Method and system for multidimensional localization and for rapid magnetic resonance spectroscopic imaging

A method and system for providing prelocalization of a volume of interest and for rapidly acquiring a data set for generating spectroscopic images. Spatial prelocalization of a volume of interest is achieved by providing a presuppression sequence before a stimulated echo (STE) sequence and a suppression sequence during the mixing time (TM) interval of the STE sequence. The presuppression sequence includes a spatial suppression sequence to selectively saturate slices that intersect the plane selected by the STE sequence in order to define a boundary for the volume of interest, and this spatial suppression sequence is substantially repeated during the TM interval of the STE sequence. Spectroscopic imaging data is acquired by an oversampled echo planar spatial-spectral imaging sequence in which the gradient reversal frequency is a integer factor of n greater than the gradient reversal frequency required to sample the spectral width.

TECHNICAL FIELD 
The present invention relates generally to the acquisition of nuclear 
magnetic resonance data and, more particularly, to a method for spatial 
localization and for rapidly acquiring magnetic resonance information 
either for magnetic resonance spectroscopy (MRS) or magnetic resonance 
imaging (MRI). 
BACKGROUND OF THE INVENTION 
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, thus providing 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 
the brain have shown locally altered metabolite levels in different 
pathologies, including brain tumors, multiple sclerosis, chronic and acute 
brain infraction, epilepsy and acquired immunodeficiency syndrome. 
HSI requires spatial prelocalization of a volume of interest (VOI) to 
suppress overwhelming water and fat signals from superficial structures 
(bone, muscle, skin) which may lead to spectral artifacts in the region of 
interest. This prelocalization may be done by selective excitation of a 
rectangular volume using three-pulse localization schemes (e.g., using 
stimulated echo methods "STEAM"), by spatial suppression of outside 
volumes or by a combination of both. The degree of prelocalization with 
these methods is limited by imperfections in the field homogeneity of the 
radiofrequency coil, by intrinsic static magnetic field inhomogeneities in 
vivo, by relaxation processes and by limitations in gradient power. To 
improve the prelocalization a combination of selective excitation of the 
volume of interest and presaturatton of outside volumes has been proposed. 
However, this approach is limited to the selection of rectangular volumes 
which do not follow the contours of organs such as the brain and is motion 
sensitive due to the application of strong gradient dephasing pulses for 
the STEAM localization sequence. To obtain a more flexible volume 
preselection a different technique has been proposed recently where 
selective excitation with a spin echo pulse sequence is used in one 
dimension and spatial presaturation of peripheral regions is used in the 
other dimensions. However, with this approach the degree of volumes 
prelocalization is limited, since spatial presaturation in vivo provides 
suppression factors of less than 100 (under favorable conditions) which 
leaves strong residual water and fat signals from peripheral regions. The 
technique has thus been used only at long echo times (272 ms) where water 
and fat signals strongly reduced as compared to metabolite signals due to 
their shorter transverse relaxation times and J-coupling. However, many 
metabolite signals also suffer strong signal losses at long echo times for 
the same reasons, and further, additional information is available at 
short echo times that is not present at long echo times. Concomitantly, 
most prior art localization techniques are not applicable to acquiring 
multiple volume data from nuclei that have a short T.sub.2 relaxation 
time, such as the phosphorus-31 (31P) nucleus or the sodium-23 (23Na) 
nucleus. Similarly, 31P and 23Na MRI and spectroscopic imaging provide 
additional information not found in HSI. 
Thus, there is a need for improvements in techniques for prelocalizing a 
volume of interest, and preferably such improved techniques should render 
short echo time spectroscopic imaging practicable in order to elucidate 
additional spectral information which is not available at long echo times. 
Another inherent requirement of spectroscopic imaging is that the pulse 
sequence for acquiring data encodes spectral information in addition to 
spatial information. This requirement presents additional problems and 
challenges in order to make spectroscopic imaging practicable. Some of the 
problems that must be addressed include: minimizing the deleterious 
coupling or overlap effects between the spectral encoding and the spatial 
encoding; minimizing the time required for acquiring the spectral and 
spatial information; increasing the signal-to-noise ratio; and increasing 
the spatial resolution of the spectral information. It is well recognized 
that many of these problems are related in a fundamental and/or practical 
manner, and thus, improving a given parameter may result in, or require, 
compromising another parameter. Indeed, however, one of the foremost 
limitations to clinical application of HSI is the length of time needed 
for data acquisition in order to provide images with sufficient spatial 
and spectral resolution. Particularly in the case of severely ill or 
instable patients, such study lengths are prohibitive. 
