The discussion about 90, 180, and 360 pulse times that aren't related
by a factors of two has gone on so long, and with so many contributors,
that I think no one should mind if I put in my two cents.
A while back, I did some (unpublished) calculations of the effect of rf
inhomogeneity on pulse length calibrations. Typical rf field profiles
tend to give a 180 that is longer than half of the 360.
Another interesting feature is in the relative magnitudes of the 90,
270, 450, and so on. No one is likely to be surprised that with rf
inhomogeneity, the signal from a 270-degree pulse is not as large as
that obtained with a 90-degree pulse. What is surprising to me is that
the 450-degree signal is larger than the 270-degree signal. In other
words, the sine wave has a positive offset.
Pulse shape problems (risetime, overshoot, phase transient, ...) can
also lead to non-commensurate 180's and 360's, but I think they cannot
produce the positive offset of the sine wave.
My simulations suggest that the 360-degree pulse is a better indicator
of the average rf field than is the 180. For this and other reasons
(less problem with long T1's, and a better null in the presence of
resonance offsets), I almost always use the 360.
Probes have rf inhomogeneity, of course, but keep in mind also that
samples which are electrically conductive (for example, salt water) can
induce rf inhomogeneity.
Tony Bielecki
-- ------------------------------------------------------- Anthony Bielecki, Ph.D. ajb@bruker.com Applications Scientist Bruker Instruments, Inc. 44 Manning Road Tel. 508-667-9580, x145 Billerica, MA 01821 Fax 508-667-0985 -------------------------------------------------------