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Physics A LEVEL (NEW) > Medical Physics > Flashcards

Medical Physics Flashcards

1
Q

Fundamental things to know about X - rays?

A

-high energy electromagnetic (em) waves.
- ionising
- produced by decelerating electrons.
-produced when electrons drop to inner energy levels of atoms.
-tend to travel in straight lines.
-absorbed by dense matter or high atomic number elements
-tend to pass through soft tissue.
- can be detected by photographic plates/film.

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2
Q

How is the x-ray spectrum produced?

A

-electrons fired at a metal target
-some of the electrons in the metal are displaced by the fired electrons
-Electrons from higher energy levels drop down to the gaps left.
-These falling electrons produce X-rays of very specific wavelengths.

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3
Q

How to calculate maximum energy of a photon from X-ray?

A

-hfmax = eV
- hc/ λmin = eV

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4
Q

How to increase intensity in X-ray?

A

-more incident electrons (higher current, by increasing temp)
means a higher intensity (as more incident photons) but the pattern will remain the same
- or increase PD

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5
Q

How to increase photon energy in an X-ray?

A
  • increasing PD
  • using equation hfmax = eV
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6
Q

What does the interaction of X-ray energy to matter depend on?

A
  • x -ray energy and atomic number of material
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7
Q

When and how does photoelectric effect happen in X-ray?

A

-occurs with lower energy x-rays and high atomic numbers
-most ionising
-creates clearest images
-orbital electron is ejected, a photoelectron
-and the photon is absorbed

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8
Q

When and how does pair-production happen in X-ray?

A

-occurs with high energy X-rays (>1.02 MeV)
-happens only close to nucleus
-photon produces an electron-positron pair (E=mc^2)

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8
Q

What effect does X-rays have on living tissue?

A

-ionizing can damage DNA
-lead to mutations in living cells (Cause cancer)

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9
Q

Why are X-rays used on cancer?

A

-High energy X-rays are used to treat cancer as they are ionising and can penetrate the tissues to affect the DNA in the cancer cells
-To reduce the effect on healthy cells, the X-rays are focussed into beams from different
directions.

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10
Q

Why is lower energy used in X-rays in diagnosis?

A

-The attenuation of the X-rays depends on the density of electrons in a material
-they do not penetrate bone as easily as tissue and
the difference in attenuation can be used to create an image

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10
Q

What is X-ray beam intensity?

A

-the X-ray energy per unit area, per unit time, passing
normally through a surface

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11
Q

Why is increasing intensity of X-ray inneficient?

A
  • most of input energy ends up as wasted heat
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12
Q

X ray intensity decrease?

A

I/Io x 100

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13
Q

X- ray absorb percentage

A

1- (I/Io x 100)

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14
Q

Why is fluoroscopy used?

A

-provides moving images of the patient’s innards that a surgeon uses as she carries out an operation

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15
Q

-What is contrast media?

A

-substances introduced into parts of the body to give
greater contrast between them and surrounding areas. Usually
they have high attenuation constants

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16
Q

Examples of contrast media?

A
  • barium meals - stomach and intestine
    -inject iodine - blood flow
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17
Q

Types of image intesification for X-rays?

A

-X-ray cassette
-fluoroscopy

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18
Q

How does an X-ray cassette work?

A

-can be perceived as a sandwich. -two outer layers are scintillators and the centre is the photographic plate
-Most of the X-rays pass straight through the first scintillator
- we can almost double the exposure by placing a second scintillator on the opposite side of the plate
-Image intensification means that the X-ray dosage for an X-ray is reduced depending on the resolution required in the image (thicker scintillators provide more intensification but they also blur the image).

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19
Q

Problem with thicker scintillators?

A
  • more intensification but also blur the image
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20
Q

What is image intesification?

A
  • using crystals called scintillators to absorb X-rays
  • scintillators then convert the energy of one X-ray into multiple, visible photons.
  • a lot easier to detect these visible photons than it is to detect one X-ray photon - hence, the image is said to be intensified.
  • Image intensification can reduce X-ray dosages
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21
Q

What is a CT scanner?

A

-an X-ray image that utilises a fan shaped beam opposite a line of digital detectors. The beam and detectors are rotated about the patient in a helical pattern. In effect, this takes multiple “slices” of the patient to produce a 3D image.
-CT stands for computed tomography.

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22
Q

Advantages a CT scanner has over a X-ray?

A

-produces 3D images.
-produces better soft tissue contrast.

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23
Q

Disadvantages a CT scanner has over an X-ray?

A
  • higher exposure to radiation
    -It is more expensive and can take longer for a detailed scan
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24
Q

What is a Piezoelectric transducer?

A
  • a piezoelectric crystal used either to generate ultrasound when an alternating voltage of ultrasonic frequency is applied, or to sense ultrasound by producing an alternating voltage in response.
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25
Q

What is a Piezoelectric crystal?

