th  - 1987 - 51st ENC Conference

WF64- POSTERS
NMR FLOW IMAGING
C. L. Dumoulln*, S. P. Souza, H. R. Hart Jr.
General Electric Research and Development Center
PO gox 8, Schenectady, New York 12301
Many flow sensitive NMR procedures have been proposed and demonstrated.
All of these methods rely on either longitudinal magnetization or transverse
magnetization for flow discrimination. Time-of-flight and spin washout are
examples of flow sensitive techniques which make use of longitudinal magneti-
zation. In these methods, the longitudinal magnetization is changed in one
location and monitored in another downstream from the first. Techniqdes which
make use of transverse magnetization such as the one described below typically
monitor the phase of spin magnetization and thus are not constrained by the
geometry of flow.
The most significant problem in blood flow imaging in animals or humans
is the suppression of signals arising from non-moving spins. This is a par-
ticularly severe problem because blood vessels are relatively small when com-
pared to the volume of the surrounding tissue and blood flow is usually pulsa-
tile rather than constant. The rapid-scan, phase contrast technique presented
here circumvents these problems and has proven very useful in obtaining non-
invasive angiograms of healthy and diseased patients.
The flow-induced phase shift of a bi-polar gradient pulse is simply
- vVTA G
where ~ is the flow-induced phase shift, 7 is the gryomagnetic ratio, V is the
component of spin velocity in the direction of the applied magnetic field gra-
dient, T is the time between the centers of the lobes of the bipolar gradient
and A G is the area of the first lobe in the bi-polar pulse (the area of the
second lobe is -AG). To selectively detect flow in an imaging or spectroscopy
procedure one can simply acquire two sets of data under different conditions
of V, T or A G. We have found that the inversion of the bi-polar gradient
pulse gives the best results. Thus, two echoes are obtained, one with
- 7VTA C and the other with # - 7VT(-Ac). Moving spins are selectively
detected ~ and stationary spins suppresse~ by taking the difference of the two
echoes. Since the flow sensitivity of such a procedure is only in the direc-
tion of the applied field gradient, two flow images are obtained with orthogo-
nal flow sensitivities and combined to give the total flow image. The inten-
sity of each pixel in the image is a quantitative measure of flow provided the
flow-induced phase shift, ~, is less than about 1 radian.
Suppression of non-moving spins can be enhanced by a weak dephasing gra-
dient applied in the direction of the image projection. Such a gradient
dephases spin magnetization over the large volume of non-moving spins and has
little effect in the relatively small vessels. Additional suppression can be
obtained by acquiring data very quickly and with short pulse-to-pulse inter-
vals. Rapid scanning saturates non-moving spins while spins which move into
the detection region are fully relaxed. Fast scanning also makes the flo~
imaging procedure much less sensitive to patient motion since differences of
data acquired in very short intervals are taken.