U.S. patent number 5,570,019 [Application Number 08/538,786] was granted by the patent office on 1996-10-29 for method for magnetic resonance spectroscopic imaging with multiple spin-echoes.
This patent grant is currently assigned to The United States of America as represented by the Department of Health. Invention is credited to Jeff Duyn, Chrit T. W. Moonen.
United States Patent |
5,570,019 |
Moonen , et al. |
October 29, 1996 |
Method for magnetic resonance spectroscopic imaging with multiple
spin-echoes
Abstract
A nuclear magnetic resonance pulse sequence to provide spectral
encoding so that the resulting series of spin-echoes each include
both spatial and spectral information for spectroscopic imaging.
Atoms within the object are excited and may then be spatially
encoded, as by a phase encoding gradient. A series of refocusing
pulses is then applied, inducing a respective series of
spin-echoes. Spectral information is directly encoded in the
spin-echo signals. The multiple spin-echoes may be used for
sampling different points of k-space, and/or for increasing the
signal-to-noise ratio by averaging. In an alternative embodiment,
the present invention produces compound weighted spectroscopic
images by selecting the period between refocussing pulses according
to the coupling constant of a group contained in the compound;
thereby, the signal of the selected compounds modulate with a known
frequency different for compounds with different coupling
constants.
Inventors: |
Moonen; Chrit T. W. (Silver
Spring, MD), Duyn; Jeff (Kensington, MD) |
Assignee: |
The United States of America as
represented by the Department of Health (Washington,
DC)
|
Family
ID: |
22311078 |
Appl.
No.: |
08/538,786 |
Filed: |
October 3, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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106377 |
Aug 13, 1993 |
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Current U.S.
Class: |
324/309;
324/307 |
Current CPC
Class: |
G01R
33/4833 (20130101); G01R 33/5615 (20130101); G01R
33/485 (20130101); G01R 33/5617 (20130101) |
Current International
Class: |
G01R
33/48 (20060101); G01R 33/561 (20060101); G01R
33/54 (20060101); G01R 33/483 (20060101); G01R
33/485 (20060101); G01V 003/00 () |
Field of
Search: |
;324/309,307,311,312,306,300,318 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Arana; Louis M.
Attorney, Agent or Firm: Morgan & Finnegan L.L.P.
Parent Case Text
This continuation of application Ser. No. 08/106,377filed on Aug.
13, 1993, now abandoned.
Claims
We claim:
1. A method for acquiring nuclear magnetic resonance information,
comprising the steps of:
exciting at least some of the atoms of an object;
providing a plurality of refocusing pulses, each for generating a
respective spin echo signal from a common region of the object,
successive refocussing pulses separated by a respective refocussing
repetition time period, and each of said refocussing pulses
associated with a respective phase encoding gradient for
individually phase encoding each of the respective spin echo
signals to provide spatial information; and
receiving, in the absence of an externally applied magnetic field
gradient, the respective spin echo signals from the object after at
least one of the refocusing pulses.
2. The method according to claim 1, wherein exciting the atoms
comprises exciting the atoms of the object by applying at least one
excitation pulse to the object.
3. The method according to claim 1, wherein said spin echo signals
are directly encoded with spectral information.
4. The method according to claim 1, exciting the atoms further
comprising applying a slice selecting magnetic gradient field to
the object while applying the excitation pulse, said slice
selecting magnetic gradient field being applied in a first
direction.
5. The method according to claim 1, further comprising, before
receiving said respective spin echo signals, applying a phase
encoding magnetic gradient field in a second direction orthogonal
to the first direction for spatially encoding the excited atoms in
said second direction.
6. The method according to claim 5, further comprising, before
receiving said respective spin echo signals, applying a phase
encoding magnetic gradient field in a third direction orthogonal to
the first direction for spatially encoding the excited atoms in the
plane containing said second direction and said third
direction.
7. The method according to claim 1, wherein the step of exciting
the atoms comprises applying a plurality of excitation pluses to
the object; and wherein a plurality of the refocusing pluses are
provided after each excitation pulse.
8. The method according to claim 1, wherein said refocussing
repetition period is varied depending on the field homogeneity of
said object.