A

-One of several types of crystal which deform when a voltage is
applied between two of their faces, and which, conversely,
produce a voltage when deformed.

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26
Q

How to calculate intensity of X-ray?

A

-power/ cross sectional area

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27
Q

How is an ultrasound generated and detected using piezoelectric crystals ?

A
  • By sending a high frequency alternating pd to the electrodes
    -the crystal will deform at SAME frequency producing ultrasound waves of required frequency.
    -As ultrasound is reflected when it meets a boundary between 2 materials, the reflected waves go back to PIEZOELECTRIC crystal, the sound waves deform the crystal and the piezoelectric effect mean an alternating pd will be detected across the electrodes and used to create an image
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28
Q

How does the piezoelectric effect work?

A

-Piezoelectric crystals produce a pd when put under strain however they also undergo the reverse piezoelectric effect - they deform when a pd is applied

29
Q

What is an A-scan?

A

-Scan in which the strength of reflections of an ultrasound pulse from interfaces in the body is shown by the amplitude of a trace

30
Q

What is a B-scan?

A

Scan in which the strength of reflection of an ultrasound pulse from interfaces in the body is shown by the brightness of a trace
An array of transducers can produce a two dimensional picture

31
Q

How to calculate acoustic impedance (Z)?

A

-cp
- c = speed of ultrasound in tissue ms^-1
- p = density of tissue

32
Q

Acoustic impendence units?

A
  • kgm^-2 s^-1
33
Q

Equation for ratio of inflected intensity (z1 is ultrasound)

A

Ir/ Io = (z 2 - z 1)^2 /
(z 2 + z 1 )^2

34
Q

A scan examples and use?

A
  • used to accurately measure distance
  • scan the eye for a detached retina or to check the thickness of the lens before an operation
  • a tumour would change expected length
  • one dimensional information not used for imaging
35
Q

B scan examples and use?

A
  • imaging foetuses
36
Q

What is a coupling medium and why is it used?

A

-Gel or oil used to exclude air between the skin and the ultrasound transducer. It reduces the mismatch in Z, and enables more ultrasound to enter the body instead of being reflected off the skin

37
Q

Doppler effect (two equations)?

A

-Δf/ fo = 2v cosθ /c
​- -Δλ/λo = 2v cos θ/ c

c- speed of ultrasound
v- speed of blood flow

38
Q

Precession of spin of proton?

A

Protons have ‘spin’ and behave like tiny magnets. In a strong magnetic field
-their spins wobble or precess around the magnetic
field direction
There is a natural frequency of wobble (the Larmor
frequency).

39
Q

Magnetic resonance of protons

A

-the magnetic resonance of protons, is the strong absorption of radio waves of the Larmor frequency by protons in a magnetic field. It results in the spin- flipping direction.

40
Q

Relaxation time of spin- flipped protons

A
  • is a characteristic time for spin-flipped protons in hydrogen atoms in a magnetic field to return to their original spin orientation when the radio frequency is removed.
  • It depends on, and so tells us about, the tissue which contains the hydrogen atoms.
41
Q

How does an MRI work?

A

-Protons have ‘spin’ and behave like tiny magnets. In a strong magnetic field their spins wobble or precess around the magnetic field direction
-There is a natural frequency of wobble (the Larmor frequency).
- the magnetic resonance of protons, is the strong absorption of radio waves of the Larmor frequency by protons in a magnetic field. It results in the spin-flipping direction.
-Relaxation time of spin- flipped
protons is a characteristic time for spin-flipped protons in hydrogen atoms in a magnetic field to return to their original spin orientation when the radio frequency is removed. It depends on, and so tells us about, the tissue which contains the hydrogen atoms.

42
Q

How to calculate Lamor Frequency (f)?

A

-f = B× 42.6 x 10^6

43
Q

How to produce MRI IMAGES?

A

-we can image one slice at a time.
-done by having a B-field that varies from head to toe
- As the B-field varies from head to toe, we can “tune in” to different slices of the body by varying the frequency of the radio waves
-Only the slice with the correct resonance frequency will respond to the radio waves and only that slice will be imaged

44
Q

What is PET?

A

-Positron emission tomography
-The positrons annihilate electrons in adjacent tissue and send out pairs of gamma photons which are detected outside the body.

45
Q

Process of PET scan?

A
  • patient is injected with a positron-emitting (B+ decaying) radionuclide attached to a substance used by the region of the body under investigation
  • radionuclide will be absorbed and broken down, releasing positrons which will collide with electrons in the body causing them to be annihilated
  • releasing 2 high energy gamma rays moving in opposite directions, recorded by detectors and creates an image of radioactivity in that region
46
Q

How to calculate energy of photon/gamma rays in an PET scan?

A
  • E = mc^2
    (using mass of positron + electron) and giving COMBINED energy
47
Q

How to calculate where along the line an annihilation of positron and electron in a PET scan happens?

