Unit 2 Module 4 - Medical imaging

Question 1

How were x-rays determined to be electromagnetic waves?

Answer 1

They can be polarised, and diffracted, establishing the wavelength

Question 2

How are x-rays produced?

Answer 2

A high voltage and good vacuum is needed in a tube with a cathode and anode. Electrons are accelerated from the cathode to the anode. If they have enough energy when hitting the anode, x-rays are produced.

Question 3

Explain the graph of x-ray intensity against wavelength.

Answer 3

There is a minimum wavelength which is associated with the energy of the electrons.
There are sharp peaks at certain wavelengths, similar to line spectra. They are characteristic of the elements of the anode. The high speed electrons hitting the anode dislodge electrons from atoms in the anode. When other electrons fall back into the spaces left behind, they emit x-rays of these characteristic wavelengths.

Question 4

How can x-rays create photoelectrons?

Answer 4

The electrons absorb the energy of the photon and gain enough energy to be released from the surface of the metal. The work function is so small it can be ignored, so energy of photon = energy of electron.

Question 5

What is pair production?

Answer 5

When an x-ray photon collides with a particle, it spontaneously produces an electron and a positron.
energy of x-ray photon = positron + electron
Due to E = mc^2 we can work out the required energy

Question 6

What is the Compton effect?

Answer 6

When an x-ray hits an electron it is deflected with a longer wavelength, as it has lost energy. The larger the angle the greater energy lost.

Question 7

How do the energy required for x-ray interactions increase?

Answer 7

Photoelectrons < compton effect < pair production

Question 8

What is intensity?

Answer 8

The power per unit cross - sectional area

Question 9

How does intensity decrease when spreading out from a source?

Answer 9

The intensity is inversely proportional to the square of the distance traveled

Question 10

What is collimated beam of x-rays?

Answer 10

A parallel beam

Question 11

How does the absorption of x-rays vary?

Answer 11

The frequency of the x-rays

Question 12

Explain the equation I=I0e-ux

Answer 12

I is intensity
I0 is initial intensity
u is the attenuation coefficient
x is the distance traveled through the medium

Question 13

What is half thickness?

Answer 13

The distance for halving the intensity

Question 14

How can x-ray images be enhanced?

Answer 14

• Photographic film which is more sensitive to x-rays, by putting a fluorescent plate behind the film
• Use a contrast medium which absorbs the x-rays improving contrast
• use an image intensifier. Have a screen of phosphor that glows when a x-ray photon hits it. This can be recorded by the a digital camera.

Question 15

What are the difficulties with 2D x-rays?

Answer 15

• If the break is a hairline fracture

- If bones block each other

Question 16

What are angiograms?

Answer 16

Xray is taken then a contrast medium is injected into the bloodstream and a second xray is taken. The images are subtracted to get an image of the constract

Question 17

What are computerised axial tomography scans?

Answer 17

It has a fan shaped xray beam which is moved around the patient. There are thousands of detectors in a ring around the patient. It produces thin slices which can be made into a 3D image.

Question 18

What are the advantages and disadvantages of CT scans?

Answer 18

Advantages - can be done quickly and initial cost is less than MRI
Disadvantages - dose of radiation to patient

Question 19

Explain the equation I=I0e-ux

Answer 19

I is intensity
I0 is initial intensity
u is the attenuation coefficient
x is the distance traveled through the medium

Question 20

What is half thickness?

Answer 20

The distance for halving the intensity

Question 21

How can x-ray images be enhanced?

Answer 21

• Photographic film which is more sensitive to x-rays, by putting a fluorescent plate behind the film
• Use a contrast medium which absorbs the x-rays improving contrast
• use an image intensifier. Have a screen of phosphor that glows when a x-ray photon hits it. This can be recorded by the a digital camera.

Question 22

What are the difficulties with 2D x-rays?

Answer 22

• If the break is a hairline fracture

- If bones block each other

Question 23

What are angiograms?

Answer 23

Xray is taken then a contrast medium is injected into the bloodstream and a second xray is taken. The images are subtracted to get an image of the constract

Question 24

What are computerised axial tomography scans?

Answer 24

It has a fan shaped xray beam which is moved around the patient. There are thousands of detectors in a ring around the patient. It produces thin slices which can be made into a 3D image.

Question 25

What are the advantages and disadvantages of CT scans?

Answer 25

Advantages - can be done quickly and initial cost is less than MRI
Disadvantages - dose of radiation to patient

Question 26

What are the advantages and disadvantages of MRI scans?

Answer 26

Advantages - quality of information is high. No ionising radiation used. Bone is no barrier.
Disadvantages - high magnetic field potentially dangerous, expensive equipment and scan takes a long time. No metal objects can be scanned.

Question 27

What is a gamma camera?

Answer 27

It is a detector of gamma photons emitted by a source inside the patient.
It is a block of lead with thousands of vertical parallel holes, collimating the beam so any going sideways will be absorbed.
The photons enter a large crystal of sodium iodide which emits a tiny pinprick of visible light. There are photomultiplier tubes which give an electrical pulse for every flash. All the tubes give a signal, those further away give a weak output. An image gradually builds up on the screen.

Question 28

What is the use of the gamma camera?

Answer 28

The radioactive material can be incorporated into many kinds of molecules so can be directed to a number of organs. It can indicate function of these organs. A 3D image can be built up using two gamma cameras at right angles to each other.

Question 29

What is positron emission tomography?

Answer 29

A positron is emitted and an electron and itself are annihilated forming two gamma photons emitted into opposite directions. These are detected and a scan is produced.

Question 30

Explain nuclear precession.

