C27 - Medical Imaging

Question 1

How are X-ray photons produced?

Answer 1

When fast-moving electrons are decelerated by interaction with atoms of a metal such as tungsten.

The kinetic energy of the electrons is transform into X-ray photons.

Question 2

How does an X-ray tube work?

Answer 2

It consists of an evacuated tube containing 2 electrodes.
(The tube is evacuated so that electrons pass through the tube without interacting with gas atoms).

An external power source is used to create a large (30-100kV) p.d. between the electrodes.

The cathode (-) is a heater which produces electrons by thermionic emission. These e- are accelerated towards the anode (+). 
The anode is made from a metal, known as the target metal. (E.g. tungsten that has a high melting point). 

X-ray photons are produced when the e- are decelerated by the anode.
(Energy output of X-rays is less than 1% Ek of incident e-. Remaining energy is transferred to thermal energy of the anode).

The tube is lined with lead.

Question 3

How can the wavelength from an X-ray tube be found?

Answer 3

Maximum energy of X-ray photons = maximum kinetic energy of electron

Therefore:
hc/λ = eV

So λ = hc/eV

Wavelength from an X-ray tube is inversely proportional to the accelerating potential difference.

Question 4

What happens when current in an X-ray tube is increased?

Answer 4

Increasing tube current just increases intensity of the X-rays.

Question 5

What’s attenuation?

Answer 5

The decrease in intensity of electromagnetic radiation (e.g. X-ray) as it passes through matter and/or space.

(Bone attenuated X-rays more than soft tissues).

Question 6

What are the 4 attenuation mechanisms (when X-ray photos interact with atoms)?

Answer 6

Simple scatter - X-ray photon scattered elastically by an electron

Photoelectric effect - X-ray photon disappears and removes an electron from the atom

Compton scattering - X-ray photon is scattered by an electron. It’s energy is reduced and the electron is ejected from the atom

Pair production - X-ray photon disappears to produce an electron-positron pair

Question 7

What happens in the X-ray ‘simple scatter’ attenuation mechanism?

Answer 7

The X-ray photon interacts with an electron in the atom, but has less energy than the energy required to remove the electron.
Therefore the photon simply bounces off (is scattered) without any change to its energy.

(This is important for photons with 1-20keV energy - not used for hospital radiography).

Question 8

What happens in the X-ray ‘photoelectric effect’ attenuation mechanism?

Answer 8

The X-ray photon is absorbed by one of the electrons in the atom.
The electron uses this energy to escape from the atom.

(Significant for photons with energy below 100 keV. Attenuation of X-rays by this mechanism is dominant when an X-ray image is taken, because hospital X-ray machines use 30-100 kV supplies).

Question 9

What happens in the X-ray ‘compton scattering’ attenuation mechanism?

Answer 9

The incoming X-ray photon interacts with an electron in the atom.
The electron is ejected from the atom, but the X-ray photon does disappear completely. Instead, it is scattered with reduced energy.

In the interaction, both energy and momentum are conserved.

(Significant for photons with 0.5-5 MeV energy).

Question 10

What happens in the X-ray ‘pair production’ attenuation mechanism?

Answer 10

An X-ray photon interacts with the nucleus of the atom.

It disappears and the electromagnetic energy of the photon is used to create an electron and its antiparticle, a positron.

(Only occurs when X-ray photons have energy equal to / greater than 1.02 MeV)

Question 11

How is intensity related to the thickness of a substance, for a given substance and energy of photons?

Answer 11

Intensity falls exponentially with thickness of a substance.

I = I₀^e-μx

Question 12

What equation shows the relationship between intensity and thickness in attenuation?

Answer 12

I = I₀^e-μx

Where:
I₀ is the intensity before any absorption
x is the thickness of the substance
μ is the attenuation coefficient / absorption coefficient of the substance

(Bone is better absorber or X-rays than muscle so has a larger μ value, measured in m⁻¹).

Question 13

Why are contrast mediums used?

Answer 13

Soft tissues have low absorption coefficients. Contract mediums improve visibility of their internal structures in X-ray.
Two of the most common are iodine and barium compounds.

Question 14

Why are barium and iodine used as contrast mediums?

Answer 14

Why have large atomic numbers, Z. (Average Z for soft tissues is 7. I = 53. Ba = 56)

For X-ray imaging, he dominant attenuation mechanism is the photoelectric effect, where the attenuation coefficient is proportional to the atomic number cubed.
(μ ∝ Z³)

Therefore they’re 430-510 times more absorbent than soft tissues.

