Medical Physics

Question 0

Larmor frequency

Answer 0

The frequency of precession of nuclei in an external magnetic field

Question 1

Define acoustic impedance

Answer 1

The property of a material that determines the intensity of ultrasound refracted at a boundary with another material

Z = pc (measured in kg m^-2 s^-1)

Question 2

How are X-rays produced?

Answer 2

Bombarding tungsten with high energy particles

Question 3

What is the wavelength of X-rays?

Answer 3

10^-13 to 10^-8

Question 4

What is the typical frequency of ultrasound used in medicine?

Answer 4

1-15MHz

Question 5

What is the name of the frequency at which the protons precess?

Answer 5

Larmor Frequence

Question 6

After the pulses of radio waves has ceased the nuclei relax and emmit what kind of wave?

Answer 6

Radio

Question 7

Gamma Cameras are used with what?

Answer 7

Radioactive tracers

Question 8

Describe X-ray production

Answer 8

A tungsten cathode is heated so that it releases electrons

These electrons are fired across a vacuum by a voltage generated by the mains

These electrons hit the negative anode and some of their energy (about 1%) is converted into X-ray photons

These X-ray photons can only exit through a window in the casing and there may be sheets of metal either side of the window to absorb X-rays not travelling straight, forming a collimated beam

The rest of the energy of the electron is gained by the anode as heat, so it rotates very fast to get rid of excess heat

The energy of one X-ray photon is virtually the same as the K.E. of the electrons, as the work function is negligible

Question 9

The energy of an electron is eV and the energy of an X-ray photon is hf. Therefore…

Answer 9

hf = eV

And as λ = v/f, λ = c/ (eV/h), λ = hc/ eV

Question 10

Describe X-ray absorption, with reference to the photoelectric effect

Answer 10

As with light rays, X-rays are also capable of producing photoelectrons

Energy of incoming photon = work function + KE of photoelectron

hf = ϕ + 1/2 mv^2

The work function is the energy required to break the bonds holding an electron in place

Because the energy of X-rays is so high, the work function is considered negligible

Therefore the photoelectron s produced have virtually the same energy as the X-ray photons that caused them

This is the main way that low energy X-rays are absorbed

Question 11

Describe Compton Scattering

Answer 11

Occurs for higher energy X-rays (0.5-5MeV)

Instead of giving all their energy to an electron, the X-ray photon only loses a small amount of energy when it hits one

The ‘Compton Electron’ is knocked off its orbit and moves of in a different direction, ionising the atom

The X-ray photon now has slightly less energy, so a smaller frequency (E=hf) and hence a longer wavelength (λ = v/f) and is also deflected

Question 12

Describe pair production with X-rays

Answer 12

Pair production occurs when an X-ray photon with energy above 1.02 MeV spontaneously splits into a positron and an electron when entering the electric field around the nucleus of an atom

The energy of the X-ray photon is converted into the mass of the electron and positron as E = mc^2 applies

The positron is soon annihilated when it collides with an electron, producing two gamma-ray photons that move off in opposite directions to each other

Question 13

Describe film intensifiers

Answer 13

To intensify the image produced from a traditional film X-ray, intensifier sheets are used

These sheets are of material that contain a phosphor – a substance that emits visible light photons when an X-ray photon hits it

The film is placed behind an intensifier screen and many light photons produced by the screen blacken the film

This dramatically reduces the energy of the X-rays that needs to be used, so reducing the risk to the patient

Question 14

Describe digital intensifiers

Answer 14

Incoming X-rays strike a phosphor screen which releases thousands of visible light photons for each X-ray photon that hits it

These then hit a photocathode, which releases an electron for each light photon, via the photoelectric effect

These electrons are then focused onto a screen which is another phosphor that gives out visible light

Image intensifies are used to reduce the length of time the patient has to be exposed to X-rays, and so that a lower intensity can be used

Question 15

Describe contrast media

Answer 15

Used to show up a certain tissue that has a similar attenuation coefficient to other tissues in the body, which would normally not show up well on an ordinary X-ray scan

The medium used has to be a good absorber of X-rays, so has to have lots of electrons and hence a high atomic number (Z)

The medium, often barium, is injected into the tissue of interest, causing the tissue to become a better absorber of X-rays, so it’s edges are more clearly defined on the final image

Question 16

Describe X-ray attenuation

Answer 16

Attenuation is the decrease in the intensity of X-rays as they travel through matter

