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EPR imaging
JOURNAL
OF MAGNETIC
RESONANCE
59,414-411
(1984)
EPR Imaging SANDRAS.EATON Department of Chemistry, University of Colorado, Denver, Colorado 80202 AND
GARETH R.EATON Department of Chemistry, University of Denver, Denver, Colorado 80208 Received March 12, 1984; revised April 19, 1984 Useful EPR imaging has been achieved using simple gradient coils on a standard spectrometer. Resolution of less than 1 mm is possible without deconvolution of the resulting spectra. Examples are presented using DPPH and nitroxyl radicals.
The many important applications of NMR imaging in biological research and clinical medicine (1-4) have stimulated several research groups to explore the feasibility of EPR imaging (5-15). The shorter wavelengths, larger dielectric loss, and broader lines for EPR relative to NMR result in limitations on sample size, need for larger magnetic field gradients, and a promise of much higher ultimate resolution. Hoch (5, 6) used oil-cooled anti-Helmholtz coils to achieve magnetic field gradients of up to 60 G/cm in the z direction (the direction defined by the main Zeeman field; in this paper we will take the x direction to be the vertical axis). Two diamonds (ca. 2.5 mm each) or two DPPH crystals (ca. l-2 mm each) were mounted on a rod and rotated in the yz plane. Fourier transform deconvolution and image reconstruction techniques were used to recover the yz cross section of the samples. Resolution of 0.1 mm was achieved. Karthe and co-workers (7, 8) used larger field gradients. Current through the coils was pulsed to achieve 1500 G/cm, which provided 30 G separation between the EPR lines of two DPPH granules 0.2 mm apart. A resolution of 0.012 mm was claimed (7). EPR zeugmatography, the use of a time-dependent modulation of the magnetic field gradient, with gradients of up to 250 G/cm was used to study spatial distribution of DPPH and Mn(I1) in various samples. The emphasis was on the projection of the species distribution on the z axis, rather than on the spectral shapes (8). Ohno studied the inhomogeneity of H atom distribution in irradiated sulfuric acid ices, using pulsed field gradients of ca. 100 G/cm (9). Three DPPH samples 2 mm apart were used to test the resolution and linearity of the -field gradient. Fourier transform deconvolution of the observed spectra was required to analyze the results (9). A gradient of ca. 3.5 G/cm along the x (vertical) direction was used 0022-2364184 S3.00 Copyright Q 1984 by Academic Press. Inc. All rights of reproduction in any form -cd.
474
EPR
IMAGING
475
to study the spatial distribution of ascorbic acid radicals in a flow system (10). The field gradient was linear for about 15 mm, and a maximum gradient of 54 G/cm could be attained. Since the samples used were large enough that the gradient over the sample was large relative to the linewidth, deconvolution methods were required to analyze the results. Details of this analysis have been reported (11). Yakimchenko et al. used dispersion adiabatic fast passage in inhomogeneous fields to study spatial distribution of paramagnetic defects in solids (12). They used an irradiated quartz sample, and generated magnetic field gradients by using ferromagnetic wedges and rods. The nature of their field gradients made the calculation of experimental lineshape beyond the scope of their paper, but they expressed the judgement that the spatial resolution of their experiments was not worse than 0.05 mm. Galtseva et al. used a ferromagnetic prism to create a field gradient of 130 G/cm along the x direction. They measured diffusion of perchlorotriphenylmethyl radicals in liquid decalin and of 02 in polytetraIluoroethylene. Their data analysis assumed that the EPR linewidth was small relative to the field change over the diffusion distance (23). Berliner is using lower frequency EPR (1.85 GHz) to explore the imaging of high dielectric samples, such as aqueous nitroxides (14). In the early 1970s Professor Paul Lauterbur and Reginald Dias used gradient coils on the outside of a Varian E-4 cavity and rotated samples of Fremy’s salt in capillaries and on filter paper. A series of projections was converted to pictures of the samples via image reconstruction techniques. This work has not been published (15). In this paper we report a simple experimental arrangement which provides routine imaging capability for samples of the dimensions commonly used in EPR, and demonstrate its capability. EXPERIMENTAL
