The ability of single-shot magnetic resonance imaging (MRI) to image faster than one slice per second has created several clinical applications. This article describes the principles of single-shot MRI with respect to conventional multi-shot techniques, and emphasizes the differences between single-shot images acquired with gradient echoes and spin echoes. Six examples illustrate how these differences can be used to choose a single-shot imaging method for a specific clinical application.
After receiving a BA in mathematics from the University
of Pennsylvania (1988), Frank Rybicki received a combined MD, PhD
degree from the Harvard-MIT Division of Health Sciences and
Technology (1994). Between 1994 and 1996, he served as a Research
Associate in the Department of Radiology, Brigham and Women's
Hospital, Boston, MA. In 1996 Dr. Rybicki completed a medical
internship at Mount Auburn Hospital, Cambridge, MA. He is
currently a fourth-year resident in diagnostic radiology at
Brigham and Women's Hospital.
Dr. Mulkern is an Associate Professor of Radiology in the
Department of Radiology, Children's Hospital and Harvard Medical
School, Boston, MA.
Despite the wide availability of magnetic resonance imaging
(MRI) techniques which can image faster than one slice per second,
sub-second imaging, and specifically "single-shot" MRI, has not
replaced slower "multi-shot" MRI in routine practice. In this
article, "single-shot" refers to imaging a single slice in a single
repetition time (TR). Specifically, the entire image is obtained
via a train of echoes after a single slice selective excitation.
Two types of echoes are described in this paper, gradient echoes
(GE) and spin echoes (SE). Single-shot MRI (SS-MRI) sequences that
use GE trains are generally termed Echo Planar Imaging (EPI);
sequences that use SE trains
have a variety of acronyms including single-shot fast-spin echo
(SSFSE), rapid acquisition with relaxation enhancement (RARE), and
Half-fourier turbo spin echo (HASTE). As defined, SS-MRI excludes
an important fast imaging alternative: short TR gradient echo
imaging in the steady state. This technique has comparable scan
times and clinical roles that can be found in the literature.
Before discussing the principles of SS-MRI, we answer the
question, "Why hasn't SS-MRI completely replaced multi-shot MRI
(MS-MRI) sequences that utilize many TR periods to acquire image
data over several minutes?" The main reason is that, in general,
multi-shot methods yield images of higher quality than single-shot
methods. To illustrate, imagine yourself in a unique art gallery,
and your task is to see the information in 64 paintings. The
gallery is unique because the only light is overhead, and it is
turned on for a brief moment and then quickly dimmed. While the
gallery is illuminated, you must obtain information from all 64
paintings, one at a time. This is nearly analogous to SS-MRI.
Turning on the overhead light represents the excitation, and the
MRI signal begins at a maximum and rapidly decays (or dims). The
information in the 64 paintings represents the patient data with
each painting corresponding to an echo.
The analogy requires a modification based on the fact that in
MRI, each echo must be sampled at fixed rate, for example, 128
samples per echo. This translates into a requirement that you blink
your eyes 128 times per painting! Consequently, your task in the
unique gallery (and SS-MRI) is not a leisurely exercise. Instead,
SS-MRI requires the acquisition of a complete image data set (in
this example 64 echoes, each sampled 128 times) after only a single
The same image obtained with MS-MRI shares the requirement of
128 samples for each of the 64 echoes. However, MS-MRI uses
multiple excitations or "shots," and each excitation corresponds to
turning on the overhead light. Thus, MS-MRI may use 64 excitations
for 64 echoes, corresponding to turning on the overhead light for
each painting. Therefore, the sampling (blinking) rate is slower
rate for each echo (painting). In the gallery, you obtain more
overall information. In MRI, multi-shot images generally have
better image quality, but at the price of a longer imaging time.
When more time is acceptable, SS-MRI has not replaced MS-MRI.
However, when rapid imaging is needed, SS-MRI has become
invaluable. The goals of this article are to describe the
principles of SS-MRI with an emphasis on how the choice of GE or SE
trains affects the image properties, and to illustrate how the
properties of SS-MRI sequences are used in six clinical
Principles of single shot MRI
MRI data acquisition
--To form an image, data (e.g., 64 echoes, each with 128 samples)
is used to generate the "k-space" representation of the image.
