Monte Carlo Methods in Quantitative Photoacoustic Tomography
Monte Carlo Methods in Quantitative Photoacoustic Tomography
复制标题
定量光声断层扫描中的蒙特卡罗方法
DOI：
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发表时间：
2016
期刊：
影响因子：
--
通讯作者：
R. Hochuli
中科院分区：
文献类型：
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作者：
R. Hochuli
定量光声层析成像（QPAT）是一种混合生物医学成像技术，其特异性来自近红外/可见激光的波长相关吸收，其灵敏度来自超声波。这种有前途的技术不仅可以揭示结构信息，还可以探测组织功能。具体而言，QPAT能够估计内源性发色团的浓度，例如氧化和脱氧血红蛋白的浓度（从中可以计算血氧），以及外源性发色团的浓度，例如近红外染料或金属纳米颗粒。该过程由于以下事实而复杂化：光声图像不经由吸收系数直接与组织性质相关，而是与波长相关的吸收系数乘以内部光通量成比例，内部光通量也是波长相关的并且通常是未知的。本论文从两个角度来处理这个问题，首先，是否某些实验条件允许的影响的通量被忽略的问题，假设它是恒定的波长，一个“线性反演”，是解决。它表明，线性反演仅适用于某些波段的照明波长和有限的深度。在这种假设是不准确的，提出了一种替代方法，其中组织内的通量建模使用一种新的蒙特卡罗模型的光传输。该模型通过将场存储在2D傅立叶谐波或3D球谐中来计算角度相关的辐射分布。本论文证明了以这种方式计算辐射率的一个关键优点是，当吸收和散射系数的估计被转换为非线性最小二乘问题时，它简化了函数梯度的计算。使用这种方法，它是证明在2D的吸收系数的估计可以执行到一个有用的精度水平，尽管在重建的散射系数的精度有限。
Quantitative photoacoustic tomography (QPAT) is a hybrid biomedical imaging technique that derives its specificity from the wavelength-dependent absorption of near-infrared/visible laser light, and its sensitivity from ultrasonic waves. This promising technique has the potential to reveal more than just structural information, it can also probe tissue function. Specifically, QPAT has the capability to estimate concentrations of endogenous chromophores, such as the concentrations of oxygenated and deoxygenated haemoglobin (from which blood oxygenation can be calculated), as well as the concentrations of exogenous chromophore, e.g. near-infrared dyes or metallic nanoparticles. This process is complicated by the fact that a photoacoustic image is not directly related to the tissue properties via the absorption coefficient, but is proportional to the wavelength-dependent absorption coefficient times the internal light fluence, which is also wavelength-dependent and is in general unknown. This thesis tackles this issue from two angles; firstly, the question of whether certain experimental conditions allow the impact of the fluence to be neglected by assuming it is constant with wavelength, a `linear inversion', is addressed. It is demonstrated that a linear inversion is appropriate only for certain bands of illumination wavelengths and for limited depth. Where this assumption is not accurate, an alternative approach is proposed, whereby the fluence inside the tissue is modelled using a novel Monte Carlo model of light transport. This model calculates the angle-dependent radiance distribution by storing the field in Fourier harmonics, in 2D, or spherical harmonics, in 3D. This thesis demonstrates that a key advantage of computing the radiance in this way is that it simplifies the computation of functional gradients when the estimation of the absorption and scattering coefficients is cast as a nonlinear least-squares problem. Using this approach, it is demonstrated in 2D that the estimation of the absorption coefficient can be performed to a useful level of accuracy, despite the limited accuracy in reconstruction of the scattering coefficient.
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