9. The method according to claim 1, wherein said refocussing
repetition period is varied depending of the coupling constant of a
group contained in a component of said object, thereby modulating
the phase of said spin-echo.
10. A method of acquiring spectroscopic magnetic resonance imaging
information for an object, comprising the steps of:
exciting some of the atoms in the object;
providing a plurality of refocussing pulses for stimulating a
respective plurality of spin echo signals from a common region of
the object;
selectively generating a plurality of phase encoding gradients in
at least two directions for spatially encoding said plurality of
spin echo signals to provide spatial information; and
receiving, in the absence of an externally applied magnetic field
gradient, said spin echo signals, wherein spectroscopic information
is directly encoded in said spin echo signal.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to the acquisition of
nuclear magnetic resonance data and, more particularly, to a method
for rapidly acquiring spectroscopic magnetic resonance information
from which a spectroscopic image may be formed.
Nuclear magnetic resonance (NMR) techniques have long been used to
obtain spectroscopic information about substances, revealing the
substance's chemical composition. More recently, spectroscopic
imaging techniques have been developed which combine magnetic
resonance imaging (MRI) techniques with NMR spectroscopic
techniques. Spectroscopic imaging techniques provide a spatial
image of the chemical composition.
In recent years there has been increasing interest in the study of
brain metabolism using proton MR spectroscopy and spectroscopic
imaging because of its noninvasive assessment of regional
biochemistry. While proton spectroscopy measures metabolite levels
in a single volume, proton spectroscopic imaging (HSI) measures the
spatial distribution of metabolites (e.g., N-acetylaspartate (NAA),
total choline, total creatine, and lactate) over a predetermined
volume of interest (VOI). HSI studies of diseased brain have shown
locally altered metabolite levels in chronic and acute brain
infarction, multiple sclerosis, epilepsy, brain tumors, and
acquired immunodeficiency syndrome.
Spectroscopic imaging inherently requires that the pulse sequence
encodes spectral information in addition to spatial information.
Thus, a central problem in spectroscopic imaging is the encoding of
the spectral information. For example, to perform Fourier imaging,
in which the spatial image is ultimately obtained by taking a
Fourier transform, the pulse sequence (i.e., both RF pulses an
gradient field pulses) shown in FIG. 1 may be used. During interval
1, a 90.degree. excitation pulse is applied in the presence of
gradient G.sub.z applied along the z axis. This combination excites
the atoms in a single slice of an object causing the atoms of that
slice to begin to decay from an excited state. As the spins of the
atoms decay or evolve, a reversed gradient G.sub.z is applied
during interval 2 and phase encoding gradients G.sub.x and G.sub.y
are applied during intervals 2 and 3. Gradients G.sub.x and G.sub.y
provide an encoding according to the location of the atoms in the
selected slice. During interval 4, a 180.degree. refocusing pulse
is applied, after which, during interval 5, a spin echo is received
and sampled. A data set is built up by applying a number of
combinations of the values of G.sub.x and G.sub.y, and the spin
echoes of this data set may then be transformed together through a
5 three-dimensional Fourier transform to provide a spectroscopic
image of the x-y plane in which the third dimension shows the high
resolution spectrum, as explained below. The spatial resolution can
be increased by sampling a large number of spin echo signals of
increasing magnitudes of the phase encoding gradients G.sub.x and
G.sub.y.
In the example shown in FIG. 1, the spectral information is encoded
directly in the spin echo, and emerges when a Fourier transform of
the spin echo is taken. Each chemical compound of a specific
element will have one or more characteristic resonances offset in
frequency from the basic resonance of that element. Therefore, the
frequencies contained in the spin echo will correspond to the
compounds of the element being imaged. When a Fourier transform of
the spin echo is taken, the resulting spectrum will have a peak
corresponding to each compound, the amplitude of the peak
reflecting the concentration of that compound. If a data set
containing several spin echoes as described above is transformed,
an image showing the high resolution spectrum at each location will
be produced, showing the concentrations of the compounds at each
spatial location.
