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Echo-planar microscopy of porous rocks
Magnetic
Imaging,
Vol. 14, Nos. 7/8, pp. 875-877, 1996 Copyright 0 1996 Elsevier Science Inc. Printed in the USA. All rights reserved 0730-725X/96 $15.00 + .OO
PI1 $50730-725X( 96) 00170-4
ELSEVIER
l
Resonance
Short Communication ECHO-PLANAR
MICROSCOPY
OF POROUS
ROCKS
A.M. PETERS, P.S. ROBYR, R.W. BOWTELL, AND P. MANSFIELD Magnetic Resonance Centre, University of Nottingham, Nottingham, NG7 2RD, UK A modiied version of the echo-planar imaging sequence incorporating multiple 180” RF pulses (PEPI) has been implemented at 11.7 T and used to generate high resolution images of porous solids. By preceding the PEP1 sequence with an inversion recovery or flow-encoding sequence, rapid T1, and velocity mapping is possible in the microscopic domain. Copyright 0 1996 Elsevier Science Inc. Keywords:
EPI; NMR microscopy;
Porous rocks.
INTRODUCTION
tion decay (FID). However, the low frequency per point of the technique makes it very sensitive to the effect of magnetic field inhomogeneities. In porous rocks saturated with water, variation in magnetic susceptibility causes large internal magnetic field gradients. At high magnetic field these give rise to significant frequency offsets and rapid Tz* relaxation, which lead to serious distortion and blurring in EP images. PEP1 (7r-EPI) 3,4 is a modification of the EPI sequence in which the repeated gradient reversals are replaced by 180” RF pulses. In PEPI, therefore, the dephasing due to magnetic field inho-
For many years, NMR techniques have been used in the investigation of porous media. Measurements of NMR relaxation times and of the diffusive behaviour of fluids in the porous microstructure have proved particularly useful. In heterogeneous samples a greater amount of information can of course be gathered by combining these measurements with imaging. To date, the resolution in most imaging experiments carried out on porous media, has been considerably coarser than the pore scale. MvIR microscopy potentially offers the possibility of making NMR measurements at a resolution approaching the pore size.’ However, the strong magnetic fields that are usually employed in high resolution NMR microscopy can lead to the presence of large magnetic field inhomogeneities, whose effects must be overcome if this potential is to be realised. Building on work carried out at 0.5 T, 3X4we have applied NMR microscopy at 11.7 T to the investigation of porous rock samples. Techniques that allow the generation of images that are relatively insensitive to field inhomogeneity, while having reasonable image acquisition times, have been implemented and measurements of T, and flow have been made. MATERIALS
AND METHODS Fig. 1. ( a) 3D inversion recovery image of a water saturated Bentheimer sandstone. (b) MO map and (c) the correspond-
Echo-planar imaging (EPI)’ allows the acquisition of a complete 2D image from a single free induc-
ing T, map, calculated from a set of 15 such images. ham, NG7 2RD, UK.
Address correspondence to R.W. Bowtell, Magnetic Resonance Centre, University
of Nottingham,
Notting875
876
Magnetic Resonance Imaging l Volume 14, Numbers 7/8, 1996
0.2
0.4
0.6
v/(mms-I) 0
0.2 0.4 v/(mms-I)
0.6
Fig. 2. (a) Velocity map, and (b) velocity distribution of water flowing at a rate of 0.2 mL/min through a cylindrical Bentheimer sandstone, calculated from an average of 16 images.
mogeneities is periodically refocussed. Consequently, image distortion is much reduced and the decay of the echo train largely depends on T2 relaxation rather than T2 * decay. The presence of the 180” RF pulses in the sequence alters the order of echo acquisition and therefore the k-space trajectory. In MBEST EPI,5 k-space is scanned in a raster fashion with alternate echoes sampled under gradients of different polarity. In PEPI, the centre echo is the first to be sampled, with the k-space scan alternating between -f-k, and -k,, stepping out by one increment for every two echoes sampled. Data reordering is, therefore, required before Fourier transformation. If the 180” RF pulses in the sequence are all of the same phase, unwanted transverse magnetisation resulting from pulse angle imperfections gives rise to a central line artifact. By alternating the phase of the 180” RF pulses, it is possible to move the artifact by half of the field of view with respect to the image and to reduce its intensity.‘j By including a phase encoding gradient pulse in the slice select direction, the sequence may be extended to three dimensional imaging. While this is
no longer a single shot technique, it does allow the acquisition of an image of N X N X N pixels in N excitations.7 Inversion recovery imaging can be performed by preceding the PEP1 experiment with a 180” RF pulse. By acquiring a set of images with different inversion times a map of the T, relaxation time can then be calculated. For a 643 image matrix and an interexperimental delay of 10 s, a set of 14 images may be obtained in under 3 h. This compares to 160 h for the equivalent 3DFT experiment. Rapid flow measurements have been made using the flow encoding sequence proposed by Guilfoyle et al.3 This consists of gradient pulses of duration, ro, flanked by 180” RF pulses. At high magnetic fields, careful choice of the timing parameters is essential if sufficient flow encoding is to be generated without severe loss of transverse magnetisation. The time between two 180” RF pulses is limited by the signal decay due to diffusion in the local gradients within the rock. The optimal duration of the velocity encoding sequence, 2r, and the gradient strength are determined by the T2 relaxation time and the signal decay due to diffusion in the applied gradients.
