Mei Wu1,2, Mingxin Chen1, Yajun Ma1, Akhil Kasibhatla1, Lidi Wan1, Saeed Jerban1, Hyungseok Jang1, Eric Y Chang1,3, and Jiang Du1
1Department of Radiology, University of California, San Diego, San Diego, CA, United States, 2Department of Radiology, Guangzhou First People’s Hospital, School of Medicine, South China University of Technology, Guangzhou, China, 3Radiology Service, VA San Diego Healthcare System, San Diego, CA, United States
Synopsis
This study investigated
the magic angle effect in three-dimensional ultrashort echo time Cones Adiabatic
T1ρ (3D UTE Cones-AdiabT1ρ) imaging of the Achilles
tendon on a clinical 3T scanner. The magic angle effect was investigated by
repeated UTE Cones-AdiabT1ρ imaging of five human Achilles tendon samples at five angular orientations ranging
from 0° to 90° relative to the B0 field. Conventional Cones continuous
wave T1ρ (Cones-CW-T1ρ) and Cones-T2*
sequences were also applied for comparison. Cones-AdiabT1ρ showed a much reduced
magic angle effect as compared to regular Cones-CW-T1ρ and Cones-T2*,
suggesting its potential use as a novel biomarker for musculoskeletal
(MSK) imaging.
Introduction
A major
confounding factor in quantitative magnetic resonance imaging of MSK tissues is
the magic angle effect (1-3). The ordered collagen fibers in MSK tissues are
subject to dipole-dipole interactions that are modulated by the term 3cos2(θ)-1,
where θ is the angle between the fiber orientation and B0. Both T2
and continuous wave T1ρ (CW-T1ρ) show a strong
magic angle effect, demonstrating up to several-fold increase when θ is changed
from 0° to 55° (3). Adiabatic T1ρ (AdiabT1ρ) relaxation has
been proposed to address this challenge (4-10). Regular AdiabT1ρ sequences cannot
evaluate many of the MSK tissues with short T2 relaxation. Ultrashort echo time
(UTE) sequences can image short T2 tissues (11). More recently, the combination
of 3D-UTE-Cones data acquisition and adiabatic T1ρ preparation (3D
UTE Cones-AdiabT1ρ)
has been proposed for magic angle insensitive imaging of both short and long T2
tissues in the MSK system. The Achilles tendon has a more organized collagen
fiber structure than most other MSK tissues, and is thus expected to be subject
to a greater magic angle effect. The purpose of this study was to investigate
the magic angle effect in 3D UTE Cones-AdiabT1ρ imaging of the Achilles
tendon on a clinical 3T scanner.Methods
The
3D UTE Cones-AdiabT1ρ sequence employed an even number of adiabatic inversion recovery (NIR) pulses
followed by regular UTE Cones imaging, during which a short rectangular pulse
excitation was followed by Cones sampling. Following each adiabatic T1ρ preparation, fast
Cones data acquisition was performed using a number of spokes (Nsp) with an
equal time interval τ. The spin lock time (TSL) is defined as the total
duration of the train of adiabatic IR pulses, i.e. TSL=NIR×Tp (Tp is duration
of a single adiabatic IR pulse). Accurate T1 measurement is needed for T1ρ calculation
because of the use of a relatively short TR. 3D UTE Cones actual flip angle
imaging (AFI) was used to map B1 inhomogeneity, which, together with a variable
flip angle (VFA) method (3D UTE Cones-AFI-VFA), was used for accurate T1
mapping (11,12). Features of the sequences used in this study are shown in
Figure 1. Typical imaging parameters included a field of view (FOV) of 5×5 cm2,
a slice thickness of 0.4 mm, and a receiver bandwidth (BW) of 105 kHz. Other
sequence parameters were: 1) UTE Cones-AFI (11): TR1/TR2 = 20/100 ms, flip
angle (FA) = 45°; 2) UTE Cones-VFA (13): TR = 20 ms; FA = 4°,7°,10°,15°, 20°,
25°, and 30°; 3) UTE Cones-AdiabT1ρ (13): TR = 1000
ms; FA = 10°; Nsp = 11; NIR = 0, 2, 4, 6, 8, 12, 16, and 20; 4) UTE Cones-CW-T1ρ (13): TR = 1000
ms; FA = 10°; Nsp = 11; TSL = 0, 1, 3, 6, 10, and 15 ms; 5) UTE Cones-T2*:
TR = 80 ms, FA = 15°, fat saturation, one set of multi-echo acquisitions (TEs =
0, 2.2, 4.4, 8.8, 14, 20 ms). The imaging protocol was
repeated on five human Achilles tendon samples (five donors aged 28-84 years,
mean age 60.4±27.2 years; 2 males, 3 females) five times, each with a different
orientation (0°, 30°, 55°, 70°, and 90° relative to B0). The rotating scheme is
shown in Figure 2. Single-component model was applied to fit T1, T1ρ, AdiabT1ρ, and T2*.
The angular dependence of each biomarker was analyzed. Results
Figure
3 shows representative images from 3D UTE Cones-AdiabT1ρ imaging, regular
UTE Cones CW-T1ρ imaging, and UTE Cones-T2*
imaging of the same Achilles tendon sample oriented 0° and 55° relative to the B0 field,
respectively. Signal from the Achilles tendon decayed much faster at 0° than at 55° when scanned using the regular UTE
Cones-CW-T1ρ and UTE Cones-T2*
sequences, but slower when scanned using the 3D UTE Cones-AdiabT1ρ sequence.
Figure
4 shows exponential fitting curves for a global ROI of an Achilles tendon
sample oriented 0°, 30°, 55°,
70°, and 90° to the B0 field using 3D
UTE Cones-AdiabT1ρ,
regular UTE Cones CW-T1ρ,
and UTE Cones-T2* imaging, respectively. AdiabT1ρ values show the
smallest magic angle effect with a 3.5-fold increase through the minimization
of dipolar interaction at 55°. In comparison,
CW-T1ρ and T2*
showed much stronger magic angle effects with 5.3-fold and 13.8-fold increase,
respectively.
Figure
5 shows the angular dependence of 3D UTE Cones-AdiabT1ρ, regular UTE Cones
CW-T1ρ, and UTE Cones-T2*
for a representative human Achilles tendon sample. The
UTE Cones-AdiabT1ρ values show a much reduced magic angle effect as
compared with the regular UTE Cones CW-T1ρ (3.7-fold
reduction) and UTE Cones-T2* values (6.6-fold
reduction).
The
average AdiabT1ρ values show the
smallest magic angle effect, with a 3.6-fold increase from 13.6 ms at 0° to 48.4 ms at 55°. The average CW-T1ρ values show much
increased magic angle effect, with a 6.1-fold increase from 7.0 ms at 0° to 42.0 ms at 55°, while the average T2* values show
the strongest magic angle effect, with a 12.3-fold increase from 2.9 ms at 0° to 35.8 ms at 55° .Conclusion
The
3D UTE Cones-AdiabT1ρ sequence is less sensitive to the magic angle effect than Cones-CW-T1ρ and Cones-T2*,
and may be used as a novel biomarker for MSK imaging. Acknowledgements
The authors are
thankful for support from R01AR075825, 1R01NS092650, 2R01AR062581,
1R01AR068987, I01CX001388, and I01RX002604.References
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