Dr. Lin is a Clinical Assistant Professor of Radiology,
Department of Radiology at the University of Washington Medical Center
and an Attending Physician at the Virginia Mason Medical Center,
Body magnetic resonance imaging (MRI) typically
presents more challenges, from the perspective of imaging sequences,
than MR imaging of the neurological and musculoskeletal systems. This is
because to adequately image many of the organs, the patient must hold
his breath, which limits the amount of time available for imaging, or
the organ itself (the heart) is moving. Many of the sequences used in
body MRI may be less familiar to radiologists than the sequences used in
the neurological and musculoskeletal systems. The goal of this article
is to present a broad conceptual and nontechnical framework that can
serve as a basis for understanding these sequences. For the purposes of
brevity, this article will assume a basic knowledge of the physics of
and sequences employed in MRI.
In the author’s experience, a major
barrier for radiologists learning body MRI sequences is the
proliferation of trade names used by different manufacturers. The use of
these trade names in the published literature may contribute to the
confusion. However, the number of ways to generate an image in body MRI
is actually limited, and the radiologist can avoid much of the potential
confusion by thinking of the sequences generically.1 For
example, the same 3-dimensional spoiled gradient-echo sequence will have
numerous trade names attached to it by different manufacturers.
However, referring to the sequence as a 3-dimensional spoiled
gradient-echo sequence avoids potential confusion and is usually much
more descriptive. That is, referring to a sequence generically tells us
almost everything one needs to know about the sequence, while the trade
names often give little information about the sequence other than a
memorable catchphrase. Table 1 lists generic sequences and common trade
names. The sequences we want to use should be fast, robust (meaning they
generate diagnostic-quality images the
majority of the time), and have the proper weighting.
Fast T2-weighted sequences
start with finding a T2-weighted sequence, which is fast, robust, and
has the proper weighting. While there are many ways to increase the
speed of a sequence once we have decided upon the sequence—eg,
increasing bandwidth or decreasing repetition time (TR)—starting from
the basic spin-echo sequence, there are 2 fundamental modifications that
can be made. One of these is to eliminate the 180-degree refocusing
pulse — that is, gradient-echo imaging. Without the refocusing pulse,
the TR and echo time (TE) can be substantially decreased, which greatly
decreases imaging time. However, for T2-weighted imaging, gradient-echo
imaging presents several problems. Without the refocusing pulse, the
images are now T2* rather than T2-weighted, and we no longer have the
proper weighting. In addition, as relatively high TE values are needed
for T2-weighted imaging, the T2* weighting will often result in
substantial susceptibility artifacts and the sequence may no longer be
robust. So it appears that in order to generate a robust, truly
T2-weighted image, the 180-degree refocusing pulse should be retained.
other basic modification to the spin-echo sequence that can be made is
to acquire multiple lines of k-space during the TE interval. In a
spin-echo sequence with a TE of milliseconds (msec), the time between 0
and 100 msec, when one line of k-space is filled, is “wasted” time in
which nothing is being acquired. If 10 lines of k-space (an “echo train”
of 10) are filled during this time (eg, at 10, 20, and up to 100 msec),
the sequence can be completed 10 times as quickly. This is called fast
spin-echo imaging. I will use this more common generic term, although it
is also a trade name; a more generic but less familiar term for the
same sequence is rapid relaxation with relaxation enhancement (RARE).
How well does this work for T2-weighted imaging? Very well, as the
retention of the 180-degree pulse gives us both T2-weighting and
decreases susceptibility artifact, and the sequence can be very fast
with a long-echo train. This technique can also be used for
gradient-echo imaging. In a single shot, echoplanar gradient-echo
sequence, all of the k-space is filled in one excitation. Echoplanar
sequences tend not to be very robust.