Recent technical developments have sought to reduce the generally long 
acquisition times necessary for spectroscopic imaging. Three-dimensional 
phase encoding is desirable, since it yields complete volume coverage, 
permits thin slices and avoids chemical shift artifacts. However, phase 
encoding is very rime consuming. Multislice techniques have been 
introduced as an alternative to reduce data acquisition times, but the 
number of slices with these techniques is limited due to the long data 
acquisition window. More recently, shorter acquisition times have been 
achieved by acquiring multiple individually phase encoded echoes during a 
single excitation. This method increases the signal-to-noise per unit time 
and unit volume, but introduces variable T2-weighting in k-space and is 
not compatible with short echo time acquisitions. Alternative approaches 
using fast imaging techniques with a Dixon-type echo time shifting to 
encode spectral information have been shown to be feasible, albeit at the 
expense of spectral resolution. In sum, these methods are not suitable for 
short echo time acquisitions. 
Echo-planar spectroscopic imaging (EPSI), a much faster method proposed by 
Mansfield, (P. Mansfield, "Spatial Mapping of the Chemical Shift in NMR", 
J. of Physics D: Applied Physics, vol. 16 (1983), pp. L235-L238), and 
further developed by others, avoids these limitations by using a series of 
periodically inverted gradients to generate a train of echoes which 
contain both spatial and spectral information, thus permitting complete 
three-dimensional spatial encoding in a clinically reasonable time frame. 
EPSI, however, requires strong fast switching gradients with excellent 
eddy current performance. Further, due to the inherent convolution of the 
spatial and spectral information, spectral aliasing artifacts and 
localization constraints have precluded applications beyond initial 
feasibility studies. 
Therefore, there is also a need for improved methods for acquiring 
spectroscopic data rapidly, without loss in spectral resolution or spatial 
localization. 
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 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. Spatial 
prelocalization of a volume of interest is achieved by providing a 
presuppression sequence before a stimulated echo (STE) sequence and a 
suppression sequence during the mixing time (TM) interval of the STE 
sequence. Preferably, the STE sequence includes three substantially 
90.degree. RF pulses each applied in combination with a respective 
gradient magnetic field to select the same slice in a desired orientation. 
The presuppression sequence includes a spatial suppression sequence to 
selectively saturate slices not in the same plane as the STE slice in 
order to define a boundary for the volume of interest, and this spatial 
suppression sequence is substantially repeated during the TM interval of 
the STE sequence. Preferably, the presuppression and the suppression 
sequences each include a chemical shift saturation for water suppression. 
Also in accordance with the present invention, method for acquiring 
spectroscopic imaging data is based on an oversampled echo planar 
spatial-spectral imaging sequence. In order to sample a predetermined 
spectral width in individual spectra, a gradient reversal frequency that 
is an integer factor of n greater than the gradient reversal frequency 
required to sample the spectral width is used. Spatial resolution and 
field of view is maintained by adjusting the gradient magnetic field 
strength, specifically requiring an n-fold increase in gradient magnetic 
field strength as compared to a prior art echo planar spectroscopic 
imaging sequence that does not include oversampling (i.e., n=1). For 
three-dimensional imaging, the other two spatial dimensions are encoded by 
conventional phase encoding. The n series of positive and negative echoes 
are reconstructed separately and then combined.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Before consideration in detail of the present invention, a brief overview 
of a conventional MRI system is in order. In particular, FIG. 1 shows in 
block diagram form an in vivo NMR imaging system which is capable of 
receiving a patient 14. The system includes a magnet 12 for generating a 
large static magnetic field. The magnet is sufficiently large to have a 
bore into which a patient 14 can fit. The patient 14 is positioned and the 
magnetic field is generated by a magnetic field generator indicated at 13 
by block B.sub.o. RF pulses are generated utilizing RF generator 16, and 
the RF pulses are shaped using modulator 17. The shape of a modulated 
pulse could be any predetermined shape, and for example may be Gaussian or 
Sinc (i.e., sin(bt)/bt, where b is a constant, and t is time). Shaped 
pulses are usually employed in order to shape and limit the bandwidth of 
the pulse, thereby restricting excitation by the RF pulse to spins that 
have Larmor frequencies within the RF pulse bandwidth. A RF pulse signal 
is transmitted to coils in the magnet assembly which are not shown. The 
coils may be surface coils, body coils, limb coils or head coils, for 
example. The duration and amplitude of the RF pulse determine the amount 
which the net magnetization is "tipped". As will be described below, in a 
preferred embodiment of the invention tip angles of substantially 
90.degree. are employed for a stimulated echo pulse sequence. 
Gradient generators 25, 20, and 19, which include respective gradient 
coils, produce the G.sub.x, G.sub.y, and G.sub.z magnetic fields in the 
direction of the polarizing magnetic field B.sub.o, but with gradients 
directed in the x, y, and z directions, respectively. The use of the 
G.sub.x, G.sub.y, and G.sub.z magnetic field gradients is well known in 
the art, including such uses as dephasing or rephasing excited spins, 
spatial phase encoding or spatial gradient encoding acquired signals, and 
spatial encoding of the Larmor frequency of nuclei for slice selection. 
