TomoTherapy, Inc., 1240 Deming Way, Madison, Wisconsin 53717, USA.
Med Phys. 2011 Jun;38(6):2841-9. doi: 10.1118/1.3589133.
To obtain accurate x-ray source profile measurements using a slit-collimator, the slit-collimator should have a narrow width, large height, and be positioned near the source. However, these conditions may not always be met. In this paper, the authors provide a detailed analysis of the slit measurement geometry and the relationship between the slit parameters and the measured x-ray source profile. The slit model allows the use of a shorter and more easily available slit-collimator, while accurate source profile measurements can still be obtained.
Measurements were performed with a variety of slit widths and/or slit to source distances. The relationship derived between the slit parameters and the measured profile was used to determine the true focal spot profile through a least square fit of the profile data. The model was verified by comparing the predicted profiles at a variety of slit-collimator parameters with the measured results on the TomoTherapy Hi-Art system.
Both the treatment beam and the imaging beam were measured. For treatment mode, it was found that a source consisting of one Gaussian with a 0.75 mm full-width-half-maximum (FWHM) and 72% peak amplitude and a second Gaussian with a 2.27 mm FWHM and 18% peak amplitude matched measurement profiles. The overall source profile has a FWHM of 0.93 mm, but with a higher amplitude in the tail region than a single Gaussian. For imaging mode, the source consists of one Gaussian with a 0.68 mm FWHM and 82% peak amplitude and a second Gaussian with a 1.83 mm FWHM and 18% peak amplitude. The overall source profile has a FWHM of 0.77 mm.
Our study of the focal spot measurement using slit-collimators showed that accurate source profile measurements can be achieved through fitting of measurement results at different slit widths and source-to-slit distances (SSD). Quantitative measurements of the TomoTherapy linac focal spot showed that the source distribution could be better described with a model consisting of two Gaussian components rather than a single Gaussian model as assumed in previous studies.
为了使用狭缝准直器获得准确的 X 射线源轮廓测量，狭缝准直器应具有较窄的宽度、较大的高度，并靠近源定位。然而，这些条件并不总是满足的。本文作者详细分析了狭缝测量几何形状以及狭缝参数与测量的 X 射线源轮廓之间的关系。狭缝模型允许使用更短且更容易获得的狭缝准直器，同时仍然可以获得准确的源轮廓测量结果。
使用各种狭缝宽度和/或狭缝到源的距离进行了测量。在通过对轮廓数据进行最小二乘拟合来确定真实焦点轮廓之前，使用得出的狭缝参数与测量轮廓之间的关系。通过将各种狭缝准直器参数下的预测轮廓与 TomoTherapy Hi-Art 系统上的测量结果进行比较，验证了该模型。
分别测量了治疗束和成像束。对于治疗模式，发现由一个半高全宽（FWHM）为 0.75 毫米、峰值幅度为 72%的高斯和一个 FWHM 为 2.27 毫米、峰值幅度为 18%的高斯组成的源与测量轮廓匹配。总体源轮廓的 FWHM 为 0.93 毫米，但在尾部区域的幅度更高，而非单个高斯。对于成像模式，源由一个 FWHM 为 0.68 毫米、峰值幅度为 82%的高斯和一个 FWHM 为 1.83 毫米、峰值幅度为 18%的高斯组成。总体源轮廓的 FWHM 为 0.77 毫米。
我们使用狭缝准直器进行焦点测量的研究表明，可以通过拟合不同狭缝宽度和源到狭缝距离（SSD）下的测量结果来实现准确的源轮廓测量。对 TomoTherapy 直线加速器焦点的定量测量表明，与以前研究中假设的单个高斯模型相比，源分布可以更好地用由两个高斯分量组成的模型来描述。
