NASA Technical Reports Server (NTRS) 20100001353: Reducing Field Distortion in Magnetic Resonance Imaging

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that in some cases, the EPR signal may Jet Propulsion Laboratory. Mail Stop 202-233
be strong enough to be detectable even In accordance with Public Law 96-517, 4800 Oak Grove Drive
in the absence of prepolarization. the contractor has elected to retain title to this Pasadena, CA 91109-8099
This work was done by Inseob Hahn, Peter invention. Inquiries concerning rights for its E-mail: [email protected]
Day, Konstantin Penanen, and Byeong Ho commercial use should be addressed to: Refer to NPO-44656, volume and number
Eom of Caltech and Mark Cohen of UCLA Innovative Technology Assets Management of this NASA Tech Briefs issue, and the
Center for Cognitive Neuroscience for NASA’s JPL page number.
Reducing Field Distortion in Magnetic Resonance Imaging
An unconventional magnetic-field configuration would be used in SQUID MRI.
NASA’s Jet Propulsion Laboratory, Pasadena, California
A concept for a magnetic resonance field. One of the problems in develop- level while using a low-strength mag-
imaging (MRI) system that would utilize ing low-magnetic-field-strength MRI netic field, without sacrificing resolution
a relatively weak magnetic field provides arises from the relationship among and without need for excessively long
for several design features that differ sig- image resolution, image-acquisition image-acquisition time. There would be
nificantly from the corresponding fea- time, and the gradient of the magnetic no applied static, homogeneous field be-
tures of conventional MRI systems. No- field. In general, the resolution (that is, cause in MRI, it is not required that all
table among these features are a the reciprocal of the smallest size of an the magnetic moments precess about
magnetic-field configuration that re- the same axis. The encoding mag-
duces (relative to the conventional netic field, B, would be configured to
configuration) distortion and blur- lie in the x,y plane of x,y,z Cartesian
ring of the image, the use of a super- coordinate system: the field would be
conducting quantum interference given by
device (SQUID) magnetometer as
the detector, and an imaging proce- B= B0ex/a[xˆcos(y/a) – yˆsin(y/a)]
dure suited for the unconventional
field configuration and sensor. where xˆ and yˆare unit vectors along
In a typical application of MRI, a the xand yaxes and B0and aare arbi-
radio-frequency pulse is used to ex- trary constants to be chosen by design
cite precession of the magnetic mo- (see figure). Inasmuch as the strength
ments of protons in an applied mag- of this field is given by |B| = B0ex/a ,
netic field, and the decaying this field would encode position
precession is detected for a short along the xaxis as a function of reso-
time following the pulse. The preces- nance frequency. Although x would
sion occurs at a resonance frequency be a nonlinear (specifically, a loga-
proportional to the strengths of the rithmic) function of resonance fre-
Field Configurationis shown in x,yplane.
magnetic field and the proton mag- quency, it would be possible, through
netic moment. The magnetic field is image feature that can be resolved) is suitable choice of the arbitrary constants,
configured to vary with position in a proportional to the strength of the mag- to reduce the deviation from linearity
known way; hence, by virtue of the afore- netic-field gradient and the image-acqui- and the concomitant spatial variation of
said proportionality, the resonance fre- sition time. Because of fundamental resolution over the image region to toler-
quency varies with position in a known properties of a magnetic field, an image- able levels.
way. In other words, position is encoded position-encoding gradient necessarily A close approximation of the mag-
as resonance frequency. entails undesired concomitant field netic field described above could be gen-
MRI using magnetic fields weaker components proportional to the image- erated by electric currents flowing axially
than those of conventional MRI offers position-encoding gradient. These com- on a z-oriented circular cylinder having
several advantages, including cheaper ponents are undesired because they a length much greater than its radius.
and smaller equipment, greater compat- cause distortion or blurring of the The current density (K) needed to gen-
ibility with metallic objects, and higher image. The distortion can be reduced by erate the magnetic field is a known con-
image quality because of low susceptibil- weakening the gradient so that the total tinuous function of the azimuthal angle
ity distortion and enhanced spin-lattice- field variation in the magnetic field is a (θ). In practice, the required continuous
relaxation-time contrast. SQUID MRI is small fraction of the static homogeneous current density K(θ) would be approxi-
being developed into a practical MRI field. However, in weakening the gradi- mated by use of multiple discrete wires
method for applied magnetic flux densi- ent, one must either accept lower resolu- positioned at suitable angular intervals
ties of the order of only 100 µT. tion or else increase image-acquisition and driven by suitable currents. To en-
In conventional MRI, it is required time to retain resolution. code position along an axis at an angle α
that all of the magnetic moments pre- According to the present concept, it with respect to the xaxis in the x,yplane
cess around the same axis defined by an should be possible to reduce distortion so that back projection reconstruction
applied static, homogeneous magnetic to a very low (in principle, negligible) could be used, one would shift the cur-
NASA Tech Briefs, January 2010 35
rent-density pattern by electronically main polarized and would start to precess invention. Inquiries concerning rights for its
changing the pattern of drive currents to about the x,y-oriented encoding magnetic commercial use should be addressed to:
make it approximate K(θ+ α). field. The sensing loop of the SQUID Innovative Technology Assets Management
In SQUID MRI, the magnetic moments magnetometer would be placed to detect JPL
are prepolarized in a strong magnetic the zcomponent of magnetization. Mail Stop 202-233
field separate from the encoding field. In This work was done by Byeong Ho Eom, 4800 Oak Grove Drive
a system according to the present con- Konstantin Penanen, and Inseob Hahn of Pasadena, CA 91109-8099
cept, the prepolarizing magnetic field Caltech for NASA’s Jet Propulsion Laboratory. E-mail: [email protected]
would be oriented along the z axis. The Further information is contained in a TSP Refer to NPO-44593, volume and number
prepolarizing magnetic would then be (see page 1). of this NASA Tech Briefsissue, and the page
turned off so rapidly that immediately af- In accordance with Public Law 96-517, number.
terward, the magnetic moments would re- the contractor has elected to retain title to this
36 NASA Tech Briefs, January 2010