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A2 Level - Physics > Chapter 27 - Medical Imaging > Flashcards

Chapter 27 - Medical Imaging Flashcards

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1
Q

What does the cathode filament do in an X-Ray Tube

A

It is made up of a high melting point, and the electrons are boiled off of it.
This is called thermionic emission.
The more the current, the more the electrons per unit time, and the more X-Rays are created.

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2
Q

Use of the vacuum chamber in an X-Ray Tube

A

Removes all obstructions to the electrons, because the electrons are scattered by the nuclei of other air molecules, which decreases the efficiency, and there must be a minimal number of atoms in the tube.

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3
Q

Use of the anode or target in an X-Ray Tube

A

The anode is positively charged and it attracts the electrons.
The kinetic energy of the electrons turns into the X-Ray photon and heat.
So, the anode is rotated and there might be extra cooling as well to distribute the heat.

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4
Q

Use of the lead shielding in an X-Ray Tube

A

Contains any stray electrons or X-Ray photons.

Lead has a high electron density cloud.

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5
Q

Use of the filter in an X-Ray Tube

A
  • The anode is pointed towards the filter/ window.

- The filter absorbs the low energy photons

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6
Q

Why does the filter absorb low energy photons

A

Because they just add an extra dose of X-Rays, but no work is done in treating the actual patient.

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7
Q

How are K lines produced in an X-Ray spectrum for any material

A
  • The electrons can remove electrons in the metal’s atoms, close to the nuclei
  • So, the gaps created in the lower energy levels are quickly filled by electrons dropping from higher energy levels.
  • Transitions release photons of a specific amount of energy, and hence a corresponding wavelength as well.
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8
Q

Define attenuation

A

The decrease in intensity of an electromagnetic radiation as it passes through matter.
Ex: bones attenuate X-Rays more than soft tissues

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9
Q

What are the 4 attenuation mechanisms

A

Simple scatter
Photoelectric effect
Compton scattering
Pair production

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10
Q

Attenuation mechanisms: Simple scatter

A

For X-Ray photons with energy in the range of 1-20keV.
The incident X-Ray photon approaches the electron without energy to remove it, so it just bounces off without any change to its energy.
This type is insignificant in hospitals

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11
Q

Attenuation mechanisms: Photoelectric effect

A

For X-Ray photons with energy less than 100keV.
X-Ray photon is completely absorbed by the orbital electron, causing it to leave the atom.
It is used in hospitals and X-Ray machines.

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12
Q

Attenuation mechanisms: Compton Scattering

A

Energy range of photons 0.5-5MeV.
The electron is ejected from the atom and the X-Ray photon is scattered with reduced energy.
Energy and momentum are conserved.

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13
Q

Attenuation mechanisms: Pair production

A

Only occurs when the X-Ray photons have equal to or more energy than 1.2MeV.
An X-ray photon interacts with the nucleus of the atom.
It then produces an electron and positron pair.
So, hf= 2m.
Where m = Mass of electron.

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14
Q

Attenuation coefficients equation

A

I = I0 e ^-μx.
x - thickness of the substance.
μ - attenuation coefficient

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15
Q

Why are contrast mediums used

A

Because soft tissues have low absorption coefficients, so contrast mediums are required.
Most common are iodine and barium. The contrast medium help by increasing the attenuation coefficient of the soft tissue.

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16
Q

Contrast mediums: Iodine

A

Used in liquids to view blood flow.

It is injected into the blood vessels.

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17
Q

Contrast mediums: Barium

A

Image digestive systems

Given as a barium meal to swallow.

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18
Q

Contrast mediums: Therapeutic use

A

Used for Linacs, to kill of cancerous cells.

By Compton scattering and pair production.

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19
Q

What does CAT stand for

A

Computerised Axial Tomography

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20
Q

CAT

A

Takes many cross-sectional X-Rays from different angles and assembles.
“Axial” images takin in the axial plane from cross-sections through the patient.

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21
Q

Advantages of CAT

A
  • 3D Scans - which helps to assess sizes, shapes and positions of tumors, leading to a better diagnosis.
  • Distinguish between soft tissues of similar attenuation coefficients
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22
Q

Disadvantages of CAT

A
  • Can expose patients to a lot of radiation.
  • Patient have to remain still
  • More expensive than X-Ray scanners.
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23
Q

Define Brachytherapy

A

Treating cancer by inserting radioactive material directly into the tumour.

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24
Q

What does a gamma camera do

A

Detects gamma photons emitted from a radioactive tracer injected into the patient.

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25
Q

Advantages of Gamma emitting source for medical imaging

A
  • Least ionising
  • Can penetrate through the patient well.
  • Have a short half life.
  • The patient isn’t subjected to a high dosage of radiation that continues after the procedure.
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26
Q

Why is Technetium-99m a good radioisotope

A

It is extremely versatile isotope and it made from molybdenum-99.

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27
Q

How does molybdenum-99 turn into technetium-99m?

