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A new method for the decoupling of multiple-coil NMR Probes
JOURNAL
OF MAGNETIC
RESONANCE
61, 130- I36 ( 1985)
NOTES A New Method for the Decoupling of Multiple-Coil AXEL
NMR
Probes
HAASE
Max-Planck-Inslitut ftir biophysikalische Chemie, PostJach2841, D-3400 Giittingen, Federal Republic of Germany Received March 20, 1984; revised June 12, 1984
NMR probes with crossed radiofrequency transmitter and receiver coils are not in use in modem Ff NMR spectrometers. Recently, however, NMR imaging and in vivo spectroscopy have focused attention on an alternative probe design with two or multiple coils. For example, surface coil NMR detectors in combination with homogeneous radiofrequency transmitter coils for ‘H imaging or localized highresolution 3’P spectroscopy turn out to be superior to single-coil probes with either a surface coil or a homogeneous coil (1, 2). This design combines the advantages of both a homogeneous radiofrequency NMR excitation for the whole volume and an excellent filling factor of the surface-coil detector for the volume of interest. Quadrature detection coils (3) surface-coil experiments with two coaxial coils (4) and rotating frame imaging experiments (5) are further examples of multiple-coil NMR probes. In spite of their advantages, the applications are up to now limited, mainly due to technical problems caused by electromagnetic coupling between coils which are tuned to the same frequency. By transmitting radiofrequency power with one coil, alternating voltage is induced in the other coils. Consequently, the original B1 field of the transmitter coil can be completely changed, and in some cases it might even be canceled. Some decoupling schemes for multiple coil probes have been described in the literature (6-8). Of course, magnetic coupling can be minimized by orthogonal orientation of two coils. However, if one of the coils is a surface coil, this procedure is extremely difficult and fails in most cases due to their three-dimensional inhomogeneous radiofrequency distribution (9). Other decoupling schemes employ passive detuning of the coils by series or parallel crossed diodes within the resonant circuit (4) active detuning by changing the variable condensers within short time intervals, or by providing a compensation voltage within the coils (6). It has been found that passive detuning methods are not optimal, mainly because the detuning broadbands the coils (low Q values) (4), which then couple extremely well. In this communication an alternative method for decoupling multiple-coil NMR probes is proposed and its effect is tested in practical NMR experiments. The method employs the quality of a quarterwave coaxial cable for the effective decoupling of coils. Suppose both coils are tuned to the same frequency and impedance is matched to a 50 D cable. The length of cable 1, connected to the coils should be exactly (k + 1/2)X/2 (k = 0, 1, 2, * . .). First let us consider the case in 0022-2364185 $3.00 Copyright 0 1985 by Academic Press. Inc. All rights of reproduction in any form reserved.
130
NOTES
131
which the end of one cable has a short circuit. A quarterwave cable acts as an impedance transformer with the relation:
with Z the characteristic impedance of the cable (e.g., 50 Q), Zi and Z, the input and output impedances. A short circuit at the end of a quarterwave cable (output) (Z, = 0) will be transformed to an open circuit with high impedance at the other end of the cable (input) (Zi = co). In this cable a standing wave pattern develops, with a voltage maximum (minimum) at the input (output) and a current minimum (maximum) at the input (output). At the input which is connected to the probe, no current is possible. If the first coil is transmitting, the induced current in the second coil will be zero. Accordingly no radiofrequency can be transmitted and the B, distribution of the transmitting coil remains unchanged. For an NMR experiment both coils look like completely independent probes. Second, let us consider the case in which one cable is open-ended. This open end is transformed to a short circuit with low series impedance within the coil, leading to large induced currents when the other coil is transmitting. This coil transmits phase shifted radiofrequency and distorts the B1 field distribution of the other coil. For decoupling of multiple-coil probes it is therefore proposed to short the X/4 cable end connected to those coils for short time intervals in which they are not working (transmitting or receiving). It should be possible to short a cable either passively by parallel crossed diodes, in the case where a high radiofrequency voltage is present (if a high voltage >0.5 V is applied to crossed semiconductor diodes, current flows