Medical Physics Flashcards
Define piezoelectric effect
It is the ability of particular materials to generate a potential difference by transferring mechanical energy to electrical energy
What happens to a pizocrystal with pd
When the pd is applied to a pizpocrystal, it deforms
If it’s in the opp direction, it expands
What happens to a piezoelectric crystal when alternating pd is applied
The crystal vibrates at the same frequency as the alternating voltage
The crystal should be cut to a specific size to create resonance
Where are piezoelectric transducers used
1) Microphone it detects the pressure variations of sound waves and converts them to electrical signals
2) Ultrasound: An alternating pd is applied to produce ultrasound waves and sent into the patients body. The returning ultra sound waves induce a pd in the transducer for analysis
Define ultrasound
A high frequency sound above the range of human hearing
How do we stop the piezoelectric crystal from vibrating too much in ultrasound
By damping with epoxy resin
How does a pizoelectric crystal act as a transmitter of ultra sound
● When it transmitts ultrasound, it converts an alternating pd into sound waves
● An alternating pd is applied across a piezoelectric crystal, causing it to change shape
● The alternating pd causes the crystal to vibrate and produce ultrasound waves
● The crystal vibrates at the same frequency of the alternating pd, so the crystal must be cut into a specific size in order to produce resonance
How does a pizoelectric crystal act as a reciever of ultra sound
● It converts the sound waves into an alternating pd
● When the ultrasound wave returns, the crystal vibrates, which produces an alternating pd across the crystal
● The received signal can be processed and used for medical diagnosis
How do we use ultrasound in medical imaging to produce a 2D image
● A pulse of ultra sound is emitted by a pizoelectric crystal
● That is reflected from the boundaries between the media (tissue, bone)
● The reflected pulse is detected by the ultrasound transmitter they generate electrical signals
● Using the speed of sound and the time of each echoes return the distance can be calculated to form an image
● The intensity of reflection gives information about the nature of the boundary
● The time between transmission and receipt of the pulse (time delay) gives information about the depth of the boundary
Define depth
The time between transmission and receipt of the pulse (the time delay)
Define nature
Amount of transmitted intensity received
Define specific acoustic impedance
The product of the speed of the ultrasound in the medium and the density of the medium
Equation of specific acoustic impedance (Z)
Z=pc
What would happen if there is a large difference in acoustic impedance
The greater the reflection
How do we prevent ultrasound from being reflected too much
By using coupling gel since it gave a similar Z value to the skin so less ultrasound transmitted
Define intensity reflection coefficient
The ratio of the intensity of the reflected waves relative to the incident (transmitted) wave
Equation on intensity reflection coefficient
Ir/Io=(Z2-Z1)^2/(Z2+Z1)^2
Define the attenuation of ultrasound
The reduction of energy due to absorption of ultrasound as it travels through a material
Equation of of attenuation
I=Ioe^-mewx
How to produce x rays
● At the cathode, electrons are released by thermionic emission
● The electrons are accelerated towards the anode at high speed
● When the electrons bombard the metal target, they lose some of their KE by transferring it to photons
● The electrons in the outershell of the atoms move into the spaces in the lower energy levels
● As they move to the lower energy levels, they release energy in the form of x-ray photons
What is the equation of energy in ultrasound
Emax=ev
Emax=hfmax
Emax=hc/wavelengthmin
What are the aims of X rays in medical imaging
1) Reduce the exposure of radiation as much as possible
2) Improve the contrast of the image
How to reduce exposure in x-rays
X rays are ionising, meaning they can damage live organs and tissues
In order to reduce exposure
Aluminium filters are used
● This is because many wavelengths of x-rays are emitted, and the longer wavelengths are more penetrating, so they are easily absorbed by the body
● This means they don’t contribute to the image and pose more of a health hazard
● The aluminium sheet absorbs these long wavelength x rays, making them safer
Define contrast
The difference in degree of blackening between structures
How to improve contrast
○ using the correct level of x ray hardness
○ using a contrast media
Define sharpness
How well-defined the edges of structure are
How to improve sharpness
○ using a narrow x ray beam
○ reducing x ray scattering using a collimator
○ smaller pixel size
How does a CT scan work
● An x-ray tube rotates around the stationary patient
● A CT scanner takes an x-ray image of the same slice at many different angles
● This process is repeated, and the image of successive slices is combined together
● A computer pieces the images together to build a 3D image, which can be rotated and viewed from different angles
List advantages of CT scan
● Produces much more detailed images
● Can distinguish between tissues with similar attenuation coefficients
● Produces a 3D image of the body by combing the images at each direction
Disadvantages of CT scan
● The patient receives a much higher dosage than a normal x ray
● Possible side effects from contrast media
Define radioactive tracer
A radioactive substance that can be absorbed by tissue in order to study the structure and function of organs in the body
What radioactive isotopes can act as radioactive tracers
Texhnetium-99m
Fluorine-18
Both have short half life
Define PET scanning
A type of nuclear medical procedure that images tissues and organs by measuring the metabolic activity of the cells of the body tissues
How do we use tracers
The radioactive tracer is injected or swallowed into the patient and flows around the body
Once the tracer has been absorbed by the tissues and organs, they appear on the screen as bright areas for a diagnosis
What I’d Annihilation
When a particle meets its equivalent antiparticle, they are both destroyed and their masses are converted to energy
What happend in PET scanning
● Positrons are emitted by the decays of the tracer
● They travel a small distance and annihilate when they interact with electrons in the tissue
● The annihilation produces a pair of gamma ray photons with travel in the opposite direction
How to detect gamma rays from PET scanning
The detector is made of 2 parts:
Crystal scintillator: When the gamma ray photon is incident on a crystal, an electron in the Crystal is excited to a higher energy state as it travels through the Crystal it excited more electrons and when the excited electrons move back to their original state the lost energy is transmitted as a visible light photons
Photomultiplyer: The photons produced are very faint, so they need to be amplified and converted to an electrical signal by a photomultiplier tube
Calculating energy of gamma ray photons equation
E=hf =m(of electron)c^2
Momentum of a photons equation
P=E/c
