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Physics A2 > Medical > Flashcards

Medical Flashcards

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1
Q

How does the magnet in an MR scanner become superconducting?

A

It is cooled with liquid helium to below its transitional temperature

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2
Q

What type of field in produced in an MR scanner?

A

A uniform magnetic field

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3
Q

Describe and explain how protons in the body made to ‘wobble’ when a patient is in an MR scanner

What is the scientific term for ‘wobble’?

A
  1. Initially, protons in body are randomly orientated and have spin
  2. Uniform magnetic field is applied, protons align parallel or antiparallel to magnetic field lines and the protons precess

Precession

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4
Q

What is precession frequency proportional to?

A

Magnetic field strength

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5
Q

What is the purpose of gradient field coils in an MR scanner?

A

Gradient field coils are superimposed perpendicular to the main magnetic field to create a gradient of magnetic field strength across the patient. Different sections have different precession frequencies

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6
Q

How is a signal detected and produced in an MR scanner? What is produced?

(4 points)

A
  1. Short radio frequency pulses are incident on parallel protons
  2. The RF pulses cause proton, causing a change of spin state
  3. Protons de-excite, reverting to original spin state and emit an RF signal
  4. Signals are detected which are processed to produce a 3D visual image?
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7
Q

What are rapid pulses of radio frequency in an MR scanner used for?

A

Detecting large molecules

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8
Q

What are slow pulses in of radio frequency in an MR scanner used for?

A

Detecting watery substances (diseased area)

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9
Q

Advantages of an MR scan

3 points

A

No known side effects

Produces non-ionizing radiation

Higher quality image and resolution for soft tissue than CT and ultrasound

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10
Q

Disadvantages of MR scanner

3 points

A

Poor imaging of bones compared to CT

Noisy scanner and takes a long time to scan

Scanners are narrow - sufferers of claustrophobia may struggle to use it

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11
Q

Describe how a rotating anode X-ray tube works

A

Electrons are emitted from a heated filament with a high current across the filament
Electrons are accelerated through a high voltage across the evacuated glass tube at a rotating tungsten anode with a bevelled edge
Characteristic and continuous X-ray emission occur

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12
Q

Why does the rotating tungsten anode have a bevelled edge?

A

It gives it a larger target area, but a smaller source area so sharpness increases

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13
Q

Why is the glass tube evacuated?

Why is there a low voltage across the filament?

A

Vacuum so electrons do not collide with air particles

So there is a high current across the filament generating heat for thermionic emission

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14
Q

How is a continuous spectrum formed?

A

Electrons undergo slingshot effect when passing a positive nucleus
When it passes, due to electrostatic attraction, it is decelerated
Loses Ke, emitting an X-ray photon

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15
Q

How is characteristic emission produced?

A

Beam of electrons collides with inner electrons, knocks it off
Outer electron de-excites to lower energy level subshell
Emits X-ray photon of fixed energies because electrons are at fixed energy levels

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16
Q

How is overheating avoided?

A

Tungsten anode rotates at 3000rpm
Tungsten is mounted on to copper which conducts heat
Focal point kept above a certain size

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17
Q

Why is the tungsten anode rotating?

Not overheating

A

To spread heat over a greater area which allows for more energetic x-rays to be produced

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18
Q

What is the relevance of spikes on the continuous spectra?

A

Characteristic line spectra is super imposed on to continuous
Spikes are specific to anode elements and occur because of energy level transitions

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19
Q

How is image sharpness improved?

3 points

A

Increase distance between anode and object
Decrease distance between object and screen
Decrease width of focal point by decreasing slope of anode

20
Q

How do you improve image contrast (with explanations)

2 points

A

Decrease tube voltage - lowers energy of x rays. Attenuation of X ray with lower energy is affected more by changes in tissue thickness however do not decrease too low or x rays do not pass through object
Use a lead collimator between patient and film to stop scattering radiation from fogging image

21
Q

Define intensity

A

Energy per second per unit area passing through a surface

22
Q

What happens to the electrons and x ray photons when the tube voltage is increased?

A

Electrons have more kinetic energy and electrons are knocked out deeper within tungsten atoms so more spikes are on the spectra

X rays photons have higher max energies because Intensity is directly proportional to V^2

23
Q

What happens, in terms of electrons and photons, when the current supplied to the filament is increased?

A

More electrons are liberated per second so more x ray photons are produced per second
Individual photons have the same energy
Intensity increases (Intensity is prop to current)

24
Q

How do you reduce patient dosage when using a rotating anode x ray tube?