A
  • distance = velocity x time
    (using speed of light)
48
Q

Advantaged and disadvantages of X-ray and C-T scan?

A
  • ADVANTAGES - clear images and low cost
  • DISADVANTAGES - high radiation dose
49
Q

Advantaged and disadvantages of ultrasound?

A
  • ADVANTAGES- no radiation
  • DISADVANTAGES- cannot be used to study the brain or lungs , low resolution
50
Q

Advantaged and disadvantages of MRI?

A
  • ADVANTAGES - no side effects, high quality images , can image any part of the body
  • DISADVANTAGES - high cost
51
Q

How do alpha particles affect body?

A
  • highly ionising and harmful when SWALLOWED , INHALED or ABSORBED as cannot penetrate most matter and will stay in body
52
Q

How do beta particles affect body?

A
  • capable of penetrating skin and causing radiation damage (skin burns)
  • most hazardous when swallowed inhaled or absorbed into bloodstream through wounds
53
Q

How do gamma particles affect body?

A
  • radiation hazard for whole body
  • some fraction of energy will ALWAYS be absorbed by body tissue
54
Q

How to calculate blood volume using a radioactive tracer?

A
  • activity of tracer =( activity of blood x total volume) / taken volume
55
Q

What is absorbed dose?

A

-This is the radiation energy absorbed per kilogram of tissue.

56
Q

What is the Gray (Gy)?

A

-the unit of absorbed dose.
-1 Gy = 1 joule per kilogram

57
Q

What is sievert (Sv)?

A

-the unit of equivalent dose and effective dose.

58
Q

Equivalent dose equation?

A

-absorbed dose × (radiation) weighting factor
- H = D x Wr

59
Q

Effective dose equation?

A

-equivalent dose × tissue weighting factor
- E = H x Wt

60
Q

How are radioactive tracers used in imaging?

A

-radioactive chemicals which are put inside a patient (either injected or ingested)
- They must emit body-penetrating gamma rays that can be viewed by a gamma camera outside the body. The radioactive tracer should also have a half-life of a few hours - long enough to take an image but short enough not to give a large radioactive dose to the patient. The most common radioactive tracer is technetium-99m - it has a half-life of 6 hours and emits gamma rays. The production of technetium-99m requires a cyclotron and so is only produced in specialised hospitals.

61
Q

What properties do radioactive tracers have and which one is most commonly used?

A

-the radioactive tracer should also have a half-life of a few hours - long enough to take an image but short enough not to give a large radioactive dose to the patient
-The most common radioactive tracer is technetium-99m - half-life of 6 hours and emits gamma rays
The production of technetium-99m requires a cyclotron and so is only produced in specialised hospitals.

62
Q

What is a Collimator?

A

-Device for producing (or selecting) a parallel beam of
gamma radiation).

63
Q

What are Scintillations?

A

-flashes of light (or ultraviolet radiation) given out by
certain crystals when high energy
particles, e.g. gamma ray photons, strike them.

64
Q

What kind of crystals have scintillations?

A

-sodium iodide

65
Q

What us a Photomultiplier?

A

-an arrangement of electrodes, with different voltages
applied to them, so that electrons emitted from one electrode by the photoelectric effect are effectively multiplied in number to make a much larger current.

66
Q

How is gamma camera used?

A

-patient will be emitting gamma rays in all directions, so we need a collimator to collect only the rays moving vertically upward
- the gamma rays are absorbed by the scintillator
-A large 2D network of photomultiplier tubes provide an electric pulse each time light is detected in front of them
- The scintillator counter counts these pulses and passes the information to a computer which displays a 2D image on the screen

67
Q

Which imaging techniques have radiation and which do not?

A
  • radiation exposure - X-ray , CT scam
  • no radiation - Ultrasound and MRI
68
Q

When are the imaging techniques applied?

A
  • Ultrasound - soft tissue, foetus or skeletal joints
  • X-ray- bone breakages , (contrast agent used for soft tissue)
  • CT-Scan- Bone injuries, lung head and chest imaging and cancer investigation
  • MRI- all soft tissue imaging (brain, injuries or tumours)
69
Q

How long are the process of the imaging techniques?

A
  • Ultrasound - short
  • X-ray- short
  • CT-Scan - short but no movement
  • MRI - long , uncomfortable and no movement (noisy and claustrophobic)
70
Q

Do the imaging techniques have high definition?

A
  • X-ray , CT-scan and MRI good definition
    -ultrasound not high definition
71
Q

Which imaging techniques do or do not have 3D imaging?

A
  • 2D - ultrasound and X-ray
  • 3D - CT- Scan (using HELICAL SCAN) and MRI
72
Q

Absorbed dose equation?

A
  • D = total energy of radiation absorbed/ mass

Physics A LEVEL (NEW) (13 decks)

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