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A strategy for performing volume-selected multipulse NMR spectroscopy in vivo
JOURNAL
OF MAGNETIC
RESONANCE
63, 6 12-6 19 ( 1985)
A Strategy for Performing Volume-SelectedMultipulse NMR Spectroscopyin Viva JAMESFIELD, WILLIAMM.
BROOK&J. MARKBULSING, ANDDAVIDM. DODDRELL
MICHAELG.
IRVING,
School of Science, Gr@th University, Nathan 4111, Queensland, Australia Received November 8, 1984; revised March 11, 1985
A wealth of biochemical information is potentially available from the successful application of in vivo multipulse NMR spectroscopy. The major experimental difficulty encountered when attempting to implement such studies using surface coils for volume selection is their inherently poor rf pulse homogeneity (I). This, of course, runs counter to the most fundamental requirement of multipulse NMR, the need for homogeneous pulses. Although such coils may be used to effect a degree of depth selection (2), in reality, when working with animals the issue is whether the surface can be eliminated to reveal signals from underlying tissue; that is, one requires a depth selection procedure to filter approximately the first 10 mm. Our experimental strategy is based upon a separation of the problems associated with volume selectivity and pulse characteristics. This is a more natural approach because, to a first approximation, pulse error compensation schemes may be tailored to the problem of efficient manipulation of spin coherence. In experiments where volume selection is performed by more sophisticated means (field gradients) the experience gained from this strategy remains directly applicable. It is well known that the use of homogeneous rf pulses for spin manipulation increases both the spectral signalto-noise ratio and the ease of performing the experiment. Indeed much effort is taken by the user of multipulse NMR spectroscopy to ensure that this condition is met. The remaining problem is to marry this requirement to that of volume selection. To achieve this we assume we have available a probe fitted with a surface coil and a second coil (saddle) having high rf pulse homogeneity. Both coils are available for pulsing, with the surface coil acting as the receiver coil. In the following, (n/2)[x] means an x-phased a/2 rf pulse applied to the saddle coil (main coil) while (r/2)(x) is the equivalent pulse applied to the surface coil. For a single-spin system the initial magnetization found at thermal equilibrium can be represented by the operator S,. At the end of a pulse train the magnetization of interest may be represented by SL (or Sb). Here, SL contains the desired information available from the specific pulse sequence, for example, chemical-shift correlation, chemical exchange, and so on. Using our main coil to implement the pulse sequence the change in the spin coherence from S, to S” occurs for the whole sample. The surface coil could now be used for detection thus yielding a degree of volume selectivity. However, to increase the volume selectivity we include a surface0022-2364/85 $3.00 Copyright 0 1985 by Academic Press. Inc. AU rights of reproduction in any form reserved.
612
613
COMMUNICATIONS
coil pulse which is phase coherent with saddle-coil pulses, yielding strategy
the overall
where 0(x) is a volume-selection procedure and may also incorporate a Bodenhausentype exorcycled pulse scheme (3). To effect the ideas outlined above experimentally, a probe tuned for 31P NMR spectroscopy was constructed. This was fitted with two coils; the first was a saddle type in approximate Helmholtz configuration, suitable for handling rats of about 350 g (dimensions 60 X 60 X 55 mm), while the second was a surface coil (diameter 27 mm) wound with its axis orthogonal to the saddle-coil axis. It is well known that two circuits tuned to the same frequency in close proximity will couple, leading to signal loss. The problem was solved (see Fig. 1) by “humbucking” the saddle coil which was wound in a parallel fashion, in the region of the surface coil. Alternatively, crossed diodes, acting as a short, may be used (4) to effect decoupling of coils, but this procedure provides an inferior solution to the problem as the Q of the coil varies with pulse power and the spectral signal-to-noise is markedly reduced. This technically less satisfactory method (4) also results in longer pulse times for constant pulse power. Our method, which allows construction of high Q circuits resonant at the same frequency in close proximity, represents a simple and highly effective solution to the problems of interacting coil geometries. For our probe, both coils had a Q of approximately 400 and no sign of coil coupling could be detected. Detailed measurements using a small test coil showed the improved rf homogeneity obtained from the saddle coil compared with that obtained from the surface coil. Frequency generation for the two rf channels was achieved as shown in Fig. 2. A “phase corrector” was used to adjust the phase of the surface-coil circuit path relative to that for the saddle coil. Phase coherence was set by looking for a null signal response using the pulse pair (7r/2)[x](a/2)(x’) applied to a small sample. This
/
Sample
tube
Bo RG. 1. Diagram showing the current flow in a parallel-wound saddle coil. (A) Surface-coil connections; (B) saddle-coil connections. The two halves of the coil have been drawn flat. In practice, of course, the halves are perpendicular to the page. Because of the current flow, the surface coil is clearly in a region where the field from the saddle coil should cancel. Complete cancellation can be achieved by changing the spatial arrangement of the horizontal length C. This is best done by transmitting on the saddle coil and, using the surface coil as a pickup coil, moving C until there is minimum (undetectable in our case) induced signal. This is known as “humbucking.”
