In-Vivo Spatially Encoded Magnetic Resonance Spectroscopy With Solvent Suppression

The present invention relates to the art of magnetic resonance spectroscopy. It finds particular application in conjunction with in-vivo examinations and will be described with particular reference thereto. However, it is to be appreciated that the invention may find further application in conjunction with the magnetic resonance spectroscopic examination of localized regions for imaging, chemical shift analysis, and the like.

In non-imaging magnetic resonance spectroscopy, samples in a test tube may be immersed or dissolved in a solvent, such as water, before undergoing analysis. When analysing a sample for chemical compositions containing hydrogen, magnetic resonance signals from a water solvent are often several magnitudes greater than the magnetic resonance signals from the composition of interest. Similarly, signals from solvents other than water often drown out the signals of interest.

One technique for separating the solvent signals from the signals attributable to the composition of interest includes the use of binomial magnetic resonance excitation pulses. As set forth in "Solvent Suppression in Fourier Transform Nuclear Magnetic Resonance," J. Mag. Res., Vol. 55, pages 283-300 (1983), P. J. Hore, the binomial pulses selectively excite nuclei within limited ranges of resonance frequencies relative to the main magnetic field. That is, the binomial pulses excite magnetic resonance in specific, limited frequency ranges and suppress magnetic resonance excitation in other resonance frequency ranges. In this manner, resonance can be selectively excited in the nuclei of interest while suppressing resonance signals from the solvent.

Heretofore, the use of binomial pulses has been limited to chemical shift spectroscopy. In magnetic resonance imaging, gradient pulses are commonly applied concurrently with the excitation pulses. The gradients across the main magnetic field cause nuclei to have different resonance frequencies at different spatial positions along the gradient. The application of a binomial excitation pulse concurrently with a magnetic field gradient would shift the excitation frquency bands spatially such that the range of excited frequencies is different at each spatial position. The spatial shift would undo the selective excitation of only specific chemical compositions to the exclusion of others.

Also, in the field of chemical shift spectroscopy, various techniques have been developed to localize the examined region, such as the technique illustrated in U.S. Pat. No. 4,480,228 issued Oct. 30, 1984 to P. A. Bottomley. In the Bottomley patent, a 90.degree. magnetic resonance excitation pulse is applied in the presence of a positive z-axis gradient. A first 180.degree. inversion pulse is applied in the presence of a y-axis magnetic field gradient which causes a first spin echo. Subsequent to the first spin echo, a second 180.degree. inversion pulse is applied in the presence of an x-axis magnetic field gradient. The second 180.degree. pulse refocuses only the magnetization of resonating nuclei at the intersection of the three gradient planes. In this manner, only nuclei at the intersection of the planes contribute to a second spin echo. Chemical shift data acquired during the second spin echo is descriptive only of the nuclei at the intersection.

The Bottomley selective spatial localization technique is not amenable to solvent suppression. The application of the necessary gradient magnetic field concurrent with the excitation pulse renders the Bottomley technique incompatible with the Hore technique.

The present invention provides a new and improved technique which provides both solvent suppression and spatial region localization which is suitable for both chemical shift and imaging examinations.

In accordance with a first aspect of the present invention, a method of localized magnetic resonance spectroscopy is provided. Resonance is selectively excited in nuclei of interest substantially without exciting nuclei of a solvent associated therewith. Continued coherent resonance of the excited nuclei is limited to nuclei in a preselected volume of interest. Resonance data is collected from resonating nuclei in the volume of interest. In this manner, the acquired resonance data is limited to nuclei of interest in the selected spatial volume. The acquired data may optionally be utilized for chemical shift spectroscopic analysis or imaging.

In accordance with another aspect of the present invention, a method of selectively limiting of magnetic resonance to a preselected volumetric region is provided. An excitation pulse is applied in the absence of magnetic field gradients to excite magnetic resonance. A first inversion pulse is applied concurrently with a first magnetic field gradient to rephase the magnetization of the excited nuclei which lie generally along a first planar region defined by the first gradient. In this manner, only the nuclei in the planar region are rephased and would contribute to a first spin echo. A second inversion pulse is applied concurrently with a second magnetic field gradient to rephase the nuclei generally along a linear region at the intersection of planar regions defined by the first and second gradients. In this manner, only the nuclei along the generally linear region are rephased and would contribute to a second spin echo. A third inversion pulse is applied concurrently with a third magnetic field gradient to rephase nuclei in a volumetric element at the intersection of generally planar regions defined by the three magnetic field gradients such that the resonating nuclei in the volumetric element are rephased and contribute to a third spin echo. In this manner, data acquired during the third spin echo is only from the nuclei in the volumetric element.

