Hyungseok Jang1,2, Curtis N Wiens1, and Alan B McMillan1
1Department of Radiology, University of Wisconsin, Madison, WI, United States, 2Department of Electrical and Computer Engineering, University of Wisconsin, Madison, WI, United States
Synopsis
In this study, we propose a highly time efficient quantitative imaging
scheme where short and long T2* components can be simultaneously estimated. This
method is based on a multi-echo UTE hybrid encoding scheme, where the central
SPI region is oversampled to allow measurement of short T2* across a wide range
of TEs. The UTE acquisition is immediately followed by a gradient echo train to
measure long T2*. We show the proposed method can obtain an extensive number of
images (e.g., approximately 750 images) within a single acquisition and
reasonable scan time.Purpose
With recent advances in ultrashort time echo (UTE) imaging, it has
become feasible to perform quantitative measurements of short T2* decay to draw
clinically meaningful information (i.e. meniscus
1 and tendon
2).
However, the acquisition of multiple UTE and gradient echo images at different
TEs with sufficient spatial resolution is very time consuming (e.g., 20-80
minutes per scan). In this study we propose a novel multi-echo UTE imaging
scheme based on hybrid encoding, which allows simultaneous measurement of short
and long T2* components with highly efficient sampling.
Methods
In the
proposed method, we use a ramped hybrid encoding (RHE)
3 scheme where
gradients are applied before the RF pulse and ramped up to the maximum gradient
amplitude immediately after RF excitation to optimize per-excitation encoding
time as the pulse sequence diagram (PSD) shows in Figure 1-a. The missing data in
central k-space (during RF coil deadtime) is measured by Cartesian single point
imaging (SPI), and the outer k-space is acquired by radial frequency encoding
as shown in Figure 1-b. The diameter of SPI sampled region (N
SPI) is
required to be larger than $$γ/πFOV_D ∫_o^{TE}G_{max}(t)dt$$ to avoid aliasing (fold-over) artifact at the desired
time echo (TE), where FOV
D denotes a desired FOV, and G
max
is maximum phase encoding gradient amplitude. Conversely, the
density of SPI encoding (determined by NSPI) determines the availability
of multiple TEs; where a larger N
SPI enables a wider range of TEs available
without aliasing, and vice versa. In the proposed method, the SPI region is
intentionally oversampled to allow a wide selection of TEs at which images can
be reconstructed without aliasing, so that T2* decay can be resolved using
those images. The DTE is equivalent
to the data sampling rate (2µs in the utillized MR system), and is vastly lower
than conventional multi-echo imaging. In addition, a gradient echo (GRE)
imaging scheme is applied to acquire images at later TEs, which allows
estimation of longer T2* components. Figure 2-a shows the PSD for UTE acquisitions
followed by a gradient echo train. Note that a extremely large number of TEs
are available owing to the oversampled SPI in both UTE and GRE acquisitions as
indicated as green color in Figure 2-b, which is beneficial in parameter
estimation for multi exponential decay.
To
evaluate the proposed method, knee imaging was performed in 3.0T MRI scanner
(GE MR750) using a 8 channel T/R knee coil. Spherical SPI encoding was
performed with NSPI=49x49x81 with # of SPI encoding = 96,417, which
yields asymmetric FOV (larger in S-I direction), and the number of frequency
encoding was 45,000. Scan parameters were as following: 24µs hard pulse with
flip angle=6 degree, G
max=30mT/m, G
RF=0%, TR=5.96ms, scan time=14min
3sec, readout BW=500kHz, image resolution=1x1x1mm, and FOV=200mm. In practice, approximately
750 images can be reconstructed without aliasing artifact; however, only 83
images were reconstructed over 70µs~4080µs: utilizing 37 UTE images between 70µs
and 220µs, and 46 images at later echoes. After reconstruction pixel-wise T2*
was estimated by fitting the resulting magnitude image to a bi-exponential
signal decay model.
Results
Figure 3 shows reconstructed images at UTE and later echoes with matching
FOV, without any aliasing artifacts. Figure 4 shows estimated short T2* and
long T2* in (a) medial meniscus and (b) femoris tendon. The short T2* estimate
in the medial meniscus was 118.2±125.9µs, and the long T2* was 8.49±3.97ms. In femoris
tendon, short T2* estimate was 548.4±233.9µs, while long T2* estimate was
7.24±4.59ms. Short T2* fraction was 32.7±21.3% and 64.5±22.5% in meniscus and tendon,
respectively.
Discussion and Conclusion
In this study, we proposed a new acquisition to simultaneously estimate
short and long T2* components. The use of oversampled SPI with hybrid encoding permitted
acquisitions with highly efficient multi-TE sampling. In future work, the
abundance of temporal data will allow signal fitting with more complex models
such as multi exponential decay model including correction of fat and water
signal components. Addtionally, while scan time is reduced compared to
conventional methods, the additional scan time required by SPI encoding can be
reduced by employing acceleration techniques such as parallel imaging or
model-based 4D compressed sensing
4.
Acknowledgements
We acknowledge support from NIH EB013770 and GE Healthcare.References
1. Williams
et al. (2012). Osteoarthritis and Cartilage. 20(6): 486–494.
2. Chang et
al. (2015). Journal of Magn Reson Imaging. 42:114–120 (2015)
3. Jang et
al. (2015). Magn Reson Med. Epub ahead of print. PMID: 26381890.
4. Jang et
al. (2015). Magn Reson Med. 73(4): 1692–1701.