SPECTROSCOPY METHODS

Abstract: Several magnetic resonance imaging (MRI) methods using adiabatic excitation are disclosed. One method accomplishes slice selection with gradient modulated adiabatic excitation. Another method employs slice selection with adiabatic excitation despite large variations in B1 magnitude. There is also described <1>H spectroscopy using solvent suppressive adiabatic pulses.

Description: SPECTROSCOPΫ METHODS Technical Field of the Invention The present invention relates generally to spectroscopy, and more particularly to methods for slice selection and solvent suppression using adiabatic exci- tation. Background of the Invention Magnetic resonance imaging (MRI) is now an impor¬ tant imaging technique in medicine. There are described herein several MRI methods using adiabatic excitation. One method accomplishes slice selection with gradient modulated adiabatic excitation. Another method employs slice selection with adiabatic excitation despite large variations in B^ magnitude. There is also described !R spectroscopy using solvent suppressive adiabatic pulses. The methods .described herein relate to the adiabatic pulses and methods described in U.S. Patent Application Serial No. 032,059, entitled "Amplitude and Frequency/Phase Modulated Pulses to Achieve Plane Rotations of Nuclear, Spin Magnetization Vectors, with Inhomogeneous Bi Fields", the entire disclosure of which is hereby incorporated by reference herein. Brief Description of the Drawings Fig. 1 is a plot of B e (t) generated by a simple version of GMAX consisting of tanh modulated Bτ_ ampli¬ tude, and sech modulated pulse frequency and gradient magnitude; Fig. 2 is a plot of a computer calculated slice profile obtained with the tanh/sech version of GMAX; Fig. 3A is a plot of signal-to-noise obtained with Solvent Suppressive Adiabatic Pulses; Fig. 3B is a plot of signal-to-noise obtained with binomial pulses. Detailed Description of the Invention SLICE SELECTION WITH GRADIENT MODULATED ADIABATIC EXCITATION Because of their extreme tolerance to RF inhomoge- neity, frequency selective adiabatic inversion pulses are becoming increasingly popular for localized spectroscopic and imaging studies carried out with sur¬ face coils. They offer superior slice definition and do not suffer from sensitivity loss caused by non-uniform spin excitation, a problem inherent with conventional pulses. With frequency selective Bi insensitive inver¬ sion pulses, slice selection is achieved by applying the pulse in the presence of a constant B Q gradient field while the pulse amplitude and frequency (or phase) are modulated. We describe here a new gradient modulated adiabatic excitation pulse (GMAX) which, in addition to amplitude and frequency modulation, employs time- dependent gradient modulation in order to achieve slice selective excitation. To clearly illustrate the principles of GMAX, con- sider a frame of reference (axes x* , y' , z*) precessing at the instantaneous frequency of the pulse. Under ideal adiabatic conditions, motion of the initial longi¬ tudinal magnetization will parallel the trajectory of the effective field B e (t), which is defined in this frame by the vectorial sum of B 1(t) and u)(t). Bχ ( t ) is RF amplitude in rads/sec and AuD(t) equals the dif¬ ference between the instantaneous ulse and spin fre¬ quencies (rads/sec). Fig. 1 shows B e (t) generated by a simple version of GMAX consisting of tanh modulated Bi amplitude, and sech modulated pulse frequency and gra¬ dient magnitude. The terms A, v, T, and B are, respec¬ tively, pulse frequency modulation amplitude (in Hz), a unitless term accounting for spatial variation in Bτ_ magnitude, pulse duration, and the limit of modulation. The quantity g(r) in the expression for cO(t) repre¬ sents the maximum B Q gradient strength (rads/sec) as a function of position, r. Unlike adiabatic inversion (Silver, M.S., R.I. Joseph, and D.I. Hoult, J. Maqn. Reson. 59,347,1984) or excitation (Johnson, A.J., K. Ugurbil, and M. Garwood. Abstract submitted for 7th Annual Society of Magnetic Resonance in Medicine meeting, 1988) pulses where the field gradient is time-invariant, GMAX creates a gradient-dependent node in ΛL (t) by executing equiva¬ lent time modulation of both pulse frequency and BQ gradient strength. This node exists in a plane lying perpendicular, to the gradient direction in a region where the field gradient magnitude exactly equals the pulse frequency amplitude, i.e. when g(r)=2TT . Since Δt (t) will be positive on one side of this plane and negative on the other, the final transverse magnetiza- tion on opposite sides will differ in phase by 180°. A second implementation of GMAX with reversed gradient modulation yields a mirror image of the first response reflected about a point where the gradient modulation is zero. Addition of these two signals defines a slice of excitation of width 4ItA. Slice position and width can be varied by manipulating A and/or gradient magnitude. By repeating this sequence with three orthogonal field gradients, 3-dimensional localization can be achieved in a manner analogous to 3-dimensional ISIS (Ordidge, R.J., A. Connelly, and J.A.B. Lohman. J. Maqn. Reson. 