One advantage of the present invention is that it facilitates the observation of hydrogen spectra of low concentration species in the presence of high concentrations of water.

Another advantage of the present invention is that it facilitates the spectroscopic examination of selectable regions of humans and other living tissue.

Yet another advantage of the present invention is that it simplifies data acquisition and handling.

Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiment.

The present invention may be embodied in various steps and arrangements of steps and in various components and arrangements of components. The drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention.

FIG. 1 is a diagrammatic illustration of a magnetic resonance apparatus in accordance with the present invention;

FIG. 2 is a diagrammatic illustration of a radio frequency and magnetic field gradient pulse sequence applied by the apparatus of FIG. 1 to implement chemical shift spectrum data acquistion aspects of the present invention; and,

FIG. 3 is a diagrammatic illustration of excitation, inversion, and magnetic field gradient pulse sequence in conjunction with the present invention which is particularly adapted to in the collection of imaging data.

With reference to FIG. 1, a magnetic resonance imaging apparatus includes a plurality of main field magnets 10 and a control circuit 12. The main magnetic field control circuit 12 controls the main field magnets such that a substantially uniform magnetic field is generated longitudinally through an image region 14. The uniform field causes a preferential alignment of the magnetization of nuclei in the image region. A gradient coil 20 includes the appropriate windings for causing gradients across the main magnetic field in the image region. A gradient control means or circuit 22 selectively applies electrical power to the gradient coil assembly to select the angle, duration, and periodicity with which magnetic field gradients are applied. In the preferred embodiment, the gradients are applied along a set of orthogonal axes, arbitrarily designated x, y, and z axis, where the z axis extends longitudinally along the main magnetic field. Each gradient field varies linearly across the image region 14 and defines a corresponding continuum of planar regions thereacross each with a common magnetic field strength. The strength of each magnetic field gradient pulse determines the rate of change or steepness of the gradient. A phase encoder means or control 24 selectively adjusts the applied gradients so as to cause a phase shift which varies with the spatial position for spatially encoding the resultant resonance signals along a selected one or more of the x, y, and z axes. A read gradient control 26 selectively applies a read gradient along the one or more of the axes.

A radio frequency (RF) coil 30 is connected with a binomial excitation pulse generator or control 32. The binomial pulse generator selectively causes the RF coil to transmit binomial excitation pulses in the absence of magnetic field gradients in accordance with the sequences and patterns set forth herein below.

The resonance frequency for a given nuclei is proportional to the product of its gyromagnetic ratio and the strength of the surrounding magnetic field. Each binomial excitation pulse includes bands of frequencies relative to the surrounding magnetic field strength which are excited and bands of frequencies which are not excited or suppressed. The strength of the surrounding main magnetic field generated by the main magnets 10 is selected relative to the binomial pulse frequency bands and the gyromagnetic ratio of nuclei in the image region such that resonance is excited in the nuclei of interest and suppressed in the nuclei of the solvent or other preselected nuclei.

An inversion pulse generator or control 34 selectively causes the RF coil to transmit 180.degree. inversion pulses. The inversion frequency for a given nucleus is again proportional to the gyromagnetic ratio of the given nucleus and the surrounding magnetic field. Because the inversion pulses are applied in the presence of a magnetic field gradient, the surrounding magnetic field is the sum of the main and gradient magnetic fields. The 180.degree. inversion pulses have a range or spectrum of frequencies which is selected to invert the magnetization of resonating nuclei only in a limited region, which limited region is determined by the magnetic field gradients generated by the gradient field control 22 and the RF coil 20. More specifically, each gradient varies the magnetic field strength linearly therealong, hence, varies the resonance frequency of the given nuclei linearly therealong. The inversion pulses invert resonating nuclei in a selected region along the gradient, i.e. a corresponding planar region transverse to the direction of the gradient. The position of the planar region along the gradient is determined by the frequencies of the inversion pulse relative to the amplitude of the gradient. The width or thickness of the planar region is determined by the breadth of the frequency spectrum and the strength or steepness of the magnetic field gradient. For simplicity of consistent operation in the preferred embodiment, the strength of the main magnetic field and the frequency spectrum of the inversion pulses are held constant. The steepness of the magnetic field gradient and the midpoint of the inversion pulse frequency spectrum are selectively varied.

