower scatter (more energy goes to recoil electron which gets absorbed)

At 50keV Compton & Photoelectric interactions occur equally

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2
Q

What is Elastic Scatter (also known as Coherent or Rayleigh Scatter)

A

Occurs where photon energy is less than binding energy

Photon bounces off electron

No contribution to dose

More likely to occur with high atomic number elements (as electrons more tightly bound) and low photon energy

  • No ionisation takes place
  • No energy lost
  • No absorption takes place
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3
Q

Bremmstrahlung Radiation

What is bremmstrahlung proportional to ?

A

Probability of bremmstrahlung is proportional to:

Atomic number2

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4
Q

Curve Shape

What effect on the curve does changing target do?

A

Changing target:

  • changes average beam energy
  • changes the characteristic peaks
  • does NOT affect peak beam energy
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5
Q

X-ray Quantity (Number of photons)

What is x ray quantity proportional to?

A

Directly proportional to the tube current

Proportional to the peak kv2

Dependent on the choice of target material (higher atomic number = more photons)

(Target material does not affect energyO

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6
Q

Computed Radiography

What colour light stimulates and what colour is released?

A

Red light laser stimulates electrons

Blue light released

Barium fluorohalide doped with europium is schintillator layer

  • The log of the light output from the screen is directly proportional to the log of the x-ray dose
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7
Q

Electromagnetic vs Sound waves

How do both waves move?

A

Electromagmentic waves move as TRANSVERSE WAVES

Sound waves are longitudinal waves that go through periods of rarefaction and compression

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8
Q

Geometric Unsharpness

What effect does exposure time have on geometric unsharpness?

A

Shortening of exposure time causes INCREASED geometric unsharpness

(shorter exposure means higher mA and blooming in size of focal spot)

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9
Q

Grids

What is usual line density?

How thick are lead strips?

What is grid ratio?

A

Line density of a grid usually 30-80cm (number of lead strips per cm)

Lead strips are 0.05 - 0.07mm

Grid ratio is depth divided by width

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10
Q

What is tube output proportional to?

A

The square of the peak kV

(in lower dose systems like mammography, it is equal to the peak kV cubed)

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11
Q

Tube kV tested with?

A

Digital kV meter

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12
Q

Cathode Filament voltage?

A

10v

4A

(Power dissipation of 40 watts)

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13
Q

What are the Focal Spot sizes?

Mammography

Radiography

Fluoro

A
  1. 1mm - magnification mammography
  2. 3mm - normal mammography and magnification radiography
  3. 6mm - conventional radiography (small focal spot) and fluoro
  4. 2mm - conventional radiography (large focal spot)
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14
Q

DSA

What matrix size used in DSA?

A

Image intensifiers are used to reconstruct images

DSA matrix size is 1024 x 1024

Spatial resolution is 2-3lp/mm

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15
Q

Target angle

What is target angle?

Do large or small target angles improve fine detail?

A

Usually 7-20 degrees

Smaller target angles improve fine detail imaging (reduces geometric unsharpness)

Smaller target angle used in mammography

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16
Q

Heat Dissipation in the tube

How is heat dispersed from tube?

A

Radiation is the primary method

Convection currents flow in the oil

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17
Q

Wave Rectification

How many diodes needed?

AC or DC current?

A

Converts AC to DC

Uses diodes

Full wave rectification needs at least 4 diodes

18
Q

How to calculate Beam Intensity?

What is it proportional to ?

Does filtration affect beam intensity?

A

Number of photons per unit area x mean photon energy

/

Unit time

Beam intensity is directly proportional to the tube current (number of electrons flowing from cathode to anode)

Approximately proportional to kV squared

Intensity will DECREASE after filtration as some of the photons are filtered out

19
Q

Unwanted Radiation

A

Made up of

  1. Leakage radiation - is what is transmitted throught tube housing
  2. Scattered radiation - is what has changed direction after leaving tube

Leakage radiation should not exceed 1mGy/hr at 1m from the focus

20
Q

Heat Loading - what are units of measurement?

How is it calculated?

A

Measured in joules or heat units (old term)

For constant three phase potential it is equal to:

kV x mA

For single phase its is equal to:

0.7 x kV x mA

21
Q

X-ray Spectrum

What effect on curve will increasing kV and mA have?

