Image Processing Flashcards
How does windowing and levelling work?
Window is how much of the greyscale you will display and level is the centre
Everything outside of this window will be black or white
How does thresholding work and when does it work best?
Binary windowing
Works best for robustly quantitative imaging modalities with large intensity differences
What are some clinical examples of thresholding?
Setting patient external
Bone outlining
Why does thresholding not work well for MR?
Image intensities are variable, same sequence: different intensity spread and different threshold required
What are kernel operators used for?
To change image in a desired way: smoothed or sharpened
How are kernel operations done?
A matrix of pixel weighting factors are applied to each pixel in an image using a convolution:
Centre kernel on a pixel
Multiply the overlying values and add together
Divide by sum of kernel values
What is the purpose of registration?
To align two images so that the same anatomy represented in each image is overlaid
Optimisation problem: no analytical solution
What are the four ingredients of a registration algorithm?
Metric: measure of image similarity
Transform: function which maps moving image to fixed image
Optimiser: method used to find the transform which optimises the metric
Interpolator: for resampling during transforms
What is the simplest metric for registration?
Mean square difference
Minimise mean square difference
Difference between fixed image intensity at a position and the intensity of the moving image at that position
What is the simplest transform for registration?
A translation
What does the optimiser do (msd example)?
Calculate all possible transforms and mean square difference values for each
Find the minimum value for the optimum registration
Not practical to calculate all values in more complex methods
How can interpolation be done?
Nearest neighbour
Linear interpolation
N-th order B-spline
Moving image needs to be resampled to match fixed image frame of reference
When can mean square difference be used?
On unimodal registrations (CT-CT)
What is mean square difference sensitive to?
Small number of voxels with large intensity differences
What is the most common metric used in RT?
Mutual information
Intensities don’t have to be the same: multi-modal