Yajun Ma1, Hyungseok Jang1, Zhao Wei1, Zhenyu Cai1, Yanping Xue1, Eric Y Chang2, Jody Corey-Bloom1, and Jiang Du1
1UC San Diego, San Diego, CA, United States, 2VA health system, San Diego, CA, United States
Synopsis
To image myelin in
brain more robustly on clinical scanners, we propose a Short TR Adiabatic
Inversion Recovery prepared UTE (STAIR-UTE) sequence for volumetric myelin
imaging in vivo. With STAIR
technique, long T2 tissues with a broad range of T1s
can be sufficiently suppressed. High myelin contrast can be robustly obtained with a TR less than 250
ms. The resultant myelin imaging shows a clear loss of myelin signal in
multiple sclerosis (MS)
lesions.
Introduction
Ultrashort echo time (UTE) sequences with echo times less
than 50 µs can directly detect signals from myelin protons (1-4). However, over
90% of the UTE signal originates from long T2 water protons, even in
myelin-rich white matter. Thus, sufficient long T2 suppression is
crucial for direct myelin imaging. One
commonly used approach is to use an adiabatic full passage (AFP) pulse to
invert the longitudinal magnetizations of long T2 tissues, and UTE
data is acquired at an appropriate inversion time (TI) with long T2 signal
nulled (4-6). Thus,
long T2 suppression is sensitive to the selection of TI in current
IR-UTE techniques (3-7). A
slight TI offset may cause significant long T2 contamination, as the
residual long T2 signals may be higher than that of myelin.
Moreover, T1 relaxation may change with age and pathology in the brain
(8,9). This makes the corresponding TI determination more challenging. Furthermore,
a single TI may not be sufficient to null all long T2 components due
to T1 variations in different white matter regions in the same brain
(10-11). To address the aforementioned problems in myelin imaging, we propose a
Short TR Adiabatic Inversion Recovery prepared UTE (STAIR-UTE) technique for
more robust long T2 suppression regardless of T1
variations in white matter.Methods
Figure 1A shows
features of the 3D STAIR-UTE sequence (12,13). Following an AFP pulse, Nsp separate k-space spokes with
an identical time interval, τ, are used for fast data acquisition (Figure 1B). Figure 1C shows the contrast mechanism of myelin imaging. The
longitudinal magnetizations of long T2 components are inverted by the AFP pulse and
start to recover right after the AFP pulse. In contrast, the longitudinal
magnetization of myelin is not inverted, but largely saturated, due to the fast
T2 relaxation of
myelin protons during the relatively long AFP pulse (14,15). Thus, the myelin
magnetization recovers from zero, and pure myelin signals can be acquired at a
specific TI when the long T2 components are nulled.
Numerical simulation was performed to investigate
the efficiency of long T2 suppression for the STAIR scheme with different TRs. The simulated T1
values of the long T2
components ranged from 20 to 2500 ms,
which cover T1 values for
both white matter and gray matter. The TIs were determined by Eq. [8] in Ma et
al. (13) in order to minimize the average signals from the long T2
components with T1
values ranging from 500 to 1500
ms. The Sratio is defined as the signal
intensity ratio between myelin and long T2s.
A series of water
phantoms with different T1s were prepared. Nine 5-mL water phantoms were prepared with MnCl2˙4H2O
concentrations of 0.0055, 0.01, 0.015, 0.0195, 0.0265, 0.0375, 0.085, 0.18, and
1.4828 g/L. The nine phantoms were then placed in parallel in a cylinder
container filled with 1% agarose. These phantoms were scanned using the
STAIR-UTE sequence with different TRs of 50, 100, 150, 200, 250, 500, and 800
ms. Other sequence parameters were as follows: field of view (FOV)=15×15×16
cm3, acquisition matrix=160×160×40, TE=32 μs, Nsp= 5, τ=5 ms, flip angle=20°, and bandwidth=125 kHz.
In vivo brain
imaging was performed on ten healthy volunteers (19-45
years of age, 6 males and 4 females) and ten
patients with multiple sclerosis (MS) (35-70 years of age, 3 males and 7
females). Informed
consent was obtained from all subjects in accordance with guidelines of the
institutional review board. The
sequence parameters were as follows: FOV=22×22×30 cm3,
matrix=140×140×60, TR/TI=140/61 ms, TE=32 μs, Nsp=5, τ=3.2 ms, flip
angle=32°, bandwidth=125 kHz, oversampling factor=2.3, scan time=9.5 min.
Clinical T2-FLAIR
sequence was used for comparison.Results and Discussion
The numerical
simulation study suggests that signal suppression for long T2
tissues with different T1s using the conventional IR scheme with a
relatively long TR (e.g. TR = 800 ms) is sensitive to the selection of TI
(Figure 2A). Additional efforts are needed to determine the optimal TI for
nulling specific long T2 components. In comparison, the proposed
STAIR scheme with TR<250 ms can effectively suppress long T2
tissues with a broad range of T1s with a single TR/TI combination
and a high myelin contrast can always be obtained (Figure 2B). The phantom study further demonstrated the
sequence’s ability to efficiently suppress long T2 signals with a
wide range of T1s (Figures 2C and 2D).
The volunteer study demonstrated the robustness of the
STAIR-UTE sequence in efficient suppression of long T2 components in
white matter and selective imaging of myelin, as confirmed by the short T2*
of 0.22±0.01 ms (Figure 3). The
representative whole brain myelin images acquired with the 3D STAIR-UTE sequence
from the same volunteer are shown in Figure 4. Figure 5 shows comparisons of STAIR-UTE and clinical T2-FLAIR images for three representative
patients with MS. Hyperintense lesions detected in T2-FLAIR images show as signal loss in
corresponding STAIR-UTE images, which demonstrates short T2 myelin signal loss and, thus, demyelination
of MS lesions. Conclusion
The 3D STAIR-UTE
sequence with a short TR/TI combination provides robust suppression of long T2
components in white matter, allowing selective volumetric imaging of myelin in
white matter of the brain in vivo. Acknowledgements
The authors acknowledge grant support from NIH (1R01NS092650) and VA Clinical Science and Rehabilitation R&D Awards (I01CX001388 and I01RX002604).References
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