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ANDANTE, a Novel Frequency-Selective Adiabatic Pulse
JOURNAL OF MAGNETIC RESONANCE,
Series B 110, 69–74 (1996)
Article No. 0009
ANDANTE, a Novel Frequency-Selective Adiabatic Pulse MANOJKUMAR SARANATHAN *
AND
MARTIN J. KUSHMERICK * ,†
*Center for Bioengineering, and †Departments of Radiology and Physiology and Biophysics, University of Washington, Seattle, Washington 98195 Received April 11, 1995; revised August 10, 1995
Surface coils are widely used in localized spectroscopy and imaging experiments in vivo due to the improved sensitivity they afford compared to volume coils. However, the RF homogeneity of surface coils is poor, off-setting some of the signal to noise ratio (SNR) gain over volume coils. Amplitude- and phase-modulated RF pulses called adiabatic half-passage pulses (AHP) have been devised to achieve uniform excitation of spins. These pulses perform optimally despite 10-fold variations in RF field (B1 ) amplitude; however, these pulses can achieve only tip angles of 907 or 1807. B1-insensitive rotation (BIR-4) pulses, which are composed of sections of AHP pulses, achieve plane rotations of magnetization vectors through arbitrary angles (1). Owing to their symmetry, they have an increased bandwidth as well as a symmetric off-resonance performance compared to AHP. A number of complex RF pulses have been crafted using BIR-4 pulses and AHPs as building blocks (2–5). Adiabatic pulses, in general, have either an asymmetric frequency response (like AHPs) or a uniform response over a wide bandwidth (like BIRs). In a variety of spectroscopy applications like solvent suppression or selective irradiation, there is a need for frequency-selective pulses. The solvent-suppressive adiabatic pulse (SSAP), which is similar to a jump–return sequence, addresses the problem of frequency selectivity but has a sinusoidal response which is inadequate for certain applications. In this paper, we propose ANDANTE (adiabatically nutated DANTE), a B1-insensitive frequency-selective plane-rotation pulse that can achieve arbitrary flip angles. As an application, we use this pulse to selectively excite the lactate methine protons in a double-quantum (DQ) gradientfiltered sequence. A detailed derivation and analysis of the BIR-4 pulse has been described elsewhere (1). To summarize briefly, the BIR-4 pulse is a composite pulse composed of four sections, each being either an adiabatic half-passage or its reverse (Fig. 1a). The middle two sections effectively constitute an adiabatic inversion pulse. The flip angle is determined by the phase jumps Df1 and Df2 . These are usually set to ( p / a /2) and 0 ( p / a /2) respectively, yielding a net flip angle of a. The amplitude- and frequency-modulation functions, v1 (t) and Dv(t) are tanh and tan functions which
closely approximate the numerically optimized functions (6, 7) and, as a function of time t, are given by
FS
v1 (t) Å v1max tanh z 1 0
Dv(t) Å Dvmax
12-15-95 10:46:04
DG
tan[ kt/Tp ] , tan[ k]
[1]
[2]
where v1max and Dvmax are the modulation amplitudes and Tp is the duration of each half-passage segment; z and k are constants that influence the adiabaticity conditions of the pulse, and following (1), are chosen to be 10 and arctan(20), respectively. If we use phase modulation, [2] is integrated to yield f(t) Å 0
DvmaxTp ln(cos[ k]) . k tan[ k]
[3]
If the adiabatic conditions are satisfied, plane rotations of any arbitrary angle can be achieved and these are relatively insensitive to RF inhomogeneities. The SSAP (2) is a BIR-4 pulse with a time delay t inserted between the first and the second segments and the phase jumps Df1 , Df2 set to /p and 0 p, respectively. On resonance, this yields a null; i.e., it returns the magnetization to the /z axis. By adjusting the delay t to 1/4V, one obtains a maximum excitation (907 pulse) at offset V. In general, the transverse magnetization vector varies as sin( Vt ), where V is the frequency offset in rad/sec. Note that the SSAP pulse and its frequency response on either side of the carrier are no longer symmetric. The ANDANTE pulse is essentially a DANTE pulse ( 8) with the hard pulses replaced by BIR-4 pulses (Fig. 1b). A train of N BIR-4 pulses of duration T, spaced t apart can be written as s(t) Å [III(t) ∗ p(t)]rect
69
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t Tp
magas
SD t L
,
[4]
1064-1866/96 $12.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
AP: Mag Res
70
NOTES
FIG. 1. Amplitude ( v1 ) and phase ( f ) modulation functions for (a) BIR-4 pulse and (b) ANDANTE pulse. The phase jumps Df1 and Df2 , are set to ( p / a /2) and 0 ( p / a /2) to achieve a tip angle of a. For the ANDANTE pulse, T is the duration of each BIR-4 pulse segment and t is the interpulse duration.
