Method for selective NMR imaging of chemically-shifted nuclei

A method for providing NMR images free of chemical shift artifacts, obtained from selected nuclei of a sample containing nuclei having chemically-shifted NMR frequencies is described. Non-selected nuclei of the sample are selectively saturated prior to applying an NMR imaging sequence by applying to the sample an RF pulse containing frequencies equal to the chemically-shifted frequencies of the non-selected nuclei and having a narrow frequency spectrum about the chemically-shifted frequencies. Alternatively, selected nuclei are directly excited during application of an NMR imaging sequence by applying to the sample an RF pulse containing frequencies equal to the chemically-shifted frequencies of the selected nuclei, and having a narrow frequency spectrum about the chemically-shifted frequencies so as to exclude signal contributions from the non-selected nuclei. These selective pulses are applied in the absence of the imaging magnetic field gradients.

BACKGROUND OF THE INVENTION 
This invention relates to nuclear magnetic resonance (NMR) imaging and, 
more particularly, to methods for overcoming chemical shift artifacts in 
NMR imaging by imaging with selected nuclei of a sample containing nuclei 
of the same, or different, species having chemically-shifted NMR 
frequencies. 
As is well known, the nuclear magnetic resonance phenomenon is exhibited by 
atomic nuclei with an odd number of either protons or neutrons. Such 
nuclei possess spin, which endows them with a small magnetic field. When 
placed in an externally applied static main magnetic field, B.sub.o, the 
nuclei tend to align themselves with the applied field and produce a net 
magnetization, M, in the direction of the applied field. The nuclei 
oscillate or precess about the axis of the applied field with a 
characteristic NMR frequency, .omega., given by the Larmor equation: 
EQU .omega.=.gamma.B.sub.o ( 1) 
where .gamma. is the gyromagnetic ratio and is constant for each NMR 
isotope. If a time-dependent (RF) magnetic field, having frequency 
components equal to the Larmor frequencies of the nuclei, is applied in a 
direction orthogonal to the main field, then the nuclei will absorb energy 
and nutate away from the axis of the main field and commence to precess at 
the Larmor frequency about the new net applied field direction. If the RF 
pulse is turned off precisely when the angle of nutation reaches 
90.degree., the magnetization is left in the transverse, or x-y, plane and 
the net magnetization now precesses about B.sub.o in the transverse plane 
at the Larmor frequency. Such a pulse is termed a 90.degree. pulse. A 
180.degree. pulse is one which nutates the magnetization through 
180.degree., inverting it. These two types of RF pulse form the basic 
tools of the NMR spectroscopist. 
Experimentally, the NMR signal is detected by a tuned RF coil with axis 
perpendicular to B.sub.o. The same coil used for excitation is also 
suitable for detection, or alternatively, a separate, mutually orthogonal, 
coil can be used. The oscillating NMR magnetization induces a voltage in 
the coil, analogous to the principle of an electric generator. The induced 
signal immediately following an RF pulse is termed a free induction decay 
(FID), reflecting the decay in the time signal as nuclei relax back to 
equilibrium in alignment with the main field or which signal decays due to 
dephasing caused by inhomogeneities in the main field. These NMR signals 
may be detected and Fourier-transformed to derive the frequency components 
of the NMR signals characteristic of the excited nuclei. 
Nuclei of the same isotope can exhibit minute variations in their NMR 
frequencies, which are referred to as chemical shifts, because of 
differences in their chemical environments which cause differences in 
their local magnetic field environments. Chemical shifts result from 
alterations of the magnetic field around nuclei as a result of the 
shielding currents that are associated with the distribution of electrons 
around adjacent atoms. The degree of shielding is characteristic of the 
environment of the nucleus, and thus the chemical shift spectrum of a 
given molecule is unique and can be used for identification. In 
conventional NMR spectroscopy, chemically-shifted signals are observed 
from the whole of an NMR sample for studying the chemical structure of the 
sample. Because the resonant frequency and the absolute chemical shift are 
dependent upon the strength of the field, the chemical shift is expressed 
as a fractional shift in parts-per-million (ppm) of the resonant frequency 
relative to an arbitrary reference compound. 
