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Construction of a high-resolution NMR probe for imaging with submillimeter spatial resolution
JOURNAL
OF MAGNETIC
RESONANCE
66,349-35
1
(1986)
Construction of a High-Resolution NMR Probe for Imaging with Submillimeter Spatial Resolution LAURANCED.HALL,STANLEYLUCK,ANDVASANTHANRAJANAYAGAM Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Y6, Canada
ReceivedJuly 10, 1985 The pursuit of a nuclear magnetic resonance system for imaging small objects has prompted a number of groups to use the coils and associated power supplies usually ursed for shimming the magnetic field to provide the linear field gradients for NMR irnaging (Z-3). The coils available on our 6.2 T, 54 mm bore Oxford Instrument superconducting magnet provide linear fields of approximately 1.1 mT m-‘, which enable imaging of axially symmetric phantoms containing water (T2 = 2.5 s) with spatial resolution of about 0.5 mm (4). The significant improvement in resolution o’bserved in images with higher gradients (vide infia) is evidence for the detrimental e&t of magnetic field inhomogeneity in experiments using such small gradients. For imaging studies of inhomogeneous objects which may contain nuclei with large natural resonance linewidths, larger gradients are necessary (5). We now describe how a completely standard high-resolution NMR probe can be fitted with coils to produce the orthogonal gradients of sufficient magnitude to provide spatial resolution better than 100 pm for a simple phantom. The probe used in this study consists of the usual components of a high-resolution device, fitted inside an aluminum tube of 37 mm outer diameter. The outer glass &war, which normally provides thermal insulation for variable temperature operation, was removed and replaced by a Perspex cylinder of outer diameter 34 mm and wall thickness 5 mm. Grooves, 3 mm deep, machined into the outer surface provide the physical localization of the 27 gauge, insulated copper wire used for the gradient coils. The dimensions of the coils used to generate the x and y gradients were determined according to the criteria of Golay (6, 7); the radius was 14.5 mm and 8 turns were used. The z gradient was produced by a Helmholtz pair, 23 mm separation, symmetrically located with respect to the midpoint of the probe; again 8 turns were used. The coils were connected to an Amphenol 7 pin hex-connector attached to the bottom of the probe. It was found necessary to have a filter to eliminate coupling between the radiofrequency and gradient coils. In static-current tests, the x and y coils were found to produce 37.1 and 39.5 mT m-‘, respectively, and the z coil produced 25 mT m-l, in good agreement with the calculated values of 37.0 and 24 mT m-', respectively. The temperature rise due to the current flow was measured using an iron-constantin thermocouple placed in contact with the aluminum can of the probe and also with the coils. A constant current of 1 A through one coil for an hour produced 349
0022-2364186 $3.00
350 a temperature rise of 6°C at the aluminum can, and 12°C at the coils, respectively. Running a single gradient at 4 A for the same amount of time with a 5% duty cycle (50 ms on, 9050 ms off) produced a temperature rise of 4°C at the can; however, a duty cycle of 10% caused the temperatures to rise by 7’C in 15 min. In view of their very low inductance (x, y, 35 PH; z, 12 rH) it is hardly surprising that these coils have short gradient rise times. These were determined from the ap pearance of NMR spectra obtained in two experiments. The first, in which the gradient is turned on well before the observed rf pulse, produces a projection spectrum which serves as the reference. In the second, the time at which the gradient is turned on prior to the observed pulse is varied; the rise time of the gradient is obtained when the projection from the second experiment resembles that of the reference. We have not yet chosen the power supply which will be included in the final configuration, but with an Amcron audio amplifier (model M-600) used to produce a 1 A current-pulse, the rise time which resulted in a minimally distorted spectrum is 10 ms. Although we have deferred the production of more complex images until we have a suitable display device, the contour plots shown in Fig. 1 for glass capillary tubes located inside a 5 mm NMR tube serve to indicate the resolution attainable using a variant of the two-dimensional .spin-warp method (8). All imaging software were written here at UBC (9, 10). Figure 1A indicates the results from four melting point capillaries (1.2 mm i.d.) and Fig. 1B represents that from finer capillaries (250-300 pm i.d.). The image given in Figure IC is from a phantom assembled inside a 1.2 mm internal diameter melting point capillary tube, which in turn is placed inside a 5 mm NMR tube. The larger of the capillaries has an inner diameter of ca. 220 pm, the smaller 140 Km. No z-axis slice selection was used to produce these images but, based on the dimensions of the radiofrequency coil, the slice thickness was ca. 11 mm. On that basis this probe, which has a S/N ratio of 350: 1 for water in a 200 pm diameter glass
B
C
Q
6
0.2 mm I
0
I
1.0
2.0 mm
I
0
I
1.0
2.0
mm
FIG. I. Two dimensional spin-warp variant images of glass capillary tubes containing water located inside a 5 mm NMR tube. (A) I .2 mm i.d. tubes;(B) 250-300 pm i.d.; 1.2 mm o.d.; (C) 140-220 am i.d. Experimental parameters; (A) sweep width = 4000 Hz; acquisition time = 64.12 ms; number of scans = 4; rr pulse width = 28 as; relaxation delay = 4 s; pgradient increment = 0.5 mT m-‘; maximum x gradient = 9.3 mT m-‘. (B) pgradient increment = 0.6 mT m-‘; maximum x gradient = 11.7 mT m-‘. (C) Sweep width = 6000 Hz; acquisition time = 42.57 ms; pgradient increment = 0.6 mT m-‘; maximum x gradient = 38.9 mT m-r.
351
NOTES
capillary tube for a single transient, should be capable of detecting a 0.1 mm thick slice with a S/N ratio of 10: 1 in 10 transients. geveral points concerning the use of this probe seem pertinent. First, it provides a very simple and convenient means for adapting a high-resolution NMR spectrometer for chemical microscopy. The fact that the gradients produced are about lo-fold greater than those from the shim coils makes feasible many imaging experiments of “real” objects (as distinct from “phantoms”). Even with the present probe, which has modest sensitivity by contemporary standards, we believe that it will be possible to detect signals from volumes as small as (150 pm)3 with a S/N ratio of 10: 1 in 10 transients; this provides adequate signal intensity to image objects as small as 1.O mm in diameter. At first we were concerned that the aluminum can of the probe might unduly damp the rise times of gradients by induction of eddy currents, and we eliminated it. However, more mature reflection led us to recognize that the inner bore tube of the magnet Dewar is sufficiently close to cause such effects. And in any event, the gradient rise tirnes turned out to be adequate. ACKNOWLEDGMENTS ‘This work was made possible by an equipment grant in the New Research Ideas Category, jointly funded by the Natural Sciences and Engineering Research Council of Canada and the Medical Research Council of Canada. One of us (S.L.) thanks NSERC for a Graduate Fellowship and V.R. thanks UBC for a University Graduate Fellowship. REFERENCES 1. A. KUMAR, 2. 3. 4. 5.
D. WELTI, AND R. R. ERNST, J. Magn. Reson. 18,69 (1975). P.BENDEL,C.M.LAI, AND P.C.LAUTERBUR,J. Magn.Reson.38,343 (1980). J. L.DELAYRE,J.S.INGWALL,C. MALLOY, ANDE.T.FOSSEL, Science212,935 (1981). L.D.HALL,V.RAJANAYAGAM, ANDS.SUKUMAR,J. Magn. Reson.60, 199(1984). P. MANSFIELD AND P. G. MORRIS, “NMR Imaging in Biomedicine,” p. 174, Academic
Press, New
York, 1982. 6. I’. A. Bol-TOMLEY,J.
Phys. E 14, 1081 (1981). 7. D. I. HOULT .~ND R. E. RICHARDS. Pm. R. Sot. London Ser. A 344,311 (1975). 8. W. A.EDELDSTEIN,J. M.S. HUTCHISON,G.JOHNSON, AND T. W. REDPATH, Phys. 751 (1980). 9. L. D. HALL AND S. SUKUMAR, J. Magn. Resort. 50, 161 (1982). 10. L.D.HALLAND S.SUKUMAR, .l. Magn.Reson.56,3 14(1984).
