医用电子直线加速器机头散射X射线的分析

本文介绍了一种用于医用电子直线加速器机头的结构部件上产生的散射X射线能量注量分布的解析计算方法。该方法是在蒙特卡罗模拟计算得到的初级X射线能谱的基础上，通过康普顿散射的Klein-Nishina公式计算得到的。考虑的主要散射源为机头内的初级准直器和均整块。我们用这个方法研究了北京医疗器械研究所生产的6MeV BJ-6加速器的机头散射情况并和实验测量结果进行了比较，比较的结果还包括均整块和初级准直器的散射与射野大小及离开X射线源距离之间的关系一比较结果表明：该解析计算方法对计算机头散射问题是可行的。     

基于二维矩阵透射剂量重建入射原射线注量分布

目的:通过电子射野影像装置(Electronic Portal Imaging Device,EPID)平面处测量得到的透射剂量分布重建到模体表面前的原射线注量分布。方法:使用SUN NUCLEAR公司的二维半导体阵列(Mapcheck2)和PTW公司的OCTAVIUS Detector 1000SRS测量有模体和无模体时不同大小射野在EPID平面处的透射剂量,通过笔者编写的算法重建入射原射线注量分布,并与三维水箱探头加带平衡帽测量的原射线注量分布比较。结果:采用Gamma法(3 mm/3%)进行评估,在射野内Mapcheck2和OCTAVIUS Detector 1000SRS探测器通过率为100%。结论:本次研究建立的模型得出的原射线离轴比的准确性可满足临床要求,可以用于剂量验证。     

医用加速器8MVX射线在水模体中的一阶散射X射线能谱的蒙特卡罗计算

目的：分析医用电子直线加速器的高能X射线与水模体相互作用过程中所产生的一次散射光子的能谱角分布和光子强度角分布。方法：利用蒙特卡罗粒子输运程序Geant4,模拟粒子输运过程．计算加速器8MeV高能X射线能谱,并根据在水模体中实际测量的PDD吸收曲线为依据,修正蒙特卡洛计算的能谱;并以此能谱为虚拟源能谱,通过对X射线与水模体相互作用后的光子一电子联合输运过程进行蒙特卡罗模拟的方法获取有关散射X线能谱数据。结果：用蒙特卡洛方法得到加速器8MV初始X射线与水模体作用产生的一次散射光子的散射光子强度和散射光子能量随散射角度变化的规律。结论：根据ICRP85出版物、ICRU44报告给出的数据,可以用组织平均原子序数作为组织等效原子序数;因此,组织密度变化在物理上反映了组织的原子密度的变化,当入射光子注量改变,模体密度变化时。仅引起相互作用的总截面相对于原子微分截面的线性变化,并不影响一阶散射X射线的散射光子的相对强度角分布和散射光子能量角分布。而散射光子发射的绝对量与初始X射线强度、组织的原子密度成正比。因此,一次散射光子的注量角分布、平均能量角分布结果可形成可调用的数据库,对快速蒙特卡洛计算很有意义。     

模体大小对CBCT图像剂量计算的影响

目的：锥形束CT（CBCT）使用宽束X-射线,探测板获取的信号受散射线的影响很大。该文对扫描模体大小及散射体积对CBCT重建图像HU值及剂量计算的影响进行研究。方法：在Elekta Synergy-XVICBCT系统中对不同深度及散射体积的均匀水模和非均匀密度参考模体扫描,并测量感兴趣区域的HU值;建立考虑和不考虑散射的两组HU-物理密度曲线应用于水模及头颈部仿真模体CBCT图像进行剂量计算,与常规CT图像（FBCT）计算结果比较。结果：均匀水模CBCT图像的HU值随水模深度增加先增大后略有减少,随纵轴散射长度增加而减少,变化幅度最大均接近10%。随散射长度增加,非均匀密度参考模体CBCT图像的高密度组织的HU值减少而低密度组织HU值增加,对1.609 g/cm3致密度骨最大减少约1422 HU。均匀水模和头颈部仿真模体CBCT图像使用考虑散射的HU-物理密度修正曲线计算与FBCT图像比较结果为：点绝对剂量（cGy/MU）最大偏差小于1.5%,等剂量线偏差小于2 mm~3 mm,2%/2 mm DTA指数的通过率平均大于97%,明显优于不做散射修正的结果。结论：Elekta Synergy-XVI系统获取CBCT图像的HU值受扫描模体的几何大小及散射体积影响很大,应选择与扫描患者近似几何大小及人体组织等效的HU-密度校准模体。考虑模体大小及散射修正的头颈部模体CBCT图像用于剂量计算能满足临床要求。     

