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As Physics Imaging > Imaging Definitions > Flashcards

Imaging Definitions Flashcards

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0
Q

What decides that colour or shade of a pixel?

A

The number assigned to each pixel.

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1
Q

What are pixels?

A

Small square picture elements,each darker or lighter shade of grey.

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2
Q

What is the resolution of an image?

A

The resolution of an image is the smallest size of thing which can be distinguished.

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3
Q

Why is the resolution important?

A

The resolution must be good enough to reveal important details.

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4
Q

How does the resolution of an image apply to the grey levels in an image.

A

In a bad image just 256 possible can be recorded. A change in intensity smaller than this will not be detected.

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5
Q

What is an ultrasound pulse?

A

An ultrasound pulse is a pulse containing just a few oscillations, it is produced by a piezoelectric crystal.

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6
Q

What is resolution?

A

The resolution of any instrument is the smallest difference which is detectable.

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7
Q

How does an ultrasound scanner make an image of a baby?

A

The delay times of reflections tell I the scanner where the denser tissue of the baby is. The strengths of the reflections tell the scanner how dense various tissues are. From delay times and strengths of reflected pulses a picture of the baby in the womb can be built up.

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8
Q

What is tomography?

A

Computer tomography is a big computing job to work out all the reflections and what’s producing them. CT is shorthand.

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9
Q

What is displacement and it’s unit?

A

Displacement, X, metres-how for a point on the wave has moved from it undisturbed position.

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10
Q

What is amplitude and unit?

A

Amplitude,a,metres- maximum displacement

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11
Q

What is wavelength and it’s unit?

A

Wavelength is in metres- the length of one while wave,from crest to crest or trough to trough.

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12
Q

What is a period and it’s units?

A

Period,T, seconds- the time taken for a whole vibration.

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13
Q

What is frequency and it’s units?

A

Frequency,f, hertz–the number of vibrations per second passing a given point.

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14
Q

What is the equation which relates speed v, frequency f and wavelength (highlander unit)?

A

Speed=frequency x wavelength.

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15
Q

How many oscillations in 10 seconds from a source of frequency 1MHz?How many complete cycles in a pulse of ultrasound 3 microseconds long if the frequency is 2MHz?

A

1) 10^7

2) 6

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16
Q

If ultrasound takes 70 microseconds to go 100mm in soft tissue, how fast does it travel in metres per second? In air, sound travels at about 340ms^-1.How many microseconds does it take to go 100mm?

A

1) about 1400ms^-1

2) about 300micro-seconds

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17
Q

Suppose one part of a baby is 200mm in front of nan other. What is the difference in time between reflections from each part,from a pulse travelling at 70micro-seconds per 100mm?

A

1) 285 micro-seconds.

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18
Q

If the ultrasound pulses are 3.5 micro-seconds long, and travel 100mm in 70 micro-seconds, what is the smallest distance one part of a baby can be behind another if the reflections can just be told apart?

A

1) 2.5 mm

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19
Q

What is the wavelength in of soft tissue of ultrasound of frequency 3MHz travelling at 1500 ms^-1? Suggest why detail is less than one wavelength in size cannot be seen.

A

1) 0.5 mm
2) Because we only measure how many wavelengths a pulse has travelled in, we have no way to measure smaller than a wavelength.

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20
Q

The ultrasound image of a baby is a square array of 256x256 pixels. If the area imaged is about 250mm across and high, what is the resolution?

A

1) each pixel is about 1mm by 1mm

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21
Q

From increasing order in the electromagnetic spectrum of:
Radio waves —>Micro waves—> visible and infrared —> X-ray and gamma rays.
How does the frequency and wavelength change?

A

1) The frequency and photon energy increase.

2) the wavelength decreases.

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22
Q

What is a charge coupled device or ccd?

A

A modern optical telescope uses a light sensitive microchip instead of a photographic plate. One type is called a charge coupled device. It is a screen covered by a million or more tiny silicon picture elements, each of which stores an electric charge when light falls on it. Charge-coupled devices can be made which are sensitive to wavelengths outside the visible range. The image- a set of values of potential difference, one on each picture element- is ‘read out’ element by element to be stored as a string of numbers.

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23
Q

How does a charge-coupled device work?

A

Light hits the picture elements, the elements storing charge proportional to the light falling on it. When an image is formed each element stores a charge in proportion to the light which has fallen on it. To record the image, the charges are shunted in sequence from one element to the next, until they reach the edge where the value is read as a potential difference. Thus the whole image can be recorded as a sequence of values of potential difference representing charges and so brightness. This picture becomes an array of numbers.

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24
Q

If you could freeze a 3GHz electromagnetic wave as it travels through space, how many complete waves would there be in a distance of 1m?( take into account that the speed of electromagnetic radiation is 3 x 10^8)

A

10

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25
Q

What is a scanning tunnelling microscope?

