2017 Exam Flashcards
1
Q
How does the addition of an aluminium filter affect the o/p? (1, 3)
A
- Attenuator preferentially absorbs lower energy (soft) photons
- Peak of spectrum and mean shifted to higher energy
- N.B - overall lower energy
2
Q
How does the addition of an aluminium filter affect the o/p? (2, 2)
A
- Unwanted low energy radiation removed to minimise patient dose, mainly to skin
- Lowest energy photons mostly give patient does w/o contributing to image
3
Q
List 4 scintillator materials used in PET and adv/disadv
A
- Stopping Power + Light o/p - - HIGH
- Decay time + energy res - - LOW
4
Q
Why does the discrete histogram equalisation technique not yield a flat histogram? (3)
A
- 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
5
Q
What does AWGN mean and how is each individual pixel effectd? (2)
A
- Noise values is added to the true pixel
- Noise drawn from the gaussian probability density function
6
Q
What is the characteristic about Speckle Noise affecting US images? (2)
A
- 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
7
Q
How to reduce effects of speckle noise in US?
A
- 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
8
Q
What type of noise could you use a Median Filter for?
A
Impulse noise
