Nuklearmedizin, Klinikum Rechts der Isar, Technische Universität München, 81675 München, Germany.
Med Phys. 2010 Jun;37(6):2804-12. doi: 10.1118/1.3431576.
A critical question in the development of combined MR/PET scanners is whether MR can provide the tissue attenuation data required for PET reconstruction. Unfortunately, MR images are often unable to encompass the entire patient. The resulting truncation in the transverse plane leads to incomplete attenuation maps, causing artifacts in the reconstructed PET image. This article describes the experiments performed to quantify these artifacts. A method to compensate the missing data was evaluated to determine whether software correction is possible or whether additional transmission hardware has to be included in the scanner.
Three studies were made. First, simulated PET data were used to quantify the bias due to an incomplete attenuation map. A set of spherical lesions was simulated in the lungs and mediastinum of a patient. The data were reconstructed with complete and partial attenuation maps and the uptake differences were evaluated. Second, clinical data from PET/CT oncology patients were used. To reproduce the expected conditions in an MR/PET scanner, only patients scanned with the arms resting along the body were considered. These scans were then used to create maps of the reconstruction bias due to field of view (FOV) limitations. Lastly, a PET reconstruction with incomplete attenuation data was evaluated as a means to obtain attenuation information beyond the MR FOV. The patient outline was automatically segmented with a three-dimensional snake algorithm and used to fill the truncated data in the attenuation map.
Average bias up to 15% and local biases up to 50% were estimated when PET data were reconstructed with incomplete attenuation information. Completing the attenuation map with data extracted from a PET prereconstruction globally reduced these biases to below 10%. This correction proved to be tolerant to inaccuracies in positioning and attenuation values. However, local artifacts up to 20% could still be found near the edges of the MR FOV.
MR FOV restrictions can indeed make the reconstructed PET data unacceptable for diagnostic purposes. Biases can be globally compensated by automatic preprocessing of the attenuation map. However, inaccuracies in the correction will result in small artifacts near the periphery of the image that could lead to false-positive findings.
在开发组合式磁共振/正电子发射断层扫描（MR/PET）扫描仪时，一个关键问题是磁共振是否能够提供用于 PET 重建的组织衰减数据。不幸的是，磁共振图像往往无法涵盖整个患者。这种在横断面上的截断会导致不完整的衰减图，从而在重建的 PET 图像中产生伪影。本文描述了为量化这些伪影而进行的实验。评估了一种补偿缺失数据的方法，以确定是否可以通过软件校正，或者是否必须在扫描仪中增加额外的透射硬件。
进行了三项研究。首先，使用模拟的 PET 数据来量化由于不完全衰减图引起的偏差。在一个患者的肺部和纵隔中模拟了一组球形病变。使用完整和部分衰减图重建数据，并评估摄取差异。其次，使用来自 PET/CT 肿瘤学患者的临床数据。为了在 MR/PET 扫描仪中重现预期的条件，仅考虑了手臂沿身体放置的患者进行扫描。然后使用这些扫描来创建由于视野（FOV）限制引起的重建偏差的图。最后，评估了使用不完全衰减数据的 PET 重建作为获取超出磁共振 FOV 衰减信息的方法。使用三维蛇算法自动分割患者轮廓，并用于填充衰减图中的截断数据。
当使用不完全衰减信息重建 PET 数据时，估计平均偏差高达 15%，局部偏差高达 50%。使用从 PET 预重建中提取的数据全局填充衰减图可以将这些偏差降低到 10%以下。这种校正被证明对定位和衰减值的不准确具有耐受性。然而，仍然可以在磁共振 FOV 的边缘附近发现高达 20%的局部伪影。
磁共振 FOV 的限制确实会使重建的 PET 数据无法用于诊断目的。通过自动预处理衰减图可以全局补偿偏差。然而，校正中的不准确会导致图像边缘附近出现小的伪影，这可能导致假阳性结果。
