Dr. Lighvani is a Fellow in the Section of Musculoskeletal Radiology and Dr. Melhem is
the Chief of the Division of Neuroradiology in the Department of
Radiology, Hospital of the University of Pennsylvania, Philadelphia, PA.
Magnetic resonance imaging (MRI) of the spine is currently
one of the most frequently requested MR examinations. The demand for
this exam will probably increase with the advancing age of the patient
population coupled with the prevalence of degenerative spine disease and
lower back pain. There is a great need, therefore, to optimize imaging
protocols and to minimize imaging time, while obtaining clinically
The traditional approach to spinal MRI has
relied upon imaging in both axial and sagittal planes to adequately
evaluate the central canal and neural foramina. Application of
3-dimensional imaging with multiplanar reconstruction could obviate the
need for obtaining images in 2 planes, decreasing the examination time
and associated patient discomfort and motion artifact.1 This
article reviews several advances in image acquisition and processing,
combined with the clinical introduction of 3 Tesla (3T) magnets, which
permit the application of 3-dimensional spinal MRI for routine clinical
Two different approaches to 3-dimensional imaging of the
spine are gradient echo (GRE)-based and fast spin echo (FSE)-based
imaging.1 Previous attempts at 3-dimensional spinal imaging
had been unsuccessful: GRE-based imaging was susceptibile to artifacts
and had inadequate delineation of spinal cord abnormalities;2,3 FSE-based imaging had relatively long imaging times.4
The inherent sensitivity of GRE sequences to susceptibility artifact is
further exacerbated at 3T ﬁeld strength, rendering this technique of
limited value in spine imaging at high ﬁeld strengths. Therefore,
FSE-based imaging remains the most promising approach for obtaining
clinically relevant volumetric imaging.
In the past, clinical
application of FSE-based 3-dimensional imaging was limited by long
acquisition times. Cerebrospinal ﬂuid (CSF) pulsation artifacts were
also problematic, resulting in CSF heterogeneity.
solutions to the long acquisition times for 3-dimensional FSE imaging
focused on obtaining more echoes per magnetization pulse (i.e.,
increasing the echo train length, ETL) and reducing the time between
successive magnetization pulses (decreasing repetition time, TR).4,5
However, decreasing the TR lowers the time for protons to regain their
longitudinal magnetization, resulting in a lower steady-state value of
longitudinal magnetization and decreased signal. The new steady-state
value of magnetization depends on the efﬁciency of the proton to give up
its energy (characterized by the T1 relaxation time). Consequently, CSF
will demonstrate relative signal loss compared with the spinal cord
because of its longer T1 with the end result being a loss of
“myelographic effect.” Additionally, increasing the ETL ampliﬁes the
inherent blurring of FSE MR imaging related to T2 decay-induced
modulation in the amplitude of the echo within k-space.1,6
Another way of reducing long acquisition times is to decrease the
phase-encoding steps; however, this would result in decreased spatial
resolution if other parameters, such as the ﬁeld-of-view, are not
The following will provide a brief overview of new
imaging solutions including driven equilibrium, SPACE (Sampling
Perfection with Application optimized Contrasts using ﬂip-angle
Evolutions) readout and parallel imaging which have, in part, remedied
the problems of decreased CSF signal; increased blurring and decreased
spatial resolution related to decreased TR; and, increased the ETL and
decreased the number of phase-encoding steps, respectively (Table 1).
These new imaging solutions combined with relatively increased
signal-to-noise at 3T, compared with 1.5T, now permit clinically
relevant 3-dimensional volumetric FSE imaging of the spine.
