Medical physics Flashcards
How are x-rays formed
simple definition
By the electron bombardment of a metal target
Explain how x-rays are produced for use in medical diagnosis.
3
electrons are accelerated (by an applied p.d.)
electrons hit target
X-rays produced when electrons decelerate
txtbook question
collimated beam
beam with parallel sides that doesn’t spread out
min wavelength of x-ray formula
=hc/eV
Why is a continuous stream of wavelength produced?
3 points
- 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
why is there a cutoff point
a cut off occurs because each electron produces a single photon so all energy is given up in one collision
Sharp peaks why
characteristic of the target because they correspond to the emission line spectrum of the metal object
Sharpness
ease with which edges can be distinguished
How to increase sharpness of image
3
- increase size of x-ray beam
- use of lead grid
- scattering of x-ray beam
Intensity (3)
- wave power per unit area
- affects degree of blackening on an image
- increased by current (more electrons emitted)
Hardness (2)
- penetration of x-ray beam
- determines fraction of intensity beam that can penetrate part of body
Two factors affecting hardness
- p.d, direct proportional
- frequency, direct proportional
Contrast
Difference in degrees of blackening between regions
How is x-ray used in imaging of internal body structures
beam incident on body part, x-ray passes through body and intensity creates image
Soft tissue- effect of x-ray
little loss of intensity so dark area
Bone- effect of x-ray
reduces intensity by greater intensity so light area
x-ray attenuation formula
I= Inought e^-ux
x-ray attenuation
gradual decrease in the intensity of a beam of x-rays as it passes through matter
txtbook
What is half-thickness
thickness of material that will reduce transmitted intensity of an x-ray beam to half its original value
How have radiographers reduced dosage of x-rays
use intensifier screens instad of photographic film.
this means that x-rays are absorbed by phosphor which emits light photons
creating an image
past paper question
explain principles behind the use of x-rays for imaging internal body structures
4 marks
- 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
explain why long wavelength x-ray radiation is more harmful than short-wavelength radiation
how is it minimised
long wavelength radiation is more likely to be absorbed by the body
- using an aluminium filter
Why is it advantageous to filter out low-energy photons from x-ray beam
3
- absorbed more readily
- do not contribute to x-ray image
- can cause tissue damage
How does CT work (4)
- angles of section combined
- 2D image formed
- Repeat for different axis
- 3D image formed
Two advantages of CT scans
- produces images that show 3D relationships between tissues
- distinguish tissues with similar densities
Differences between CT and x-ray scan image
5 marks
- x-ray image is 2D
- CT takes many images of a slice at different angles
- series of images of slices is made
- so that 3D image is made
- this image can be rotated
What is a piezo-electric crystal
substance that generates electric charge after mechanical stress
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
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
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
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
What influences the resolution of an image
in relation to ultrasound
the wavelength of reflected waves
Acoustic impedence formula
z= density * speed of sound in the medium
Intensity reflection coefficient formula
Reflected I/ Original I
Attenuation meaning
Reduction of energy due to absorption of ultrasound as it travels through the material
Radioactive tracer meaning
a substance containing radioactive nuclei that can be introduced into the body and is then absorbed by the tissue being studied
What type of tracer is used?
a tracer that decays by beta positive decay such as flourine-18
Annihilation?
Occurs when a particle interacts with its anti particle.
Mass energy and momentum is conserved
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
Energy is calculated how (2)
E= mc²
E= hf
What is the frequency range of ultrasound waves
above 20kHz
Define specific acoustic impedence
2 marks
- product of density and speed
- speed of ultrasound in medium
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
Explain principles of the detection of ultrasound waves, 3
Piezo electric crystal
Ultrasound makes the crystal vibrate
Vibration produces alternating emf
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
Why could we not do CT without a powerful computer
computers are needed to store and process huge quantity of data
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
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
