PURPOSE Prostate imaging is a major application of hybrid positron emission tomography/magnetic resonance imaging (PET/MRI). Currently, MRI-based attenuation correction (MRAC) for whole-body PET/MRI in which the bony structures are ignored is the main obstacle to successful implementation of the hybrid modality in the clinical work flow. Ultrashort echo time sequence captures bone signal but needs specific hardware-software and is challenging in large field of view (FOV) regions, such as pelvis. The main aims of the work are (1) to capture a part of the bone signal in pelvis using short echo time (STE) imaging based on time-resolved angiography with interleaved stochastic trajectories (TWIST) sequence and (2) to consider the bone in pelvis attenuation map (µ-map) to MRAC for PET/MRI systems. PROCEDURES Time-resolved angiography with interleaved stochastic trajectories, which is routinely used for MR angiography with high temporal and spatial resolution, was employed for fast/STE MR imaging. Data acquisition was performed in a TE of 0.88 milliseconds (STE) and 4.86 milliseconds (long echo time [LTE]) in pelvis region. Region of interest (ROI)-based analysis was used for comparing the signal-to-noise ratio (SNR) of cortical bone in STE and LTE images. A hybrid segmentation protocol, which is comprised of image subtraction, a Fuzzy-based segmentation, and a dedicated morphologic operation, was used for generating a 5-class µ-map consisting of cortical bone, air cavity, fat, soft tissue, and background (µ-mapMR-5c). A MR-based 4-class µ-map (µ-mapMR-4c) that considered soft tissue rather than bone was generated. As such, a bilinear (µ-mapCT-ref), 5 (µ-mapCT-5c), and 4 class µ-map (µ-mapCT-4c) based on computed tomography (CT) images were generated. Finally, simulated PET data were corrected using µ-mapMR-5c (PET-MRAC5c), µ-mapMR-4c (PET-MRAC4c), µ-mapCT-5c (PET-CTAC5c), and µ-mapCT-ref (PET-CTAC). RESULTS The ratio of SNRbone to SNRair cavity in LTE images was 0.8, this factor was increased to 4.4 in STE images. The Dice, Sensitivity, and Accuracy metrics for bone segmentation in proposed method were 72.4% ± 5.5%, 69.6% ± 7.5%, and 96.5% ± 3.5%, respectively, where the segmented CT served as reference. The mean relative error in bone regions in the simulated PET images were -13.98% ± 15%, -35.59% ± 15.41%, and 1.81% ± 12.2%, respectively, in PET-MRAC5c, PET-MRAC4c, and PET-CTAC5c where PET-CTAC served as the reference. Despite poor correlation in the joint histogram of µ-mapMR-4c versus µ-mapCT-5c (R2 > 0.78) and PET-MRAC4c versus PET-CTAC5c (R2 = 0.83), high correlations were observed in µ-mapMR-5c versus µ-mapCT-5c (R2 > 0.94) and PET-MRAC5c versus PET-CTAC5c (R2 > 0.96). CONCLUSIONS According to the SNRSTE, pelvic bone, the cortical bone can be separate from air cavity in STE imaging based on TWIST sequence. The proposed method generated an MRI-based µ-map containing bone and air cavity that led to more accurate tracer uptake estimation than MRAC4c. Uptake estimation in hybrid PET/MRI can be improved by employing the proposed method.

中文翻译：

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使用TWIST序列捕获骨盆MRI的骨信号（视场较大），并为前列腺PET / MRI成像生成5级衰减图。

目的前列腺成像是混合正电子发射断层扫描/磁共振成像（PET / MRI）的主要应用。当前，对于忽略了骨结构的全身PET / MRI，基于MRI的衰减校正（MRAC）是在临床工作流程中成功实施混合模式的主要障碍。超短回波时间序列可捕获骨骼信号，但需要特定的硬件软件，并且在诸如骨盆等大视野（FOV）区域中具有挑战性。这项工作的主要目的是（1）基于时间分辨血管造影和交错的随机轨迹（TWIST）序列的短回波时间（STE）成像，以捕获骨盆中的一部分骨信号；（2）考虑骨骼用于PET / MRI系统的骨盆衰减图（µ-map）到MRAC。程序随机交错轨迹的时间分辨血管造影通常用于具有高时空分辨率的MR血管造影，通常用于快速/ STE MR成像。在骨盆区域以0.88毫秒（STE）和4.86毫秒（长回波时间[LTE]）的TE进行数据采集。基于兴趣区（ROI）的分析用于比较STE和LTE图像中皮质骨的信噪比（SNR）。混合分割协议由图像减法，基于模糊的分割和专用的形态学运算组成，用于生成由皮质骨，气孔，脂肪，软组织和背景组成的5类µ-map （µ-mapMR-5c）。生成了一个基于MR的4类µ-map（µ-mapMR-4c），它考虑了软组织而不是骨骼。因此，基于计算机断层扫描（CT）图像生成了双线性（µ-mapCT-ref），5（µ-mapCT-5c）和4类µ-map（µ-mapCT-4c）。最后，使用µ-mapMR-5c（PET-MRAC5c），µ-mapMR-4c（PET-MRAC4c），µ-mapCT-5c（PET-CTAC5c）和µ-mapCT-ref（PET- CTAC）。结果LTE图像中SNRbone与SNRair腔的比率为0.8，而STE图像中该因子增加到4.4。提议的方法中，骨分割的骰子，灵敏度和准确度指标分别为72.4％±5.5％，69.6％±7.5％和96.5％±3.5％，其中以CT分割作为参考。在PET-MRAC5c，PET-MRAC4c和PET-CTAC5c中，模拟PET图像中骨骼区域的平均相对误差分别为-13.98％±15％，-35.59％±15.41％和1.81％±12.2％，其中PET-CTAC作为参考。尽管µ-mapMR-4c与µ-mapCT-5c的关节直方图之间的相关性较差（R2> 0.78）以及PET-MRAC4c与PET-CTAC5c的关节直方图之间的相关性（R2 = 0.83），但在µ-mapMR-5c与µ-mapMR-5c之间的相关性很高-mapCT-5c（R2> 0.94）和PET-MRAC5c与PET-CTAC5c（R2> 0.96）。结论根据SNRSTE，骨盆骨，在基于TWIST序列的STE成像中，皮质骨可与气腔分离。所提出的方法生成了一个基于MRI的µ-map，其中包含骨骼和气腔，从而导致比MRAC4c更加精确的示踪剂吸收估算。通过采用提出的方法可以改善混合PET / MRI中的摄取估计。96）。结论根据SNRSTE，骨盆骨，在基于TWIST序列的STE成像中，皮质骨可与气腔分离。所提出的方法生成了一个基于MRI的µ-map，其中包含骨骼和气腔，与MRAC4c相比，它可以更准确地估算示踪剂的摄取。通过采用提出的方法可以改善混合PET / MRI中的摄取估计。96）。结论根据SNRSTE，骨盆骨，在基于TWIST序列的STE成像中，皮质骨可与气腔分离。所提出的方法生成了一个基于MRI的µ-map，其中包含骨骼和气腔，从而导致比MRAC4c更加精确的示踪剂吸收估算。通过采用所提出的方法可以改善混合PET / MRI中的摄取估计。