光子射野剂量计算参数研究

目的：介绍医用加速器常规光子射线的机器数据测量方法及剂量计算模型中基本参数的计算过程。以百分深度剂量与散射因子为基础数据,根据原散射线模型通过测量数据推导出原射线组织最大剂量比、散射最大剂量比、原射线在水中线性衰减系数、能量注量等,为进一步还原射野在水模体中的剂量分布提供方法与理论。方法：用Blue Phantom三维水箱在医科达Synergy加速器上测量6MV光子线的百分深度剂量、离轴比剂量、总散射因子、准直器散射因子,先从测量的百分深度剂量曲线中按照原散射模型剥离出原射线百分深度剂量,然后在Matlab软件中拟合处理测量的散射因子数据,外推出零野的模体散射因子,从而按照给定公式计算出组织最大剂量比、散射最大剂量比。按照离轴比剂量,利用平方反比规律推出最大开野在模体表面的能量注量。结果：计算出准直器散射因子、总散射因子的拟合公式,外推零野模体散射因子（s。）、根据原射线的百分深度剂量曲线计算出原射线在水中线性衰减系数,组织最大剂量比（TMR）、散射最大剂量比（SMR）、以及射野能量注量分布（Fluence Matrix）。结论：这些基本参数是剂量计算建模的关键,也是进一步研究各种剂量计算模型的基础。     

基于非晶硅电子射野影像装置的剂量响应研究

目的：临床条件下研究探讨非晶硅电子射野影像装置（a-Si EPID）的剂量响应特性。方法 ：本实验在Elekta Precise直线加速器上X射线能量分别为6 MV和10 MV,采用PTW电离室、等效固体水和不同厚度铜板条件下实施测量。首先,通过EPID信号和模体中电离室的测量比较,确定出EPID剂量响应的建成厚度。其次,临床条件下利用模体的不同厚度测量分析有关剂量、每脉冲剂量和脉冲重复频率（PRF）函数的EPID信号响应情况。结果：在不增加建成材料、10 cm～60cm空气间隙条件下EPID显示了最大11.6%的过响应信号变化。临床上额外将3 mm铜建成区置于EPID上方,空气间隙大于40 cm条件下EPID响应变化将会降至1%以内。在测量范围内随MU数、PRF和每脉冲剂量变化的EPID信号响应是非线性的,最大信号变化接近于3%。因假峰和图像滞后效应等影响,短时间照射EPID会明显地产生出低剂量响应。结论：采用合适的建成层和实施对每脉冲剂量、PRF等校正,非晶硅EPID剂量响应变化可控制在1%以内,从而建立起较为理想的剂量响应曲线。     