Understanding the mathematical relationship between the k-space
representation and the image itself is less important than
understanding three basic points: 1) the k-space representation of
the image exists. That is, there are two representations for each
slice: the anatomic representation viewed by the radiologist, and
the k-space representation; 2) like the anatomic representation,
the k-space representation is a two dimensional matrix of pixel
values. In the gallery example, k-space is a 64 * 128 matrix. Each
of the 64 lines is occupied by one echo, and each of 128 entries
per line is occupied by a sample; 3) each pixel in the k-space
matrix is an individual sample of the signal, or transverse
magnetization, measured after the slice selective excitation. The
excitation (turning on the overhead light) tips the equilibrium
longitudinal magnetization from a slice of spins into the
"transverse" plane, the plane perpendicular to the main magnetic
field. Once there, the precession of spins forms the signal which
is acquired using appropriately oriented radiofrequency (RF) coils.
The signal is acquired as a series of echoes used to construct the
k-space representation of the image. The MRI terminology for this
process is "filling k-space." While numerous books and articles
this paper focuses on the echoes that fill k-space and how the
choice of an echo (GE versus SE) leads to several image
Gradient echoes and spin echoes
--The two basic types of echoes used to fill k-space are GE and SE.
A third type, "stimulated echoes," has a role in clinical MRI and
is described in the literature.
A GE is generated by successive application of two magnetic field
gradients of opposite polarity. The first gradient dephases the
spins; the second gradient, applied shortly after the first,
rephases the spins. The GE peaks when the effects of the dephasing
and rephasing gradients are balanced. The generation of a GE is
often illustrated with a racing analogy.
This article uses a horse race to describe how spins can form
echoes. Several horses (spins), each with a different but constant
speed, begin a race at the starting line. During the first half of
the race (the dephasing gradient), the horses spread along the
track. At the midpoint of the race (the application of the
rephasing gradient), the horses reverse their direction. Since each
horse maintains its constant speed, they return to the starting
line together (the peak of the GE).
The generation of a SE begins like a GE with a gradient
dephasing the spins. However, instead of a rephasing gradient, a
180 degree refocusing pulse is applied. The refocusing pulse
effectively reverses the polarity of all gradients that are
present, including the dephasing gradient. This refocusing or
reversal leads to the generation of the SE. In the horse race
analogy, the first half of the race is identical. However, at the
midpoint of the SE race, the refocusing pulse instantaneously
transports each horse to its mirror position on the opposite side
of the starting line. Each horse continues to run in the same
direction at its constant speed, but after the refocusing pulse the
slower horses are ahead of the faster ones. The SE peaks when the
spins are refocused, corresponding to the time when the horses all
return to the starting line.
Gradient and spin echo trains determine the image properties
--Unlike MS-MRI where multiple excitations are performed to fill
k-space, in SS-MRI every line of k-space is filled after a single
excitation by collecting multiple echoes as so-called "echo
trains." A GE train produces a GE-SS-MRI sequence while a SE train
produces a SE-SS-MRI sequence (figure 1). The fact that either echo
train can fill all of k-space in one excitation was noted by
Mansfield who coined the term "echo planar imaging" or EPI.
However, the term EPI has become linked to GE-SS-MRI while
sequences that use SE trains have acronyms such as SSFSE, RARE, and
HASTE. In the following comparison of GE and SE methods, all four
acronyms (EPI, SSFSE, RARE, HASTE) refer to single-shot
There are three differences between GE and SE trains that
contribute to the image characteristics and ultimately to the
clinical uses GE-SS-MRI and SE-SS-MRI.
Gradient echoes can be collected approximately four times as
fast as spin echoes (figure 1). The reason is that SE trains
require refocusing pulses; the time for each refocusing pulse,
associated gradients, and the spin echo is approximately 4 msec. In
contrast, GEs can be sequentially sampled a 1 msec intervals.
Consequently, GE-SS-MRI is faster than SE-SS-MRI for an equivalent
number of echoes.
The rate at which GEs fade (T2*) is faster than the rate at
which SE fades (T2). In the gallery analogy, the overhead light
dims faster for GE trains. The difference between T2 and T2* is
based on two sources of signal loss, "reversible" and
The etiology of "reversible" signal loss is unwanted magnetic field
gradients that can be reversed by the refocusing pulse in the SE.