66, 283, 1986). Fig. 2 shows a computer calculated slice profile obtained with the tanh/sech version of GMAX, p=5.3. Although the included example of GMAX consists of a tanh/sech modulation pair, other functions can be used provided the boundary conditions remain unchanged; spe¬ cifically, numerically optimized modulation schemes can be used to enhance the pulse with respect to off- resonance performance and B^ insensitivity (Ugurbil, K., M. Garwood, and A. Rath J. Maqn. Reson. in press, 1988). The Silver, Johnson, Ordidge and Ugurbil references spe¬ cified above are hereby incorporated by reference herein. SLICE SELECTION WITH ADIABATIC EXCITATION DESPITE LARGE VARIATIONS IN Bi MAGNITUDE Frequency selective excitation pulses used in NMR imaging and spectroscopy are Bj sensitive and cannot induce uniform excitation, or maintain a uniform slice profile in the presence of large variation in Bτ_ magni¬ tude. Consequently, in surface coil studies using these pulses, it is virtually impossible to select a distor¬ tion free slice that is perpendicular to the plane of the surface coil. The only methods capable of defining uniform slices over large variations in B ] _ have relied on adiabatic inversion (Silver, M.S., R.I. Joseph, and D.I. Hoult, J. Maqn. Reson. 59, 347, 1984) or excitation (Johnson, A.J. , M. Garwood, and K. Ugurbil. Abstract submitted for 7th Annual Society of Magnetic Resonance in Medicine meeting, 1988) pulses that require the addi¬ tion of two separate acquisitions. These pulses are unsuitable for multislice imaging or spectroscopic and imaging studies in the presence of motion. Here, we report a new adiabatic excitation pulse that is both slice selective and B^ insensitive-and does not require multiple acquisitions. Described in terms of amplitude and frequency modu¬ lation, this slice selective adiabatic excitation or SSAX pulse consists of two functionally distinct but contiguous segments. The initial segment combines constant Bτ_ amplitude with a large frequency sweep; the subsequent segment ' couples decaying Bτ_ amplitude with zero frequency modulation. Taking B^ to be along x 1 in a reference frame which rotates at the instantaneous frequency of the pulse" (axes x', y', and z Λ ) , the effect of SSAX can be visualized as follows: SSAX first creates a distribution of spin orientations in the x'z' plane as a function of frequency offset so that only spins with zero offset point along x* . Subsequently, spins not along x*- are taken either back up to z' (positive offset) -or down to -z 1 (negative offset) whereas those along x? (zero offset) are allowed to remain in the- transverse plane. For illustrative purposes, the following example is a simple version of a SSAX pulse constructed with tangent and sech functions. Bl ( t ) = 2TTA V β ech £f±(t - • £) " (i ^ t ^ T ) ^uθ(t) = 0 Bi is RF amplitude in rads/sec, & > is the difference between the instantaneous pulse and spin Larmor frequen¬ cies in rads/sec, A*tan[ TTq/2] is the frequency modula¬ tion amplitude in Hz, v is a unitless parameter equal to the (peak Bι)/2TTA ratio, T is pulse duration, and q and p are chosen to set the modulation limits. When B j is inhomogeneous, both B]_ and v depend on spatial coor¬ dinates. While the above example is written using tan and sech functions, provided the boundary values remain the same, other modulation schemes can be used; in par- ticular, numerically optimized modulation routines can be used to .improve off-resonance performance and B_ insensitivity of the pulse (Ugurbil, K., M. Garwood, and A. Rath. J..Maqn. Reson. in press, 1988). Projected, applications include combining SSAX with a single adiabatic spin excitation to achieve solvent suppression while exciting other frequencies, combining two SSAX. pulses with a 90° adiabatic rotation from the x'y' plane to the z' -axis to .define a 2-dimensional column" of excitation in one pulse train. Similarly, three SSAX pulses can be combined with two 90° adiabatic rotations to define in a single pulse train a 3-dimensional volume of excitation which can be used in spectroscopic localization. The slice profile of the current version of this pulse is not square. However, there are no Bτ_ insensitive and slice selective pulses which, do not require multiple acquisitions. Therefore, this pulse would be preferred in all applications in the presence of inhomogeneous Bτ_'s where multislice capabi- lity is desired, motion is present, and/or subtraction errors are significant. The disclosures of the above- specified Silver, Johnson and Ugurbil references are hereby incorporated by reference herein. X H SPECTROSCOPY USING SOLVENT SUPPRESSIVE ADIABATIC PULSES (SSAP) In vivo ^H NMR spectroscopy is usually executed with surface coils, which provide enhanced sensitivity for most applications although the B^ field is extremely inhomogeneous. Pulse sequences designed to selectively suppress the H2O signal have been based on RF pulses which are sensitive to variations in B^ magnitude (P.J. Hore. J.Magn. Reson. 