Gurney-Champion Oliver J, Lens Eelco, van der Horst Astrid, Houweling Antonetta C, Klaassen Remy, van Hooft Jeanin E, Stoker Jaap, van Tienhoven Geertjan, Nederveen Aart J, Bel Arjan
Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands and Department of Radiology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands.
Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands.
Med Phys. 2015 May;42(5):2638-47. doi: 10.1118/1.4918753.
In radiation therapy of pancreatic cancer, tumor alignment prior to each treatment fraction is improved when intratumoral gold fiducial markers (from here onwards: markers), which are visible on computed tomography (CT) and cone beam CT, are used. Visibility of these markers on magnetic resonance imaging (MRI) might improve image registration between CT and magnetic resonance (MR) images for tumor delineation purposes. However, concomitant image artifacts induced by markers are undesirable. The extent of visibility and artifact size depend on MRI-sequence parameters. The authors' goal was to determine for various markers their potential to be visible and to generate artifacts, using measures that are independent of the MRI-sequence parameters.
The authors selected ten different markers suitable for endoscopic placement in the pancreas and placed them into a phantom. The markers varied in diameter (0.28-0.6 mm), shape, and iron content (0%-0.5%). For each marker, the authors calculated T2 (∗)-maps and ΔB0-maps using MRI measurements. A decrease in relaxation time T2 (∗) can cause signal voids, associated with visibility, while a change in the magnetic field B0 can cause signal shifts, which are associated with artifacts. These shifts inhibit accurate tumor delineation. As a measure for potential visibility, the authors used the volume of low T2 (∗), i.e., the volume for which T2 (∗) differed from the background by >15 ms. As a measure for potential artifacts, the authors used the volume for which |ΔB0| > 9.4 × 10(-8) T (4 Hz). To test whether there is a correlation between visibility and artifact size, the authors calculated the Spearman's correlation coefficient (Rs) between the volume of low T2 (∗) and the volume of high |ΔB0|. The authors compared the maps with images obtained using a clinical MR-sequence. Finally, for the best visible marker as well as the marker that showed the smallest artifact, the authors compared the phantom data with in vivo MR-images in four pancreatic cancer patients.
The authors found a strong correlation (Rs = 1.00, p < 0.01) between the volume of low T2 (∗) and the volume with high |ΔB0|. Visibility in clinical MR-images increased with lower T2 (∗). Signal shift artifacts became worse for markers with high |ΔB0|. The marker that was best visible in the phantom, a folded marker with 0.5% iron content, was also visible in vivo, but showed artifacts on diffusion weighted images. The marker with the smallest artifact in the phantom, a small, stretched, ironless marker, was indiscernible on in vivo MR-images.
Changes in T2 (∗) and ΔB0 are sequence-independent measures for potential visibility and artifact size, respectively. Improved visibility of markers correlates strongly to signal shift artifacts; therefore, marker choice will depend on the clinical purpose. When visibility of the markers is most important, markers that contain iron are optimal, preferably in a folded configuration. For artifact sensitive imaging, small ironless markers are best, preferably in a stretched configuration.
在胰腺癌放射治疗中，当使用在计算机断层扫描（CT）和锥形束CT上可见的瘤内金基准标记物（以下简称：标记物）时，每次治疗分次前的肿瘤对准情况会得到改善。这些标记物在磁共振成像（MRI）上的可见性可能会改善CT与磁共振（MR）图像之间的图像配准，以用于肿瘤轮廓勾画。然而，由标记物引起的伴随图像伪影是不理想的。可见性程度和伪影大小取决于MRI序列参数。作者的目标是使用独立于MRI序列参数的测量方法，确定各种标记物的可见潜力和产生伪影的可能性。
作者选择了十种适合内镜放置在胰腺中的不同标记物，并将它们放入体模中。这些标记物在直径（0.28 - 0.6毫米）、形状和铁含量（0% - 0.5%）方面有所不同。对于每个标记物，作者使用MRI测量计算T2（∗）图和ΔB0图。弛豫时间T2（∗）的降低会导致信号缺失，与可见性相关，而磁场B0的变化会导致信号偏移，与伪影相关。这些偏移会抑制准确的肿瘤轮廓勾画。作为潜在可见性的度量，作者使用低T2（∗）的体积，即T2（∗）与背景相差>15毫秒的体积。作为潜在伪影的度量，作者使用|ΔB0| > 9.4×10（-8）T（4赫兹）的体积。为了测试可见性与伪影大小之间是否存在相关性，作者计算了低T2（∗）体积与高|ΔB0|体积之间的斯皮尔曼相关系数（Rs）。作者将这些图与使用临床MR序列获得的图像进行比较。最后，对于可见性最佳的标记物以及伪影最小的标记物，作者将体模数据与四名胰腺癌患者的体内MR图像进行比较。
作者发现低T2（∗）体积与高|ΔB0|体积之间存在强相关性（Rs = 1.00，p < 0.01）。临床MR图像中的可见性随着T2（∗）降低而增加。对于|ΔB0|高的标记物，信号偏移伪影更严重。在体模中可见性最佳的标记物，即铁含量为0.5%的折叠标记物，在体内也可见，但在扩散加权图像上显示有伪影。在体模中伪影最小的标记物，即小的、拉伸的、无铁标记物，在体内MR图像上无法分辨。
T2（∗）和ΔB0的变化分别是潜在可见性和伪影大小的与序列无关的度量。标记物可见性的改善与信号偏移伪影密切相关；因此，标记物的选择将取决于临床目的。当标记物的可见性最重要时，含铁的标记物是最佳选择，最好是折叠结构。对于对伪影敏感的成像，小的无铁标记物最好，最好是拉伸结构。
