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CPMG Program

(for RadioProcessor and iSpin-NMR)


The 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 CPMG experiment program is available for download at the bottom of this page, as well as 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


The CPMG program accepts command line parameters to specify the values for each of the pulse program parameters. It takes the following arguments as inputs:


Argument
Parameter
Units/Values
1
File Name

2
Board Number
#
3
De-blank Bit
0, 1, 2, or 3
4
Debug 0 or 1
5
ADC Frequency
MHz
6
Spectrometer Frequency
MHz
7
Spectral Width
kHz
8
Number of Echoes
#
9
Amplitude
0.0 - 1.0
10
90 Degree Pulse Time
us
11
90 Degree Pulse Phase
degrees
12
180 Degree Pulse Time
us
13
Include 90
0 or 1
14
Bypass FIR
0 or 1
15
Number of Echo Points
#
16
Number of Scans
#
17
Tau
us
18
De-blanking Delay
ms
19
Transient Delay
us
20
Repetition Delay
s
Table1. CPMG Input Arguments

CPMG Acquisition Tips

Several suggestions for acquiring CPMG data are listed below:
  • Start with a simple single-pulse NMR experiment. Find the resonance frequency and adjust the transmitter output phase to achieve the largest received signal amplitude for the real channel (the imaginary channel amplitude should be approximately zero).
  • Find the 90 degree pulse length and verify that the 180 degree pulse length is twice that of the 90 degree pulse length.
  • Copy the relevant parameters from the single-pulse NMR experiment over to the CPMG program. The relevant parameters are:
    • BOARD_NUMBER
    • ADC_FREQUENCY
    • SPECTROMETER_FREQUENCY
    • SPECTRAL_WIDTH
    • AMPLITUDE
    • P90_TIME
    • P90_PHASE
    • TRANSIENT_DELAY
    • REPETITION_DELAY
  • Estimate the TAU time. This should be approximately equal to the time it takes for your single-pulse test FID to decay to zero.
  • Run the CPMG test initially with NUMBER_OF_POINTS_PER_ECHO equal to zero and NUMBER_OF_ECHOES equal to one. Make sure your SPECTROMETER_FREQUENCY is set to resonance. You should see the initial FID from the 90 degree pulse time, followed by a large spike (the 180 degree pulse), and then an echo.  When scaled correctly, the FID from the 90 degree pulse should look nearly identical to the signal acquired using the single-pulse NMR test.
  • Adjust the TAU and SPECTRAL_WIDTH parameters as necessary to produce the desired echo.
  • After setting all of the above parameters you can adjust the NUMBER_OF_ECHOES to produce more echoes, and the NUMBER_OF_POINTS_PER_ECHO to only acquire the echo peaks.

Here are a few other tips:
  • Narrowing the SPECTRAL_WIDTH parameter may improve the apparent signal-to-noise ratio. Keep in mind that reducing the spectral width will reduce the number of points sampled per echo which may require you to reduce the NUMBER_OF_POINTS_PER_ECHO parameter.
  • Try running multiple scans to perform signal averaging. This should also improve the signal-to-noise ratio. If the magnetic field drifts quickly you will need to use a small REPETITION_DELAY and be sure to set the SPECTROMETER_FREQUENCY to the resonance frequency shortly before running the experiment.

Download

The CPMG is available as part of the RadioProcessor Examples.
  • cpmg.exe (64-bit)
  • cpmg.bat
  • Source Code : 
  •      cpmg.c
         cpmg.h


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