Fat and Water Magnetic Resonance Imaging
Thorsten A. Bley, MD,1,2Oliver Wieben, PhD,3,4Christopher J. Franc ¸ois, MD,1
Jean H. Brittain, PhD,5and Scott B. Reeder, MD, PhD1,3,4,6*
A wide variety of fat suppression and water–fat separation
methods are used to suppress fat signal and improve visu-
alization of abnormalities. This article reviews the most
commonly used techniques for fat suppression and fat–
water imaging including 1) chemically selective fat sup-
pression pulses “FAT-SAT”; 2) spatial-spectral pulses (wa-
ter excitation); 3) short inversion time (TI) inversion
recovery (STIR) imaging; 4) chemical shift based water–fat
separation methods; and finally 5) fat suppression and
balanced steady-state free precession (SSFP) sequences.
The basic physical background of these techniques includ-
ing their specific advantages and disadvantages is given
and related to clinical applications. This enables the reader
to understand the reasons why some fat suppression
methods work better than others in specific clinical set-
Key Words: fat and water MRI; fat suppression; spatial
spectral pulses; STIR; chemical shift imaging; SSFP
J. Magn. Reson. Imaging 2010;31:4–18.
© 2009 Wiley-Liss, Inc.
MOST CLINICAL magnetic resonance imaging (MRI) ap-
plications detect the signal from protons, which com-
promise over 90% of nuclei in the human body. The
detected protons are either part of water, bound to
molecules such as proteins or carbohydrates, or fat.
Their respective signal intensities in imaging voxels re-
sults from a combination of their spin density, longitu-
dinal and transverse relaxation times (T1and T2, re-
spectively), and theparametersof the imaging
sequence used. By exploiting the particular character-
istics of hydrogen atoms, MRI can provide excellent
contrast between soft tissues, according to whether
they are bound to water or to lipid molecules.
With its relatively short T1relaxation time, fat signal
often appears bright in many important clinical imag-
ing sequences and can obscure underlying pathology
such as edema, inflammation, or enhancing tumors.
For this reason, most clinical protocols use fat suppres-
sion methods to suppress fat signal and improve visu-
alization of these abnormalities. This is particularly
true for standard imaging sequences such as fast spin-
echo (FSE), spoiled gradient echo (SPGR), and steady-
state free precession (SSFP). Reliable fat-suppression
has the added benefit of eliminating chemical shift ar-
tifact, by virtue of the fact that fat signal is no longer
present, and lower bandwidths may be used with fat-
suppressed applications to improve signal-to-noise ra-
tio (SNR). In addition, there are several pathologies
where direct visualization of fat may be desirable, such
as fatty tumors, including adrenal adenomas, angio-
myolipomas, liposarcomas, and other fat-containing
mesenchymal tumors. There is also tremendous cur-
rent interest in the quantification of the amount of
visceral adipose tissue, as well as fatty infiltrative dis-
eases such as hepatic steatosis (1–6). For any of these
applications, separation of water and fat signals may be
highly desirable with acquisition of “water-only” and/or
This article reviews the most commonly used tech-
niques for fat suppression and fat–water imaging in-
cluding 1) chemically selective fat suppression pulses
“FAT-SAT”; 2) spatial-spectral pulses (water excitation);
3) short inversion time (TI) inversion recovery (STIR)
imaging; 4) chemical shift based water–fat separation
methods; and finally 5) fat suppression and balanced
SSFP sequences (Table 1).
PHYSICS OF WATER–FAT IMAGING
NMR Spectrum of Water and Fat
The electronic shielding of the protons in the triglycer-
ide molecules of fat is greater than that experienced by
protons in water molecules, resulting in different mi-
croscopic magnetic fields, and subsequently different
proton resonance frequencies. Fat has a complex spec-
1Department of Radiology, University of Wisconsin, Madison, Wiscon-
2Department of Diagnostic and Interventional Radiology, University
Medical Center Hamburg-Eppendorf, Hamburg, Germany.
3Department of Medical Physics, University of Wisconsin, Madison,
4Department of Biomedical Engineering, University of Wisconsin, Mad-
ison, Wisconsin, USA.
5Global MR Applied Science Laboratory, GE Healthcare, Madison, Wis-
6Department of Medicine, University of Wisconsin, Madison, Wiscon-
*Address reprint requests to: S.B.R., 600 Highland Avenue, CSC E1/
374, Madison, WI, 53792-3252. E-mail: firstname.lastname@example.org
Received October 30, 2008; Accepted July 6, 2009.
Published online in Wiley InterScience (www.interscience.wiley.com).
JOURNAL OF MAGNETIC RESONANCE IMAGING 31:4–18 (2010)
© 2009 Wiley-Liss, Inc.
trum with multiple peaks, the largest of which is shifted
downfield by ?3.5 ppm from the water peak (Fig. 1). For
simplicity and practicality, we will limit the majority of
the discussions in this article to this single fat reso-
nance peak, which usually contains more than 10
times the signal energy of any other fat peak (7). It is
important to realize, however, that advanced quantita-
tive applications may need to consider these additional
peaks of fat.