FIG. 2 illustrates an alternative technique for encoding the
spectral information, in which a selective 90.degree. excitation
pulse is similarly applied in the presence of gradient G.sub.z
during interval 1. Then, during interval 2, gradient G.sub.z is
reversed, and during intervals 2 and 3, phase encoding gradient
G.sub.y is applied. Also during intervals 2 and 3, an initial
prephasing pulse gradient G.sub.x is applied in preparation for
applying G.sub.x as an observation gradient during the spin echoes.
During interval 4, the 180.degree. refocusing pulse is applied.
During interval 5, the observation gradient G.sub.x is again
applied for a time period which is centered around the center point
of the sampling period of the spin echo. As shown in FIG. 2,
several different values of gradient G.sub.y are applied in the
Fourier method of imaging to obtain a data set to be
transformed.
To encode the spectral information in the sequence of FIG. 2, the
timing of the 180.degree. pulse may be changed by an increment dt,
as shown in rf sequence (b). This introduces a phase error for all
spins which are not resonating at a frequency equal to the
detection reference frequency, and the phase error is proportional
to the frequency offset, resulting in a phase encoding of the
spectral information.
The above methods for obtaining spectral information each require a
large number of data acquisition sequences or shots, each beginning
with an excitation, typically a selective 90.degree. excitation
pulse, followed by a phase encoding interval, an echo generating
waveform such as a refocusing pulse, and a spin echo sampling
interval. The delay between sequence or shots is typically
relatively long in relation to the length of each sequence. As a
result, spectroscopic magnetic resonance techniques using
conventional equipment have usually been restricted to imaging
small volumes or small areas.
For example, most existing HSI techniques for human brain are based
on preselection of a volume of interest (VOI) within the skull in
order to reduce the undesirable resonances of water and lipid
originating from areas outside the VOI. This preselection is
generally achieved by a double spin echo technique or a stimulated
echo technique. One or two dimensions of gradient phase encoding
are employed to spatially discriminate within the VOI. Most
recently, the HSI experiment has been extended to three dimensions
of phase encoding, allowing the VOI to extend in all three
dimensions and thereby obtaining metabolic information from a
larger brain volume. Due to the large number of phase encoding
steps in this experiment, the clinical limitation on the total
measurement time becomes a severe restriction. For example, an
experiment with 16.times.16.times.12 phase encoding steps and a
repetition time (TR) of 2 seconds would require 1.5 hour of total
measurement time. In the case of severely ill or instable patients,
such study lengths are prohibitive. Therefore, it would be
advantageous if more than one spin echo could be sampled for
spectral information in each data acquisition sequence.
Techniques used in other areas of NMR gather data from a series of
echoes. For instance, multiple spin-echoes per excitation pulse
have been used in conventional imaging. Also, echo-planar
techniques employ a pulse sequence which samples the entire k-space
by a series of gradient reversals following only one excitation
pulse; however, this method requires equipment modification to
permit rapid gradient reversals so that all the samples are
acquired within the transverse relaxation time. (Stehling et al.,
Science, 250, 53-60, (1990)). P. Mansfield, "Spatial Mapping of the
Chemical Shift in NMR", J. of Physics D: Applied Physics, vol. 16
(1983), pp. L235-L238, discusses the application of echo-planar NMR
imaging methods to the mapping of chemical shift data spectra. This
technique, as noted above, requires equipment modification in order
to rapidly reverse the gradient field.
It would be advantageous, however, to have a technique for
acquiring NMR information including both spatial and spectral
information from a series of echoes without the need for repeated
reversal of a magnetic field gradient, which is relatively
difficult to achieve in practice. It would also be advantageous to
have such a technique which could obtain greater spatial and
spectral resolution. Further, since the spectral bandwidth for
chemical information is typically only 3-8 ppm, spectroscopic
techniques are very sensitive to the deleterious residual effects
of switched fields. Moreover, in known spin-echo methods the
readout gradient typically has a short duration, thereby limiting
spectral resolution. Thus, it would be advantageous to have a
spectral encoding technique which does not require a readout
gradient.
