苹果组织内部的病变会导致其光学参数发生变化。 用频域近红外光学成像法（FD-DOT）对苹果组织进行吸收系数和约化散射系数的检测, 并结合三维重构技术得到的重构图像可以直观地了解苹果内部的病变情况, 从而实现对苹果内部病变的无损检测。 选择可最大程度区分苹果正常组织与病变组织所对应的波长为740 nm的光作为激光光源。 当FD-DOT的入射光调制频率不同、 苹果内部病变的程度不同、 病变位置和大小不同时, 会导致成像精度的变化, 设计了一系列模拟仿真实验研究以上因素对苹果内部病变检测精度的影响: 设定不同的激光调制频率, 研究调制频率对重构图像精度的影响; 在苹果模型中某一位置添加不同大小的球形病变, 研究病变区域大小对重构图像精度的影响; 在苹果模型中不同位置添加一定大小的异质体, 研究病变位置不同对重构图像精度的影响。 首先用Abaqus建立苹果有限元网格模型, 设计了12个740 nm的近红外激光光源和6个检测器均匀排布在苹果模型表面, 根据实验需要, 在组织体模型中添加代表病变的球形异质体, 用经过高频调制的光源照射进苹果, 检测出射光的交流幅度和相位延迟, 然后借助开源软件NIRFAST计算并反推出待测苹果内部的吸收系数和约化散射系数分布并进行三维重构, 重构结果可以用重构图像的吸收系数对比度噪声比（CNR值）和吸收系数分布图进行评价。 实验结果表明, 想要检测到尺寸较大苹果的深处病变, 需要较高的入射光调制频率; 该方法可以检测到大小适宜的苹果中大部分半径大于5 mm的球形病变区域, 且随着病变区域在一定范围内扩大, 重构图像的精度逐渐增加, 但病变区域过大时, 图像精度开始降低; 病变区域距离检测器越来越近时, 重构图像的精度逐渐增加, 但当病变区域与检测器距离过小时, 重构图像的精度有降低的趋势; 病变区域距离检测器平面的垂直距离越近, 重构图像的精度越高。 以上实验结果将为应用频域近红外光学成像法对苹果进行无损检测奠定良好基础。

Lesions inside the apple tissue cause changes in their optical parameters. Frequency domain diffuse optical tomography（FD-DOT） is used to detect the absorption coefficient and the reduced scattering coefficient of apple tissue, and the reconstructed image obtained by combining the 3D reconstruction technology can intuitively understand the internal condition of the apple. In this way, non-destructive detection of internal lesions in apple was realized. According to the visible near-infrared transmission spectrum of normal and rotten apple samples, the wavelength of 740nm can be chosen as the laser light source to distinguish normal and diseased apple tissues furthest. The imaging precision will vary with the frequency of incident light modulation, the degree, location, and size of lesions within the apple. In this paper, a series of simulation experiments are designed to study the effect of the above factors on the detection accuracy: to study the influence of modulation frequency on the accuracy of reconstructed images, setting different laser modulation frequencies; to study the influence of the size of lesion on the accuracy of the reconstructed image, adding spherical heteroplasmon of different sizes at a certain position in the apple model ; A heteroplasmon of a certain size was added at different positions to study the influence of different lesion positions on the accuracy of the reconstructed image. Firstly, finite element mesh model of apple was established by Abaqus. Twelve 740nm near-infrared laser sources and six detectors were designed to be evenly arranged on the surface of the apple model. Then according to the experimental needs, a spherical heteroplasmon representing lesion was added to the tissue model. After that irradiation into interior of the apple with a high-frequency modulated light source, and detect the AC amplitude and phase delay of the emitted light. The software NIRFAST is used to calculate and inversely derive the absorption coefficient and the reduced scattering coefficient distribution of the apple to be tested. Finally, the reconstructed image is obtained by using 3D reconstruction method, reconstruction results can be evaluated using the absorption coefficient contrast-to-noise ratio (CNR) and absorption coefficient distribution of the reconstructed image. The experimental results show that in order to detect deep lesions of larger apples, a higher incident light modulation frequency is required; this method can detect most spherical lesions with a radius greater than 5 mm in suitable apples, and as the size of lesions is enlarged within a certain range, the accuracy of the reconstructed image is gradually increased. However, when lesion area is too large, the image accuracy begins to decrease. When lesion is closer and closer to the detector, the accuracy of the reconstructed image gradually increases, but when distance between lesion and detector is too small, the accuracy of the reconstructed image has a tendency to decrease; the closer the vertical distance of lesion region to the detector plane is, the higher the accuracy of the reconstructed image is. The above experimental results will lay a good foundation for the application of FD-DOT in non-destructive testing of apples.