A

Mo-99 isotope decays by beta minus decay with a half life of 67 hours.
Tc-99m is a daughter nucleus and it is too unstable.
It then loses its energy by emitting a gamma photon of exactly 140keV, with a half life of 6 hours.
It leaves Tc-99, which is extremely stable with a half-life of 210,000 years.

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28
Q

What does the m stand for in Tc-99m?

A

Metastable and stays in a state of extremely high energy.

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29
Q

What does the Tc-99 decay into?

A

It undergoes beta minus decay to form a 99 Ry 42.

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30
Q

Define a radioactive tracer

A

A radioactive tracer is a chemical compound, in which 1 or more atoms have been replaced with a radioisotope.

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31
Q

How does a gamma camera work

A
  1. The gamma photons travel to the collimator. Only the photons travelling in the same axis as the tubes reach the scintillator.
  2. 1 single gamma photon striking the scintillator produces thousands of photons of visible light.
  3. The photons of visible light travel through the light guide into the photomultiplier tubes. 1 photon of light entering a tube is converted into a electrical pulse.
  4. The pulse travels to a computer and Software is used to convert those pulses into an image.
32
Q

Define a collimator

A

A honeycomb of long, thin tubes made from lead.

33
Q

How are the photomultiplier tubes arranged

A

In a hexagonal tube.

34
Q

What is the scintillator made of

A

Often sodium iodide.

35
Q

Information about fluorine-18

A
  • Versatile medical tracer
  • Positron emitter with a half life of 110 mins.
  • Made on-site or a specialist lab.
36
Q

How does fluorine decay

A

Into an 18 Oxygen atom, a positron, neutrino and a gamma photon.

37
Q

How can fluorine- 18 be made

A

Collide high speed protons with oxygen-18 nuclei and you form a fluorine-18 and a neutron

38
Q

FDG

A

Used in PET Scans.
It is like naturally occurring glucose, but tagged with a radioactive isotope Fluorine-18 rather than an oxygen atom.
Monitored using a gamma detector..

39
Q

Carbon monoxide

A

Used in PET Scans
Made using the C-11 isotope.
Has a half-life of 20 minutes
Is good at clinging onto the haemoglobin molecule.

40
Q

How does a PET Scan work

A
  1. Patient is injected with FDG
  2. PET Scanners detect gamma photons emitted when the positron from decaying fluorine-18 nuclei annihilate with the electrons in the patient.
  3. Annihilation produces 2 gamma photons. A ring of detectors connected to a computer registers a positron emission when the opposite detectors detect a gamma photon.
  4. Computer can detect the position of annihilation.
  5. Produce image.
41
Q

Advantages of PET Scans

A
  • Non- invasive
  • Diagnose cancers
  • Onset of disorders in the brain
  • Assess the effect of drugs on organs
42
Q

Disadvantages of PET Scans

A
  • Expensive
  • Poor resolution images (in comparison to CT Scans)
  • Safety- Gamma radiation poses possible risks to health.
43
Q

Frequency of ultrasound

A

Higher than 20KHz.

Because humans can only hear from 20Hz- 20kHz.

44
Q

Advantages of ultrasound scans

A
  • Non-ionising
  • Harmless
  • Non-invasive
  • Quick
45
Q

An ultrasound transducer

A

A device used to generate and receive ultrasound. It changes electrical energy into ultrasound, and vice versa using the piezoelectric effect

46
Q

Piezoelectric effect

A

It was discovered that some crystals can produce EMF when they are distorted.
Then, when you apply an EMF on them, they can stretch and compress.

47
Q

Ultrasound transducers

To emit ultrasound

A
  1. A high frequency alternating p.d is applied across the crystals.
  2. It repeatedly expands and compresses the crystals
  3. The frequency is the same as the natural frequency of the crystals, so that they resonate and produce oscillations of intense ultrasounds signal.
  4. It emits pulses of ultrasound (frequency of 20kHz)
48
Q

Ultrasound transducers

To detect ultrasound

A
  1. Any ultrasound incident on the crystal will vibrate, so the crystals is compressed and expanded.
  2. This vibration generates an AC EMF, which is detected by their circuits.
49
Q

Material for piezoelectric effect

A
  • Lead zirconate titanate.
  • Polyvinylidene fluoride.
  • Quartz
50
Q

A-Scans

A
  • Used to record along a straight line through the patient.
  • Each pulse is transmitted back and the return one will have less energy than the original, as it loses energy between the boundaries.
  • Produces a voltage-time graph.
  • Time is the time to travel there and back.
51
Q

B- Scans

A
  • A 2D scans
  • Transducer is moved and for each position of transducer, the computer produces a row and dots on a screen.
  • Each dot= 1 boundary between 2 tissues.
  • Brightens of dot is proportional to the intensity of the reflected ultrasound.
52
Q

Define acoustic impedance

A

The product of density of a substance and the speed c of the ultrasounds in that substances.

53
Q

SI units for acoustic impedance

A

kg m-2 s-1

54
Q

What is the intensity reflection coefficient

A

Intensity of the reflected / intensity of the incident.