and the diodes appear as a low impedance), or actively by means of PIN-diode switches. To test the proposal, a two-coil NMR probe has been investigated more closely in four different experiments. The probe combines a large saddle-shaped transmitter coil with a height of 3 cm and a diameter of 2 cm, a one turn circular surface coil of 1 cm diameter, which is coaxially mounted within one winding of the large coil, and a phantom (see Fig. 1). The surface coil is oriented such that maximum voltage is induced within the coil corresponding to maximum coupling. Both coils are made of 0.8 mm insulated copper wire and connected by copper wires of 5 cm length to the tuning and matching capacitor-y network. The probe was tuned and matched (109.343 MHz for 31PNMR in a 6.3 T Bruker WH-270 spectrometer) with a bridge arrangement (10) for both coils separately. It was found that the tuning procedure was dependent on the cable length as well as on the type of termination of the cables (e.g., short circuit, open circuit or termination by a 50 s2 load), so that the probe has been retuned for each different experiment. The B1 field strength of the large coil was measured by a 3’P NMR experiment on a phantom composed of a glass cylinder with 1.5 cm diameter and 2 cm height, filled with 0.1 M NaCl aqueous solution and containing a 1 mm diameter bulb filled with 80% phosphoric acid. To probe the B, field the bulb can be fixed at four different positions on the symmetry axis of the phantom, as shown in Fig. 1. The B1 field strength of the large coil is determined by the variation of the 31P signal with respect to the transmitter pulse length. For maximum signal (the 90” pulse) one obtains
132
NOTES
FIG. 1. The two-coil probe including a saddle-shaped (LC) and a surface coil (SC) used to test the coildecoupling scheme. One-half of the saddle-shaped coil is coplanar with the surface coil. The numbers l-4 indicate the positions where the B, field has been measured using a 1 m m diameter glass bulb filled with 80% phosphoric acid.
B1 = CWp) with y the magnetogyric ratio for the 3’P nucleus, and t, the pulse length. In a first experiment, the B, field of the large coil without the surface coil was measured. The coil is used for transmitting and receiving the NMR signal as usual (II). As expected the pulse-length variation of the signal turns out to be sinusoidal (see Fig. 2a) and the B, field strength is nearly constant for the four points within the probe (see Fig. 3a). Thus at least on this axis perpendicular to Bo, the coil transmits homogeneous radiofrequency intensity. In a second experiment both the surface coil and the large coil were connected to a X/4 coaxial cable (type: RG 58/U, 50 Q) with a cable length of 42 cm. The cable for the surface coil was open ended, and the cable for the large saddle-shaped coil was connected to the transmitter cable, which was terminated by a serial crossed diode set. The large coil was tuned, while the surface-coil cable was open ended and the surface coil was tuned while the cable for the large coil was connected to the crossed diode set. Again the B, field for the large coil was measured, by transmitting and receiving with this coil. In Fig. 2b the resulting dependence of the 3’P signal on the pulse duration is shown. The pulse length for the maximum signal is remarkably lengthened and a signal inversion for longer pulse lengths is not obtained. In this case the definition of a 90” pulse length becomes ambiguous and the interpretation of the pulse length t, for maximum signal as a 90” pulse seems arbitrary. However, taking this value as a reference the four B1 values are sharply reduced and the field is no longer constant over the volume. After Fourier transformation of the free induction decay a phase shift of nearly r compared to the first experiment was obtained. This reflects the fact that the surface coil transmits
NOTES
133
4 0.5
b) 0.5
MO
0 -0.5
c) 0.5
MO
0
-0.5
FIG. 2. Pulse length variation of the “P NMR intensity of a bulb of 1 mm diameter filled with 80% phosphoric acid and fixed at position 2 within the probe (see Fig. 1). The saddle-shaped coil is used as transmitter and receiver (except for c)). The pulse length fr, is given in ps, the intensity Ma is calculated relative to the maximum intensity obtained for each curve. The probe is composed by (a) the saddleshaped coil, (b) the saddle-shaped coil and the surface coil connected to a A/4 line, which is open-ended (0) or terminated by a 50 Q load (B), (c) saddle-shaped coil and surface coil connected to a X/4 line, which is shunted (o), or terminated by a crossed-diode short (m). In this case the surface coil is used as a receiver, the saddle-shaped coil is the transmitter, or with an open-ended X/2 line connected to the surface coil (0).
