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A level physics > 27 - Medical Imaging > Flashcards

27 - Medical Imaging Flashcards

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1
Q

How are x-rays produced in an x-ray tube?

A
  • Cathode = hot filament, produces electrons by thermionic emission
  • Electrons are accelerated across large p.d in a vacuum so there are no interactions
  • Electrons hit anode (target metal) and as they slow down, they produce x-ray photons
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2
Q

Why is the anode in an x-ray tube angled?

A

So that the x-rays leave the tube through a desired window.

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3
Q

How is the anode prevented from overheating?

A
  • Cooling it with oil

- Rotating it to spread the electrons over the surface area

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4
Q

Why is a material with a high melting point used for the anode?

A

Because it gets very hot with the amount of energy that is transferred to it by the electrons

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5
Q

What is an x-ray tube lined with and why?

A

Lead - to protect the radiographer from xrays emitted in other directions.

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6
Q

How can we apply the conservation of energy to an x-ray tube?

A

The maximum kinetic energy of one electron must be equal to the maximum energy of one x-ray photon.

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7
Q

What does attenuation describe?

A

The decrease in intensity of electromagnetic radiation as it passes through matter.

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8
Q

What happens in simple scattering?

A

The x-ray photon interacts with an electron but does not have enough kinetic energy so it bounces off without any change in energy.

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9
Q

What is the energy range of simple scattering?

A

<20KeV

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10
Q

What happens in the photoelectric effect?

A

The x-ray photon transfers its energy to an electron in a one to one reaction. This causes the electron to be freed from the atom with any excess kinetic energy from the photon.

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11
Q

What is the energy range of the photoelectric effect?

A

<100KeV

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12
Q

What is the most commonly found type of attenuation mechanism found in hospitals and why?

A

Photoelectric effect

Hospital x-ray machines use p.ds in the range 30-100 KeV

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13
Q

What happens in Compton scattering?

A

The x-ray photon transfers energy to the electron, freeing it but does not disappear. The photon is scattered with a much lower energy than before.

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14
Q

What is conserved in Compton scattering?

A

Energy and momentum

Photons have momentum :0

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15
Q

What is the energy range of Compton scattering?

A

0.5 - 5 MeV

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16
Q

What happens in pair production?

A

The photon interacts with the nucleus of the atom.

The photon disappears and its energy is used to form an electron positron pair.

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17
Q

What is the energy range of pair production?

A

> = 1.02MeV

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18
Q

What does μ stand for?

A

The attenuation coefficient / absorbtion coefficient

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19
Q

What does I stand for in terms of attenuation?

A

Intensity

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20
Q

What does x stand for in terms of attenuation?

A

Thickness of material.

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21
Q

What is a contrast medium?

A

A chemical that is inserted into a patients body which has a much higher attenuation coefficient than tissue in order to make certain areas of the body clearly visible with x-rays.

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22
Q

Why are iodine and barium commonly used contrast mediums?

A

They have a high atomic number compared to soft tissue so they are much more absorbent than tissue.

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23
Q

What is iodine commonly used to detect?

A

Blood flow - it is injected into the bloodstream to highlight blood vessels and the structure of the heart.

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24
Q

What is barium commonly used to detect?

A

Digestive system - eaten in the form of a barium meal, it highlights the digestive system.

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25
Q

What two attenuation methods are used to kill cancerous cells?

A

Compton scattering

Pair production

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26
Q

How does a CAT scanner work?

A

An x-ray source is rotated around a body with detectors at the other side.
As it rotates, it moves up the body.
This creates lots of thin “slices” of an image which can be put together by advanced software to produce a 3D image.

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27
Q

What does CAT stand for?

A

Computerised Axial Tomography

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28
Q

What are the advantages of CAT scans?

A
  • 3D image makes it easier to diagnose

- Can distinguish between different soft tissues

29
Q

What are the disadvantages of CAT scans?

A
  • More expensive

- Longer exposal time to ionising radiation

30
Q

Why is gamma radiation ideal for radioisotopes?

A

Because they are low ionising but highly penetrative

31
Q

What must the half life of a radio isotope be?

A

Short enough so that the patient is not emitting radiation after more than a day
Long enough so that radiation is detectable for a few hours.

32
Q

What are two commonly used radioisotopes?

A

Technetium 99m

Fluorine 18

33
Q

What does m stand for in Technetium 99m?

A

Meta stable - The nucleus stays in a high energy, unstable state for longer than expected

Makes it extremely useful for medical imaging

34
Q

What must radioisotopes be combined with in order to form pharmaceuticals?

A

Elements that will target specific organs / tissues.

e.g. glucose will target cancer cells

35
Q

How does a gamma camera work?

A

Gamma photons from the decay of radioisotopes reach the collimator and travel up to the scintillator.
The scintillator produces thousands of visible light photons when a single gamma photon interacts with it.
These visible light photons travel along the photomultiplier tubes and converted to a voltage.
The computer detects this voltage and uses the data to form a detailed image.

36
Q

What is the collimator?

A

Thin lead tubes.

