Liu Langechuan, Antonuk Larry E, El-Mohri Youcef, Zhao Qihua, Jiang Hao
Department of Radiation Oncology, University of Michigan, Ann Arbor, Michigan 48109.
Med Phys. 2015 Apr;42(4):2072-84. doi: 10.1118/1.4915120.
In modern radiotherapy treatment rooms, megavoltage (MV) portal imaging and kilovoltage (kV) cone-beam CT (CBCT) imaging are performed using various active matrix flat-panel imager (AMFPI) designs. To expand the clinical utility of MV and kV imaging, MV AMFPIs incorporating thick, segmented scintillators and, separately, kV imaging using a beam's eye view geometry have been investigated by a number of groups. Motivated by these previous studies, it is of interest to explore to what extent it is possible to preserve the benefits of kV and MV imaging using a single AMFPI design, given the considerably different x ray energy spectra used for kV and MV imaging. In this paper, considerations for the design of such a dual energy imager are explored through examination of the performance of a variety of hypothetical AMFPIs based on x ray converters employing segmented scintillators.
Contrast, noise, and contrast-to-noise ratio performances were characterized through simulation modeling of CBCT imaging, while modulation transfer function, Swank factor, and signal performance were characterized through simulation modeling of planar imaging. The simulations were based on a previously reported hybrid modeling technique (accounting for both radiation and optical effects), augmented through modeling of electronic additive noise. All designs employed BGO scintillator material with thicknesses ranging from 0.25 to 4 cm and element-to-element pitches ranging from 0.508 to 1.016 mm. A series of studies were performed under both kV and MV imaging conditions to determine the most advantageous imager configuration (involving front or rear x ray illumination and use of a mirror or black reflector), converter design (pitch and thickness), and operating mode (pitch-binning combination).
Under the assumptions of the present study, the most advantageous imager design was found to employ rear illumination of the converter in combination with a black reflector, incorporate a BGO converter with a 0.508 mm pitch and a 2 cm thickness, and operate at full resolution for kV imaging and 2 × 2 binning mode for MV imaging. Such a dual energy imager design should provide soft tissue visualization at low, clinically practical doses under MV conditions, while helping to preserve the high spatial resolution and high contrast offered by kV imaging.
The authors' theoretical investigation suggests that a dual energy imager capable of largely preserving the desirable characteristics of both kV and MV imaging is feasible. Such an imager, when coupled to a dual energy radiation source, could facilitate simplification of current treatment room imaging systems (as well as their associated quality assurance), and facilitate more precise integration of kV and MV imaging information by virtue of reduced geometric uncertainties.
在现代放射治疗机房中，兆伏（MV）门静脉成像和千伏（kV）锥束CT（CBCT）成像使用各种有源矩阵平板成像器（AMFPI）设计来进行。为了扩大MV和kV成像的临床应用，多个研究小组对采用厚的、分段闪烁体的MV AMFPI以及单独使用射野方向观几何的kV成像进行了研究。受这些先前研究的启发，鉴于用于kV和MV成像的X射线能谱有很大不同，探索使用单一AMFPI设计在多大程度上能够保留kV和MV成像的优势是很有意义的。在本文中，通过研究基于采用分段闪烁体的X射线转换器的各种假设AMFPI的性能，探讨了这种双能成像器设计的考量因素。
通过CBCT成像的模拟建模来表征对比度、噪声和对比度噪声比性能，而通过平面成像的模拟建模来表征调制传递函数、斯旺克因子和信号性能。模拟基于先前报道的混合建模技术（同时考虑辐射和光学效应），并通过电子加性噪声建模进行了增强。所有设计均采用BGO闪烁体材料，厚度范围为0.25至4 cm，元件间距范围为0.508至1.016 mm。在kV和MV成像条件下进行了一系列研究，以确定最有利的成像器配置（包括前向或后向X射线照明以及使用镜子或黑色反射器）、转换器设计（间距和厚度）和操作模式（间距合并组合）。
在本研究的假设下，发现最有利的成像器设计是采用转换器的后向照明并结合黑色反射器，采用间距为0.508 mm、厚度为2 cm的BGO转换器，并在kV成像时以全分辨率操作，在MV成像时以2×2合并模式操作。这样的双能成像器设计应能在MV条件下以低的、临床实用剂量提供软组织可视化，同时有助于保留kV成像提供的高空间分辨率和高对比度。
作者的理论研究表明，一种能够在很大程度上保留kV和MV成像理想特性的双能成像器是可行的。这样的成像器与双能辐射源耦合时，可以简化当前的治疗机房成像系统（及其相关的质量保证），并由于减少了几何不确定性而有助于更精确地整合kV和MV成像信息。