where III(t) is the comb function with impulse spacing t given by `
III(t) Å ∑ d (t 0 nt ),
[5]
n Å0`
p(t) is the adiabatic modulation function, and rect(t/L), a rectangular function of length L. L Å NT / (N 0 1) t and ∗ represents convolution. Invoking the linearity of the smallflip-angle approximation, the transverse magnetization Mxy is proportional to the Fourier transform of the RF irradiation and, as a function of offset frequency, is given by Mxy ( v ) } [III( v )P( v )] ∗ sinc( vL),
[6]
where P( v ) and III( v ) are the Fourier transforms of p(t) and III(t). If P( v ) is reasonably flat over the spectral region of interest, a reasonable assumption for BIR-4 pulses, then Eq. [3] can be approximated by a finite train of sinc functions of main lobe width 2/L, separated by 1/ t. By varying t, one can move the harmonics of the sinc function away from the spectral region of interest. The theoretical performances of ANDANTE and SSAP were compared using PENCIL, a simulation program developed at the University of Washington by Jonathan Callahan
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(Figs. 2a–2f). All flip angles were set to 907 and relaxation was ignored. The overall length of ANDANTE was 13 ms ( t Å 0.325 ms; T Å 4Tp Å 1 ms; N Å 10), and that of SSAP was 2 ms ( t Å 1 ms). It can be seen from Figs. 2a–2d that the frequency selectivity of ANDANTE is high compared to SSAP. Figures 2e and 2f show that the adiabaticity of the BIR-4 pulse is still preserved by ANDANTE. Phase errors are restricted to within {57 over a fivefold range of B1 amplitude, minimizing any signal loss. We tested the performance of the sequence by acquiring spectra on a GE CSI-II system (85 MHz 1H frequency; horizontal bore). A homebuilt surface coil of 5 cm inner diameter was used for both transmission and reception. The first sample consisted of a vial of water. The frequency response of ANDANTE was calculated by sweeping the carrier frequency over 2000 Hz in steps of 25 Hz. Figure 3 shows a stacked plot of the magnitude spectra versus frequency offset for an ANDANTE pulse train consisting of 10 97 pulses of duration 1 ms separated by 325 ms (L Å 13 ms). The main lobe width was 600 Hz as predicted by the simulations. The second sample consisted of a polythene bag filled with 20 ml olive oil wrapped around a bottle containing 50 ml of a 17 mM solution of lactic acid. The bag containing the oil was closest to the surface of the coil. Lactate measurements in vivo are complicated by the presence of overlapping lipid methylene resonances (1.2–1.3 ppm). Of the different
AP: Mag Res
71
NOTES
FIG. 2. Bloch-equation simulations of ANDANTE and SSAP. Transverse (dotted lines) and longitudinal (bold lines) magnetization as a function of transmitter offset for (a) ANDANTE and (b) SSAP. The corresponding phases are shown in (c) and (d), respectively. (e) Transverse (dotted lines) and longitudinal (bold lines) magnetization and (f ) phase as a function of B1 amplitude for ANDANTE. All simulations were carried out using PENCIL. For ANDANTE, N Å 10; T Å 1 ms; t Å 0.325 ms. For the SSAP, t Å 1 ms and T Å 1 ms.
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AP: Mag Res
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NOTES
FIG. 3. Frequency profile of ANDANTE obtained from a sample of water by sweeping the carrier frequency through 1000 Hz in steps of 25 Hz. The stacked plot is composed of individual magnitude spectra. Pulse parameters for ANDANTE: N Å 10; T Å 1 ms; t Å 0.325 ms. Single acquisitions were collected every 4 s and the magnitude spectra were computed with no line broadening.