Med. Bioi. 25,
OF MAGNETIC
RESONANCE
66,349-35
1
(1986)
Construction of a High-Resolution NMR Probe for Imaging with Submillimeter Spatial Resolution LAURANCED.HALL,STANLEYLUCK,ANDVASANTHANRAJANAYAGAM Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Y6, Canada
ReceivedJuly 10, 1985 The pursuit of a nuclear magnetic resonance system for imaging small objects has prompted a number of groups to use the coils and associated power supplies usually ursed for shimming the magnetic field to provide the linear field gradients for NMR irnaging (Z-3). The coils available on our 6.2 T, 54 mm bore Oxford Instrument superconducting magnet provide linear fields of approximately 1.1 mT m-‘, which enable imaging of axially symmetric phantoms containing water (T2 = 2.5 s) with spatial resolution of about 0.5 mm (4). The significant improvement in resolution o’bserved in images with higher gradients (vide infia) is evidence for the detrimental e&t of magnetic field inhomogeneity in experiments using such small gradients. For imaging studies of inhomogeneous objects which may contain nuclei with large natural resonance linewidths, larger gradients are necessary (5). We now describe how a completely standard high-resolution NMR probe can be fitted with coils to produce the orthogonal gradients of sufficient magnitude to provide spatial resolution better than 100 pm for a simple phantom. The probe used in this study consists of the usual components of a high-resolution device, fitted inside an aluminum tube of 37 mm outer diameter. The outer glass &war, which normally provides thermal insulation for variable temperature operation, was removed and replaced by a Perspex cylinder of outer diameter 34 mm and wall thickness 5 mm. Grooves, 3 mm deep, machined into the outer surface provide the physical localization of the 27 gauge, insulated copper wire used for the gradient coils. The dimensions of the coils used to generate the x and y gradients were determined according to the criteria of Golay (6, 7); the radius was 14.5 mm and 8 turns were used. The z gradient was produced by a Helmholtz pair, 23 mm separation, symmetrically located with respect to the midpoint of the probe; again 8 turns were used. The coils were connected to an Amphenol 7 pin hex-connector attached to the bottom of the probe. It was found necessary to have a filter to eliminate coupling between the radiofrequency and gradient coils. In static-current tests, the x and y coils were found to produce 37.1 and 39.5 mT m-‘, respectively, and the z coil produced 25 mT m-l, in good agreement with the calculated values of 37.0 and 24 mT m-', respectively. The temperature rise due to the current flow was measured using an iron-constantin thermocouple placed in contact with the aluminum can of the probe and also with the coils. A constant current of 1 A through one coil for an hour produced 349
0022-2364186 $3.00
350 a temperature rise of 6°C at the aluminum can, and 12°C at the coils, respectively. Running a single gradient at 4 A for the same amount of time with a 5% duty cycle (50 ms on, 9050 ms off) produced a temperature rise of 4°C at the can; however, a duty cycle of 10% caused the temperatures to rise by 7’C in 15 min. In view of their very low inductance (x, y, 35 PH; z, 12 rH) it is hardly surprising that these coils have short gradient rise times. These were determined from the ap pearance of NMR spectra obtained in two experiments. The first, in which the gradient is turned on well before the observed rf pulse, produces a projection spectrum which serves as the reference. In the second, the time at which the gradient is turned on prior to the observed pulse is varied; the rise time of the gradient is obtained when the projection from the second experiment resembles that of the reference. We have not yet chosen the power supply which will be included in the final configuration, but with an Amcron audio amplifier (model M-600) used to produce a 1 A current-pulse, the rise time which