Induced nuclear magnetic resonance signals are detected by receiver coils 
in the magnet which are not shown. The receiver coils and the transmitter 
coils may be the same, with a transmit/receive (T/R) switch being used to 
select transmission or reception of radio frequency signals to or from the 
coils, respectively. The received signal is demodulated by demodulator 27, 
and the demodulated signal is amplified and processed in the 
analog-to-digital processing unit 28 to provide data as indicated at 29. 
The entire process is monitored and controlled by the processor 31 which, 
according to the functional block diagram of FIG. 1 and to components 
found in known commercial or experimental systems that are used to control 
and monitor the entire process, includes components necessary to control 
the timing, amplitudes and shapes of the control signals for the various 
elements of the MRI system and typically includes programming, computing, 
and interfacing means. 
In FIG. 2A, a three-dimensional slice (or slab) 38, represents a 
three-dimensional section of the patient 14. It is an aim of the system 
and method described herein to provide spatial prelocalization of a region 
or volume of interest (VOI), such as VOI 36, within the slice 38. 
Moreover, the system and method as will be described is especially 
designed to obtain spectroscopic data for multiple localized regions 
within the VOI 36. The volume of interest, the boundaries of which are 
defined by a process herein referred to as "prelocalization", is typically 
a slice or slab, and a localized region, may be either a two-dimensionally 
localized region or a three-dimensionally localized volume element. 
Referring now to FIG. 3, a pulse sequence in accordance with an embodiment 
of the present invention is shown. The pulse sequence, which is further 
described hereinbelow, consists essentially of three parts: a 
spectral-spatial presuppression sequence, a basic three-pulse stimulated 
echo (STE) sequence which includes a spectral-spatial suppression sequence 
during the TM interval, and an echo-planar spatial-spectral encoding 
gradient sequence. 
STIMULATED ECHO (STE) LOCALIZATION 
In accordance with the present invention, spatial prelocalization of a VOI 
is provided by a spectral-spatial presuppression sequence and a basic 
three-pulse STE sequence which includes a spectral-spatial suppression 
sequence during the T.sub.m interval. 
A basic three-pulse STE sequence, comprising substantially 90.degree. RF 
pulses P1, P2, and P3 results in the generation of an echo signal delayed 
after pulse P3 by a time interval approximately equal to the time interval 
between pulse P1 and pulse F2 (i.e., TE/2). The interval between P2 and P3 
is referred to as the mixing time interval or TM interval. In an 
embodiment of the present invention, each pulse P1, P2, and P3 is a 
frequency selective (e.g., shaped) pulse, and a magnetic gradient field is 
applied in conjunction with each 90.degree. RF pulse in order to select a 
slice or slab oriented in a plane of preferred orientation. Although any 
spatial orientation of the plane is possible, for purposes of discussion 
it is assumed that the STE selected slice is oriented in a plane that is 
perpendicular to the direction of the applied magnetic gradient field. 
Preferably, in accordance with an embodiment of the present invention, the 
same slice is selected by each slice selection combination of a frequency 
selective RF pulse and an applied gradient magnetic field, and 
accordingly, in the pulse sequence of FIG. 3 G.sub.x1, G.sub.x2, and 
G.sub.x3 are all applied in the z-direction. It is noted that during the 
TM interval, the spins excited by P1 and P1 within the VOI are directed 
along the z axis, and therefore, the spin-spin interaction, characterized 
by the T.sub.2 relaxation time, is in abeyance. 
Prior to application of the basic STE pulse sequence, a presuppression 
sequence is applied. Preferably, the presuppression sequence includes a 
water suppression sequence, shown as interval WS1, and a spatial 
suppression sequence, shown as interval SS1. Interval WS1 includes a 
chemical shift selective (CHESS) excitation pulse CS1 for water 
saturation, which is preferably followed by one or more gradients, shown 
as DX1, DY1, and DZ1, to dephase the spins excited by the pulse CS1. 
According to known methods, the pulse CS1 is specifically shaped or 
modulated to excite only a narrow frequency band around the water 
resonance, and its amplitude is sufficient to cause saturation. 
The presuppression sequence also includes a spatial suppression interval 
SS1 during which time n slices are selected for suppression, where n is 
generally any integer value. Preferably, these suppression pulses are 
directed to lipid suppression. As shown schematically in FIG. 3, a 
suppression slice is selected by applying a shaped RF pulse V.sub.n in 
conjunction with a linear combination of orthogonal gradient magnetic 
fields G.sub.nx, G.sub.ny, G.sub.nz, followed by an appropriate dephasing 
gradient D.sub.nx, D.sub.ny, D.sub.nz. Preferably, for localization of a 
VOI such as VOI 36 of FIG. 2A, a series of selective suppression pulses 
are generated, each of which suppresses a different slice positioned along 
the periphery of the VOI and orthogonal to the slice to be selected by the 
STE sequence, the overlap of the selective suppression pulses encompassing 
the VOI. The intersection of suppression slices 21 to encompass the VOI 36 
is schematically shown in FIG. 2B. 