FIG. 4. Double-quantum gradient-filtered sequence (9) used to measure lactate. The third 907 pulse is a frequency-selective pulse. t 0 Å 40 ms, t1 Å 11 ms, t1 Å 9 ms, t2 Å 31 ms. The gradient strengths are Gc Å 1.22 G cm01 and G2 Å 2G1 Å 9.6 G cm01 and their durations are 2 and 3 ms, respectively.
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AP: Mag Res
FIG. 5. 1H spectra from a sample of lactic acid using the pulse sequence of Fig. 4 with the third 907 pulse (frequency selective) being (a) SSAP and (b) ANDANTE. All other 907 and 1807 pulses were replaced with BIR-4 pulses of 1 ms duration. The transmitter frequency was set at 1.3 ppm for (a) and 4.3 ppm for (b). A 1 ms delay ( t ) was inserted in a 1 ms BIR-4 pulse to create the SSAP. For ANDANTE, a cascade of 10 97 BIR-4 pulses of 1 ms duration were used with an interpulse spacing of 0.325 ms. Sweep width was 3000 Hz and 2K complex points were acquired. Both spectra are single-scan acquisitions, Fourier transformed with no line broadening and phase corrected. No phase cycling was applied. The durations and gradient strengths are as described in the legend to Fig. 4.
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AP: Mag Res
74
NOTES
lactate editing sequences that have been proposed for in vivo studies, sequences based on multiple-quantum coherences have been shown to be the most efficient (10). A doublequantum gradient sequence with a frequency-selective read pulse (9) was used to acquire lactate spectra (Fig. 4). This sequence has been described in detail by Hurd and Freeman (9). Briefly, the first and the second 907 pulses create the DQ coherences which then evolve during the time interval t1 . The first 1807 pulse refocuses the effects of chemical shifts and field inhomogeneities. The third 907 pulse converts the DQ coherence into observable magnetization which is then detected after it is allowed to refocus under the action of the second 1807 pulse. The fact that DQ coherences are twice as sensitive to field variations (or gradients) is used to edit coupled spins like those of the lactate methyl group from uncoupled spins like those of water. If the gradient strength of G2 is twice that of G1 , only spins that have undergone DQ transitions (coupled spins) are refocused while uncoupled spins are dephased. The crusher gradients Gc serve to remove pulse imperfections. In DQ sequences where coherence selection is achieved using field gradients instead of phase cycling, only 25% of the lactate signal is observable (10). Hurd et al. have shown that one can gain a factor of two improvement in signal intensity by making the third 907 pulse (read pulse) frequency selective (9). In our implementation of this sequence, we replaced all the hard pulses with BIR-4 pulses and the frequency-selective 907 pulse with either ANDANTE (Fig. 5a) or SSAP (Fig. 5b). For good lipid suppression, it is important that the frequency-selective read pulse irradiate only the lactate methine protons (4.3 ppm) and none of the lipid methylene protons (1.2–1.3 ppm). Hence, the duration of ANDANTE was chosen to be 13 ms and the carrier placed at 4.3 ppm to effect a null at 1.3 ppm, corresponding to the lipid methylene resonances. For the SSAP, the carrier was placed at 1.3 ppm and t was chosen so that 1/4t is 250 Hz, creating a null on resonance (lipid) and maximum excitation (907 ) of the lactate methine resonance (4.3 ppm). Figures 5a and 5b demonstrate the superior lipid-suppression and selective excitation properties of ANDANTE. The reason is that the excitation profile of ANDANTE is a sinc function as opposed to the sine function of SSAP. Inspection of Figs. 1a
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and 1b shows that ANDANTE excites the lipid regions less because of the broadness of its nulls, and, hence, the broad multicomponent lipid peaks will always be suppressed more, compared to SSAP. Note the reduced lipid intensity in Fig. 5b, compared to Fig. 5a with similar lactate peak intensity at the same noise level. Thus, the suppression of lipids is better because of the frequency selectivity of ANDANTE and because the DQ filter works better. In conclusion, a new frequency-selective adiabatic pulse has been proposed that shows promise for use in vivo for selective excitation of metabolites as well as for solvent suppression. Unlike the previously proposed adiabatic DANTE sequence (11), ANDANTE can achieve arbitrary tip angles. The frequency selectivity can be varied by altering the T/ t ratio of the pulse. ACKNOWLEDGMENTS We thank Dr. Michael Garwood and Dr. Eric Shankland for helpful discussions and Dr. Jonathan Callahan for help with the simulations on PENCIL. PENCIL is available from the WWW site at http://strange. engr.washington.edu/pencil_home.html. This work was supported by NIH Grants AR36281 and AR41928.
