Chapter 27 Medical Imaging

Question 1

Describe the basic structure of an X-ray tube

Answer 1

• There is a VACUUM contains a CATHODE.
  There is a vacuum to prevent electrons from decelerating
• Cathode is heated which produces a beam of electrons (THERMIONIC EMISSION)
• Using VERY HIGH VOLTAGES, the electrons are accelerated to VERY HIGH VELOCITIES
• These accelerated electrons collide with a metal target called an ANODE
• The electrons rapidly decelerate and convert some of their Kinetic energy into X-rays.

Question 2

Problems in X-ray production + solutions

Answer 2

• Only little (1%) of the electron energy is used to produce the x-rays, the rest heats up the anode.
• -Anode is cooled by oil flowing around it, it also has a high SHC.
• Operators need to be protected
• • X-ray tube is surrounded by a lead shielding and the anode is shaped so that the xrays are detected through a window.

Question 3

The energy of the emitted photons depends on…

Answer 3

the energy of the incident electron

Question 4

T or F? One electron releases one X-ray photon.

Answer 4

True

Question 5

Which principle does this follow:
“Maximum energy of a photon from an X-ray tube must equal the max energy of a single electron”

Emax = hf = eV = hc/lander

Answer 5

principle of conservation of energy

Question 6

Describe the X-ray spectrum graph;

Intensity of X-rays/ Wavelength

Answer 6

The spikes (K-Lines) are produced due to the
rearrangement of electrons within an atom in the anode
following the removal of an inner shell electron.

Question 7

If the x-ray has energy below work function, then what happens?

Answer 7

It will be simple scattered

does not lose energy

Question 8

Define the attenuation mechanism: Simple Scatter

Answer 8

• occurs for X-ray energies 1-20keV
• X-ray interacts with an electron in the atom but does not ionise the atom
• energy of the scattered x-ray does not change
• hospital x-ray machines generate x-ray > 20keV so this effect is very insignificant in radiography.

Question 9

explain Compton scattering

Answer 9

• high energy photons
• electron is removed
• still some energy
• x-ray scattered with a lower energy

Question 10

explain pair production

Answer 10

• x ray of high incoming energy
• interacts with nucleus
• electron positron-pair is produced
• the positron then annihilate
• and produce a PAIR of lower energy photons

Question 11

Define the attenuation mechanism for: Photoelectric absorption

Answer 11

• x ray absorbed by an electron
• electron uses this energy to escape the atom
• an electron is released with some kinetic energy

Question 12

define attenuation

Answer 12

the reduction in intensity as a photon beam passes through matter

(caused by absorption and scattering)

Question 13

define x-ray intensity

Answer 13

the power per unit cross-sectional area

Question 14

what does collimating x-ray beams do?

Answer 14

ensures that intensity of the x-ray is constant, if travelling through a vacuum.

Question 15

explain I = Io x e^-mue x x

Answer 15

I = final x ray intensity
Io = original x-ray intensity
mue = attenuation coefficient 
x = distance travelled through material (thickness)

Question 16

absorption of x-rays varies with

Answer 16

frequency.

Question 17

explain absorption & Z

Answer 17

Z = proton number

Question 18

higher mue =

Answer 18

greater absorption