Reversible gradients are either global (main field inhomogeneity)
or local (air-tissue interfaces or metallic implants). Since
reversible gradients can not be refocused in a GE, the associated
signal is lost in GE-SS-MRI. "Irreversible" signal losses are
random in nature; they can not be refocused by the 180 degree
refocusing pulse and therefore contribute to signal loss in both
GE-SS-MRI (T2*) and SE-SS-MRI (T2). As shown below, the T2* decay
in GE trains makes perfusion and functional MRI (fMRI) of the brain
possible. Therefore, these applications use predominantly EPI.
The artifact created from susceptibility shifts and the chemical
shift between fat and water is refocused with SE trains. In EPI,
both shifts cause oscillations of the signal which lead to spatial
distortions. Complete descriptions
include the fact that these distortions are primarily along the
dimension of the image known as the "phase encoding" dimension.
Both susceptibility (from bone and air) and chemical shift (from
fat) provide obstacles for MRI in abdomen and contribute to the
fact that SE-SS-MRI is more often used for abdominal applications
such as fetal imaging and MRCP.
Before the applications of SS-MRI, we discuss one important
feature that is independent of the type of echo train: the echo
time (TE) which ultimately determines the degree of T2 or T2*
weighting in the image. Since a single-shot MR image is generated
from an echo train and each echo has a different T2* or T2
weighting, what is the "effective TE" of the image? Or, to make the
question more specific, how can a SE-SS-MRI sequence be tailored
for fetal MRI (which requires intermediate T2-weighting) and MRCP
(which requires heavy T2 weighting)? The answer is related to how
individual echoes are used to fill k-space,
but ultimately the radiologist chooses an effective TE which
determines the T2 weighting.
Applications of SS-MRI
Echo planar imaging
Perfusion imaging of the brain:
Perfusion imaging evaluates relative cerebral blood volume and flow
and has the potential to contribute to the management of patients
with cerebral ischemia.
Imaging is usually performed by the bolus tracking method, which is
enabled by two features of EPI: image speed (for example, 10 slices
of the brain every 2.5 seconds) and T2* decay. Between 15 and 30
seconds after bolus intravenous administration of a gadolinium
chelate contrast agent, the concentration in the cerebral vessels
peaks, and the vessels achieve a markedly different susceptibility
than the surrounding tissue.
This difference in susceptibility creates local microscopic field
gradients around the vessels. Similar to local gradients at an
air-tissue interfaces, these gradients cause a reversible loss
which contributes to T2* decay to a far greater extent than T2
decay. In perfusion imaging, the degree of signal loss is captured
over time by EPI and is then analyzed
to determine relative cerebral blood flow (figure 2).
Diffusion imaging of the brain:
Diffusion-weighted MR images provide the earliest imaging of acute
cerebral infarction with intense signal generated by the decreased
apparent diffusion coefficient (ADC) at the infarct site. The
feature of EPI that enables diffusion MRI is speed. Diffusion
weighting in EPI is achieved by a modification to the pulse
sequence as shown in figure 1A. Specifically, additional gradients
(diffusion sensitization gradients) are placed around an additional
which only minimally compromises the image speed that is required
to freeze the macroscopic motion of the normal brain pulsation from
the beating heart. With macroscopic motion frozen, MRI can detect
the microscopic random motion of water molecules within and between
cells. In acute infarction, there is decreased microscopic motion
and thus a decreased ADC.
On diffusion-weighted images, regions of low ADC are hyperintense
(figure 3). One interpretation of the decreased ADC in infarcted
brain is cell swelling (cytotoxic edema), leading to more
intracelluar water within a given voxel. Because speed is the
essential requirement for diffusion imaging, SE-SS-MRI can also
been used. However, for technical reasons,
EPI is more commonly employed in clinical practice.
Functional MRI of the brain:
Functional MRI (fMRI) maps regions of the brain that become
"activated" by a task performed by the patient, for example finger
tapping or viewing a strobe light. These tasks would be expected to
activate the motor strip and visual centers, respectively. fMRI has
expanded from research protocols to a promising tool for
presurgical diagnosis. For example, fMRI maps identify the motor
cortex or primary visual centers preoperatively so these areas can
be avoided as much as possible during surgery.