REFERENCES 1. R. S. Staewen, A. J. Johnson, B. D. Ross, T. Parrish, H. Merkle, and M. Garwood, Invest. Radiol. 25, 559 (1990). 2. B. D. Ross, H. Merkle, K. Hendrich, R. S. Staewen, and M. Garwood, Magn. Reson. Med. 23, 96 (1992). 3. H. Merkle, H. Wei, M. Garwood, and K. Ugurbil, J. Magn. Reson. 99, 480 (1992). 4. M. Garwood and H. Merkle, J. Magn. Reson. 94, 180 (1991). 5. A. J. Johnson, M. Garwood, and K. Ugurbil, J. Magn. Reson. 81, 653 (1989). 6. M. Garwood and Y. Ke, J. Magn. Reson. 94, 511 (1991). 7. K. Ugurbil, M. Garwood, and A. R. Rath, J. Magn. Reson. 80, 448 (1988). 8. G. A. Morris and R. Freeman, J. Magn. Reson. 29, 433 (1978). 9. R. E. Hurd and D. M. Freeman, Proc. Natl. Acad. Sci. USA 86, 4402 (1989). 10. J. E. van Dijk, A. F. Mehlkopf, and W. M. M. J. Bovee´, NMR Biomed. 5, 75 (1992). 11. Y. Ke, M. Garwood, and D. G. Schupp, J. Magn. Reson. 96, 633 (1992).
AP: Mag Res
Series B 110, 69–74 (1996)
Article No. 0009
ANDANTE, a Novel Frequency-Selective Adiabatic Pulse MANOJKUMAR SARANATHAN *
AND
MARTIN J. KUSHMERICK * ,†
*Center for Bioengineering, and †Departments of Radiology and Physiology and Biophysics, University of Washington, Seattle, Washington 98195 Received April 11, 1995; revised August 10, 1995
Surface coils are widely used in localized spectroscopy and imaging experiments in vivo due to the improved sensitivity they afford compared to volume coils. However, the RF homogeneity of surface coils is poor, off-setting some of the signal to noise ratio (SNR) gain over volume coils. Amplitude- and phase-modulated RF pulses called adiabatic half-passage pulses (AHP) have been devised to achieve uniform excitation of spins. These pulses perform optimally despite 10-fold variations in RF field (B1 ) amplitude; however, these pulses can achieve only tip angles of 907 or 1807. B1-insensitive rotation (BIR-4) pulses, which are composed of sections of AHP pulses, achieve plane rotations of magnetization vectors through arbitrary angles (1). Owing to their symmetry, they have an increased bandwidth as well as a symmetric off-resonance performance compared to AHP. A number of complex RF pulses have been crafted using BIR-4 pulses and AHPs as building blocks (2–5). Adiabatic pulses, in general, have either an asymmetric frequency response (like AHPs) or a uniform response over a wide bandwidth (like BIRs). In a variety of spectroscopy applications like solvent suppression or selective irradiation, there is a need for frequency-selective pulses. The solvent-suppressive adiabatic pulse (SSAP), which is similar to a jump–return sequence, addresses the problem of frequency selectivity but has a sinusoidal response which is inadequate for certain applications. In this paper, we propose ANDANTE (adiabatically nutated DANTE), a B1-insensitive frequency-selective plane-rotation pulse that can achieve arbitrary flip angles. As an application, we use this pulse to selectively excite the lactate methine protons in a double-quantum (DQ) gradientfiltered sequence. A detailed derivation and analysis of the BIR-4 pulse has been described elsewhere (1). To summarize briefly, the BIR-4 pulse is a composite pulse composed of four sections, each being either an adiabatic half-passage or its reverse (Fig. 1a). The middle two sections effectively constitute an adiabatic inversion pulse. The flip angle is determined by the phase jumps Df1 and Df2 . These are usually set to ( p / a /2) and 0 ( p / a /2) respectively, yielding a net flip angle of a. The amplitude- and frequency-modulation functions, v1 (t) and Dv(t) are tanh and tan functions which
