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Medical Physics Flashcards

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1
Q

How are X-rays produced?

A

When charged particles are rapidly decelerated (or accelerated), kinetic energy is transformed into high frequency photons.

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2
Q

What is the Piezoelectric Effect?

A

A material generates a voltage as it contracts and expands or will contract/ expand when a voltage is applied.

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3
Q

What is acoustic impedance?

A

the product of density of the material and speed of sound in that material

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4
Q

Does sound travel faster in denser or less dense mediums?

A

Denser

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5
Q

What do Piezoelectric materials tend to be made from?

A

Quartz, Polymeric or Ceramic materials

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6
Q

Which radioactive isotope gives out beta plus radiation, which can be used in PET scans?

A

Fluorine-18

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7
Q

What is the half life of flourine 18?

A

110 mins

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8
Q

Which element gives off gamma radiation as a radio isotope?

A

Technetium 99m

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9
Q

What is the half life of technetium-99 m?

A

67 hours

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10
Q

What device is Technetium 99-m used in?

A

Gamma Cameras

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11
Q

What role does a collimator play in a gamma camera?

A

Only rays moving directly upwards are detected by the detectors behyond this.

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12
Q

The Gamma Camera: What is a scintillator?

A

Crystal that gives out a small flash of visible light when hit by a gamma photon.

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13
Q

What is the job of a photomultiplier in a gamma camera?

A

Causes a cascade of electrons from a singular electron, producing a measurable current/voltage

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14
Q

Name the parts of the gamma camera.

A

Collimator, Scintillator, Light guide, Photomultiplier.

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15
Q

Why is the collimator in a gamma camera made of a high density material such a lead?

A

To ensure gamma radiation is absorbed.

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16
Q

What is a PET scanner?

A

A ring of gamma cameras placed around a patient so that a 3D image can be generated from the emission of gamma photons.

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17
Q

What are disadvantages of PET scans?

A

Expensive, requires tracers to be synthesised on site.

18
Q

In a pet scan how many photons are produced in each electron-positron annihilation?

A

2

19
Q

How is the site of a tracer estimated in a PET scan?

A

A pair of gamma photons are emitted from the electron-positron annihilation. These travel in opposite directions (conservation of momentum) so are detected at opposite sides of the ring. The speed of these photons is known allowing us to locate the annihilation event.

20
Q

How much energy is each gamma photon produced by beta plus and beta minus annihilation in a PET scan?

A

0.51 MeV

21
Q

Gamma Rays and X-rays may have the same frequency, how do we tell which a wave is?

A

Depends on how they are formed, X-rays formed through decelerating electrons, Gamma released from an atomic nucleus.

22
Q

In X-ray production electrons are accelerated from the cathode to the

A

anode

23
Q

In X-ray production electrons are accelerated in a vacuum towards a metal anode, what is an example of a metal the anode may be made from?

A

Tungsten

24
Q

In X-ray production when electrons hit the anode how do they loose their energy?

A

In the form of heat and as X-rays.

25
Q

How do you work out max energy of X-ray photons assuming no energy released as heat?

A

eV=hf

26
Q

What is the most common way X-rays interact with matter?

A

The photoelectric effect

27
Q

Describe the Compton Effect.

A

High energy photon bounces off an electron, this changes the direction and energy of the photon (lower energy X-ray photons are emitted)

28
Q

What kind of voltage is needed for pair production?

A

Above 1 million volts.

29
Q

What is pair production?

A

An electron-positron pair is spontaneously produced via mass-energy relation.

30
Q

Describe simple scattering

A

X-rays reflect off layers of atoms when they do not have enough energy for more complex processes.

31
Q

What are the four attenuation mechanisms?

A

Simple Scattering, Photoelectric Effect, Compton effect, Pair Production.

32
Q

What are contrast media? (Used in X-ray imaging)

A

High attenuation materials with large proton numbers. (Far better at absorbing X-rays than soft tissues).

33
Q

Give two examples of contrast media used in X-ray imaging.

A

Iodine - Used in liquids (Eg. observing blood flow).
Barium Sulphate - Used in the digestive system.

34
Q

For the photoelectric effect, the attenuation coefficient is proportional to the proton number…

A

cubed.

35
Q

What relationship does the intensity of a collimated beam have with thickness?

A

Decreases exponentially.
(I= Io e^ux)

36
Q

What is an ultrasound B scan?

A

A series of A scans stitched together to form a 2D image, accomplished by moving the transducer over the skin.

37
Q

With materials with similar acoustic impedances a small amount of the sound is…

A

reflected.

38
Q

What is the Doppler technique used for?

A

Measuring the speed of blood flow (aspirin can reflect waves back to the transducer).

39
Q

What is the Doppler effect?

A

Changing frequency of a wave due to relative motion of the observer and emitter,

40
Q

Describe the principles of PET scanning.

A

Body surrounded by a ring of gamma cameras. F-18 decays releasing positrons which annihilate electrons Annihilation of a positron and electron releases gamma rays travelling in opposite directions. Delay time between gamma rays is used to determine the location of F-18. Computer is connected to detectors an image is formed from these.

41
Q

How would you define attenuation?

A

The decrease in intensity of EM radiation as it travels through matter/space

42
Q

What are the important diagnostic or therapeutic uses of X rays?

A

Diagnosis of blood flow issues, digestive system imaging, killing cancerous cells

Physics (20 decks)

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