614
COMMUNICATIONS
zj +:;&.,I Q
gE m E m x i FIG. 2. Block diagram of the frequency-generation experiments.
CC
8 B 2 E 8
surface coil connected to receiver
path used to perform the double transmitter coil
ensures x = x’ or x = x’ + r. In either case, phase coherence exists, so there should be no signal loss following the pulse pair O(x)(71./2)[&~] and this proved to be the case. To test this arrangement for volume-selected multipulse NMR spectroscopy, the following experiments were carried out. First, two 5 mm tubes were filled with concentrated aqueous solutions of either sodium hydrogen phosphate (Pi) or glucose6-phosphate (G-6-P) (Sample A). The tubes were filled to a depth of approximately one-half of the diameter of the surface coil and centrally placed side-by-side along the axis defined by the field direction at the face of the surface coil and were bisected by this axis (Fig. 1). The samples were a distance of 7 and 12 mm from the surface-coil center, the phosphate sample being closer to the coil. (This has a much sharper resonance than that for glucosed-phosphate which is broadened by long-range 31P-‘H coupling). In Fig. 3a we show a spectrum from test sample A acquired following a 48 ps pulse applied to the surface coil. Eight scans were averaged with a recycle time of 25 s. Both signals are observed with a signal-to-noise ratio comparable to that obtained with excitation applied via the saddle coil only (see insert to Fig. 4). Using a 96 ps pulse on the surface coil the phosphate signal is significantly reduced Fig. 3b. The first multipulse experiment attempted was a selective T1 measurement using the inversion-recovery technique, the overall pulse train employed being
(a/2)[xl~[yl(a/2)[Xl--7--e(~+X), receiver f.
PI
Here, /3 was set to give a null for the phosphate signal. A composite inversion pulse (5) on the saddle coil gave significantly better results than a single hard P pulse. A
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COMMUNICATIONS
b
FIG. 3. (a) 80.98 MHz 31P NMR spectrum from test Sample A using a 48 ps pulse on the surface coil: 8 scans, recycle time 25 s, 1000 Hz spectral width. (b) Same as (a) but using a 96 ~LSpulse.
set of volume-selected partially relaxed (PRFT) spectra are shown in Fig. 4. A set of spectra determined without volume selection gave an identical T, value for the glucosed-phosphate signal: Tl(PJ = 4.9 S, Tr(G-6-P) = 7.3 S. To study the volume-selection capabilities of our experimental arrangement using a more difficult test, the G-6-P solution was replaced with a concentrated aqueous solution of ATP (Sample B). (The pH of this solution was about 3 leading to the unusual chemical shifts displayed in Fig. 5.) Because of the low signal strength of the ATP resonances relative to that of the phosphate (Fig. 5a), to perform volumeselected NMR means that the demands on signal suppression are far more stringent. Furthermore, the lengths of the samples were now extended to be equal to the coil diameter. Again they were centered with the coil axis. This procedure is to be
616
COMMUNICATIONS
FIG. 4. A set of volume-selective PRFT spectra obtained from Sample A using pulse train [3]. Eight scans, 1000 Hz spectral width, recycle time 30 s; (7r/2)[x] = 72 ps, 19(x)set at 96 ps. The inserted spectrum is obtained from Sample A following eight ?r/2 pulses applied to the saddle coil. The signal-to-noise ratio obtained here represents the maximum that can be achieved using our experimental setup.