closely approximate the numerically optimized functions (6, 7) and, as a function of time t, are given by
FS
v1 (t) Å v1max tanh z 1 0
Dv(t) Å Dvmax
12-15-95 10:46:04
DG
tan[ kt/Tp ] , tan[ k]
[1]
[2]
where v1max and Dvmax are the modulation amplitudes and Tp is the duration of each half-passage segment; z and k are constants that influence the adiabaticity conditions of the pulse, and following (1), are chosen to be 10 and arctan(20), respectively. If we use phase modulation, [2] is integrated to yield f(t) Å 0
DvmaxTp ln(cos[ k]) . k tan[ k]
[3]
If the adiabatic conditions are satisfied, plane rotations of any arbitrary angle can be achieved and these are relatively insensitive to RF inhomogeneities. The SSAP (2) is a BIR-4 pulse with a time delay t inserted between the first and the second segments and the phase jumps Df1 , Df2 set to /p and 0 p, respectively. On resonance, this yields a null; i.e., it returns the magnetization to the /z axis. By adjusting the delay t to 1/4V, one obtains a maximum excitation (907 pulse) at offset V. In general, the transverse magnetization vector varies as sin( Vt ), where V is the frequency offset in rad/sec. Note that the SSAP pulse and its frequency response on either side of the carrier are no longer symmetric. The ANDANTE pulse is essentially a DANTE pulse ( 8) with the hard pulses replaced by BIR-4 pulses (Fig. 1b). A train of N BIR-4 pulses of duration T, spaced t apart can be written as s(t) Å [III(t) ∗ p(t)]rect
69
m4435$7297
t Tp
magas
SD t L
,
[4]
1064-1866/96 $12.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
AP: Mag Res
70
NOTES
FIG. 1. Amplitude ( v1 ) and phase ( f ) modulation functions for (a) BIR-4 pulse and (b) ANDANTE pulse. The phase jumps Df1 and Df2 , are set to ( p / a /2) and 0 ( p / a /2) to achieve a tip angle of a. For the ANDANTE pulse, T is the duration of each BIR-4 pulse segment and t is the interpulse duration.
where III(t) is the comb function with impulse spacing t given by `
III(t) Å ∑ d (t 0 nt ),
[5]
n Å0`
p(t) is the adiabatic modulation function, and rect(t/L), a rectangular function of length L. L Å NT / (N 0 1) t and ∗ represents convolution. Invoking the linearity of the smallflip-angle approximation, the transverse magnetization Mxy is proportional to the Fourier transform of the RF irradiation and, as a function of offset frequency, is given by Mxy ( v ) } [III( v )P( v )] ∗ sinc( vL),
[6]
where P( v ) and III( v ) are the Fourier transforms of p(t) and III(t). If P( v ) is reasonably flat over the spectral region of interest, a reasonable assumption for BIR-4 pulses, then Eq. [3] can be approximated by a finite train of sinc functions of main lobe width 2/L, separated by 1/ t. By varying t, one can move the harmonics of the sinc function away from the spectral region of interest. The theoretical performances of ANDANTE and SSAP were compared using PENCIL, a simulation program developed at the University of Washington by Jonathan Callahan
m4435$7297
12-15-95 10:46:04
magas