radiofrequency which is phase-shifted with respect to the large coil. Of course, there must be regions within the probe volume where both radiofrequency intensities are nearly canceled leading to extremely long 90” pulse lengths. The effect is only slightly changed when the X/4 cable is terminated by a 50 D load. The pulse length for maximum signal is again at least 3.5 times as long as for the single coil and only a small signal inversion can be obtained for longer pulse lengths (see Fig. 2b). In a third experiment the end of the X/4 cable for the surface coil was shunted. The probe was retuned again as described above. In Fig. 2c the pulse length variation of the “P NMR intensity is shown. The pulse length for the 90” flip angle is longer
134
NOTES
1
2
FIG. 3. B, field distribution of the saddle-shaped indicated by the numbers l-4. (a) B, field distribution (b) B, field distribution of the saddle-shaped coil with to the crossed diode set of the preamplifier input, (d) the surface coil X/4 line open-ended.
3
4
coil. The B, field is measured at four positions, of the saddle-shaped coil without the surface coil, the surface coil X/4 line shunted, or (c) connected B, field distribution of the saddle-shaped coil with
by 10% than in the first experiment. The intensity variation is sinusoidal with excellent signal inversion. The B, field strength variation over the volume shows the same quality as for the large coil without the second coil, and the absolute B, field strength is reduced by 10%. The second coil shows no effect on the homogeneity of the large coil and only a small effect on the overall B, field strength. The shunted X/4 cable at the surface coil provides sufficient decoupling for the use in NMR experiments. In testing other cable lengths it has been found that the length of the cable is a very critical parameter and should be determined very carefully. For example, increasing the X/4 line by only 5% (2 cm) yields a 90” pulse length which is 50% longer. Especially a shunted X/2 cable gives the same results as the second experiment. Of course, in this case the low impedance at the end of the cable is transformed to a low series impedance within the coil (for a X/2 cable the input/ output impedance relation is Zi = Z,). On the other hand, if this cable is open ended, the coil should have a high series impedance. In fact in this case the B, field distribution of the large coil was the same as with the shunted X/4 cable (see Fig. 2~). Since the experimental arrangement described above cannot be used directly for NMR studies, (e.g., it is useless to mechanically short the cable for a receiving surface coil), a more practical electronic short circuit has been tested in a fourth experiment. The X/4 cable for the surface coil was terminated by a parallel crossed diode set (type: lN9 14). This introduces low impedance at the end of the cable as long as a high voltage is induced within the system, i.e., the surface coil and the cable. Thus the transmitting B, field strength is not influenced by the surface coil. The serial crossed diode set for the transmitter cable is placed at a distance of h/2 away from the large coil (see Fig. 4). This transforms the virtual open circuit at low
NOTES
135
FIG. 4. Block diagram of a decoupled saddle-shaped coil transmitter and surface coil receiver probe using crossed diodes (type: lN914). Abbreviations: P, preamplifier; T, transmitter; SC, surface coil; LC, saddle-shaped coil. The length of the cable SC-A is exactly X/4, the length of the cable LC-B is X/2.
voltage to an open circuit at the large coil. Therefore the Bi field strength of the surface coil is not influenced by the large coil. By transmitting with the large coil and receiving with the surface coil the pulse length variation of the 3’P intensity is identical to the third experiment, except that the absolute intensity falls off rapidly for points far away from the surface coil. The Bi field homogeneity for the large coil is again not influenced by the surface coil, but a further reduction of 10% of the overall B, field strength has been obtained (compared to the third experiment). Concluding, the experiments show that the influence on the Bi field distribution of a transmitting coil by other coils within the probe is minimized as long as these coils are connected to a quartet-wave cable the end of which is shunted (or a halfwave cable the end of which is open) for the time of the transmitting pulse. The short or open circuits can be constructed either by parallel or serial crossed diodes or by PIN-diode circuits which can be switched within a few microseconds. Since most NMR preamplifiers are protected at the input by a crossed diode short for high voltages, it is often sufficient to connect the receiving surface coil by a X/4 or 3X/4 cable to the preamplifier input. With this arrangement all possible surface-coil orientations with respect to a large homogeneous coil are possible. Nevertheless, the application of this double-coil experiment has some difficulties. When the sample magnetization is excited by a uniform Bi field, the surface coil receives a signal which results from a superposition of signals of varying phases, due to the fact that the surface coil provides an inhomogeneous B, field (9) with varying phases, depending on the spatial position with respect to the surface coil. The result of this effect is a signal cancellation from a part of the sample if the double-coil experiment is applied for in viva spectroscopy. The sensitive region of the surface coil is therefore changed and should be determined carefully for performing in vivo spectroscopy. For imaging experiments this problem is minimized, as the voxel size limits the variation of phases within the voxel. In spite of these problems, the proposed decoupling scheme has great advantages either for high-resolution spectroscopy using receiving surface coils, or for localized imaging of surface tissue or organs by surface-coil detectors.