5__, 383(1983)). When using surface coils to transmit rectangular or amplitude modulated pulses, sample regions inevitably experience nutation angles, & . that are multiples of 180°, and signals arising in regions where θ=90° may be partially can¬ celled by signals produced where θ=270°. In addition, the frequency response of such pulses depends upon θ, and is therefore a function of spatial coordinates when inhomogeneous RF coils are employed for RF transmission. Recently, we described amplitude and frequency/phase modulated pulses which are highly insen¬ sitive to Bi inhomogeneity and can achieve uniform 90° (K. Ugurbil, M. Garwood, and M.R. Bendall, J. Magn. Reson. 72, 177(1987)) and 180° (M.R. Bendall, M. Garwood, K. Ugurbil, and D.T. Pegg, Magn. Reson. Med. 4, 498(1987), K. Ugurbil, M. Garwood, A.R. Rath, and M.R. Bendall, J. .Maqn. Reson. , in press) plane rotations across the surface coil active volume provided the adiabatic condition (i.e., | B e /(d
< /dt)| >> 1) is satisfied throughout the pulse. These pulses are com¬ posed of segments during which the effective field, B e , rotates 90° with respect to a frame rotating with the instantaneous frequency of the pulse. The 90° and 180° plane rotation pulses, BIR-2 and BIREF-1, are composed of 4 and 2 such segments, respectively. When a delay period, ~ V, equal to iv is placed between the first and second 90° segments of BIR-2, spins with precessional frequencies equal to +nv are returned to the z'-axis at the end of the pulse, where n is an odd integer and v is the frequency offset (in Hz) in the rotating frame. Likewise, spins with frequencies qual to +nv do not refocus when a delay period equal to i is placed in the middle of refocusing pulse BIREF-1. Consequently, pulses BIR-2 and BIREF-1 can be modified to achieve solvent suppression, where the frequency response (cos(2fι~vT ) and cos(lTvT'), respectively) and spatial dependence are highly invariant across a wide range of Bτ_ magnitude. BIR-2 and BIREF-1 solvent suppressive adiabatic pulses (SSAP) were used for excitation and refocusing in a spin echo sequence to obtain _in vivo ^-H spectra of rat brain. Spectra were acquired using a GE CSI-II spectro¬ meter equipped with a 40 cm, 4.7 T magnet. An 8 mm diameter surface coil placed over the rat head was used. " was adjusted to produce a null at the H2O resonance and yield maximal signal 500 Hz either side of H2O. Fig. 3A shows the results obtained with SSAP after 48 scans using a repetition time of 2 sec and an echo time (TE) of 136 msec. For comparison. Fig. 3B shows the results obtained from the same animal using a spin echo with binominal pulses 1-3-3-1 and 2-6-6-2 (P.J. Hore J. Maqn. Reson. 55, 383(1983)). For this latter experi¬ ment, the pulse lengths were adjusted so that the cumu¬ lative excitation flip angle equaled 135° on resonance at the coil center. All other parameters remained constant. Because adiabatic pulses can uniformly excite spins throughout the surface coil active volume, the spectral signal-to-noise obtained with SSAP (Fig. 3A) was increased relative to that obtained with the bino¬ mial pulses (Fig. 3B). The disclosures of the above- specified Bendall, Hore and two Ugurbil references are hereby incorporated by reference herein. Although the invention has been described herein in its preferred form those skilled in the art will readily recognize that many modifications- -and changes may be made thereto without departing from the spirit and scope of the claims appended hereto.

Claims: What is claimed is: 1. Method for slice selection in a sample under the influence of a constant B Q gradient field comprising the steps of: a) providing a surface coil proximate said sample; b) energizing said surface coil with an adiabatic excitation pulse; and c) modulating the gradient of said pulse over time in order to achieve slice selective excitation. 2. A method of slice selection in a sample under the influence of a constant Bg gradient field comprising the steps of: a) providing a surface coil proximate said sample; and b) exciting said surface coil to produce an adiaba¬ tic excitation pulse comprising two functionally distinct but contiguous segments, and an initial segment of said pulse including a constant Bτ_ amplitude with a large frequency sweep, the sub¬ sequent segment of said pulse coupling a decaying Bi amplitude with zero frequency modulation. 3. A method for solvent suppression in ^H spectroscopy in a sample under the influence of a constant B Q gra¬ dient field comprising the steps: a) providing a surface coil proximate said sample; and b) energizing said coil to produce an adiabatic pulse comprising segments separated by a predeter¬ mined delay period to achieve solvent suppression.

Inventors: GARWOOD MICHAEL, UGURBIL KAMIL, GARWOOD, MICHAEL, UGURBIL, KAMIL
Applicants: UNIV MINNESOTA, REGENTS OF THE UNIVERSITY OF MINNESOTA
Application year: 1989
Main classification: G01R33
IPC codes: A61B5, G01R33