Varian Edge均整和非均整模式下6 MV和10 MV光子线能谱研究

目的:研究Varian Edge均整(FF)和非均整(FFF)模式下6 MV和10 MV光子线能谱并对比其差异。方法:利用蒙特卡洛程序软件包EGSnrc/Beamnrc建立Varian Edge 6 MV FF和FFF、10 MV FF和FFF的加速器模型,模拟所对应的相空间文件,而后以相空间作为输入源,利用DOSXYZnrc计算其在水体模中的剂量分布,并与三维水箱的测量数据比对,当模拟值与测量值之间的差异在1%之内时,利用Beamdp分析此时的相空间文件,得到对应的光子线能谱,并比较相互之间的差异。结果:模拟的百分深度剂量曲线和离轴比曲线与测量值之间的差异在1%之内。相对于FF模式,FFF模式的能谱"软化",其中6 MV FFF的平均能量从1.587 MeV下降至1.172 MeV,低能(能量≤1 MeV)光子所占的份额由41.06%上升至60.04%;而10 MV FFF的平均能量从2.796 MeV下降至1.956 MeV,低能光子所占的份额由21.22%上升至44.63%。同一射野内FFF模式的能谱随离轴距离的改变较小,同时每初始粒子所引起的能量注量是FF模式的2～4倍,射野内的能量注量分布变得不均匀,非平坦度F上升;分析不同射野下的能谱发现FFF模式的机头散射较少。结论:本研究结果对理解FFF模式下光子线的物理特性提供了非常好的参考价值。     

Acuros XB、各向异性解析算法与蒙特卡罗算法在非均匀组织中剂量计算准确性对比研究

目的:对比Acuros XB算法(AXB)、各向异性解析算法(AAA)和蒙特卡罗(MC)算法在非均匀组织中剂量计算准确性。方法:在Eclipse计划系统上分别设置两种类型的非均匀模体(水-肺-水模体、水-骨-水模体),并设定3个不同大小的0°照射野,源皮距=100 cm。采用AXB、AAA及MC算法进行剂量计算,提取射野中心轴百分深度剂量,以MC计算结果为基准,计算AXB和AAA两种算法与MC算法的相对偏差,提取非均匀组织及高梯度区(即4.5~15.5 cm)的数据做对比分析。结果:AXB算法3个射野相对偏差绝对值分别为4.186±1.451、0.834±0.300、0.726±0.165(水-肺-水模体)和1.694±0.374、1.325±0.328、0.343±0.244(水-骨-水模体)。AAA算法在两模体的对应值分别为6.679±4.694、4.151±1.789、4.353±2.546(肺)和3.270±0.826、5.971±1.587、2.406±0.574(骨)。采用配对样本t检验,P值均小于0.05。结论:在非均匀组织及其边界,AXB算法计算精度比AAA算法更为准确,基本接近MC算法。     

基于压缩感知的超声逆散射成像研究

目的逆散射超声成像具有较高的空间分辨率和图像对比度,有着重大的临床应用价值。传统的基于逆问题理论的超声逆散射重建算法存在重建系统稳定性差,设备数据采集量大等问题。本文基于超声逆散射理论,利用压缩感知原理,对目标进行重建,以降低设备数据处理量,增强重建过程的稳定性。方法首先分别从时域和频率两个角度建立超声散射正向模型;再根据压缩感知原理,提出数据采集方案,获得投影观测数据;然后,利用目标的稀疏性,建立基于压缩感知的超声逆散射重建逆问题;最后,以囊肿体模和点目标为例,求解逆问题,重建图像。结果提出的基于压缩感知超声逆散射重建算法,对囊肿体模采样率降低50%,对点目标采样率降低76%。结论基于压缩感知的超声逆散射重建,能够降低设备数据处理量,与传统的延时叠加、合成孔径等成像算法相比,重建的图像具有更高的空间分辨率和对比度。     

DPM蒙特卡罗剂量计算算法在均匀组织和非均匀组织剂量精确性的验证

验证DPM蒙特卡罗剂量计算算法预测均匀组织和非均匀组织剂量的精确性。DPM分别计算:①6 MeV单能光子3cm×3cm照射野和Varian 60℃加速器源水模体百分深度剂量曲线和10cm深度处离轴比;②6 MeV单能光子3cm×3cm、10cm×10cm照射野分别在水(6cm)/肺(6cm)/水(8cm)、水(6cm)/骨骼(2cm)/水(12cm)非均匀组织的百分深度剂量曲线;③6MeV单能光子6cm×6cm照射野人体头部和腹部组织在射野内和射野外的百分深度剂量曲线。比较DPM计算值与DOSXYZnrc/EGSnrc系统在相同条件下的计算值。结果显示二者计算值在水模中的误差在±3%以内,在非均匀组织中,除了个别点,误差都在±3%以内。DPM能够精确计算均匀组织和非均匀组织剂量。     