U.S. Pat. No. 4,628,262 to Maudsley, which is herein incorporated
by reference, discloses a method for acquiring a data set for
generating a spectroscopic image by generating multiple spin-echoes
per excitation pulse by using a series of 180.degree. refocussing
pulses. After slice selection, a series of refocussing pulses, each
followed by a readout gradient (i.e., G.sub.x), is provided with
the timing between the refocussing pulse and the readout gradient
controlled to produce spectral encoding. Each combination of a
refocussing pulse and a readout gradient induces a spin echo
signal, and the readout gradient controls the time of occurrence of
spin echo signal. Spectral encoding is achieved by displacing the
time between at least one refocussing pulse and the readout
gradient, thereby phase encoding the spectral information.
Evidently, the pulse sequence which generates the spin echo signal
also encodes the spectral information. For a given observed
bandwidth, the delay interval, dt, must satisfy the Nyquist
sampling theorem, and increased spectral resolution is achieved by
increasing the number of spin echoes for a given phase encoding
gradient G.sub.y.
There remains, however, a need for further improvements in
spectroscopic NMR techniques.
SUMMARY OF THE INVENTION
The present invention provides a method for acquiring a data set
for generating spectroscopic images which is not limited by the
disadvantages of the prior art. The invention further provides a
method for acquiring spectroscopic information which has high
efficiency and high signal-to-noise ratio per unit time. The
invention involves the application of a pulse sequence to a
conventional magnetic resonance imaging (MRI) apparatus in order to
rapidly acquire data for generating spectroscopic images. For each
90 degree excitation pulse that is applied, a plurality of spin
echoes are generated by a respective plurality of 180 degree
refocussing pulses, and spectroscopic information is directly
encoded in the spin echo signals. A pulse sequence for practicing
the present invention comprises the steps of: slice selection by
the combination of a selective 90 degree RF pulse and a magnetic
field gradient, followed by applying a series of refocussing 180
degree RF pulses for stimulating a respective series of spin
echoes. Each spin echo is phase encoded according to a series of
gradients applied in mutually orthogonal directions, (and
orthogonal to the slice selection magnetic field gradient), for
encoding according to the location of the atoms in the selected
slice. A spectroscopic image is obtained by performing a Fourier
transform of the acquired spin echo data over k-space, wherein the
spectroscopic information is directly encoded in the time domain
spin echo signals. Another embodiment of the present invention
produces compound weighted spectroscopic images by selecting the
period between refocussing pulses according to the coupling
constant of a group contained in the compound; thereby, the signal
of the selected compounds modulate with a known frequency different
for compounds with different coupling constants.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in greater detail below by way of
reference to the accompanying drawings, wherein:
FIG. 1 is a prior art pulse sequence timing diagram for acquiring
spectroscopic information;
FIG. 2 is another prior art pulse sequence timing diagram for
acquiring spectroscopic information;
FIG. 3 is a pulse sequence for acquiring spectroscopic information,
according to the present invention;
FIG. 4A is a pulse sequence for acquiring spectroscopic
information, according to an example of the present invention.
FIG. 4B is a pulse sequence for outer volume suppression used in
conjunction with the pulse sequence of FIG. 4A, according to an
example of the present invention;
FIG. 4C shows schematically the k-space scanning trajectory
according to the phase encoding sequence used in FIG. 4A;
FIG. 5A shows magnetic resonance images shows magnetic resonance
images obtained for an example according to the present
invention;
FIG. 5B shows an array of spectra, obtained for an example
according to the present invention; and
FIG. 5C a spectrum from the array of spectra of FIG. 5B, obtained
for an example according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the accompanying drawings, wherein like reference
characters refer to like parts throughout the various views, there
are shown in FIGS. 1-8 the preferred embodiments of the method for
acquiring NMR information, according to the present invention. All
references cited hereinabove and hereinbelow are incorporated by
reference.
FIG. 3 shows one embodiment of the pulse and gradient switching
sequence of the present invention. The process of acquiring
spectroscopic imaging data typically includes several distinct
intervals in which specific magnetic fields are applied to the
object being imaged. During the first of these intervals,
designated interval 1, a group of atoms is excited. A slice
selecting gradient field G.sub.(slice) is applied along the z axis
during interval 1, and a selective 90 degree excitation pulse is
applied during G.sub.(slice), causing the atoms in a selected slice
of the object to be excited. From this excited state, the atoms
begin to decay according to the well-known time constants T.sub.1
(spin-lattice relaxation time) and T.sub.2 (spin-spin relaxation
time). During the initial decay, also known as the free induction
decay (FID), a negative gradient is applied along the z axis to
reverse the dephasing caused by the gradient G.sub.z after the
center of the excitation pulse applied during interval 1.