The resulting chemical shift, ?fcs, in the resonance
frequency is linearly related to the magnetic field
2?B0? ???ppm? ? 10?6
where ? is the gyromagnetic ratio and ?/2? ? 42.58
MHz/T. At 1.5T, the resonance frequency for protons is
?63.9 MHz. At body temperature the chemical shift
between protons and fat is approximately ?210 Hz (fat
precesses more slowly than water) (8). As explicitly writ-
ten in Eq. 1, the chemical shift is directly proportional
to the main magnetic field B0. Therefore, at 3.0T, an
increasingly important clinical field strength, the chem-
Most Commonly Used Techniques for Fat Suppression and Fat-Water Imaging
Method Advantages Disadvantages Suggested applications
Chemically selective fat
● Relatively fast
● Applicable to most pulse
● Sensitive to B0and B1
● Low sequence efficiency
● Most applications except:
● Head and neck
● Extremities with metal implants
● Insensitive to B1
● Relatively fast
● Practical to most pulse
sequences except FSE
● Sensitive to B0
● Low sequence efficiency
● Longer excitation pulses
● 3D imaging of cartilage in knee
● Most applications except:
● Head and neck
● Robust to B0and B1
● Reliable fat suppression
● Mixed contrast
● Inherent T1weighting
● Only works with PD and
● Low SNR efficiency
● Suppresses short T1
species and enhancing
tissue after contrast
● Head and neck
● Large field of view
● Inhomogeneous B0
● T2/PD applications
Chemical shift based
● Robust fat saturation
● Provides fat/water images
● Allows recombined images
● Corrects for chemical shift
● Universal compatibility
● Quantitative applications
● High SNR efficiency
● Long scan times
● More complex
● Anywhere fat saturation and
water excitation fail
● Anywhere fat information/
quantification is needed
● Hepatic steatosis
● Adnexal masses
● High SNR
● Bright fluid
● T2/T1mixed contrast
● Banding and flow artifacts
● “India Ink” artifact
● Currently under
● Limited clinical experience
● NCE vascular imaging
● Bowel imaging
● New applications being
Figure 1. NMR spectrum of water–fat mixture acquired at 3T
demonstrates a single resonance for water. The main fat peak
resonates 420 Hz more slowly than the water (on resonance in
this figure). It is important to note that fat actually has several
additional peaks (*), some of which lie near the water reso-
nance. For the purpose of this work, we will consider fat a
single peak at ?420 Hz at 3T (or ?210 Hz at 1.5T).
Fat and Water MRI5
ical shift between water and the main fat peak doubles
to ?420 Hz.
It is important to note that the resonance frequency of
water is dependent on the temperature of the tissue
being imaged, and therefore the apparent chemical
shift between water and fat is dependent on the local
temperature (9). For example, the chemical shift be-
tween water and the main fat peak is ?210 Hz at body
temperature, and ?224 Hz at 22°C. These temperature
differences may be important for specific fat suppres-
sion and water–fat separation applications in tissue or
phantoms not at 37°C.
Main Magnetic Field (B0) Inhomogeneities
Several of the fat–water separation techniques dis-
cussed below rely on the assumption that there are
constant resonance frequencies for fat and water across
the image. However, in practical applications many fac-
tors can create inhomogeneities in the main magnetic
field (B0) that violate this assumption and result in
imperfect suppression of fat.
The main magnet itself may have an imperfect mag-
netic homogeneity, although this is usually a minor
effect in modern MR scanners that are usually
shimmed to homogeneity within 1 ppm across the field
of view (FOV). Magnetic susceptibility introduced by the
patient leads to more significant field distortions. For
example, susceptibility differences at air/tissue inter-
faces such as the nasal cavities, ears, lungs, or skin can
lead to large B0inhomogeneities. This also holds true
for bowel gas and adjacent intraluminal fluid or ascites.
Susceptibility differences, in general, are proportional
to the main magnetic field, and therefore worsen at
higher field strengths. Furthermore, implanted ferro-
magnetic objects such as dentures, surgical clips, sta-
ples, prosthetic joints, etc., may cause more severe dis-
tortions in the static magnetic field. The susceptibilities
lead to geometrically dependent field inhomogeneities
that shift the resonance frequencies of water and fat
relative to the MR system’s transmit and receive fre-
For traditional Cartesian applications (ie, spin-warp
k-space sampling), B0inhomogeneities have three main
effects: 1) distortion in the readout direction, 2) accel-
erated T2* decay for gradient echo imaging, and 3) failed
fat suppression, which is the most relevant for this
article. As discussed in more detail below, field inhomo-
geneities shift the position of the water and fat peaks
with respect to the frequency-selective profile of the
fat-saturation pulse, resulting in failure of the desired
fat suppression and possibly leading to inadvertent loss
of water signal.
Radiofrequency (RF) (B1) Inhomogeneities
Receive B1inhomogeneities can cause intensity varia-
tions across the image. However, these are infrequently
a problem in clinical routine, and B1sensitivity correc-
tion algorithms are widely used and available on most
modern scanners. Transmit B1inhomogeneities, how-
ever, lead to inhomogeneous flip angles across the im-
age and can be problematic for some fat suppression
methods. This problem is often minimized by using
body or bird cage coils that provide a more homogenous
transmit field in combination with (relatively inhomo-
geneous) receive phased arrays of surface coils. At 3.0T,
dielectric effects can also lead to variations in the
achieved flip angle, particularly in abdominal imaging
applications and in the presence of ascites. Fortu-
nately, advanced design techniques, such as adiabatic
approaches, can generate RF pulses that are relatively
insensitive to B1variations, addressing this concern for
CHEMICALLY SELECTIVE FAT SUPPRESSION
In 1985 Haase et al (10) introduced a chemical shift
selective (CHESS) imaging technique that can be used
to selectively excite certain spin species such as fat or
water, irrespective of their spatial location. Using this
technique, the desired component of longitudinal mag-
netization (eg, water) remains unaffected while the un-
wanted component (eg, fat) is left with no net magneti-
Typically, the saturation pulse carrier frequency is
centered at the main fat peak and the RF amplitude
envelope is designed as a “sinc” function or related
function. In the Fourier (spectral) domain the RF sinc
pulse becomes a “rect” function (which has a “rectan-
gular” shape, hence the name) with a bandwidth deter-
mined by the width of the sinc lobes in such a way that
it does not affect the water frequency. This leads to a
rect function of RF energy at the location of the main fat
peak (Fig. 2). If the flip angle of this pulse is adjusted to
90°, all longitudinal magnetization in the fat peak will
be tipped into the transverse plane. In reality, the RF
pulse is a truncated sinc and the spectral sensitivity is
altered slightly from the idealized “rect” function. The
RF pulse is followed by a crusher gradient to spoil this
transverse magnetization, saturating all transverse
magnetization of fat. Such fat-saturation pulses (chem-
ically selective RF pulses and crusher gradients) are
followed immediately by a standard imaging sequence.