Question 15

How are iodine and barium used as contrast mediums?

Answer 15

Iodine - used in liquids.
An organic compound of iodine is injected into blood vessels.

BaSO4 given in form of white liquid mixture. Patient swallows before X-ray taken.

Question 16

What’s a CAT scan?

Answer 16

Computerised axial tomography

A large number of X-ray images are taken from different angles and assembled to a 3D image.

Question 17

What happens in a CAT scan?

Answer 17

An X-ray tube, and the detectors opposite it, rotate around the patient.

The X-ray tube produces a fan-shaped beam of X-rays, 1-10mm thick.
The thin beam irradiates a thin slice of the patient, and the X-rays are attenuated by different amounts by different tissues.
The intensity of the transmitted X-rays are recorded by detectors which send electrical signals to a computer.

Each time the X-ray tube and detectors make a 360° rotation, a 2D image is produced and the patient moves along 1cm to produce the next ‘slice’ of the image.

Question 18

What are the advantages and disadvantages of CAT scans?

Answer 18

+ Create 3D images
+ Can differentiate between soft tissues of similar attenuation coefficients

• Traditional X-ray is quicker and cheaper
• X-rays are ionising and harmful
• Patients have to remain very still, otherwise image will be blurred

Question 19

What isotopes are used as medical tracers?

Answer 19

Fluorine - 18

Technetium/Tc - 99m

Question 20

What’s a radiopharmaceutical?

Answer 20

A radioisotope chemically combined with elements that will target particular tissues in order to ensure that the radioisotope reaches the correct organ or tumour (for diagnosis or treatment).

(Also known as a medial tracer)

Question 21

What’s a medical tracer?

Answer 21

A radiopharmaceutical / a compound labelled with a radioisotope that can be traced inside the body using a gamma camera.

Question 22

What does a gamma camera do?

Answer 22

Detect gamma photons emitted from the medical tracer injected into the patient.
An image is produced indicating the concentration of the tracer within the patient’s body.

Question 23

What happens in a gamma camera?

Answer 23

Gamma photons travel towards a collimator (a honey comb of long, thin tubes made of lead).
Any photons arriving at an angle to the axis of the tubes are absorbed by the tube. Thus only those travelling along the axis of the tubes reach the scintillator.

Photons of visible light travel through the light guide into photomultiplier tubes that are connected to a computer.

Question 24

What are the parts of a gamma camera?

Answer 24

Collimator
Scintillator
Light guide
Photomultiplier tubes
Computer 
This then produces an image

Question 25

What’s a collimator?

Answer 25

Part of a gamma camera - a honeycomb of long, thin tubes of lead which absorb photons that arrive at at an angle at the axis of the tubes.

This is to produce a clear image.

Question 26

What’s a scintillator?

Answer 26

Part of a gamma camera, often made of sodium iodide, which produces thousands of photons of visible light when struck by a single gamma photon.

Question 27

What is a photomultiplier tube?

Answer 27

Apparatus which converts a photon of visible light into an electrical pulse.

Question 28

How does a gamma camera differ from X-ray imaging?

Answer 28

A gamma camera produces an image that shows the function and processes of the body rather than its anatomy.

Question 29

How does medical tracer fluorine-18 decay?

Answer 29

F-18 decays into an oxygen - 18 molecule, a positron, a neutrino and a gamma photon (which is detected outside the body).

Question 30

What medical tracers are commonly used for PET scans?

Answer 30

FDG - fluorodeoxyglucose. (Similar to natural glucose however it is tagged with fluorine-18).

Carbon monoxide (with C-11 isotope).

Question 31

How does a PET scanner work?

Answer 31

The patient lies on a table surrounded by a ring of gamma detectors.
Each detector consists of a photomultiplier tube and a sodium iodide scintillator, connected to a computer. It produces a voltage pulse / a signal for every gamma photon incident at its scintillator.

The patient is injected with tracer (FDG). The PET scanner detects gamma photons emitted when positrons from decaying fluorine-18 annihilate with electrons in the patient (NOT gamma photons emitted by F-18 decay only).

The annihilation of a positron and electron produces 2 gamma photons in opposite directions (momentum conserved).

The computer can then determine point of annihilation from the difference in arrival times from two detectors opposite one another.

Image is generated (shows different colours and brightness).

Question 32

What does PET (scan) stand for?

