Synopsis

To propose a combination of a three dimensional ultrashort echo time sequence employing cones trajectories with an AdiabT1rho preparation (3D AdiabT1rho UTE-Cones) for volumetric T1rho assessment of both short and long T2 tissues in the knee joint on a clinical 3T scanner. Simulation, phantom, ex vivo and in vivo studies were carried out in this feasibility study.

Introduction

Method

Features of the 3D AdiabT_1rho UTE-Cones pulse sequence used in this study are shown in Figure 1. An even number (N_IR) of adiabatic inversion pulses (IR) are used for AdiabT_1rho preparation. When N_IR is a multiple of four, then every four consecutive adiabatic IR pulses follow a MLEV4 phase cycling scheme ⁵. Following the AdiabT_1rho preparation are N_sp separate k-space spokes or acquisitions with an equal time interval for fast data acquisition. A short rectangular pulse (duration of 26 to 52 µs) is used for non-selective signal excitation in each spoke (Fig. 1B), followed by 3D spiral trajectories with conical view ordering (Fig. 1C) ⁶.

At steady state, the signal equation is expressed as follows when a single acquisition (N_sp = 1) is obtained after AdiabT_1rho preparation:

\[S(TSL)=M_0\sin(\alpha)\frac{e^{-TSL/T_{1\rho}}(1-e^{-(TR-TSL)/T_{1}})}{1-e^{-TSL/T_{1\rho}}e^{-(TR-TSL)/T_{1}}\cos(\alpha)}+C[1]\]

Where M₀ is the equilibrium state magnetization and α is the excitation flip angle. A constant C is induced to account for noise and artifacts. For multispoke acquisitions, we modified Eq. [1] by simply changing cos(α) to cos^N_sp(α) to account for the saturation effect of N_sp acquisitions, which is expressed as follows ⁷:

\[S(TSL)=M_0\sin(\alpha)\frac{e^{-TSL/T_{1\rho}}(1-e^{-(TR-TSL)/T_{1}})}{1-e^{-TSL/T_{1\rho}}e^{-(TR-TSL)/T_{1}}\cos^{N_{sp}}(\alpha)}+C[2]\]

A phantom was prepared as 2% w/v agarose gel containing 0.1 mM MnCl₂. A 3D UTE-Cones actual flip angle imaging (AFI) sequence and a variable flip angle (VFA) sequence were employed for B₁ mapping and T₁ measurement, respectively ⁸. The following parameters were used for 3D AdiabT_1rho UTE-Cones: TR=500 ms, FA=10°, matrix=128×128×20, N_IR=0,2,4,6,8,12 and 16, N_sp=1,5,15,25,35 and 45.

Magic angle effect study for both T_1rho and T₂ was carried out by imaging a sliced human patellar sample (around 4 mm thick). The sample was imaged with five angular orientations (i.e. 0°, 30°, 55°, 70° and 90°) between the primary collagen fiber direction in the middle zone of the articular cartilage and the main field . For each angular orientation, 3D AdiabT_1rho UTE-Cones and 2D CPMG sequences were used for T_1rho and T₂ measurements, respectively. The sequence parameters were: FOV=5×5×2 cm3, matrix=128×128×10, bandwidth=83.3 kHz, TR=1000 ms, FA=10°, N_sp=5, N_IR=0,4,8,12,16 and 20.

Ex vivo (n=4) and in vivo (n=6) knees were imaged with the following parameters: FOV=15×15×10.8 cm³, bandwidth=166 kHz, TR=500 ms, FA=10°, matrix=256×256×36, Nsp=25, N_IR=0,2,4,6,8,12 and 16 each with a scan time of 2 min 34 sec. UTE-Cones AFI data were acquired with TR₁/TR₂=20/100 ms, FA=45°, matrix=128×128×18, scan time of 4 min 57 sec. VFA UTE-Cones data were acquired with TR=20 ms, FA=5°,10°,20° and 30°, matrix=256×256×36, scan time of 9 min 28 sec;

Results and Discussion

Figure 2 shows the phantom results. The original model shows increased errors with higher N_sp, while the modified model only increased very slightly with higher N_sp.

Figure 3 shows the magic angle effect in AdiabT_1rho UTE-Cones imaging. T₂ increased by approximately 200 % as the angle increased from 0° to 55°. AdiabT_1rho only increased by approximately 50%.

Figure 4 shows knee study of a 23y volunteer. Excellent AdiabT1rho fitting was achieved demonstrating a T_1rho of 13.7±1.0 ms for the quadriceps tendon, 22.5±1.2 ms for the PCL, 21.5±1.1 ms for the meniscus and 43.5±5.9 ms for the patellar cartilage.

Table 1 summarizes the T_1rho values for quadriceps tendon, patellar tendon, ACL, PCL, meniscus, patellar cartilage and muscle of the six in vivo knees.

Conclusion

Acknowledgements

References

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