Photon scatter in portal images: accuracy of a fluence based pencil beam superposition algorithm

The accuracy of a pencil beam algorithm to predict scattered photon fluence into portal imaging systems was studied. A data base of pencil beam kernels describing scattered photon fluence behind homogeneous water slabs (1-50 cm thick) at various air gap distances (0-100 cm) was generated using the EGS Monte Carlo code. Scatter kernels were partitioned according to particle history: singly-scattered, multiply-scattered, and bremsstrahlung and positron annihilation photons. Mean energy and mean angle with respect to the incident photon pencil beam were also scored. This data allows fluence, mean energy, and mean angular data for each history type to be predicted using the pencil beam algorithm. Pencil beam algorithm predictions for 6 and 24 MV incident photon beams were compared against full Monte Carlo simulations for several inhomogeneous phantoms, including approximations to a lateral neck, and a mediastinum treatment. The accuracy of predicted scattered photon fluence, mean energy, and mean angle was investigated as a function of air gap, field size, photon history, incident beam resolution, and phantom geometry. Maximum errors in mean energies were 0.65 and 0.25 MeV for the higher and lower energy spectra, respectively, and 15 degrees for mean angles. The ability of the pencil beam algorithm to predict scatter fluence decreases with decreasing air gap, with the largest error for each phantom occurring at the exit surface. The maximum predictive error was found to be 6.9% with respect to the total fluence on the central axis. By maintaining even a small air gap (approximately 10 cm), the error in predicted scatter fluence may be kept under 3% for the phantoms and beam energies studied here. It is concluded that this pencil beam algorithm is sufficiently accurate (using International Commission on Radiation Units and Measurements Report No. 24 guidelines for absorbed dose) over the majority of clinically relevant air gaps, for further investigation in a portal dose prediction algorithm.     

Photon scatter in portal images: physical characteristics of pencil beam kernels generated using the EGS Monte Carlo code

Pencil beam kernels describing scattered photon fluence behind homogeneous water slabs at various air gap distances were generated using the EGS Monte Carlo code. Photon scatter fluence was scored in separate bins based on the particle's history: singly scattered, multiply scattered, and bremsstrahlung and positron annihilation photons. Simultaneously, the mean energy and mean angle with respect to the incident photon pencil beam were tallied. Kernels were generated for incident photon pencil beams exhibiting monoenergetic spectra of 2.0 and 10.0 MeV, and polyenergetic spectra representative of 6 and 24 MV beams. Reciprocity was used to generate scatter fractions on the central axis for various field sizes, phantom thicknesses, and air gaps. The scatter kernels were further characterized by full width at half-maximum estimates. Modulation transfer functions were calculated, providing theoretical estimates of the limit of performance of portal imaging systems due to the intrinsic scattering of photon radiation through the patient.     

A two-step algorithm for predicting portal dose images in arbitrary detectors

Recently, portal imaging systems have been successfully demonstrated in dosimetric treatment verification applications, where measured and predicted images are quantitatively compared. To advance this approach to dosimetric verification, a two-step model which predicts dose deposition in arbitrary portal image detectors is presented. The algorithm requires patient CT data, source-detector distance, and knowledge of the incident beam fluence. The first step predicts the fluence entering a portal imaging detector located behind the patient. Primary fluence is obtained through ray-tracing techniques, while scatter fluence prediction requires a library of Monte Carlo-generated scatter fluence kernels. These kernels allow prediction of basic radiation transport parameters characterizing the scattered photons, including fluence and mean energy. The second step of the algorithm involves a superposition of Monte Carlo-generated pencil beam kernels, describing dose deposition in a specific detector, with the predicted incident fluence. This process is performed separately for primary and scatter fluence, and yields a predicted dose image. A small but noticeable improvement in prediction is obtained by explicitly modeling the off-axis energy spectrum softening due to the flattening filter. The algorithm is tested on a slab phantom and a simple lung phantom (6 MV). Furthermore, an anthropomorphic phantom is utilized for a simulated lung treatment (6 MV), and simulated pelvis treatment (23 MV). Data were collected over a range of air gaps (10-80 cm). Detectors incorporating both low and high atomic number buildup are used to measure portal image profiles. Agreement between predicted and measured portal dose is better than 3% in areas of low dose gradient (<30%/cm) for all phantoms, air gaps, beam energies, and detector configurations tested here. It is concluded that this portal dose prediction algorithm is fast, accurate, allows separation of primary and scatter dose, and can model arbitrary detectors.     

