extra phase factor in a 2D spin echo experiment

[trina]

Hi Mikhail and all,

I am trying to simulate singlets with a regular 2D spin echo with TE =
4ms and ppm_ref_offset at 4.721ppm but can't seem to obtain how a
spectrum with singlets should look like. IE with the real spectrum
even and symmetric, and the imag part odd. Instead I obtained
spectrums that looked like there's some phase factor involved. I
generated the plot (labelled "Cr singlets from spinev simulation")
which can be seen at http://www.mit.edu/~trina/singlet.JPG . The plot
I obtained from a physical MRI scan at 3T is shown on the left as "Cr
singlets from 3T scan".

Did I write something wrong in the below or I might have missed
something. Please help.

Thanks.

*****************************************
** 2DJ for 2D Creatine spectrum at 3T **
****** The System ***********************
spectrometer(MHz) 128
spinning_freq(kHz) *
channels H1
nuclei H1 H1 H1 H1 H1
atomic_coords *
cs_isotropic 3.0270 3.0270 3.0270 3.9130 3.9130 ppm
csa_parameters *
j_coupling *
quadrupole *
dip_switchboard *
csa_switchboard *
exchange_nuclei *
bond_len_nuclei *
bond_ang_nuclei *
tors_ang_nuclei *
groups_nuclei *
******* Pulse Sequence ******************************
CHN 1
timing(usec) 0.5 2000 0.5 2000 (1000)1024D1
power(kHz) 500 0 1000 0 0
phase(deg) 0 0 90 0 0
freq_offs(kHz) 0 0 0 0 0
******* Variables **********************************
ppm_ref_offs_1 = 0.6066565
******* Options *************************************
rho0 F1z
observables F1p
EulerAngles *
n_gamma *
line_broaden(Hz) 0 2
zerofill *
FFT_dimensions 1 ppm


* ppm_ref_offs is the offset, in kHz, of the reference (0 ppm) with
respect
to the reference frequency (0 kHz, at the center of the spectrum)

* Run plot2Dppm('cosy_re.dat') to plot the spectrum in MATLAB

[Mikhail Veshtort]

Hi Trina,

I ran the simulation quoted in your post and obtained the exact result
one should expect from it:
two purely absorptive gaussian peaks in the imaginary part, and two
purely dispersive peaks in the real part of the spectrum. I have no
idea how you could get the poorly phased peaks in the matlab images
you show. Try it again please.

Mikhail