Fig. 1. Images of the three E-field cells. Two cells fit inside a 5 mm NMR tube (a) and produce and E-field (b) and // (c) to the Bo field. The third cell (d) is located inside an MRI phantom.

1 cmNMR Tube

Copper Wires

Teflon RodPlatinum Electrodes

Propylene Carbonate Tube Spinner

a

EBo

Electrodes

b E//Bo

Electrodes

c

Electric Cell

d

Experimental MethodsTwo electric field cells were constructed to fit inside a standard 5 mm NMR tube. (See Fig. 1.) One produced an electric field perpendicular () to Bo and had platinum electrodes separated by 3 mm. The second cell produced an electric field parallel (//) to Bo with copper electrodes separated by 2 mm. We estimate that ~90% of the sample experiences the applied E in the cell while 100% does in the // cell. The signal was calculated as the integrated absorption signal. NMR measurements from these cells were made on a 7 T NMR spectrometer (Bruker Biospin). R1 values were measured using a 32 or 128 point inversion recovery (IR) sequence. R2 measurements were made using a 32 point CPMG with t=500 s and single-echo, spin-echo (SE) sequences. Another larger, 7.6 ml volume cell with copper electrodes separated by 2 cm was constructed inside a Teflon® container and imaged with a 1.5 T imager (GE Healthcare) within an MRI resolution phantom with additional R2 standards. R2 measurements were made on this cell using a series of four-echo SE sequences and their magnitude images. All cells were filled with pure propylene carbonate (Sigma-Aldrich) and measurements made at 20 °C. All the cells were connected to a battery pack located outside the magnet which could supply 0 to 235 V DC. Relaxation rates were determined using either mono- or bi-exponential fits or an inverse Laplace transform.[1]

Magnetic Resonance Laboratory OCHESTER

MEDICAL CENTER

UNIVERSITY of RYujie Qiu,1 Wing-Chi E. Kwok,2 Joseph P. Hornak1

1Rochester Institute of Technology, Rochester, NY 14623; 2University of Rochester, Rochester, NY 14642

Electric Field NMR and Spin Relaxation in Propylene Carbonate

Motivation & BackgroundWe were motivated to study the effect of an electric field on the 1H spin-lattice and spin-spin relaxation rates (R1 and R2 respectively) in our search for a dynamic functional MRI (fMRI) phantom. Our goal is to rapidly and reversibly change the R2 of a polar liquid by the application of an electrical potential (E), thus mimicking the change in R2* from the blood oxygen level dependent (BOLD) response used in fMRI. Electric fields can change the dipole-dipole interaction, anisotropic part of the scalar interaction, and chemical-shift anisotropy of spins in polar molecules, and hence R1 and R2.

Results & DiscussionThe R1 and R2 values were different for the four 1H types in propylene carbonate without E. (See Fig. 2.) The four proton chemical shifts were not resolvable in any of the three cells due to Bo inhomogeneities. In the NMR cell, the inhomogeneities were from the electrodes. The mean R1 value did not change with the application of E, but their distribution became broader. This might explain the conflicting literature on R1.[2,3] The CPMG signal decay for E//Bo and EBo was clearly bi-exponential with R2 values of approximately 0.77 and 21.5 s-

1. Both R2 values remained approximately the same for E//Bo, but for EBo the small R2 value increased at a linear dR2/dE = 1.5×10-5 ms-1V-1 while the large R2 value decreased at -4.9×10-5 ms-1V-1. E is having an effect on R1 and R2, however, these changes are insufficient to base an fMRI phantom on.[4,5]

The spin echo signal intensity decreases with the R2 rate constant values in Table 1, and is also modulated by the four J couplings between the four unique proton types. (See Fig. 3.) The echo modulation causes the signal in our inhomogeneous Bo systems to decrease as a function of the echo time (TE). The echo modulation is not visible and the decay appears mono-exponential for the NMR data when 7.5 < TE < 60 ms and for the MRI data when 15 < TE < 150 ms. (See Fig. 4.) These decays were fit with an apparent R2 (R’2).