Answer 32

Positron emission tomography scan

Question 33

What do PET scanners detect?

Answer 33

The PET scanner detects gamma photons emitted when positrons from decaying fluorine-18 annihilate with electrons in the patient (NOT gamma photons emitted by F-18 decay only).

The annihilation of a positron and electron produces 2 gamma photons in opposite directions (momentum conserved).

e− + e+ → 2γ

Question 34

What are the advantages and disadvantages of PET?

Answer 34

+ Non-invasive technique
+ Used to help diagnose different types of cancers, plan heart surgery and observe brain function
+ Used to assess the effect of new medicines on organs

• Very expensive
• Only found at larger hospitals for those with complex problems

Question 35

What’s an ultrasound transducer?

Answer 35

A device used to generate and receive ultrasound.

It changes electrical energy to sound and sound energy to electrical via piezoelectric effect.

Question 36

What’s the piezoelectric effect?

Answer 36

The production of an e.m.f. by some crystals (e.g. quartz) when they are compressed, stretched, twisted or distorted.

The reverse is also possible, an applied external p.d. can cause the crystal to stretch or compress.

Question 37

How does an ultrasound transducer work?

Answer 37

A high freq (5MHz) alternating p.d. is applied across opposite faces of a crystal. This repeatedly compresses and expands the crystal.
The freq chosen should be the same as the natural freq of the crystal therefore, the crustal resonates and produces an intense ultrasound signal. (Pulses of ultrasound emitted)

The same transducer will also detect ultrasound.
Ultrasound incident on the crystal will make it vibrate (by tiny amounts).
This generates an alternating e.m.f. across the ends of the crystal.

Question 38

What’s an A-scan?

Answer 38

Simplest type of ultrasound scan.
Produces time against voltage graph.

A single transducer is used to record along a straight line, and can be used to determine thickness of bone or distance between retina and lens.

Question 39

What a B-scan?

Answer 39

A type of ultrasound scan which produces a 2D image.

The transducer gets moved over the patient’s skin and, for each position, the computer produces a row of dots, each corresponding to the boundary between 2 tissues.

Brightness of the dots is proportional to intensity of reflected ultrasound pulse.

Question 40

What happens when ultrasound is incident at a boundary between 2 substances?

Answer 40

A proportion of it’s intensity will be reflected and the remainder will be refracted.
The fraction of ultrasound reflected depends on the acoustic impedance of both media.

Question 41

What is acoustic impedance, Z?

Answer 41

It’s defined as the product of the density, p, of the substance and the speed, c, of ultrasound in that substance.

Z = pc

(Its units are kg/m2 s)

Question 42

What’s the intensity reflection coefficient? *

Answer 42

The ratio Ir / I0

The ratio of reflected intensity to the incident intensity.

(There will be greater reflection when the values of acoustic impedances are very different).

Question 43

What equation shows the the ratio of reflected to incident intensity?

Answer 43

Ir / I0 = (Z2−Z1)2 / (Z2 +Z1)2

Where:
Ir is reflected intensity
I0 is incident intensity
Z1 and Z2 are the acoustic impedances of the The substances (e.g. bone and skin).

(There will be greater reflection when the values of acoustic impedances are very different).

Question 44

What does impedance matching / acoustic matching suggest?

Answer 44

That 2 substances (e.g. coupling gel and skin) have similar values of acoustic impedance.

Question 45

Why is a coupling gel used for ultrasound?

Answer 45

It has a similar value of acoustic impedance to skin.
It fills air gaps between the transducer and skin and ensures that almost all of the ultrasound enters the patient’s body; otherwise the waves would be reflected.

Question 46

How is a colour Doppler (ultrasound) scan carried out?

Answer 46

The ultrasound transducer is pressed light over the skin above blood vessels.

The transducer sends pulses of ultrasound and receives the reflected pulses.
Ultrasound reflected of tissues will return with the same frequency and wavelength.
However waves reflected off many moving blood cells will have a changed frequency. (Freq increased when blood moves towards the transducer and vice versa).

The change in frequency is proportional to the speed of the blood.

Question 47

What equation is used to show the change in observed ultrasound frequency?

Answer 47

∆f / f = 2vcosθ / c

Where:
f is the original ultrasound frequency
v is the speed of the moving blood cells
c is the speed of the ultrasound in the blood

f is directly proportional to the speed of blood flow

Question 48

What’s an ultrasound?

Answer 48

Longitudinal wave with a frequency above 20kHz