Medical Physics

Question 1

Define piezoelectric effect

Answer 1

It is the ability of particular materials to generate a potential difference by transferring mechanical energy to electrical energy

Question 2

What happens to a pizocrystal with pd

Answer 2

When the pd is applied to a pizpocrystal, it deforms
If it’s in the opp direction, it expands

Question 3

What happens to a piezoelectric crystal when alternating pd is applied

Answer 3

The crystal vibrates at the same frequency as the alternating voltage
The crystal should be cut to a specific size to create resonance

Question 4

Where are piezoelectric transducers used

Answer 4

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

Question 5

Define ultrasound

Answer 5

A high frequency sound above the range of human hearing

Question 6

How do we stop the piezoelectric crystal from vibrating too much in ultrasound

Answer 6

By damping with epoxy resin

Question 7

How does a pizoelectric crystal act as a transmitter of ultra sound

Answer 7

● 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

Question 8

How does a pizoelectric crystal act as a reciever of ultra sound

Answer 8

● 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

Question 9

How do we use ultrasound in medical imaging to produce a 2D image

Answer 9

● 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

Question 10

Define depth

Answer 10

The time between transmission and receipt of the pulse (the time delay)

Question 11

Define nature

Answer 11

Amount of transmitted intensity received

Question 12

Define specific acoustic impedance

Answer 12

The product of the speed of the ultrasound in the medium and the density of the medium

Question 13

Equation of specific acoustic impedance (Z)

Answer 13

Z=pc

Question 14

What would happen if there is a large difference in acoustic impedance

Answer 14

The greater the reflection

Question 15

How do we prevent ultrasound from being reflected too much

Answer 15

By using coupling gel since it gave a similar Z value to the skin so less ultrasound transmitted

Question 16

Define intensity reflection coefficient

Answer 16

The ratio of the intensity of the reflected waves relative to the incident (transmitted) wave

Question 17

Equation on intensity reflection coefficient

Answer 17

Ir/Io=(Z2-Z1)^2/(Z2+Z1)^2

Question 18

Define the attenuation of ultrasound

Answer 18

The reduction of energy due to absorption of ultrasound as it travels through a material

Question 19

Equation of of attenuation

Answer 19

I=Ioe^-mewx

Question 20

How to produce x rays

Answer 20

● 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

Question 21

What is the equation of energy in ultrasound

Answer 21

Emax=ev
Emax=hfmax
Emax=hc/wavelengthmin

Question 22

What are the aims of X rays in medical imaging

Answer 22

1) Reduce the exposure of radiation as much as possible
2) Improve the contrast of the image

Question 23

How to reduce exposure in x-rays

Answer 23

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

Question 24

Define contrast

Answer 24

The difference in degree of blackening between structures

Question 25

How to improve contrast

Answer 25

○ using the correct level of x ray hardness
○ using a contrast media

Question 26

Define sharpness

Answer 26

How well-defined the edges of structure are

Question 27

How to improve sharpness

Answer 27

○ using a narrow x ray beam
○ reducing x ray scattering using a collimator
○ smaller pixel size

Question 28

How does a CT scan work

Answer 28

● 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

Question 29

List advantages of CT scan

Answer 29

● 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

Question 30

Disadvantages of CT scan

Answer 30

● The patient receives a much higher dosage than a normal x ray
● Possible side effects from contrast media

Question 31

Define radioactive tracer

Answer 31

A radioactive substance that can be absorbed by tissue in order to study the structure and function of organs in the body

Question 32

What radioactive isotopes can act as radioactive tracers

Answer 32

Texhnetium-99m
Fluorine-18
Both have short half life

Question 33

Define PET scanning

Answer 33

A type of nuclear medical procedure that images tissues and organs by measuring the metabolic activity of the cells of the body tissues

Question 34

How do we use tracers

Answer 34

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

Question 35

What I’d Annihilation

Answer 35

When a particle meets its equivalent antiparticle, they are both destroyed and their masses are converted to energy

Question 36

What happend in PET scanning

Answer 36

● 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

Question 37

How to detect gamma rays from PET scanning

Answer 37

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

Question 38

Calculating energy of gamma ray photons equation

Answer 38

E=hf =m(of electron)c^2

Question 39

Momentum of a photons equation

Answer 39

P=E/c