contrasted with the experimental methodology adopted by Bendall and co-workers (3, 6) who test volume selection by using a single disk-shaped sample, small relative to the dimensions of their surface coil. We consider our arrangement to be a more realistic test of our probe design because it allows us to investigate the problems associated with signals arising from spins which are remote from the axis perpendicular to the surface-coil plane. It seems likely to us that in experiments involving real samples (animals, humans), where volume selection is most important, these offaxis effects will demand serious consideration. Figure 5b shows a PRFT spectrum using pulse train [2] with T set equal to 100 ps. Note, that although the error signal from the phosphate solution is reduced compared to the normal spectrum, Fig. 5a, it is still large and it displays significant out-of-phase error components. However, because d(x) and (a/2)[~4 are phase coherent, such out-of-phase signal components can be eliminated by the use of a purging pulse (7) from the saddle coil. The pulse pair ?r(x)(?r/2)[+y] and receiver add/add should give superior volume selection than simply X(X). Our inversionrecovery sequence becomes
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COMMUNICATIONS
FIG. 5. (a) Normal spectrum from Sample B: spectral width 2000 Hz, 8 scans acquired using a recycle time of 25 s. (b) Volume-selected PRFT spectrum from Sample B using pulse train [2], 7 set at 100 hs. (c) Same as (b) but using pulse train [3]. Note the improved volume selection. The phases of spectra (b) and (c) have been inverted.
(~/2>[xl~[vl(a/2)[xl-?-8(+x)(~/2)[fyl,
receiver ++--.
[31
A purged PRFT spectrum is shown in Fig. 5c. Note the excellent volume selection achieved by this simple procedure. It is well known that inversion-recovery is the method of choice for T, determinations (8); we are confident that the procedure described here has applicability to volume-selected in vivo Tl measurements and we are investigating it further.
618
COMMUNTCATI~NS
Of course the ability to perform a volume-selective T, measurement does not necessarily mean that our experimental setup is capable of general multiple-pulse NMR spectroscopy. For this, we must be able to form spin ethos. We have therefore considered the modified spin-echo pulse train 8(x)(a/2)[ky]-r-(7r/2)[x]7r[y](r/2)[x]-T-Acquire.
[41
Again the pulse pair ti(x)(?r/2)[+y] gives initial volume selection with the echo generated using the composite ?r pulse applied to the saddle coil. If this sequence is used in a volume-selected two-dimensional experiment, the F, projection yields the natural Tz. Figure 6 shows two-dimensional contour maps from test sample A with (a) and without (c) volume selection. The inserts (b) and (d) are the Fi projections. For these experiments, absolute-value calculations were performed. In spite of this, the suppression of the unwanted signal (phosphate) is excellent. The use of the absolute-value calculation ensures that the maximum response from the phosphate error signal is observed. The results presented in this paper demonstrate that our strategy has the potential of permitting the application of multipulse NMR in vivo in a relatively simple way. Our method does not suffer from the inherent complexities when only surface coils are available (9). However, our approach will not be easy to implement on the
a
L : \-
b
cl
FIG. 6. (a) Volume-selected two-dimensional contour map obtained from Sample A following pulse tram [4]. The F2 spectral width is 1000 Hz; the FI spectral width is 62.5 Hz. 64 spectra were acquired in the F2 dimension and processed with approximately 1 Hz line broadening. Each F2 spectrum represented the average of 8 scans using a recycle time of 25 s. (b) FI cross section from the G-6-P signal. (c) Same as (a) but without volume selection. (d) F, cross section from the phosphate signal. Both cross-section linewidths are digitally limited and (d) is truncated in t,
619
COMMUNICATIONS
current generation of in vivo NMR spectrometers because of the technical demands. These include (a) the availability of a suitable probe, (b) the need for two phasecontrol modulators, (c) sufficient rf pulse control lines (minimum of eight), (d) the availability of two high-power amplifiers, and (e) a procedure for adjusting the relative phase of the two t-f channels. Our measurements were undertaken using a 4.7 T 130 mm vertical-bore magnet manufactured by Oxford Instruments. A CXP (Bruker) pulse programmer, two CXP phase modulators, and a CXP receiver system were employed. The “phase corrector” was constructed from the normal phase controller used in the Bruker lock circuits, and was adapted to provide a 360” phase shift at our frequency of interest, 80.98 MHz. The main tuning capacitor in our probe was manufactured by Jennings. This capacitor is capable of accepting the 1.2 kW of pulse power applied to the saddle coil from a CXP type high-power amplifier. The amplifier used to provide excitation to the surface coil was homemade. ACKNOWLEDGMENTS This research was supported and the J. P. Kelly Foundation
by the Australian Research Grants of the Mater Hospital, Brisbane.