Table 1. R1 and R2 values of propylene carbonate with

E=0.

H

(ppm)R1 (s

-1) R2(CPMG) (s-1)

a b

a 1.497 0.67 0.63 42.4

B 4.031 0.46 0.58 30.2

C 4.561 0.44 0.56 19.4

D 4.863 0.32 0.47 32.2

(d)

(c)

(b)

(a)

ConclusionsWe conclude the following about the propylene carbonate system. An E field has an effect on the R1 and R2. The effect is largest for R2 with EBo, but too small to use as an fMRI dynamic phantom. On the other hand, the echo modulation frequencies change with the application of EBo, but not E//Bo, causing a change in the effective R2. This change is suitable to mimic the BOLD response in fMRI and use in a dynamic fMRI phantom.

References1. SW Provencher, Phys Comm 27:229-242 (1982).2. GP Jones, A Bradbury, PA Bradley. Mol Cryst Liq Cryst 55:143-150 (1979).3. TM Plantenga, HA Lopes Cardozo, J Bulthuis, C. Maclean. Chem Phys Lett 81:223-226 (1981).4. S Ogawa, DW Tank, et al., Proc Natl Acad Sci 89:5951-5955 (1992).5. J Olsrud, A Nilsson, et al.,  Magn Reson Imag 26:279-286 (2008).

Results & Discussion

For EBo, dR’2/dE = 9.2×10-4 and 4.5×10-4 ms-1V-1 for 7 and 1.5 T respectively. At both field strengths, dR’2/dE 0 for E//Bo. We believe the application of EBo changes the echo modulation frequencies slightly thus changing R’2.

Fig. 5. is a magnetic resonance image of the fMRI phantom with center electric cell. The artifact from the electric wire leading to the cell is visible in the mid upper right of the image.

Table 2. Relaxation rates of propylene carbonate and their change with E.

Relaxation Type

R1 R2 R’2

Bo (T) 7 7 7 1.5

Sequence IR CPMG SE SE

E:Bo

Orientation

EBo E//Bo EBo E//Bo EBo E//Bo EBo

Ri (s-1) 0.421 0.78 21.13 0.75 21.80 57.83 65.35 20.25 22.47

dRi/dE

(m s-1V-1)

0 1.6×10-8 4.5×10-6 1.5×10-5 -4.9×10-5 8.5×10-6 9.2×10-4 6.4×10-5 4.5×10-4

Electric CellR2 Standard

-1

0

1

0 0.2 0.4 0.6 0.8 1 t (s)Sig

na

l

H(a)

-1

0

1

0 0.2 0.4 0.6 0.8 1 t (s)Sig

na

l

H(b)

-1

0

1

0 0.2 0.4 0.6 0.8 1 t (s)Sig

na

l

H(d)

-1

0

1

0 0.2 0.4 0.6 0.8 1 t (s)Sig

na

l

H(c)

-1

0

1

0 0.2 0.4 0.6 0.8 1 t (s)Sig

na

l

H(a)+H(b)+H(c)+H(d) Fig. 3. Echo modulation pattern for the four chemical shift com-ponents and the summation. Highlighted portion represents R2’ fit region .

0

100

200

300

400

0 0.05 0.1 0.15

TE

NM

R S

ign

al

E = 0 V/cm, T2 = 45 ms

E = 118 V/cm, T2 = 36 ms

Fig. 4. Spin echo signal of propylene carbonate as a function of TE with and without electric field at 1.5 T.

Fig. 5. Spin echo signal of the fMRI phantom showing electric field cell, R2

standards, and resolution grid.

Fig. 2. Structure of propylene carbonate indicating the four proton types.

For a poster copy, see http://www.cis.rit.edu/people/faculty/hornak/conf-pre/ENC-2012/.