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A-Level Physics > Medical Physics > Flashcards

Medical Physics Flashcards

1
Q

What is an X-ray tube?

A

A device that converts an electrical input into X-rays

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2
Q

What are the 4 main components of an X-ray tube?

A
  • A heated cathode
  • An anode
  • A metal target
  • A high voltage power tube
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3
Q

What does an x-ray tube look like?

A
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4
Q

What is the role of a heated cathode?

A
  • At one end of the end of the tube is the cathode (negative terminal) which is heated by en electric current
  • The heat causes electrons to be liberated from the cathode, gathering in a cloud near the surface
  • This process of thermoionic emission is the source of the electrons
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5
Q

What is the role of the anode?

A

At the other end oft the tube, an anode (positive terminal) is connect to a high voltage supply
- This allows the electrons to be accelerated up to a voltage of 200kV - when the electron arrives at the anode =, its KE is 200 keV (by definition of electronvolt)
- Only about 1% of its KE is converted into x-rays and the rest is converted into heat energy. Therefore, the anode is spun at 3000 rpm and sometimes water cooled

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6
Q

What is the role of a metal target?

A
  • When the electrons hit the target at high speed, the lose some of their KE and this is emitted as X-ray photons
  • A heat resistant block of metal is embedded at the end of the anode, facing the cathode - this is the material that the electrons collide with, and X-rays are generated in
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7
Q

What is the role of the high voltage power supply?

A
  • The high voltage power supply creates a large pd between the cathode and the target
  • This causes electrons in the cloud around the cathode to be accelerated to a high velocity towards the target, which they strike, creating X-rays
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8
Q

What is the role of lead shielding?

A

X-rays are produced in all directions, so the tube is surrounded by lead shielding. This is to ensure the safety of the operators and recipients of the x-rays. An adjustable window allows a concentrated beam of x-rays to escape and be controlled safely

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9
Q

What is the role of the vacuum chamber?

A

Where the anode and cathode are stored. This is to ensure the electrons don’t collide with any particles in transit to the metal target

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10
Q

What are the 2 methods of x-ray production when electrons collide with the metal target?

A
  • Bremstahling
  • Characteristic radiation
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11
Q

What is the Bremstahling method to produce X-rays?

A
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12
Q

What is the maximum energy that an X-ray photon can have?

A

Equal to the electronvolt

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13
Q

What is the smallest possible wavelength an x-ray can have equal to?

A

The highest possible frequency and therefore the highest possible energy ; assuming that all the electrons KE has turned into EM radiation

Emax = eV = hc / λmin

f max = eV / h

λmin = hc / eV

where V = pd between anode and cathode

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14
Q

What is characteristic radiation method to produce x-rays?

A
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15
Q

What is x-ray attenuation?

A

The reduction in energy, or intensity, of a beam of x-rays due to the their interaction with matter

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16
Q

What are the 4 main ways x-rays can be attenuated?

A
  • Simple scattering
  • Photoelectric effect
  • Compton scattering
  • Pair production
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17
Q

How does simple scattering work?

A

A low energy x-ray photon encounters an electron in an atom causing it to be scattered without a change in energy

Photons are deflected from their initial by interaction of the materials atoms. There is no change in energy of the X-ray photon and no absorption of it. It causes blurring or noise in x-ray imaging as the scattered x-rays arrive at the detector from several angles

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18
Q

How does the photoelectric effect work (x-ray)?

A

An x-ray photon is absorbed by an inner shell electron, causing it to be ejected from the atom as a photoelectron

As a result, the x-ray photon is completely absorbed and all its energy is converted to the photoelectron

The same equation from quantum physics topic apply!!!!

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19
Q

What is the Compton effect?

A

An x-ray photon id deflected by an interaction with an orbital electron causing the wavelength of the photon to increase and the ejection of the electron from the atom at a higher speed

  • The x-ray is deflected from its initial path
  • The x-rays wavelength increases and energy decreases
  • The electron involved is ejected from the atom involved in the interaction (energy lost from photon does to ejecting it)

The electron and x-ray are deflected in different directions due to conservation of momentum

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20
Q

What is pair production (x-ray)?

A

A high-energy x-ray photon passes closes to the nucleus of an atom causing the production of an electron-positron pair

This can only occur with high energy x-rays. This is because the energy of the x-ray photon must be above a certain value to provide the total rest mass energy of the electron-positron pair

As a result of pair production, the X-ray photon is completely absorbed and all its energy is imported to the electron-positron pair

Equations from nuclear exist here!!!

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21
Q

What does this show?

A

Simple scattering

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22
Q

What does this show?

A

The photoelectric effect

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23
Q

What does this show?

A

Compton effect

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24
Q

What does this show?

A

Pair production

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25
Q

What do bones absorb?

