Week Three - Medical Data Processing & Analysis

Question 1

Define contrast.

Answer 1

The ratio of a region’s intensity compared to that of its background.

Question 2

What is normal contrast?

Answer 2

C = (f-b)/(f+b)

Question 3

What is simultaneous contrast?

Answer 3

Cs = (f-b)/b

Question 4

What is the law of simultaneous contrast?

Answer 4

All colours appear to be altered by those around them. Relates to the effect of background on the perception of an object.

Question 5

What is one key advantage of using normal contrast?

Answer 5

Values are limited to the range [-1,1].
Negative contrast value = object darker than background.
Positive contrast value = object lighter than background.

Question 6

What are Mach Bands?

Answer 6

An optical illusion that shows that perceived brightness is not a simple function of intensity, but the HVS tends to undershoot or overshoot around the boundary of regions of different intensities.

Question 7

What is just noticeable difference?

Answer 7

The amount something must be changed in order for a difference to be noticeable at least half of the time. For contrast = 2%.

Question 8

What is Weber’s Law?

Answer 8

The size of JND is proportional to the intensity of the stimulus. 2%.

Question 9

Why is contrast important?

Answer 9

Something wrong (calcification) against low density tissue (fat) = high contrast (easy to detect).

Something wrong (calcification) against high density tissue (breast tissue) = low contrast (hard to detect)

Question 10

What is a histogram?

Answer 10

A graph that provides a view of the intensity profile of an image by plotting pixel intensity against frequency of pixel intensity.

Question 11

What is noise?

Answer 11

A signal other than that of interest.

Question 12

What are 3 common sources of noise?

Answer 12

Physiological - breathing in a chest x-ray
Instrumentation
Environmental

Question 13

What are 3 common types of noise?

Answer 13

Salt and pepper - random b&w pixels.
Impulsive - random white pixels
Gaussian - lots of small variations in intensity (Gaussian normal distribution)

Question 14

What is the concept behind spatial domain image processing?

Answer 14

Sub-image (3x3 neighbourhood etc) is moved around an image as an operator T is applied at each stage.

Question 15

What is the simplest form of spatial domain processing?

Answer 15

Pixel point processing: new pixel = T(old pixel). No regard for neighbours.

Question 16

What are 5 types of grey level transformation functions?

Answer 16

Linear
Log
Power-law
Window-level
Pseudo-colour table

Question 17

What is the purpose of finding the negative of an image?

Answer 17

To enhance white or grey detail embedded in dark regions - especially when black areas are dominant in size.

Question 18

What is the purpose of log GLT?

Answer 18

Dynamic range compression: sometimes only the brightest parts of an image are displayed.

Question 19

What is the difference between log and inverse log GLT?

Answer 19

Log: dark pixels expanded, bright pixels compressed. (Darks get brighter)
Inverse log: dark pixels compressed, bright pixels expanded (darks get darker)

Question 20

What is the difference between log GLT and power-law GLT?

Answer 20

Using the power-law function - a family of possible transformation curves can be obtained just by varying gamma.

Question 21

Why is the window level operator necessary?

Answer 21

Hounsfield scale = 4000 different densities but our eyes can only perceive about 20 different shades of grey. So an interval (or window) is selected for viewing.

Question 22

What does a window level operation involve?

Answer 22

Contrast outside the window is lost completely, while the portion of the range lying inside the window is stretched to the original grey range (e.g. 0 to 255)

Question 23

Why are pseudo-colour table transformations necessary?

Answer 23

HVS can only see about 20 different shades of grey but thousands of colours.

Question 24

What is temporal subtraction?

Answer 24

Subtracts two images in a pixel wise manner, to get rid of the background. (DIFFERENT STUFF YOU SEE) e.g. angiography with and without contrast agent.

Question 25

What is temporal average?

Answer 25

Adding two images in a pixel wise manner. (SAME STUFF YOU SEE) To decrease noise.

Question 26

Why is it temporal averaging and not temporal addition?

Answer 26

So pixel intensities don’t overflow.

Question 27

Why is histogram equalisation used?

Answer 27

To increase the contrast in an image by stretching its histogram to be approximately uniformly distributed.

Question 28

What are the steps involved in a histogram equalisation?

Answer 28

1. Pixel intensities
2. Number of pixels
3. Probability
4. Cumulative probability
5. C.P. * desired range
6. Round
7. Use o.g. intensities as a look-up table

Question 29

What is the purpose of a box filter?

Answer 29

To reduce the amount of intensity variation between one pixel and the next - in order to smooth images and reduce noise. Trade off between noise removal and detail preservation.

Question 30

What is the difference between a box filter and a weighted average filter?

Answer 30

Instead of averaging all the pixel values in the window, give the closer pixels higher weighting and far away pixels lower weighting.

Question 31

What is the purpose of a median filter?

Answer 31

To force points with distinct grey levels to be more like their neighbours.

Question 32

Why is a median filter better than an averaging filter? (2 reasons)

Answer 32

1. Median is a robust average (not affected by outliers)

2. Median value is actually a neighbourhood value - so you’re not making up a new number

Question 33

What is Taylor Swift’s best album?

Answer 33

Speak Now/Red

up 2 u
but wrong if u didn’t pick one of these