Ordinary X-rays decrease in intensity according to the inverse square law

For a collimated (does not spread out – parallel beams) beam of X-rays, the intensity varies

This is a form of exponential decay

Bone is a better absorber of X-rays than flesh, so it has a higher attenuation coefficient

Question 17

Describe a CAT scan

Answer 17

CAT stands for computerised axial tomography

The patient lies inside a ring of X-rays detectors whilst an X-ray tube rotates around them

In modern CAT scanners the patient is moved through the ring of detectors so that a picture of the whole body can be built up

The X-ray tube exposes the patient to a fan shape beam of X-rays, and the intensities of the X-rays after passing through the body are determined by the detectors opposite the X-ray tube

This information is sent to a computer which builds up a 3D picture of the inside of the patient

Slices or cross-sections through the patient can then be viewed

Question 18

What are the advantages and disadvantages of CAT scans

Answer 18

Advantages

You can see 3D images compared to 2D images produced by conventional X-rays

Tumours can be located accurately

They are better at distinguishing between tissues with similar attenuation

Disadvantage

Exposes the patient to several years worth of background ionising radiation

Question 19

Describe the gamma camera

Answer 19

Used to detect gamma ray photons

The photons pass through a collimator (a series of small lead tubes that ensures only photons travelling straight that are passed through it) The rest are absorbed by the lead

The gamma ray photons hit the crystal, which is a scintillator as it produces flashes of visible light (light photons) when hit by a gamma ray photons

A visible light photon is used by a photocathode to release an electron via e photoelectric effect

The electron enters a photomultiplier tube and is accelerated to a series of dynodes at increasing voltages. Each time it collides with a dynode it releases two or three secondary electrons which are also accelerated to the next dynode. By the time it gets to the final dynode there are thousands of electrons hitting it

This forms an electrical pulse which is processed by a computer to display a dot on the screen

Each pixel on the screen corresponding to one photomultiplier tube

Question 20

How is the gamma camera used?

Answer 20

The image can be coloured to show areas where lots of gamma ray photons were emitted compared to areas that only emitted a few

It is often used to check blood flow through organs, especially kidneys, as a damaged kidney will not show up as well as a normally functioning kidney as it has reduced blood flow through it

To produce the image a person has to be given a beta positive or gamma ray emitting substance

Question 21

What is a PET scan?

Answer 21

PET stands for positron emission tomography

It is another technique that utilises gamma ray photons

The radiopharmaceuticals (radioactive substances that can be taken into the body) used to emit positrons (beta-positive radiation)

These positrons soon collide with electrons and the two annihilate each other producing two gamma photons in the process

These gamma ray photons travel in completely opposite directions

Each pair of gamma photons produced is picked up by opposite detectors, and by measuring the tiny time difference in the detection of the photons, the position of their emission can be calculated

Gradually a 3D image of the distribution of the tracer can be built up by the computer

This is often used to show up tumours, but can also be used to monitor brain activity

The tracer used is usually glucose tagged with Fluorine-18, so it will be taken up more in tissues with a high rate of respiration

Question 22

Describe ultrasound

Answer 22

Ultrasound is a sound wave with a frequency above the limit of human hearing

Ultrasound is produced by applying a voltage across a piezoelectric crystal

A piezoelectric crystal contracts by around 1% when a pd is applied across it, and this creates an ultrasonic wave

Ultrasound waves incident on the crystal also cause it to contract, and this produces an emf across it

This effect is used to detect reflected ultrasound waves

Question 23

What is a transducer?

Answer 23

A device that converts a non-electrical signal into an electrical signal

Question 24

Why is ultrasound used in medical imaging?

Answer 24

Because the ultrasound waves are partially reflected at the boundary between two different materials (e.g. Bone and tissue)

The fraction of ultrasound wave reflected can be calculated if we know the acoustic impedance

The fraction of the original intensity can be worked out.

Question 25

Why is gel used with ultrasounds?

