Image Reconstruction Flashcards
What are ITERATIVE ALGORYTHIMS?
- An estimation of 2D patterns of x-rays absorption
- Projection data is calculated and compared to the experimental results
- Discrepancies are measured and removed through iterative improvements of estimations
What are ANALYTIC ALOGRITHIMS?
- The image is reconstructed directly from projection data
What is PROJECTION?
A received set of intensities from a given angle. Multiple projections are reconstructed into a single 2D slice
What is BACK-PROJECTION?
An approach permitting the reconstruction of an attenuation distribution for an object from a series of projections
* the signal is passed through the target, the difference in the signal sent and the signal received is due to the attenuation of the material
Why are MULTIPLE ANGLES USED FOR PROJECTIONS?
From one perspective we cannot state for certain the construction of the target, for example you wouldn’t be able to tell how many layers there are
What are the STEPS TO A SIMPLE BACK-PROJECTION process (6)?
1) Determine the size of the resultant matrix from the length of the projection matrices
2) Select you starting direction and use the values of this initial slice as the values for your matrix pixels
3) Set the values for all pixels in the line to be equal to the projection pixel value
4) Iterate (clockwise/anticlockwise) until opposite the start point (don’t use the final position though)
5) Subtract the sum of all values in the initial slice from all pixels
6) Divide all pixel values by the # of projections minus 1
What is an ARTIFACT?
A feature appearing in an image that isn’t present in the real target
What CAUSES ARTIFACTS?
- Quirks in the modality
- Reconstruction and processing errors
- Motion during imaging
What does a HIGH VALUE MEAN IN THE RECONSTRUCTION MATRICIES?
The higher the number the higher the attenuation experienced at this point - therefore a higher density material is present.