EPR spectra were obtained on a Varian E-9 X-band spectrometer. Samples were supported using standard quartz and Pyrex cylindrical tubes, or the flat cell assembly we described previously (16). Magnetic field gradients in the z direction were generated by homemade auxiliary coils connected in series. There were two pairs of coils. The dimensions of the coils, using the notation of Ref. (I 7), were (a) large coils: inner radius p, = 14.5, outer radius p2 = 24.5; (b) small coils: p1 = 6.25, p2 = 11.25 mm. The small coils were arranged to oppose the large coils. All coils were made of 24 awg Belden magnet wire, were 4.8 mm thick and were 25 mm apart. This was not the optimum spacing for field linearity-it was chosen to determine the quality of spectra that could be obtained by simply reversing the Helmholtz arrangement that would most commonly be available to researchers. The coils were arranged in anti-Helmholtz configuration and fastened to the sides of a Varian E23 1 cavity. The total resistance of the coil assembly was about 3.5 Q. The power supply used provided about 1.86 amp across these coils. A field gradient was estimated (I 7) in reasonable agreement with the observed gradient of about 27 G/cm. This coil and power supply arrangement is sufficiently similar to the Varian E271A Rapid Scan accessory that anyone with the Varian or Bruker rapid scan units could expect to achieve results comparable to ours by merely reversing one of the rapid scan coils and using a constant, rather than a rapidly changing, current.
476
EATON
AND EATON
FIG. 1. EPR spectra of two DPPH samples. (A) shows the lineshape without applied field gradient, (B), (C), and (D) show the spectral separations achieved for DPPH samples 0.9, 3.0, and 5.0 mm apart, respectively. The gradient used was about 2.7 G/mm. Scan width, 40 G. RESULTS
AND DISCUSSION
The spectral resolution achieved is illustrated in Fig. 1 for samples consisting of two DPPH particles at 0.9, 3.0, and 5.0 mm separations. Note that the line shapes are not very distorted. Figure 2 shows comparable results for an assembly of four DPPH particles. Application to nitroxyl radicals in different environments is illustrated in Fig. 3. One of the aqueous nitroxyl samples contained chromium(III)
FIG. 2. EPR spectrum of an array of four DPPH samples at positions of 0, 0.6, 3.6, and 5.0 mm relative to the sample which gave the low-field peak. Scan width, 40 G with a gradient of 2.7 G/mm.
EPR
477
IMAGING
25 G FIG. 3. EPR spectra of 2,2,6,6-tetramethylpiperidinooxy (tempo) I mM in water in two 0.95 mm id. capillary tubes. One tube also contained 2.1 mM chromium(III) oxalate. The centers of the tubes were about 6.5 mm apart. The spectrum shown was obtained with a gradient of 1 G/mm. The larger linewidth of the nitroxyl in the presence of chromium(II1) oxalate is evident in the presence of the gradient, but the fact that the sample contains nitroxyl in two different environments is not evident in the absence of the gradient. Also, in the presence of the gradient power saturation curves were obtained and shown to be different for the nitroxyl in the two different environments.
oxalate, an effective spin probe broadening reagent (18, 19). Without a field gradient applied the spectra of the two samples superimposed, but with a field gradient the two environments could be studied separately. For example, the different power saturation curves of nitroxyl in the presence and absence of chromium(II1) oxalate were obtained with the gradient applied. The field gradient over the samples was large enough to cause some line broadening. If the relaxation times were measured with a spin-echo spectrometer the inhomogeneous broadening due to the gradient coils would not be a problem (20). ACKNOWLEDGMENTS Mr. Richard Quine assisted in matching the power This work was supported in part by NIH GM26566.
supply
to the coils
to obtain
the desired
gradient.
REFERENCES
1. R. DAMDIAN, 2. 3. 4.
5. 6. 7. 8. 9.