This leaves little time for recovery of fat longitudinal
magnetization, resulting in suppression of signal from
Figure 2. Schematic spectrum of water and fat peaks and the
positioning of a spectrally selective “rect” function (“fat-sat
pulse”) with a particular bandwidth centered on the main fat
peak, ?210 Hz from the water peak at 1.5T, before (A) and
after (B) the application of the fat-saturation pulse. RF en-
ergy ? Fourier spectrum of the fat-saturation pulse.
6 Bley et al.
fat-containing tissues. Fat saturation pulses are highly
effective in regions where both the main magnetic field
(B0) and the transmit RF field (B1) are relatively homo-
geneous (Fig. 3). Typical areas where fat-saturation
pulses are effective include targeted volumes of the
knee, pelvis, and abdomen. Fat-saturation is more
challenging over large FOVs, in head and neck imaging,
and in areas with metallic implants.
The primary disadvantage of fat-saturation pulses is
that they are relatively sensitive to B0inhomogeneities
that shift the position of the water and fat peaks with
respect to the frequency of the fat-saturation pulse (Fig.
3B,C). This can result in failed fat suppression, and can
even cause inadvertent suppression of water signal.
Therefore, high-quality fat suppression with fat satura-
tion pulses requires a relatively homogeneous magnetic
field over the entire FOV, to allow the lipid protons to
resonate at the same frequency throughout the imaging
volume. Even with optimal shimming, magnetic sus-
ceptibility variations in the patient may impair the qual-
ity of fat suppression.
Furthermore, fat-saturation pulses are sensitive to B1
inhomogeneities, because robust fat-saturation re-
quires a relatively accurate 90° pulse (that can typically
be achieved within ?5–10° degrees) in order to saturate
the longitudinal magnetization of fat (Fig. 3D). A 45°
pulse, for example, would provide only partial suppres-
sion of fat. Typical and acceptable levels for B0-unifor-
mity are approximately ?1 ppm. Since the signal of all
lipid protons needs to be saturated without affecting
the water signal, fat sat does not perform well with
transmit surface coils. Coils with uniform RF fields,
such as head, body, and extremity coils are preferred.
At higher field strengths the chemical shift increases
between water and fat. At 3.0T, for example, the chem-
ical shift of the fat and water spectrum is approximately
?420 Hz. In principal, field variations due to suscepti-
bility should also double, proportional to the main mag-
netic field, which will widen the distribution of magnetic
field inhomogeneities. One might expect the increased
susceptibility to offset the increased chemical shift be-
tween water and fat, and there should be no overall
improvement in the performance of fat suppression at
higher field strengths. In practice, however, fat sup-
pression performs better at higher field strengths and it
is easier to suppress the fat signal without affecting the
water signal. This may be due to the fact that a wider
spectral bandwidth of RF energy is used to suppress
fat, which requires shorter RF pulses in the time do-
main. Shorter RF pulses generally perform better and
are easier to design, and this may result in overall
improved fat suppression. Following the same logic at
lower field strengths, where the chemical shift between
the water and fat signals is reduced, longer RF pulses
are needed to obtain a good spectral profile. Tradeoffs
are necessary to minimize the duration of the RF pulses
and performance can be degraded relative to higher
field strengths, with an increased opportunity for failed
fat suppression and/or suppressed water signal.
SPATIAL-SPECTRAL PULSES (WATER
A more advanced approach for fat suppression is to
excite the water peak directly, rather than suppress the
fat peak (11,12). One of the earliest implementations,
known as “spatial-spectral” pulses, invoked the con-
cept of adding a spectral dimension to excitation k-
space (11). Spatial-spectral pulses are unique in that
they simultaneously excite a spatial region of spins (eg,
a slice) as well as a spectral band (eg, water) in order to
provide excitation of the water signal only within the
slice of interest.
The mechanism behind spatial-spectral pulses can
be explained in a simplified manner as follows. Con-
sider a train of slice-selective “?” pulses, each with a
small tip angle, ?, eg, 5–10°. These ? pulses are sepa-
rated by the time needed for fat to precess 180° relative
B1inhomogeneities on fat sat pulses. The rect pulse played for
fat saturation placed correctly at the fat signal establishes the
desired fat saturation (A). With B0inhomogeneities in the field
the SINC pulse may result positive or negative to the fat water
spectrum which may lead to missed or incomplete fat suppres-
sion (B) or inadvertent water suppression (C), respectively.
