problem with the filtered back projection method

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Dear all,
I have some problems to create a procedure that allows me to reconstruct an image from projections data, by using the filtered back projection method.

I found in the example experiments the Radon Transform demo, but I found it a bit complicated...

Could you please help me?
Hello Sheeba,

[The following email was sent to you directly by email. It is included here for the benefit of others who might be interested in back-projections.]

I don't know which part of the demo experiment was incomprehensible; maybe it is just a matter of understanding basic IGOR concepts in which case the Getting Started Tutorial may be helpful. You can find the tutorial under Windows Menu->Help Windows->Getting Started.

> My datas correspond to a matrix of 640 (rows)x1200(columns), so I guess that these datas are not 2D wave…

I do not understand what you mean by this statement. Do you have a single 2D wave or 1200 waves of 640 rows?

If you have a single wave that contains the matrix -- just to be sure, execute the command:

Print WaveDims(nameOfYourWave)

where you'd substitute for nameOfYourWave the name of the wave containing your data.

If IGOR prints 2 then indeed you have a 2D wave and you can proceed. If not, you need to build a matrix wave. In most cases it is just easier to reload the data as a matrix instead of individual waves.

Next, make sure that your 2D wave is either single or double precision. You can check that in the Data Browser or Data Menu->Browse Waves dialog. If your wave is not SP or DP you can convert it by executing on the command line:

Redimension/D nameOfYourWave

At this point you need to make sure that the input wave's scaling encodes the angular resolution that was used to create it. The demo uses an output of 256 x 256 so the row scaling is -128 to 127. Similarly, the demo used a projection slice that was between 0 and pi so that was set to the wave scaling of the input columns. You should choose the values as appropriate for your application. To set them use Data Menu->Change Wave Scaling dialog.



> ImageTransform /BPJ={640, 1200} /D =M_backProj_Lung backProjection Matrix_norm_Lung


The /D flag does not apply in this case. As the documentation states: the output is stored in the wave M_BackProjection in the current data folder.

I hope this helps,

A.G.
WaveMetrics, Inc.

September 6, 2011 at 10:25 am - Permalink