Medical Physics Graduate Program, Duke University, Durham, NC 27710, USA.
Med Phys. 2012 Oct;39(10):6056-64. doi: 10.1118/1.4752212.
To implement dual-energy imaging technique for virtual monochromatic (VM) and linearly mixed (LM) cone beam CTs (CBCTs) and to demonstrate their potential applications in metal artifact reduction and contrast enhancement in image-guided radiation therapy (IGRT).
A bench-top CBCT system was used to acquire 80 kVp and 150 kVp projections, with an additional 0.8 mm tin filtration. To implement the VM technique, these projections were first decomposed into acrylic and aluminum basis material projections to synthesize VM projections, which were then used to reconstruct VM CBCTs. The effect of VM CBCT on the metal artifact reduction was evaluated with an in-house titanium-BB phantom. The optimal VM energy to maximize contrast-to-noise ratio (CNR) for iodine contrast and minimize beam hardening in VM CBCT was determined using a water phantom containing two iodine concentrations. The LM technique was implemented by linearly combining the low-energy (80 kVp) and high-energy (150 kVp) CBCTs. The dose partitioning between low-energy and high-energy CBCTs was varied (20%, 40%, 60%, and 80% for low-energy) while keeping total dose approximately equal to single-energy CBCTs, measured using an ion chamber. Noise levels and CNRs for four tissue types were investigated for dual-energy LM CBCTs in comparison with single-energy CBCTs at 80, 100, 125, and 150 kVp.
The VM technique showed substantial reduction of metal artifacts at 100 keV with a 40% reduction in the background standard deviation compared to a 125 kVp single-energy scan of equal dose. The VM energy to maximize CNR for both iodine concentrations and minimize beam hardening in the metal-free object was 50 keV and 60 keV, respectively. The difference of average noise levels measured in the phantom background was 1.2% between dual-energy LM CBCTs and equivalent-dose single-energy CBCTs. CNR values in the LM CBCTs of any dose partitioning are better than those of 150 kVp single-energy CBCTs. The average CNR for four tissue types with 80% dose fraction at low-energy showed 9.0% and 4.1% improvement relative to 100 kVp and 125 kVp single-energy CBCTs, respectively. CNRs for low-contrast objects improved as dose partitioning was more heavily weighted toward low-energy (80 kVp) for LM CBCTs.
Dual-energy CBCT imaging techniques were implemented to synthesize VM CBCT and LM CBCTs. VM CBCT was effective at achieving metal artifact reduction. Depending on the dose-partitioning scheme, LM CBCT demonstrated the potential to improve CNR for low contrast objects compared to single-energy CBCT acquired with equivalent dose.
实现虚拟单能（VM）和线性混合（LM）锥形束 CT（CBCT）的双能成像技术，并展示其在金属伪影减少和图像引导放射治疗（IGRT）中的对比度增强方面的潜在应用。
使用台式 CBCT 系统获取 80 kVp 和 150 kVp 投影，并额外添加 0.8 毫米锡过滤。为了实现 VM 技术，首先将这些投影分解为丙烯酸和铝基底材料投影，以合成 VM 投影，然后使用这些投影重建 VM CBCT。使用内部钛-BB 体模评估 VM CBCT 对金属伪影减少的影响。使用包含两种碘浓度的水体模确定最佳 VM 能量，以最大化碘对比度的对比噪声比（CNR）并最小化 VM CBCT 中的束硬化。LM 技术通过线性组合低能（80 kVp）和高能（150 kVp）CBCT 来实现。在保持总剂量与单能 CBCT 大致相等的情况下，通过离子室测量，改变低能和高能 CBCT 之间的剂量分配（低能为 20%、40%、60%和 80%）。比较了四种组织类型的双能 LM CBCT 和 80、100、125 和 150 kVp 单能 CBCT 的噪声水平和 CNR。
VM 技术在 100 keV 时显示出金属伪影的显著减少，与同等剂量的 125 kVp 单能扫描相比，背景标准偏差降低了 40%。对于无金属物体的碘浓度和最小束硬化的最大 CNR 能量分别为 50 keV 和 60 keV。在体模背景中测量的平均噪声水平差异在双能 LM CBCT 和等效剂量单能 CBCT 之间为 1.2%。任何剂量分配的 LM CBCT 的 CNR 值均优于 150 kVp 单能 CBCT。对于低能的 80%剂量分数，四种组织类型的平均 CNR 分别比 100 kVp 和 125 kVp 单能 CBCT 提高了 9.0%和 4.1%。随着 LM CBCT 中低能（80 kVp）的剂量分配更加偏重，低对比度物体的 CNR 提高。
实现了双能 CBCT 成像技术来合成 VM CBCT 和 LM CBCT。VM CBCT 有效地实现了金属伪影减少。根据剂量分配方案，与使用等效剂量获取的单能 CBCT 相比，LM CBCT 有可能提高低对比度物体的 CNR。