Only photons travelling along the axis of the tubes can go through.
Photons at an angle will be absorbed

37
Q

What is the scintillator?

A

A material which has a 1 in 10 chance to produce thousands of visible light photons (flashes) for each gamma photon that that strikes it.

38
Q

What are photomultiplier tubes?

A

A photon hits a photocathode and releases an electron (photoelectric effect).

This electron is bounced between electrodes, accelerated by potential differences and producing more and more electrons at each collsion due to the high energy.

At the end of the tube, there can be around 1 million electrons which are detected as a tiny electrical pulse.

39
Q

What does a gamma camera detect?

A

The function of organs in the body.

NOT the structure

40
Q

How is fluorine-18 formed?

A

In a particle accelerator - colliding an Oxygen-8 nucleus with a proton, releasing a neutron.

41
Q

What chemical do we use to look at organs with high rates of respiration in PET scans?

A

Glucose

42
Q

What chemical do we use to look at blood flow with in PET scans?

A

Carbon monoxide

43
Q

How are the gamma photons that are detected by a gamma camera produced?

A

Positrons are produced in Beta+ decay of the Fluorine-18.

When these positrons annihilate with electrons inside the body, they produce a pair of gamma photons.

44
Q

In what direction do the two gamma photons move once they are produced by electron-positron annihilation?

A

Away from each other.

45
Q

How does the computer determine the point of annihilation in a PET Scan?

A

By calculating the difference in arrival time and therefore distance that a pair of gamma photons have traveled.

46
Q

How does a PET scan work?

A

The patient lies down and is surrounded by a ring of gamma cameras. Cameras opposite each other detect one of a pair of gamma photons produced from annihilation.

47
Q

What is the scintillator commonly made from?

A

Sodium iodide

48
Q

What are the advantages of a PET scan?

A
  • They can help to diagnose cancers
  • Highlight abnormal functions in the body e.g. tumor would show high concentration of tracer as it uses lots of glucose
  • Plan complex surgery
49
Q

What are the disadvantages of PET scans?

A

Very expensive as requires particle accelerator nearby to create medical isotopes.

50
Q

What is ultrasound?

A

Sound waves that are over 20KHz

51
Q

What frequencies of ultrasound are used for imaging?

A

1-15 MHz

52
Q

What sort of wave is ultrasound?

A

Longitudinal

53
Q

What is an ultrasound transducer?

A

A device that both emits and recieves ultrasound

54
Q

What is the piezoelectric effect?

A

When an emf is placed across some cystals, they either stretch or compress (depending on the polarity).

When these crystals are compressed or stretched by an external force, they produce an emf.

55
Q

What is the percentage of strain experienced by piezoelectric crystals?

A

< 0.1%

56
Q

How are piezoelectric crystals used in ultrasound transducers to produce ultrasound?

A

An alternating p.d. is applied to the crystals at the desired frequency of the ultrasound.
This causes the crystals to stretch and compress at this frequency and produce sound waves.

57
Q

Ultrasound is emitted from transducers in _.

A

Pulses

Typically 5000 pulses per second

(Do not confuse with frequency of ultrasound)

58
Q

How are piezoelectric crystals used in ultrasound transducers to receive ultrasound?

A

Reflected sound causes the crystal to compress and expand. This produces a tiny emf which can be detected.

59
Q

What is an ultrasound A scan?

A

When the transducer is not moved during the scan and measures a straight line. An image is not formed, only a measurement of distance between boundaries.

60
Q

How is the amplitude of ultrasound increased when it is produced?

A

The piezoelectric crystals are made to oscillate at their natural frequency.

61
Q

How is the distance between boundaries between mediums calculated from an ultrasound?

A

The time taken for the ultrasound to travel to a boundary and back x the speed of ultrasound in the medium.

62
Q

What is a B scan?

A

When the transducer is moved while capturing an image.
This is connected to a high speed computer which combines the measurements, a series of bright dots at boundaries, into a 2d image.

63
Q

What is acoustic impedance?

A

The product of the density of a material and the speed of ultrasound in it.

Z

64
Q

When will reflection be maximum?

A

Where the acoustic impedance of the two materials is very different.

65
Q

When will reflection be minimum?

A

Where the acoustic impedance of the two materials is very similar.

66
Q

What does the radio Iᵣ/I₀ tell us?

A

The proportion of ultrasound that is reflected at a boundary.

67
Q

Why is coupling gel used at the transducer to skin boundary.

A

Removes air pockets.
Air to skin has a large Z difference so 99% is reflected.
Coupling gel has similar Z to skin so almost all ultrasound passes through.

68
Q

How can ultrasound be used to measure the speed of blood?

A

Ultrasound transducer placed in plane of blood vessel (or 60 degrees)
Ultrasound reflected off of moving blood will have a changed frequency due to the Doppler effect.
Δf ∝ v
Computer can colour blood on screen based on relative speed.

69
Q

Why can we not hold the transducer perpendicular to the skin when measuring blood flow?

A

Because cos90 = 0

There is no component of ultrasound in the plane of the motion of the blood so there will be no change in frequency.

A level physics (26 decks)

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