Reconstruction from Raw Data

 

Then the file selection form should appear similar in appearance to the following figure:

  

Figure 3: Select First Data Image

The user can select the first file in the sequence of projection files .

You are then asked to select the rectangle to reconstruct . The program sets the volume to reconstruct by using this projection and one rotated by 90 degree. For both a rectangle region is interactively defined. For a data-set which has been collected with a vertical axis of rotation, the first image sets the width of the volume to reconstruct, and the 90 degree rotation image sets the depth. The minimum of both sets the minimum of the height range to reconstruct, and the maximum of both sets the maximum of the height range to reconstruct. (Note: By editing the output parameter, the size and position of the volume to reconstruct can be changed.)

  

Figure 4: Select Region to Reconstruct

Figure 4, Page  shows a ``typical'' projection from a well controlled experiment. For the back-projection method to be valid the projection of the object must be fully within the view of the detector at all rotation angles. Hence the absorption should be very low, and the recorded X-ray intensity maximal, at the edges of the image. In this case the rotation axis was vertical, so the left and right-hand edges should not contain the object, which is the case. By selecting the smallest rectangle which just completely contains the object, then reconstruction time can be minimised.

If you realise that the image selected is not the correct image, you can CANCEL the operation and the program will stop.

Next you are prompted to SELECT LAST DATA IMAGE FILE, with the same file selection utility which you used to select the first data file (see Figure 3, Page ). This file should normally be the last file before the 180 degree rotation image.