Since the Larmor frequency is proportional to the magnetic field, if the 
magnetic field varies spatially in a sample, then so does the resonant 
frequency of the nuclei. In NMR imaging, at least one magnetic field 
gradient is applied to the sample to spatially encode the emitted NMR 
signals. If, in the presence of gradients, an RF excitation pulse having a 
narrow range of frequency components is applied to the nuclei in a 
selected region, e.g., a slice or a selected point of the sample, this 
region is selectively excited and the NMR signals from the selected region 
can be detected. The data collected from different regions or points of 
the sample can be processed in a well-known manner to construct an image. 
NMR imaging in the past has typically been performed in rather low magnetic 
fields and chemical shifts have not been a significant problem. In 
magnetic fields below about 0.5 T, chemical shifts are difficult to 
observe because the chemical shifts are comparable to the natural 
linewidths of the resonances and the low sensitivity of nuclei other than 
hydrogen (.sup.1 H). It is desirable, however, to perform NMR imaging in 
higher magnetic fields, e.g. in fields in excess of 1 T, because of the 
improved signal-to-noise ratios realized; recent advances in magnet 
technology permit the use of higher magnetic fields of the order of 1-1.5 
T in medical and biological NMR imaging. As the magnetic field increases, 
the chemical shift increases proportionately and becomes a greater 
problem. Chemical shift can produce the same effect as a spatial variation 
in the NMR signal. This results in chemical shift artifacts which are 
manifested as rings in multiple-angle-projection imaging and ghosts in 
two-dimensional-Fourier-transform (2DFT) imaging. Ghost artifacts, for 
example, may appear as a faint ring or ghost at one side of an image, and 
they obliterate some of the spatial information present. In proton imaging 
of the body, the chemical shift observed is principally between the 
hydrogens attached to oxygen in water (H.sub.2 O) and the hydrogens 
attached to carbon in alkyl --CH.sub.2 -- groups found in lipid (fat) and 
other tissues. The effect of the chemical shift is to produce two 
superimposed images, with one image being the water image and the other 
image being the lipid image shifted by an amount corresponding to the 
chemical shift. 
The observation and recognition of chemical shift artifacts in NMR imaging 
was first published by the present inventor, Paul A. Bottomley, in "A 
Versatile Magnetic Field Gradient Control System For NMR Imaging", J. 
Phys. E: Sci. Instrum., Vol. 14, 1981, where the expression: 
##EQU1## 
is proposed for the minimum imaging gradient, g, required to resolve N 
pixels of a sample of spatial extent, a, containing a spectrum of chemical 
shifts, .delta.*, measured in frequency units to avoid chemical shift 
artifact. Since the chemical shift range in frequency units increases 
linearly with magnetic field strength and it is desirable to increase N to 
maximize spatial resolution, a condition is rapidly approached where 
practical gradient strengths are insufficient to satisfy equation (2). 
Furthermore, it is disadvantageous to increase the gradient strength 
beyond what is absolutely necessary due to inherent main field 
inhomogeneity, because this increases the frequency bandwidth of the NMR 
signal and therefore reduces the signal-to-noise ratio. 
In general, it is not possible to correct for chemical shift artifacts from 
a single NMR scan by calculation unless prior knowledge is possessed of 
either the spatial information or the chemical shift spectral information, 
including the amplitudes of the peaks. Such knowledge would be contrary to 
the aim of an imaging experiment, which is to investigate the interior of 
an unknown object. Thus, chemical shift artifacts are a significant 
problem, particularly for high field imaging in homogeneous main magnetic 
fields. 
It is therefore desirable to provide NMR imaging methods that overcome 
chemical shift artifacts. It is also desirable to provide NMR imaging 
methods that enable resolution of chemically-shifted species. For example, 
an image constructed from CH.sub.2 lipid alone may prove useful for 
looking at fat or atheroscelerotic lesions or plaques in blood vessels, as 
well as for the evaluation of heart disease. 
