DOI:10.2214/AJR.06.0556
AJR 2007; 188:W199-W201
© American Roentgen Ray Society
Isotropic 3D Fast Spin-Echo with Proton-Density-Like Contrast: A Comprehensive Approach to Musculoskeletal MRI
Lawrence Yao1,
John T. Pitts2 and
David Thomasson1
1 Diagnostic Radiology Department, Clinical Center, National Institutes of
Health, 10 Center Dr., Rm. 1C360, Bethesda, MD 20892.
2 InVivo Corporation, Orlando, FL.
Received April 23, 2006;
accepted after revision July 17, 2006.
Address correspondence to L. Yao.
J. T. Pitts is employed by InVivo Corporation, Orlando, FL.
WEB This is a Web exclusive article.
Abstract
OBJECTIVE. Scanning time considerations have restricted routine use
of 3D Fourier transform (3DFT)-encoded MRI to gradient-recalled echo
sequences. We sought to combine isotropic 3DFT acquisition with fast spin-echo
at a practical scan duration. This strategy offers versatile image contrast
for musculoskeletal evaluation and facilitates image reformation tailored to
the depiction of small anatomic features.
CONCLUSION. Isotropic 3DFT fast spin-echo is feasible on current MRI
scanners and has the potential to improve musculoskeletal evaluation.
Keywords: ankle • high resolution • MRI • MR technique
Introduction
Currently, the most versatile type of sequence for musculoskeletal
MRI evaluation is arguably a fast spin-echo acquisition with proton
density-weighting. Fast spin-echo sequences are preferred over conventional
spin-echo sequences for reasons of efficiency. This type of sequence has
favorable contrast characteristics for assessment of hyaline cartilage,
ligaments, tendons, and fibrocartilage
[1-3].
Proton density-weighting also confers favorable signal-to-noise ratio
characteristics, which facilitates the high image resolution that is necessary
for evaluation of small body parts.
The nature of small body parts evaluation poses special challenges for
routine MRI scan plane selection and prescription. Depiction of important
structures may require complex oblique scan planes. Variances in body part
positioning may alter the relationships between structures of interest and
anatomic landmarks. Optimal oblique or double oblique scan plane prescription
therefore requires detailed knowledge of clinical anatomy.
For these reasons, 3D Fourier transform (3DFT) acquisition is conceptually
attractive for small body parts MRI evaluation, both because of
signal-to-noise ratio efficiency
[3] and the facilitation of
image reformation in complex, oblique planes after scan acquisition. Practical
application of 3DFT imaging has been largely restricted to gradient-recalled
echo (GRE) sequences, which can be implemented at high resolution in
reasonable scanning times. Despite varied strategies for GRE scan acquisition,
GRE sequences typically lack the tissue contrast necessary for broad
musculoskeletal diagnosis [4].
These sequences are therefore more commonly adjunctive to spin-echo imaging or
used for imaging after IV contrast administration.
In this article, we illustrate the emerging feasibility of 3DFT acquisition
of turbo spin-echo (TSE) images for routine musculoskeletal evaluation. The
efficiencies of parallel imaging have brought the scanning duration for this
strategy into a clinically acceptable range. We illustrate how this sequence
can be implemented with an efficient, relatively short repetition time and a
driven equilibrium pulse [5] to
create tissue contrast that is similar to the proton density-weighting
preferred for many musculoskeletal applications.
Materials and Methods
An isotropic 3DFT TSE, proton density-weighted sequence was implemented for
musculoskeletal evaluation on a commercial 3-T MRI system (Intera, Philips
Medical Systems). Scanning parameters for 3DFT TSE were TR/TE effective,
720/40; echo-train length, 11; slice partitions, 142; imaging matrix, 256
x 256. The k-space sampling order was low to high (centric). A terminal
-90° radiofrequency pulse was used for restoration of longitudinal
magnetization (DRIVen Equilibrium pulse, [DRIVE]). Refocusing radiofrequency
pulses were reduced in amplitude (140°) to reduce the specific absorption
rate (SAR). These refocusing pulses were spatially broad to enable echo
spacings of 7 milliseconds at a receiver bandwidth of ± 56.3 kHz.
Scanning acceleration was achieved by using sensitivity encoding (SENSE) in
both the slice and in-plane phase-encoding dimensions, with an acceleration
factor of two. Half-Fourier sampling was also used in the in-plane phase
direction. The total scanning time was 7 minutes 20 seconds.