10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
Vol. 19, “NMR in Medicine,” SpringerEd., “NMR Basic Principles and Progress,” Verlag, Berlin, 198 1. P. MANSFIELD AND P. G. MORRIS, “Advances in Magnetic Resonance,” Supplement 2, “NMR Imaging in Biomedicine,” Academic Press, New York/London, 1982. C. C. JAFFE, Am. Sci. 70, 576 (1982). Z. H. CHO, H. S. KIM, H. B. SONG, AND J. C~MMING, Proc. IEEE 70, 1152 (1982). M. J. R. HOCH AND A. R. DAY, Solid State Commun. 30, 2 11 (1979). M. J. R. HOCH, J. Phys. C 14, 5659 (1981). W. KARTHE AND E. WEHRSDORFER, J. Magn. Reson. 33, 107 (1979). T. HERRLING, N. KLIMES, W. KARTHE, U. EWERT, AND B. EBERT, J. Magn. Reson. 49,203 (1982). K. OHNO, Jpn. J. Appl. Phys. 20, L179 (1981). K. OHNO, J. Magn. Reson. 49, 56 (1982). K. OHNO, J. Magn. Reson. 50, 145 (1982). 0. E. YAKIMCHENKO, I. N. KARTSIVADZE, B. V. OZHERELEV, AND YA. S. LEBEDEV, Dokl. Akad. Nauk SSSR 268,384 (1983). E. V. GALTSEVA, 0. YE. YAKIMCHENKO, AND YA. S. LEBEDEV, Chem. Phys. Left. 99,301 (1983). L. J. BERLINER AND H. NISHIKAWA, Abstracts, 25th Rocky Mountain Conference, Denver, Colorado, 1983; abstract 155. P. LAUTERBUR, personal communication, March 1984. S. S. EATON AND G. R. EATON, Anal. Chem. 49, 1277 (1977). E. R. ANDREW, I. ROBERTS, AND R. C. GUPTA, J. Sci. Instrum. 43,936 (1966). T. D. YAGER, G. R. EATON, AND S. S. EATON, J. Chem. Sot. Chem. Commun. 944, (1978). T. D. YAGER, G. R. EATON, AND S. S. EATON, Inorg. Chem. 18,725 (1979). J. R. NORRIS, M. C. THURNAUER, AND M. K. BOWMAN, Adv. Biol. Med. Phys. 17, 365 (1980).
OF MAGNETIC
RESONANCE
59,414-411
(1984)
EPR Imaging SANDRAS.EATON Department of Chemistry, University of Colorado, Denver, Colorado 80202 AND
GARETH R.EATON Department of Chemistry, University of Denver, Denver, Colorado 80208 Received March 12, 1984; revised April 19, 1984 Useful EPR imaging has been achieved using simple gradient coils on a standard spectrometer. Resolution of less than 1 mm is possible without deconvolution of the resulting spectra. Examples are presented using DPPH and nitroxyl radicals.
The many important applications of NMR imaging in biological research and clinical medicine (1-4) have stimulated several research groups to explore the feasibility of EPR imaging (5-15). The shorter wavelengths, larger dielectric loss, and broader lines for EPR relative to NMR result in limitations on sample size, need for larger magnetic field gradients, and a promise of much higher ultimate resolution. Hoch (5, 6) used oil-cooled anti-Helmholtz coils to achieve magnetic field gradients of up to 60 G/cm in the z direction (the direction defined by the main Zeeman field; in this paper we will take the x direction to be the vertical axis). Two diamonds (ca. 2.5 mm each) or two DPPH crystals (ca. l-2 mm each) were mounted on a rod and rotated in the yz plane. Fourier transform deconvolution and image reconstruction techniques were used to recover the yz cross section of the samples. Resolution of 0.1 mm was achieved. Karthe and co-workers (7, 8) used larger field gradients. Current through the coils was pulsed to achieve 1500 G/cm, which provided 30 G separation between the EPR lines of two DPPH granules 0.2 mm apart. A resolution of 0.012 mm was claimed (7). EPR zeugmatography, the use of a time-dependent modulation of the magnetic field gradient, with gradients of up to 250 G/cm was used to study spatial distribution of DPPH and Mn(I1) in various samples. The emphasis was on the projection of the species distribution on the z axis, rather than on the spectral shapes (8). Ohno studied the inhomogeneity of H atom distribution in irradiated sulfuric acid ices, using pulsed field gradients of ca. 100 G/cm (9). Three DPPH samples 2 mm apart were used to test the resolution and linearity of the -field gradient. Fourier transform deconvolution of the observed spectra was required to analyze the results (9). A gradient of ca. 3.5 G/cm along the x (vertical) direction was used 0022-2364184 S3.00 Copyright Q 1984 by Academic Press. Inc. All rights of reproduction in any form -cd.
