Image Algebra - Introduction

The Image Algebra tool can be used to perform algebraic operations on images. You can perform transforms on single images, or combine multiple images, and even perform simple linear regression on images. You can view the result, or you can save it to disk as a new image, for later retrieval, viewing and analysis.

Start the Image Algebra tool from the Process menu:

The Image Algebra frame will now appear as shown below:

The procedure for performing image algebra can be summarised as follows:

• Decide how many images you want to work with. For example, for an magnetisation transfer ratio (MTR) calculation, you will use two images (the images without and with off-resonance saturation pulses). For a simple image subtraction you will be working with two images. Click on the button until you have the correct number of image selection panels in the "Input images" pane. If you have too many images, you can remove any of them by clicking on the icon.
• The pixel intensity in each input image has a name by which it is referred to in the formula. By default, these names are I1, I2, I3 etc. However, you can change the name to reflect the meaning of these images. For example, you if are performing an MTR calculation, you might want to refer to the image without the saturation pulses as "M0", and the image with saturation pulses as "Ms". If you wish, alter the name in the Name field at the right hand side of each input image.
• Enter your formula. The formula can use:
  • The operators *, /, +, - and ^ that can be seen in the Operators menu. You can also use parentheses (..).
  • Any of the built in functions that can be seen in Functions menu.
  • The constants pi and e that can be seen in the Constants menu.
  • The variable names that you set for your input images (for example M0 and Ms in the example above).
  You can either type in the formula, or use the Operators, Functions and Constants menu to insert text into the formula.
• For each of the input images, set an input image file by clicking on the load image icon: . This will bring up a File Chooser that you can use to set the input image corresponding to the variable name on the right. Alternatively, you simply type in the folder and image file name in the text fields, or by pressing the right mouse button and selecting from the menu of recently-used images.

Each input image must be of the same dimensionality (i.e., the same number of slices, and rows and columns within the slice). The Algebra tool then works through every pixel in the set of input images, and applies the formula to produce a single output image derived from a combination of the input images.