Spatially varying flip angles across the FOV may lead to B1
inhomogeneities that result in an insufficient fat sat pulse and
thus lead to incompletely saturated fat signal (D).
Schematic displaying the effect of B0 and
Fat and Water MRI7
to water, or T ? 1/(2?f), about 2.3 msec at 1.5T. After
the first ? pulse, both fat and water are tipped ? degree
from the z-axis. Immediately before the next pulse,
however, the fat has precessed 180° in the transverse
plane, and is subsequently tipped back along the Mz
axis, while the water continues its trajectory toward the
transverse plane (Fig. 4). In this way the water reso-
nance is selectively excited, while the fat resonance
remains unaffected. Details of spatial-spectral pulses
have been described in greater detail elsewhere (11).
Spatial-spectral pulses are a very effective means of
selectively exciting water within an image and are most
commonly used in conjunction with spiral and echo
planar imaging (EPI) (13,14) (Fig. 4C,D). They can also
be combined with other sequences such as spoiled gra-
dient echo and fast spin-echo imaging (15,16). The ma-
jor drawback of spatial-spectral pulses are their sensi-
tivity to B0 inhomogeneities, which is similar to
conventional fat-saturation pulses (11,14). The other
major drawback is the need for relatively lengthy
pulses, which can reduce overall sequence efficiency.
For this reason spatial-spectral pulses are most com-
monly used with sequences requiring only a few excita-
tions and longer TRs.
Binomial water excitation pulses have been proven to
be beneficial for cartilage imaging (17), where uniform
fat suppression with high spatial resolution is required.
Cartilage imaging with single frequency selective RF
pulses at ultrahigh-fields has been shown to be easily
applicable for fat suppression or fat–water separation
An advantage of spatial-spectral pulses compared to
fat-saturation pulses is that they are relatively insensi-
tive to B1inhomogeneity because they directly excite
the water signal, and avoid incomplete fat suppression
associated with fat-saturation pulses that require uni-
form RF flip angles across the sample.
Spatial-spectral pulses work particularly well at
higher field strengths because shorter RF pulses can be
used, which facilitates a wider spectral bandwidth of RF
energy centered at the main fat peak, making fat satu-
ration less sensitive to B0 inhomogeneities. Shorter
pulses can be achieved because there is increased fre-
quency separation between the water and fat reso-
nances. This allows a shorter time between the multiple
excitation pulses, as shown by time period “T” in Fig.
4A. For example, 3.0T cardiac MR utilizing spatial-
spectral imaging in healthy volunteers has been shown
to result in improved image quality, improved blood
SNR efficiency, and significant improvement in blood-
myocardium contrast-to-noise ratio (CNR) efficiency
compared to 1.5T. Taking full advantage of the gradient
and RF capabilities the spatial-spectral excitation du-
ration was as short as 3.6 msec (19). Figure 4B shows a
clinical example of the right coronary artery acquired at
3T using this technique. Figure 4C,D show examples of
spatial-spectral excitations in combination with diffu-
sion-weighted imaging in the brain. This approach is
used by most EPI methods.
SHORT TI INVERSION RECOVERY (STIR)
Inversion recovery (IR) pulses are known to produce
heavily T1-weighted images. With the use of higher
magnetic field strength which evolved in the 1980s, the
tissue T1increased and a longer time was required to
Figure 4. Schematic of spatial-spectral pulses. A: ? pulses are separated by a time, T, sufficient to create a 180° phase shift
between water and fat. T is ?2.3 msec at 1.5T. Immediately after the first ? pulse, water and fat are tipped together. However,
immediately before the next ? pulse the fat has precessed 180° such that after the second ? pulse, the fat is tipped back up along
the Mzaxis and the water is tipped further toward the transverse plane. 3.0T cardiac MR utilizing spatial-spectral imaging of the
right coronary artery suppresses the surrounding fat tissue in the right atrioventricular groove (B). Image courtesy Krishna
Nayak, PhD (Department of Electrical Engineering, University of Southern California, Los Angeles). Using spatial-spectral pulses
all fat in the subcutaneous tissue has been effectively suppressed in a 70-year-old woman with left MCA infarct. Restricted
diffusion can be readily revealed on the EPI image with a b-value of 0 mm2/s (C) and 1000 mm2/s (arrow in D).
8 Bley et al.
produce equivalent image contrast with IR sequences.
In the mid 1980s Bydder et al (20,21) introduced an
important variant of the conventional IR sequence in
which the initial inversion pulse was timed in a way
that the signal from fat (short T1) was nulled. This
“short TI inversion-recovery” (STIR) method suppresses
the signal of tissues with short T1values such as fat.
STIR imaging has been used to increase the contrast
between tumor and surrounding normal tissues mainly
by nulling the fat signal (21). However, STIR images also
introduce unwanted T1contrast into the remaining wa-
ter signal, which may be a confounding factor when
interpreting STIR images. Due to the risk of nulling
contrast enhancing tissue, STIR imaging is used pri-
marily for T2and proton density (PD) weighted imaging,
and T1weighted STIR imaging should be avoided espe-
cially when using extrinsic contrast agents. Utilizing IR
imaging, the signal intensity of any tissue can be nulled
by variations in the inversion time (TI) (22). By choosing
a short inversion time, the short T1 tissue signal is
suppressed (Fig. 5A). The TI corresponding to fat signal
suppression in STIR is ?100–200 msec at 1.5T as com-
pared to 1500–2500 msec for the long-TI fluid attenu-
ated inversion recovery sequence (FLAIR) for brain im-
aging. In general the TI should be increased when
imaging at 3.0T due to the longer T1of tissue at higher
The primary advantage of the STIR pulse sequence
lies in its ability to produce uniform fat suppression.