A

Increasing the tube current (mA) will NOT affect the shape of the x-ray spectrum

  • will increase amount of characteristic and bremmstrahlung radiation

Increasing the kvP will shift curve up and to the right

22
Q

Are the max energy of x-ray photons affected by the anode or three phase supply?

A

NOT affected by:

  • anode material
  • voltage waveform (this affects mean energy)
23
Q

How to calculate HVL?

A

HVL = 0.693/LAC

It is the thickness of material to attenuate a POLYENERGETIC beam by 50%

LAC and MAC are used for monoenergetic beams

24
Q

What is Mass Attenuation Coefficient ?

What are units?

A

The fraction of photons removed from a monochromatic energy beam by a homogenous absorber per unit mass

Is LAC/Density

Units are cm squared/kg or cm squared/g

Attenuation of the beam = MAC x mass thickness

Depends on the radiation interaction

25
Q

Focal Spot Size Quality Control

How to measure focal spot size and resolution?

A

Focal Spot size measured using Pinhole imaging

Resolution measured using a Star Test Pattern

26
Q

Tomography

What does narrow angle tomography produce?

Angle?

Blur?

A

Tomography most useful when imaging structures with High inherent contrast

Contrast depends on slice thickness

  • thicker slices produce better contrast

Structures to the right angle of the film appear blurred

  • those in line of sight between tube and detector are sharp

In Narrow Angle (thicker slices)

  • reduces blur of structures outside focal plane
  • preferred for imaging tissues with low contrast
  • increased tendance for phantom image formation
27
Q

How is heat lost from anode??

A

RADIATION MAINLY

28
Q

What is Inherent Filtration equal to?

A

Mainly occurs at glass envelope but includes oil, tube housing, target material

Equivilent to approx 0.5 - 1mm of aluminium

Inherent filtration reduces tissue contrast

29
Q

What is an Auger Electron ?

A

Results after the photoelectric effect where an additional electron is released

  • Are an important source of dose in radiology
  • Occurs as an alternative process to characteristic radiation release

The kinetic energy of auger electron is the binding energy of the K shell minus twice the energy of the L shell

30
Q

What is normal Bit Depth?

Compression techniques

A

For DR:

  • usually 12 - 16bit depth
  • wide the dynamic range the wider the bit depth needed

Compression:

  1. Lossless
  • reversible
  • can be restored into an identical version of the original
  • can reduce file size by a factor of 2 to 3
  1. Lossy
  • reduces file size by up to 40
  • irreversible
31
Q

Detector Dose indicators (in CR)

A

Provide a measure of the signal from the image plate AVERAGED over a broad region of the plate

  • equivilent to the optical density of the film
  • doubling dose does NOT double DDI
32
Q

What is Noise calculation?

A

Quantum Noise: proportional to the square root of number of photons

Noise is inversley proportional to the square root of number of photons

SNR = M/ square root of M (M = number of photons)

Signal to noise ratio is PROPORTIONAL to square root of number of photons

33
Q

What is an electron volt?

A

When an electron is accelerated through 1 volt the unit used is an electron volt

The rest mass of an electron is 511keV

34
Q

Three phase supply effect on photon quality and quantity

A

Increases both quality and quantity

Shifts curve up and to right

35
Q

What is Isotropic Scatter ?

A

Where at low energies in compton scatter:

  • photons are given off in all directions EQUALLY
36
Q

What increases contrast in radiography?

A
  • Increasing thickness of structure

Contast is proportional to the difference in LAC of tissues x tissue thickness

Contrast decreases as kV increases

37
Q

How to reduce scatter and improve contrast?

A
  1. Reduce field size
  2. Decrease patient thickness
  3. Reduce kV
  4. Use of antiscatter grid
  5. Increased Focus to detector distance
38
Q

What is grid ratio?

A

Depth of interspace / width

Typical ratio is 8:1

Higher the ratio the better the contrast

39
Q

What 3 factors are needed for focus grids?

A
  1. Must be at specified distance from anode
  2. Should be tilted progressively to point towards tube focus
  3. Tube must be accurately angle over the grid

Can result in grid cutoff if above measures arent adhered to

40
Q

What is standard number of bits used in radiography?

A

Usually 12 to 14 bits

41
Q

What is Nyquist frequency?

A

The max frequency that can be sampled

Equal to half the sampling frequency

42
Q

What size of anode disc is used in radiography?

A

Usually 20 - 30cm in diameter

Physics (10 decks)

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