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The Angular Dependence of 3D Ultrashort Echo Time Cones Adiabatic T1ρ (3D UTE Cones-AdiabT1ρ) Imaging of the Achilles Tendon
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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Figures

Figure 1. Quantitative 3D UTE Cones sequences include the basic 3D UTE Cones sequence (A), which employs a short rectangular pulse for signal excitation followed by 3D spiral sampling with a minimal nominal TE of 32 µs and conical view ordering (B), the 3D UTE Cones actual flip angle imaging (AFI) sequence with dual-TR acquisitions for B1 mapping (C), the conventional 3D UTE Cones sequence with a single TR for T1 measurement with the variable flip angle (VFA) or variable repetition time (VTR) method (D), and the 3D UTE Cones-AdiabT1ρ sequences for AdiabT1ρ measurement (E).

Figure 2. The rotating scheme in the magic angle study of cadaveric human Achilles tendon samples. Achilles tendon samples were rotated 0°, 30°, 55°, 70°, and 90° relative to the B0 field.

Figure 3. 3D UTE Cones-AdiabT1ρ imaging of an Achilles tendon sample oriented 0° (A-D) and 55° (a-d) with a series of TSLs of 0 ms, 24 ms, 48 ms, and 72 ms. Regular 3D UTE Cones-CW-T1ρ imaging of the same Achilles tendon sample oriented 0° (E-H) and 55° (e-h) with a series of TSLs of 0 ms, 6 ms, 10 ms, and 15 ms. 3D UTE Cones-T2* imaging of the same Achilles tendon sample oriented 0° (I-L) and 55° (i-l) with a series of TEs of 0 ms, 4.4 ms, 8.8 ms, and 14 ms. Signal for the Achilles tendon acquired by 3D UTE Cones-AdiabT1ρ imaging decays much slower at 0° than that acquired by the latter two sequences.

Figure 4. 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 imaging (A-E), regular 3D UTE Cones-CW-T1ρ imaging (F-J), and 3D UTE Cones-T2* imaging (K-O). AdiabT1ρ values show the smallest magic angle effect with 3.5-fold increase from 13.9 ms at 0° to 48.7 ms at 55°. Regular CW-T1ρ values show increased magic angle effect with a 5.3-fold increase from 7.9 ms at 0° to 37.9 ms at 55°. T2*values show the largest magic angle effect with a 13.8-fold increase from 2.3 ms at 0° to 31.7 ms at 55°.

Figure 5. The angular dependence of 3D UTE Cones-AdiabT1ρ (A), regular 3D UTE Cones CW-T1ρ (B), and 3D UTE Cones-T2* (C), as well as the fold changes of all three biomarkers (D) for a human Achilles tendon sample. The mean and standard deviation for AdiabT1ρ, CW-T1ρ, and T2* values at 0°, 30°, 55°, 70°, and 90° relative to the B0 field are displayed. The AdiabT1ρ values show the least angular dependence with a 3.3-fold increase from 0° to 55°. The CW-T1ρ and T2* values increased by 7.0-fold and 9.9-fold, respectively.

Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)
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