Even in difficult areas of the body the STIR sequence
has proven to be extremely reliable, with strong insen-
sitivity to B0inhomogeneities (Fig. 5B,C). Furthermore,
respiratory artifacts are less pronounced and confusion
with intraabdominal fat can be avoided, allowing bowel
loops to be reliably identified (20).
Disadvantages of STIR pulses include the inherent
T1-weighting that limits its use to T2-weighted imaging
or proton density-weighted imaging. It is also relatively
inefficient, requiring an inversion time of ?200 msec.
The inversion also degrades the SNR of the remaining
water signal by ?40%–50%. Practically, STIR is only
used with fast spin-echo or spin-echo techniques. Even
though it is insensitive to B0inhomogeneities, it may be
sensitive to B1inhomogeneities if the inversion pulse is
not an adiabatic pulse, particularly at higher field
strengths, where B1may be more inhomogeneous.
It is important to stress that the inversion pulse and
subsequent recovery of longitudinal magnetization al-
ters the image contrast. Because gadolinium reduces
the relaxation time T1, the signal from gadolinium-en-
hancing lesions may be inadvertently suppressed.
Therefore, T1-weighted STIR imaging should always be
avoided, particularly with contrast-enhanced imaging.
However, since STIR sequences provide excellent fat
suppression over large FOVs while being very insensi-
tive to B0inhomogeneities, it is clinically a very practi-
cal method and is widely used.
CHEMICAL SHIFT BASED WATER–FAT
Chemical shift based water–fat separation methods
comprise a class of approaches commonly known as
“Dixon” water–fat separation (8,23–26). Unlike the
methods described above, which suppress fat signal or
selectively excite water, Dixon methods rely on the
phase shifts created by fat–water resonance frequency
differences to separate water from fat. Phase informa-
tion is encoded by acquiring images at slightly different
echo times (TE), exploiting the difference in resonance
frequency between water and fat. By strategically ac-
quiring images at specific TE values, the combined sig-
nal from a voxel containing water and fat signals can be
decomposed into separate water and fat images. In this
way, Dixon methods provide a “fat-suppressed” water-
only image and a “water-suppressed” fat-only image.
This approach was first proposed by Dixon (23), with
two images acquired at different TEs such that water
and fat were “in-phase” (Sin? W?F) or “out of phase”
(Sout? W–F). By adding and subtracting Sinand Sout,
water (W) and fat (F) images are easily separated, since
W ? (Sin? Sout)/2, and F ? (Sin? Sout)/2, (Fig. 6). The
original approach required only two images, thus it was
considered a “two-point” method.
Figure 5. Schematic displaying the longitudinal magnetization of fat and water in relation to the inversion pulse (A). T1of fat
is shorter than T1of water tissue. Short TI inversion recovery imaging acquires images ?200 msec after the inversion pulse,
during the fat zero-crossing (fat is nulled at this time), providing robust fat suppression, but less SNR performance. Insufficient
fat suppression occurred in the cervical subcutaneous fat and at apex of the lungs (arrows in B) in T2-weighted fat saturated fast
spin echo (B). Homogenous fat suppression was achieved in the corresponding STIR images (light arrows in C). Note mixed
contrast displayed in the depicted part of the brain in the STIR image.
Fat and Water MRI9
Unfortunately, Dixon’s original approach was sensi-
tive to B0inhomogeneities that resulted in water–fat
“swapping” in the image. As discussed below, this oc-
curs because B0inhomogeneities create a natural am-
biguity when only one chemical species (water, fat)
dominates the signal from the pixel: fat signal is indis-
tinguishable from water signal that is off-resonance by
?210 Hz, etc. Unwrapping algorithms are needed to
avoid this ambiguity.
The Dixon two-point method was subsequently mod-
ified by Glover and Schneider (24), by acquiring a third
image that was used to compensate for B0inhomoge-
neities. This “three-point” approach acquired images at
TE values that generated phase shifts of 0, ? ?, and – ?
between the water and fat (24). The additional informa-
tion can be used to calculate a B0field inhomogeneity
image (“field map”). By using phase unwrapping algo-
rithms, this approach can remove the effects of the field
map, thereby avoiding fat–water swapping. By address-
ing the problems involved with B0inhomogeneity this
technique became a more robust method, particularly
at areas of high susceptibility. Glover (27) also sug-
gested a four-point method with phase shifts of 0, ?, 2?,
and 3? and an additional measurement of the spectral
width of the fat resonance.
Dixon methods are also insensitive to B1inhomoge-
neities and provide robust water–fat separation. They
are compatible with a wide variety of pulse sequences
including T2-weighted FSE, T1-weighted FSE, gradient
echo, and SSFP. Fat-saturation typically fails in certain
regions of the body because of unfavorable geometry,
eg, in the lung apices and in the neck where large
susceptibility differences create severe magnetic field
inhomogeneities. Chemical shift methods are more ro-
bust in these areas (Fig. 7). In addition to the water-only
images, fat images and recombined in-phase and out-
of-phase images are also available for review using
chemical shift artifact, which causes a spatial shift of
fat signal in the readout direction, can be removed by
realigning the separated water and fat images before
recombination to form these synthesized in-phase and
out-of-phase images (28). This opens interesting possi-
bilities for improved SNR performance through low-
bandwidth imaging, free from chemical shift artifact.