A

Has an ultra sharp needle point which is scanned in zig-zags across the surface of a material. When the tip is close to the atoms of the the surface, electrons can ‘tunnel’ across the gap so hush at there is an electric current between the surface and the tip. The tip is moved up or down so as to keep the tunnelling current constant. A record of ups and downs at the atomic scale of the surface. The smaller the gap, the bigger the tunnelling current. Signals go to a piezoelectric crystal move the needle up and down.

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26
Q

How is the cross sectional distance of an atom?

A

1nm or 10^-9 m

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27
Q

What is the binary system?

A

The numbers in computers stored as on- or off- values. ‘On’ May be a high potential difference, ‘off’ May be low.

28
Q

What was on of Claude Shannon’s ideas?

A

One of his ideas was that the amount of information can be measured by the amount of storage that it needs.

29
Q

What is one ‘bit’?

A

A memory location storing a 1 or 0.

30
Q

How much information does one bit give and how much information does 8 bits give?

A

One bit of information is one choice: 0 or 1. Eight bits, giving 256 possibilities, is a useful slice of memory. It is enough to give a code number to every key on the keyboard, including capitals and lower case,numbers and punctuation marks. Eight bits is also referred to as a byte.

31
Q

An image with better resolution will contain what?

A

More information.

32
Q

How many alternatives are there if 8 bits of information are used?

A

2^8 so 256 possibilities

33
Q

In general what is the amount of information measured in?

A

The amount of information is measured in I, which is the number of bits.
Knowing the number of bits you can work out the number of alternatives which is 2^I

34
Q

Why is the information used on a plus scale while the number of latter natives is measured on a times scale?

A

If the number of bits increases by one, the number of alternatives doubles.

35
Q

What is needed to be taken into account on a ‘Plus’ scale ( linear)?

A
  • It is the amount of information
  • it increases be equal additions
  • amount is measured in I
  • amount = log2 N
36
Q

What is needed to be taken into account on a ‘Times’ scale (logarithmic)?

A
  • it is the number of latter natives
  • increase in equal multiples
  • number of alternatives= 2^I
  • number of alternatives= N
37
Q

Explain what is log2?

A

Log2(number of alternatives)= information
-Information= number of bits

Example-

What is the amount of information needed to to give 256 alternatives?

Log2(256)= 8. The answer is 8 bits.

38
Q

Why is it good to plot in logarithms?

A

They are often good to make plots of quantities which span over many orders of magnitude.

39
Q

What base logarithms are used for distances?

A

These are logarithms(base 10) of distances in metres.

For example

  • 3 is the power of ten in 10^3m= 1km
  • 6 is the power of ten in 10^-6m=1micrometer.
40
Q

How do you smooth sharp edges in image processing?

A

Rule:
- Replace each pixel by the mean of its value and those of its neighbours

Example:

3 3 3 –>
3 2 3 –> (3+3+3+3+3+3+3+3+2)/9= 2.9 The value 2 and all the
3 3 3 –> values surrounding 2 will become 2.9. But values not surrounded by 2 will get a different mean.

41
Q

How is noise removed in image processing?

A

Rule: Replace each pixel by the median of its value and those of its neighbours.

Example-
3 3 3 –>
3 2 3 –> 2,3,3,3,3,3,3,3,3. The median is the value in the
3 3 3 –> middle of value of those lined up which here is 3.

42
Q

How are edges found in image processing?

A

Rule:Subtract the N,S,E and W neighbours from 4 times the value of each pixel.

Example-

2 2 2 –> multiply 4 by 4 to give 16 then take away (2+2+2+2)
2 4 2 –> giving the middle value 8 as 16–8=8.
2 2 2 –>

This is called the Laplace rule, it detects changes in gradient of brightness.

43
Q

What is the cornea?

A

The cornea is curved front surface of the eye, basically the lens. Much of the bending the light happens here. A lens inside the eye adjusts Teheran focus to get a sharp image on the retina.

44
Q

Describe the human eye.l

A

Picturing the eye as a sphere the back half has the retina which contains the cell which detect light. At the back of the eye there is the optic nerve taking the information to the brain.

45
Q

How does the eye work?

A

Inside the eye is an adjustable lens,which adds to the bending of light by the curved surface of the cornea. Muscles attached to the lens can stretch the lens, making it less curved, helping to adjust the eye to focus at a given distance.

46
Q

Describe the retina.

A

They retina of the eye is built of light sensitive elements which emit signals that go to the brain. One kind, the ‘cones’, detect colours and work best in bright light. A second kind, the ‘rods’ are sensitive to dim light but do not distinguish colours. There are more than 100 million rods in the eye. Rods greatly outnumber cones, except in one special region, the fovea, where there are only cones. This is where the eye detects fine detail.

47
Q

What are the the cells connecting the rods and cones to the optic nerve?

A

The horizontal cells and ganglion cell.

48
Q

Describe the distribution of cones and rods throughout the retina.

