Medical Imaging

Question 1

State the range of wavelengths for X-rays

Answer 1

10^-8 to 10^-13m

Question 2

State the main property of X-rays and it’s uses

Answer 2

• they have very high energy
• they can kill cells at high exposure
• they can be used to treat cancer

Question 3

Explain how X-rays can be produced

Answer 3

draw diagram
- the high voltage accelerates the electrons - they gain Ek
- electrical energy -> kinetic energy
- W = QV = eV = 1/2mv²
- when the electrons hit the target metal, Ek becomes the energy of an EM wave
- eV -> 1/2mv² -> hc/λ
- eV = hc/λ
λ = hc/eV

Question 4

In X-ray imaging what forms the image?

Answer 4

It is the effect of different media on the intensity of the X-rays which forms the image

Question 5

State and describe the four X-ray attenuation mechanisms

Answer 5

Simple scatter - the X-ray photon is scattered elastically by an electron
Photoelectric effect - the X-ray photon disappears and removes an electron from the atom
Compton scattering - the incident X-ray photon is absorbed by an electron, a lower energy photon is emitted, and the electron is ejected from the atom
Pair production - the X-ray photon disappears to produce and electron-positron pair

Question 6

At what photon energy does simple scatter occur?

Answer 6

1-20keV

Question 7

At what photon energy does the photoelectric effect occur?

Answer 7

1-100keV

Question 8

At what photon energy does Compton scattering occur?

Answer 8

500-5000keV

Question 9

At what photon energy does pair production occur?

Answer 9

> or equal to 1.02MeV

Question 10

What is the typical value for a hospital X-ray?

Answer 10

30keV - photoelectric is what dominates

Question 11

What are disadvantages of X-ray scans?

Answer 11

• cannot resolve overlaps
• cannot distinguish between soft tissues

Question 12

State the advantages and disadvantages of CAT scans

Answer 12

Advantages:
- gives a 3d image
- can resolve overlaps
- can distinguish between soft tissues of similar attenuation coefficient
Disadvantages:
- patient must stay still
- very high exposure
- expensive

Question 13

State two common tracers used to monitor processes in the body

Answer 13

Fluorine - 18
Technetium - 99m

Question 14

State the list of components in a gamma camera in order from closest to patient to furthest away

Answer 14

Collimator
Scintillator
Light guide
Array of photomultiplier tubes (learn the diagram for these)
Circuits
Computer
Display

Question 15

State the properties of ultrasound waves

Answer 15

They are longitudinal waves with a frequency greater than 20kHz

Question 16

Describe the piezoelectric effect

Answer 16

Some crystals produce an e.m.f if they are compressed - an effect which is reversible

Question 17

What does an ultrasound transducer produce?

Answer 17

5000 pulses per second of 5MHz ultrasound

Question 18

State and describe the two types of ultrasound scan

Answer 18

A-scans - 1 directional
B-scans - multiple A-scans combined to form a 2d image

Question 19

What is the equation for acoustic impedance of a medium?

Answer 19

Acoustic impedance = density of medium x speed of ultrasound in the medium
z = ρc

Question 20

What can happen to ultrasound at boundaries between media?

Answer 20

It can be:
- reflected
- refracted
- transmitted

Question 21

What equation can be used when considering only ultrasound travelling along the normal?

Answer 21

Ir/Io = ((z2-z1)/(z2+z1))²

Question 22

How can we avoid reflection before the ultrasound reaches the body?

Answer 22

We can use an acoustic matching gel - or coupling gel - with a similar z (acoustic impedance) to skin

Question 23

What are some of the advantages and disadvantages of using MRI techniques?

Answer 23

• method does not use ionising radiation, so no radiation hazard to patient or staff
• gives better soft tissue congrats than CT scans
• generates data from a 3D volume simultaneously
• information can be displayed on a screen as a section in any direction
• there are no moving mechanisms involved in MRI
• strong magnetic field could draw steel objects into the magnet
• metallic objects may become heated
• cardiac pacemakers may be affected by the magnetic fields
• CT scanners better for viewing bony structures

Question 24

Define intensity reflection coefficient

Answer 24

The ratio of the reflected intensity Ir to the incident intensity Io (Ir/Io)