Module 6 - 6.5.1 Using X-Rays

Question 1

X Ray Production (5)

Answer 1

• electrons emitted from a filament heater (heated by an electric current)
• the high p.d. accelerates the electrons between the filament heater (cathode) and the anode (metal with high m.p.)
• if the electrons have enough energy, when they hit the anode, some x ray photons will be emitted
• due to the electron interacting with the electric field around the nucleus
• the energy of the photon is equal to the Ek lost by the electron when decelerating rapidly by smashing into the anode

Question 2

Features of the X Ray Production Process (4)

Answer 2

• happens in a vacuum: electrons would lose energy when colliding with gas molecules
• anode is shaped: x rays come off in desired directions
• tube surrounded by lead shielding: precent exposure to radiation
• anode cooled by oil flowing through it: Ek of electron transferred as heat to anode

Question 3

X Ray Production: Energy of Photons (5)

Answer 3

• photons can have less then the max energy, so photons have a distribution of energies
• spikes at specific photon energies
• caused by accelerated electron knocking an orbiting electron out of an energy level
• another electron from a higher level moves to occupy the vacancy
• this releases an x ray photon of wavelength corresponding to the difference in energy levels

Question 4

The intensity of x rays emitted is

Answer 4

directly proportional to the number of electrons emitted per second from the filament heater

Question 5

Interaction of X Rays with Matter (4)

Answer 5

Simple Scattering
- interacts with orbiting electron
- photon scattered: no change in energy or photon absorption
The Photoelectric Effect
- interacts with orbiting electron
- photoelectron emitted with max Ek, photon no longer exists
Compton Scattering
- interacts with orbiting electron
- electron emitted at high speed, photon continues with less energy and lower frequency
Pair Production
- interacts with nucleus
- electron and positron produced, photon no longer exists

Question 6

X rays are ionising and the electrons produced

Answer 6

can cause further ionisations

Question 7

Attenuation (definition)

Answer 7

the exponential decrease in intensity

Question 8

Collimated (definition)

Answer 8

Beams are travelling in parallel lines

Question 9

Intensity of collimated X Rays (2)

Answer 9

• hardly changes with distance when not interacting with different materials
• decreases with distance when passing through a substance

Question 10

A higher attenuation coefficient means

Answer 10

it is better at stopping x rays

Question 11

A Contract Material is and is used for …

Answer 11

• a material with a high attenuation coefficient (barium/iodine)
• to see detail of different types of soft tissues with similar attenuation coefficients

Question 12

CAT Scans can (3)

Answer 12

• provide very accurate pictures of the position of internal organs without being obscured by other structures in the body
• show soft tissues clearly
• produce 3D images

Question 13

Production of a CAT Scan Image (6)

Answer 13

• a thin fan shaped x ray beam is passed though patient
• beam detected by a ring of detectors
• x ray source is rotated around patient
• source and detectors move up about a cm
• next revolution looks at next slice of body
• computer processes data to produce 3D image