A radio frequency receiver 36 receives radio frequency spin echo or other resonance signal components generated by the resonating nuclei in the image region. Separate transmit and receive coils may be provided or the RF coil 30 may function alternately as a transmitting and receiving antenna.

A chemical shift spectrum analysis means 40 analyzes received chemical shift spectrum spin echo data from the receiver to provide chemical composition and other spectrographic information. A transform means 50 transforms or maps discretely sampled values of phase encoded resonance signals received in the presence of a read gradient from a phase domain to a spatial domain. In the preferred embodiment, a fast, two-dimensional Fourier transform algorithm transforms or maps the received resonance signals from each selected spin echo to generate a corresponding view in the spatial domain.

A processor 52 processes the image views to create an electronic image representation of resonating nuclei. The image representation may be displayed on a video monitor 54 or other appropriate display. The processor 52 may perform other computer enhancement operations on the views or images. For example, the views or images may be color or gray scale encoded in accordance with the spectrographic data derived by the spectrographic analyzer 40. Alternately, the spectrographic data may be displayed directly on the video monitor or other appropriate display 54 along with any image representation. The spectrographic data, image representation, and any computer enhancements may be stored on disc or tape for later display, further processing, computer evaluation, electronic enhancement, or the like.

With reference to FIG. 2, the binomial pulse generator or resonance exciting means 32 selectively excites resonance in the nuclei of interest substantially without exciting nuclei of a solvent associated therewith. In the preferred embodiment, the resonance exciting means applies a binomial 90.degree. excitation pulse 60, such as a 1331 pulse. The numerals of the pulse designation indicate the relative pulse lengths and the bars indicate a 180.degree. phase shifted pulse. Equal delays separate each pulse of the sequence of pulses which are run together to provide the 1331 pulse. The binomial excitation pulse rotates the magnetization vectors of nuclei in a limited range or ranges of resonance frequencies by 90.degree.. Nuclei which resonate at other frequencies are suppressed, i.e. not excited. The frequency at which any given nucleus resonates is dictated by the Larmor equation, i.e. is proportional to the product of the gyromagnetic ratio of the nuclei and the strength of the magnetic field in which it resides. Thus, by adjusting or appropriately selecting the strength of the main magnetic field generated by magnets 10 relative to the excitation and suppression bands of the binomial pulse, the nuclei which are to be excited and suppressed are selected. In the preferred embodiment, hydrogen nuclei in compositions other than water are excited and hydrogen nuclei in water are suppressed.

After resonance has been excited, the generation of magnetic resonance signals is limited to a selected volume of interest. Specifically, the inversion pulse generator 34 generates a first inversion pulse 70 concurrently with the gradient magnetic field control 22 generating a first gradient magnetic field gradient along a first direction or axis. The frequency spectrum of the first inversion pulse 70 and the first magnetic field gradient 72 cause the magnetization vectors of resonating nuclei located in a first planar region defined by the first magnetic field gradient to be inverted. Upon inversion, the dephasing magnetization vectors of the excited nuclei are flipped 180.degree. and commence rephasing. Normally, the rephasing magnetizations would come completely back into phase creating a first spin echo. However, in the preferred embodiment the first spin echo data is not collected. Accordingly, it has been found advantageous to suppress or dephase the first spin echo to simplify data acquisition.

After a duration in which the first spin echo would have occurred had it not been dephased, the inversion pulse generator 34 causes the generation of a second inversion pulse 74 concurrently with the gradient control 22 causing a second magnetic field gradient 76. The second magnetic field gradient extends generally linearly through the image region to limit the inversion of nuclei by the frequency spectrum of the second inversion pulse to a second generally planar region defined thereby. Normally, a second spin echo would be caused by rephasing nuclei in a generally linear region at the intersection of the first and second planes. However, because the second spin echo does not contain data of interest in the preferred embodiment, the second spin echo is dephased or suppressed.

It is to be appreciated that the first inversion pulse 70 only inverted the magnetization vectors in the first planar region defined by the first magnetic field gradient 72. Magnetizations in other areas of the image region 14 continued to dephase. The second inversion pulse inverts the magnetizations in the second planar region defined by the second magnetic field gradient 76 to commence rephasing. The magnetizations in the second will refocus into a spin echo the same duration after the second inversion pulse as the duration between when they were last in phase and the second inversion pulse. The magnetizations lying in the second planar region which are not also lying in the first planar region have been dephasing since the excitation pulse 60. Those magnetizations which were first inverted by the first inversion pulse 70 and subsequently inverted by the second inversion pulse 74 will refocus the same duration after the second inversion pulse as the duration between when the first spin echo would have occurred and the second inversion pulse. The magnetization of the nuclei in the linear region in which the first and second planes intersect will thus refocus into a spin echo at a different time from the other nuclei in the image region and are readily segregated.