Now it might be asked that
if the length of the echo train determines the speed of the sequence,
why not always use the longest echo train possible? One problem is that
if we consider the previous example of a sequence with a TE of 100 msec
and echo train of 10, each of the 10 lines of k-space filled actually
has a different TE, ranging from 10 to 100 msec and thus a different
signal intensity, and there is no actual TE for the sequence as a whole.
In practice, the actual weighting of a fast-spin echo sequence is
determined by the effective TE. As the center of k-space is important
for signal-to-noise ratio and the periphery of k-space is important for
image detail, the TE at which the center of k-space is obtained (the
effective TE) will determine the weighting. However, there is still a
problem with the 9 other lines of k-space, which contributes to image
detail. As these are obtained at different TEs and will have different
signal intensities, the image will be blurred. If the echo train is
longer, the image blurring will increase as the number of different
signal intensities increases.
Thus, increasing the echo train to
decrease imaging time will typically increase image blurring. How
important is this blurring? It depends on what is being imaged. Let’s
consider fluid. Fluid has a long T2-relaxation time, so during a
fast-spin echo sequence with a TE of 100 msec and an echo train of 10,
there is actually little difference in the signal intensity of fluid
between 10 and 100 msec, as there has not been much T2 relaxation in
this time interval. Therefore, the effects of image blurring on fluid
are minimal. This suggests that the use of very long echo trains to
image fluid is optimal. On the other hand, solid tissue has a shorter
T2-relaxation time, and the difference in signal intensity of solid
tissue between 10 and 100 msec would be substantially greater than that
of fluid. So image blurring in solid tissues might increase
substantially at higher echo trains. During magnetic resonance
cholangiopancreatography (MRCP) (Figure 1), a long echo train allows for
very fast imaging of fluid, which is not substantially blurred. Solid
tissue is substantially blurred, but this is not what is being primarily
imaged. For other applications, shorter echo trains at the expense of
imaging time may be desirable, if image detail of solid tissues is
A supplemental way of decreasing imaging time is the variety of techniques that rely on alternate ways of filling k-space.2
These are essentially shortcuts in k-space and decrease image time at
the expense of signal-to-noise ratio (SNR). These techniques can be used
to decrease imaging time in both fast spin-echo and gradient-echo
imaging, and are best thought of in the framework of those sequences
rather than in isolation. One class of techniques uses nontraditional
trajectories for filling k-space, eg, spiral
or radial. In another class of techniques, not all of the k-space lines
are acquired and the remainder of k-space is filled with a partial
Fourier method. Half-Fourier acquisition is a commonly used technique
that relies on acquiring slightly more than half of the k-space lines
and filling in the missing data using the conjugate symmetry of k-space.
Fluid-imaging techniques, such as MRCP, typically will use this
sequence: single-shot fast spin-echo with half-Fourier acquisition. If
we learn the generic name of the sequence, it will tell us the
essentials of the sequence. In a fast spin-echo technique, multiple
lines of k-space are acquired in one excitation. A single-shot sequence
is one in which all of the k-space is acquired in one excitation to
maximize imaging speed and thus a large number of k-space lines must be
filled. In order to accomplish this, a technique, half-Fourier
acquisition, where only half of k-space is acquired is employed.
summary, T2-weighted sequences in body MRI are typically performed with
a fast spin-echo sequence. In these sequences, there is a trade-off
between imaging time and image blurring, which is determined by the echo
train. Very fast T2-weighted images can be obtained using a single-shot
technique. Additional k-space shortcuts can be used with fast spin-echo
sequences, which further decrease imaging time at the expense of SNR.
Fast T1-weighted sequences
we want to acquire a fast, robust study with T1-weighted image
contrast, one option is a fast spin-echo sequence, as used for
T2-weighted imaging. However, this does not work nearly as well as with
T2-weighted imaging. The substantially shorter TEs needed for
T1-weighted image contrast limit the increase in acquisition time
possible. For example, with a T1-weighted sequence with a TE of 20
milliseconds, only 2 lines of k-space might be filled during 20
milliseconds. This would halve the imaging time, but is still not nearly
enough for breath-hold imaging.