136
NOTES REFERENCES
1. L. AXEL, Abstracts of the 2nd Annual Meeting of the Society of Magnetic Resonance in Medicine. San Francisco, 1983, p. 16. 2. P. STYLES, Ann. Is!. Super. Sunifri 19, 61 (1983). 3. C.-N. CHEN, D. 1. HOULT, AND V. .I. SANK, J. Mugn. Reson. 54, 324 (1983). 4. M. R. BENDALL, Chem. Phys. Left. 99, 310 (1983). 5. D. I. HOULT, J. Magn. Reson. 33, 183 (1979). 6. R. J. BLUME, Rev. Sci. Instrum. 33, 1472 (1962). 7. M. A. PACKARD, Rev. Sci. Instrum. 19,435 (1948). 8. R. R. LEMB~ AND V. J. KOWALEWSKI, J. Phys. E 8,632 (1975). 9. A. HAASE, W. HKNICKE, AND J. FRAHM, J. Mugn. Reson. 56, 401 (1984). 10. D. I. HOULT, Prog. Nucl. Magn. Reson. Spectrosc. 12, 4 1 (1978). Il. J. L. LOWE AND C. E. TARR, J. Phys. E 1, 320 (1968).
OF MAGNETIC
RESONANCE
61, 130- I36 ( 1985)
NOTES A New Method for the Decoupling of Multiple-Coil AXEL
NMR
Probes
HAASE
Max-Planck-Inslitut ftir biophysikalische Chemie, PostJach2841, D-3400 Giittingen, Federal Republic of Germany Received March 20, 1984; revised June 12, 1984
NMR probes with crossed radiofrequency transmitter and receiver coils are not in use in modem Ff NMR spectrometers. Recently, however, NMR imaging and in vivo spectroscopy have focused attention on an alternative probe design with two or multiple coils. For example, surface coil NMR detectors in combination with homogeneous radiofrequency transmitter coils for ‘H imaging or localized highresolution 3’P spectroscopy turn out to be superior to single-coil probes with either a surface coil or a homogeneous coil (1, 2). This design combines the advantages of both a homogeneous radiofrequency NMR excitation for the whole volume and an excellent filling factor of the surface-coil detector for the volume of interest. Quadrature detection coils (3) surface-coil experiments with two coaxial coils (4) and rotating frame imaging experiments (5) are further examples of multiple-coil NMR probes. In spite of their advantages, the applications are up to now limited, mainly due to technical problems caused by electromagnetic coupling between coils which are tuned to the same frequency. By transmitting radiofrequency power with one coil, alternating voltage is induced in the other coils. Consequently, the original B1 field of the transmitter coil can be completely changed, and in some cases it might even be canceled. Some decoupling schemes for multiple coil probes have been described in the literature (6-8). Of course, magnetic coupling can be minimized by orthogonal orientation of two coils. However, if one of the coils is a surface coil, this procedure is extremely difficult and fails in most cases due to their three-dimensional inhomogeneous radiofrequency distribution (9). Other decoupling schemes employ passive detuning of the coils by series or parallel crossed diodes within the resonant circuit (4) active detuning by changing the variable condensers within short time intervals, or by providing a compensation voltage within the coils (6). It has been found that passive detuning methods are not optimal, mainly because the detuning broadbands the coils (low Q values) (4), which then couple extremely well. In this communication an alternative method for decoupling multiple-coil NMR probes is proposed and its effect is tested in practical NMR experiments. The method employs the quality of a quarterwave coaxial cable for the effective decoupling of coils. Suppose both coils are tuned to the same frequency and impedance is matched to a 50 D cable. The length of cable 1, connected to the coils should be exactly (k + 1/2)X/2 (k = 0, 1, 2, * . .). First let us consider the case in 0022-2364185 $3.00 Copyright 0 1985 by Academic Press. Inc. All rights of reproduction in any form reserved.