Why does it reduce patient dosage? (/why advantageous?)

A

Aluminum filter between x ray tube and patient

Decreases intensity of low energy photons
Hardly changes intensity of high energy photons
Reducing low energy photons reduces dose received by patient

25
Q

Describe and explain how a flat panel detector works

A
  1. X rays fired at patient. FTP detector behind them
  2. scintillator material produces light when hit by x rays. Light intensity is direct prop to energy of incident x ray photons
  3. Photodiode pixels generate V from incident scintillator light > V is prop to light intensity
  4. Each pixel has thin film transistor which reads digital image from generated V
26
Q

Why is using a FTP detector advantageous?

A

High resolution
Less image distortion
Is lightweight and compact
X ray image can be copied, stored or changed

27
Q

Describe and explain how fluoroscopy works to produce a fluorescent viewing screen

A

X rays hit fluorescent screen which emits scintillated light
Light caused electrons to be emitted from photocathode
Electrons are accelerated by a high voltage and gain Ke whilst focusing on viewing screen

28
Q

What is the 5 part structure of an intensifying screen?

A

Plastic cover

Front intensifying screen

Double side film

Back intensity screen

Metal back

29
Q

Why is an intensifying screen advantageous? (4 points)

A

Contains crystals that fluoresce so photographic image develops quickly
I-screen is close to film so little image quality is lost
Shorter exposure time needed
Reduces patient radiation dose

30
Q

Define linear attentiation coefficient

Unit?

A

The fraction of x rays removed per unit thickness of a material for x rays of specific energy

m-1

31
Q

Define half value thickness

A

The thickness of a material required to reduce the intensity to half of its original value for x rays of specific energy

32
Q

Define mass attenuation coefficient

Unit?

A

The amount of radiation absorbed per unit mass for x rays of specific energy

m^2 kg^-1

33
Q

What is the cause of x ray attenuation

A

Scattering or absorption of x rays

34
Q

What is used to differentiate between areas with similar attenuation coefficients?

Give an example of this utility

A

Artificial contrast media

Barium meal. It has a high Ar so it is an X-ray opaque material

35
Q

How does a CT scan work?

4 steps

A

X ray tube produces a narrow, monochromatic x ray beam using lead collimators
Tube rotates around the body
X rays are picked up by detector
Detector feeds signal to computer

36
Q

What are the advantages of CT?

A

Good resolution and clear imaging of bones
Cheaper than MRI
Quicker than MRI

37
Q

What are the disadvantages of CT?

A

Ionizing radiation damages cells and produces cancer
Investigating soft tissue with fluoroscopy results in extra dosage of radiation
Not suitable for pregnant women
Patients have to lie still

38
Q

What is a medical tracer?

A

A gamma emitting radioactive isotope used to show tissue and organ function

39
Q

Why are the following used as medical tracers:

Technetium - 99m
Indium-111
Iodine-131

A

Has a long enough half life to be detected but short enough that patients exposure time is small

Labels antibodies and red blood cells to find infections

Detects problems in thyroid because thyroid naturally uses iodine

40
Q

What is the molybdenum-technetium generator used for? Why?

A

Transportation of technetium because molybdenum had a half life of 66 hours whereas technetium has a half life of 6 hours

41
Q

What is the molybdenum combined with?

A

Strongly bonded aluminum oxide which decays into technetium 99m

42
Q

How is technetium-99m washed out of the m-t generator?

A

Saline solution

43
Q

Describe and explain how a PET scan work

A

Patient is injected with substance used by body + a positron emitting tracer with a short half life
Radio tracer travels through body to organs
Electron + positron collision annihilation occurs emitting a high energy gamma photon
Gamma ray detected and computer builds image

44
Q

Definition of biological half life

A

The time taken for the body to eliminate half of the radioactive isotope through excretion or metabolism

45
Q

Describe and explain how a gamma camera works

A
  1. Gamma rays which are parallel to lead collimators enter gamma camera
  2. Fluorescent crystals emits a flash of scintillated light when gamma ray hits it
  3. Photomultiplier tubes contain photocathodes which release an electron via photoelectric effect due to incident light
  4. Electron is converted to a cascade of electrons, so the flash of light is converted to an electricity pulse
  5. Electronic circuit collects signals and sends to PC to process image
46
Q

Explain why the effective half life of a radionuclide in a biological system is always less than the physical half life

A

The physical half life depends only on the properties of the radionuclide

Biological removal of the nuclide occurs

Physics A2 (11 decks)

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