Institute of Radiooncology - OncoRay, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany.
OncoRay - National Center for Radiation Research in Oncology, Faculty of Medicine, University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany.
Med Phys. 2018 Jan;45(1):37-47. doi: 10.1002/mp.12670. Epub 2017 Dec 10.
X-ray-based position verification of the target volume in image-guided radiation therapy (IGRT) of patients with pancreatic ductal adenocarcinoma (PDAC) is currently performed on solid fiducial markers that are implanted under endoscopic ultrasonography. A new biodegradable liquid fiducial marker has recently been introduced. To assess its potential use for magnetic resonance imaging (MRI)-guided photon or proton radiotherapy of PDAC, the MRI visibility and artifacts of this marker were quantified and compared against solid gold markers.
Different spherical volumes (10 μL, 25 μL, 50 μL, and 100 μL) of a biodegradable liquid fiducial marker as well as seven differently sized and oriented solid gold (0.35 mm diameter; 5 mm and 10 mm length) and iron-gold alloy fiducial markers (0.28 mm diameter; 1 cm and 2 cm length) were implanted in a spherical gel phantom, mimicking the proton spin relaxation properties of healthy pancreatic tissue at 3 Tesla. MR relaxometry was performed to quantify the size and magnitude of the decrease in the effective transversal relaxation time T2∗ and relative proton density ρ(H) as a measure of potential visibility and to quantify the size and magnitude of the increase in magnetic field inhomogeneity ΔB as a measure of potential signal artifacts. The phantom was scanned in a 3.0 T PET/MR scanner with an eight-channel head coil.
The solid fiducial markers showed a direct linear relationship between the potentially visible size and artifact size. The liquid fiducial marker showed a tendency toward a potentially visible size at smaller artifacts. Liquid markers from 25 to 100 μL generated visible volumes comparable to the size of the solid markers. The magnitude of visibility was the highest for the liquid fiducial marker with volumes of 25-100 μL showing no correlation with the magnitude of artifact. The solid markers showed a strong nonlinear correlation between magnitude of visibility and artifact, whereas the solid marker consisting of a gold-iron alloy induced the strongest artifacts.
The liquid fiducial marker causes signal voids on MRI due to its absence of water hydrogen atoms without strongly affecting the magnetic field in the surrounding tissue. The alteration of the static magnetic field was found to be the main effect leading to the visibility of the solid fiducial markers. Hence, especially when a low level of image distortion is required, MRI characteristics of the liquid marker surpass those of solid gold markers currently being used for IGRT of PDAC.
在胰腺导管腺癌（PDAC）的图像引导放射治疗（IGRT）中，目前通过内窥镜超声植入的固体基准标记物对靶区体积进行基于 X 射线的位置验证。最近引入了一种新型可生物降解的液体基准标记物。为了评估其在 PDAC 的磁共振成像（MRI）引导光子或质子放疗中的潜在用途，对这种标记物的 MRI 可见度和伪影进行了量化，并与固体金标记物进行了比较。
在一个模拟 3T 下健康胰腺组织质子自旋弛豫特性的球形凝胶体模中，植入了不同体积（10 μL、25 μL、50 μL 和 100 μL）的可生物降解液体基准标记物以及七个不同尺寸和方向的固体金（0.35 毫米直径；5 毫米和 10 毫米长）和铁金合金基准标记物（0.28 毫米直径；1 厘米和 2 厘米长）。进行磁共振弛豫测量以量化有效横向弛豫时间 T2∗减小的大小和幅度以及质子密度 ρ（H）的相对减小，作为潜在可见度的度量，量化磁场不均匀性ΔB 增大的大小和幅度，作为潜在信号伪影的度量。该体模在配备八通道头部线圈的 3.0T PET/MR 扫描仪中进行了扫描。
固体基准标记物的潜在可见尺寸与伪影尺寸之间呈直接线性关系。液体基准标记物表现出较小伪影时潜在可见尺寸的趋势。25 至 100 μL 的液体标记物产生的可见体积与固体标记物的尺寸相当。25-100 μL 体积的液体基准标记物的可见度最高，与伪影的幅度无关。固体标记物的可见度与伪影幅度之间存在强烈的非线性相关性，而由金铁合金组成的固体标记物则产生最强的伪影。
由于液体基准标记物不含水氢原子，因此在 MRI 上会产生信号空洞，而不会强烈影响周围组织中的磁场。研究发现，改变静磁场是导致固体基准标记物可见度的主要影响因素。因此，特别是当需要较低的图像失真水平时，液体标记物的 MRI 特性优于目前用于 PDAC 的 IGRT 的固体金标记物。