Therefore, gradient-echo imaging
must be used to acquire fast, robust, T1-weighted images. However, the
requirement for speed now presents a problem. Fast gradient-echo
sequences need to use a short TR. If the TR is shorter than the
T2-relaxation times of the imaged tissues, which will typically be case
for breathhold imaging, the transverse magnetization will not fully
decay before the next radiofrequency (RF) pulse. After multiple RF
pulses, there will be a “steady-state” residual-transverse
magnetization,3 which is constant (Figure 2). This
residual-transverse magnetization contributes T2-weighting to the image.
The sequence is now fast and robust, but it no longer has strict
More specifically, we have a steady-state, or
coherent, sequence in which the image contrast depends upon the T2- to
T1-ratio. What does this actually look like, and is this a positive or
negative factor? Fat and fluid will have high signal, as fat and fluid
have comparable T1- and T2-relaxation times. Other tissues will have
substantially lower signals, as T2-relaxation times are much shorter
than T1-relaxation times. So we will obtain images where fat and fluid
is bright, and everything else is isointense (Figure 3). This isn’t the
type of image contrast needed for most body MRI applications.
contrast is thus the default condition for fast gradient-echo imaging.
To remove this contrast, the TR could be increased so the transverse
magnetization will relax fully after each RF pulse. However, the
sequence would no longer be fast enough for breathhold imaging. Instead,
the residual-transverse magnetization is removed through “spoiling,”4
which can be done with a gradient or RF pulse (Figure 4). Removal of
the residual transverse magnetization does result in a fast, robust
sequence, which is T1-weighted.
Thus the majority of fast
T1-weighted sequences in body MRI applications are spoiled gradient-echo
images. These can be acquired 2-dimensionally or 3-dimensionally (in
MRI, 3-dimensional means that there is phase-encoding in 2 axes). Often
chemical shift (in- and out-of-phase) imaging is used in conjunction
(Figure 5). For applications in which a very short acquisition time is
needed (eg, MR angiography) these techniques are usually combined with a
The 3-dimensional spoiled gradient-echo
sequence is a versatile sequence that can be used both for MR
angiography and imaging of the parenchymal organs (Figure 6), simply by
changing the flip angle. The Ernest angle5 is the flip angle in spoiled gradient-echo imaging where signal intensity is maximized:
cos α = exp (-TR/T1).
the T1-relaxation time of intravascular contrast is shorter than that
of a parenchymal organ, the 3-dimensional spoiled gradient-echo sequence
would use a larger flip angle for MR angiography and a smaller flip
angle for parenchymal organ imaging. The flip angle must also be
adjusted for the TR (the higher the TR, the greater the flip angle).
gradient-echo sequences are another way to obtain fast T1-weighted
sequences that will not be addressed here as these sequences are not
commonly used for body MRI. In these sequences, the T1 weighting is
provided by a preparation pulse. Typically these sequences are used to
obtain multiple rapid T1-weighted images at the same level; eg, for
In summary, the spoiled gradient-echo
sequence is the workhorse sequence for fast T1-weighted imaging. Fast
gradient-echo sequences, without additional modification, are in the
steady-state and have T2/T1 weighting; they must be spoiled to obtain
T1-weighting. Spoiled gradient-echo sequences can be modified for a
variety of applications by adjusting the flip angle.
return to the steady-state gradient-echo sequence. This sequence is
both fast and robust. It has higher SNR than a spoiled gradient-echo
sequence, as the unspoiled residual-transverse magnetization contributes
to the signal. The intrinsically high SNR can be parlayed into a faster
imaging time by increasing the bandwidth and decreasing the TR. So
what’s not to like about a fast sequence with high SNR? We also need the
correct image contrast, and T2/T1-image contrast is not the contrast
needed for most body imaging applications. In the past, sequences were
always designed to avoid T2/T1 contrast. However, with increased
gradient strengths and shorter imaging times, there were applications in
which strict T1 or T2 weighting was not needed, as long as imaging
could be performed quickly. A good example is cine cardiac imaging, in
which what is required for image contrast is not strict T1 or T2
weighting but simply that the blood appears much brighter than the
myocardium. So once MRI scanners were fast enough, steady-state
sequences, which were previously explicitly avoided, turned out to be
There are a few different steady-state sequences.1
These are designed to maintain the steady state rather than spoil it.