BRIEF SUMMARY OF THE INVENTION 
In accordance with a first embodiment of the invention, a method for NMR 
imaging that enables the NMR frequencies of selected chemically-shifted 
nuclei of the same species to be resolved, to allow images from the 
selected nuclei to be constructed without chemical shift artifacts, 
utilizes a step of selectively saturating or inverting the non-selected 
nuclei of a sample, by applying to the sample an RF pulse (termed a 
"sizzler" pulse) which has a narrow frequency spectrum about the 
chemically-shifted NMR frequency of the non-selected nuclei, prior to a 
conventional imaging sequence and adjusting the amplitude of the 
narrow-frequency-spectrum RF pulse so as to saturate or invert the 
non-selected nuclei such that these nuclei do not produce NMR signals 
during data acquisition in the imaging sequence. 
An RF pulse having the desired spectral content may be formed by providing 
an RF carrier having a frequency equal, for example, to the NMR frequency 
of the nuclei it is desired to saturate, and amplitude-modulating that RF 
carrier with a waveform that controls the width and the shape of a 
resulting RF carrier pulse, so as to limit the frequency spectrum of the 
RF carrier pulse to a narrow band of frequencies about the NMR frequency 
of the nuclei to be saturated. The RF carrier pulse is applied to the 
sample in the absence of magnetic field gradients so as to saturate (or 
invert) the non-selected nuclei in the sample. For samples containing two 
chemically-shifted nuclei, the nuclei can be individually resolved and 
images constructed from each chemically-shifted nuclear species by 
sequentially saturating (or inverting) one nuclei and then the other. For 
samples containing more than two chemically-shifted nuclei, composite RF 
carrier pulses may be employed for simultaneously saturating, or 
inverting, all but the selected nuclei. 
A second embodiment of the invention that enables resolution of different 
chemically-shifted species and enables NMR images of those species to be 
constructed without chemical shift artifacts, utilizes the chemical shift 
selective "sizzler" pulse in an otherwise conventional NMR imaging 
sequence of the form of either a selective excitation 90.degree. or 
180.degree. RF pulse as part of a 90.degree.-.tau.-180.degree. spin echo 
or a 90.degree.-.tau.-180.degree.-2.tau.-180.degree.-2.tau.-180.degree. . 
. . Carr-Purcell RF pulse sequence, where .tau. is a preselected fixed 
time interval between pulses. In both cases, the selective excitation 
pulse is applied in the absence of applied imaging magnetic field 
gradients and tailored to select only the chemically-shifted nuclear 
species that it is desired to image. In cases where selective excitation 
is also used in the imaging sequence, for plane selection for example, 
image plane selection must be performed using a different RF pulse than 
that pulse used for selecting the chemical shift species.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention is particularly well adapted for use with NMR proton imaging 
for resolving water and lipid peaks, i.e., NMR signals, and for overcoming 
chemical shift artifacts produced thereby, and will be described in that 
environment. However, as will be appreciated, this is illustrative of only 
one utility of the invention. 
Prior to describing the invention, it will be helpful to review briefly 
some of the basics of NMR imaging. FIG. 1 illustrates an NMR sample 10, 
which may, for purposes of illustration and explanation, be cylindrically 
shaped. Sample 10 is positioned in a static homogeneous main magnetic 
field B.sub.o directed along the positive z-axis direction of a 
conventional Cartesian coordinate system, the z-axis being selected to be 
coincident with the axis 11 of the sample. The origin of the coordinate 
system is chosen as the center of the sample, which is also the center of 
a thin planar slab or imaging volume 12 of the sample selected by 
selective irradiation of the sample in the presence of magnetic field 
gradients. Typically, three such gradients are employed for imaging: 
EQU G.sub.x (t)=.differential.Bo/.differential.x (3) 
EQU G.sub.y (t)=.differential.Bo/.differential.y (4) 
EQU G.sub.z (t)=.differential.Bo/.differential.z (5) 
The gradients G.sub.x, G.sub.y and G.sub.z are generally different 
functions of time t, and have associated magnetic fields b.sub.x, b.sub.y 
and b.sub.z, respectively, wherein 
EQU b.sub.x =G.sub.x (t)x (6) 
EQU b.sub.y =G.sub.y (t)y (7) 
EQU b.sub.z =G.sub.z (t)z (8) 
As will be described more fully hereinbelow, the gradients are applied to 
the sample along with RF magnetic field pulses as part of an imaging pulse 
sequence to selectively excite and detect NMR signals from different 
portions of the sample. The RF pulses are directed orthogonal to the 
B.sub.o field and are used, for example, in combination with the magnetic 
field gradients to excite nuclei in a specific planar region to resonance. 