55
Q

For normal Incidence:

Ir/Io formula

A

(Z2-Z1)^2 / (Z2+Z1)^2

56
Q

Acoustic impedance- when is the highest amount of reflection

A

When the acoustic impedances are very different.

57
Q

acoustic impedance- coupling gel

A

There are often air pockets between the transducer and the skin.
So, coupling gel is used to reduce the amount of waves reflected and the acoustic impedance of the gel is similar to the Ultrasound.

58
Q

Impedance matching

A

When 2 substances have similar values of acoustic impedance to reduce the amount of reflection that occurs.

59
Q

Advantages of doppler imaging

A

It’s non-invasive

60
Q

What is doppler imaging used for

A

To reveal blood clots, narrowing of walls, accumulation of fatty deposits.

61
Q

How is Doppler Ultrasounds used

A

Transducer is pressed over the skin and it’ll send and receive pulses.
So, the waves that are reflected off of tissues have the same frequency and wavelength, but those that reflect off of moving blood cells change in frequency.

62
Q

Doppler Ultrasound equation

A

Δf= (2fv cosθ) /c

63
Q

In an X-Ray tube, how is the heat distributed

A
  1. You have the rotor to spin the anode.
  2. You add oil to absorb the heat.
  3. You have the anode slanted, so the electron actually collides with more area, increasing heat distribution.
64
Q

What is CCD

A

A charged couple device. It is placed underneath the organ for the X-Rays to land on (used instead of photographic plates)

65
Q

Advantages of CCD

A
  • Portability

- And easier to store than photographic plates.

66
Q

X-Ray tube description

A

The electrons are boiled off of the filament and they then accelerate towards the anode. When they hit the target, the electrons interact with the atoms of the tungsten target and are decelerated. So, the electrons lose their energy as heat (99% of it) and X-Rays (1%)

67
Q

Exact steps for how does a CAT Scanner work?

A
  1. Patient lies on a stationary bed.
  2. The X-Ray tube moves around the ring, directing a thin, fan shaped beam of X-Rays in a short pulse towards the patient.
  3. Detectors opposite the X-Ray tube detect signals, which are fed into a computer.
  4. 1 Revolution of the X-Ray tube, all the signals from the detectors are added together producing 1 2D Slice.
  5. The bed is slowly moved through the scanner as the X-ray tube rotates, allowing a new cross-sectional image to be made.
  6. All the slices are put together by the computer, which produces a 3D image.
68
Q

Photomultiplier tubes - and then explain how they work

A

They are extremely sensitive detectors of light.

  1. A single visible photon travels from the scintillator, is incident upon a photocathode and produces 1 single electron.
  2. The emitted electron is accelerated towards a dynode, strikes it and produces a number of secondary electrons.
  3. The secondary electrons are accelerated towards a subsequent dynode held at a higher potential, giving rise to the emission of tertiary electrons.
  4. This process is repeated with several dynodes, creating a measurable voltage at the final anode.
69
Q

How do the dynodes produces lots of electrons from 1 single photon

A

Because the first electrons knocks some of their energy to the electrons in the dynode and so they leave, and become even more electrons.
Because this is interaction between electron-electron, they can share the energy amongst them and don’t have 1 electron means t emission of only 1 other electron.

70
Q

How are radioactive tracers made

A
  1. Cyclotrons are used to bombard stable nuclei with protons/ neutrons, forming radioisotopes.
  2. The radioisotopes are attached to a specific biologically active compound by a chemical reaction.
  3. Radioactive tracer is absorbed by specific tissues/ organs
71
Q

What are the properties of Radioactive tracers?

A
  1. Gamma emitters (as they are the most penetrating and the least ionising)
  2. Short half life
    (This leads to high activity, so that a small dose can be given. And safety and convenience, so that the patient doesn’t have to be kept too long)
72
Q

Explain how an X-Ray machine works

A

A small current is run through the filament, and then electrons are emitted through thermionic emission.. Since the tube is evacuated, a large p.d exists between the filament and the tungsten target.
Upon striking the target, the electrons interact with the atoms or particles of the tungsten target and they are decelerated. Some of the energy of the electrons is manifested as heat, and the other as X-Ray photons.

73
Q

X- Ray machines - How can you get the shortest wavelength of X-Ray photon produced?

A

The shortest wavelength of X-Ray produced can be calculated by assuming all the energy goes into producing an X-Ray photon.

74
Q

How does a CAT Scanner work?

A

The patient lies on the bed. The X-Ray tube moves around the ring, directing a thin, fan shaped beam of X-Rays in a short pulse towards the patient. The detectors opposite the X-Ray tub detect the signal. Following a complete revolution of the X-Ray tube, signals from all the detectors are added together, producing a single 2D image. The bed is slowly moved in the scanner, as the X-Ray tube rotates, allowing a new cross-sectional image in an adjacent plane to be made.
The computer then puts everything together.

75
Q

Doppler Ultrasound: Calculating the speed of blood flow

A

The probe must be held at an angle to the skin, so that the Ultrasound waves have a component in the same plane as the blood flow’s velocity.

A2 Level - Physics (11 decks)

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