Chapter 27 - Medical Physics

Question 1

X-ray wavelengths

Answer 1

10^-13 –> 10^-8 m

Question 2

Producing x-rays

Answer 2

Eject electrons from a cathode in thermionic emission with a p.d. of 50 to 200kV
They are attracted to an anode (tungsten), reducing the kinetic energy of electrons and converting to photons

Question 3

Bremsstrahlung

Answer 3

A continuous spectrum of energies of x-rays emitted as electrons lose kinetic energy

Question 4

Characteristic x-rays

Answer 4

Emitted from the anode as electrons move between energy levels in the anode

Question 5

Shortest wavelength of x-rays

Answer 5

λ = hc/eV

Question 6

Hard and soft x-rays

Answer 6

Hard (0.10 to 0.01nm) are more penetrative than soft (10 to 0.1nm)
An aluminium sheet is used to filter out ionising but less useful soft x-rays

Question 7

Collimated x-rays

Answer 7

A parallel beam

Question 8

μ

Answer 8

The linear attenuation coefficient (the percentage reduction in intensity per unit length)

Question 9

Attenuation

Answer 9

The decrease in intensity of EM radiation

Question 10

X-ray intensity formula

Answer 10

I = I0 e^-μx

Question 11

Simple scattering

Answer 11

X-rays <20keV cannot remove electrons from atoms so they are scattered and intensity is reduced
Soft x-rays are filtered so not significant in radiography

Question 12

Photoelectric effect

Answer 12

If hf > Φ, electrons absorb a photon and are emitted, meaning the intensity is reduced as photons are lost

Question 13

Compton effect

Answer 13

0.5 to 50 MeV energy
A photon with momentum collides with an electron, knocking it out of the nucleus, reducing its own momentum
The energy is reduced and likely no longer an x-ray photon

Question 14

Pair production

Answer 14

Energy >= 1.02 MeV

A photon exchanges its energy for mass, producing an electron-positron pair

Question 15

Contrast media

Answer 15

Compounds with high atomic numbers create scattering in soft tissue
μ ∝ Z^3
e.g. barium
Used to detect the boundaries between media

Question 16

CAT scan

Answer 16

Computerised Axial Topography
An x-ray tube is mounted on a ring with a sensing detector array opposite
The tube and detector rotate around the ring and a patient moves slowly through it to create a “spiral” of slices
The images are overlayed by a computer

Question 17

CAT scan advantages and disadvantages

Answer 17

+ much higher contrast resolution than x-rays, much more precise than x-rays
- much more ionising than x-rays, take longer

Question 18

Tracers

Answer 18

Small amounts of radioactive material into the body to follow blood flow or to investigate metabolic pathways

Question 19

Tracer uses

Answer 19

To replace a stable atom and act as labels to follow the path of a compound

Question 20

Tc-99

Answer 20

Tc-99m is a gamma emitter with a 6 hour half life
(m is for metastate before gamma is emitted)
Stable Tc-99 then emits beta minus

Question 21

Gamma camera

Answer 21

Detects gamma photons from the body and a collimator ensures only photons from the body are detected
A scintillator converts the gamma photons to visible photons and a photomultiplier converts photons to many electrons

Question 22

Photomultipliers

Answer 22

• Photon emits a photoelectron
• Increasing pds attract these to release electrons from dynodes
• The amount of electrons continually increases

Question 23

PET scan

Answer 23

Positron Emission Topography

Uses F-18, a beta plus emitter

Question 24

PET scan process

Answer 24

The F-18 decay will release a positron that will travel a few mm before annihilating with an electron in the body. This releases a pair of gamma photons in opposite directions so this and the time difference tells you the location

Question 25

PET scan F-18 production process

Answer 25

O-18 + proton –> F-18 + neutron

Must be done on site due to the short half life

Question 26

Ultrasound frequencies

Answer 26

Above the limit of human hearing 20 kHz

1-3 MHz are used in scans

Question 27

Piezoelectric effect

Answer 27

Piezoelectric crystals produce a net p.d. across its sides as it is stretched or compressed
An imbalance of charge causes a net charge on opposite sides
Passing an A/C causes the crystal to stretch and compress as the charge on each side changes, choose a p.d. and frequency to generate ultrasound

Question 28

Ultrasound transducer

Answer 28

A piezoelectric crystal between electrodes, connected to a computer by a cable
Surrounded by damping material
An acoustic window focuses the sound waves

Question 29

A-scans

Answer 29

Surfaces between media of different densities partially reflect ultrasound
The reflected ultrasound is detected by the transducer and causes the crystal to change shape
average speed x distance gives twice the distance from the scanner

Question 30

B-scans

Answer 30

A 2D image built up from A-scans

Each A-scan produces a series of dots between boundaries between tissues

Question 31

Acoustic impedance

Answer 31

Z = ρc

Acoustic impedance = density x speed of sound in a material

Question 32

Reflected intensity formula

Answer 32

I(r)/I(0) = (Z2 - Z1)^2/(Z2 + Z1)^2

Question 33

Intensity-reflection coefficient

Answer 33

I(r)/I(0)

Question 34

Impedance matching

Answer 34

99.9% of the ultrasound should be reflected by the skin based on the acoustic impedance difference to air
A coupling gel with a similar acoustic impedance to skin must be used to remove the air gap

Question 35

Doppler ultrasound

Answer 35

If an ultrasound wave is reflected from red blood cells, the reflected wave has a different frequency to the initial due to the doppler effect. Only possible for blood vessels with fast blood flow
A computer can colour-code the images

Question 36

Doppler ultrasound formula

Answer 36

Δf/f = 2vcosθ/c

θ is the angle between the pulse and the motion of the blood
c is the speed of sound in the medium