X-ray & CT

Question 1

What are the different types of imaging that are achieved using X-rays

Answer 1

2D - static Radiography & Dynamic Flouroscopy - pixels
3D - CT - images has voxels

Question 2

What are the different X-ray equipment for surgery and IR suites

Answer 2

Surgery Mobile C-arm / Robotic C-arm
IR - single plane fixed C-arm / Bi-plane fixed C-Arm

Question 3

Briefly describe how an X-ray tube works

Answer 3

In a vaccum, Electron source (cathode filament) emits electrons when heated (by current up to 7amps) towards target (Anode). electrons interact with anode material tungston and produce X-rays and heat.

Question 4

what material is used as a target and why?

Answer 4

Tungsten due to high melting point and atomic number. In order to prevent melting the anode will rotate to limit time in the electron beam.

Question 5

Describe briefly the interactions that occur in the anode.

Answer 5

1. Multiple scattering of the incoming electrons within the material with a trail of ionisations
   and excitations – energy dissipated as heat
2. Incoming electrons occasionally eject inner shell electron from the target atom. Outer shell
   electron falls down to fill the gap leading to emission of a characteristic X-ray. Energy of
   characteristic X-ray is the difference in energy between the shells
3. Incoming electrons may travel close to the nucleus of target atom and interact with its
   electric field and decelerate, changing energy and direction - loss of energy is emitted as Xray (bremsstrahlung radiation)
4. Rarely incoming electron is completely stopped in one collision and all its energy is emitted
   as an X-ray – maximum X-ray energy

Question 6

Describe the focal spot and effective focal spot

Answer 6

Focal spot – area of target material bombarded by electrons from the
filaments
* Effective focal spot – area of the X-ray beam projected onto the patient
* Effective focal spot depends on angles of the target and filament size
* Most X-ray tubes equipped with two filament sizes to provide two focal
spots sizes (fine focus FF & broad focus BF)

Question 7

What is the anode heel effect

Answer 7

More photons are reflected from the anode towards the cathode than away. Therefore there will be a more intense X-ray beam at the side of the window towards the cathode than the anode.

Question 8

Describe breifly how a transformer works

Answer 8

Transform a voltage or current level into another
* 2 wires wound around an iron core
* Current flows through one winding (1° circuit), a magnetic
field is created within the iron core
* Magnetic field induces a voltage in the second winding (2°
circuit) – current will then flow (“mutual induction”)
* Voltage proportional to no. of windings in each coil

Question 9

What are the different types of transformers

Answer 9

• Step up transformer
  – Increases voltage and decreases current
  – Accelerate electrons from cathode to anode
• Step down transformer
  – Increases current and decreases voltage
  – Provide high current to the filament
• Transformer is earthed (very high voltages)
• Auto-transformer: single coil wrapped around iron core (self
  induction – position of the input and output wires can be
  varied so ratio Input/Output can be varied

Question 10

Describe how Direct fourier transforms work in Ct reconstruction

Answer 10

• Obtain 1D projections
• fourier transform each to produce 1D datasets
• Use results to fill a 2D fourier space
• convert to a cartesian grid
• apply an inverse fourier transform to reconstruct in the spatial domain

Question 11

What are the pros and cons of direct fourier reconstruction

Answer 11

Pros - computationally simply and quick

cons - produces streak artefacts due to under smapling at the high frequency areas in the fourier space.
- Not robust to noise