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Fig. 1A —Several anatomic structures in ankle and hindfoot of healthy
43-year-old male volunteer. Parameters for axial 2D turbo spin-echo (TSE)
image, A, were TR/TE, 2,800/30 and echo-train length, 7. Parameters for
various optimized, view planes reformatted from single 3D Fourier transform
TSE acquisitions, B-F, were 720/40 and echo-train length, 11.
Axial 2D TSE image shows calcaneofibular ligament (arrowhead) and
portion of anterior talofibular ligament (arrow). Synovial fluid in
anterolateral gutter is nearly isointense to cartilage and inconspicuous.
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Fig. 1B —Several anatomic structures in ankle and hindfoot of healthy
43-year-old male volunteer. Parameters for axial 2D turbo spin-echo (TSE)
image, A, were TR/TE, 2,800/30 and echo-train length, 7. Parameters for
various optimized, view planes reformatted from single 3D Fourier transform
TSE acquisitions, B-F, were 720/40 and echo-train length, 11.
Axial oblique 3D TSE image reformatted to show anterior talofibular ligament
(arrow) to advantage. Joint fluid deep in relation to this ligament
is relatively bright because of driven equilibrium technique, creating useful
positive contrast relative to cartilage.
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Fig. 1C —Several anatomic structures in ankle and hindfoot of healthy
43-year-old male volunteer. Parameters for axial 2D turbo spin-echo (TSE)
image, A, were TR/TE, 2,800/30 and echo-train length, 7. Parameters for
various optimized, view planes reformatted from single 3D Fourier transform
TSE acquisitions, B-F, were 720/40 and echo-train length, 11.
Second axial oblique 3D TSE image is reformatted to show calcaneofibular
ligament (arrow) to advantage. This ligament is typically not well
seen on axial images oriented for best depiction of anterior talofibular
ligament.
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Fig. 1D —Several anatomic structures in ankle and hindfoot of healthy
43-year-old male volunteer. Parameters for axial 2D turbo spin-echo (TSE)
image, A, were TR/TE, 2,800/30 and echo-train length, 7. Parameters for
various optimized, view planes reformatted from single 3D Fourier transform
TSE acquisitions, B-F, were 720/40 and echo-train length, 11.
Sagittal oblique 3D TSE image reformatted to course of flexor hallucis longus
tendon (arrowheads) as it traverses tarsal tunnel.
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Fig. 1E —Several anatomic structures in ankle and hindfoot of healthy
43-year-old male volunteer. Parameters for axial 2D turbo spin-echo (TSE)
image, A, were TR/TE, 2,800/30 and echo-train length, 7. Parameters for
various optimized, view planes reformatted from single 3D Fourier transform
TSE acquisitions, B-F, were 720/40 and echo-train length, 11.
Axial oblique 3D TSE image reformatted to show both inferoplantar,
longitudinal (arrowhead), and medioplantar oblique (arrow)
portions of spring ligament. These structures are not consistently visualized
on routine axial or sagittal 2D image acquisitions.
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Fig. 1F —Several anatomic structures in ankle and hindfoot of healthy
43-year-old male volunteer. Parameters for axial 2D turbo spin-echo (TSE)
image, A, were TR/TE, 2,800/30 and echo-train length, 7. Parameters for
various optimized, view planes reformatted from single 3D Fourier transform
TSE acquisitions, B-F, were 720/40 and echo-train length, 11.
Coronal oblique 3D TSE image reformatted to show separate fascicles of
anterior tibiofibular ligament (arrowhead). This structure, which is
deranged in high ankle sprain, is challenging diagnosis on routine 2D imaging.
Tibiospring fibers of deltoid ligament (arrow) and tibiotalar joint
are also well shown in this viewing plane.
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Conventional 2D TSE imaging with proton density-weighting was also
performed in the axial plane with the following scan parameters: 2,800/30;
echo-train length, 7; imaging matrix, 320 x 256; slice thickness, 3 mm;
field of view, 14 cm; averages, 2. The total scanning time for the 2D
acquisition was 3 minutes 25 seconds.
The utility of this scanning strategy was illustrated in the ankle and
hindfoot. The ankles of healthy volunteers (four men; age range 25-57 years)
were scanned in the prone position using a 6-channel, 8-element volume
extremity receiver coil. Images were acquired in the oblique sagittal plane
using a 14-cm field of view. The voxel dimensions were 0.5 x 0.55
x 0.55 mm. Frequency encoding was oriented along the craniocaudal axis.