resulted in a minimally distorted spectrum is 10 ms. Although we have deferred the production of more complex images until we have a suitable display device, the contour plots shown in Fig. 1 for glass capillary tubes located inside a 5 mm NMR tube serve to indicate the resolution attainable using a variant of the two-dimensional .spin-warp method (8). All imaging software were written here at UBC (9, 10). Figure 1A indicates the results from four melting point capillaries (1.2 mm i.d.) and Fig. 1B represents that from finer capillaries (250-300 pm i.d.). The image given in Figure IC is from a phantom assembled inside a 1.2 mm internal diameter melting point capillary tube, which in turn is placed inside a 5 mm NMR tube. The larger of the capillaries has an inner diameter of ca. 220 pm, the smaller 140 Km. No z-axis slice selection was used to produce these images but, based on the dimensions of the radiofrequency coil, the slice thickness was ca. 11 mm. On that basis this probe, which has a S/N ratio of 350: 1 for water in a 200 pm diameter glass
B
C
Q
6
0.2 mm I
0
I
1.0
2.0 mm
I
0
I
1.0
2.0
mm
FIG. I. Two dimensional spin-warp variant images of glass capillary tubes containing water located inside a 5 mm NMR tube. (A) I .2 mm i.d. tubes;(B) 250-300 pm i.d.; 1.2 mm o.d.; (C) 140-220 am i.d. Experimental parameters; (A) sweep width = 4000 Hz; acquisition time = 64.12 ms; number of scans = 4; rr pulse width = 28 as; relaxation delay = 4 s; pgradient increment = 0.5 mT m-‘; maximum x gradient = 9.3 mT m-‘. (B) pgradient increment = 0.6 mT m-‘; maximum x gradient = 11.7 mT m-‘. (C) Sweep width = 6000 Hz; acquisition time = 42.57 ms; pgradient increment = 0.6 mT m-‘; maximum x gradient = 38.9 mT m-r.
351
NOTES
capillary tube for a single transient, should be capable of detecting a 0.1 mm thick slice with a S/N ratio of 10: 1 in 10 transients. geveral points concerning the use of this probe seem pertinent. First, it provides a very simple and convenient means for adapting a high-resolution NMR spectrometer for chemical microscopy. The fact that the gradients produced are about lo-fold greater than those from the shim coils makes feasible many imaging experiments of “real” objects (as distinct from “phantoms”). Even with the present probe, which has modest sensitivity by contemporary standards, we believe that it will be possible to detect signals from volumes as small as (150 pm)3 with a S/N ratio of 10: 1 in 10 transients; this provides adequate signal intensity to image objects as small as 1.O mm in diameter. At first we were concerned that the aluminum can of the probe might unduly damp the rise times of gradients by induction of eddy currents, and we eliminated it. However, more mature reflection led us to recognize that the inner bore tube of the magnet Dewar is sufficiently close to cause such effects. And in any event, the gradient rise tirnes turned out to be adequate. ACKNOWLEDGMENTS ‘This work was made possible by an equipment grant in the New Research Ideas Category, jointly funded by the Natural Sciences and Engineering Research Council of Canada and the Medical Research Council of Canada. One of us (S.L.) thanks NSERC for a Graduate Fellowship and V.R. thanks UBC for a University Graduate Fellowship. REFERENCES 1. A. KUMAR, 2. 3. 4. 5.
D. WELTI, AND R. R. ERNST, J. Magn. Reson. 18,69 (1975). P.BENDEL,C.M.LAI, AND P.C.LAUTERBUR,J. Magn.Reson.38,343 (1980). J. L.DELAYRE,J.S.INGWALL,C. MALLOY, ANDE.T.FOSSEL, Science212,935 (1981). L.D.HALL,V.RAJANAYAGAM, ANDS.SUKUMAR,J. Magn. Reson.60, 199(1984). P. MANSFIELD AND P. G. MORRIS, “NMR Imaging in Biomedicine,” p. 174, Academic
Press, New
York, 1982. 6. I’. A. Bol-TOMLEY,J.
Phys. E 14, 1081 (1981). 7. D. I. HOULT .~ND R. E. RICHARDS. Pm. R. Sot. London Ser. A 344,311 (1975). 8. W. A.EDELDSTEIN,J. M.S. HUTCHISON,G.JOHNSON, AND T. W. REDPATH, Phys. 751 (1980). 9. L. D. HALL AND S. SUKUMAR, J. Magn. Resort. 50, 161 (1982). 10. L.D.HALLAND S.SUKUMAR, .l. Magn.Reson.56,3 14(1984).
Med. Bioi. 25,