Berg A, Pernkopf M, Waldhäusl C, Schmidt W, Moser E
Institut für Medizinische Physik, Medizinische Universität Wien, Währingerstr 13, A-1090 Wien, Austria.
Phys Med Biol. 2004 Sep 7;49(17):4087-108. doi: 10.1088/0031-9155/49/17/018.
Precise methods of modem radiation therapy such as intensity modulated radiotherapy (IMRT), brachytherapy (BT) and high LET irradiation allow for high dose localization in volumes of a few mm3. However, most dosimetry methods-ionization chambers, TLD arrangements or silicon detectors, for example-are not capable of detecting sub-mm dose variations or do not allow for simple dose imaging. Magnetic resonance based polymer dosimetry (MRPD) appears to be well suited to three-dimensional high resolution relative dosimetry but the spatial resolution based on a systematic modulation transfer function (MTF) approach has not yet been investigated. We offer a theoretical construct for addressing the spatial resolution in different dose imaging systems, i.e. the dose modulation transfer function (DMTF) approach, an experimental realization of this concept with a phantom and quantitative comparisons between two dosimetric systems: polymer gel and film dosimetry. Polymer gel samples were irradiated by Co-60 photons through an absorber grid which is characterized by periodic structures of different spatial period (a), the smallest one at width of a/2 = 280 microm. The modulation in dose under the grid is visualized via calibrated, high resolution, parameter-selective (T2) and dose images based on multi-echo MR imaging. The DMTF is obtained from the modulation depth of the spin-spin relaxation time (T2) after calibration. Voxel sizes below 0.04 mm3 could be achieved, which are significantly smaller than those reported in MR based dose imaging on polymer gels elsewhere, using a powerful gradient system and a highly sensitive small birdcage resonator on a whole-body 3T MR scanner. Dose modulations at 22% of maximum dose amplitude could be observed at about 2 line pairs per mm. The polymer DMTF results are compared to those of a typical clinical film-scanner system. This study demonstrates that MR based gel dosimetry at 200 microm pixel resolution might even be superior, with reference to relative spatial resolution, to the results of a standard film-scanner system offering a nominal scan resolution of 200 microm.
现代放射治疗的精确方法，如调强放射治疗（IMRT）、近距离放射治疗（BT）和高传能线密度照射，能够在几立方毫米的体积内实现高剂量定位。然而，大多数剂量测定方法，例如电离室、热释光剂量计装置或硅探测器，无法检测亚毫米级的剂量变化，或者不允许进行简单的剂量成像。基于磁共振的聚合物剂量测定法（MRPD）似乎非常适合三维高分辨率相对剂量测定，但基于系统调制传递函数（MTF）方法的空间分辨率尚未得到研究。我们提出了一种理论结构，用于解决不同剂量成像系统中的空间分辨率问题，即剂量调制传递函数（DMTF）方法，通过体模对这一概念进行实验实现，并对两种剂量测定系统：聚合物凝胶和胶片剂量测定法进行定量比较。聚合物凝胶样品通过一个吸收器网格由钴-60光子进行照射，该吸收器网格具有不同空间周期（a）的周期性结构，最小的周期在a/2 = 280微米宽度处。通过基于多回波磁共振成像的校准、高分辨率、参数选择性（T2）和剂量图像，可以观察到网格下剂量的调制情况。在校准后，从自旋-自旋弛豫时间（T2）的调制深度获得DMTF。使用全身3T磁共振扫描仪上的强大梯度系统和高灵敏度小鸟笼谐振器，可以实现低于0.04立方毫米的体素大小，这明显小于其他地方基于聚合物凝胶的磁共振剂量成像所报道的体素大小。在每毫米约2线对的情况下，可以观察到最大剂量幅度22%的剂量调制。将聚合物DMTF结果与典型临床胶片扫描系统的结果进行比较。这项研究表明，就相对空间分辨率而言，基于磁共振的凝胶剂量测定法在200微米像素分辨率下甚至可能优于标称扫描分辨率为200微米的标准胶片扫描系统的结果。