Analytical calculation of the portal scatter to primary dose ratio: an EGS4 Monte Carlo and experimental validation at large air gaps.

An analytical approximation for the scatter to primary dose ratio (SPR) on the central axis was validated against Monte Carlo results and experimental measurements for homogeneous and inhomogeneous phantoms. The analytical approximation only included first-order Compton scatter. The contribution to the total SPR from first-order Compton scatter, multiply scattered photons and electron scatter was investigated using Monte Carlo simulation for homogeneous phantoms (up to 30 cm thick for 6 and 18 MV beams; source to detector distances from 150 to 230 cm) as well as for a neck, thorax and pelvis phantom. SPRs were measured on the central axis with an ionization chamber for water phantoms (up to 20 cm thick at 4 MV, 30 cm for 6 MV and 10 MV and 40 cm for 18 MV; source to detector distances of 185 and 200 cm) and for phantoms representing the neck, thorax and pelvis (for air gaps of 50 cm and larger). The mean difference between the experimental and analytical SPRs on the central axis for source to detector distances of 170 cm or greater was within: -0.003 (neck); -0.012 (thorax); -0.028 (pelvis, 10 MV) and 0.008 (pelvis, 18 MV) respectively.     

Calculation of photon energy deposition kernels and electron dose point kernels in water

Effects of changes in the physics of EGSnrc compared to EGS4/PRESTA on energy deposition kernels for monoenergetic photons and on dose point kernels for beta sources in water are investigated. In the diagnostic energy range, Compton binding corrections were found to increase the primary energy fraction up to 4.5% at 30 keV with a corresponding reduction of the scatter component of the kernels. Rayleigh scattered photons significantly increase the scatter component of the kernels and reduce the primary energy fraction with a maximum 12% reduction also at 30 keV where the Rayleigh cross section in water has its maximum value. Sampling the photo-electron angular distribution produces a redistribution of the energy deposited by primaries around the interaction site causing differences of up to 2.7 times in the backscattered energy fraction at 20 keV. Above the pair production threshold, the dose distribution versus angle of the primary dose component is significantly different from the EGS4 results. This is related to the more accurate angular sampling of the electron-positron pair direction in EGSnrc as opposed to using a fixed angle approximation in default EGS4. Total energy fractions for photon beams obtained with EGSnrc and EGS4 are almost the same within 0.2%. This fact suggests that the estimate of the total dose at a given point inside an infinite homogeneous water phantom irradiated by broad beams of photons will be very similar for kernels calculated with both codes. However, at interfaces or near boundaries results can be very different especially in the diagnostic energy range. EGSnrc calculated kernels for monoenergetic electrons (50 keV, 100 keV, and 1 MeV) and beta spectra (32P and 90Y) are in excellent agreement with reported EGS4 values except at 1 MeV where inclusion of spin effects in EGSnrc produces an increase of the effective range of electrons. Comparison at 1 MeV with an ETRAN calculation of the electron dose point kernel shows excellent agreement.     

