What’s Your Filter?

As a result of the SPECT filters audit the NMSQG have produced some graphs to aid users who wish to convert Butterworth filters from one nuclear medicine computer system to another. The following link will open an Excel spreadsheet that contains the conversion graphs and instructions for their use. Please read the ‘Read this first’ sheet and then the ‘Instructions’ sheet. Excel Spreadsheet

Alternatively, if you wish to use the IDL program to test your filters it can be found here:  Filter Program

If you do not already have IDL on your computer you will need their Virtual Machine from this link (note extremely large file!): IDLVirtualMachine

You will also need the test data sets: either 64×64 or 128×128 matrix size

To use the program, process one of the test sets using your normal parameters and save some transaxial slices (the 5 central slices would be ideal) as a dicom file. Start the SPECT filter tool and press “import file” to select the required file. If you want to compare your filter to a standard Butterworth filter complete the input parameters in the pop-up box, however, the program will run and give the ‘Half amplitude frequency’ and ‘Sharpness’ without any of these parameters being input. Note these two values for your standard filter.

Now process the same test data using the new system or filter and save transaxial slices in the same way. Run these through the SPECT filtering tool to see if the ‘Half amplitude frequency’ and ‘Sharpness’ are equivalent. If you are comparing implementation of Butterworth filters from different manufacturers, the spreadsheets on this page will give you a good starting point for what to use as your new filter, otherwise you will need some trial and error to get an equivalent filter.

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