FIGS. 2a and 2b illustrate 64 MHz .sup.1 H spectra 14 and 16, respectively, 
recorded from an entire human head and thigh, respectively, in a magnetic 
field of 1.5 T. The two peaks 14a and 14b or 16a and 16b of each spectra 
correspond to the protons on water (H.sub.2 O) (peaks 14a and 16a)s and 
alkyl groups (--CH.sub.2 --) (peaks 14b and 16b), respectively. It will be 
seen that the relative intensities of the peaks vary with the tissue 
studied and with the individual: there is less --CH.sub.2 -- alkyl 
component observed in the brain spectrum 14b, compared to the thigh 
spectrum 16b, for example. However, the --CH.sub.2 -- NMR component in the 
head is still sufficient to generate a ghost artifact at the periphery of 
2DFT head images, as previously described. From FIGS. 2a and 2b, it can be 
seen that the --CH.sub.2 -- peaks 14b and 16b are shifted approximately 
+3.5 ppm from the respective water peaks 14a and 16a (e.g. roughly about 
220 Hz) and have a linewidth of the order of 1 ppm (i.e. about 64 Hz) at 
their midpoints. 
In accordance with a first embodiment of the invention, the NMR signals 
corresponding to the lipid may be suppressed by applying to the sample, 
prior, but close in time, to the normal imaging sequence, an RF selective 
"sizzler" saturation pulse having: (1) frequency components in a narrow 
frequency band centered on the --CH.sub.2 -- resonance frequency; and (2) 
an amplitude sufficient to saturate the --CH.sub.2 -- nuclei, i.e., 
sufficient to cause the spins to go into an excited state so that they do 
not produce NMR signals during the imaging sequence. The bandwidth of the 
sizzler pulse should be wide enough to cover the frequency spread of the 
--CH.sub.2 -- peak 14b or 16b, but narrow enough to avoid saturating the 
frequency region associated with the water nuclei peaks 14a and 16a. For 
the spectra illustrated in FIGS. 2a and 2b, the sizzler pulse may have a 
bandwidth BW.sub.s of the order of 1 ppm, e.g. about 64 Hz (at 1.5 T). 
The sizzler pulse may be produced in a number of different ways. For 
example, an RF generator set to the frequency of the nuclei which are to 
be saturated, inverted or otherwise excited, may be pulse-modulated or 
gated to provide an RF pulse of the correct frequency and bandwidth. The 
bandwidth of the sizzler pulse can be controlled by controlling either or 
both of the pulse duration and the pulse shape. As is well known, a 
square-shaped RF pulse having a width, i.e., duration, of t seconds will 
irradiate approximately a 2/t frequency band centered on the RF frequency 
of the pulse. Thus, a pulse having a duration t of 32 msec. will have a 
bandwidth of about 62.5 Hz, which is approximately equal to the 1 ppm 
bandwidth of the --CH.sub.2 -- peaks 14b or 16b of FIGS. 2a or 2b. 
However, a square pulse undesirably has frequency components which extend 
beyond this frequency range, because such a pulse has the well-known 
frequency profile that varies as (sin .omega.)/.omega.. It is desirable to 
utilize, as a sizzler pulse, a pulse having much sharper bandwidth edges, 
so as to excite a more sharply-defined response. Pulses having the desired 
bandwidth may be produced by amplitude-modulating an RF pulse with an 
appropriate waveform so as to confine the frequency components of the 
pulse to the desired bandwidth. Many such waveforms are available and may 
be employed. One such waveform, for example, is a Gaussian waveform, which 
produces a Gaussian frequency distribution. Another waveform which may be 
employed is (sin bt)/bt, where b is a constant, since such a modulated RF 
pulse has frequency components contained within a rectangular profile in 
the frequency domain. To compensate for ringing in the frequency domain 
due to finite truncation of the (sin bt)/bt modulation, a Hanning window 
may also be applied to the RF pulse to attenuate its side lobes by 
modulating the pulse envelope with a Hanning function. The Gaussian and 
(sin bt)/bt modulation waveforms are examples of two of many different 
types of waveforms which may be employed and which are contemplated by the 
present invention. 