Answer 25

The fraction of the original intensity that is reflected at the air-skin boundary is around 99.95%, hence most ultrasound would be lost before even entering the body

Ultrasound gel with a similar acoustic impedance to skin is used between the transducer and the skin to minimise this effect

This is called impedance matching

Question 26

Describe an A-scan

Answer 26

An A-scan is the simplest type of scan

A pulse of ultrasound is sent into the body and the reflected waves are displayed as pulses on a voltage-time graph

The depth of the tissues can be determined by the time taken for the pulse to be received

The type of reflecting tissue can be gathered from the amplitude of each pulse

The thickness of a certain material can also be calculated by working out the distance travelled by the ultrasound and dividing it by 2, as the reflected wave has to travel back through the material again

Question 27

Describe a B-scan

Answer 27

A B-scan is a detailed image of the area of interest built up by moving the ultrasound transducer across the area

Each reflected pulse is analysed to determine the depth of the reflecting material (the time taken for the reflection to be detected) and the type of reflecting material, from the p.d. generated by the reflected wave

Surfaces that strongly reflect ultrasound waves appear brighter (bone) than areas that don’t (tissue)

This is the scan that is typically used to show a foetus in the womb

Question 28

Describe a Doppler Ultrasound

Answer 28

This is used to check that blood is flowing smoothly (at the same speed) through a blood vessel, and that the heart is functioning properly

If the blood is flowing at different speeds in different area, it indicates that a blockage may be forming

The procedure uses the Doppler Effect - blood cells moving away from an incident ultrasonic wave reflect the waves at slightly longer wavelength (and consequently lower frequency), as each wave is reflected at a distance slightly further away from the source, so the string of reflected waves is slightly longer

The opposite applies if the blood cells are moving towards the source of the ultrasonic waves

Question 29

Describe an MRI scan

Answer 29

When protons spin (which they do all the time) they behave as tiny magnets when placed in an external magnetic field (as they are charged, so when they spin they create a current, which creates a magnetic field)

Hence when a strong external magnetic field is applied, they align themselves with it

However they do not align directly with it, rather they process around it at an angle, like a gyroscope in a gravitational field

The angular frequency of the precession is called the Lamor frequency

When the protons are stimulated with a radio wave, they will resonate, absorb the energy and ‘flip’ up into a higher energy state where their magnetic fields are aligned antiparallel to the external magnetic field

When the radio frequency waves are switched off, the protons ‘relax’ back to their lower energy state, emitting their excess energy in the form of radio waves of the same frequency as the ones they absorbed

The rate of relaxation is different in different tissues

This allows the different tissues to be distinguished by the rate at which they relax.

A false colour image can then be produced by a computer, showing the different tissues

Question 30

What do MRI scanners consist of

Answer 30

A large superconducting magnet to produce the strong external magnetic field. Kept around 4K by liquid helium (to reduce resistance to virtually 0)

One radio frequency coil to transmit and one to receive radio waves

A set of gradient coils, to produce small differences in the external magnetic field across the patient

Question 31

What are the advantages of MRI?

Answer 31

Doesn’t use ionising radiation

Non-invasive

Gives better contrast between soft tissues than CAT scans

3D images and cross sections can be produced

Easy to locate tumours

Question 32

What are the disadvantages of MRI?

Answer 32

Patients with pacemakers or surgical pins cannot undergo an MRI as the metal heats up

The scanners are expensive and take a long time to produce an image

They have to be shielded from any external radio waves

Question 33

Explain how a collimator in a Gamma Camera works

Answer 33

Gamma ray photons travel along the axis of lead tubes, and this allows parallel gamma rays

Having narrow lead tubes makes the image sharper

Question 34

Explain how a scintillator in a gamma camera works

Answer 34

Gamma ray photons hit the scintillator and produce thousands of visible light photons

Question 35

Explain how photomultiplier tubes work in a gamma camera

Answer 35

A visible light photon is used by a photocathode to release an electron via the photoelectric effect

The electron enters the photomultiplier tube, where the electrons produce an electric pulse

Question 36

Explain how a computer is use in a gamma camera

Answer 36

Signal from the photomultiplier tubes are used to produce an image

Each pixel on the screen corresponds to one photomultiplier tube

Question 37

Describe how an ultrasound is used to determine the speed of the blood in an artery

Answer 37

Ultrasound is reflected by moving blood cells

The frequency / wavelength is changed

The change in frequency/wavelength is repeated to the speed of the blood

Question 38

Describe the use of medical tracers to diagnose the condition of organs

Answer 38

A radioactive substance is injected/ingested by the patient

The tracer is absorbed by the organs and shows any blockages or damage to the organs

A beta detector or gamma camera is used to detect the radiation in the body

Question 39

State the properties of X-rays

Answer 39

They are EM waves

They travel at the speed of light

They travel in a vacuum

They are transverse waves

Can cause ionisation

They are high energy photons