This is particularly true at higher field strengths, where
relatively high bandwidths are typically used to avoid
chemical shift artifact.
The primary disadvantage of Dixon methods is the
increased scan time. Despite this, these methods are
generally highly SNR-efficient if the correct choice of
TEs are used (29). With the correct choice of echoes, the
signal from the three images can achieve the maximum
possible SNR in the water and fat images. Methods for
scan time reduction have been investigated through
partial k-space acquisition methods and parallel imag-
ing (30,31). The latter approach is highly complemen-
tary with Dixon methods, because SNR penalties of
parallel imaging are offset by gains in the SNR from the
Dixon reconstruction when using well-defined phased
array coils and optimized parallel imaging algorithms
that minimize local noise amplification (g-factor) (30).
Figure 6. Signal vector diagram describing the conventional
Dixon method. The signal of water (W) and fat (F) is added and
the resultant signal is observed. Water and fat images can be
generated by adding and subtracting the “in phase” (upper
row) and the “out of phase” images, respectively (lower row).
Figure 7. Sagittal T1W-FSE images of the cervical spine utilizing conventional fat-saturation method (A) and Dixon imaging (B).
Fat suppression failures of the subcutaneous fat at the anterior and posterior aspect of the neck in areas of air/tissue interfaces
are present with the conventional fat-saturation method (arrows in A). Utilizing Dixon imaging a homogenous suppression of fat
can be readily appreciated in the same patient as in A. Note that even in the areas of air/tissue interfaces homogenous fat
suppression is achieved (arrows in B).
10Bley et al.
Inherent to all Dixon water–fat separation methods is
a natural ambiguity that leads to “water–fat swapping”
(Fig. 8). This occurs because a voxel containing only
water (fat) “looks” like fat (water) that is off-resonance
by ?210 Hz (?210 Hz), at 1.5T. To solve this ambiguity,
additional information is needed for region-growing re-
construction algorithms to avoid this swapping. Most
unwrapping algorithms exploit the fact that B0 field
inhomogeneities vary smoothly across the image and
are continuous. Sophisticated algorithms are needed to
avoid these swaps, and most reconstruction algorithms
usea variety ofphase
(24,32,33) or region-growing methods that estimate the
correct field inhomogeneity map (34,35). The success of
a water–fat separation method largely depends on its
ability to avoid water–fat swaps.
Over the past decade, several methods have been
proposed to improve on the methods proposed by Dixon
and Glover. Xiang and An (8) proposed a three-point
method which included a general direct phase encoding
Figure 9. Large enhancing soft tissue mass in the left gluteal muscles after administration of gadolinium contrast, imaged with
the three-point direct phase encoding method of Xiang et al. Uniform separation of water and fat is demonstrated. Selected
acquisition parameters include: 1.5T, spin-echo, TR/TE ? 800/24 msec, body coil. Images courtesy Q.S. Xiang, PhD (University
of British Columbia, Vancouver, BC).
“Water–fat swapping” can be avoided by utilizing reconstruction algorithms with region growing methods that take the field
inhomogeneity into account (light arrows in B,D).
Sagittal water images (A,B) and fat images (C,D) of a foot demonstrate “water–fat swapping” (arrows in A,C).
Fat and Water MRI11
(DPE) of the chemical shift information in 1997. A gen-
eral asymmetric sampling scheme allowed an analytical
solution for pixel-level water–fat separation using a re-
gion-growing method, rather than phase unwrapping,
which can be challenging. A clinical example using the
DPE approach is shown in Fig. 9. More recently, Xiang
(36) reported an efficient and robust asymmetric two-
point Dixon method with a partially opposed phase
In 2002 Ma et al (25) introduced an FSE-based Dixon
method that achieves an echo shift without requiring
an increase in echo spacing, collects generally asym-
metric echoes, and includes a phase-sensitive partial
Fourier image reconstruction. In order to further de-
crease imaging time, Ma et al (37) then proposed a fast
spin echo two-point Dixon method (fast 2PD) that uses
two interleaved FSE images.
In 2004 Ma (32) also presented a novel phase correc-
tion algorithm that was based on a fast gradient echo
scheme that uses precalculated spatial gradients of the
signal phase to guide the growth sequence and thus
avoids manual selection of the seeds or usage of an
empirical angular threshold. With this technique the
signal direction of a given pixel is determined by the
amplitude and phase of the surrounding pixels, whose
direction has already been determined. This has proven
to be very computationally efficient and robust, even in
pixels with large phase uncertainty (Fig. 10) (38).
Iterative decomposition of water and fat with echo
asymmetry and least squares estimation (IDEAL) is a
multi-point water–fat separation method that has also
been recently described (5,29,39–45) (Fig. 11). This
technique allows for three or more echoes at arbitrary
echo times that can be subsequently optimized to max-
imize the SNR performance of the water–fat separation.
The optimal echo times, expressed in terms of the rel-
ative phase shift between water and fat, have been in-
vestigated both theoretically (46) and experimentally
(29). In this work, it was shown that for a three-point
acquisition, the center echo should have a phase be-
tween water and fat that is in quadrature (ie, perpen-
dicular) in the first and third echo or subsequently
2/3? before and after the center echo, respectively. This
gives an effective signal averaging of 3.0 for all combi-
nations of water and fat, and therefore it is highly SNR-
efficient, achieving the best possible SNR performance
of the water–fat separation (Fig. 12). Reeder et al (29,43)
showed that symmetrically acquired echoes lead to ar-
tifacts at water–fat tissue interfaces resulting from SNR
Figure 10. Uniform fat–water
separation is achieved with a
rapid contrast enhanced spoiled
gradient acquisition using the z-
point method described by Ma
for robust fat–water separation
over a large FOV. Excellent de-
piction of hemangioma is noted
in the liver (arrow in A,C,D). A ?
water image, B ? fat image, C ?
in-phase, and D ? opposed-
phase. Images courtesy Russell
Low, MD (Sharps Children’s
Hospital, San Diego, CA).