Echo-planar microscopy of
porous
RESULTS The sequences described above have been implemented on an 11.7 T NMR microscope, 8equipped with actively screened gradient coils. Figure 1a shows a 3D inversion recovery image of a Bentheimer sandstone sample of 9 mm diameter. A series of 15 such images with different inversion times ranging from 3 ms to 4 s were obtained in 20 min with a repetition time of 10 s. The in-plane resolution was 180 pm and slice thickness was 0.5 mmi. From these, both an MO map (Fig. lb) and a T, map (Fig. lc) were calculated. Despite the large variation in signal intensity in both the base images and the A.&, map, the TI map is relatively uniform. The T, values measured at 11.7 T are comparable with those observed at 0.5 T,4 in both mean and standard de.viation. A preliminary flow map (Fig. 2a) was measured in a Bentheimer sandstone sample (9 mm diameter, 18 mm length) at a flow rate of 0.2 mL/min. The in-plane resolution was 180 /Irn and the slice thickness was 1.5 mm. The flow encoding sequence consisted of 16 gradient pulses of strength 79 mT/m with 7 = 10.08 ms and ro = 740 ps. The velocity distribution is shown in Fig. 2b. The mean value, 0.23 mL/min agrees well with the theoretical bulk velocity of 0.22 mL/min.
CONCLUSIONS PEP1 allows the acquisition of an entire image from a single FID with a resolution approaching the pore scale. The technique is relatively insensitive to the magnetic field inhomogeneities found in porous media at the high fields used in NMR microscopy. The rapid nature of the technique allows the, acquisition of volumar T, maps in a reasonable imaging time. The addition of a suitable spin preparation sequence allows the monitoring of dy-
rocks
0
A.M.
PETERS
ET AL.
877
namic flow processes. Current effort is focused on the generation of flow maps similar to those presented here, but at different flow rates with the aim of investigating the parametric dependence of the stochastic flow tl-~eory~~‘~as a function of resolution. REFERENCES 1. Nesbitt, G.J.; Fens, T.W.; vanden Brink, J.S.; Roberts, N. Evaluation of fluid displacement in porous media using NMR microscopy. In: B. Blumich; W. Kuhn (Eds) . Magnetic Resonance Microscopy. Weinheim: VCH; 1992: p. 287. 2. Mansfield, P. Multi-planar image formation using NMR spin echoes. J. Phys. C. lO:L55; 1977. 3. Guilfoyle, D.N.; Mansfield, P.; Packer, K.J. Fluid flow measurements in porous media by echo-planar imaging. J. Magn. Reson. 971342-358; 1992. 4. Issa, B.; Mansfield, P. Proc. 12th Ann. Meet. Sot. Magn. Reson. Med. 3:1236; 1993. 5. Howseman, A.M.; Stehling, M.K.; Chapman, B.; Coxon, R.; Turner, R.; Ordidge, R.J.; Cawley, M.G.; Glover, P.; Mansfield, P.; Coupland, R.E. Improvements in snap shot nuclear magnetic resonance imaging. Br. J. Radiol. 61:882; 1988. 6. Hennel, F. Modification of the Carr-Purcell sequence for single shot echo planar imaging. Magn. Reson. Med. 26: 116; 1992. 7. Guilfoyle, D.N.; Issa, B.; Mansfield, P. Rapid volumetric NMR imaging of fluids in porous solids using a 3D ITEPI (PEPI) hybrid. J. Magn. Reson. A. 119:151; 1996. 8. Bowtell, R.W.; Brown, G.D.; Glover, P.M.; McJury, M.; Mansfield, P. Resolution of cellular structures by NMR microscopy at 11.7 T. Philos. Trans. R. Sot. 333:457467; 1990. 9. Mansfield, P.; Issa, B. Studies of fluid transport in porous rocks by echo-planar MRI. Magn. Reson. Imaging 12:275; 1994. 10. Mansfield, P.; Issa, B. A microscopic model of fluid transport in porous rocks. Magn. Resort. Imaging 14:71 I-714; 1996.