The volume of interest is further selected when the inversion pulse generator 34 applies a third inversion pulse 78 in the presence of a third magnetic field gradient 80. In this manner, the magnetization of only those dipoles are located both in the intersection of the first and second planes and a third plane defined by the third magnetic field gradient 80 are again inverted. The intersection of the first, second, and third planes thus defines a volumetric element or region which is the only region in which the magnetizations are sequentially inverted such that the magnetizations commence rephasing to form a third spin echo 82 when the magnetizations come into coincidence. The magnetization of vectors in other areas of the image region do not come into coincidence concurrently with the third spin echo 82 and, accordingly, do not contribute to the acquired data.

During the third spin echo 82, the receiver 86 acquires magnetic resonance data. The received magnetic resonance data is processed by the chemical spectrum analyzer 40 to determine the chemical compositions within the volumetric element. In the preferred embodiment, hydrogen bearing compositions are determined except water which is suppressed.

The strength of the gradients applied by the gradient control means 32 determines the thickness of the corresponding planar region. The steeper the gradient, the thinner the planar region. Thus, by adjusting the relative gradient strengths, the size and dimensions of the volumetric element can be selected and adjusted. It is to be appreciated that adjusting the strength of the gradients adjusts the total strength of the magnetic field in the image region, hence, the resonance frequency of the dipoles of interest. To compensate for this resonance frequency shift, the midpoints of the frequency spectrum of the first, second, and third inversion pulses are adjusted accordingly. Alternately, the dimensions of the volumetric element can be adjusted by adjusting the frequency spectrum or band width of the inversion pulses.

Adjusting the frequency of an inversion pulse adjusts the spatial position of the corresponding plane along the gradient axis. That is, by increasing or decreasing the midpoint frequency of the inversion pulse, the planar region along the gradient axis in which magnetizations are inverted is shifted to areas of stronger or weaker magnetic field strength. By appropriately selecting or adjusting the midpoint frequencies of each of the first, second, and third inversion pulses, the position of the volumetric element can be selected in three dimensions.

It is often convenient to provide an image representation of the nuclei in the selected volumetric element. In the preferred embodiment, the images are projections of the resonating dipoles in the volumetric element relative to one or more of the gradient axes.

With reference to FIG. 3, the image of the volumetric element may be made before collection of the chemical spectrum data, after collection of the chemical spectrum data, or in the midst of collecting chemical data. To collect image data, the excitation pulse generator 32 causes the generation of a like binomial pulse 60' in the absence of gradient magnetic fields. This causes the same dipoles to be excited and suppressed as in the chemical shift experiment.

Following the excitation pulse, the phase encoding means 24 provides a selected one of a plurality of phase encoding pulses 90 along one or more of the axis. Thereafter, a first inversion pulse 70' is applied concurrently with a first magnetic field gradient 72'. The first spin echo, which would normally be generated by nuclei in the first planar region, is again dephased or suppressed. A second inversion pulse 74' and a second magnetic field gradient 76' are concurrently applied. The second spin echo, which would normally be generated by nuclei at the intersection of the first and second planes, is again dephased or suppressed. A third inversion pulse 78' is applied concurrently with a third magnetic field gradient 80' to cause a third spin echo 82'. A read gradient pulse sequence 92 is applied along a selected read direction. The read gradient sequence is selected such that a first portion 94 of the read gradient sequence is applied before the third spin echo 82' and a second portion 96 of the sequence is applied during the third spin echo. In this manner, the data acquired by the receiver 36 during the third spin echo represents a projection of the resonating nuclei in the volumetric element projected into the plane which is defined by the axes along which the phase encode and read gradient pulses are applied, in the illustrated sequence the x,y plane. By appropriately placing the phase encode and read gradients along others of the axis, projections into the x,z and y,z planes may also be produced. The data collected during the third spin echo is transformed by a two-dimensional Fourier transform means 50 into the image representation for display on the video monitor 54. By using like excitation, inversion, and gradient pulses in both the chemical shift and the imaging sequence, the image will be of the same region as the chemical shift data.

The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding specification. It is intended that the invention be construed as including all such alterations and modifications insofar as they come within the scope of the appended claims or the equivalents thereof.