A

X-ray radiation. This is why they appear white on X-ray photograph. When collimated beam of x-rays passes through the patient body, they are absorbed and scattered

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26
Q

How can the attenuation of x-rays be calculated?

A
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27
Q

What is the attenuation coefficient dependant on?

A

The energy of the x-ray photons

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28
Q

How does the intensity of the x-rays decay?

A

Exponentially

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29
Q

What is half thickness?

A

The thickness of the material that will reduce the x-ray beam or a particular frequency to half its original value

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30
Q

What is contrast media?

A

A substance, such as barium or iodine, which is a good absorber of x-rays. A patient is given so it can give a better contrast on an x-ray image.

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31
Q

Why is contrast media used sometimes?

A
  • Some soft tissue organs do not show up on the x-rays when the organ has a similar attenuation coefficient to the other tissues in the area
  • They are good absorbers of x-rays as they have a large attenuation coefficient
  • Hence when contrast media enter an organ, the image of the organ is enhanced when imaged using x-rays
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32
Q

When is iodine used as the contrast media?

A

In liquids eg. to observe blood flow
It is usually injected into the patient

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33
Q

When is barium sulfate used as contrast media?

A

In the digestive system - this is usually ingested by the patient

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34
Q

What is the large attenuation coefficient of contrast media due to?

A

The large atomic numbers of these elements

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35
Q

What is needed, over an x-ray, for a more comprehensive image?

A

A CAT scan (computerised axial tomography)

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36
Q

How does a CAT scan work?

A
  • An x-ray tube rotates around the stationary patient
  • A CAT scanner takes x-ray images of the same slice at many different angles
  • This process is repeated, then images of successive slices are combined together
  • A computer pieces that images together to build a 3D image
  • This 3D image can be rotated and viewed from different angles
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37
Q

What are the advantages of a CAT scan?

A
  • Produces more detailed images - the software can add colour and sharpen images, and parts can be edited out
  • Can distinguish between tissues with similar attenuation coefficients, giving a higher resolution image
  • Soft tissue and bone can be imaged in a single process
  • Produces a 3D image of the body by combining the image at each direction
  • No overlapping images - no bones obscuring organs
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38
Q

What are the disadvantages of CAT scans?

A
  • The patient receives a much higher does of norma x-ray
  • There are possible side effects from the contrast media
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39
Q

What are the consequences of x-rays being ionising?

A

They can cause damage to tissue and can potentially lead to cancerous mutations

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40
Q

How do healthcare professionals ensure x-rays cause as little damage as possible to the patient?

A

They make sure they receive the minimum dosage possible using aluminium filters

  • Many wavelengths are emitted
  • Longer wavelengths of x-ray are more penetrating, therefore, they are more likely to be absorbed by the body
  • This means they do not contribute to the image and pose more of a health hazard
  • The aluminium sheets absorbs these long wavelength x-rays making them safer
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41
Q

What is contrast?

A

The difference in degree of blackening between structures

42
Q

How can image contrast be improved?

A
  • Using the correct level of x-ray hardness ; hard x-rays for bones, soft x-rays for tissues
  • Using a contrast media
43
Q

What is sharpness?

A

How well defined the edges of structures are

44
Q

How can image sharpness be improved?

A
  • Using a narrow x-ray beam
  • Reducing x-ray scattering by using a collimator or lead grid
  • Smaller pixel size
45
Q

What is a radioactive tracer?

A

A radioactive substance that can be absorbed by tissue in order to study the the structure and function of organs in the body

46
Q

What are good examples of radioactive tracers?

A

Technetium-99 or fluorine-18

47
Q

Why are Technetium-99 and fluorine-18 good radioactive tracers?

A
  • They both bind to organic molecules, such as glucose or water, which are readily available in the body
  • They both emit gamma radiation and decay into stable isotopes
  • Technetium-99 has a short half-life of 6 hours
  • Fluorine-18 has an even shorter half-life of 110 minutes, so the patient is exposed to radiation for a shorter time
48
Q

What is a common tracer used in PET scanning?

A

A glucose molecule with radioactive fluorine attached called a flurodeoxyglucose. The fluorine undergoes beta plus decay, emitting a positron

49
Q

How does a radioactive tracer work?

A

It is injected or swallowed by the patient and flows around the body. Once the tissues and organs gave absorbed the tracer, then they appear on the screen as area for a diagnosis. This allows doctors to determine the progress of a disease and how effective any treatments have been.

50
Q

What are the purposes of radioactive tracers?

A

Diagnosis of cancer, also for the heart and detecting areas of decreased blood flow and brain injuries, including dementia

51
Q

How can a medical tracers progress around the body be detected?

A

Using a gamma camera

52
Q

What are the 4 main components of a gamma camera?