A

The rods and cones are less densely packed nearer to the edges of the retina, so the resolution of off-centre vision is poor. The fovea in the centre of the retina is packed with cones and gives the best image but is not very sensitive to light as there are no rods.

49
Q

How do the rods and cones improve vision?

A

The rods and cones in the erections are connected to those near them. The connections are inhibitory, a lot of light on one rod or cone turns down the sensitivity of its neighbours. This sharpens our sight as the eye exaggerates the contrast in bright and dark at an edge.

50
Q

What are the two ways of thinking about light?

A
  • Ray point of view

- wave point of view

51
Q

Describe the ray point of view.

A

In their ray point of view, light is tracked along the rays which go straight, until the light is reflected at a surface or is bent as it passes from one substance into another. Rays from a. Point source spread out getting further and further apart. Rays are convenient construction lines,not real light paths,though the beam from a laser does stay quite close to a single straight line.

52
Q

Describe wave point of view.

A

From wave point of view, waves spread out in all directions from a point source like the circular ripples. From a stone thrown in a pond. The connection between the two points of view is at a ray is a line pointing along the direction of motion of the wavefront.

53
Q

In ray point of view describe how light travels from a point source and how light travels from a very distant source.

A

1) From a small source the light travels out in straight lines towards all directions.
2) From a distant source, the rays are parallel to one another.

54
Q

Describe how light travels from a small source and from a. Distant source in wave point of view.

A

1) Light spreads out in spherical wavefronts from a small source
2) Wave fronts in parallel beam or from a distant source are straight ( not curved) and parallel.

55
Q

Describe what happens to light in ray point of view and wave point of view when going through ‘burning glass’.

A

Wave point of view- The lens adds curvature to the wave,centring it on the focus.( look at the image in the book.)

Ray point of view- The lens bends the rays, bringing them to a focus.

56
Q

How does a lens focus waves?

A

The lens makes the waves more curved by slowing down the middle more than the edges. Waves going slowly through thick glass in the middle of the lens get delayed more than waves going through the thinner glass near the edges of the lens. Waves through the middle of the lens get left behind a little.

57
Q

What is focal length and dioptre and what is their relationship?

A

The distance from lens to focus is called the focal length f. A powerful lens has a small focal length. The power of the lens, measured in dioptres, is 1/f.
Thus a camera with a focal length of 50mm has a power of 1/(50x10^-3) = 20 dioptre.
1/r gives the focal length were r is the power of the lens.

58
Q

How is the curvature of the sphere of the waves emerging from a lens calculated?

A

A lens changes how curved the waves are. The radius if the spherical wave fronts just after getting through the lens is f. The curvature of a sphere of radius r is 1/r. The curvature is smaller when r is bigger.

59
Q

Does a lens affect the general direction of light?

A

No, because the general direction of travel of the light is not altered by the lens, but only the curvature of the waves, points high up on the object come low down on the image! and points right of the object come left of the image.

60
Q

What is the rule for how lenses shape light?

A

Curvature of waves going out = curvature of the waves coming in + curvature added by lens.

1/v = 1/u + 1/f
v= The distance of the image of the source from the lens.
u= distance from the source from the lens.
f= the focal length.
61
Q

What are some rules for using the equation 1/v= 1/u+1/f?

A

1) Measure from the lens
2) Count distances to the right positive.
3) Count distances to the left negative.

These rules are called the Cartesian sign convention.

62
Q

What is the equation for linear magnification?

A

Linear magnification m = image distance/ object distance.
m=v/u

  • Negative sign of m indicates inversion of image.
  • Magnitude of m les than 1 indicates dimunition.
63
Q

Why does light passing through a lens clock up exactly at the same time on all paths from a point on a source to the corresponding point on the image?

A

The angled outer paths are obviously longer than ones through the centre. But the light through the centre has to travel through more glass since the centre of the lens is the fattest. Since glass slows down light more it takes the light longer to travel that given distance. The two effects balance out.

64
Q

When having to use the equation 1/v= 1/u + 1/f what do you do when the light coming towards the lens is parallel?

A

If the light coming towards the lens is parallel then it has no curvature so u= 0 and so 1/0 = 0.

So essentially the equation becomes 1/v = 1/f as there was no previous curvature to the waves before it hit the lens.

65
Q

What happens to the equation 1/v= 1/u+1/f when the light coming out of the lens is effectively parallel?

A

Since the curvature of the the waves after passing through the lens is 0 v=0 and 1/v = 0.
So 0= 1/u + 1/f –> u= -f

66
Q

A camera with a 50mm focal length is focused on a person face 1 m from the lens. How far must the film be behind the lens?

A

53mm

67
Q

With a zoom lens, you can vary the power of the lens.What power is required to focus a flower 250mm in front of the lens on film 50 mm behind the lens?

A

24 dioptre

Ask why it’s not -24

As Physics Imaging (2 decks)

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