As previously noted for the first embodiment, the sizzler pulse is applied 
to the sample in the absence of gradients and prior, but close in time, to 
the normal imaging sequence. The effect of saturating the lipid nuclei is 
to remove the alkyl --CH.sub.2 -- frequencies from the NMR signals 
produced during the normal imaging sequence, thereby enabling the water 
peak 14a or 16a to be resolved and chemical shift artifacts, produced by 
peaks 14b or 16b, to be avoided. Moreover, the invention conveniently 
enables and further extends to the saturation of the water nuclei response 
peaks 14a or 16a so that the --CH.sub.2 -- response peak regions 14b and 
16b can be resolved to construct a --CH.sub.2 -- image, which, being 
sensitive to fat content, may be useful for locating atherosclerotic 
lesions or plaques in blood vessels and for the evaluation of heart 
disease. 
In the second embodiment, the sizzler pulse is incorporated into the 
imaging sequence to select and excite the desired nuclear species for 
observation, rather than remove it by saturation; centering the sizzler 
pulse on the water peak 14a or 16a in FIGS. 2a or 2b, respectively will 
preclude the appearance in the image of chemical shift artifacts from 
other species in the spectrum, and generate an image of only the water 
component. An image reflecting the --CH.sub.2 -- component can be obtained 
by selecting only the lipid component 14b or 16b in the same manner. 
The invention, of course, may be also be applied to other than proton 
imaging. For example, for imaging other nuclei having multiple 
chemically-shifted peaks, composite pulses may be used to saturate all but 
selected nuclei to enable resolution of their respective peak (or peaks) 
for imaging. 
The invention is not restricted to any particular NMR imaging sequence, 
many of which are known, and in general may be employed with any of the 
known imaging sequences. FIGS. 3 and 4 are exemplary of two different 
imaging sequences with which the invention may be employed. FIG. 3 
illustrates the first embodiment of the invention employed with a 
multiple-angle-projection imaging sequence, and FIG. 4 illustrates another 
embodiment of the invention employed with a 
two-dimensional-Fourier-transform (spin-warp) imaging sequence. These two 
imaging sequences are described in detail in U.S. patent application Ser. 
No. 345,444, filed Feb. 3, 1982, now U.S. Pat. No. 4,471,306, issued Sept. 
11, 1984, assigned to the same assignee as the present invention, and 
incorporated in its entirety by reference herein. Accordingly, the imaging 
sequence of FIGS. 3 and 4 will be described only generally. 
Referring to FIG. 3, a rather long sizzler saturation pulse 20, constructed 
in the manner previously described, is applied to the sample in the 
absence of applied imaging magnetic field gradients, e.g. while the 
G.sub.x, G.sub.y and G.sub.z gradient signal portions 22a, 23a and 24a all 
have zero magnitudes, during a first time period q.sub.1. The amplitude 
envelope and frequency of pulse 20 are selected to saturate undesired, 
e.g., lipid, nuclei in the sample. 
Following the sizzler saturation pulse 20, a normal 
multiple-angle-projection imaging sequence, of gradient signals 22b, 23b 
and 24b and RF pulse signal 26, is applied to the sample during time 
period q.sub.2 -q.sub.6 to excite the nuclei in a selected x-y plane, and 
the resulting NMR signals 28 are Fourier-transformed to provide a 
projection along a radial line in the plane defined by the magnitudes of 
the x and y gradients. 