12 Bley et al.
ophthalmic arteries can be readily revealed by IDEAL time of flight imaging (arrows in B). As in the previous figure the fat signal
is entirely eliminated and appears black rather than hypointense as seen with suppression methods.
Conventional time of flight imaging with flow compensation fails to depict the ophthalmic arteries (A). Both
Figure 12. Transversal T2-weighted, fat-saturated TSE of the pelvis of a 28-year-old woman with ovarian teratoma. Note failed
fat suppression in the sacrum and dorsal subcutaneous fat (light arrows in A) and within the teratoma (arrow in A) due to lumbar
metallic fixation rods cranial to the imaging plane. The water image of a T2-weighted IDEAL fat-sat reveals homogeneous fat
suppression in the respective location (asterisk in B). The ovarian teratoma is readily visualized on the water-image (arrow in B),
the fat-image (arrow in C), and the in-phase-image (arrow in D). In fact, direct visualization of the fatty components within the
teratoma is feasible in the water-image (arrow in B).
Fat and Water MRI 13
performance that depended on the amount of water and
fat in the voxel. The main disadvantage of IDEAL, like
other three-point Dixon methods, is that it triples the
scan time. Methods to reduce the scan time penalty
include partial k-space acquisition (39,47), parallel im-
aging (48,49), and multiecho acquisition (50). An im-
portant advantage of the IDEAL method is that it can be
modified to separate more than two species that have
widely spaced resonance peaks, eg, fat-water-silicone
(26,51), or13C labeled compounds (52,53).
Finally, IDEAL has the important advantage that the
signal model can be easily modified to include multiple
spectral peaks of fat (54) (Fig. 13). If the resonance
frequencies and relative amplitudes of these peaks are
known a priori, their contributions to the total fat signal
can be more accurately modeled than a single reso-
nance peak. This was recently shown to improve the
quality of fat suppression, avoiding residual “gray” fat
that results from incomplete separation of water and fat
(54). As described below, accurate spectral modeling of
fat has also been shown to improve the accuracy of
water–fat separation methods that attempt to quantify
The description of chemical shift based water–fat sep-
aration methods given above is certainly not complete.
A more detailed discussion would be beyond the scope
of this article and is found elsewhere in the literature
FAT SUPPRESSION AND BALANCED SSFP
In the past 7–8 years, there has been considerable
effort placed on new fat suppression methods with
SSFP. SSFP is a rapid pulse sequence with very high
SNR performance, and is commonly used for cardiac
and abdominal imaging. SSFP has a mixed contrast
that depends on the ratio of T2/T1, such that fat ap-
pears very bright, and robust fat suppression with
SSFP is highly desirable.
The interesting phase behavior of SSFP with nonuni-
form spectral response allows an opportunity to develop
new fat suppression methods unique to this sequence.
As shown in Fig. 14, SSFP exhibits a repeating “pass-
band” behavior with periodic magnitude and phase be-
havior at different off-resonance frequencies (58–60).
The period of the passbands is equal to 1/TR, which is
the time needed for an off-resonance spin to nutate by
360°. Importantly, the phase of adjacent passbands will
alternate between 0 and 180° (ie, the sign of the trans-
verse magnetization alternates between plus and mi-
nus), when TE ? TR/2, which is the most commonly
used TE for SSFP acquisitions. If water and fat reso-
nances fall within adjacent passbands then this will
create opposing phase of the water and fat.
The opposing phase created by the phase behavior of
the SSFP signal will result in the familiar “India ink”
artifact that occurs at water–fat interfaces, commonly
seen in SSFP images. This occurs because of destruc-
tive interference of water and fat signals through a
partial volume effect, when water and fat resonances
fall in different passbands. It is important to realize that
this effect is not a constant feature of SSFP images and
only occurs when water and fat resonances fall within
passbands that have opposite phase. Changes in the TR
or in the field strength may result in the water and fat
peaks having the same phase (0° or 360°), will lead to
addition of the water and fat signal, and the India arti-
fact at the water–fat interface will no longer be present.
Figure 14 shows an example of this effect in cardiac
images acquired at 3.0T with different TR values, re-
sulting in variable behavior of the India ink artifact. A
schematic of the magnitude (Fig. 14C, top) and phase
behavior (Fig. 14C, bottom) of the SSFP signal response
is plotted against the phase developed by a spin during
one TR. This complex phase behavior of SSFP can also
be exploited to separate water and fat, and several ap-
proaches have been described. A linear combination
SSFP technique which is able to generate spectral se-
Figure 13. Sagittal T2-weighted fat-saturated FSE sequence of the knee of a 56-year-old man with tibial plateau fracture. Note
how extent of the bone marrow edema thins out toward the posterior portion of the tibial plateau on the fat sat FSE sequence
(light arrow in A), while it is readily revealed on the IDEAL image with multipeak reconstruction (arrow in B).
