medical physics Flashcards
what is the piezoelectric effect
The ability of particular materials to generate a potential difference by transferring mechanical energy to electric energy
applications of a piezoelectric transducer
- Microphone
-ultrasound
what is an ultrasound
A high frequency sound above the range of human hearing.
-usually above 20,000Hz
how are ultrasounds used to create a 2D image
- transducer sends out a beam of ultrasound waves into the body using piezocrystal
- sound waves are reflected back into the body by boundaries
- echoes hit transducer and generate an electric signal sent to ultrasound scanner
- distance and depth calculated using speed of sound and time between echoes
- distances used to create a 3d image
how does a higher frequency of ultrasound affect image
- higher resolution
- smaller structures can be distinguished
what is specific acoustic impedance
the product of the speed of the ultrasound in a medium and the density of the medium
how does the difference between the Specific acoustic impedance affect intensity
the larger the difference the more will be reflected
what is intensity reflection coefficient
the ratio of the intensity of the reflected wave relative to the incident wave
what is a coupling medium
medium used to reduce the difference between z as air and skin have a very large difference
what is attenuation of ultrasound matter
the reduction of energy due to the absorption of ultrasound as it travels through a material
- the greater the value the faster the rate of decay
how are X-rays produced
-electrons are accelerated nu the accelerating p.d
- the target is hit through electron bombardment
- x-rays are produced when the energy of the electrons is converted into photons
why do the x-rays produced have a continuous spectrum of energies
-electrons decelerate on hitting the target
- there are a range of decelerations
-photon energy depends on the magnitude of deceleration
what are the two aims when treating patients
- reducing exposure
- contrast and sharpness
how can exposure be reduced in x-rays
using aluminium filters to absorb longer wavelength x-rays since x-rays are ionsing patients should recieve minimum dosage
what is contrast and how can it be improved
the difference in the degree of blackening
-using the correct level x-ray hardness, using a contrast media
how does the difference in the linear attenuation coefficients of different tissues affect contrast
the larger the difference (assuming thickness is equal) the different the amounts absorbed so there would be good contrast
what is sharpness and how can be improved
how well defined the edges of the structures are
- using a narrower beam, reduce x-ray scattering
what does CT and CAT scanning
Computed tomography (CT)
Computerised Axial Tomography (CAT)
main features of CT scanning
-X-ray tube rotates around the stationary patient
- CT scanner takes x-ray images of the same slice, at many different angles
- the process is repeated with the successive slices being combined together
- a computer pieces the images together to build a 3D images
Advantages of CT scanning
-Detailed images
-produced a 3D image
disadvantages of CT scanning
patient recieves a higher dose of x-ray
possible side effects from contrast media
what is a radioactive tracer
A substance containing radioactive nuclei that is introduced to the body and can be absorbed by tissues in order to study the structure and function of organs in the body
why are radioactive substances like flourine-18 suitable for this purpose
-they bind to organic molecules
-they emit gamma radiation and decay into stable isotopes
- they have short half-lifes
what does PET stand for
Positron Emission Tomograpgy