After the presuppression sequence, as described above all three STE pulses 
are available to select the same slice in the third dimension. In 
accordance with the present invention, the series of suppression pulses in 
SS1 are repeated in the SS2 interval during the TM period, thereby 
providing marked improvement in suppression. Preferably, the chemical 
shift selective excitation pulse sequence is also repeated during the TM 
period (i.e., WS2) to enhance water suppression. It is emphasized that the 
specific gradient, dephasing, and RF pulses shown in interval SS1 (as well 
as SS2) are merely schematic, and that actual pulse sequences during the 
SS1 (and SS2) interval will depend on many factors including the desired 
number and orientation of the suppression slices, as well as the preferred 
suppression technique. 
It is understood that in accordance with the multidimensional spatial 
suppression sequence, any number of spatial suppression pulses may be 
employed to shape the volume of interest with great flexibility. The 
suppression slices need not be limited to being orthogonal to the slice 
selected by the STE pulses. For instance, suppression slices may be 
selected parallel to, and above and below, the STE pulse selected slice in 
order to enclose the volume of interest in all dimensions. As a further 
example of a prior art method for selecting the suppression slices, two 
90.degree. pulses having different frequency spectra may be applied 
simultaneously during the application of a given gradient magnetic field 
in order to suppress signals from parallel slices separated according to 
the respective frequency spectrums of the two 90.degree. pulses. It is 
further understood that each suppression sequence (i.e., WS1:SS1 and 
WS2:SS2) may include any known spectral and/or spatial suppression 
techniques. 
It can also be appreciated that the pulse sequences employed during the WS1 
and SS1 intervals need not be identical to those during the WS2 and SS2 
intervals, respectively. For instance, in order to reduce losses in 
saturation due to T1 relaxation, it may be desirable to use broader band 
(i.e., "harder") RF pulses during the presuppression SS1 interval than 
during the SS2 interval in which the RF pulses would have greater 
frequency selectivity to better define the volume of interest. That is, 
because T1 relaxation may have a noticeable deleterious effect on the 
efficiency of presuppression, but has a negligible effect on the 
efficiency of suppression performed during the TM interval, for a given 
STE interval it may be preferable to reduce the time period over which T1 
decay occurs for the presuppression sequence by decreasing the duration of 
the presuppression sequence by using RF pulses of shorter duration. Of 
course, it is possible to use "harder" suppression pulses during the TM 
interval than during the presuppression interval to reduce the potential 
effects of coupling between spins on the spectra, which are more prominent 
at longer echo times. Another example of variations that may be desired is 
to include spatial suppression pulses to define the top and bottom regions 
of the selected slice (i.e., planes parallel to the selected slice) during 
the SS2 interval, but not to include these pulses during the SS1 interval. 
Or alternatively, several optimized chemical shift selective pulses may be 
employed during SS1, while only one chemical shift selective pulse may be 
employed during SS2. 
As may be appreciated by one skilled in the art, although in the embodiment 
shown in FIG. 3 the same slice is selected by each STE RF pulse and 
magnetic field gradient combination, the present invention may be 
practiced with variations of these particular pulse sequences. For 
instance, each STE RF pulse may be associated with an applied magnetic 
field gradient that is orthogonal to the other magnetic field gradients. 
Alternatively, no magnetic field gradient may be applied during the third 
RF pulse in order to avoid eddy current effects. It is also understood, 
however, that the absence of applied orthogonal magnetic field gradient 
components during the STE localization sequence avoids the deleterious 
effects due to magnetic gradient field misalignment or miscalibration, and 
that repeated selection of the same slice provides enhanced localization 
in the direction of the applied magnetic field gradient and reduces the 
need for additional outer-volume suppression or dephasing gradients for 
the non-selected region outside of the desired slice. That is, in contrast 
to conventional STEAM localization schemes, since the magnetization above 
and below the volume of interest is not touched, gradient dephasing 
requirements and motion sensitivity are reduced despite the elongated TM 
period for additional suppression. 
Due to the improved spatial prelocalization provided by the present 
invention, it is possible to use very short echo times without introducing 
localization errors. Thus, metabolites with strong J-modulation and short 
transverse relaxation times may be measured using a much more flexible 
volume prelocalization as compared to previous methodology. Due to the 
additional spatial presaturation pulses the mixing interval has to be 
elongated at expense of signal losses due to longitudinal relaxation and 
due to modulation of signal losses clue to longitudinal relaxation and due 
to modulation from J-coupling. However, these losses can be kept small 
with the use of short RF and gradient pulses and the use of short echo 
times. 
OVERSAMPLED ECHO-PLANAR SPECTROSCOPIC IMAGING (EPSI) 
In a preferred embodiment of the present invention, a sequence for 
obtaining data is an n-fold oversampled echo-planar spectroscopic imaging 
(EPSI) sequence for encoding both spatial and spectral information, shown 
generally in FIG. 3 as echo-planar sequence 99. The increased acquisition 
speed of EPSI directly results from the continuous simultaneous encoding 
of spectral and spatial information and thus, convolution of spectral and 
spatial information is inherent. 