As shown, in time period q.sub.2, a narrow-frequency-band 90.degree. 
selective RF pulse 26a is applied in the presence of a positive gradient 
G.sub.z portion 24a-1 to select the desired x-y plane. The RF pulse 26a 
may be formed by amplitude-modulating an RF carrier having a frequency 
equal to the Larmor frequency of the selected plane with a Gaussian, a 
(sin bt)/bt of the like waveform to limit the frequency spectrum of the 
pulse to a narrow band about the frequency of the selected plane. Although 
in the context of a normal imaging sequence the bandwidth of the 
90.degree. selective pulse 26a is considered to be "narrow", in relation 
to the sizzler saturation pulse, its excitation spectrum is quite large 
and may be of the order of 1 KHz or so. During time period q.sub.3, a 
negative G.sub.z gradient portion 24b-2 is applied, to rephase the spins 
excited in time period q.sub.2 along with dephasing positive gradients 
G.sub.x portion 22b-1 and G.sub.y portion 23b-1 applied in the x and y 
directions, respectively. Following a short waiting period to allow the 
currents in the gradient windings to subside, a 180.degree. nonselective 
inverting RF pulse 26b is applied to the sample during time period q.sub.4 
at a time .tau. following the 90.degree. selective pulse 26a. This 
produces a spin-echo NMR signal 28a at time .tau. following the 
180.degree. pulse 26b, which signal 28a is read out in the presence of 
gradients G.sub.x portion 22b-2 and G.sub.y portion 23b-2, having 
amplitudes respectively equal to (g cos .THETA.) and to (g sin .THETA.), 
respectively, which define the angle .THETA. of the particular radial line 
along which the projection is taken. The imaging sequence illustrated in 
FIG. 3 may be repeated a large number of times for different values of 
.THETA. (at 1.degree. intervals, for example) to cover at least a 
180.degree. arc in the imaging plane, and the resulting data (NMR signals 
28a) may be processed to construct an image. A sizzler pulse 20 preceeds 
each of the repetitions of the multiple-angle--projection sequence. 
FIG. 4 illustrates the sizzler saturation pulse 20' employed with a 
two-dimensional-Fourier-transform (2DFT) imaging sequence for spin-warp 
imaging. As before, the sizzler saturation pulse 20' is formed to saturate 
selected chemical shifted nuclei and is applied to the sample in the 
absence of gradients (e.g. G.sub.x, G.sub.y and G.sub.z portions 22a', 
23a' and 24a' are substantially of zero magnitude) during time period 
q.sub.1. During period q.sub.2, a 90.degree. selective RF pulse 26a' is 
applied in the presence of a positive G.sub.z gradient portion 24b'-1, to 
selectively excite an imaging plane. During time period q.sub.3, a 
negative G.sub.z gradient portion 24b'-2 is applied to rephase the nuclear 
spins in the planar region excited during the time period q.sub.2, along 
with a dephasing positive G gradient portion 22b'-1 and with a 
phase-encoding G.sub.y gradient portion 23b'-1. Following a short time 
interval to allow the currents in the gradient windings to subside, a 
180.degree. nonselective inverting RF pulse 26b' is applied during time 
period q.sub.4 at a time .tau. following the 90.degree. selective pulse 
26a', and a positive rephasing G.sub.x gradient 22b'-2 is applied during 
time periods q.sub.5 and q.sub.6 to obtain the spatial nuclear spin 
information along the x-axis at a time .tau. following the 180.degree. 
pulse 26b'. 
The phase-encoding G.sub.y gradient portion 23b'-1 is applied in the y-axis 
direction during time period q.sub.3 in order to introduce a twist in the 
spins in the y-axis direction. The spatial information encoded by the 
different phases of the spins causes spins at different x positions to 
precess at different frequencies, enabling separation of the return 
signals 28a' at each x position. During subsequent imaging sequences, 
different values of G.sub.y portion 23b'-1 are employed (as indicated by 
the dotted lines) to give different projections, and a complete planar 
image is reconstructed by a two-dimensional-Fourier-transformation of the 
projections. The sizzler saturation pulse 20' is applied just prior to the 
normal spin-warp imaging sequence (starting at time q.sub.2) for each 
repetition. 
FIGS. 3 and 4 are examples of but two imaging sequences with which the 
invention may be employed, and the invention is applicable to other 
imaging sequences as well. 