130
NOTES
131
which the end of one cable has a short circuit. A quarterwave cable acts as an impedance transformer with the relation:
with Z the characteristic impedance of the cable (e.g., 50 Q), Zi and Z, the input and output impedances. A short circuit at the end of a quarterwave cable (output) (Z, = 0) will be transformed to an open circuit with high impedance at the other end of the cable (input) (Zi = co). In this cable a standing wave pattern develops, with a voltage maximum (minimum) at the input (output) and a current minimum (maximum) at the input (output). At the input which is connected to the probe, no current is possible. If the first coil is transmitting, the induced current in the second coil will be zero. Accordingly no radiofrequency can be transmitted and the B, distribution of the transmitting coil remains unchanged. For an NMR experiment both coils look like completely independent probes. Second, let us consider the case in which one cable is open-ended. This open end is transformed to a short circuit with low series impedance within the coil, leading to large induced currents when the other coil is transmitting. This coil transmits phase shifted radiofrequency and distorts the B1 field distribution of the other coil. For decoupling of multiple-coil probes it is therefore proposed to short the X/4 cable end connected to those coils for short time intervals in which they are not working (transmitting or receiving). It should be possible to short a cable either passively by parallel crossed diodes, in the case where a high radiofrequency voltage is present (if a high voltage >0.5 V is applied to crossed semiconductor diodes, current flows and the diodes appear as a low impedance), or actively by means of PIN-diode switches. To test the proposal, a two-coil NMR probe has been investigated more closely in four different experiments. The probe combines a large saddle-shaped transmitter coil with a height of 3 cm and a diameter of 2 cm, a one turn circular surface coil of 1 cm diameter, which is coaxially mounted within one winding of the large coil, and a phantom (see Fig. 1). The surface coil is oriented such that maximum voltage is induced within the coil corresponding to maximum coupling. Both coils are made of 0.8 mm insulated copper wire and connected by copper wires of 5 cm length to the tuning and matching capacitor-y network. The probe was tuned and matched (109.343 MHz for 31PNMR in a 6.3 T Bruker WH-270 spectrometer) with a bridge arrangement (10) for both coils separately. It was found that the tuning procedure was dependent on the cable length as well as on the type of termination of the cables (e.g., short circuit, open circuit or termination by a 50 s2 load), so that the probe has been retuned for each different experiment. The B1 field strength of the large coil was measured by a 3’P NMR experiment on a phantom composed of a glass cylinder with 1.5 cm diameter and 2 cm height, filled with 0.1 M NaCl aqueous solution and containing a 1 mm diameter bulb filled with 80% phosphoric acid. To probe the B, field the bulb can be fixed at four different positions on the symmetry axis of the phantom, as shown in Fig. 1. The B1 field strength of the large coil is determined by the variation of the 31P signal with respect to the transmitter pulse length. For maximum signal (the 90” pulse) one obtains
132
NOTES
FIG. 1. The two-coil probe including a saddle-shaped (LC) and a surface coil (SC) used to test the coildecoupling scheme. One-half of the saddle-shaped coil is coplanar with the surface coil. The numbers l-4 indicate the positions where the B, field has been measured using a 1 m m diameter glass bulb filled with 80% phosphoric acid.
