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physics > 27) medical imaging > Flashcards

27) medical imaging Flashcards

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1
Q

What provides the energy of the X-rays in an X-ray tube?

A

The kinetic energy of the accelerated electrons.

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2
Q

What is an X-ray?

A

A photon produced by the deceleration of a fast moving electron

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3
Q

What is the typical p.d. of an X-ray tube for medical imaging?

A

30-100kV

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4
Q

What causes the emission of electrons from the cathode in an X-ray tube?

A

It is heated to produce thermionic emission.

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5
Q

What name is given to the anode in an X-ray tube?

A

The target metal.

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6
Q

What is an essential property for the target metal in an X-ray tube?

A

High melting point.

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7
Q

What causes the production of X-rays in an X-ray tube?

A

The deceleration of electrons when they hit the target metal/anode.

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8
Q

In an X-ray tube what happens to most of the kinetic energy lost by the electrons when they hit the anode?

A

It is transferred to thermal energy of the anode.

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9
Q

What measures can be taken to prevent an anode melting in an X-ray tube?

A

It is cooled by oil or rotated

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10
Q

What feature of X-ray tubes protects the radiographer?

A

Lead lining.

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11
Q

What determines the maximum frequency of X-rays produced in an X-ray tube?

A

The maximum kinetic energy of a single electron.

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12
Q

When is the equation λ = hc/eV used?

A

To find the minimum wavelengths of X-ray produced (by an X-ray tube)

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13
Q

What is V in the equation λ = hc/eV?

A

The accelerating potential difference.

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14
Q

Describe the beam used by CAT scanners.

A

A thin (1-10mm) fan.

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15
Q

What happens to the position of the X-ray tube during a CAT scan?

A

It rotates around the patient.

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16
Q

What are the key advantages of CAT scans over conventional X-rays?

A

They produce a 3D image and can distinguish between tissues with similar attenuation coefficients.

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17
Q

What are the disadvantages of CAT scans compared to conventional X-rays?

A

The radiation dose is greater and they are more expensive and take more time.

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18
Q

How does a CAT scan produce a 3D image?

A

A series of slices are imaged and processed via a computer to produce a 3D image.

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19
Q

What is X-ray attenuation?

A

The decrease in intensity as X-rays pass through matter.

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20
Q

What are the four mechanisms of X-ray attenuation?

A

Simple scatter Photoelectric Effect Compton Scattering Pair Production

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21
Q

What occurs during simple scatter of X-rays?

A

The X-ray interacts with an electron in an atom and changes direction but not energy.

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22
Q

Which method of attenuation is most significant with X-rays with energy in the range 1-20keV?

A

Simple scatter

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23
Q

What occurs during the photoelectric effect attenuation of X-rays?

A

The X-ray is absorbed by an electron in an atom which gains the energy of the photon and leaves the atom.

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24
Q

Which method of attenuation is significant with X-rays with energies up to 100keV?

A

Photoelectric effect

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25
Q

Which method of attenuation is most significant to medical X-ray imaging?

A

The photoelectric effect.

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26
Q

What occurs during Compton scattering attenuation of X-rays?

A

The X-ray interacts with an electron in the atom causing the electron to be ejected and the photon to be scattered with reduced energy.

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27
Q

Which method of attenuation is significant with X-rays with energy in the range 0.5-5MeV?

A

Compton scattering.

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28
Q

What occurs during pair production with X-rays?

A

An X-ray interacts with the nucleus of an atom converting the energy of the photon to and electron and a positron.

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29
Q

Which method of attenuation occurs with X-rays with energies 1.02MeV?

A

Pair production

30
Q

What is I in the equation I = I₀e^(-µx)?

A

Transmitted intensity

31
Q

What is I₀ in the equation I = I₀e^(-µx)?

A

Intensity before absorption

32
Q

What is µ in the equation I = I₀e^(-µx)?

A

Attenuation/absorption coefficient.

33
Q

What is x in the equation I = I₀e^(-µx)?

A

Thickness of the absorbing substance

34
Q

What are the SI units of attenuation coefficient?

A

m⁻¹

35
Q

What is a contrast medium in X-rays?

A

A material with a high attenuation coefficient used to image soft tissue.

36
Q

What contrast medium is typically used to examine blood flow?

A

Iodine

37
Q

What contrast medium is typically used to imagine the digestive system?