(Figs. 2a–2f). All flip angles were set to 907 and relaxation was ignored. The overall length of ANDANTE was 13 ms ( t Å 0.325 ms; T Å 4Tp Å 1 ms; N Å 10), and that of SSAP was 2 ms ( t Å 1 ms). It can be seen from Figs. 2a–2d that the frequency selectivity of ANDANTE is high compared to SSAP. Figures 2e and 2f show that the adiabaticity of the BIR-4 pulse is still preserved by ANDANTE. Phase errors are restricted to within {57 over a fivefold range of B1 amplitude, minimizing any signal loss. We tested the performance of the sequence by acquiring spectra on a GE CSI-II system (85 MHz 1H frequency; horizontal bore). A homebuilt surface coil of 5 cm inner diameter was used for both transmission and reception. The first sample consisted of a vial of water. The frequency response of ANDANTE was calculated by sweeping the carrier frequency over 2000 Hz in steps of 25 Hz. Figure 3 shows a stacked plot of the magnitude spectra versus frequency offset for an ANDANTE pulse train consisting of 10 97 pulses of duration 1 ms separated by 325 ms (L Å 13 ms). The main lobe width was 600 Hz as predicted by the simulations. The second sample consisted of a polythene bag filled with 20 ml olive oil wrapped around a bottle containing 50 ml of a 17 mM solution of lactic acid. The bag containing the oil was closest to the surface of the coil. Lactate measurements in vivo are complicated by the presence of overlapping lipid methylene resonances (1.2–1.3 ppm). Of the different
AP: Mag Res
71
NOTES
FIG. 2. Bloch-equation simulations of ANDANTE and SSAP. Transverse (dotted lines) and longitudinal (bold lines) magnetization as a function of transmitter offset for (a) ANDANTE and (b) SSAP. The corresponding phases are shown in (c) and (d), respectively. (e) Transverse (dotted lines) and longitudinal (bold lines) magnetization and (f ) phase as a function of B1 amplitude for ANDANTE. All simulations were carried out using PENCIL. For ANDANTE, N Å 10; T Å 1 ms; t Å 0.325 ms. For the SSAP, t Å 1 ms and T Å 1 ms.
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AP: Mag Res
72
NOTES
FIG. 3. Frequency profile of ANDANTE obtained from a sample of water by sweeping the carrier frequency through 1000 Hz in steps of 25 Hz. The stacked plot is composed of individual magnitude spectra. Pulse parameters for ANDANTE: N Å 10; T Å 1 ms; t Å 0.325 ms. Single acquisitions were collected every 4 s and the magnitude spectra were computed with no line broadening.
FIG. 4. Double-quantum gradient-filtered sequence (9) used to measure lactate. The third 907 pulse is a frequency-selective pulse. t 0 Å 40 ms, t1 Å 11 ms, t1 Å 9 ms, t2 Å 31 ms. The gradient strengths are Gc Å 1.22 G cm01 and G2 Å 2G1 Å 9.6 G cm01 and their durations are 2 and 3 ms, respectively.
m4435$7297
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magas
AP: Mag Res
FIG. 5. 1H spectra from a sample of lactic acid using the pulse sequence of Fig. 4 with the third 907 pulse (frequency selective) being (a) SSAP and (b) ANDANTE. All other 907 and 1807 pulses were replaced with BIR-4 pulses of 1 ms duration. The transmitter frequency was set at 1.3 ppm for (a) and 4.3 ppm for (b). A 1 ms delay ( t ) was inserted in a 1 ms BIR-4 pulse to create the SSAP. For ANDANTE, a cascade of 10 97 BIR-4 pulses of 1 ms duration were used with an interpulse spacing of 0.325 ms. Sweep width was 3000 Hz and 2K complex points were acquired. Both spectra are single-scan acquisitions, Fourier transformed with no line broadening and phase corrected. No phase cycling was applied. The durations and gradient strengths are as described in the legend to Fig. 4.
