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LAET A-level OCR Physics A 2023 > Medical Physics > Flashcards

Medical Physics Flashcards

1
Q

What are ultrasounds?

A

Ultrasound consists of sound waves (longitudinal waves) of such high frequency that they cannot be detected by the human ear. Strictly speaking, the ultrasound range starts at about 20 kHz, but medical applications use much higher frequencies, typically between 1 MHz and 20 MHz

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2
Q

How can ultrasounds be produced?

A

By applying an alternating voltage to a piezoelectric crystal. The crystal will expand and contract at the frequency of the voltage due to the pizolectric effect. This will produce a pressure wave (ultrasound). The piezolelctric crystal must be cut to a certain size in order to work in resonant condition

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3
Q

What is a common material for a piezoelectric crystal?

A

Quartz

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4
Q

How is an ultrasound pulse generated by a transducer?

A

A high frequency p.d. is applied at the natural frequency of the piezoelectric crystal causing the crystal to vibrate.

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5
Q

How is an ultrasound pulse detected by a transducer?

A

Sound incident on the crystal causes it to vibrate generating an EMF across its faces.

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6
Q

What is the piezoelectric effect

A

A voltage applied to a piezoelectric material induces some strain/deformation. The opposite is also true. Stress applied to a material induces a voltage

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7
Q

What is an ultrasound A-scan?

A

A single transducer is placed on the skin emits a single, short pulse. Echoes return from boundaries between different body tissues. The output is displayed as a graph on an oscilloscope, showing how the echo amplitude changes with time

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8
Q

What is an ultrasound B-scan?

A

A B-scan or brightness scan creates an image by scanning the ultrasound beam over the patient. A rapid sequence of A-scans is carried out as the beam sweeps across. The strength of the echo signal is used to control the brightness of a picture element, rather than the height of a trace on an oscilloscope

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9
Q

How does an ultrasound A-scan calculate the distance to boundaries between tissues?

A

2 x Distance = v x t, where v is the speed of the ultrasound and t is the time interval between the pulse reflected by the first boundary and the pulse reflected by the second boundary.

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10
Q

What determines the acoustic impedence of a substance?

A

The density and the speed of sound in the substance.

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11
Q

What are the units of acoustic impedance?

A

kgm⁻²s⁻¹

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12
Q

What is the intensity reflection coefficient for ultrasound?

A

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

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13
Q

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

A

The difference between the acoustic impedances of the two substances.

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14
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.

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15
Q

What is an ultrasound coupling gel?

A

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

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16
Q

What is doppler ultrasound used for?

A

Measuring the speed of blood flow.

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17
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

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18
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.

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19
Q

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

A

Change in the observed frequency of the reflected ultrasound.

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20
Q

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

A

Blood flow speed.

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21
Q

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

A

The angle between the tranducer axis and the blood flow.

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22
Q

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

A

The speed of ultrasound in blood

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23
Q

What are the four mechanisms of X-ray attenuation?

A

Simple scattering (or Elastic scattering or Thomson scattering)
Photoelectric Effect
Compton Scattering (or inelastic scattering)
Pair Production

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24
Q

What occurs during simple scattering of X-rays?

A

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

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25
Q

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

A

Simple scattering (Or Elastic scattering)

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26
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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27
Q

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

A

Photoelectric effect

28
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.

29
Q

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

A

Compton scattering

30
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.

31
Q

Which method of attenuation start to occur with X-rays with energies of 1.02MeV?

A

Pair production

32
Q

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

A

Transmitted intensity

33
Q

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

A

Intensity before absorption( Incident energy)

34
Q

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

A

Attenuation/absorption coefficient

35
Q

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

A

Thickness of the absorbing substance

36
Q

What contrast medium is typically to examine blood flow?

A

Iodine

37
Q

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

A

Barium sulfate

38
Q

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

A

20-150kV

39
Q

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

A

It is heated to produce thermionic emission.

40
Q

What connected above the anode in an X-ray tube?

A

The target metal

41
Q

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

A

High melting point

42
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 (Bremsstrahlung effect) and some descrete k-lines

43
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.

44
Q

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

A

It is cooled by water/oil and rotated

45
Q

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

A

The maximum kinetic energy of a single electron.

46
Q

When is the equation λ = hc/eV used?

A

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

47
Q

What happens to the x-ray spectra if the accelerating voltage in the x-ray tube is increased

A

The continuous spectra inreases in amplitude and shifts to the left. The K-lines do not change.

48
Q

Describe the beam used by CAT scanners

A

A thin (1-10mm) fan shaped beam

49
Q

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

A

It rotates around the patient

50
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.

51
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.

52
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.

53
Q

Which radionuclide is used for imaging with a gamma camera?

A

Technitium-99m

54
Q

Which radionuclide is used in PET scanning?

A

Fluorine-18

55
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.

56
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.

57
Q

What is a medical tracer?

A

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

58
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.

59
Q

How is a collimator in a gamma camera constructed?

A

It is a honeycomb of long, thin lead tubes.

60
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.

61
Q

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

A

They convert visible light photons to an electrical pulse. They have a photocathode first that convert visible light into electrons, The through collision with a series of dynos kep at a graudally increasing potential the electrons are exponentially multiplied

62
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.

63
Q

What process produces gamma photons in a PET scan?

A

lectron and positron annihilation.

64
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.

65
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.

66
Q

What are the disadvantages of a PET scan?

A

They are expensive and require facilities (particle accelarotor) to prepare tracers local to the scanner.

67
Q

How is Fluorine -18 produced?

A

In a small particle accelarator. Hi energy protons collide with Oxygen-18 to produce Fluorine-18

LAET A-level OCR Physics A 2023 (15 decks)

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