Answer 30

The hydrogen atom has a single proton which spins on its axis. This spinning gives the proton a very small magnetic property called its magnetic moment. When in a magnetic field the proton experiences a torque and so it precesses just like a gyroscope. The frequency of precession is called the Larmor frequency.

Question 31

What is larmor frequency?

Answer 31

4.25x10^7 x magnetic flux density

Question 32

How can a nucleus be made to resonate?

Answer 32

A magnetic field is altered using an alternating current of radio frequency fR in coils placed in the magnetic field. when the radio frequency equals the Larmor frequency a large amount of energy is absorbed by the proton, and flips into its higher energy state.
The proton relaxes into its lower energy state and releases the energy as a radio wave.

Question 33

What are the advantages and disadvantages of MRI scans?

Answer 33

Advantages - quality of information is high. No ionising radiation used.
Disadvantages - high magnetic field potentially dangerous, expensive equipment and scan takes a long time.

Question 34

How is the magnetic field created in MRI scans?

Answer 34

It is produced by coils carrying huge currents in wires kept near absolute zero, using superconductivity. It is an electromagnet..

Question 35

Explain the arrangement of magnets in an MRI scanner.

Answer 35

There is a main magnet which produces a very large magnetic field. There are additional magnets which alter the strength of the magnetic field of the main magnet slightly from place to place. This means that the Lamor frequency is different for every part of the body. This is why it takes so long - the frequency of the radiowaves must be changed so all the points are scanned.

Question 36

Why are the radiowaves sent in pulses?

Answer 36

So the coil can pick up the emitted radiowaves from the patient.

Question 37

Explain how endoscopes can be used.

Answer 37

They have optic fibres that can be inserted into the body openings. Light is passed down one set of optic fibres and then reflected back up a different set of optic fibres that are arranged so that they have exactly the same arrangement in the bundle at the bottom at the top.

Question 38

Explain the piezoelectric effect

Answer 38

When certain crystals have a potential difference applied to them they contract. When a high frequency alternating pd is applied the crystals oscillate at the frequency of the signal, sending out ultrasound. When it receives an ultrasound a pd is created across it.

Question 39

What are the principles of ultrasound scanning?

Answer 39

• Ultrasound is reflected from surfaces rather than going right through the body.
• Ultrasound must be pulsed, so there is time for reflected echoes to be picked up by the transducer.

Question 40

What is the pulse repetition frequency?

Answer 40

The maximum frequency of ultrasound pulses, to allow time for ultrasound to be sent out and received.

Question 41

What are the different types of ultrasound scanning?

Answer 41

Type A - no photo is produced but measurements can be taken from it to determine dimensions
Type B - an array of transducers are used together with fanning out of ultrasonic beam across the body. Many returning echoes are recorded and a picture is built up.

Question 42

What is acoustic impedance?

Answer 42

It is used to determine the fraction of the intensity refracted at a boundary between two materials of different acoustic impedance’s. Acoustic impedance is determined by Z = density of material x speed sound travels through material.

Question 43

What is impedance matching?

Answer 43

The need to match up similar impedance’s to get a good reflection value. Gel and skin have similar impedance values so most is transmitted not reflected.

Question 44

What are Doppler scans?

Answer 44

They send out ultrasound. If is has a greater wavelength when returning then the blood is flowing away, and if it has a shorter wavelength then the blood is flowing towards you.

Question 45

State two factors that determine the value of the acoustic impendance

Answer 45

density of material and and speed of ultrasound

Question 46

What are the advantages and disadvantages of MRI scanning?

Answer 46

Advantages: no ionising radiation, no radiation to patient or staff, better soft tissue contrast, 3D images

Disadvantages: attracts steel due to magnetic field, people with pace makers cannot use it and people with pins cannot use it because the metal will heat up

Question 47

Describe how a CT scan is produced

Answer 47

X-rays pass through patient, and x-ray source is rotated to give a slice. It is moved and repeated to create a 3D image. Different tissues have different absorptions. Bone has a high Z number. Attenuation is photoelectric effect.

Question 48

Why are CT scans not used in checking healthy patients?

Answer 48

Uses ionising radiation which causes cancer. They are expensive.

Question 49

What would happen if gel isn’t used in an ultrasound scan?

Answer 49

All the ultrasound would have been reflected at the skin because there is large differences between the acoustic impendances. It would give no peaks.

Question 50

Describe the use of contrast medium in x-rays

Answer 50

x-rays travel through the body and hit a screen with varying intensity, causing different shades to appear on the screen- xrays darken the screen. This is because different tissues have different absorptions by the photoelectric effect.
contrast medium has a high attenuation coefficient because of the high Z number. Barium meal is swallowed orally. It allows the xrays to differentiate between soft tissues because they have similar absorptions, so the barium gives a contrast.

Question 51

Explain the xray intensifying screen

Answer 51

xray photons penetrate the patient. Tissues have different absorptions, for example bone has high absorption so less xray photons reach the film. The film is blackened by the photons. xrays hit phosphor atoms, which become excited and emit light photons when returning to ground state, which causes extra fogging of the film. There is more contrast on the film giving a better image. Lower intensity of xrays can be used

Question 52

explain why a single crystal of quartz is piezoelectric

Answer 52

it changes change under alternating pd

Question 53

explain the need for coupling medium in ultrasound scanning

Answer 53

so ultrasound is transmitted rather than reflected. most ultrasound is reflected when difference in acoustic impendance is large.

Question 54

explain how xrays are generated and the energy conversions that occur

Answer 54

electrons are accelerated from cathode to anode under high potential difference. electrons undergo deceleration when they hit the anode which generates xrays. when an electron is knocked out of orbit from the other electron, other electrons drop into the energy level, which is characteristic of the target.
kinetic energy is transferred into photon energy, but mostly thermal energy. a vacuum is used so the electrons aren’t slowed down