Scheme,
the Queensland
Cancer
Fund,
REFERENCES I. J. J. H.
ACKERMAN,
T. H.
GROVE,
G. C. WONG,
D. G. GADIAN,
AND G. K.
RADDA,
Nature
(London) 283, 167 (1980). HAASE, C. MALLOY, AND G. K. RADDA, J. Magn. Reson. 55, 164 (1983). R. BENDALL AND R. E. GORDON, J. Magn. Reson. 53, 365 (1983). R. BENDALL, Chem. Phys. Lett. 99, 310 (1983). H. LEVITT AND R. FREEMAN, J. Magn. Reson. 3, 473 (1979). R. BENDALL AND W. P. AUE, J. Magn. Reson. 54, 149 (1983). M. DODDRELL, R. M. LYNDEN-BELL, AND J. M. BULSING, J. Magn. Reson. 53, 355 (1983). L. MARTIN, J.-J. DELPUECH, AND G. J. MARTIN, “Practical NMR Spectroscopy,” Heyden, London, 1980. 9. M. R. BENDALL AND D. T. PEGG, J. Magn. Reson. 57, 337 (1984).
2. 3. 4. 5. 6. 7. 8.
A. M. M. M. M. D. M.
OF MAGNETIC
RESONANCE
63, 6 12-6 19 ( 1985)
A Strategy for Performing Volume-SelectedMultipulse NMR Spectroscopyin Viva JAMESFIELD, WILLIAMM.
BROOK&J. MARKBULSING, ANDDAVIDM. DODDRELL
MICHAELG.
IRVING,
School of Science, Gr@th University, Nathan 4111, Queensland, Australia Received November 8, 1984; revised March 11, 1985
A wealth of biochemical information is potentially available from the successful application of in vivo multipulse NMR spectroscopy. The major experimental difficulty encountered when attempting to implement such studies using surface coils for volume selection is their inherently poor rf pulse homogeneity (I). This, of course, runs counter to the most fundamental requirement of multipulse NMR, the need for homogeneous pulses. Although such coils may be used to effect a degree of depth selection (2), in reality, when working with animals the issue is whether the surface can be eliminated to reveal signals from underlying tissue; that is, one requires a depth selection procedure to filter approximately the first 10 mm. Our experimental strategy is based upon a separation of the problems associated with volume selectivity and pulse characteristics. This is a more natural approach because, to a first approximation, pulse error compensation schemes may be tailored to the problem of efficient manipulation of spin coherence. In experiments where volume selection is performed by more sophisticated means (field gradients) the experience gained from this strategy remains directly applicable. It is well known that the use of homogeneous rf pulses for spin manipulation increases both the spectral signalto-noise ratio and the ease of performing the experiment. Indeed much effort is taken by the user of multipulse NMR spectroscopy to ensure that this condition is met. The remaining problem is to marry this requirement to that of volume selection. To achieve this we assume we have available a probe fitted with a surface coil and a second coil (saddle) having high rf pulse homogeneity. Both coils are available for pulsing, with the surface coil acting as the receiver coil. In the following, (n/2)[x] means an x-phased a/2 rf pulse applied to the saddle coil (main coil) while (r/2)(x) is the equivalent pulse applied to the surface coil. For a single-spin system the initial magnetization found at thermal equilibrium can be represented by the operator S,. At the end of a pulse train the magnetization of interest may be represented by SL (or Sb). Here, SL contains the desired information available from the specific pulse sequence, for example, chemical-shift correlation, chemical exchange, and so on. Using our main coil to implement the pulse sequence the change in the spin coherence from S, to S” occurs for the whole sample. The surface coil could now be used for detection thus yielding a degree of volume selectivity. However, to increase the volume selectivity we include a surface0022-2364/85 $3.00 Copyright 0 1985 by Academic Press. Inc. AU rights of reproduction in any form reserved.
