Samuel Fielden1, John Mugler2, Wilson Miller2, Alto Stemmer3, Josef Pfeuffer3, Berthold Kiefer3, and Craig Meyer1,2
1Biomedical Engineering, University of Virginia, Charlottesville, VA, United States, 2Radiology & Medical Imaging, University of Virginia, Charlottesville, VA, United States, 3Application Development, Siemens Healthcare, Erlangen, Germany
Synopsis
While
3D radial-based methods have become established in recent years for
ultrashort-echo-time (UTE) imaging, these acquisitions are generally slow due
to the inefficiency of radial k-space trajectories. The purpose of this work
was to implement a fast UTE acquisition based on an optimized 3D
stack-of-spirals acquisition and to perform a proof-of-concept evaluation of
the method for bone imaging of the skull and cartilage imaging of the knee.Introduction
Many
biological tissues exhibit such short relaxation times that their signals decay
completely by the time conventional sequences begin sampling. For this reason,
many tissues, especially bone, cartilage, ligaments, and tendons of the
musculoskeletal system have been difficult to study using MRI. To address this
issue, ultrashort-echo-time (UTE) pulse sequences have been developed to begin
sampling as closely as possible to the excitation pulse. The time delay caused
by the slice selection gradient has led to the development of specialized RF
pulses and acquisition schemes for 2D imaging [1]; however these methods are
inherently challenging and may have limited robustness. Alternate
implementations of UTE sequences are based on 3D radial acquisitions with
nonselective RF pulses [2]. While relatively easy to implement and perform, 3D
radial acquisition schemes are slow, often requiring several minutes to collect
a full 3D volume of data [3].
Qian, et al [4]
demonstrated that a 3D stack-of-spirals acquisition can achieve very short echo
times by beginning each spiral readout immediately after the through-plane
phase-encoding gradient waveform has completed, resulting in a variable TE in
the through-plane direction. For the center of k-space where the PE gradients
are small (or nonexistent), the minimum TE achievable with slab-selective
excitation pulses in that study was approximately 600 µs. This variable-TE
stack-of-spirals method introduces some blurring in the through-plane direction
in exchange for improved scan times due to the efficiency of spiral readouts.
Here, we have replaced the slab-selective excitation pulse with a short
nonselective hard RF pulse [5], reducing the minimum TE, and demonstrate the
sequence’s utility in visualizing bone (skull) and cartilage in the knee of a
normal volunteer.
Methods
A
prototype 3D spoiled gradient-echo sequence was developed to support a
stack-of-spirals acquisition. A 60 µs nonselective hard excitation pulse was
used, reducing the minimum TE to 50 µs. Maximum TE depended on number of slices
and slice resolution, and was generally in the range of 250 – 400 µs (Fig. 1).
The sequence’s operation
was demonstrated in two settings. In the first, a human head was scanned with
parameters: TR = 10 ms; TE = 50-370 µs; flip angle 5°; matrix 96x96x64; FOV 240
mm
3; 98 interleaves of 1.0 ms duration each; 67-second acquisition
time. Imaging was performed using a 12-channel head RF coil. A second
volumetric image was obtained with a TE of 5.1 ms (to preserve fat/water phase)
to provide late-TE comparison images. The second setting was a human knee,
using an extremity coil, with scan parameters adjusted slightly to achieve true
1.5x1.5x1.5 mm
3 isotropic resolution. Most notably, because
cartilage has a longer T2* than bone, the readout duration was extended to 2.5
ms and the required number of interleaves dropped to 70, resulting in a total
acquisition time of 97 seconds. All imaging was performed on a 1.5 T scanner (MAGNETOM
Avanto, Siemens Healthcare, Erlangen, Germany). Informed consent was obtained
prior to imaging.
Results
Figure
2 shows whole-head spiral UTE images alongside late-echo images to illustrate
the difference in contrast achievable with this sequence. Direct subtractions
as well as scaled subtractions [6] are shown, highlighting the bone signal.
SNR, measured in a region of the frontal bone, is 54 in the minimum-TE image,
23 in the direct-subtraction image, and 43 in the scaled-subtraction image. In
Fig. 3, cartilage and the meniscus is visible in the knee joint.
Conclusions
By
utilizing non-selective RF pulses, the minimum echo time achievable by a
stack-of-spirals UTE sequence was reduced from 600 to 50 µs, enabling capture
of signals from rapidly decaying musculoskeletal tissues. Rapid imaging may be
desirable for patients who have joint or bone pain, and the efficiency of
spiral readouts supports rapid generation of 3D UTE images; achieving
whole-head UTE images in 67s and whole-knee images in 97s.
Acknowledgements
No acknowledgement found.References
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