Schubert Tilman, Wu Yijing, Johnson Kevin M, Wieben Oliver, Maksimovic Jane, Mistretta Charles, Turski Patrick
From the *Department of Radiology, University of Wisconsin-Madison, Madison, WI; †Clinic for Radiology and Nuclear Medicine, Basel University Hospital, Basel, Switzerland; and ‡Department of Medical Physics, University of Wisconsin-Madison, Madison, WI.
Invest Radiol. 2016 Nov;51(11):706-713. doi: 10.1097/RLI.0000000000000288.
Time-of-arrival (TOA) maps can be derived from high-resolution 4-dimensional (4D) contrast-enhanced magnetic resonance angiography (MRA) data sets to provide a quantitative description of contrast material arrival time in each voxel. This information can further be processed to create a compressed time evolution curve that virtually shortens the contrast bolus (virtual bolus [VB]). The purpose of this project was to determine whether TOA-enhanced 4D MRA and/or VB imaging improve the display of contrast kinetics in patients with vascular disease.
High-resolution whole-brain contrast-enhanced 4D MRA examinations with 1.2-second temporal reconstruction were acquired by using radial acquisition and highly constrained projection reconstruction (radial 4D contrast-enhanced HYPRFlow, abbreviated as HFMRA in this article) in 10 patients (8 patients with arteriovenous malformations [AVM], 1 patient with an arteriovenous fistula, and 1 patient with a high-grade intracranial stenosis). The TOA for each voxel was defined as the time point when the signal intensity reached 20% of its maximum. In the first method, TOA maps were generated, color-encoded, and then multiplied with the time-resolved contrast-enhanced MRA images at each time frame to form new 4D MRA images (TOA-enhanced HFMRA), which contains the contrast arrival times with defined color encoding. In the second method, each time frame was weighted by a Gaussian distribution in the time domain to form a virtual 4D bolus map. This 4D bolus map was then color-coded and multiplied with the HFMRA images to form a digital subtraction angiography (DSA)-like VB, where at each time frame, only vessels with certain TOA values within the defined bolus length appear. HFMRA, TOA maps, and VB images were scored qualitatively with regard to delineation of arteries, veins, and nidus, as well as artifacts. Furthermore, diagnostic confidence and arteriovenous overlap were evaluated and compared between techniques. A comparison with DSA was performed where DSA served as the reference standard in terms of number of arterial feeders, draining veins, and Spetzler-Martin score of AVMs. In addition, TOA maps were evaluated quantitatively.
Overall, diagnostic confidence score of TOA was significantly higher compared with that of HFMRA (P = 0.03). Virtual bolus showed significantly higher scores for overall diagnostic confidence (P = 0.02) and reduced arteriovenous overlap (0.01) compared with HFMRA. Furthermore, VB-reduced arteriovenous overlap scores were significantly higher compared with TOA (P = 0.04). Agreement regarding AVM draining veins was lower between DSA and HFMRA (κ = 0.3) compared with TOA and VB (κ = 0.56). Agreement regarding Spetzler-Martin score was lower between DSA and HFMRA (κ = 0.56) compared with TOA and VB (κ = 0.74).
TOA-enhanced HFMRA provides serial images and time of arrival maps in one inclusive display. In this study, TOA mapping combined with Virtual Bolus imaging improved diagnostic confidence in AVM patients and facilitated arteriovenous separation. The VB method further reduced overlap of arterial and venous structures.
到达时间（TOA）图可从高分辨率四维（4D）对比增强磁共振血管造影（MRA）数据集中得出，以定量描述每个体素中对比剂的到达时间。该信息可进一步处理以创建压缩时间演变曲线，从而实际上缩短对比剂团注（虚拟团注[VB]）。本项目的目的是确定TOA增强的4D MRA和/或VB成像是否能改善血管疾病患者对比剂动力学的显示。
采用径向采集和高度约束投影重建（本文简称为HFMRA的径向4D对比增强HYPRFlow）对10例患者（8例患有动静脉畸形[AVM]、1例患有动静脉瘘、1例患有高级别颅内狭窄）进行了具有1.2秒时间分辨率的全脑高分辨率对比增强4D MRA检查。每个体素的TOA定义为信号强度达到其最大值的20%时的时间点。在第一种方法中，生成TOA图，进行颜色编码，然后在每个时间帧与时间分辨对比增强MRA图像相乘，以形成新的4D MRA图像（TOA增强HFMRA），其包含具有定义颜色编码的对比剂到达时间。在第二种方法中，每个时间帧在时域中由高斯分布加权，以形成虚拟4D团注图。然后对该4D团注图进行颜色编码，并与HFMRA图像相乘，以形成类似数字减影血管造影（DSA）的VB，其中在每个时间帧，仅在定义的团注长度内具有特定TOA值的血管出现。对HFMRA、TOA图和VB图像在动脉、静脉和畸形血管团的描绘以及伪影方面进行定性评分。此外，评估并比较了不同技术之间的诊断置信度和动静脉重叠情况。将其与DSA进行比较，DSA在动脉供血支数量、引流静脉数量以及AVM的Spetzler-Martin分级方面作为参考标准。此外，对TOA图进行了定量评估。
总体而言，TOA的诊断置信度评分显著高于HFMRA（P = 0.03）。与HFMRA相比，虚拟团注在总体诊断置信度方面得分显著更高（P = 0.02），动静脉重叠减少（P = 0.01）。此外，VB减少的动静脉重叠评分显著高于TOA（P = 0.04）。与TOA和VB（κ = 0.56）相比，DSA与HFMRA在AVM引流静脉方面的一致性较低（κ = 0.3）。与TOA和VB（κ = 0.74）相比，DSA与HFMRA在Spetzler-Martin分级方面的一致性较低（κ = 0.56）。
TOA增强HFMRA在一个综合显示中提供了序列图像和到达时间图。在本研究中，TOA映射与虚拟团注成像相结合提高了AVM患者的诊断置信度，并有助于动静脉分离。VB方法进一步减少了动脉和静脉结构的重叠。