As noted earlier, the sizzler saturation pulse may be employed for 
saturating first the alkyl (--CH.sub.2 --) nuclei in hydrogen (.sup.1 H) 
spectra so that the resulting image is constructed from only the water 
nuclei response and is free of chemical shift artifacts. The sizzler pulse 
may then be employed to saturate the water nuclei and an image constructed 
from the --CH.sub.2 -- nuclei. For imaging samples containing more than 
two chemically-shifted species, the sizzler pulse may be a composite pulse 
comprising two or more saturation pulses, each formed to saturate 
different nuclei to enable other selected nuclei to be imaged. 
FIGS. 5 and 6 illustrate a second embodiment of the invention wherein the 
sizzler pulse is incorporated into conventional NMR imaging pulse 
sequences to replace one of the existing RF pulses, thereby eliminating 
sizzler pulse application during interval q.sub.1. For example, FIG. 5, 
illustrates the application to the 2DFT spin-warp imaging sequence using a 
sizzler pulse 20" as the 90.degree. excitation pulse in a first interval 
q.sub.2 of the sequence 26". The 90.degree. pulse in interval q.sub.2 was 
formerly used for slice selection in FIGS. 3 and 4, but since selection of 
the desired chemically-shifted species must take place in the absence of 
applied imaging gradients 22a", 23a" and 24a", the slice selection portion 
of the sequence is shifted to a 180.degree. pulse 26b" in later time 
interval q.sub.4. The excitation bandwidth of the 90.degree. sizzler pulse 
20" is adjusted to excite only the bandwidth of the desired nuclear 
species as before, while the amplitude of 90.degree. sizzler pulse 20" is 
adjusted to produce a 90.degree. selective pulse for those species. After 
subsidence of the 90.degree. pulse, a positive G.sub.x gradient portion 
22b"-1, and a phase-encoding G.sub.y gradient portion 23b"-1 are applied 
during interval q.sub.3, similar to the sequence in FIG. 4. Thereafter, a 
slice-selective 180.degree. RF pulse 26b" is applied during interval 
q.sub.4, in the presence of a G.sub.z gradient 24b' -1 to select an 
imaging plane perpendicular to the z-axis. The NMR spin-echo signal 28" is 
again sampled in the presence of a uniform x-gradient 22b"-2 in intervals 
q.sub.5 and q.sub.6. During subsequent imaging sequences, different values 
of G.sub.y portion 23b"-1 are employed (as indicated by the dotted lines) 
to give different projections, and a complete planar image is 
reconstructed by a two-dimensional-Fourier-transformation of the 
projections. 
FIG. 6 illustrates an application of the second embodiment to the 2DFT 
spin-warp imaging sequence, using a sizzler pulse 20'" as the 180.degree. 
excitation pulse in time interval q.sub.4 of the sequence. The application 
of gradient, RF pulses and data acquisition in intervals q.sub.2, q.sub.3, 
q.sub.5 and q.sub.6 occurs in the same manner as in like time intervals of 
the sequence of FIG. 4. However, in interval q.sub.4, the 180.degree. 
pulse 26b' is replaced by a 180.degree. sizzler pulse 20'" with 
appropriately adjusted excitation bandwidth, and amplitude corresponding 
to a 180.degree. pulse for the selected nuclear species. 
Pulse schemes corresponding to the projection reconstruction method (FIG. 
3) for the second embodiment can be utilized by substituting the G.sub.x 
gradient sequence (signals 22b'"-1, 22a'" and 22b'"-2) and the G.sub.y 
gradient sequence (signals 23b'"-1 and 23a'"), similar to those of FIG. 3, 
into the same intervals of FIGS. 5 and 6. 
As noted earlier, the sizzler excitation pulse may be employed for 
selecting first and --CH.sub.2 -- nuclei in hydrogen (.sup.1 H) spectra so 
that the resulting image is constructed from only the --CH.sub.2 -- nuclei 
response and is free of chemical shift artifacts. The sizzler pulse may 
then be employed to excite the water nuclei and an image constructed from 
the water nuclei. 
FIG. 7 is a simplified block diagram of an NMR imaging system 30 for use 
with the invention. The system comprises a general-purpose computer means 
32 coupled to a disc storage means 33 and an interface means 35. An RF 
transmitter means 38, signal average means 40 and gradient power supplies 
means 42, 43 and 44 for energizing, respective, x, y and z gradient coils 
46, 47 and 48, are all coupled to the computer means 32 through the 
interface means 35. 