Imaging,
Vol. 14, Nos. 7/8, pp. 875-877, 1996 Copyright 0 1996 Elsevier Science Inc. Printed in the USA. All rights reserved 0730-725X/96 $15.00 + .OO
PI1 $50730-725X( 96) 00170-4
ELSEVIER
l
Resonance
Short Communication ECHO-PLANAR
MICROSCOPY
OF POROUS
ROCKS
A.M. PETERS, P.S. ROBYR, R.W. BOWTELL, AND P. MANSFIELD Magnetic Resonance Centre, University of Nottingham, Nottingham, NG7 2RD, UK A modiied version of the echo-planar imaging sequence incorporating multiple 180” RF pulses (PEPI) has been implemented at 11.7 T and used to generate high resolution images of porous solids. By preceding the PEP1 sequence with an inversion recovery or flow-encoding sequence, rapid T1, and velocity mapping is possible in the microscopic domain. Copyright 0 1996 Elsevier Science Inc. Keywords:
EPI; NMR microscopy;
Porous rocks.
INTRODUCTION
tion decay (FID). However, the low frequency per point of the technique makes it very sensitive to the effect of magnetic field inhomogeneities. In porous rocks saturated with water, variation in magnetic susceptibility causes large internal magnetic field gradients. At high magnetic field these give rise to significant frequency offsets and rapid Tz* relaxation, which lead to serious distortion and blurring in EP images. PEP1 (7r-EPI) 3,4 is a modification of the EPI sequence in which the repeated gradient reversals are replaced by 180” RF pulses. In PEPI, therefore, the dephasing due to magnetic field inho-
For many years, NMR techniques have been used in the investigation of porous media. Measurements of NMR relaxation times and of the diffusive behaviour of fluids in the porous microstructure have proved particularly useful. In heterogeneous samples a greater amount of information can of course be gathered by combining these measurements with imaging. To date, the resolution in most imaging experiments carried out on porous media, has been considerably coarser than the pore scale. MvIR microscopy potentially offers the possibility of making NMR measurements at a resolution approaching the pore size.’ However, the strong magnetic fields that are usually employed in high resolution NMR microscopy can lead to the presence of large magnetic field inhomogeneities, whose effects must be overcome if this potential is to be realised. Building on work carried out at 0.5 T, 3X4we have applied NMR microscopy at 11.7 T to the investigation of porous rock samples. Techniques that allow the generation of images that are relatively insensitive to field inhomogeneity, while having reasonable image acquisition times, have been implemented and measurements of T, and flow have been made. MATERIALS
AND METHODS Fig. 1. ( a) 3D inversion recovery image of a water saturated Bentheimer sandstone. (b) MO map and (c) the correspond-
Echo-planar imaging (EPI)’ allows the acquisition of a complete 2D image from a single free induc-
ing T, map, calculated from a set of 15 such images. ham, NG7 2RD, UK.
Address correspondence to R.W. Bowtell, Magnetic Resonance Centre, University
of Nottingham,
Notting875
876
Magnetic Resonance Imaging l Volume 14, Numbers 7/8, 1996
0.2
0.4
0.6
v/(mms-I) 0
0.2 0.4 v/(mms-I)
0.6
Fig. 2. (a) Velocity map, and (b) velocity distribution of water flowing at a rate of 0.2 mL/min through a cylindrical Bentheimer sandstone, calculated from an average of 16 images.
mogeneities is periodically refocussed. Consequently, image distortion is much reduced and the decay of the echo train largely depends on T2 relaxation rather than T2 * decay. The presence of the 180” RF pulses in the sequence alters the order of echo acquisition and therefore the k-space trajectory. In MBEST EPI,5 k-space is scanned in a raster fashion with alternate echoes sampled under gradients of different polarity. In PEPI, the centre echo is the first to be sampled, with the k-space scan alternating between -f-k, and -k,, stepping out by one increment for every two echoes sampled. Data reordering is, therefore, required before Fourier transformation. If the 180” RF pulses in the sequence are all of the same phase, unwanted transverse magnetisation resulting from pulse angle imperfections gives rise to a central line artifact. By alternating the phase of the 180” RF pulses, it is possible to move the artifact by half of the field of view with respect to the image and to reduce its intensity.‘j By including a phase encoding gradient pulse in the slice select direction, the sequence may be extended to three dimensional imaging. While this is
no longer a single shot technique, it does allow the acquisition of an image of N X N X N pixels in N excitations.7 Inversion recovery imaging can be performed by preceding the PEP1 experiment with a 180” RF pulse. By acquiring a set of images with different inversion times a map of the T, relaxation time can then be calculated. For a 643 image matrix and an interexperimental delay of 10 s, a set of 14 images may be obtained in under 3 h. This compares to 160 h for the equivalent 3DFT experiment. Rapid flow measurements have been made using the flow encoding sequence proposed by Guilfoyle et al.3 This consists of gradient pulses of duration, ro, flanked by 180” RF pulses. At high magnetic fields, careful choice of the timing parameters is essential if sufficient flow encoding is to be generated without severe loss of transverse magnetisation. The time between two 180” RF pulses is limited by the signal decay due to diffusion in the local gradients within the rock. The optimal duration of the velocity encoding sequence, 2r, and the gradient strength are determined by the T2 relaxation time and the signal decay due to diffusion in the applied gradients.