A
  • Collimator
  • Scintillator
  • Photomultiplier tubes
  • Computer & display
53
Q

What is the purpose of a collimator?

A

When gamma rays are emitted, they are absorbed by thin lead tubes known as collimators. They are the key to producing the sharpest and highest resolution images

  • Only photons moving parallel to the collimator will be absorbed, this improves the sharpness of the image as scattered photons are excluded
  • The narrower and longer the collimators, the more gamma rays that will be absorbed and hence the more electrons will be produced, this improves the image quality as more electrons contributing to the electrical pulse output will increase the resolution of the image
54
Q

What is the role of a scintillator?

A

When the gamma ray photon is incident on the crystal scintillator, an electron in the crystal is excited to a higher energy state. As the excited electrons travel through the crystals it excites more electrons. When the excited electrons move back down to their original state, the lost energy is transmitted as visible light photons.

It converts high energy gamma photons into visible light photons

55
Q

What is the role of the photomultiplier tube?

A

They amplify and convert the photons produced by the scintillator to an electrical signal

56
Q

How are gamma camera images formed on a computer?

A
  • The signals produced by the photomultiplier tubes are used to produce an energy using the electrical signals from the detectors
  • The tracers will emit lots of gamma rays simultaneously and the computers will use this to form an image
  • The more photons from a particular point, the more tracer that is present in the tissue being studied, and this will appear as a bight point on the image
  • An image of the tracer concentration in the tissue can be created by processing the arrival times of the gamma ray photons
57
Q

How do the chemical properties of technetium-99 allow it to be used in diagnosing issues?

A

Its chemical properties enables a small quantity to be incorporated into several tissues, so it can be used to image several organs at once

58
Q

What is positron emission tomography (PET)?

A

A type of nuclear medical procedure that images tissues and organs by measuring the metabolic activity of the cells of body tissues

59
Q

How does PET scanning work?

A

A beta-plus emitting radioactive tracer is used in order to stimulate positron-electron annihilation to produce gamma photons. There are the detected using a ring of gamma cameras

60
Q

What happens before a PET scan?

A

The patient is injected with a beta plus emitting isoptope, usually fluorine-18

61
Q

What happens during a PET scan?

A
  • The part of the body being scanned is surrounded by a ring of gamma camera
  • The positrons from the F-18 nuclei annihilate with electrons in the patient
  • The annihilation of a positron and an electrons produces 2 identical gamma photons travelling in opposite directions
  • The delay time between these 2 gamma ray photons is used to determine the location of the annihilation due to the F-18 tracer
  • Photons that do not arrive within a nanosecond on oner another are ignored since they cannot have come from the same part
62
Q

What happens after the PET scan?

A

The computer connected to the gamma cameras detect the signal and an image is formed by the computer

63
Q

In a PET scan, how far can a positron travel before it is annihilated by an electron?

A

Less than a millimetre - positron and electron are annihilated to form pure energy in the form of 2 gamma ray photons

64
Q

What are the energy and momentum for the gamma photons in PET scanning?

A

E = hf = mc^2

p = E/ c

65
Q

What are the positives about PET scanning?

A

The tracer has a short half life so it begins emitting positrons immediately. This allows for short exposure time to radiation for the pateient

66
Q

What are the negatives about PET scanning?

A

The short half life of the tracer means the patient must be scanned quickly and not all hosiptals have expensive PET scanners

67
Q

How does the image form on a computer for a PET scan?

A
  • The signals produced by the photomultiplier tubes are used to produce an image
  • The gamma rays travel in straight lines in opposite directions when formed from a positron-electron annihilation - this happens in order to conserve momentum
  • They hit the electrons in a line (known as the line of response)
  • The tracers will emit lots of gamma rays simultaneously, and the computers use this information to create an image
  • The more photons from a particular point, the more tracer that is present in the tissue being studied, and this will appear as bright point on this image
  • An image of the tracer concentration in the tissue can be created by processing the arrival time of the gamma ray photons
68
Q

What is ultrasound?

A

Sound waves with a frequency above the human hearing range of 20kHz

69
Q

What is the piezoelectric effect?

A

The ability of a particular material to generate a potential difference by transferring mechanical energy to electrical energy

70
Q

What is a transducer?

A

Any device that converts energy from one form to another

71
Q

What is in the centre of a piezoelectric transducer?

A

A piezoelectric crystal; a material which produces a p.d when they are deformed (compressed or stretched)

72
Q

What happens if a p.d. is applied to piezoelectric crystal?

A

It deforms (compresses) and if the p.d. is reversed, then it expands

73
Q

What happens if an alternating p.d is applied across a piezoelectric crystal?