612
613
COMMUNICATIONS
coil pulse which is phase coherent with saddle-coil pulses, yielding strategy
the overall
where 0(x) is a volume-selection procedure and may also incorporate a Bodenhausentype exorcycled pulse scheme (3). To effect the ideas outlined above experimentally, a probe tuned for 31P NMR spectroscopy was constructed. This was fitted with two coils; the first was a saddle type in approximate Helmholtz configuration, suitable for handling rats of about 350 g (dimensions 60 X 60 X 55 mm), while the second was a surface coil (diameter 27 mm) wound with its axis orthogonal to the saddle-coil axis. It is well known that two circuits tuned to the same frequency in close proximity will couple, leading to signal loss. The problem was solved (see Fig. 1) by “humbucking” the saddle coil which was wound in a parallel fashion, in the region of the surface coil. Alternatively, crossed diodes, acting as a short, may be used (4) to effect decoupling of coils, but this procedure provides an inferior solution to the problem as the Q of the coil varies with pulse power and the spectral signal-to-noise is markedly reduced. This technically less satisfactory method (4) also results in longer pulse times for constant pulse power. Our method, which allows construction of high Q circuits resonant at the same frequency in close proximity, represents a simple and highly effective solution to the problems of interacting coil geometries. For our probe, both coils had a Q of approximately 400 and no sign of coil coupling could be detected. Detailed measurements using a small test coil showed the improved rf homogeneity obtained from the saddle coil compared with that obtained from the surface coil. Frequency generation for the two rf channels was achieved as shown in Fig. 2. A “phase corrector” was used to adjust the phase of the surface-coil circuit path relative to that for the saddle coil. Phase coherence was set by looking for a null signal response using the pulse pair (7r/2)[x](a/2)(x’) applied to a small sample. This
/
Sample
tube
Bo RG. 1. Diagram showing the current flow in a parallel-wound saddle coil. (A) Surface-coil connections; (B) saddle-coil connections. The two halves of the coil have been drawn flat. In practice, of course, the halves are perpendicular to the page. Because of the current flow, the surface coil is clearly in a region where the field from the saddle coil should cancel. Complete cancellation can be achieved by changing the spatial arrangement of the horizontal length C. This is best done by transmitting on the saddle coil and, using the surface coil as a pickup coil, moving C until there is minimum (undetectable in our case) induced signal. This is known as “humbucking.”
614
COMMUNICATIONS
zj +:;&.,I Q
gE m E m x i FIG. 2. Block diagram of the frequency-generation experiments.
CC
8 B 2 E 8
surface coil connected to receiver
path used to perform the double transmitter coil
ensures x = x’ or x = x’ + r. In either case, phase coherence exists, so there should be no signal loss following the pulse pair O(x)(71./2)[&~] and this proved to be the case. To test this arrangement for volume-selected multipulse NMR spectroscopy, the following experiments were carried out. First, two 5 mm tubes were filled with concentrated aqueous solutions of either sodium hydrogen phosphate (Pi) or glucose6-phosphate (G-6-P) (Sample A). The tubes were filled to a depth of approximately one-half of the diameter of the surface coil and centrally placed side-by-side along the axis defined by the field direction at the face of the surface coil and were bisected by this axis (Fig. 1). The samples were a distance of 7 and 12 mm from the surface-coil center, the phosphate sample being closer to the coil. (This has a much sharper resonance than that for glucosed-phosphate which is broadened by long-range 31P-‘H coupling). In Fig. 3a we show a spectrum from test sample A acquired following a 48 ps pulse applied to the surface coil. Eight scans were averaged with a recycle time of 25 s. Both signals are observed with a signal-to-noise ratio comparable to that obtained with excitation applied via the saddle coil only (see insert to Fig. 4). Using a 96 ps pulse on the surface coil the phosphate signal is significantly reduced Fig. 3b. The first multipulse experiment attempted was a selective T1 measurement using the inversion-recovery technique, the overall pulse train employed being
(a/2)[xl~[yl(a/2)[Xl--7--e(~+X), receiver f.
