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OverviewThe CPMG experiment runs much like the Hahn-Echo experiment, however it applies additional 180 degree pulses after the first 180 degree pulse to measure the decaying echo response. The rate of this exponential decay of echo peaks is measured by T2, the Spin-Spin relaxation time. The CPMG program is available for download at the bottom of this page, as well as with the other NMR programs in the RadioProcessor section of the SpinAPI Example Downloads.
Figure1. CPMG Continuous Results (32 echoes, a sample of household cooking oil using a 10.8MHz permanent magnet)
Figure 1 shows the resultant FID response when capturing data continuously during a CPMG experiment. The large negative spikes are caused by the 180 degree pulses being applied.
Since a large number of echoes may be required to accurately measure T2, the CPMG program allows for segmented scanning acquisition. This mode reduces memory usage by allowing the RadioProcessor to only acquire a set number of points per echo (as little as a single point per echo). Figure 2 and 3, below, show multiple point and single point segmented scans.
Figure2. CPMG Segmented Results (80 echoes, 16 points per echo, a sample of household cooking oil using a 10.8MHz permanent magnet)
Figure3. CPMG Segmented Results (80 echoes,1 point per echo, a sample of household cooking oil using a 10.8MHz permanent magnet)
Figure 4, below, shows the pulse sequence used by the RadioProcessor to perform this experiment. Each vertical line is a new instruction, and the names of the durations used are shown above each time range.
Figure4. CPMG Pulse Sequence
Running the CPMG ProgramThe CPMG program accepts command line parameters
to specify the values for
each of the pulse program's parameters. It takes the following arguments as inputs:
Table1. CPMG Input Arguments
These arguments must all be passed as input parameters when launching the cpmg.exe file. Omitting or passing arguments out of order will cause the program to return an error. To simplify this process, a cpmg.bat file is provided. This file allows for easy manipulation of argument values, and ensures that they will passed in the correct order. To use the cpmg.bat file simply:
Viewing CPMG ResultsAfter execution of the CPMG program, the output files can be used with external programs to visualize the data.
The LabVIEW NMR Interface can process .txt files. To do this:
The Felix NMR program can process .fid files. To do this simply ensure that the Felix NMR program is installed properly and double-click on the .fid file.
CPMG Acquisition Tips
Setting the input parameters correctly is critical in obtaining good CPMG data. Below are suggestions for determining these parameters:
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