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UTE with Multi-Echo Radial Acquisition for PET/MRI Attenuation Correction1Engineering Physics, Tsinghua University, Beijing, China, 2Gordon Center for Medical Imaging, Radiology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, United States, 3Nuclear Medicine, West China Hospital, Sichuan University, Sichuan, China, 4Key Laboratory of Particle and Radiation Imaging, Ministry of Education, Medical Physics and Engineering Institute, Tsinghua University, Beijing, China
Synopsis
Reliable estimation of PET attenuation coefficient (AC) is a fundamental problem in PET/MR imaging. Accurate measurement of bone density variation is vital to generate reliable subject-specific AC maps using MR. Also, minimizing the data acquisition time of MR sequences for AC is essential to allow more MR scans in a single PET/MR scanning session. In this work, we propose a new acquisition scheme combining UTE and multi-echo radial acquisition to reliably estimate the composition of water,
Introduction
Reliable estimation of PET attenuation coefficient (AC) is a fundamental problem in PET/MR imaging [1-3]. One of the standard MR-based method for generating AC is to segment an MR image volume into different tissue classes (e.g., soft-tissue, fat, bone, and air) and then assign a single AC value to each segment [4-6]. However, this method does not account for intra- and inter-subject variation, which may lead to unacceptable biases especially in areas such as bone [3,7,8]. Thus, accurate measurement of bone density variation is vital to generate reliable subject-specific AC maps [9-13]. Also, minimizing the data acquisition time of MR sequences for AC [14,15] is essential to allow more MR scans in a single PET/MR scanning session. In this work, we propose a new acquisition scheme combining UTE and multi-echo radial acquisition to reliably estimate the composition of water, fat and bone in a minute scan. The performance of the proposed method is verified via an in vivo experiment.Methods
All experiments were performed on a whole-body PET-MR scanner (mMR, Siemens Healthcare, Erlangen, Germany) using head coil for both transmission and signal reception. A healthy volunteer was scanned using the proposed 3D UTE multi-echo radial sequence (Fig.1). The study protocol was approved by our local Institutional Review Board (IRB). The imaging parameters were: field-of-view (FOV) = 240$$$\times$$$240$$$\times$$$240 $$$mm^3$$$, matrix size = 128$$$\times$$$128$$$\times$$$128, TR = 9.9 $$$ms$$$, TE1/TE2/TE3/TE4/TE5/TE6/TE7 = 70/2110/2810/3550/4250/4990/5690 $$$\mu s$$$, hard pulse duration = 100 $$$\mu s$$$, gradient ramp time = 400 $$$\mu s$$$ (gradient slew rate = 48.9 $$$mT/m/ms$$$), plateau gradient amplitude = 19.57 $$$mT/m$$$, dwell time = 2.5 $$$\mu s$$$, number of TR units = 6434 (corresponding to acceleration factor of 8), and total scan time = ~1 $$$min$$$. A separate calibration scan was acquired from a water phantom to correct the k-space trajectory for each echo using a method similarly described previously [16]. Image reconstruction was performed using SENSE reconstruction [17,18] and non-uniform fast Fourier transform (NUFFT) reconstruction [19].
The composition of water and fat signals were jointly estimated as follows [20]:
$$\left\{\hat{\rho}_W,\hat{\rho}_F,\hat{R}_2^*,\triangle\hat{\omega}_0\right\}=arg\min_{\rho_W,\rho_F,R_2^*,\triangle \omega_0} \sum_{q=1}^Q\sum_{p=1}^P\mid I_q(TE_p) - (\rho_{W,q}+\rho_{F,q}\sum_{m=1}^M \alpha_m e^{-i\cdot \omega_m\cdot TE_p})\cdot e^{-TE_p\cdot R_{2,q}^*} \cdot e^{-i\cdot \triangle\omega_{0,q}\cdot TE_p} \mid^2 +\mu\sum_{q=1}^Q V(\rho_{W,q},\rho_{F,q},R_{2,q}^*,\triangle \omega_{0,q})$$
where $$$I_q(TE_p)$$$ denotes the signal at voxel $$$q = 1,…,Q$$$ and echo time $$$p=1,…7$$$, $$$\rho_{W,q}$$$ and $$$\rho_{F,q}$$$ denote the proton density signals from water and fat, respectively, $$$R_{2,q}^*$$$ denotes the single, representative relaxation rate of both water and fat, $$$\alpha_m$$$ and $$$\omega_m$$$ each denote the relative amplitudes (i.e., $$$\sum_{m=1}^M \alpha_m =1$$$) and frequency offsets of an M-peak (i.e., $$$M=6$$$) fat model [21], $$$\triangle\omega_{0,q}$$$ denotes the frequency offset due to magnetic field inhomogeneity ($$$\triangle B_0$$$), $$$\mu$$$ denotes the regularization parameter, and $$$V$$$ denotes a regularization function reflecting prior information. Subsequently, $$$\rho_W$$$ and $$$\rho_F$$$ were compared for image segmentation into water, fat, and bone components.
Results and Discussion
The proposed imaging scheme requires k-space trajectory correction to reliably reconstruct the multi-echo images (Fig.2). Water, fat, bone, R2*, and B0 maps were successfully obtained using the proposed method (Figs. 3-4). The fat images showed high values in the subcutaneous fat and the water images showed high values in the brain parenchyma (Figs. 3-4). Relatively high R2* values were shown for fat and bone regions in the brain (Figs. 3-4). Continuous signal distribution of bone was successfully obtained (Figs. 3-4), indicating potential usage of the proposed method for generating subject-specific AC maps.Conclusion
Continuous signal distribution of bone was successfully obtained, indicating potential usage of the proposed method for generating subject-specific AC maps.Acknowledgements
This work was partially supported by the National Institutes of Health (T32EB013180, R01CA165221, R21EB021710, and P41EB022544), National Natural Science Foundation of China (81471692), and Tsinghua Undergraduate Overseas Research Training Supporting Program (TOP OPEN).References
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