Department of Electrical and Computer Engineering, University of California San Diego, La Jolla, California, USA.
Department of Radiology, University of California San Diego, La Jolla, California, USA.
NMR Biomed. 2022 Aug;35(8):e4725. doi: 10.1002/nbm.4725. Epub 2022 Mar 29.
Fluorine-19 ( F) magnetic resonance imaging (MRI) is an emerging technique offering specific detection of labeled cells in vivo. Lengthy acquisition times and modest signal-to-noise ratio (SNR) makes three-dimensional spin-density-weighted F imaging challenging. Recent advances in tracer paramagnetic metallo-perfluorocarbon (MPFC) nanoemulsion probes have shown multifold SNR improvements due to an accelerated F T relaxation rate and a commensurate gain in imaging speed and averages. However, F T -reduction and increased linewidth limit the amount of metal additive in MPFC probes, thus constraining the ultimate SNR. To overcome these barriers, we describe a compressed sampling (CS) scheme, implemented using a "zero" echo time (ZTE) sequence, with data reconstructed via a sparsity-promoting algorithm. Our CS-ZTE scheme acquires k-space data using an undersampled spherical radial pattern and signal averaging. Image reconstruction employs off-the-shelf sparse solvers to solve a joint total variation and -norm regularized least square problem. To evaluate CS-ZTE, we performed simulations and acquired F MRI data at 11.7 T in phantoms and mice receiving MPFC-labeled dendritic cells. For MPFC-labeled cells in vivo, we show SNR gains of ~6.3 × with 8-fold undersampling. We show that this enhancement is due to three mechanisms including undersampling and commensurate increase in signal averaging in a fixed scan time, denoising attributes from the CS algorithm, and paramagnetic reduction of T . Importantly, F image intensity analyses yield accurate estimates of absolute quantification of F spins. Overall, the CS-ZTE method using MPFC probes achieves ultrafast imaging, a substantial boost in detection sensitivity, accurate F spin quantification, and minimal image artifacts.
氟-19（F）磁共振成像是一种新兴技术，可提供体内标记细胞的特异性检测。由于采集时间长且信噪比（SNR）适中，因此三维自旋密度加权 F 成像具有挑战性。最近，示踪剂顺磁金属全氟碳化物（MPFC）纳米乳液探针的进展表明，由于 F T弛豫率的加速以及成像速度和平均速度的相应提高，SNR 提高了多倍。但是，F T的减少和线宽的增加限制了 MPFC 探针中金属添加剂的量，从而限制了最终的 SNR。为了克服这些障碍，我们描述了一种压缩采样（CS）方案，该方案使用“零”回波时间（ZTE）序列实现，并通过稀疏性促进算法进行数据重构。我们的 CS-ZTE 方案使用欠采样的球形径向图案和信号平均来获取 k 空间数据。图像重建采用现成的稀疏求解器来解决联合全变差和范数正则化最小二乘问题。为了评估 CS-ZTE，我们在体模和接受 MPFC 标记树突状细胞的小鼠中进行了模拟和 11.7 T 的 F MRI 数据采集。对于体内的 MPFC 标记细胞，我们显示在 8 倍欠采样时 SNR 增益约为 6.3×。我们表明，这种增强归因于三种机制，包括在固定扫描时间内进行欠采样和相应增加信号平均，CS 算法的去噪属性以及 T 的顺磁减少。重要的是，F 图像强度分析可准确估计 F 自旋的绝对定量。总体而言，使用 MPFC 探针的 CS-ZTE 方法实现了超快成像，大大提高了检测灵敏度，准确的 F 自旋定量和最小的图像伪影。