Some of these sequences have minimal clinical use, others have
specialized applications (eg, DESS or CISS). The relevant steady-state
sequence for body MRI applications is a fully refocused or balanced
sequence, in which the steady state is maintained with balanced
gradients that minimize dephasing (Figure 7).
sequences have essentially replaced spoiled gradient-echo sequences for
cine cardiac imaging in recent years. Steady-state sequences are faster,
and have higher SNRs and contrast-to-noise ratios. With a spoiled
sequence, the “bright blood” comes from through plane-flow enhancement
(similar to time-of-flight angiography). Steady-state gradient-echo
sequences are not flow-sensitive, and the “bright blood” comes from
intrinsic contrast. There are substantial effects of this change in
sequences. For example, with steady-state gradient-echo sequences, the
myocardium can be better delineated from the cardiac chamber (Figure 8).
This affects determination of ventricular volumes6 and thus
ejection fraction (ventricular volumes determined by steady-state
sequences are larger than those determined by spoiled sequences).
However, not everything is advantageous. As steady-state sequences are
not flow-sensitive, it is harder to visualize turbulent flow (eg from
regurgitation) (Figure 9).
Steady-state sequences can also be
used to visualize blood outside of the heart; that is, for
nonenhanced-MR angiography. This may be more relevant with the recently
reported relationship between gadolinium and nephrogenic systemic
fibrosis. Steady-state sequences are the most common sequences used to
obtain “scout” images in body MR imaging. As very fast sequences with
high SNR, they are optimally suited for this purpose. As steady-state
sequences are very fast sequences where fluid is bright, they can be
viewed as a complement or alternative to single-shot fast spin-echo
sequences for body MRI applications (eg, for MRCP or fetal imaging).
Single-shot fast spin-echo sequences are typically preferred for most of
these applications, but one advantage of steady-state sequences is
substantially lower radiofrequency deposition.
steady-state gradient-echo sequences have become the standard for cine
cardiac imaging, and have other applications as well. These sequences
are very fast and robust, but they are limited to applications where
T2/T1-image contrast is acceptable.
- Boyle GE, Ahern M, Cooke J, et al. An interactive taxonomy of MRI sequences. Radiographics. 2006;26:e24.
- Lee VS. Fast scanning and k-space shortcuts. In: Lee VS. Cardiovascular MRI: Physical principles to practical protocols. Philadelphia, PA: Lippincott Williams and Wilkins. 2006:138-158.
- Chavan GB, Babyn PS, Jankharia BG, et al. Steady-state MR imaging
sequences: Physics, classification, and clinical applications. Radiographics. 2008;28:1147-1160.
- Mitchell DG, Cohen MS. Pulse sequences: Gradient echo and spin echo. In: MRI Principles, 2nd Edition. Philadelphia, PA: Elsevier Science; 2003:400.
- Mitchell DG, Cohen MS. MRI principles. Philadelphia, PA: Saunders; 2004:163-176.
- Mitchell DG, Cohen MS. Proton environments and T1 relaxation. Philadelphia, PA: Saunders; 2004:177-185.
- Mitchell DG, Cohen MS. MRI principles. Philadelphia, PA: Saunders; 2004:21-34.
- Hudsmith LE, Petersen SE, Tyler DJ, et al. Determination of
cardiac volumes and mass with FLASH and SSFP cine sequences at 1.5 vs 3
tesla: A validation study. J Magn Reson Imaging. 2006;24:312-318.