Echo-planar spatial-spectral encoding according to the present invention 
may be described more specifically by referring to FIGS. 4A-4C. FIG. 4A 
shows a conventional echo planar spatial-spectral encoding sequence that 
encodes one dimension of k-space (e.g., all sampled values of k.sub.z) 
corresponding to one spatial dimension, and the full spectroscopic 
dimension for each excitation (e.g., echo). Each reversal of the gradient 
magnetic field G refocuses the echo signal, thus providing an echo train 
E1, E2, E3, E4, for example. The echo train contains spatial information 
encoded in the shape of each echo and spectral information encoded in the 
signal change from echo to echo (i.e., E1 to E2 to E3, etc.). It is 
understood that the desired spectral bandwidth, .beta., is determined by 
time period between echoes in the t.sub.s dimension which for the 
conventional EPSI sequence equals one-half the readout gradient pulse 
periodicity .DELTA., the spectral resolution is determined by the number 
of echoes in the echo train, the spatial resolution is determined by the 
time integral of the magnetic field gradient over the time period between 
gradient reversals (i.e., the maximum spatial frequency), and the field of 
view is determined by the sampling frequency for acquiring a given echo. 
FIG. 5 illustrates the k-space encoding scheme for the conventional EPSI 
readout gradient of FIG. 4A, in which the length of each gradient pulse, 
.tau., corresponds to the inverse of the spectral width in the 
reconstructed spectra. 
As discussed hereinabove, chemical shift evolution during the acquisition 
of a single echo and static magnetic field inhomogeneities distort the 
localization. Echoes are time shifted depending on the strength and the 
orientation of these interactions, thereby introducing asymmetry in the 
echo train. This asymmetry can only be partly recovered during 
postprocessing by reversing the echoes acquired with negative gradients or 
by other correction schemes. More specifically, evolution in time 
convolves spectral and spatial information which leads to chemical shift 
artifacts. Also, reversal of the readout gradient in the presence of local 
magnetic field inhomogeneities and asymmetries in gradient switching 
introduce periodicities in k-space which lead to aliasing artifacts. A 
prior art technique for removing aliasing is based on separating (editing) 
the echoes obtained with positive and negative gradients at the expense of 
reducing the spectral width to 1/2.tau., where .tau. is the time duration 
of a readout gradient pulse. 
In accordance with the present invention, and as will be further understood 
hereinbelow, in order to simultaneously reduce chemical shift artifacts 
and eliminate aliasing while retaining the desired spectral width 
(1/.tau.), an EPSI sequence is employed with n-fold oversampling in the 
time domain. The n-fold oversampling comprises an n-fold increase in the 
read out gradient strength, and an n-fold increase in the read out 
gradient frequency (i.e., maintaining a constant total time integral of 
the readout gradient pulse which corresponds to the spatial resolution). 
FIG. 4B and FIG. 4C illustrate a two-fold and a four-fold oversampled EPSI 
sequence, respectively, according to the present invention. FIG. 5 
illustrates the k-space trajectory as a function of time for a two-fold 
oversampled EPSI sequence, and a four-fold oversampled EPSI sequence, 
corresponding respectively to FIG. 4B and FIG. 4C, as compared to the 
trajectory corresponding to the above-described, original encoding scheme 
proposed by Mansfield, referenced hereinabove, where the length of each 
gradient pulse (.tau.=1/.beta.) corresponds to the inverse of the spectral 
width in the reconstructed spectra. 
Since, in accordance with the present invention, the read out gradient 
strength increases n-fold, the displacement effects of chemical shift and 
internal gradients are reduced n-fold. As illustrated in FIG. 4B and FIG. 
4C, the n series of positive and negative echoes are reconstructed 
separately (edited) to avoid ghosting artifacts from gradient reversal. 
Particularly, it is understood that the n-fold oversampling provides for 
use of the known technique of separately reconstructing positive and 
negative echoes to avoid ghosting artifacts from gradient reversal, but 
without sacrificing spectral width. 
It is noted that although due to the n-fold increase in the gradient 
reversal frequency the spectral width of the whole data set increases 
n-fold with a sqrt(n)-fold increase in noise, the signal to noise ratio is 
fully recovered by combining (e.g., averaging) the n series of localized 
spectra after reconstruction. This aspect of the echo editing is important 
since, as described above, the convolution of the spectral and spatial 
information is inherent to EPSI, and it may intuitively appear that 
reducing the chemical shift effects on the spatial encoding (i.e., 
improving localization) by increasing the gradient magnetic field strength 
could only be achieved to the detriment of the chemical shift information 
(i.e., reducing signal-to-noise). Also, although the method requires 
faster gradient rise times and stronger amplitudes it is feasible with 
currently available gradient technology as shown below. 
