Imaging Physics Laboratory, Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA.
Phys Med Biol. 2010 Sep 21;55(18):5317-39. doi: 10.1088/0031-9155/55/18/005. Epub 2010 Aug 24.
The aim of the study is to determine the upper limits of the signal-to-noise ratio (SNR) in radiography and computed tomography (CT) with polyenergetic x-ray sources. In x-ray imaging, monoenergetic x-rays provide a higher SNR compared to polyenergetic x-rays. However, the SNR in polyenergetic x-ray imaging can be increased when a photon-counting detector is used and x-rays are optimally weighted according to their energies. For a particular contrast/background combination and at a fixed x-ray entrance skin exposure, the SNR in energy-weighting x-ray imaging depends on tube voltage and can be maximized by selecting the optimal tube voltage. The SNR in energy-weighted x-ray images acquired at this optimal tube voltage is the highest SNR that can be achieved with polyenergetic x-ray sources. The optimal tube voltages and the highest SNR were calculated and compared to the SNR of monoenergetic x-ray imaging. Monoenergetic, energy-weighting polyenergetic and energy-integrating polyenergetic x-ray imagings were simulated at a fixed entrance skin exposure of 20 mR. The tube voltages varied in the range of 30-140 kVp with 10 kV steps. Contrast elements of CaCO(3), iodine, adipose and tumor with thicknesses of 280 mg cm(-2), 15 mg cm(-2), 1 g cm(-2) and 1 g cm(-2), respectively, inserted in a soft tissue background with 10 cm and 20 cm thicknesses, were used. The energy weighting also improves the contrast-to-noise ratio (CNR) in CT when monoenergetic CT projections are optimally weighted prior to CT reconstruction (projection-based weighting). Alternatively, monoenergetic CT images are reconstructed, optimally weighted and composed to yield a final CT image (image-based weighting). Both projection-based and image-based weighting methods improve the CNR in CT. An analytical approach was used to determine which of these two weighting methods provides the upper limit of the CNR in CT. The energy-weighting method was generalized and expanded as a weighting method applicable in areas other than x-ray and CT. An optimal x-ray tube voltage providing the highest SNR in energy-weighting x-ray imaging was determined for each contrast/background combination. Depending on the imaging task, the highest SNR with energy-weighted polyenergetic x-rays was close to the SNR provided by monoenergetic x-rays. In CT, projection-based weighting provided higher CNR than image-based weighting, thus determining an upper limit of the CNR in CT. The weighting approach can be applied to imaging methods with contrast mechanisms other than x-ray interaction. A unique, task-dependent tube voltage exists in photon-counting x-ray and CT that provides the highest SNR with polyenergetic x-rays. The highest SNR achieved in photon-counting energy-weighted x-ray and CT can approach the SNR of monoenergetic x-ray and CT imaging, depending on the imaging task.
本研究旨在确定使用多能 X 射线源进行放射摄影和计算机断层扫描（CT）时的信噪比（SNR）上限。在 X 射线成象中，与多能 X 射线相比，单能 X 射线提供更高的 SNR。然而，当使用光子计数探测器并根据 X 射线的能量对其进行最佳加权时，多能 X 射线成像中的 SNR 可以增加。对于特定的对比度/背景组合，并在固定的 X 射线入口皮肤暴露下，能量加权 X 射线成像中的 SNR 取决于管电压，并通过选择最佳管电压来最大化。在该最佳管电压下获得的能量加权 X 射线图像的 SNR 是使用多能 X 射线源可实现的最高 SNR。计算并比较了最佳管电压和最高 SNR 与单能 X 射线成像的 SNR。在固定的入口皮肤暴露为 20 mR 的情况下，对单能、能量加权的多能和能量积分的多能 X 射线成像进行了模拟。管电压在 30-140 kVp 范围内变化，步长为 10 kV。在厚度为 10 cm 和 20 cm 的软组织背景中插入了厚度分别为 280 mg cm(-2)、15 mg cm(-2)、1 g cm(-2)和 1 g cm(-2)的 CaCO(3)、碘、脂肪和肿瘤对比元素。还使用了单能 CT 投影在 CT 重建之前进行最佳加权的情况下（基于投影的加权）改善 CT 中的对比噪声比（CNR）的情况（基于投影的加权）。或者，重建、最佳加权和组合单能 CT 图像以产生最终的 CT 图像（基于图像的加权）。基于投影的加权和基于图像的加权方法都可以改善 CT 中的 CNR。使用分析方法确定了这两种加权方法中的哪一种提供了 CT 中 CNR 的上限。能量加权方法被推广和扩展为适用于 X 射线和 CT 以外的区域的加权方法。为每个对比度/背景组合确定了在能量加权 X 射线成像中提供最高 SNR 的最佳 X 射线管电压。根据成像任务，具有能量加权的多能 X 射线的最高 SNR 接近单能 X 射线的 SNR。在 CT 中，基于投影的加权比基于图像的加权提供更高的 CNR，从而确定了 CT 中 CNR 的上限。该加权方法可应用于除 X 射线相互作用以外的对比度机制的成像方法。在具有多能 X 射线的光子计数 X 射线和 CT 中存在独特的、任务相关的管电压，该管电压可提供最高 SNR。取决于成像任务，在光子计数能量加权 X 射线和 CT 中实现的最高 SNR 可以接近单能 X 射线和 CT 成像的 SNR。