Images from the 3D acquisition were reformatted on a commercially available
workstation (Advantage Windows, GE Healthcare). Multiplanar reformations were
generated at a 1.6-mm slice thickness.
MR images of the ankle of a healthy volunteer are shown in Figure
1A,
1B,
1C,
1D,
1E,
1F.
Figure 1A is a sample axial 2D
TSE image with proton density-weighting. Figures
1B,
1C,
1D,
1E,
1F are illustrative
reformations of a 3D TSE acquisition in compound oblique planes tailored to
depict anatomic structures of routine interest. Image contrast in the 3D TSE
images is similar to the 2D TSE images, although there is relatively lower
signal in muscle and higher signal in synovial fluid. Whereas the axial 2D
image has an apparent advantage in in-plane resolution, the reformations of
the 3D acquisition show how anatomy can be retrospectively viewed in complex
view-plane orientations without a discernible compromise in spatial
resolution. In this way, a single 3DFT TSE scan facilitates a detailed
assessment of various structures that would require a multiplicity of
carefully prescribed compound oblique 2D TSE acquisitions.
Discussion
Long scan times have limited the clinical utility of 3D spin-echo imaging
implementations [6,
7]. On current high-field
scanning platforms that support multichannel coil technology, 3D TSE is now a
viable alternative to traditional multiplanar 2D scanning protocols because of
the acceleration of phase encoding afforded by parallel acquisition strategies
such as simultaneous acquisition of spatial harmonics (SMASH) or SENSE.
Although 3D TSE scanning times are longer than individual 2D TSE scanning
times, the 3D technique may decrease overall scanning time by obviating
multiple orthogonal 2D acquisitions that are routinely required for clinical
evaluation. Three-dimensional TSE acquisition may improve evaluation of
complex anatomy by facilitating multiplanar image reformation in arbitrary
planes tailored to specific anatomic regions of interest. Isotropic or near
isotropic 3D data sets offer potentially superior evaluation of small body
parts, given that optimally oriented 2D scans cannot be acquired for all
potential structures of interest in regions of complex anatomy.
Volumetric 3D TSE imaging may offer practical advantages as well.
Two-dimensional multiplanar scanning prescription requires a thorough working
knowledge of clinical anatomy. The optimal scanning plane may also be
influenced by body part position, which is particularly variable for the
extremities. Scanning prescription is therefore imperfect, subjective, and
dependent on user experience. A 3D volumetric scanning prescription is
inherently simpler and more robust.
Limitations to the 3D TSE approach are several. The technique is only
feasible for anatomic parts in which a parallel imaging technique can be
applied in two dimensions. Foldover artifact must be avoided in two orthogonal
dimensions. SAR limitations pose challenges to implementation of this
strategy, particularly at 3-T and higher magnetic field strengths. For these
reasons, the full potential of this strategy may await the availability of
send-receive, multichannel extremity, or smallvolume coils.
SAR considerations dictate the use of refocusing radiofrequency pulses of
reduced amplitude, which may have unanticipated effects on tissue contrast
[8]. These effects, and the use
of fast recovery or tip-back radiofrequency pulses
[5], may warrant a
reformulation of the scanning parameters that define a 3D TSE sequence with
the most broadly useful contrast characteristics for musculoskeletal
evaluation. In the implementation we have presented, we sought to mimic the
proton-density contrast that is popular in 2D TSE scans. The contrast in the
3D TSE sequence is not strictly a reflection of proton density weighting,
however, but a complex function of T1, T2, and the ratio of T2 to T1. The
utility of a DRIVE pulse to confer "arthrographic" contrast to
T1-weighted 2D TSE imaging, creating a mixed-contrast sequence useful for
musculoskeletal evaluation, has also been highlighted in another recent report
[9].
A volumetric, isotropic 3D approach to routine musculoskeletal diagnosis
presents new challenges for MR image review and interpretation. The strength
of the approach is the facility to view the data in arbitrary and customized
cross-sectional planes. To fully exploit the potential of 3D data sets, the
interpretative process demands real-time interactive image reformation.
Fortunately, many diagnostic workstation environments have already
incorporated these important image navigation capabilities.
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Abstract
Introduction
