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Physics 9702 Theory > Medical physics > Flashcards

Medical physics Flashcards

(48 cards)

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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

Study These Flashcards
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

Study These Flashcards
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

Study These Flashcards
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

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

How does CT work (4)

Study These Flashcards
A
  • angles of section combined
  • 2D image formed
  • Repeat for different axis
  • 3D image formed
25
Two advantages of CT scans
- produces images that show 3D relationships between tissues - distinguish tissues with similar densities
26
Differences between CT and x-ray scan **image** | 5 marks
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
What is a piezo-electric crystal
substance that generates electric charge after mechanical stress
28
Piezo-electric inducer
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
How are ultrasound waves produced (3)
- 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
How to detect ultrasound waves (5)
- 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
How is diagnostic information about internal structures obtained? (5) | in relation to ultrasound
- 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
What influences the resolution of an image | in relation to ultrasound
the wavelength of reflected waves
33
Acoustic impedence formula
z= density * speed of sound in the medium
34
Intensity reflection coefficient formula
Reflected I/ Original I
35
Attenuation meaning
Reduction of energy due to absorption of ultrasound as it travels through the material
36
Radioactive tracer meaning
a substance containing radioactive nuclei that can be introduced into the body and is then absorbed by the tissue being studied
37
What type of tracer is used?
a tracer that decays by beta positive decay such as flourine-18
38
Annihilation?
Occurs when a particle interacts with its anti particle. Mass energy and momentum is conserved
39
How does PET work (4)
- 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
Energy is calculated how (2)
E= mc² E= hf
41
What is the frequency range of ultrasound waves
above 20kHz
42
Define specific acoustic impedence | 2 marks
- product of density and speed - speed of ultrasound in medium
43
Why is ultrasound emitted in pulses (4)
- 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
Explain principles of the detection of ultrasound waves, 3
Piezo electric crystal Ultrasound makes the crystal vibrate Vibration produces alternating emf
45
Principles behind use of ultrasound to obtain diagnostic information about structures within the body | 5
- 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
Why could we not do CT without a powerful computer
computers are needed to store and process huge quantity of data
47
Why an aluminium filter may be placed in the X-ray beam when producing an X-ray image of a patient 3
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
Explain main principles behind generation of ultrasound | 6 marks
- 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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