14 Bley et al.
lectivity for water/lipid discrimination has been de-
scribed for various clinical applications (62,63). They
also introduced a fast, spectrally selective imaging
method called fluctuating equilibrium magnetic reso-
nance (FEMR), which permits simultaneous acquisition
of several images with different contrast features. This
Figure 15. Hepatic steatosis is quantified utilizing IDEAL SPGR at 3T. In-phase images (A,D) and out-of-phase images (B,E) of
two patients with fatty infiltration in the liver. Please note different signal intensity in the liver of the two patients in the R2*
images from T2*-IDEAL (C,F). The fat-signal fraction values in the liver of the patient in the upper row was 21% (C) as compared
to more severe steatosis with 56% fat fraction in the patient in the lower row (F).
Figure 14. Four-chamber view SSFP cardiac MRI without (light arrows in A) and with “India-ink” artifact at fat-water interfaces
(arrows in B). Schematic of the magnitude and phase of the SSFP signal response plotted against the phase developed by a spin
during one TR, resulting from off-resonance effects such as B0inhomogeneity and/or chemical shift from fat (C). For example,
with a TR of 4.6 msec the passbands are separated by ?210 Hz. On-resonance water spins would sit at 0 Hz in the central
passband, while fat spins would fall exactly one passband to the left, with opposite phase of water. This leads to subtraction of
water and fat signals in voxels containing both species, explaining the India-ink artifact at fat-water interfaces from partial
volume effects and the phase behavior of SSFP (B). At different values of TR or at 3T, and in the presence of a magnetic field
inhomogeneity, the relative positions of water and fat may fall into the same passband. In these situations the India ink artifact
may not be present (A).
Fat and Water MRI15
is achieved by producing an equilibrium magnetization
that fluctuates from excitation to excitation (63). Har-
greaves et al (59) proposed a single-point “phase sensi-
tive” fat-water separation method that exploits the al-
ternating phase behavior of SSFP images acquired with
a passband width equal to the chemical shift between
water and fat (eg, 210 Hz requiring TR ? 4.6 msec at
1.5T) to ensure that the phase between water and fat
are always in opposition. This group also proposed a
dual-acquisition phase sensitive SSFP that separates
fat and water by combining signal from two SSFP se-
quences that are added in quadrature and then phase
corrected (64). Several other interesting methods have
also been proposed (65–67), although a complete de-
scription is beyond the scope of this article.
Other fat suppression methods that are important for
SSFP imaging, but do not specifically rely on SSFP’s
unique phase behavior, include the approach by Schef-
fler et al (61) that interrupts the steady state of the SSFP
acquisition to play intermittent frequency selective fat
saturation pulses. In addition, Reeder et al introduced a
chemical shift-based water–fat separation method com-
bined with SSFP that uniformly separates water from
fat while being insensitive to B0 inhomogeneities for
cartilage imaging (69) and cardiac CINE imaging (40).
The ability of chemical shift-based water–fat separation
methods to accurately separate the signals from water
and fat has led to great interest in quantifying fatty
infiltration of organs in a variety of disease conditions,
including nonalcoholic fatty liver disease (NAFLD).
NAFLD is currently the leading cause of chronic liver
disease in the US, affecting up to 34% of the US popu-
lation (69,70). It is also related to the metabolic syn-
drome, which includes obesity and type II diabetes
(69,71). The current gold standard for the diagnosis of
NAFLD is liver biopsy, which is expensive, risky, and
suffers from high sampling variability, greatly limiting
its clinical utility. Therefore, there is a great need for
noninvasive biomarkers such as imaging, not only for
early detection of disease, but also to reliably quantify
the severity of disease. Unfortunately, there are several
confounding factors, such as T1-related bias (5,55),
noise-related bias (5), T2* decay (47,55), and the com-
plex spectrum of fat (54,55) that limits the ability of MRI
to accurately quantify fat. The development of water–fat
separation methods into a quantitative tool for mea-
surement of fat is an active area of research by many
groups (55,56,72–79) and may offer new perspectives in
obesity research. Figure 15 shows examples of “fat-
fraction” images acquired in two patients with steatosis
using one approach (56).
Robust fat suppression methods are essential clinical
tools necessary to improve the conspicuity of important
anatomy and pathology, allowing MRI to fully exploit
the utility of its intrinsically high soft-tissue contrast.
Fortunately, there are a wide variety of fat suppression
and water–fat separation methods available. The ad-
vantages, disadvantages, and suggested applications of
most commonly used techniques for fat suppression
and fat-water imaging are summarized in Table 1.
Chemically selective methods work very well in many
applications where good magnetic field homogeneity is
present. Spatial-spectral pulses are used in specific
clinical scenarios, such as cartilage imaging, again,
when B0homogeneity is good. STIR imaging has been
an excellent and robust conventional method that may,
however, be replaced in the near future with chemical
shift-based methods that are capable of robust fat sup-
pression over large FOVs and in areas of severe mag-
netic field inhomogeneity. Chemical shift-based meth-
ods will likely play an increasing role for a wider variety
of applications, particularly when scan time concerns
are effectively addressed. Some of the newer techniques
with chemical shift-based methods with accurate spec-
tral modeling may provide continued improvement in
the quality of fat suppression. Emerging fat quantifica-
tion methods may become increasingly useful for quan-
tification of disease severity such as hepatic steatosis
and other disorders of abnormal fat deposition. Finally,
utilizing nonuniform spectral response of different fat
bands in SSFP, fat suppression methods have been
introduced to various applications, and will help prac-
titioners fully capitalize on the diagnostic utility of this
important pulse sequence through improved suppres-
sion of fat.
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