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Physics 9702 Theory > Medical physics > Flashcards

Medical physics Flashcards

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1
Q

How are x-rays formed

simple definition

A

By the electron bombardment of a metal target

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2
Q

Explain how x-rays are produced for use in medical diagnosis.

3

A

electrons are accelerated (by an applied p.d.)
electrons hit target
X-rays produced when electrons decelerate

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3
Q

txtbook question

collimated beam

A

beam with parallel sides that doesn’t spread out

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4
Q

min wavelength of x-ray formula

A

=hc/eV

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5
Q

Why is a continuous stream of wavelength produced?

3 points

A
  • Radiation is produced whenever a charged particle is accelerated
  • The wavelength depends on the acceleration of the electrons hitting the metal object
  • the accelerations are varied therefore varied wavelengths
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6
Q

why is there a cutoff point

A

a cut off occurs because each electron produces a single photon so all energy is given up in one collision

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7
Q

Sharp peaks why

A

characteristic of the target because they correspond to the emission line spectrum of the metal object

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8
Q

Sharpness

A

ease with which edges can be distinguished

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9
Q

How to increase sharpness of image

3

A
  • increase size of x-ray beam
  • use of lead grid
  • scattering of x-ray beam
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10
Q

Intensity (3)

A
  • wave power per unit area
  • affects degree of blackening on an image
  • increased by current (more electrons emitted)
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11
Q

Hardness (2)

A
  • penetration of x-ray beam
  • determines fraction of intensity beam that can penetrate part of body
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12
Q

Two factors affecting hardness

A
  • p.d, direct proportional
  • frequency, direct proportional
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13
Q

Contrast

A

Difference in degrees of blackening between regions

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14
Q

How is x-ray used in imaging of internal body structures

A

beam incident on body part, x-ray passes through body and intensity creates image

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15
Q

Soft tissue- effect of x-ray

A

little loss of intensity so dark area

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16
Q

Bone- effect of x-ray

A

reduces intensity by greater intensity so light area

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17
Q

x-ray attenuation formula

A

I= Inought e^-ux

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18
Q

x-ray attenuation

A

gradual decrease in the intensity of a beam of x-rays as it passes through matter

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19
Q

txtbook

What is half-thickness

A

thickness of material that will reduce transmitted intensity of an x-ray beam to half its original value

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20
Q

How have radiographers reduced dosage of x-rays

A

use intensifier screens instad of photographic film.
this means that x-rays are absorbed by phosphor which emits light photons
creating an image

21
Q

past paper question

explain principles behind the use of x-rays for imaging internal body structures

4 marks

A
  • x-ray beam directed through body onto detector
  • different tissues abosrb beam by different amounts
  • giving ‘shadow’ image of structures
  • the resolution of the image is determined by the sharpness of the beam
22
Q

explain why long wavelength x-ray radiation is more harmful than short-wavelength radiation

how is it minimised

A

long wavelength radiation is more likely to be absorbed by the body

- using an aluminium filter

23
Q

Why is it advantageous to filter out low-energy photons from x-ray beam

3

A
  • absorbed more readily
  • do not contribute to x-ray image
  • can cause tissue damage
24
Q

How does CT work (4)

A
  • angles of section combined
  • 2D image formed
  • Repeat for different axis
  • 3D image formed
25
Q

Two advantages of CT scans

A
  • produces images that show 3D relationships between tissues
  • distinguish tissues with similar densities
26
Q

Differences between CT and x-ray scan image

5 marks

A
  1. x-ray image is 2D
  2. CT takes many images of a slice at different angles
  3. series of images of slices is made
  4. so that 3D image is made
  5. this image can be rotated
27
Q

What is a piezo-electric crystal

A

substance that generates electric charge after mechanical stress

28
Q

Piezo-electric inducer

A

A device that uses mechanic stress/strain for the generation of an electrical voltage proportional to the applied stress

it also acts as detector of waves

29
Q

How are ultrasound waves produced (3)

A
  • alternating voltage is applied across a piezo-electric material
  • this causes it to be compressed and extended
  • the crystal becomes a vibrating source of ultrasound waves at the same frequency as the alternating voltage
30
Q

How to detect ultrasound waves (5)

A
  • ultrasound waves change pressure in medium
  • atoms shift positions closer to the plate
  • opposite charges are induced on the silver plate
  • this induces emf across the plate causing a fluctuating pd
  • this causes processable pulses
31
Q

How is diagnostic information about internal structures obtained? (5)

in relation to ultrasound

A
  • ultrasound pulses are produced
  • these pulses are reflected at the boundaries
  • gel is used to minimise reflection at skin
  • time delay between generation and detection gives information about depth
  • intensity of reflected waves gives information about the nature of the wave
32
Q

What influences the resolution of an image

in relation to ultrasound

A

the wavelength of reflected waves

33
Q

Acoustic impedence formula

A

z= density * speed of sound in the medium

34
Q

Intensity reflection coefficient formula

A

Reflected I/ Original I

35
Q

Attenuation meaning

A

Reduction of energy due to absorption of ultrasound as it travels through the material

36
Q

Radioactive tracer meaning

A

a substance containing radioactive nuclei that can be introduced into the body and is then absorbed by the tissue being studied

37
Q

What type of tracer is used?

A

a tracer that decays by beta positive decay such as flourine-18

38
Q

Annihilation?

A

Occurs when a particle interacts with its anti particle.
Mass energy and momentum is conserved

39
Q

How does PET work (4)

A
  • positrons emitted by decay of the tracer annihilate when they interact with electrons in the tissue
  • this produces a pair of gamma-ray photons travelling in opp directions
  • these travel outside the body and are detected
  • an image of the tracer concentrated on the tissue can be created by processing arrival time of the gamma-ray photons
40
Q

Energy is calculated how (2)

A

E= mc²
E= hf

41
Q

What is the frequency range of ultrasound waves

A

above 20kHz

42
Q

Define specific acoustic impedence

2 marks

A
  • product of density and speed
  • speed of ultrasound in medium
43
Q

Why is ultrasound emitted in pulses (4)

A
  • allows the reflected signal to be distinguished from the emitted signal
  • detection occurs in the time between emitted pulses
  • cannot emit and detect at the same time
  • reflection and emission detected by same probe
44
Q

Explain principles of the detection of ultrasound waves, 3

A

Piezo electric crystal
Ultrasound makes the crystal vibrate
Vibration produces alternating emf

45
Q

Principles behind use of ultrasound to obtain diagnostic information about structures within the body

5

A
  • pulses of ultrasound directed into the body
  • reflected at the boundary between tissues
  • reflected pulse is detected and processed
  • time for return of echo gives info on depth
  • amount of reflection gives info on tissue structures
46
Q

Why could we not do CT without a powerful computer

A

computers are needed to store and process huge quantity of data

47
Q

Why an aluminium filter may be placed in the X-ray beam when producing an X-ray image of a patient 3

A

Aluminium absorbs most low energy X-rays
X-ray beam contains many wavelengths
Low energy X-rays can cause harm but do not contribute to the image

48
Q

Explain main principles behind generation of ultrasound

6 marks

A
  • peizo-electric crystal used
  • opposite sides coated with silver to act as electrodes
  • pd across crystal causes crystal to change shape
  • alternating voltage applied across crystal
  • causing it to oscillate
  • crystal cut so that it vibrates at resonant frequency

Physics 9702 Theory (13 decks)

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