PI
Here, /3 was set to give a null for the phosphate signal. A composite inversion pulse (5) on the saddle coil gave significantly better results than a single hard P pulse. A
615
COMMUNICATIONS
b
FIG. 3. (a) 80.98 MHz 31P NMR spectrum from test Sample A using a 48 ps pulse on the surface coil: 8 scans, recycle time 25 s, 1000 Hz spectral width. (b) Same as (a) but using a 96 ~LSpulse.
set of volume-selected partially relaxed (PRFT) spectra are shown in Fig. 4. A set of spectra determined without volume selection gave an identical T, value for the glucosed-phosphate signal: Tl(PJ = 4.9 S, Tr(G-6-P) = 7.3 S. To study the volume-selection capabilities of our experimental arrangement using a more difficult test, the G-6-P solution was replaced with a concentrated aqueous solution of ATP (Sample B). (The pH of this solution was about 3 leading to the unusual chemical shifts displayed in Fig. 5.) Because of the low signal strength of the ATP resonances relative to that of the phosphate (Fig. 5a), to perform volumeselected NMR means that the demands on signal suppression are far more stringent. Furthermore, the lengths of the samples were now extended to be equal to the coil diameter. Again they were centered with the coil axis. This procedure is to be
616
COMMUNICATIONS
FIG. 4. A set of volume-selective PRFT spectra obtained from Sample A using pulse train [3]. Eight scans, 1000 Hz spectral width, recycle time 30 s; (7r/2)[x] = 72 ps, 19(x)set at 96 ps. The inserted spectrum is obtained from Sample A following eight ?r/2 pulses applied to the saddle coil. The signal-to-noise ratio obtained here represents the maximum that can be achieved using our experimental setup.
contrasted with the experimental methodology adopted by Bendall and co-workers (3, 6) who test volume selection by using a single disk-shaped sample, small relative to the dimensions of their surface coil. We consider our arrangement to be a more realistic test of our probe design because it allows us to investigate the problems associated with signals arising from spins which are remote from the axis perpendicular to the surface-coil plane. It seems likely to us that in experiments involving real samples (animals, humans), where volume selection is most important, these offaxis effects will demand serious consideration. Figure 5b shows a PRFT spectrum using pulse train [2] with T set equal to 100 ps. Note, that although the error signal from the phosphate solution is reduced compared to the normal spectrum, Fig. 5a, it is still large and it displays significant out-of-phase error components. However, because d(x) and (a/2)[~4 are phase coherent, such out-of-phase signal components can be eliminated by the use of a purging pulse (7) from the saddle coil. The pulse pair ?r(x)(?r/2)[+y] and receiver add/add should give superior volume selection than simply X(X). Our inversionrecovery sequence becomes
617
COMMUNICATIONS
FIG. 5. (a) Normal spectrum from Sample B: spectral width 2000 Hz, 8 scans acquired using a recycle time of 25 s. (b) Volume-selected PRFT spectrum from Sample B using pulse train [2], 7 set at 100 hs. (c) Same as (b) but using pulse train [3]. Note the improved volume selection. The phases of spectra (b) and (c) have been inverted.
(~/2>[xl~[vl(a/2)[xl-?-8(+x)(~/2)[fyl,
receiver ++--.
[31
A purged PRFT spectrum is shown in Fig. 5c. Note the excellent volume selection achieved by this simple procedure. It is well known that inversion-recovery is the method of choice for T, determinations (8); we are confident that the procedure described here has applicability to volume-selected in vivo Tl measurements and we are investigating it further.