B1 = CWp) with y the magnetogyric ratio for the 3’P nucleus, and t, the pulse length. In a first experiment, the B, field of the large coil without the surface coil was measured. The coil is used for transmitting and receiving the NMR signal as usual (II). As expected the pulse-length variation of the signal turns out to be sinusoidal (see Fig. 2a) and the B, field strength is nearly constant for the four points within the probe (see Fig. 3a). Thus at least on this axis perpendicular to Bo, the coil transmits homogeneous radiofrequency intensity. In a second experiment both the surface coil and the large coil were connected to a X/4 coaxial cable (type: RG 58/U, 50 Q) with a cable length of 42 cm. The cable for the surface coil was open ended, and the cable for the large saddle-shaped coil was connected to the transmitter cable, which was terminated by a serial crossed diode set. The large coil was tuned, while the surface-coil cable was open ended and the surface coil was tuned while the cable for the large coil was connected to the crossed diode set. Again the B, field for the large coil was measured, by transmitting and receiving with this coil. In Fig. 2b the resulting dependence of the 3’P signal on the pulse duration is shown. The pulse length for the maximum signal is remarkably lengthened and a signal inversion for longer pulse lengths is not obtained. In this case the definition of a 90” pulse length becomes ambiguous and the interpretation of the pulse length t, for maximum signal as a 90” pulse seems arbitrary. However, taking this value as a reference the four B1 values are sharply reduced and the field is no longer constant over the volume. After Fourier transformation of the free induction decay a phase shift of nearly r compared to the first experiment was obtained. This reflects the fact that the surface coil transmits
NOTES
133
4 0.5
b) 0.5
MO
0 -0.5
c) 0.5
MO
0
-0.5
FIG. 2. Pulse length variation of the “P NMR intensity of a bulb of 1 mm diameter filled with 80% phosphoric acid and fixed at position 2 within the probe (see Fig. 1). The saddle-shaped coil is used as transmitter and receiver (except for c)). The pulse length fr, is given in ps, the intensity Ma is calculated relative to the maximum intensity obtained for each curve. The probe is composed by (a) the saddleshaped coil, (b) the saddle-shaped coil and the surface coil connected to a A/4 line, which is open-ended (0) or terminated by a 50 Q load (B), (c) saddle-shaped coil and surface coil connected to a X/4 line, which is shunted (o), or terminated by a crossed-diode short (m). In this case the surface coil is used as a receiver, the saddle-shaped coil is the transmitter, or with an open-ended X/2 line connected to the surface coil (0).
radiofrequency which is phase-shifted with respect to the large coil. Of course, there must be regions within the probe volume where both radiofrequency intensities are nearly canceled leading to extremely long 90” pulse lengths. The effect is only slightly changed when the X/4 cable is terminated by a 50 D load. The pulse length for maximum signal is again at least 3.5 times as long as for the single coil and only a small signal inversion can be obtained for longer pulse lengths (see Fig. 2b). In a third experiment the end of the X/4 cable for the surface coil was shunted. The probe was retuned again as described above. In Fig. 2c the pulse length variation of the “P NMR intensity is shown. The pulse length for the 90” flip angle is longer
134
NOTES
1
2
FIG. 3. B, field distribution of the saddle-shaped indicated by the numbers l-4. (a) B, field distribution (b) B, field distribution of the saddle-shaped coil with to the crossed diode set of the preamplifier input, (d) the surface coil X/4 line open-ended.
3
4
coil. The B, field is measured at four positions, of the saddle-shaped coil without the surface coil, the surface coil X/4 line shunted, or (c) connected B, field distribution of the saddle-shaped coil with
by 10% than in the first experiment. The intensity variation is sinusoidal with excellent signal inversion. The B, field strength variation over the volume shows the same quality as for the large coil without the second coil, and the absolute B, field strength is reduced by 10%. The second coil shows no effect on the homogeneity of the large coil and only a small effect on the overall B, field strength. The shunted X/4 cable at the surface coil provides sufficient decoupling for the use in NMR experiments. In testing other cable lengths it has been found that the length of the cable is a very critical parameter and should be determined very carefully. For example, increasing the X/4 line by only 5% (2 cm) yields a 90” pulse length which is 50% longer. Especially a shunted X/2 cable gives the same results as the second experiment. Of course, in this case the low impedance at the end of the cable is transformed to a low series impedance within the coil (for a X/2 cable the input/ output impedance relation is Zi = Z,). On the other hand, if this cable is open ended, the coil should have a high series impedance. In fact in this case the B, field distribution of the large coil was the same as with the shunted X/4 cable (see Fig. 2~). Since the experimental arrangement described above cannot be used directly for NMR studies, (e.g., it is useless to mechanically short the cable for a receiving surface coil), a more practical electronic short circuit has been tested in a fourth experiment. The X/4 cable for the surface coil was terminated by a parallel crossed diode set (type: lN9 14). This introduces low impedance at the end of the cable as long as a high voltage is induced within the system, i.e., the surface coil and the cable. Thus the transmitting B, field strength is not influenced by the surface coil. The serial crossed diode set for the transmitter cable is placed at a distance of h/2 away from the large coil (see Fig. 4). This transforms the virtual open circuit at low
NOTES
135
FIG. 4. Block diagram of a decoupled saddle-shaped coil transmitter and surface coil receiver probe using crossed diodes (type: lN914). Abbreviations: P, preamplifier; T, transmitter; SC, surface coil; LC, saddle-shaped coil. The length of the cable SC-A is exactly X/4, the length of the cable LC-B is X/2.