In accordance with the present invention, then, echo planar spectroscopic 
imaging in one dimension with n-fold oversampling in the time domain and 
subsequent echo editing is provided to reduce displacement artifacts 
n-fold and to avoid aliasing without sacrificing spectral width or signal 
to noise. Thus, the rapid acquisition associated with echo-planar imaging 
is made practicable for spectroscopic imaging, concomitantly rendering 
spectroscopic imaging more practicable as a diagnostic modality. 
In accordance with volume encoding, the n-fold oversampled echo-planar 
spectroscopic imaging sequence is performed in one direction to encode the 
spatial information for that direction, in addition to spectroscopic 
information (e.g., the z direction), while the other two spatial 
dimensions are encoded by conventional phase encoding, as shown in FIG. 3. 
It may be understood that in accordance with the present invention, the 
spatial encoding direction of the echo-planar sequence may be in any one 
of the magnetic field gradient directions. The orientation of the 
echo-planar pulse sequence may be selected in accordance with the desired 
resolution, and other spectral and spatial parameters. For example, 
provided that a sufficient sampling rate is possible, the gradient 
direction of the echo-planar spectroscopic sequence may be selected as the 
spatial direction requiring the largest field of view. 
It may also be appreciated that in accordance with the echo-planar 
acquisition sequence of the present invention, three-dimensional encoding 
is not necessary, and it may sometimes be advantageous or preferable for 
purposes of rapid acquisition, for example, to acquire spectroscopic 
information for a two-dimensional spatial slice, or multiple 
two-dimensional spatial slices (e.g., using multi-slice techniques). FIG. 
6 illustrates a preferred embodiment of the present invention for 
acquiring echo planar spectroscopic imaging (EPSI) data for a slice 
defined by two spatial dimensions. As indicated, STE localization is first 
performed in accordance with the present invention in order to localize 
the region of interest within a selected slice. The EPSI sequence 
spatially encodes the y-direction, and conventional phase encoding is used 
in the x-direction. It is noted, however, that three-dimensional spatial 
encoding generally has advantages over multi-slice methods, since 
continuous volume coverage and very thin slices can be obtained, and since 
chemical shift artifacts in the slice selection direction are avoided. 
It is further noted that although a preferred embodiment of the present 
invention is directed to spectroscopic imaging, the oversampling technique 
can be extended to conventional echo planar MR imaging to eliminate 
aliasing artifacts. 
In accordance with the present invention, then, numerous advantages and 
attendant advantages are realized. For instance, STE localization with 
spatial presaturation applied both before the STE sequence and during the 
TM interval of STE sequence improves the degree of suppression by several 
orders of magnitude. Moreover, more presaturation pulses can be applied to 
shape the volume of interest with greater flexibility than prior art 
methods, and without sacrificing saturation efficiency due to T1 
relaxation, for example. Further, the STE localization sequence allows the 
use of short echo times. In addition, n-fold oversampled EPSI sequences 
result in increased immunity to spectral-spatial convolution effects, and 
are well suited for separating even and odd echoes, thus further improving 
localization and rendering EPSI more practicable. These and other related 
features related to improved spatial localization and acquisition speed, 
result in a preferred embodiment for high speed EPSI. 
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 
The present invention has been implemented on a conventional, clinical 1.5 
Tesla whole body MRI scanner (GE Medical Systems, Milwaukee, Wis.) 
equipped with 10 mT/m actively shielded whole body gradients. Measurements 
with water suppression have been performed on phantoms and in vivo in the 
human brain. 
In such an experiment, an HSI pulse sequence corresponding to FIG. 3 was 
employed, except that the echo planar spatial-spectral encoding was 
performed along the y-axis, corresponding to an in-plane direction. STE RF 
pulses of 2 ms length, gradient ramps of 500 us and 2 ms gradient crusher 
pulses were employed, resulting in 10 msec echo times. All three 
STE--slice selective pulses selected the same axial slice. 
Sixteen 3 ms long spatial suppression pulses (eight in each suppression 
interval SS1,SS2) with subsequent 4 ms gradient dephasing pulses were 
employed for flexible suppression of multiple 4 cm thick slices around the 
volume of interest, orthogonal to the STE-selected slice. Four optional 
pulses (two in each suppression interval) for additional suppression above 
and below the slice of interest were available, but not used in this 
study. The suppression slices were positioned along the contours of the 
brain to include as much grey matter as possible inside the spectroscopic 
volume preselection. An example of the spatial suppression on a phantom is 
shown in FIG. 7. Chemical shift selective (CHESS) pulses with a bandwidth 
of 75 Hz were used for water suppression in the WS1 and WS2 intervals, 
typically using three pulses in WS1 and only one pulse in WS2. The flip 
angles and timing sequence of the water suppression pulses were 
numerically optimized according to known techniques. A user-friendly 
interface, which was supported by graphic prescription, was used to 
independently enter the position, orientation, and width of each 
suppression slice. The water suppression and spatial suppression pulses 
were adjusted to compensate for T1-relaxation effects. On phantoms, 
uniform several hundred fold spatial suppression was obtained. The width 
of the transition regions from minimum to maximum suppression was less 
than 10% of the saturation slice width, as shown in FIG. 8. 
