Medical Physics

Question 1

Piezoelectric effect

Answer 1

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

Question 2

Transducer

Answer 2

any device that converts energy from one form to another

Question 3

Ultrasound

Answer 3

a high frequency sound above the range of human hearing (above 20kHz)

Question 4

Ultrasound generation and detection

Answer 4

Generation:
1. Alternating p.d applied across piezo-electric crystal causing it vibrate
2. Crystal vibrate and produces ultrasound waves

Detection:
1. When ultrasound wave returns, the crystal vibrates and produces an alternating p.d across the crystal

Question 5

Ultrasound in Medical Imaging

Answer 5

1. A pulse of ultrasound is emitted by the transducer
2. The pulse reflects at boundaries in the body
3. The reflected pulse is detected by transducer and generates electrical signals sent to ultrasound scanner
4. Computer calculates the distance from transducer to the tissue boundary and generates 2D images of tissues and organs

Question 6

How to create a clearer ultrasound image?

Answer 6

The higher the frequency of the ultrasound, the higher the resolution and the smaller structures that can be distinguished

Question 7

Ultrasound gives 2 info about the boundary

Answer 7

1. Depth - time between transmission and receipt of the pulse
2. Nature - amount of transmitted intensity received

Question 8

Acoustic impendance

Answer 8

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

z = pc

Question 9

Greater acoustic impendance needs 2 things:

Answer 9

• higher density
• how fast the speed of ultrasound is

Question 10

Between the media of different acoustic impedance…

Answer 10

the greater the difference, the greater the reflection and the smaller the transmission

if impedance is the same, there is no reflection

Question 11

Intensity reflection

Answer 11

the ratio of the intensity of the reflected wave relative to the incident wave

Ir / Io = (z2 - z1)^2 / (z2 + z1)^2

Question 12

Attenuation

Answer 12

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

Question 13

Graph of attenuation of ultrasound

Answer 13

Decreases linearly

Question 14

Producing x-rays

Answer 14

1. At the cathode, the electrons are released by thermonic emission
2. electrons are accelerated towards the anode at high speed
3. when electrons hit the metal target, they lose Ek and transfer energy to photons
4. electrons in outershells move into spaces with lower energy levels
5. x-ray photons are released in low energy levels

Question 15

X-rays and medical imaging

Answer 15

1. Reducing exposure -> aluminum filters
   - the aluminium filters absorb the wavelength of x-rays
   - long wavelengths are highly penetrating and ionising
2. Contrast and sharpness
   contrast - the difference in degree of blackening between structures
   - improved by using the correct hardness of x-rays
   sharpness - how well defined the edges of structures are
   - improved by using a narrower x-ray beam, use collinator, smaller pixel size

Question 16

Attenuation of x-ray equation

Answer 16

I = Io e^- µx

Question 17

CTS how it works

Answer 17

1. x-ray tube rotates around the stationary patient
2. a CT scanner takes x-ray images of the same size at many different angles
3. process is repeated and images are combined together
4. images created a 3d image that can be rotated and viewed in different angles

Question 18

CTS advantages and disadvantages

Answer 18

adv:
- more detailed images
- distinguish tissues with similar attenuation coefficients
- produces a 3d image

disadv:
- higher dosage of radiation
- side effects from contrast media

Question 19

Radioactive tracers

Answer 19

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

Question 20

PET scanners

Answer 20

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

Question 21

PET scanner annihilation process

Answer 21

1. fluorodeoxyglucose undergoes β+ decay and emits a positron
2. positron collides with electron and annihilates
3. annihilation: mass becomes pure energy in the form of two gamma rays that move in opposite direction

Question 22

annihilation

Answer 22

when a particle meets its equivalent antiparticle, both destroyed, mass converts into energy

Question 23

energy of gamma ray photons equation

Answer 23

E = hf = mc^2

momentum of photon: p = m/c

Question 24

PET scanner how it works

Answer 24

1. patient lies down with a ring of detectors
2. detector has crystal scintillator and photomultiplier
3. when gamma rays hit the crystal, electrons inside the crystal become excited and transmit as light photons. the photmultipler amplies photons and converts into electrical signal
4. the two gamma rays that hit detectors in line form a line of response
5. tracers that get hit by gamma ray form an image
6. image is formed by processing annual times of x-ray photon