Driven equilibrium pulse is a novel approach to the decreased CSF signal associated with imaging with a shorter TR.7
This technique involves the application of a resonant 90 degree
radiofrequency (RF) pulse at the time when the transverse magnetization
is refocused by the last 180 degree RF pulse. The residual transverse
magnetization, which is relatively high in the case of CSF and
characterized by its long T2 relaxation time, is converted to
longitudinal magnetization (Figure 1).
artiﬁcial recovery of longitudinal magnetization effectively compensates
for the relative loss of CSF signal associated with shorter TR and
restores the myelographic effect. Ultimately, the combination of a
traditional FSE technique and driven equilibrium pulse results in faster
imaging by eliminating the need for long TR while maintaining relative
The driven equilibrium pulse is critical at higher
ﬁeld strength and increasing ETL, as T1 relaxation time prolongs with
increasing ﬁeldstrength (it is approximately 15% to 20% greater at 3T
vs. 1.5T) and recovery of longitudinal magnetization is halted by every
refocusing 180 degree RF pulse. Therefore, relying on the natural
recovery of the longitudinal magnetization in order to maintain the
myelographic effect would require signiﬁcantly prolonged TR and thus an
increase of the imaging time.8
Experiments conducted at
3T comparing traditional FSE with FSE/driven equilibrium (RESTORE,
Siemens Healthcare, Malvern, PA) demonstrate the impact of this pulse
sequence on myelographic effect at various TR values (Figure 2). As
expected, the driven equilibrium pulse results in higher CSF signal
intensity compared with traditional FSE throughout the range of
clinically relevant TR values (Figure 3A). There is a dramatic increase
in the relative CSF signal intensity with decreasing TR values,
particularly at the lower TR values (Figure 3B), which would be used in
3-dimensional FSE imaging.
For example, clinically relevant
images of the cervical spine can be obtained by combining 3-dimensional
FSE and driven equilibrium imaging of the cervical spine utilizing a
single sagittal 3-dimensional FSE slab with 70 partitions. By reducing
the TR to 210 ms, a 1 mm3isotropic voxel size can be achieved in a total imaging time of 2:40 min. (Figure 4).8
is important, however, to keep in mind that images obtained with driven
equilibrium pulse are not equivalent to standard T2-weighted images.
While driven equilibrium is able to restore the myelographic effect of a
standard T2 sequence by augmenting CSF signal, its ability to depict
intrinsic cord lesions with prolonged T2 relaxation is diminished,
particularly at low TR. Preliminary work suggests that application of
longer TR when assessing intrinsic cord lesions on the order of 2000 ms
would yield adequate contrast to allow for detection and
characterization of more subtle cord lesions.
Another critical component of clinically viable imaging is the application of SPACE readout.9
This modiﬁcation of FSE involves applying variable ﬂip-angle RF pulses.
The nonselective RF pulse permits ultra-short echo spacing and the
variable ﬂip angle results in a pseudo steady-state with maintained
relative tissue contrast (Figure 5) as well as a reduction of blurring
associated with high ETL. Another advantage of SPACE readout is a
reduction in speciﬁc absorption rate (SAR) as a consequence of the
application of lower ﬂip angles. This is critical at 3T, as FSE pulse
sequences are inherently SAR intensive and SAR is proportional to the
square of the magnetic ﬁeld.Therefore, SAR becomes a limiting factor, as
it quadruples with the doubling of the magnetic ﬁeld and increases
along with increasing ETL and decreasing TR.
imaging allows for faster image acquisition and potentially improved
image quality by undersampling k-space. By using the inherent spatial
sensitivity information that is present in phased-array RF receiver
coils, only a fraction of phase-encoding steps need to be acquired for
image construction while maintaining image contrast and spatial
The coil sensitivity information can
be obtained either through a prescan calibration during initial patient
setup, resembling a low-resolution image, or through “auto calibration”
by obtaining additional lines of k-space with each sequence.10,11 Subsequently, image reconstruction can be performed either in the space domain (SENSE)12 or in the spatial frequency domain with simultaneous acquisition of spatial harmonics (SMASH).13
With SENSE, images obtained from each independent receiver coil/channel
are combined to unfold the ﬁnal image (Figure 6A). With SMASH,
information from each independent receiver coil/channel is used to
calculate the missing k-space data and then to generate the ﬁnal image
Shorter imaging times can be achieved by reducing the
number of phase-encoding steps needed to generate the ﬁnal image. The
maximum parallel imaging factor, reﬂecting a reduction in imaging time,
is 2 to 3 in each phase-encoding direction. The ampliﬁcation of
spatially dependent noise associated with the undersampling of the
phase-encoding steps currently prohibits the use of a parallel imaging
factor >3(Figure 7). In 3-dimensional imaging, the phase-encoding
steps can be reduced in 2 spatial dimensions allowing for a maximum
parallel imaging factor of 6 to 9.10
The increased signal-to-noise ratio at 3T14
partially counterbalances the reduction in signal-to-noise ratio
associated with higher parallel imaging factors which would not be
otherwise practical at 1.5T.15
This imaging protocol,
incorporating driven equilibrium, SPACE readout and parallel imaging,
has demonstrated its effectiveness inevaluating degenerative disease in
the cervical, thoracic and lumbar spine (Figures 8, 9 and 10). The
combination of high contrast and improved spatial resolution allows the
radiologist to characterize disc pathology, assess for presence of cord
and nerve root impingement, and evaluate neural foraminal patency.
