Jaffray D A, Battista J J, Fenster A, Munro P
Department of Medical Physics, University of Western Ontario, Canada.
Med Phys. 1993 Sep-Oct;20(5):1417-27. doi: 10.1118/1.597106.
A computerized tomography (CT) reconstruction technique has been used to make quantitative measurements of the size and shape of the focal spot in medical linear accelerators. Using this technique, we have measured the focal spots in a total of nine accelerators, including (i) two Varian Clinac 2100c's, (ii) two Atomic Energy of Canada Ltd. (AECL) Therac-25's, (iii) two AECL Therac 6's, (iv) a Siemens KD-2, (v) a Varian Clinac 600c (4 MV), and (vi) an AECL Therac-20. Some of these focal spots were monitored for changes over a 2-yr period. It has been found that (i) the size and shape of the source spot varies greatly between accelerators of different design ranging from 0.5 to 3.4 mm in full width at half maximum (FWHM); and (ii) for accelerators of the same design, the focal spots are very similar. In addition to the measurements of the focal spot, a new technique for measuring the magnitude and distribution of extra-focal radiation originating from the linear accelerator head (flattening filter, primary collimator) has also been developed. The extra-focal radiation produced by a Varian Clinac 2100c accelerator was measured using this technique and it was found that the extra-focal radiation accounts for as much as 8% of the total photon fluence reaching the isocenter. The majority (75%) of this extra-focal radiation originates from within a circle 6 cm in diameter at the target plane. The source MTFs for each of the measured focal spots have been calculated in order to assess their influence on the spatial resolution of verification images. The limiting spatial resolution (i.e., 10% modulation) for all the source MTFs is 1.8 mm-1 or greater when used for transmission radiography at a magnification of 1.2. The extra-focal radiation, which produces a low-frequency drop in the source MTFs of up to 8%, changes with field size. As a result, the source MTFs of linear accelerators depend not only on the design of individual accelerators and image magnification, but also on the field size used when forming an image.
一种计算机断层扫描（CT）重建技术已被用于对医用直线加速器中焦点的大小和形状进行定量测量。使用该技术，我们总共测量了九台加速器中的焦点，包括：（i）两台瓦里安Clinac 2100c；（ii）两台加拿大原子能有限公司（AECL）的Therac - 25；（iii）两台AECL Therac 6；（iv）一台西门子KD - 2；（v）一台瓦里安Clinac 600c（4兆伏）；以及（vi）一台AECL Therac - 20。其中一些焦点在两年时间内被监测其变化情况。已发现：（i）不同设计的加速器之间源焦点的大小和形状差异很大，半高宽（FWHM）范围从0.5毫米到3.4毫米；（ii）对于相同设计的加速器，焦点非常相似。除了焦点测量外，还开发了一种新技术，用于测量源自直线加速器头部（均整器、初级准直器）的焦点外辐射大小和分布。使用该技术测量了瓦里安Clinac 2100c加速器产生的焦点外辐射，发现焦点外辐射占到达等中心的总光子注量的比例高达8%。这种焦点外辐射的大部分（75%）源自靶平面上直径6厘米的圆内。已计算出每个测量焦点的源调制传递函数（MTF），以评估它们对验证图像空间分辨率的影响。当用于1.2倍放大率的透射式射线照相时，所有源MTF的极限空间分辨率（即10%调制）为1.8毫米⁻¹或更高。产生源MTF中高达8%的低频下降的焦点外辐射随射野大小而变化。因此，直线加速器的源MTF不仅取决于单个加速器的设计和图像放大率，还取决于成像时使用的射野大小。