During interval 2, a 180 degree refocussing pulse is applied for
refocussing the excited spins to induce the spin echo shown in
interval 3. In conjunction with the refocussing pulse, a slice
selection gradient is applied to select approximately the same
slice as in interval 1.
During interval 3, first spatial encoding gradients (i.e.,
G.sub.(phase1) and G.sub.(phase 2) may be applied, as shown, along
the x and y axes, in order to phase encode the spatial information
of the selected slice (i.e., sample a point of k-space). The
induced spin-echo is then acquired without the application of any
further gradient fields. Thus, the spin-echo is directly encoded
with spectroscopic information according to the chemical shifts
found in the selected slice. Then, in interval 4, gradients may be
applied along the x and y directions with an integrated intensity
equal to the previously applied phase encode gradients, but in
opposite direction, in order to cancel (i.e., rewind) the
phase.
In accordance with the present invention, as shown in FIG. 3, after
the first spin-echo is acquired, at least 0 one other refocussing
pulse is applied for inducing at least one additional spin-echo.
Each spin-echo is associated with phase-encode gradients.
Preferably, in order to maximize the data acquisition rate each
spin-echo samples a separate point of k-space; however, more than
one spin-echo can sample the same k-space point, for example, to
increase the signal-to-noise ratio.
Ancillary pulse sequences (i.e., shaded gradient pulses) are
applied for dephasing undesired magnetization and preventing
unwanted stimulated echoes or gradient echoes.
In sum, multiple spin echo signals are acquired for each excitation
pulse, wherein the spectroscopic information is directly encoded in
the spin echo signals, thereby increasing the spin echo acquisition
rate. Once a complete data set is acquired, a three-dimensional
Fourier transform yields a spectroscopic image for the selected
slice.
The number of spin echoes that may be induced after a given
excitation pulse is limited by the spin-spin relaxation time
T.sub.2, which varies for different media. In practice, the
frequency of the refocussing pulses is dependent on the field
homogeneity of the medium. For instance, a large field homogeneity
increases the dephasing of the excited spins, and thus, the
refocussing pulse frequency should be increased. It is not
necessary to wait for the spin-echo signal to decay completely
before applying subsequent refocussing pulses. In fact, a
repetition schemed with a period of approximately one T2* value or
less may be employed.
In a further embodiment of the present invention, the pulse
sequence of FIG. 3 is adapted to acquire a specific compound
weighted signal based on the property that the phase of acquired
signal will modulate when using a period between refocusing pulses
that is determined by the coupling constant of a particular group
which makes up the compound. For example, this embodiment has been
used to separate fat signals from lactate signals, yielding a
lactate weighted signal, by using a period of 136 msec between
refocussing pulses which enhances the sensitivity to lactate
compounds. For the lactate, the 136 msec period is based on the
coupling constant of the lactate methyl group.
The following example is presented to illustrate features and
characteristics of the present invention which is not to be
construed as being limited thereto.
Example
Measurements were performed on a normal volunteer using a
conventional 1.5 T GE/SIGNA whole-body scanner (GE Medical Systems,
Milwaukee) with 10 mT/m actively shielded gradients and a standard
quadrature head coil. The multi-spin-echo sequence consisted (FIGS.
4A and 4B) of three parts: a chemical shift selective saturation
(CHESS) sequence for water suppression, a multi-slice outer volume
suppression (OVS), and a multi spin-echo spectroscopic imaging (SI)
sequence in accordance with the present invention. The CHESS
sequence was identical as used in a single-echo method reported by
J. H. Duyn, et al., Radiology, (July 1993), which is expressly
incorporated by reference.