Additionally, the ability to obtain coronal reconstructions (Figure 9B)
without added imaging time provides the radiologist the opportunity to
examine degenerative pathology in a new way. Coronal reconstructions may
be particularly helpful in the evaluation of patients with scoliosis or
pronounced extra-foraminal disease by providing a quick global
overview. Whether the additional imaging plane will improve the
radiologist’s ability to detect disease or better characterize it still
needs to be determined. Also, the volumetric nature of 3-dimensional
images lends itself to theoretical future integration of computer-aided
diagnosis (CAD) software to assist the radiologist in assessing central
canal and foraminal narrowing.
Moreover, the 3-dimensional imaging
protocol can signiﬁcantly improve the radiologist’s ability to evaluate
the postoperative spine. The susceptibility artifact related to
metallic hardware is minimized through the application of ultrashort
echo spacing achieved with nonselective RF pulses used in SPACE readout
(Figure 11), which otherwise would severely limit the diagnostic value
of traditional imaging sequences. Furthermore, some of the more
frequently seen intrinsic cord lesions, such as syringomyelia and
transverse myelitis (Figure 12), are easily detectable on these
sequences as well.
The combination of
higher signal-to-noise ratio at high-ﬁeld imaging, faster image
acquisition with parallel imaging, effective T2 with SPACE readout and
elimination of CSF pulsation artifacts with driven equilibrium make
3-dimensional FSE spinal MRI feasible for routine clinical practice.
Further investigation of these novel techniques may focus on assessing
the ability to detect and characterize more subtle cord lesions.
- Melhem ER. Technical challenges in MR imaging of the cervical spine and cord. Magn Reson Imaging Clin N Am. 2000;8:435-452. Review.
- Czervionke LF, Daniels KL, Wehrli FW, et al. Magnetic susceptibility artifacts in gradient-recalled echo MR imaging. AJNR Am J Neuroradiol. 1988;9:1149-1155.
JS, Remley K. Effects of magnetic susceptibility artifacts and motion
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- Yuan C, Schmiedl UP, Weinberger E, et al. Three-dimensional fast
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- Yousem DM, Atlas SW, Goldberg HI, Grossman RI. Degenerative
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Sze G, Kawamura Y, Negishi C, et al. Fast spin-echo MR imaging of the
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- van Uijen CM, den Boef JH. Driven-equilibrium radiofrequency pulses in NMR imaging. Magn Reson Med. 1984;1(4):502-507.
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Lichy MP, Wietek BM, Mugler JP 3rd, et al. Magnetic resonance imaging
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- Blaimer M, Breuer F, Mueller M, et al. SMASH, SENSE, PILS, GRAPPA: How to choose the optimal method. Top Magn Reson Imaging. 2004; 15(4):223-236. Review.
- Glockner JF, Hu HH, Stanley DW, et al. Parallel MR imaging: A user's guide. Radiographics. 2005;25(5):1279-1297. Review.
Pruessmann KP, Weiger M, Scheidegger MB, et al. SENSE: Sensitivity
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- Sodickson DK, Manning WJ. Simultaneous acquisition of spatial harmonics (SMASH): Fast imaging with radiofrequency coil arrays. Magn Reson Med. 1997;38(4):591-603.
- Ocali O, Atalar E. Ultimate intrinsic signal-to-noise ratio in MRI. Magn Reson Med. 1998;39(3): 462-473.
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