Department of Radiology, Stanford University, California 94305, USA.
Med Phys. 2010 Aug;37(8):4029-37. doi: 10.1118/1.3457472.
An optimal material selection for primary modulator is proposed in order to minimize beam hardening of the modulator in x-ray cone-beam computed tomography (CBCT). Recently, a measurement-based scatter correction method using primary modulation has been developed and experimentally verified. In the practical implementation, beam hardening of the modulator blocker is a limiting factor because it causes inconsistency in the primary signal and therefore degrades the accuracy of scatter correction.
This inconsistency can be purposely assigned to the effective transmission factor of the modulator whose variation as a function of object filtration represents the magnitude of beam hardening of the modulator. In this work, the authors show that the variation reaches a minimum when the K-edge of the modulator material is near the mean energy of the system spectrum. Accordingly, an optimal material selection can be carried out in three steps. First, estimate and evaluate the polychromatic spectrum for a given x-ray system including both source and detector; second, calculate the mean energy of the spectrum and decide the candidate materials whose K-edge energies are near the mean energy; third, select the optimal material from the candidates after considering both the magnitude of beam hardening and the physical and chemical properties.
A tabletop x-ray CBCT system operated at 120 kVp is used to validate the material selection method in both simulations and experiments, from which the optimal material for this x-ray system is then chosen. With the transmission factor initially being 0.905 and 0.818, simulations show that erbium provides the least amount of variation as a function of object filtrations (maximum variations are 2.2% and 4.3%, respectively, only one-third of that for copper). With different combinations of aluminum and copper filtrations (simulating a range of object thicknesses), measured overall variations are 2.5%, 1.0%, and 8.6% for 25.4 microm of copper, erbium, and tungsten, respectively. With and without 300 microm of copper in the beam, the measured variations for 25.4 microm of copper, erbium, and tungsten, 1 mm of aluminum, as well as 406 microm of copper, are 1.8%, 0.2%, 5.5%, 1.9%, and 7.5%, respectively.
The spatial variation in the effective transmission factor of the modulator blocker due to beam hardening caused by the modulator itself reaches a minimum when the K-edge of the modulator material is near the mean energy of the spectrum. An optimal modulator material selection using the K-edge discontinuity is therefore proposed.
为了最大限度地减少调制器的束硬化，提出了一种用于初级调制器的最佳材料选择，以用于 X 射线锥形束计算机断层扫描（CBCT）。最近，已经开发并实验验证了一种基于初级调制的基于测量的散射校正方法。在实际实现中，调制器阻挡器的束硬化是一个限制因素，因为它会导致初级信号不一致，从而降低散射校正的准确性。
这种不一致性可以有意分配给调制器的有效传输因子，其作为对象滤波函数的变化代表调制器的束硬化程度。在这项工作中，作者表明，当调制器材料的 K 边接近系统光谱的平均能量时，变化达到最小值。因此，可以分三步进行最佳材料选择。首先，估计和评估给定 X 射线系统（包括源和探测器）的多色光谱；其次，计算光谱的平均能量，并决定 K 边能量接近平均能量的候选材料；第三，在考虑束硬化程度和物理化学性质的情况下，从候选材料中选择最佳材料。
使用台式 X 射线 CBCT 系统在 120 kVp 下进行验证，该系统在模拟和实验中都验证了材料选择方法，然后选择了该 X 射线系统的最佳材料。在初始传输因子分别为 0.905 和 0.818 的情况下，模拟表明，铒的变化最小，作为对象过滤函数的函数（最大变化分别为 2.2%和 4.3%，仅为铜的三分之一）。对于不同的铝和铜过滤组合（模拟一系列物体厚度），对于 25.4 µm 的铜、铒和钨，测量的总变化分别为 2.5%、1.0%和 8.6%。在光束中带有和不带有 300 µm 的铜的情况下，对于 25.4 µm 的铜、铒和钨、1 mm 的铝以及 406 µm 的铜，测量的变化分别为 1.8%、0.2%、5.5%、1.9%和 7.5%。
由于调制器本身引起的束硬化，调制器阻挡器的有效传输因子的空间变化在调制器材料的 K 边接近光谱的平均能量时达到最小值。因此，提出了一种使用 K 边不连续性的最佳调制器材料选择。