As with perfusion imaging, the speed and T2* sensitivity makes
EPI the most widely used sequence for fMRI. The physiology measured
by fMRI is the activation of regions in the brain that receive more
oxygenated blood than neighboring or "unactivated" regions.
Since deoxyhemoglobin is paramagnetic and oxyhemoglobin is not, the
brain surrounding vessels with a higher concentration of
deoxyhemoglobin has more rapid T2* decay than brain surrounding
vessels with more oxyhemoglobin. The change in T2* of brain tissue
surrounding a vessel secondary to differences in deoxyhemoglobin
concentration (fMRI) is less than the change in T2* from perfusion
MRI with the bolus tracking method. This translates into relatively
less contrast in fMRI. Thus, measurements (20 EPI slices every 2
seconds) are repeated with and without the activation task; for
example, 10 cycles of 20 seconds of finger tapping (the "on state")
followed by 20 seconds without tapping (the "off state"). Various
can then be used to determine which parts of the brain are
responding to the on/off states, and color maps of those regions
can be overlaid on the anatomical images (figure 4).
Limitations of EPI:
Each of the three applications of EPI provides complementary
information to routine brain MRI protocols that rely heavily on
MS-MRI. The fact that each application images the brain may suggest
a limitation of EPI in other body parts. Although EPI is capable of
freezing motion in the abdomen,
artifacts from susceptibility and chemical shift has limited the
clinical use of EPI outside the brain (figure 5). In clinical
evaluation of the abdomen (and chest), SE-SS-MRI, although slightly
slower than EPI, has predominated, primarily because of the better
Spin echo SS-MRI
Applications of obstetric MRI include evaluation of maternal
disease and evaluation of the fetus when sonography is either
limited or indeterminate for an anomaly. When obstetric MRI is
performed for the fetus,
SE-SS-MRI is used to meet three requirements: speed (to freeze
fetal motion), the removal of susceptibility and chemical shift
artifact, and intermediate T2 weighting. The need for intermediate
T2 weighting in fetal MRI introduces T2-related signal loss in the
later echoes. This obstacle is overcome by utilizing a mathematical
relationship between the information contained in the first half of
the echoes and the information in the second half of the echoes.
Consequently, only the first half of the echoes (or, in practice,
slightly greater than the first half) is required in so-called
which freezes fetal motion (figure 6).
MR cholangiopancreatography (MRCP)
--Single-shot MRCP provides a noninvasive breath-hold evaluation of
the biliary tract.
The imaging requirements for MRCP are similar to fetal imaging:
speed (to freeze motion), the removal of chemical shift and
susceptibility artifact (typically from air in the duodenum or
surgical clips), and T2 weighting. Because of its very long T2,
bile is discriminated from shorter T2 tissues such as liver and
pancreas. Clinical MRCP is typically performed with breath-hold
half-Fourier SE-SS-MRI with a single thick slice (e.g., 5 cm)
acquisition followed by multiple thin slice (e.g., 3 mm)
acquisitions which can be viewed after maximum-intensity projection
(MIP) reconstruction (figure 7).
MRI of the pulmonary parenchyma
--The many challenges imposed by MRI of the pulmonary parenchyma
include motion and large local gradients created from the alveolar
air/soft-tissue interfaces. Although the many attempts at pulmonary
MRI have included multi-shot methods, the speed and insensitivity
to susceptibility of SE-SS-MRI has been used for breath hold,
ECG-triggered MRI (figure 8) of many pulmonary disorders.
SS-MRI uses only a single excitation to generate a complete
image data set. This data is acquired via a train of either
gradient echoes or spin echoes, and the type of echo train used in
a single-shot method determines several imaging properties: image
speed, T2* versus T2 decay, and artifact from susceptibility and
chemical shifts. We hope that this paper has provided a concise
review of SS-MRI, image properties in single-shot imaging methods,
and clinical applications.
We thank Drs. Clare Tempany, Mary Frates, Gregory Sica, Liangge
Hsu, Richard Schwartz, Russel Blinder, Carol Barnewolt, and Michael
Rivkin for their assistance with the figures.