Magnetic field correction factors are needed for absolute dosimetry in magnetic resonance (MR)-linacs. Currently experimental data for magnetic field correction factors, especially for small volume ionization chambers, are largely lacking. The purpose of this work is to establish, independent methods for the experimental determination of magnetic field correction factors in an orientation in which the ionization chamber is parallel to the magnetic field. The aim is to confirm previous experiments on the determination of Farmer type ionization chamber correction factors and to gather information about the usability of small-volume ionization chambers for absolute dosimetry in MR-linacs. The first approach to determine is based on a cross-calibration of measurements using a conventional linac with an electromagnet and an MR-linac. The absolute influence of the magnetic field in perpendicular orientation is quantified with the help of the conventional linac and the electromagnet. The correction factors for the parallel orientation are then derived by combining these measurements with relative measurements in the MR-linac. The second technique utilizes alanine electron paramagnetic resonance dosimetry. The alanine system as well as several ionization chambers were directly calibrated with the German primary standard for absorbed dose to water. Magnetic field correction factors for the ionization chambers were determined by a cross-calibration with the alanine in an MR-linac. Important quantities like for Farmer type ionization chambers in parallel orientation and the change of the dose to water due the magnetic field have been confirmed. In addition, magnetic field correction factors have been determined for small volume ionization chambers in parallel orientation. The electromagnet-based measurements of for MR-linacs and parallel ionization chamber orientations resulted in 0.9926(22), 0.9935(31) and 0.9841(27) for the PTW 30013, the PTW 31010 and the PTW 31021, respectively. The measurements based on the second technique resulted in values for of 0.9901(72), 0.9955(72), and 0.9885(71). Both methods show excellent accuracy and reproducibility and are therefore suitable for the determination of magnetic field correction factors. Small-volume ionization chambers showed a variation in the resulting values for and should be cross-calibrated instead of using tabulated values for correction factors.

磁共振 (MR) 直线加速器中的绝对剂量测定需要磁场校正因子。目前，磁场校正因子的实验数据，特别是小体积电离室的实验数据，在很大程度上是缺乏的。这项工作的目的是建立独立的方法，用于在电离室与磁场平行的方向上通过实验确定磁场校正因子。目的是确认以前关于确定 Farmer 型电离室校正因子的实验，并收集有关小容量电离室在 MR 直线加速器中绝对剂量测定的可用性的信息。第一种方法确定是基于使用带有电磁体的传统直线加速器和 MR-直线加速器的测量交叉校准。在传统直线加速器和电磁铁的帮助下，可以量化垂直方向磁场的绝对影响。然后通过将这些测量值与 MR-直线加速器中的相对测量值相结合，得出平行方向的校正因子。第二种技术利用丙氨酸电子顺磁共振剂量测定法。丙氨酸系统和几个电离室直接用德国水吸收剂量的主要标准进行校准。电离室的磁场校正因子通过与 MR-linac 中的丙氨酸进行交叉校准来确定。重要数量如对于平行取向的 Farmer 型电离室和由于磁场对水的剂量变化已得到证实。此外，已经确定了平行方向的小体积电离室的磁场校正因子。基于电磁铁的 MR-直线加速器和平行电离室方向的测量结果分别为PTW 30013、PTW 31010 和 PTW 31021 的 0.9926(22)、0.9935(31) 和 0.9841(27)。基于第二种技术的测量结果为0.9901(72)、0.9955(72) 和 0.9885(71)。这两种方法都显示出出色的准确性和重现性，因此适用于确定磁场校正因子。小体积电离室的结果值存在差异，应进行交叉校准，而不是使用表格值作为校正因子。