The OVS sequence (FIG. 4B) suppresses water and lipid signals from
octagonally grouped slices around the VOI, using 8 slice-selective
RF pulses (.alpha.1-.alpha.8). The pulses re combined in two groups
of four. The first group suppresses slices left, right, anterior
and posterior with respect to the VOI and is followed by a 6 ms
gradient crusher. The second group of four pulses suppresses the
diagonal oriented slices and is followed by an 8 ms gradient
crusher. The crushers have the specific orientation indicated in
the diagram in order to minimize creation of coherences. The
amplitudes of the RF pulses are fine-tuned to compensate for T1
relaxation. A delay of 6 msec was appended to minimize the effects
of short term eddy currents on the spin-echo sequence.
The multi spin-echo sequence acquired 4 spin-echoes at equidistant
intervals of 148 ms, the first one occurring at 200 ms. In order to
position the first echo at the desired echo time, an additional
180.degree. RF pulse was used. Thus, one slice selective 90.degree.
and five slice selective 180.degree. RF pulses were used. The
bandwidth of the pulses was 2 kHz. The 180.degree. pulses selected
a 10% thicker slice than the 90.degree. RF, resulting in an
improved slice profile. The resulting slice profile had a full
width at half maximum of 13 mm. A 7 mm slice gap was maintained
between adjacent slices. The 180.degree. RF pulses were flanked by
4 ms gradient crusher pulses in x, y, and z direction, the x and y
components being each of 10 mT/m strength. The strength of the
z-crusher was varied between -9, -3, 3, 9, and -6 mT/m on
subsequent crusher pairs in order to suppress stimulated echoes and
higher order spin-echoes. Each echo signal was individually phase
encoded, applying phase encoding gradients after each 180.degree.
RF pulse and using rewinders were to minimize artifacts. A 128 ms
echo was symmetrically acquired using a spectral width of 2000 Hz.
A 32.times.32 circular k-space sampling scheme was applied, using a
24 cm.times.24 cm field of view (FOV). FIG. 4C shows schematically
how k-space was scanned by phase encoding each of the spin-echoes.
The center of k-space was scanned using the first echo, subsequent
echoes scanned higher k-space points. The particular scanning
scheme resulted in an effective echo time of 200 ms. During each
experiment data during one TR period was acquired without phase
encoding gradients in order to provide correction factors for
signal losses due to T2 relaxation and to RF pulse angle and slice
profile imperfections. Three slices were acquired within a total TR
of 2700 ms. The total measurement time was 9 minutes.
Prior to the HSI experiment, axial gradient-echo (GRASS) images
were obtained with TE=30 ms, TR=600 ms, to locate the skull/brain
interface. On the basis of these images, the VOI was selected and
the OVS pulses were positioned. Localized shimming was then
performed over the VOI and the RF amplifier output was adjusted
using a single spin-echo sequence. The RF pulse amplitude of the
CHESS water suppression pulse was adjusted using the multi-echo
sequence, by minimizing water signals in the first echo (TE=200
ms). Then the multi-echo HSI experiment was started, and upon
completion, a series of axial GRASS MR images was obtained with
TE=30 ms, TR=60 ms, using a slice thickness and location
corresponding with the HSI images. The total measurement time,
including GRASS MRI scans, was 20 minutes. For comparison, a
standard single-echo multi-slice acquisition was performed on the
same volunteer, with TE=272 ms and slightly modified amplitudes of
CHESS and OVS RF pulses. A TR of 2200 ms was chosen, resulting in
an amount of T.sub.1 weighting similar as in the multi-echo
experiment. The total measurement time for the single-echo
experiment was 27 minutes.
Data processing was performed off-line on a SUN-SPARC workstation
using IDL.TM. software (Research Systems, Boulder, Colo.). Each
slice was processed separately.
