X-ray

Question 1

glass tube

Answer 1

It is sealed and evacuated. It allows electrons to travel unimpeded

Question 2

rotating anode

Answer 2

Rotation of the anode spreads out the heated area on the target It is made of metal (tugsten or molybdenum) and emits X-rays when hit by (energetic) electrons

Question 3

filament [or cathode]

Answer 3

It is the heat source necessary for the release of electrons from surface of cathode by thermionic emission

Question 4

lead housing

Answer 4

It prevents X-rays from escaping in unwanted directions

Question 5

Explain why the anode is rotated

Answer 5

The anode is rotated so that the heat produced is spread over a greater volume/area/section This allows more energetic X-rays to be produced [or allows X-rays to be generated for longer] without risk of melting the anode.

Question 6

Explain why the anode has a bevelled edge.

Answer 6

The bevelled edge gives larger target area for the electrons
but a smaller source area from which the X-rays are produced. This gives a sharper image beam of X-rays and produces a sharper image.

Question 7

Define for a material, the linear attenuation coefficient

Answer 7

The linear attenuation coefficient, is the fraction of X-rays removed per unit thickness of the material.

Question 8

Define for a material, the half-value thickness.

Answer 8

The thickness of the material which will reduce the intensity to half its original level for a specified energy of the X-rays

Question 9

State the purpose of the fluorescent screen,

Answer 9

It converts X rays to visible photons

Question 10

state the purpose of photocathode

Answer 10

It converts photons to emission of electrons

Question 11

State the purpose the anodes,

Answer 11

They increases the kinetic energy of electrons travelling from cathode to anode and they also focus the rays of electrons to produce a faithful image

Question 12

state the purpose he fluorescent screen

Answer 12

It converts (increased) electron energy into light photons

Question 13

Give one example of a medical application for which an image intensifier might be used. Explain why the use of an image intensifier is required.

Answer 13

It might be used for a dynamic process - such as fluid flow It cuts the radiation dose whilst still providing good image [or allows multiple or continuous use of X rays]

Question 14

State two methods which can be used to increase the intensity of the X-ray beam produced by the tube.

Answer 14

Method 1: increasing the potential difference (voltage) across the tube
Method 2: increasing the tube current or increasing the filament temperature

Question 15

For each method of increasing intensity, state the effect on the maximum X-ray photon energy.

Answer 15

(Method 1: increasing the potential difference (voltage) across the tube)
Method 1: will increase the maximum photon energy

(Method 2: increasing the tube current or increasing the filament temperature )
Method 2: will not change the maximum photon energy

Question 16

Before taking an X-ray photograph, the X-ray beam emerging from the tube is passed through an aluminium filter. State and explain the reason for filtering the X-rays

Answer 16

The aluminium filter reduces the intensity of low energy photons but hardly changes intensity of high energy photons at all. We need high energy for the X-ray picture production [low energy tend to be attenuated by the body] Therefore reducing low energy photons reduces the dose received by patient without affacting picture quality.

Question 17

When an X-ray image is obtained of certain organs, image contrast enhancement is necessary. Explain why image contrast enhancement is needed and describe how this might be achieved.

Answer 17

For a clear image you need a large difference in densities between part being investigated and parts around it. When this is not natural, you can add material to the part under investigation which has high density to provide good attenuation of X-rays An example of a suitable contrast material for a barium meal is barium sulfate

Question 18

In the course of diagnosis and treatment of a child’s broken arm, several images of the arm are required.
In each case, state which imaging technique would probably be used and give two reasons for the choice.
(a) Broken arm

Answer 18

You would use an X-ray because an x-ray image gives:
a sharp image
with good contrast
and good resolution.

Question 19

Similarly, to check the progress of a woman’s pregnancy, several images of the foetus are required.
(b) Foetus

Answer 19

You would use an ultrasound scan because ultrasound:
is non-ionising and therefore safe to use on the unborn foetus,
detects change in tissue type and so allows the development to be monitored and
allows real-time imaging to check on heart development and movement.

Question 20

Explain why the spectrum has spikes at specific photon energies.

Answer 20

The spikes are specific to the anode target element. The energy level transitions of electrons within the atoms are fixed. That is why they are called characteristic spikes.

Question 21

scintillator crystal(s)/fluorescent screen

Answer 21

It converts X-ray photons into light photons

Question 22

photocathode

Answer 22

Incident light energy on the photocathose releases electrons. The number of electrons released is proportional to the X-ray intensity

Question 23

anodes

Answer 23

They increase the kinetic energy of the electrons by accelerating them towards their positive charge. The are also positioned to focus the electrons to form an image

Question 24

fluorescent screen

Answer 24

This converts the kinetic energy of the incident electrons into light photons

Question 25

Explain what is meant by the half-value thickness of lead for X-rays.

Answer 25

The half-value thickness is the thickness of lead needed to reduce intensity of the X-rays of a specific energy by half

Question 26

The X-ray spectrum for a certain X-ray tube target is shown below. Explain the process which gives rise to spikes at certain photon energies.

Answer 26

Electrons strike the metal anode and ionise/excite the target atoms. This causes vacancies in inner electron levels. Excited/higher electrons fall to inner/lower energy level states and the change in energy is released as a electromagnetic photon. Most of these photons are in the infra-red energy range but some are of X-Ray energy. The gaps between the energy levels in a particular atom are fixed. Therefore the transitions that produce X-rays will be fixed for a particular target element. This is why the X-rays produced will be of a certain energy - these X-rays form the characteristic peaks.

Question 27

Explain how the intensifying screens in the film cassette achieve their purpose and state their benefit to the patient.

Answer 27

Most X-rays pass through the cassette and do not interact with it at all. Including the two intensifying screen layers as well as the film increases the chance of X-ray interaction being detected. Therefore the use of such screens reduces the radiation dose to the patient by making the exposure time shorter. X-rays that interact with the film cause exposure. The intensifying screens have a fluorescent coating; this converts any X-ray photons that interact with it into light photons. The light photons produced, that are emitted in the direction of the film, expose the film in the correct place due to closeness of the screens to the film. You therefore do not need such a big dose of X-rays to achieve the required exposure.

Question 28

X-rays are used in a CT scanner. Describe briefly how a CT scanner produces an image

Answer 28

Narrow fan-shaped beam of X-rays X ray generator rotated (in circular path) around patient on a ring Detectors are positioned on the ring opposite the generators in an arc long enough to receive the fan of X-rays. The detectors register the transmitted intensity from a single blast of X-rays The transmitter and detectors are then rotated so that the next section can be scanned. This is repeated while the body slowly moves on a motorised platform. In this way a spiral scan pattern is obtained. Detector output is sent to a computer which (over time) produces a series of cross sectional images that can be analysed

Question 29

Explain the term contrast enhancement when applied to X-ray photographic imaging and explain how such enhancement is achieved

Answer 29

The clearest images are obtained when there is a large difference in density/proton number between the part of the body being investigated and the surrounding parts. You can improve contrast by introducing a high proton number material into a bady cavity, such as barium sulphate - this is done as a 'barium meal' or 'barium enema'.

Question 30

What is meant by half-value thickness?

Answer 30

Half-value thickness is the thickness of material needed to reduce the transmitted intensity of the X-rays to half of the intensity incident on the material.