Folks,
Earlier tonight, I put together the leads from about 3 or 4 mysterious
things happening when using shaped rf pulses on our system, and found a
major problem.
The system:
AMX console on 4.7T horizontal imaging system. Console has CPU4 and MCI 3
channel interface. (I know, really old....); BSV10 and BLTX amplifiers
Computer: SGI O2 running IRIX 6.5; XWIN-NMR 2.6 patchlevel5
The problem:
Shaped rf pulses do not appear shaped, but mostly rectangular. In fact, they
appear as if the shape may be correct, but is truncated at some very low
level, maybe +/- a few % of intended shape amplitude.
This is characterized by the following oddities that have been building up
over the last few weeks:
- on completion of any experiment using a soft pulse, an error message
appears: WriteShape failed for /u/exp/stan/nmr/lists/wave/sinc3.512
Giving sinc3.512 write permissions, resulted in the file being overwritten
by a much larger file, which then generated a ReadShape error at the next
zg. Changing the filename to SINC3.512 (as documented for bug in gradient
shape names) had no effect. Copying over binary shape files from other AMX
or DRX systems running older version of XWIN-NMR resulted in ReadShape
errors, and the zg aborting.
- inconsistent slice thicknesses, and a slice profile (from the imslice
sequence) that looks like a real mess; unless you zf to a few k points
before FTing, in which case the slice profile is a sinc shape, not a
rectangle!!
- no transmitter power indicator on the AMX router, regardless of the
setting of TP0, while a low power hard pulse, with TL0 = 10dB, gives ~ 30W
power display on the router
- these 3 problems occurred, regardless of whether the BSV10 or BLTX
amplifier was used. (The console was originally an AMX500, therefore we use
the X channel for all frequencies, including 1H)
The above finally prompted me to look at the rf shape on the oscilloscope -
something I probably should have done a few weeks back. The rf signal was
naturally very low - in haste I neglected to measure the actual power, but I
had knocked down the attenuation to less than 20 dB on the scope input
before I could see anything. The pulse appeared "square", giving the sinc
profile I had seen. It also appeared to have random positive and negative
components, and a few zero points - making believe it is the intended shape,
but truncated at a very low level. I could not detect noticeable changes on
the scope display by eye, for any of the different rf shapes from the
wave.imag directory, except for the triangle.512 shape.
I was nominally using a sinc3.512 pulse, duration 4ms, tp0 18-30 dB, but I
doubt these parameters are having any effect on the pulse shape.
If anyone has dealt with XWIN-NMR 2.6 on an AMX, or has seen any of these
problems, or if there are known bugs for this combination, I would be
grateful to know. Are shape files for XWIN 2.6 supposed to be incompatable
with older releases, and vice-versa??
Hopefully, the shape files in wave.imag (the only ones I have used so far
this sytem) are damaged and can be replaced. However, I fear something in
the rf generation area may be truncating the shape.
Many Thanks,
Ross Mair
-- Ross Mair Staff Scientist Office Phone: (617) 495 7218 Harvard-Smithsonian Centre for Astrophysics Office Fax: (617) 496 7690 60 Garden St, MS 59, email: rmair@cfa.harvard.edu Cambridge, MA 02138 NMR lab at MIT (617) 252 7011 USA
This archive was generated by hypermail 2b29 : Sun Dec 31 2000 - 01:15:03 PST