Duke University Medical Center, Durham, NC, USA.
Phys Med Biol. 2011 Jul 21;56(14):4433-51. doi: 10.1088/0031-9155/56/14/013. Epub 2011 Jun 30.
Radiochromic plastic and gel materials have recently emerged which can yield 3D dose information over clinical volumes in high resolution. These dosimeters can provide a much more comprehensive verification of complex radiation therapy treatments than can be achieved by conventional planar and point dosimeters. To achieve full clinical potential, these dosimeters require a fast and accurate read-out technology. Broad-beam optical-computed tomography (optical-CT) systems have shown promise, but can be sensitive to stray light artifacts originating in the imaging chain. In this work we present and evaluate a method to correct for stray light artifacts by deconvolving a measured, spatially invariant, point spread function (PSF). The correction was developed for the DLOS (Duke large field-of-view optical-CT scanner) in conjunction with radiochromic PRESAGE® dosimeters. The PSF was constructed from a series of acquisitions of projection images of various sized apertures placed in the optical imaging chain. Images were acquired with a range of exposure times, and for a range of aperture sizes (0.2-11 mm). The PSF is investigated under a variety of conditions, and found to be robust and spatially invariant, key factors enabling the viability of the deconvolution approach. The spatial invariance and robustness of the PSF are facilitated by telecentric imaging, which produces a collimated light beam and removes stray light originating upstream of the imaging lens. The telecentric capability of the DLOS therefore represents a significant advantage, both in keeping stray light levels to a minimum and enabling viability of an accurate PSF deconvolution method to correct for the residual. The performance of the correction method was evaluated on projection images containing known optical-density variations, and also on known 3D dose distributions. The method is shown to accurately account for stray light on small field dosimetry with corrections up to 3% in magnitude shown here although corrections of >10% have been observed in extreme cases. The dominant source of stray light was found to be within the imaging lens. Correcting for stray light extended the dynamic range of the system from ∼30 to ∼60 dB. The correction should be used when measurements need to be accurate within 3%.
近年来出现了一些可以在高分辨率下提供临床体积内的 3D 剂量信息的放射色塑料和凝胶材料。这些剂量计可以提供比传统的平面和点剂量计更全面的复杂放射治疗验证。为了实现充分的临床潜力，这些剂量计需要快速、准确的读取技术。宽束光学计算机断层扫描（光学 CT）系统显示出了潜力，但可能对成像链中产生的杂散光伪影敏感。在这项工作中，我们提出并评估了一种通过反卷积测量的、空间不变的点扩散函数（PSF）来校正杂散光伪影的方法。该方法是与放射色 PRESAGE®剂量计一起为 DLOS（杜克大学大视场光学 CT 扫描仪）开发的。PSF 是由一系列放置在光学成像链中的不同大小孔径的投影图像的采集构建而成的。图像是在不同的曝光时间和孔径大小（0.2-11mm）下采集的。研究了 PSF 在各种条件下的表现，发现其具有稳健性和空间不变性，这是反卷积方法可行性的关键因素。PSF 的空间不变性和稳健性得益于远心成像，它产生了一个平行光光束，并消除了成像透镜上游产生的杂散光。因此，DLOS 的远心能力不仅可以将杂散光水平降至最低，还可以实现准确的 PSF 反卷积方法来校正残余杂散光，这是一个显著的优势。在包含已知光密度变化的投影图像和已知的 3D 剂量分布上评估了校正方法的性能。该方法在小场剂量测量中能够准确地考虑杂散光，校正后，光密度变化幅度最大可达 3%，尽管在极端情况下观察到了超过 10%的校正。杂散光的主要来源被发现是在成像透镜内。校正杂散光后，系统的动态范围从大约 30dB 扩展到大约 60dB。当测量需要在 3%以内准确时，应该使用该校正方法。
