Dear
Please read pp.107-114 before
the next lecture on Friday November 30. We will finish our coverage of NMR
imaging, consider how NMR imaging and spectroscopy come together in localized
spectroscopy, and look briefly at how gradients can also be used for liquids
spectroscopy.
FOR THOSE IN THE
You will have an opportunity
to take the lab check-out exam in the final lab period December 7 or at another
time arranged by your instructor. Please arrange a time slot with your lab
instructor (if you have not already).
See:
http://www.nmr.ucdavis.edu/BCM230_Fall2007/LabCheckOutInfo.html
for more info.
FOR THOSE TAKING THE COURSE
FOR CREDIT:
The second problem set is due
at the next class meeting, 12:10 PM Friday November 30.
Now some points about NMR
IMAGING:
1) Query: What are the key
differences between NMR spectroscopy and NMR imaging?
a. Spectroscopy can be done
with constant B0; imaging requires the use of uniformly varying B0 (a field
gradient).
b. In spectroscopy different
Larmor frequencies represent different chemical shifts; in imaging different
Larmor frequencies represent different macroscopic LOCATIONS of nuclei within
the sample.
c. Spectroscopy thus probes
molecular structure; imaging produces an image (picture) of the sample. Spectroscopy
is "molecular"; imaging is "macroscopic".
d. In spectroscopy intensity
at a given Larmor frequency represents the number of nuclei in that chemical
environment; in imaging peak intensity at a given Larmor frequency is a measure
of the number of nuclei at a physical position within the sample.
2) How does a frequency
selective pulse select a "slice" from a sample?
Consider again the diagram
near the top of p.101. In the absence of a field gradient, all nuclei (lets say
we are looking at the protons of water) within the cylinder would have the same
Larmor frequency. Applying a field gradient along the z-axis (in the lab frame)
will cause a range of Larmor frequencies along the lab frame z-axis. A
frequency selective pulse (lets say its 2 kHz "wide") will ONLY tip
down those nuclei that are +-1 kHz from the carrier, thus tipping only those
nuclei in a thin plane with a width defined by the equation just to the left of
the picture of the cylinder: z = (delta omega)/(gamma * gradient strength). With
delta omega = 2000 Hz and gamma = 4000 Hz/gauss a gradient of 2.5 gauss/cm in
the z-direction in the lab frame produces a slice 0.2 cm thick.
Note that the WIDTH of the
slice can be controlled by varying by varying the gradient strength; larger
z-gradient strengths will narrow the slice.
Note also that the POSITION of
the slice along the z-axis can be altered by changing the carrier frequency.
3) Note that in the
macroscopic lab frame axes are defined for an imaging magnet as shown in the
bottom figure of p.98. y is the vertical axis, x the
horizontal, and z comes out of the paper at the viewer. This is because imaging
magnets have the magnet bore (the z-axis) parallel to the floor of the room.
4) Although the primary use of
field gradients is for NMR imaging, they can also be used for spectroscopy in
two ways. One is for localized spectroscopy as we shall discuss in the next
lecture. The second increasingly common use is for some kinds of liquids
spectroscopy like we do on the 500 and 600; we will look briefly at one example
of this in the next lecture.