A spectral width .beta. of 488 Hz in individual spectra was encoded. The 
echo planar gradient in the y-direction was periodically inverted every 
1024 .mu.sec (1/2.beta.) with two-fold oversampling during the entire 
spectroscopic acquisition to resolve the y dimension. The gradient ramp 
time from zero to maximum gradient amplitude was hardware-limited and 
varied with spatial resolution and gradient axis. For a nominal spatial 
resolution of 5 mm and a gradient amplitude of 4.6 mT/m the ramp time was 
approximately 160 us. For nominal resolutions of 7.5 mm and 10 mm the ramp 
time were approximately 110 and 80 us, respectively. For each data trace, 
16384 complex data points (i.e., 512 spectral points*32 spatial points) 
were sampled continuously with a data bandwidth of 32 kHz to yield a 
frequency resolution of 1.95 Hz. No gradient tuning was required. Data 
acquisition and echo-planar gradient encoding started 1 ms prior to the 
top of the stimulated echo to minimize first order phase errors in the 
spectra. The other dimensions (i.e., x and z) were localized with 
conventional phase encoding. 
Data processing was performed using the SA/GE software (GE Medical Systems, 
Milwaukee, Wis.) on a SUN Spare II workstation. Data representing "even" 
and "odd" echoes were rearranged to yield separate data sets and processed 
separately. Each edited echo-planar data trace was reformatted into a 
2-dimensional submatrix to separate spatial and spectral information. 
Spectral filtering consisted of a 2 Hz exponential line-broadening. Mild 
Fermi filtering in the spatial domains (radius: 0.9*k.sub.max, width: 
0.2*k.sub.max) was employed to reduce Gibb's ringing. Residual water 
signals were removed by low frequency filtering in the time domain as 
follows: A binomial filter of 131 points width was applied to the 
spatially localized time domain data and the result was subtracted from 
the original. This filtering strongly reduced residual water signals with 
negligible effect on metabolite signals outside of a spectral range 
between 3.9 and 4.5 ppm. Local shifts in peak position due to 
inhomogeneities in the magnetic field strength were automatically 
corrected by referencing to the position of n-acetyl aspartate (NAA). Zero 
order phase correction was performed automatically. "Even" and "odd" echo 
absorption mode spectra were added to maintain signal-to-noise. 
Spectroscopic images were created in magnitude mode by spectral 
integration over a spectral width of 12 Hz. 
For the in vivo studies, rapid multi-slice gradient-recalled echo scans 
(TR: 100 ms, TE: 5 ms) were obtained in order to select the volume of 
interest. Since the stimulated echo part of the pulse sequence may be 
converted into a volume selective pulse sequence by changing the 
orientations of the slice selection gradients, this technique was 
sometimes used to move the selected volume into regions containing lipids 
for localized tuning of outer volume suppression pulses. Typically, a 4.5 
cm thick axial slab at the level of the lateral ventricle was preselected. 
In accordance with the above described experimental parameters, 
echo-planar read-out gradients were employed along the y-axis (i.e., 
orthogonal to the 4.5 cm thick axial direction) to encode 32 slices with a 
nominal spatial resolution of 7.5 min. The nominal x-z cross-sectional 
resolution was 1 cm. First and second order shimming on the volume of 
interest was performed manually. EPSI data were acquired at TR: 2000 msec, 
TE: 13 msec, and TM: 120 msec. A 32*32*8 k-space was sampled with a 
32*32*6 cm.sup.3 field of view using four averages. Accordingly, the 
nominal voxel size was 0.75 cm.sup.3. The total data acquisition time was 
34 minutes. 