A

Barium sulfate

38
Q

How are X-rays used for cancer therapy?

A

A linear accelerator produces high energy X-rays that destroy cells.

39
Q

Which radionuclide is used for imaging with a gamma camera?

A

Technitium-99m

40
Q

Which radionuclide is used in PET scanning?

A

Fluorine-18

41
Q

Why are gamma emitting sources useful for imaging inside the body?

A

They are the least ionising and can penetrate through a patient to be detected externally.

42
Q

Why are short half-lives necessary for the radioisotopes used for medical imaging?

A

To ensure a large activity from a small amount and to minimise radiation dose after the procedure.

43
Q

What is a medical tracer?

A

A compound containing a radioisotope that is put into the patients body.

44
Q

What is the function of a collimator in a gamma camera?

A

To absorb any photons not travelling along the axis of the tubes so the source location of gamma radiation can be idenitifed.

45
Q

How is a collimator in a gamma camera constructed?

A

It is a honeycomb of long, thin lead tubes.

46
Q

What is the function of a scintillator in a gamma camera?

A

It produces many photons of visible light when a gamma photon interacts with it.

47
Q

What is the function of the photomultiplier tubes in a gamma camera?

A

They convert visible light photons to an electrical pulse.

48
Q

What is the function of the computer in a gamma camera?

A

It processes the signals from the photomultipliers to locate where the gamma ray originated to produce an image showing the concentrations of medical tracer.

49
Q

What type of radiation is detected in a PET scan?

A

Gamma

50
Q

What type of radiation is emitted by Fluorine-18 nucleus in a PET scan?

A

Beta plus/positron

51
Q

What process produces gamma photons in a PET scan?

A

Electron and positron annihilation.

52
Q

How is the the source of the gamma photons located in a PET scan?

A

By the time delay between the arrival of diametrically opposite gamma photons.

53
Q

What is the typical time delay between the arrival of gamma photons in PET scan?

A

Nanoseconds

54
Q

Why is medical imaging using gamma emitting tracers useful?

A

It can measure the function of organs in the body rather than just the structure.

55
Q

What are the disadvantages of a PET scan?

A

They are expensive and require facilities to prepare tracers local to the scanner.

56
Q

What determines the acoustic impedence of a substance?

A

The density and the speed of sound in the substance.

57
Q

What are the units of acoustic impedance?

A

kgm⁻²s⁻¹

58
Q

What is the intensity reflection coefficient for ultrasound?

A

The ratio (reflected intensity of ultrasound ÷ incident intensity of ultrasound)

59
Q

What factor determines the refelction intensity coefficient for ultrasound at a boundary?

A

The difference between the acoustic impedances of the two substances.

60
Q

Why does a coupling gel need to be used when performing an ultrasound?

A

Air pockets between the transducer and skin cause reflection of nearly all of the ultrasound at the skin-air boundary.

61
Q

What is an ultrasound coupling gel?

A

A substance applied to the skin and transducer with an impedance similar to skin.

62
Q

What is impedance/acoustic matching?

A

When two substances havce similar acoustic impedances so negligible reflection of ultrasound occurs.

63
Q

What causes ulatrasound to undergo doppler shift?

A

Reflection from moving blood cells.

64
Q

What is doppler ultrasound used for?

A

Measuring the speed (and volume) of blood flow.

65
Q

What is measured in doppler ultrasound to determine the velocity of the blood?

A

The observed change in frequency of the reflected ultrasound pulse.

66
Q

How is blood flow viewed during a doppler ultrasound?

A

The signal is processed by computer and displayed as a colour coded image on a screen.

67
Q

Why does the transducer have to placed away from the normal during a doppler ultrasound?

A

To ensure there is a component of the blood flow’s velocity in the direction of the ultrasound pulse.

68
Q

What is ∆f in the equation? ∆f = (2fvcosθ)÷c

A

Change in the observed frequency of the reflected ultrasound.

69
Q

What is v in the equation? ∆f = (2fvcosθ)/c

A

Blood flow speed.

70
Q

What is θ in the equation? ∆f = (2fvcosθ)/c

A

The angle between the tranducer axis and the blood flow.

71
Q

What is c in the equation? ∆f = (2fvcosθ)/c

A

The speed of ultrasound in blood.

physics (18 decks)

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