618
COMMUNTCATI~NS
Of course the ability to perform a volume-selective T, measurement does not necessarily mean that our experimental setup is capable of general multiple-pulse NMR spectroscopy. For this, we must be able to form spin ethos. We have therefore considered the modified spin-echo pulse train 8(x)(a/2)[ky]-r-(7r/2)[x]7r[y](r/2)[x]-T-Acquire.
[41
Again the pulse pair ti(x)(?r/2)[+y] gives initial volume selection with the echo generated using the composite ?r pulse applied to the saddle coil. If this sequence is used in a volume-selected two-dimensional experiment, the F, projection yields the natural Tz. Figure 6 shows two-dimensional contour maps from test sample A with (a) and without (c) volume selection. The inserts (b) and (d) are the Fi projections. For these experiments, absolute-value calculations were performed. In spite of this, the suppression of the unwanted signal (phosphate) is excellent. The use of the absolute-value calculation ensures that the maximum response from the phosphate error signal is observed. The results presented in this paper demonstrate that our strategy has the potential of permitting the application of multipulse NMR in vivo in a relatively simple way. Our method does not suffer from the inherent complexities when only surface coils are available (9). However, our approach will not be easy to implement on the
a
L : \-
b
cl
FIG. 6. (a) Volume-selected two-dimensional contour map obtained from Sample A following pulse tram [4]. The F2 spectral width is 1000 Hz; the FI spectral width is 62.5 Hz. 64 spectra were acquired in the F2 dimension and processed with approximately 1 Hz line broadening. Each F2 spectrum represented the average of 8 scans using a recycle time of 25 s. (b) FI cross section from the G-6-P signal. (c) Same as (a) but without volume selection. (d) F, cross section from the phosphate signal. Both cross-section linewidths are digitally limited and (d) is truncated in t,
619
COMMUNICATIONS
current generation of in vivo NMR spectrometers because of the technical demands. These include (a) the availability of a suitable probe, (b) the need for two phasecontrol modulators, (c) sufficient rf pulse control lines (minimum of eight), (d) the availability of two high-power amplifiers, and (e) a procedure for adjusting the relative phase of the two t-f channels. Our measurements were undertaken using a 4.7 T 130 mm vertical-bore magnet manufactured by Oxford Instruments. A CXP (Bruker) pulse programmer, two CXP phase modulators, and a CXP receiver system were employed. The “phase corrector” was constructed from the normal phase controller used in the Bruker lock circuits, and was adapted to provide a 360” phase shift at our frequency of interest, 80.98 MHz. The main tuning capacitor in our probe was manufactured by Jennings. This capacitor is capable of accepting the 1.2 kW of pulse power applied to the saddle coil from a CXP type high-power amplifier. The amplifier used to provide excitation to the surface coil was homemade. ACKNOWLEDGMENTS This research was supported and the J. P. Kelly Foundation
by the Australian Research Grants of the Mater Hospital, Brisbane.
Scheme,
the Queensland
Cancer
Fund,
REFERENCES I. J. J. H.
ACKERMAN,
T. H.
GROVE,
G. C. WONG,
D. G. GADIAN,
AND G. K.
RADDA,
Nature
(London) 283, 167 (1980). HAASE, C. MALLOY, AND G. K. RADDA, J. Magn. Reson. 55, 164 (1983). R. BENDALL AND R. E. GORDON, J. Magn. Reson. 53, 365 (1983). R. BENDALL, Chem. Phys. Lett. 99, 310 (1983). H. LEVITT AND R. FREEMAN, J. Magn. Reson. 3, 473 (1979). R. BENDALL AND W. P. AUE, J. Magn. Reson. 54, 149 (1983). M. DODDRELL, R. M. LYNDEN-BELL, AND J. M. BULSING, J. Magn. Reson. 53, 355 (1983). L. MARTIN, J.-J. DELPUECH, AND G. J. MARTIN, “Practical NMR Spectroscopy,” Heyden, London, 1980. 9. M. R. BENDALL AND D. T. PEGG, J. Magn. Reson. 57, 337 (1984).
2. 3. 4. 5. 6. 7. 8.
A. M. M. M. M. D. M.