Echo-planar microscopy of
porous
RESULTS The sequences described above have been implemented on an 11.7 T NMR microscope, 8equipped with actively screened gradient coils. Figure 1a shows a 3D inversion recovery image of a Bentheimer sandstone sample of 9 mm diameter. A series of 15 such images with different inversion times ranging from 3 ms to 4 s were obtained in 20 min with a repetition time of 10 s. The in-plane resolution was 180 pm and slice thickness was 0.5 mmi. From these, both an MO map (Fig. lb) and a T, map (Fig. lc) were calculated. Despite the large variation in signal intensity in both the base images and the A.&, map, the TI map is relatively uniform. The T, values measured at 11.7 T are comparable with those observed at 0.5 T,4 in both mean and standard de.viation. A preliminary flow map (Fig. 2a) was measured in a Bentheimer sandstone sample (9 mm diameter, 18 mm length) at a flow rate of 0.2 mL/min. The in-plane resolution was 180 /Irn and the slice thickness was 1.5 mm. The flow encoding sequence consisted of 16 gradient pulses of strength 79 mT/m with 7 = 10.08 ms and ro = 740 ps. The velocity distribution is shown in Fig. 2b. The mean value, 0.23 mL/min agrees well with the theoretical bulk velocity of 0.22 mL/min.
CONCLUSIONS PEP1 allows the acquisition of an entire image from a single FID with a resolution approaching the pore scale. The technique is relatively insensitive to the magnetic field inhomogeneities found in porous media at the high fields used in NMR microscopy. The rapid nature of the technique allows the, acquisition of volumar T, maps in a reasonable imaging time. The addition of a suitable spin preparation sequence allows the monitoring of dy-
rocks
0
A.M.
PETERS
ET AL.
877
namic flow processes. Current effort is focused on the generation of flow maps similar to those presented here, but at different flow rates with the aim of investigating the parametric dependence of the stochastic flow tl-~eory~~‘~as a function of resolution. REFERENCES 1. Nesbitt, G.J.; Fens, T.W.; vanden Brink, J.S.; Roberts, N. Evaluation of fluid displacement in porous media using NMR microscopy. In: B. Blumich; W. Kuhn (Eds) . Magnetic Resonance Microscopy. Weinheim: VCH; 1992: p. 287. 2. Mansfield, P. Multi-planar image formation using NMR spin echoes. J. Phys. C. lO:L55; 1977. 3. Guilfoyle, D.N.; Mansfield, P.; Packer, K.J. Fluid flow measurements in porous media by echo-planar imaging. J. Magn. Reson. 971342-358; 1992. 4. Issa, B.; Mansfield, P. Proc. 12th Ann. Meet. Sot. Magn. Reson. Med. 3:1236; 1993. 5. Howseman, A.M.; Stehling, M.K.; Chapman, B.; Coxon, R.; Turner, R.; Ordidge, R.J.; Cawley, M.G.; Glover, P.; Mansfield, P.; Coupland, R.E. Improvements in snap shot nuclear magnetic resonance imaging. Br. J. Radiol. 61:882; 1988. 6. Hennel, F. Modification of the Carr-Purcell sequence for single shot echo planar imaging. Magn. Reson. Med. 26: 116; 1992. 7. Guilfoyle, D.N.; Issa, B.; Mansfield, P. Rapid volumetric NMR imaging of fluids in porous solids using a 3D ITEPI (PEPI) hybrid. J. Magn. Reson. A. 119:151; 1996. 8. Bowtell, R.W.; Brown, G.D.; Glover, P.M.; McJury, M.; Mansfield, P. Resolution of cellular structures by NMR microscopy at 11.7 T. Philos. Trans. R. Sot. 333:457467; 1990. 9. Mansfield, P.; Issa, B. Studies of fluid transport in porous rocks by echo-planar MRI. Magn. Reson. Imaging 12:275; 1994. 10. Mansfield, P.; Issa, B. A microscopic model of fluid transport in porous rocks. Magn. Resort. Imaging 14:71 I-714; 1996.