Spectroscopic images obtained in human brain together with the 
corresponding localizer images (MRI) are shown in FIG. 9. Due to the short 
echo time used in this study it was possible not only to detect singlet 
resonances from choline (CHO), creatine (CR) and n-acetyl aspartate (NAA), 
but also to detect multiplet signals from inositol (Ino), 
glutamate/glutamine (Glx) and cytosolic proteins (Prot). In the bottom 
slice 5 the ventricular spaces are clearly visible. Residual water and 
lipid signals from superficial regions were strongly reduced to or below 
the level of the metabolite resonances. The spectral quality was very 
consistent in different voxels as evidenced by FIG. 10, which shows a 
zoomed spectral array from slice 4 in FIG. 9 for a spectral range from 1 
to 3.7 ppm with major resonances from Cho, Cr, and NAA, and which also 
evidences the excellent lipid suppression in the periphery. Moreover, the 
spectral quality was very similar to that obtained with conventional 
methods, as evidenced by FIG. 11, which shows an individual spectrum from 
the array in FIG. 10, and in which displays the whole spectral range, 
rendering visible singlet resonances from Cho (3.24 ppm), Cr (3.03 ppm) 
and NAA (2.02 ppm), as well as multiplet resonances from Ino (3.56 ppm), 
Glx (2.35 ppm) and Prot (0-2 ppm). The signal to noise ratio of NAA was 
between 10 and 15 in well resolved voxels which was consistent with 
results obtained with conventional planar HSI in the same time frame. Due 
to spectral-spatial oversampling the water signal could be positioned on 
resonance and signals that fell outside of the spectral window on the 
right side, aliased back appearing on the left side. The experimental 
results indicate that there is no loss in signal to noise per unit time 
and unit volume as compared to conventional techniques. In addition, the 
experimental results indicate that higher order shimming is important to 
limit spectral linebroadening and frequency shifts when acquiring data 
from large brain volumes. Similarly, appropriate postprocessing methods 
with automated correction of frequency shifts due to residual magnetic 
field inhomogeneities improved the representation of the metabolite signal 
distributions. 
In addition to the volume encoded HSI experiments, single slice (i.e., two 
spatial dimensions) HSI was performed using a pulse sequence corresponding 
to FIG. 6. Many of the features of the pulse sequence and protocol follow 
the volume encoded HSI example discussed above. In this single slice 
experiment, echo-planar encoding was performed along the y-axis of the 
slice to acquire 16384 complex points (=512 spectral points * 32 spatial 
points) using a spectral width of 32 kHz to yield a digital frequency 
resolution of 1.95 Hz. No gradient tuning was required. The x-dimension 
was localized with conventional 32 step phase encoding. Water-suppressed 
and non water-suppressed spectroscopic imaging data acquired at TR: 2000 
ms, TE: 13 ms and TM: 120 ms from 1 or 2 cm thick axial slices. Using 
these parameters, the acquisition time for a 32*32 spatial matrix was 
about 64 seconds. When necessary, averaging was performed to improve the 
signal to noise ratio. 
As evinced by the experimental results, currently available gradient 
hardware on commercial whole body scanners is well suited for applications 
of echo-planar HSI in human brain without introducing significant 
localization artifacts and spectral distortions. Active gradient shielding 
as well as powerful and stable gradient amplifiers are important for 
achieving the required gradient performance. For a spatial resolution of 
better than 5 mm it will be necessary to achieve faster gradient ramp 
times, for example by using dedicated head gradient sets or by employing 
more powerful gradient amplifiers. Future implementation of echo-planar 
HSI will benefit from more powerful whole body gradients which will soon 
be available for echo-planar imaging. In addition, the increased 
acquisition speed provided in accordance with the present invention may be 
more fully exploited as phased array surface coils and higher field 
strengths become available. This implementation of the present invention 
may ultimately lead to functional HSI with a time resolution of a few 
minutes. 
Thus, as illustrated through the preferred embodiment and the foregoing 
example, and as understood by further practicing the present invention, 
many advantages and attendant advantages are provided by the present 
invention, the features of which include improved spatial localization and 
rapid data acquisition for generating high resolution spectroscopic 
images. From a research and clinical point of view the short acquisitions 
time reduces the risk of motion artifacts that makes spectroscopic imaging 
very difficult to perform in some clinical conditions, for example with 
children, cognitively impaired patients and patients with movement 
disorders. It also enables multiple measurements under different 
experimental conditions and time courses. The flexibility of positioning 
and shaping the region of lipid suppression, as well as the degree of 
lipid suppression achieved even with surface coils, enhances the capacity 
of studying cortical brain structures close to lipid containing regions. 
Improved localization and increased acquisition rate renders further 
improvements and implementations feasible, such as multi-dimensional 
spectral editing (e.g., correlated spectroscopy "COSY", nuclear overhauser 
enhanced spectroscopy "NOESY", etc.), combination with other localization 
methods, and spatial localization and encoding over a volume of interest 
without the need for slice selectivity of the STE pulses. 
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, although the invention is particularly well adapted for use with 
NMR proton spectroscopy for improving localization and increasing the 
acquisition rate, it will be appreciated that this is illustrative of only 
one utility of the invention, and that the modified STE localization 
and/or the oversampled echo-planar technique may be readily adapted for 
use with conventional imaging. Also, a plurality of oversampled EPSI 
sequences may be applied in different directions in an interleaved manner; 
for example, a first oversampled EPSI sequence may be applied in the 
y-direction, and a second oversampled EPSI sequence may be applied in the 
z-direction, interleaved such that within the duration of a readout 
gradient pulse of the first oversampled EPSI sequence, the entire second 
oversampled EPSI sequence is applied. In addition, while proton 
spectroscopic imaging is described, the invention is readily applicable to 
the other elements, such as sodium or phosphorus. 
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.