For the multi-echo experiment, the four echoes of the reference
acquisition without phase encoding gradients were apodized with a
10% Hamming filter and Fourier transformed, resulting in total
volume spectra acquired by each of the four echoes. The magnitude
signals of the NAA resonance were used to calculate correction
factors for signal losses due to T.sub.2 relaxation and
imperfections in RF pulse angle and slice profiles. Signal losses
during each 180.degree. RF pulse were in the order of a few
percent, and the correction for the combined effects of relaxation
and RF pulse angle correspond to an exponential decay constant of
400 ms. The correction factors were applied to data of the phase
encoded experiment, after which a 10% Hamming filter was applied in
spectral domain, and a radial cosine filter in spatial domain
(two-dimensional filter). The data matrix was then zero-filled
eight times in spectral domain, and Fourier transformed in all
three dimensions. The resulting effective in-plane spatial
resolution was 1.3 cm, as calculated from the contour of the
2-dimensional point spread function (PSF) at half maximum height,
and included effects of the circular k-space sampling scheme.
Spectra were corrected for B.sub.o inhomogeneities by referencing
to the position of NAA. In case of NAA being below an optional
threshold, its position was copied from neighbouring voxels.
Metabolite images of NAA, and choline +creatine were created by
integrating the spectra between 1.9 and 2.1 and between 2.9 ppm and
3.3 ppm frequency regions respectively.
For the standard single-echo multi-slice experiment, a cosine
filter was applied in spectral domain, whereas the spatial domain
was filtered identically as the multi-echo data set. Also
peak-picking and integration were performed in an identical fashion
as with the multi-echo data set.
The results of the multi-echo multi-slice experiment are summarized
in FIGS. 5A, 5B, and 5C.
FIG. 5A shows, for each of the three slices studied, the
spectroscopic images of NAA and total choline total creatine,
together with the corresponding GRASS images. Clearly recognizable
in the two most inferior slices (top rows) are the ventricular
spaces. The bright intensities in the NAA images are insufficiently
suppressed signals of lipid from the skull area, which occasionally
bleed into brain regions (e.g. slice 1). The bright intensities in
the choline+creatine images from top and bottom slices are caused
by remaining water signal.
The SNR in the images of the multi-echo experiment varies between
15 and 20 for the NAA images and between 10 and 15 for
choline+creatine images. The spectral quality of the multi-echo
experiment is demonstrated in FIG. 5B, which shows an array of
spectra extracted from a region just left of the ventricles in
slice 2. An individual spectrum is taken from this array (second
from left in top row) and displayed in FIG. 5C.
The example presented above demonstrates the advantages of fast
spectroscopic imaging using the multi-spin-echo method. In the
following a few other aspects of the multi-echo technique are
discussed.
An important design criterion in the multi-echo experiment is the
number of echoes. A larger number of echoes gives a potentially
higher efficiency. Since the total acquisition time (echo duration
x number of echoes) is limited by T.sub.2, an increased number of
echoes is accompanied by a reduced echo duration. Reducing the echo
duration below T.sub.2 * (over the voxel), will lead to a reduction
in spectral resolution, and only a minimal increase in efficiency.
Therefore, the optimum number of echoes, with respect to efficiency
and spectral resolution, depends on the ratio T.sub.2
*/T.sub.2.
During initial tests of the multi-echo experiment on phantoms
artifacts were seen associated with transitions in the signal phase
going from one segment in k-space to the next. Most of these
artifacts could be explained by eddy current induced residual
gradients and B.sub.o shifts, originating from CHESS and OVS
gradient crushers, and resulting in an accumulated signal phase at
the center of the first acquisition period. This additional phase
is inverted in subsequent echoes, leading to the artifacts. For
example a net accumulation of 30.degree. phase in the interval
between the 90.degree. RF pulse and the first acquisition interval,
due to a B.sub.0 shift, leads to significant distortion of the PSF.
A method used to minimize phase distortions in the echo signals,
was to introduce a delay of 6 ms between OVS and spin-echo
sequence. Furthermore, the amplitudes of crusher gradients after
the 180.degree. RF pulses were fine-tuned to minimize net dephasing
at the centers of the acquisition intervals. This was done once for
a specific setting of timing variables (TR, delays after CHESS and
OVS, TEs) and gradient strengths (crushers, slice selection
gradients). Remaining phase differences between echoes were
negligible and cause no visible artifacts.