voltage to an open circuit at the large coil. Therefore the Bi field strength of the surface coil is not influenced by the large coil. By transmitting with the large coil and receiving with the surface coil the pulse length variation of the 3’P intensity is identical to the third experiment, except that the absolute intensity falls off rapidly for points far away from the surface coil. The Bi field homogeneity for the large coil is again not influenced by the surface coil, but a further reduction of 10% of the overall B, field strength has been obtained (compared to the third experiment). Concluding, the experiments show that the influence on the Bi field distribution of a transmitting coil by other coils within the probe is minimized as long as these coils are connected to a quartet-wave cable the end of which is shunted (or a halfwave cable the end of which is open) for the time of the transmitting pulse. The short or open circuits can be constructed either by parallel or serial crossed diodes or by PIN-diode circuits which can be switched within a few microseconds. Since most NMR preamplifiers are protected at the input by a crossed diode short for high voltages, it is often sufficient to connect the receiving surface coil by a X/4 or 3X/4 cable to the preamplifier input. With this arrangement all possible surface-coil orientations with respect to a large homogeneous coil are possible. Nevertheless, the application of this double-coil experiment has some difficulties. When the sample magnetization is excited by a uniform Bi field, the surface coil receives a signal which results from a superposition of signals of varying phases, due to the fact that the surface coil provides an inhomogeneous B, field (9) with varying phases, depending on the spatial position with respect to the surface coil. The result of this effect is a signal cancellation from a part of the sample if the double-coil experiment is applied for in viva spectroscopy. The sensitive region of the surface coil is therefore changed and should be determined carefully for performing in vivo spectroscopy. For imaging experiments this problem is minimized, as the voxel size limits the variation of phases within the voxel. In spite of these problems, the proposed decoupling scheme has great advantages either for high-resolution spectroscopy using receiving surface coils, or for localized imaging of surface tissue or organs by surface-coil detectors.
136
NOTES REFERENCES
1. L. AXEL, Abstracts of the 2nd Annual Meeting of the Society of Magnetic Resonance in Medicine. San Francisco, 1983, p. 16. 2. P. STYLES, Ann. Is!. Super. Sunifri 19, 61 (1983). 3. C.-N. CHEN, D. 1. HOULT, AND V. .I. SANK, J. Mugn. Reson. 54, 324 (1983). 4. M. R. BENDALL, Chem. Phys. Left. 99, 310 (1983). 5. D. I. HOULT, J. Magn. Reson. 33, 183 (1979). 6. R. J. BLUME, Rev. Sci. Instrum. 33, 1472 (1962). 7. M. A. PACKARD, Rev. Sci. Instrum. 19,435 (1948). 8. R. R. LEMB~ AND V. J. KOWALEWSKI, J. Phys. E 8,632 (1975). 9. A. HAASE, W. HKNICKE, AND J. FRAHM, J. Mugn. Reson. 56, 401 (1984). 10. D. I. HOULT, Prog. Nucl. Magn. Reson. Spectrosc. 12, 4 1 (1978). Il. J. L. LOWE AND C. E. TARR, J. Phys. E 1, 320 (1968).