474
EPR
IMAGING
475
to study the spatial distribution of ascorbic acid radicals in a flow system (10). The field gradient was linear for about 15 mm, and a maximum gradient of 54 G/cm could be attained. Since the samples used were large enough that the gradient over the sample was large relative to the linewidth, deconvolution methods were required to analyze the results. Details of this analysis have been reported (11). Yakimchenko et al. used dispersion adiabatic fast passage in inhomogeneous fields to study spatial distribution of paramagnetic defects in solids (12). They used an irradiated quartz sample, and generated magnetic field gradients by using ferromagnetic wedges and rods. The nature of their field gradients made the calculation of experimental lineshape beyond the scope of their paper, but they expressed the judgement that the spatial resolution of their experiments was not worse than 0.05 mm. Galtseva et al. used a ferromagnetic prism to create a field gradient of 130 G/cm along the x direction. They measured diffusion of perchlorotriphenylmethyl radicals in liquid decalin and of 02 in polytetraIluoroethylene. Their data analysis assumed that the EPR linewidth was small relative to the field change over the diffusion distance (23). Berliner is using lower frequency EPR (1.85 GHz) to explore the imaging of high dielectric samples, such as aqueous nitroxides (14). In the early 1970s Professor Paul Lauterbur and Reginald Dias used gradient coils on the outside of a Varian E-4 cavity and rotated samples of Fremy’s salt in capillaries and on filter paper. A series of projections was converted to pictures of the samples via image reconstruction techniques. This work has not been published (15). In this paper we report a simple experimental arrangement which provides routine imaging capability for samples of the dimensions commonly used in EPR, and demonstrate its capability. EXPERIMENTAL
EPR spectra were obtained on a Varian E-9 X-band spectrometer. Samples were supported using standard quartz and Pyrex cylindrical tubes, or the flat cell assembly we described previously (16). Magnetic field gradients in the z direction were generated by homemade auxiliary coils connected in series. There were two pairs of coils. The dimensions of the coils, using the notation of Ref. (I 7), were (a) large coils: inner radius p, = 14.5, outer radius p2 = 24.5; (b) small coils: p1 = 6.25, p2 = 11.25 mm. The small coils were arranged to oppose the large coils. All coils were made of 24 awg Belden magnet wire, were 4.8 mm thick and were 25 mm apart. This was not the optimum spacing for field linearity-it was chosen to determine the quality of spectra that could be obtained by simply reversing the Helmholtz arrangement that would most commonly be available to researchers. The coils were arranged in anti-Helmholtz configuration and fastened to the sides of a Varian E23 1 cavity. The total resistance of the coil assembly was about 3.5 Q. The power supply used provided about 1.86 amp across these coils. A field gradient was estimated (I 7) in reasonable agreement with the observed gradient of about 27 G/cm. This coil and power supply arrangement is sufficiently similar to the Varian E271A Rapid Scan accessory that anyone with the Varian or Bruker rapid scan units could expect to achieve results comparable to ours by merely reversing one of the rapid scan coils and using a constant, rather than a rapidly changing, current.
476
EATON
AND EATON
FIG. 1. EPR spectra of two DPPH samples. (A) shows the lineshape without applied field gradient, (B), (C), and (D) show the spectral separations achieved for DPPH samples 0.9, 3.0, and 5.0 mm apart, respectively. The gradient used was about 2.7 G/mm. Scan width, 40 G. RESULTS
AND DISCUSSION
The spectral resolution achieved is illustrated in Fig. 1 for samples consisting of two DPPH particles at 0.9, 3.0, and 5.0 mm separations. Note that the line shapes are not very distorted. Figure 2 shows comparable results for an assembly of four DPPH particles. Application to nitroxyl radicals in different environments is illustrated in Fig. 3. One of the aqueous nitroxyl samples contained chromium(III)
FIG. 2. EPR spectrum of an array of four DPPH samples at positions of 0, 0.6, 3.6, and 5.0 mm relative to the sample which gave the low-field peak. Scan width, 40 G with a gradient of 2.7 G/mm.