A

The crystal will vibrate at the same frequency as the alternating current

74
Q

What can a piezoelectric crystal act as?

A

Both a receiver and transmitter of ultrasound

75
Q

What happens in a piezoelectric crystal when its receiving ultrasound?

A

Converts the sound waves into alternating p.d.

76
Q

What happens in a piezoelectric crystal when it transmits ultrasound?

A

It converts an alternating p.d into sound waves

77
Q

What is an ultrasound transducer?

A

Transducer which is made up of a piezoelectric crystal and electrode which produces an alternating p.d - it can receive and transmit ultrasound

78
Q

What is an A-scan?

A

A one-dimensional ultrasound scan used to determine the distance or depth of an internal structure

79
Q

How is an A-scan achieved?

A
  • Measuring the time delay between generating and receiving the signal
  • Using the speed of sound in the media to calculate the distance travelled by the signal
80
Q

What is an A-scan useful for?

A
  • Determining distances from the ultrasound device to the point of reflection - this type of scan only produces measurements, not an image
  • For example, the length of an eye needs to be determined in planning surgeries or assessing the presence of abnormalities, such as tumour
81
Q

What is a B-scan?

A

An ultrasound used to build up a 2 or dimensional image of an internal structure using a number of sensors or one sensor in different positions

82
Q

How is a B-scan achieved?

A
  • Using pulsed ultrasound waves in different positions to produce several measurements of time intervals between generating and receiving pulses
  • Moving the transducer over the patients skin, or using several transducer, to produce a series of A-scans that are combined to form an image
83
Q

What is a B-scan useful for?

A
  • Creating images of internal structures for diagnostic purposes
  • For example, bones, muscles and organs or checking on the progress of a fetus
84
Q

How do you achieve the clearest image from a B-scan?

A
  • Pulsed ultrasound waves are used to allow time for the reflected waves to be received and not interfere with transmitted waves
  • Smaller wavelengths are used to give more detailed images as they will allow the sound waves to diffract around finer points of detail on the internal structure being studied
85
Q

What is acoustic impedance?

A

The product of the speed of ultrasound in the medium and the density of the medium

86
Q

What is the equation for acoustic impedence?

A

Z = pc

Z = acoustic impedance, kgm^-2 s^-1
p = density of material kg m^-3
c = speed of sound in material ms^-1

87
Q

What happens at the boundary between media of different acoustic impedances?

A

Some of the wave energy is reflected and some is transmitted

88
Q

The greater the differences in acoustic impedance between 2 media, the _______ the reflection and the _______ the transmission

A

Greater, smaller

89
Q

Two materials with the same acoustic impedance would give ________________

A

No reflection

90
Q

What is the intensity reflection coefficient?

A

The ratio of the intensity of the reflected wave relative to the incident wave

91
Q

What is the equation for the intensity reflection coefficient, a?

A

a = Ir / Io = (z2 - z1)^2 / (z2 + z1)^2

Ir = intensity of reflected wave, Wm^-2
Io = intensity of incident wave, Wm^-
Z1 = acoustic impedance of one material
Z2 = acoustic impedance of second material

92
Q

What is impedance matching?

A

2 media have a similar acoustic impedance, resulting in little to no reflection of the ultrasound wave

93
Q

What is required when ultrasound is used in medical imaging?

A

A coupler between the transducer and the body

94
Q

Why is a coupler required in ultrasound imaging?

A

The soft tissue of body are much denser than air. If air is present between the transducer and the body then almost all of the ultrasound will be reflected

95
Q

What is placed between the transducer and the body in ultrasound imaging?

A

A coupling gel

96
Q

What do lower values of the intensity reflection coefficient mean?

A

The media are impedance matched so less reflection occurs

97
Q

What do higher values of the intensity reflection coefficient mean?

A

The media are not impedance matched, so more reflection occurs

98
Q

What is the doppler effect?

A

The frequency change of a wave due to the relative motion between a source and an observer

99
Q

In medicine, what can doppler imaging be used as?

A

A non-invasive technique to measure the speed of blood flow in the heart or in an artery. This is very effective because blood contains iron which is very reflective

100
Q

What is the movement of ultrasound when dopple imaging is being used?

A
  • Pulses of ultrasound are emitted from a transducer into a blood vessel
  • The ultrasound pulses are reflected by moving blood cells
  • The moving blood causes a shift in the frequency of an ultrasound
  • The transducer detects an increase in frequency if the blood is moving towards the transducer and a decrease if it is moving away
101
Q

What is the doppler imaging equation?

A

Δf / f0 = 2v cosθ / c

Δf = difference between emitted and recieved frequencies
f0 = frequency of ultrasound emitted by the transducer Hz
v = velocity of the blood
θ = angle between the transducer and the blood vessel

A-Level Physics (23 decks)

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