Exercise 5: Image processing

Monday, October 23, 14.30-17.00 in the E-databar, build 341, ground floor, room 015.

Purpose:

The purpose of this exercise is to show how the manipulation of gray levels in CT images can reveal new details and structures, and how filtration on images can be performed.

Preparation:

Read Chapter 2 in the book: J. L. Prince & J. M. Links: Medical imaging signals and systems, Pearson Prentice Hall Bioengineering, 2006.

Go through the different exercise points and write down suggestions for your Matlab code.

Exercise:

1. Download the Shepp-Logan phantom image from the web-site at: https://courses.healthtech.dtu.dk/22485/exercises/exercise5/data/shepp_logan.mat. There are two versions of the image contained in the Matlab file: one sh) with the standard scaling of the different features in the phantom and one (sh_black) with enhanced black levels in the image. The size of the pixels is 0.86 x 0.86 mm. Display the two images with the command imagesc, and use a black and white color scale using the command colormap . Use the command axis('image') to make a square display of the image, with the correct size on the axis. Make a procedure to change the color map using the commands colormap and image, so that only a small range of the pixel values occupy the whole color scale.

2. The Shepp-Logan phantom contains some very faint details, that only can be seen by adjusting the gray scale mapping. The structures are shown on the web-page: https://courses.healthtech.dtu.dk/22485/?ct_data/shepp_logan.html. Adjust the gray scale on the image sh to obtain an image like to one on the web page. (Hint: try plotting the image values first to gain an insight into the different values).

3. Download the images: https://courses.healthtech.dtu.dk/22485/exercises/exercise5/data/brain.mat and https://courses.healthtech.dtu.dk/22485/exercises/exercise5/data/head.mat and manipulate their color scales. Select a scaling so the ventricles in the brain can be seen in the brain image. The size of the pixels in both images is 0.43 x 0.43 mm. Explain what you see.

4. Make a two-dimensional Fourier transform of the sh_black image, and make a mesh plot of the amplitude spectrum with the command mesh. Plot the spectrum with the correct spatial frequency axis. Study the symmetry relations for the Fourier transform.

5. Make a low-pass filter with a circularly symmetric transfer function that removes all frequencies above a value of 116 m^-1. Apply the filter on the image sh_black and study the effect of the filter. Try other cut-off frequencies and look at the effect. Try also to zero pad the Fourier transforms to avoid the circular convolution effect.

6. Use an edge enhancement filter given as [-1 -1 -1; -1 9 -1; -1 -1 -1] to enhance the edges in the image sh_black.

7. Try the above mentioned image processing on the clinical images downloaded previously.