m4435$7297
12-15-95 10:46:04
magas
AP: Mag Res
74
NOTES
lactate editing sequences that have been proposed for in vivo studies, sequences based on multiple-quantum coherences have been shown to be the most efficient (10). A doublequantum gradient sequence with a frequency-selective read pulse (9) was used to acquire lactate spectra (Fig. 4). This sequence has been described in detail by Hurd and Freeman (9). Briefly, the first and the second 907 pulses create the DQ coherences which then evolve during the time interval t1 . The first 1807 pulse refocuses the effects of chemical shifts and field inhomogeneities. The third 907 pulse converts the DQ coherence into observable magnetization which is then detected after it is allowed to refocus under the action of the second 1807 pulse. The fact that DQ coherences are twice as sensitive to field variations (or gradients) is used to edit coupled spins like those of the lactate methyl group from uncoupled spins like those of water. If the gradient strength of G2 is twice that of G1 , only spins that have undergone DQ transitions (coupled spins) are refocused while uncoupled spins are dephased. The crusher gradients Gc serve to remove pulse imperfections. In DQ sequences where coherence selection is achieved using field gradients instead of phase cycling, only 25% of the lactate signal is observable (10). Hurd et al. have shown that one can gain a factor of two improvement in signal intensity by making the third 907 pulse (read pulse) frequency selective (9). In our implementation of this sequence, we replaced all the hard pulses with BIR-4 pulses and the frequency-selective 907 pulse with either ANDANTE (Fig. 5a) or SSAP (Fig. 5b). For good lipid suppression, it is important that the frequency-selective read pulse irradiate only the lactate methine protons (4.3 ppm) and none of the lipid methylene protons (1.2–1.3 ppm). Hence, the duration of ANDANTE was chosen to be 13 ms and the carrier placed at 4.3 ppm to effect a null at 1.3 ppm, corresponding to the lipid methylene resonances. For the SSAP, the carrier was placed at 1.3 ppm and t was chosen so that 1/4t is 250 Hz, creating a null on resonance (lipid) and maximum excitation (907 ) of the lactate methine resonance (4.3 ppm). Figures 5a and 5b demonstrate the superior lipid-suppression and selective excitation properties of ANDANTE. The reason is that the excitation profile of ANDANTE is a sinc function as opposed to the sine function of SSAP. Inspection of Figs. 1a
m4435$7297
12-15-95 10:46:04
magas
and 1b shows that ANDANTE excites the lipid regions less because of the broadness of its nulls, and, hence, the broad multicomponent lipid peaks will always be suppressed more, compared to SSAP. Note the reduced lipid intensity in Fig. 5b, compared to Fig. 5a with similar lactate peak intensity at the same noise level. Thus, the suppression of lipids is better because of the frequency selectivity of ANDANTE and because the DQ filter works better. In conclusion, a new frequency-selective adiabatic pulse has been proposed that shows promise for use in vivo for selective excitation of metabolites as well as for solvent suppression. Unlike the previously proposed adiabatic DANTE sequence (11), ANDANTE can achieve arbitrary tip angles. The frequency selectivity can be varied by altering the T/ t ratio of the pulse. ACKNOWLEDGMENTS We thank Dr. Michael Garwood and Dr. Eric Shankland for helpful discussions and Dr. Jonathan Callahan for help with the simulations on PENCIL. PENCIL is available from the WWW site at http://strange. engr.washington.edu/pencil_home.html. This work was supported by NIH Grants AR36281 and AR41928.
REFERENCES 1. R. S. Staewen, A. J. Johnson, B. D. Ross, T. Parrish, H. Merkle, and M. Garwood, Invest. Radiol. 25, 559 (1990). 2. B. D. Ross, H. Merkle, K. Hendrich, R. S. Staewen, and M. Garwood, Magn. Reson. Med. 23, 96 (1992). 3. H. Merkle, H. Wei, M. Garwood, and K. Ugurbil, J. Magn. Reson. 99, 480 (1992). 4. M. Garwood and H. Merkle, J. Magn. Reson. 94, 180 (1991). 5. A. J. Johnson, M. Garwood, and K. Ugurbil, J. Magn. Reson. 81, 653 (1989). 6. M. Garwood and Y. Ke, J. Magn. Reson. 94, 511 (1991). 7. K. Ugurbil, M. Garwood, and A. R. Rath, J. Magn. Reson. 80, 448 (1988). 8. G. A. Morris and R. Freeman, J. Magn. Reson. 29, 433 (1978). 9. R. E. Hurd and D. M. Freeman, Proc. Natl. Acad. Sci. USA 86, 4402 (1989). 10. J. E. van Dijk, A. F. Mehlkopf, and W. M. M. J. Bovee´, NMR Biomed. 5, 75 (1992). 11. Y. Ke, M. Garwood, and D. G. Schupp, J. Magn. Reson. 96, 633 (1992).
AP: Mag Res