Option - Chapter 5

Question 1

Why are X-rays used for medical imaging?

Answer 1

Because they can penetrate matter and darken photographic film.

Question 2

Describe how a rotating X-ray tube works?

Answer 2

Electrons are emitted by the filament wire when it is heated by an electric current. The electrons are attracted by the rotating metal anode when it is positive relative to the filament. The step-up transformer provides an alternating PD for the filament wire and a much larger alternating PD between the anode and the filament.

Question 3

What is the focal spot?

Answer 3

The area of the anode from which X-rays are emitted.

Question 4

Why must the area of the focal spot be as small as possible?

Answer 4

To prevent the image from being blurred.

Question 5

Describe the anode, where it is placed etc

Answer 5

The anode surface is at an angle of about 70 degrees to the electron beam so that the x-rays effectively originate from a much smaller area than the impact area of the beam.
The anode is made to rotate at a steady speed so the point of impact continually changes to prevent overheating.

Question 6

What is the maximum kinetic energy of a beam electron at the anode equal to?

Answer 6

The workdone in eV on it by the anode pd.

Question 7

How to calculate the frequency fmax for a photon of maximum energy eV?

Answer 7

h fmax = eV

Question 8

What is the first point on the x-axis equal to on a relative intensity vs photon wavelength graph?

Answer 8

It is the wavelength min

Question 9

What do the spikes on an intensity curve represent?

Answer 9

They are due to beam electrons knocking out electrons from the innermost shells of the atoms in the anode.

Question 10

What does the anode PD control?

Answer 10

It controls the maximum energy of the X-ray photons from an X-ray tube.

Question 11

What does the electron beam current control?

Answer 11

It controls the intensity of the X-ray beam. This is because increasing the electron beam current increases the number of electrons hitting the anode each second which therefore increases the number of X-ray photons emitted per second.

Question 12

What does increasing the electron beam current not effect?

Answer 12

It does not affect the maximum kinetic energy of the electrons and therefore does not increase the maximum energy of the emitted X-ray photon.

Question 13

How to calculate the electrical power supplied to an X-ray tube?

Answer 13

= beam current I x anode pd V

Question 14

What does the X-ray energy produced per second depend on?

Answer 14

It depends on the electrical power supplied to an X-ray tube and the efficiency of the tube, which depends on factors such as the design of the anode and the filament.

Question 15

How do you calculate the efficiency of the tube?

Answer 15

Efficiency = (X-ray energy produced per second)/(electrical power supplied) x100%

Question 16

Define intensity of a beam of X-rays, and what is it’s unit?

Answer 16

Intensity is defined as the energy per second per unit area incident normally on a surface.
The unit is J s^-1 m^-2

Question 17

What does it mean when the intensity is attenuated?

Answer 17

When the intensity decreases the further it travels through the matter.

Question 18

How do you calculate intensity for a beam of X-ray that passes through an absorber?

Answer 18

I = Io e^ux
Io is incident intensity
u = attenuation coefficient
x = absorber thickness

Question 19

What is half-value thickness?

And how would you calculate it?

Answer 19

The thickness of a material to reduce the intensity by 50%
So you would substitute 0.5Io for Io
Giving you uX1/2 = ln2

Question 20

What is the mass attenuation coefficient?

And what is the unit?

Answer 20

It is attenuation coefficient of an absorber divided by the density of the absorber substance.
This is because the attenuation coefficient is proportional to the density.
The unit is m^2 kg^-1

Question 21

What does the mass attenuation coefficient depend on?

Answer 21

It depends on the atomic number Z of the absorber and the photon energy.

Question 22

How do you improve an x-rays quality?

Answer 22

By improving the sharpness and contrast of the image. The sharpness is improved by making the focal spot as small as possible.
The contrast is improved by using a lead collimator grid to prevent X-rays scattered by tissue and bone in the patient from reaching the film.

Question 23

Describe the order of a lead collimator grid?

Answer 23

From the top 
Filter
Limb
Table
Lead grid
The film in a lightproof wrapper

Question 24

What is a contrast medium?

Answer 24

A contrast medium is used to make the image of certain organs stand out. A contrast medium is something which can be swallowed and is a good absorber of X-rays.

Question 25

How do you reduce the patient’s exposure to X-rays?

Answer 25

Using a beam definer consisting of two pairs of thick lead plates that ensure the beam is no wider than it has to be.
They also use a metal filter between the X-ray tube and the patient is used to filter out low energy X-rays which would be otherwise absorbed by the tissue.

Question 26

How do you intensify an image from an x-ray?

Answer 26

By using an intensifying screen which will increase the rate of darkening and reduces the duration of exposure.

Question 27

What components make up an intensifying screen?

Answer 27

Plastic front cover
Sheet of calcium tungstate
Double-sided film
Calcium tungstate
Double-sided film 
calcium tungstate
Metal back

Question 28

Describe what happens in an image intensifier?

Answer 28

The light produced from the screen strikes the photocathode causing electrons to be emitted. The electrodes attract the electrons through a PD of about 25kV and focus them onto a small fluorescent viewing screen to form a visible image on the viewing screen. The image on the viewing screen is much more intense than the image on the first fluorescent screen.

Question 29

Why is the image on the viewing screen more intense than the image on the first fluorescent screen?

Answer 29

Because each electron emitted from the photocathode gains kinetic energy as it is attracted through the electrodes. Therefore each electron hitting the viewing screen causes many visible photons to be emitted from the viewing screen
And because the electrons from the photocathode are focused by the electrodes onto a much smaller than that of the photocathode.

Question 30

Describe what makes up a flat-panel detector

Answer 30

X-rays pass onto the scintillator
Photodiodes
Read out circuit

Question 31

What do the photodiodes do in a flat panel detector?

Answer 31

It detects light photons from the scintillator and generates an electrical signal in proportion to the number of photons detected.

Question 32

What are the advantages of a flat panel detector?

Answer 32

Greater sensitivity than a photographic film, enabling lower doses of radiation to be used
Uniform response in terms of dose variations, unlike photographic film, which is less uniform at high densities.
Less image distortion
The image can be processed
Portable, convenient and more economic

Question 33

What are the disadvantages of flat panel detectors?

Answer 33

They cant follow fast-changing images, cant observe continuous changes.

Question 34

How does a CT scanner work?

Answer 34

A CT scanner consists of an X-ray tube and a ring of thousands of small solid-state detectors. A patient lies on a bed which the X-ray tube automatically moves around inside of the ring, turning as it moves so the X-ray beam is always directed at the centre of the ring. The detector signals are recorded each time the X-ray tube moves around the ring through a fraction of a degree until the tube has moved through 180 degrees.

Question 35

What is a voxel?

Answer 35

The way that describes the body by considering it as a collection of small volume elements. The larger the number of voxels the greater the intensity is reduced by.