RF transmitter means 38 can be employed for generating both the sizzler 
saturation pulse 20, 20', 20" or 20'" and the RF pulses 26a and 26b, 26a' 
and 26b', 26a" and 26b", 26a'" and 26b'" and the like, required for the 20 
imaging sequences illustrated in FIGS. 3-6. The pulses may be amplified in 
RF power amplifier means 50 and applied to a transmitter coil 52. The NMR 
imaging signals are generated responsive to the signal applied by coil 52 
to the sample. The resulting NMR signals may be received by a receiver 
coil 54, amplified in a low noise preamplifier means 56, and filtered and 
detected by a receiver means 58. The receiver means output signal may be 
digitized and averaged by signal averager means 40, and the resulting data 
may be processed by the computer for constructing the image and for 
providing an imaging display, as on a CRT display. The preamplifier means 
56 and receiver means 58 are usually protected from the RF pulses by 
gating or blanking signals provided on lines 60 and 62 from the 
transmitter or from the computer, and also by passive filtering. A magnet 
means 64 provides the static homogeneous main magnetic field B.sub.o. 
Computer means 32 is employed for controlling the system and may provide 
the voltage waveforms for the gradient power supplies means 42-44. 
Transmitter means 38 preferably comprises at least one programmable 
frequency synthesizer means 38a, controlled by the computer, for 
generating RF carriers of the required frequencies for the sizzler 
saturation pulse and for the RF imaging pulses, gating circuitry for 
controlling the widths of the RF pulses, and modulators for applying 
envelope modulation waveforms to the RF pulses. The modulation waveforms 
may be generated in the computer. For saturating multiple 
chemically-shifted nuclei simultaneously, transmitter means 38 may contain 
multiple RF frequency modulation (FM) circuitry and amplitude modulators. 
The parameters of the sizzler saturation or excitation pulse 20, 20', 20" 
or 20'" required to saturate or excite particular chemically-shifted 
nuclei may be determined in several different ways. For example, the 
frequency of the sizzler saturation pulse may be determined by running the 
system and adjusting the frequency synthesizer 38a of transmitter means 38 
until its frequency matches the NMR frequency of the selected 
chemically-shifted nuclei. The pulse width and pulse strength of the RF 
pulse may then be adjusted until the NMR signal 28 or 28' due to the 
chemically-shifted nuclei disappears where sizzler saturation pulses are 
employed, or for a maximum FID or spin-echo signal 28" or 28'", 
respectively, where sizzler 90.degree. or 180.degree. pulses are 
respectively utilized. In proton imaging, for example, to suppress signals 
due to --CH.sub.2 -- nuclei with a sizzler saturation pulse, a bottle 
containing lipid (rich in --CH.sub.2 --) material may also be placed 
adjacent to the sample to assist in centering the frequency of the 
transmitter means synthesizer. Once the parameters of the sizzler 
saturation pulse have been determined, the computer may control the 
synthesizer frequency during imaging to switch back and forth between the 
frequency of the sizzler saturation pulse and the frequency of pulses 
26a/26b or 26a'/26b' corresponding to the seelected imaging plane. 
Alternatively, since the 90.degree. selective pulse 26a/26a' has a 
spectrum which is quite a bit larger than that of the sizzler saturation 
pulse, and the chemical shift offsets are rather small in most cases, the 
transmitter means synthesizer frequency may simply be set to the frequency 
of the chemically-shifted nuclei to be saturated and the modulation 
waveforms and pulse widths controlled in accordance with the type of pulse 
to be generated. A frequency offset of several parts per million for the 
90.degree. selective pulse, for example, will have very little detrimental 
effect since the pulse will still have frequency components about the NMR 
frequency of the selected imaging plane. 
While a preferred embodiment of the invention has been shown and described, 
it will now be apparent to those skilled in the art that many variations 
and modifications in this embodiment may be made without departing from 
the principles and spirit of the invention, and that I desire the scope of 
my invention to be defined only by the appending claims.