Date:  Wednesday, March 1, 1995
Time:  4:15 PM   Refreshments at 4:00
Location:  ERL 126




Prepolarized Magnetic Resonance Imaging 
using Low Frequency Detection


Dr. Steve Conolly
Dr. Greig Scott
ISL/Stanford University



Abstract


Conventional Magnetic Resonance Imaging (MRI) systems use a static field
that must be strong (1 T) for polarization and uniform (1 ppm) for signal
coherence.  Taken together, these two constraints dictate expensive
superconducting magnets.  Hence the magnet represents a dominant cost in
the $2 M price tag typical for commercial MRI scanners.  We are designing a
new system that polarizes with a strong but inhomogeneous pulsed magnet and
then reads the signal in a much weaker pulsed field.  The new technique is
called prepolarized MRI (PMRI).  By decoupling the polarization and readout
roles, one can use two inexpensive time-varying electromagnets to replace
the expensive static superconductor.  The polarizing coil should be strong
but need not be homogeneous.  The receiver field can be much weaker and
needs only mild homogeneity (100 ppm).  Our current experience indicates
that these two coils can be built for less than $25K.  We believe that
these systems will produce excellent quality medical images for a small
fraction of the current capital costs.

Of course, there are significant challenges with the new approach.  These
include high power switching, low-cost magnet design, and low-frequency (1
MHz) induction signal reception.  The last challenge and our design
approach will be outlined.