During data processing, each echo was multiplied by a correction
factor, to compensate for signal losses due to T.sub.2 relaxation
and RF pulse imperfections. The correction factor used in our
experiments was determined by measuring the NAA signal decay over
the four echoes, and corresponded to a T.sub.2 of 400 ms. Reports
have shown that in normal brain T.sub.2 relaxation times of
choline, creatine and NAA vary between 200 and 400 ms. Since
k-space is scanned in a segmented fashion, inappropriate correction
for T.sub.2 relaxation effects results in distortion of the spatial
PSF. The observed PSF for T.sub.2 's between 200 and 800 ms is very
similar, and suggests the adequacy of a single correction factor
for choline, creatine and NAA. In using this correction, components
of lipid resonances with short T.sub.2 (T.sub.2 <50 ms,), have a
much broadened PSF with increased sidelobe amplitudes.
As described hereinabove, as a further embodiment of the present
invention, lactate weighted imaging may be performed. The lactate
signal originates from a coupled methyl doublet, with a phase
varying with echo time. In order to measure the doublet in phase,
the echo times have to be multiples of 136 ms. For lactate to be
measured without localization errors, subsequent echo times have to
be separated a multiple of 272 ms. An experiment with echo times of
136, 408, 680 and 952 ms is also possible since lactate has a
T.sub.2 of around 1200 ms. However, in the later echoes, the other
metabolites of interest will be attenuated significantly due to
their much shorter T.sub.2 values, and will therefore likely have a
broadened PSF.
In the experiment discussed above, the increased efficiency of the
multi-echo technique was used to reduce the measurement time.
Alternatively, the increased efficiency could be used to create
T.sub.2 maps (by scanning all of k-space with each echo), or to
perform a full 3D experiment. For example an experiment with
20.times.20.times.12 encoding steps (cylindrical k-space sampling)
at TR=2 s will take 30 minutes. Using a 20 cm.times.20 cm.times.12
cm an isotropic (nominal) resolution of 1 cm.times.1 cm.times.1 cm
is achieved.
Thus, as illustrated through the preferred embodiment and the
foregoing examples, and as understood by further practicing the
present invention, many advantages are provided by the present
invention. Data for generating high resolution spectroscopic images
may be acquired rapidly. Since spectral information is directly
encoded in the spin-echoes, spectral resolution is determined by
the echo sampling period. K-space sampling, and concomitantly
spatial resolution may be independently controlled and determined
by phase encoding gradients G.sub.x and G.sub.y. The rate of data
acquisition may be increased by increasing the refocussing pulse
repetition frequency or increasing the number of spin echoes per
excitation pulse. Further, no readout gradients are employed. The
method also provides for high efficiency and high SNR per unit time
by measuring the signal over a very large part of the T2 decay
curve.
Although the above description provides many specificities, these
enabling details should not be construed as limiting the scope of
the invention, and it will be readily understood by those persons
skilled in the art that the present invention is susceptible to
many modifications, adaptations, and equivalent implementations
without departing from this scope. For example, in the above
illustrative example, the increased efficiency of the multiple
spin-echo techniques was used for reducing the measurement time.
Alternatively, or in combination, the increased efficiency could be
used to create T2 maps by scanning all of k-space for each echo
time, to acquire a separate part of phase-encode space using
additive phase-encode gradients or phase encode rewinders, to
increase the signal-to-noise ratio, or to perform a full
three-dimensional experiment. For example, as described above, an
experiment with 20.times.20.times.12 encoding steps (i.e.,
cylindrical k-space sampling) at TR=2 seconds will take 30 minutes.
Using a 20 cm.times.20 cm.times.12 cm volume, an isotropic
(nominal) resolution of 1 cm.times.1 cm.times.1 cm is achieved.
Moreover, as can be appreciated from the above illustrative pulse
sequence and examples, the present invention may be practiced in
conjunction with multi-slice spectroscopic imaging methods, with
three-dimensional spectroscopic imaging (e.g., by applying
phase-encoding gradients in the slice direction as well), with
outer volume suppression techniques, or with time-domain and
frequency domain fitting to derive concentrations of various
molecules. Further, many variations can be employed for ordering
the phase-encoding.
These and other changes can be made without departing from the
spirit and the scope of the invention and without diminishing its
attendant advantages. It is therefore intended that the present
invention is not limited to the disclosed embodiments but should be
defined in accordance with the claims which follow.
* * * * *