Energy and spatial distribution of multiple order Compton scatter in SPECT: a Monte Carlo investigation

Energy and spatial projection distributions were simulated for gamma camera imaging of multiple order Compton scattered photons. SPECT imaging of a line source of radioactivity located in a water filled cylindrical phantom was modelled using Monte Carlo techniques. Photon trajectories were followed from emission to detection including the effects of all physical interactions and the resulting energy spectra and spatial projections were sorted as a function of the number of times the photon underwent Compton scattering before detection. Analysis of energy spectra demonstrates that Compton events up to second order overlap with the non-scattered events and distributions are peaked at lower energies as the scattering order increases. Analysis of spatial projections shows that, with increasing order, Compton events produce tails on the line spread function which progress from roughly exponential to nearly flat distributions. The use of Monte Carlo modelling thus allows a detailed investigation of the spatial and energy distribution of Compton scatter which could not be performed using present experimental techniques.     

Patient-specific dosimetry of conventional and intensity modulated radiation therapy using a novel full Monte Carlo phase space reconstruction method from electronic portal images

Electronic portal imagers have promising dosimetric applications in external beam radiation therapy. In this study a patient dose computation algorithm based on Monte Carlo (MC) simulations and on portal images is developed and validated. The patient exit fluence from primary photons is obtained from the portal image after correction for scattered radiation. The scattered radiation at the portal imager and the spectral energy distribution of the primary photons are estimated from MC simulations at the treatment planning stage. The patient exit fluence and the spectral energy distribution of the primary photons are then used to ray-trace the photons from the portal image towards the source through the CT geometry of the patient. Photon weights which reflect the probability of a photon being transmitted are computed during this step. A dedicated MC code is used to transport back these photons from the source through the patient CT geometry to obtain patient dose. Only Compton interactions are considered. This code also produces a reconstructed portal image which is used as a verification tool to ensure that the dose reconstruction is reliable. The dose reconstruction algorithm is compared against MC dose calculation (MCDC) predictions and against measurements in phantom. The reconstructed absolute absorbed doses and the MCDC predictions in homogeneous and heterogeneous phantoms agree within 3% for simple open fields. Comparison with film-measured relative dose distributions for IMRT fields yields agreement within 3 mm, 5%. This novel dose reconstruction algorithm allows for daily patient-specific dosimetry and verification of patient movement.     

Monte Carlo simulation of diagnostic x-ray scatter

A Monte Carlo method was developed and implemented to simulate x-ray photon transport. Simulations consisted of a pencil beam of monoenergetic photons with energies from 50 to 110 keV incident on water and aluminum slabs. The dependence of scatter fraction and multiple scattering on x-ray energy, scatterer thickness, and material is reported in both number and energy fluence. The average energy of scattered photons reaching the detector plane is also reported. Comparisons are made to previous x-ray scatter computations.     

A Monte Carlo study of multiple scatter effects in Compton scatter densitometry

The contribution of multiple scatter to the measured signal in x- and gamma-ray Compton scatter densitometry has been investigated theoretically by the use of Monte Carlo techniques to follow individual photon life histories. A three component phantom was employed in the computer model to simulate the patient at three examination sites; the radius/ulna, the femoral neck, and the lumbar spine. Monoenergetic radiation beams of 60- and 100-keV photons and polyenergetic x-ray spectra of 100 and 140 kVp were used. Scattered events were detected over 360 degrees and classified according to their origin and frequency of scatter. The single scatter in bone to multiple scatter ratio was studied as an indication of the signal-to-noise ratio and this was found to vary with phantom size but was independent of photon energy. Correction factors to be used in a clinical densitometer to account for the inclusion of multiple scatter events were computed. These were found to be 0.65-0.58 at the optimum scattering angles for the phantoms considered.     

Generation of photon energy deposition kernels using the EGS Monte Carlo code

The EGS Monte Carlo code was used to generate photon energy deposition kernels which describe the energy deposited by charged particles set in motion by primary, first scattered, second scattered, multiple scattered and bremsstrahlung plus annihilation photons. These were calculated for a water medium irradiated with monoenergetic photons with energies in the range 0.1-50 MeV. In addition to the primary energy deposition kernels, primary charged particle transport was further characterised by computing the effective centre of the voxels, and the effective penetration depth, effective radius and effective lateral distance travelled by these particles. The dose per unit collision kerma for parallel monoenergetic primary photons beta' was calculated. Additional applications of the energy deposition kernels are discussed.