2017 Exam

Question 1

How does the addition of an aluminium filter affect the o/p? (1, 3)

Answer 1

• Attenuator preferentially absorbs lower energy (soft) photons
• Peak of spectrum and mean shifted to higher energy
• N.B - overall lower energy

Question 2

How does the addition of an aluminium filter affect the o/p? (2, 2)

Answer 2

• Unwanted low energy radiation removed to minimise patient dose, mainly to skin
  
  • Lowest energy photons mostly give patient does w/o contributing to image

Question 3

List 4 scintillator materials used in PET and adv/disadv

Answer 3

• Stopping Power + Light o/p - - HIGH
• Decay time + energy res - - LOW

Question 4

Why does the discrete histogram equalisation technique not yield a flat histogram? (3)

Answer 4

• Histogram equalisation rempas histogram components on the intensity scale
• A uniform flat histogram requires the pixel intensities to be redistributed so that there are L groups of n/L pixels with the same intensity values
• Hist equalisation method has no provisions for this tpye of intensity redistribution process

Question 5

What does AWGN mean and how is each individual pixel effectd? (2)

Answer 5

• Noise values is added to the true pixel
• Noise drawn from the gaussian probability density function

Question 6

What is the characteristic about Speckle Noise affecting US images? (2)

Answer 6

• Speckle noise is caused by random interference of scattered echoes from tissues who surface is rough on the scale of the wavelength of the incident US wave
• Contains multiplicative contamination and is therefore signal dependent

Question 7

How to reduce effects of speckle noise in US?

Answer 7

• Convert the multiplicative noise into additive by taking log of image
• Take wavelet transform of image and threshold the resulting co-effs
• Take inverse wavelet transform and exponentials

Question 8

What type of noise could you use a Median Filter for?

Answer 8

Impulse noise