EPR
477
IMAGING
25 G FIG. 3. EPR spectra of 2,2,6,6-tetramethylpiperidinooxy (tempo) I mM in water in two 0.95 mm id. capillary tubes. One tube also contained 2.1 mM chromium(III) oxalate. The centers of the tubes were about 6.5 mm apart. The spectrum shown was obtained with a gradient of 1 G/mm. The larger linewidth of the nitroxyl in the presence of chromium(II1) oxalate is evident in the presence of the gradient, but the fact that the sample contains nitroxyl in two different environments is not evident in the absence of the gradient. Also, in the presence of the gradient power saturation curves were obtained and shown to be different for the nitroxyl in the two different environments.
oxalate, an effective spin probe broadening reagent (18, 19). Without a field gradient applied the spectra of the two samples superimposed, but with a field gradient the two environments could be studied separately. For example, the different power saturation curves of nitroxyl in the presence and absence of chromium(II1) oxalate were obtained with the gradient applied. The field gradient over the samples was large enough to cause some line broadening. If the relaxation times were measured with a spin-echo spectrometer the inhomogeneous broadening due to the gradient coils would not be a problem (20). ACKNOWLEDGMENTS Mr. Richard Quine assisted in matching the power This work was supported in part by NIH GM26566.
supply
to the coils
to obtain
the desired
gradient.
REFERENCES
1. R. DAMDIAN, 2. 3. 4.
5. 6. 7. 8. 9.
10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
Vol. 19, “NMR in Medicine,” SpringerEd., “NMR Basic Principles and Progress,” Verlag, Berlin, 198 1. P. MANSFIELD AND P. G. MORRIS, “Advances in Magnetic Resonance,” Supplement 2, “NMR Imaging in Biomedicine,” Academic Press, New York/London, 1982. C. C. JAFFE, Am. Sci. 70, 576 (1982). Z. H. CHO, H. S. KIM, H. B. SONG, AND J. C~MMING, Proc. IEEE 70, 1152 (1982). M. J. R. HOCH AND A. R. DAY, Solid State Commun. 30, 2 11 (1979). M. J. R. HOCH, J. Phys. C 14, 5659 (1981). W. KARTHE AND E. WEHRSDORFER, J. Magn. Reson. 33, 107 (1979). T. HERRLING, N. KLIMES, W. KARTHE, U. EWERT, AND B. EBERT, J. Magn. Reson. 49,203 (1982). K. OHNO, Jpn. J. Appl. Phys. 20, L179 (1981). K. OHNO, J. Magn. Reson. 49, 56 (1982). K. OHNO, J. Magn. Reson. 50, 145 (1982). 0. E. YAKIMCHENKO, I. N. KARTSIVADZE, B. V. OZHERELEV, AND YA. S. LEBEDEV, Dokl. Akad. Nauk SSSR 268,384 (1983). E. V. GALTSEVA, 0. YE. YAKIMCHENKO, AND YA. S. LEBEDEV, Chem. Phys. Left. 99,301 (1983). L. J. BERLINER AND H. NISHIKAWA, Abstracts, 25th Rocky Mountain Conference, Denver, Colorado, 1983; abstract 155. P. LAUTERBUR, personal communication, March 1984. S. S. EATON AND G. R. EATON, Anal. Chem. 49, 1277 (1977). E. R. ANDREW, I. ROBERTS, AND R. C. GUPTA, J. Sci. Instrum. 43,936 (1966). T. D. YAGER, G. R. EATON, AND S. S. EATON, J. Chem. Sot. Chem. Commun. 944, (1978). T. D. YAGER, G. R. EATON, AND S. S. EATON, Inorg. Chem. 18,725 (1979). J. R. NORRIS, M. C. THURNAUER, AND M. K. BOWMAN, Adv. Biol. Med. Phys. 17, 365 (1980).