投影放射照相的一个主要限制是在二维图像平面上对三维患者体积和解剖结构的投影. In chest imaging, for example, the bony structure of the ribs, clavicle, etc. 会不会经常隐藏细微的肺软组织病变因解剖重叠而引起投影过程. 移除骨骼结构, 因此, might aid in the visualization of otherwise undetectable lesions; similarly, 去除软组织成分并强调骨骼结构可能有助于区分软组织病变和钙化病变. 与软组织相比，骨骼作为能量函数的差分衰减更高，这使得能够将以不同x射线能量拍摄的两张图像分解为组织选择的解剖表示, 即“仅限软组织”和“仅限骨骼”图像.

如上图所示, 用两种不同的x射线能量束获取同一物体的两幅图像会表现出不同的衰减特性. Note the much greater attenuation of bone at low energies, and similar attenuation of bone and soft tissue at higher energies. x轴表示单能光子能量. 光谱的有效能量等于具有相同总体衰减的单能x射线光子的能量. 图中显示的是60 kVp(绿色)和120 kVp(红色)下产生的光谱的典型x射线“有效能量”。. 如果在这些能量下获得两张x射线图像, one can "weight" one image relative to the other that when subtracted, 会因为骨头或软组织而使信号失效吗, 取决于权重因子.

上图所示是使用计算机放射成像双能系统在低能和高能下获得的两幅图像(稍后将详细介绍该技术)。. Note the higher bone-tissue contrast on the left, which represents the "low energy" image. 每张图像的下面是低能图像(左)和高能图像(右)中软组织和骨骼的相对衰减的规范化呈现。, which illustrates the larger overall bone signal (8 units in the low energy image, 高能图像中的4个单位), and less energy-dependent soft tissue signals of lower signal in the low and high energy images, 分别.

双能量处理包括根据“仅限软组织”图像中由于骨骼而产生的信号为零的愿望对每个图像进行加权, or to null the signal due to soft tissue for a "bone only" image. 在如下所示的风格化插图中, to remove bone requires that the signal due to bone be zeroed out. This can be achieved by multiplying the high energy image by 2 and the low energy image by 1, subtracting the weighted high from the weighted low image, and scaling the residual tissue signal over a range to produce a tissue-only image, 如下图所示.

Similarly, to remove soft tissue requires that the signal due to soft tissue be zeroed out. This can be achieved by multiplying the low energy image by 2 and the low energy image by 3, subtracting the weighted low energy image from the weighted high energy image, and scaling the residual bone signal over a range to produce a bone-only image, 如下图所示.

双能放射照相是如何进行的?

Currently there are two clinical systems available for dual-energy radiography

• 一种专门的射线照相系统采用“被动”光刺激存储荧光粉成像板同时获取两张图像. 成像板是堆叠的, 几何排列, 并由铜过滤器分离, 哪个优先吸收较低的x射线能量. 用单个kVp x射线束获得低能像(前成像板)和高能像(后成像板). 如下图所示.

有了这项技术, the low and high energy images are acquired simultaneously, essentially eliminating any artifacts due to patient motion, but the energy separation between the two image pairs is small, which results in a relatively low SNR for the tissue and bone images at typical patient exposures.

• Another dual-energy capable radiography system uses an "active" flat-panel detector, where a low energy image (~60 kVp) is initially acquired, 快速读出和检测器复位, followed by a high energy image (~120 kVp) immediately afterward. 如下图所示.

This dual-energy method uses a flat-panel detector with a fast readout capability, 允许使用两个独立的x射线光束，产生光束有效能量的巨大差异. The first acquisition occurs with the high (120 kVp) energy beam, 然后通过TFT平板阵列的图像读出, followed immediately by the low (60 kVp) energy beam acquisition, 然后通过图像读出. Energy separation is large, allowing for a relatively high SNR for a given patient exposure. 然而, 由于获取两幅图像所需的延迟时间和平板阵列的读出时间, 图像之间的时间差异通常会导致运动伪影，这是由于在约230 ms的图像采集时间和图像之间的读出时间内，患者的非自愿和自愿运动.

双能方法的x射线波谱

下面描述的是用于上述两种双能量方法的典型x射线光束光谱.
左图是使用CR双探测器/滤光片夹层在120 kVp下采集的单x射线束. Advantages include simpler x-ray operation and no patient motion. 缺点是相对较差的能量分离和较低的检测效率(与平板探测器相比).

On the right is the dual x-ray beam acquisition at 60 kVp and 120 kVp using the single, 快速读出薄膜晶体管阵列探测器. 优点包括更好的能量分离和更好的图像质量，在相同的剂量相比，CR探测器三明治. Disadvantages include potential for patient motion artifacts, and the need for a more complicated system interface and more costly system.

双能图像示例

Dual energy radiography can assist in the differential diagnosis of
软性病变vs钙化病变. In the dual-energy image acquisition using a CR sandwich detector pair shown in the figure below, 复合图像(左)上的病变明显钙化，与仅骨图像相同. 用于平板双能图像采集, note the clearly visible soft tissue lesion in the tissue-only image. 回想起来, the lesion is reasonably easy to detect in the conventional composite image, 但很明显，肋骨投射出解剖学上的“噪音”，干扰了肺组织中相对较大病变的显著性. Also of note is the cardiac motion visible in the bone-only image, where soft-tissue .

双能量图片库

像任何其他数字图像一样，双能量图像可以用不同的灰度表示进行操作. The next several sets of images demonstrate a variety of composite, tissue-only and bone-only images of the postero-anterior chest projection. Many of the images contain soft-tissue and calcified pulmonary lesions, 还有一些能量敏感元件的例子，它们在仅组织或仅骨骼的图像中投射特定的信号.

Example image sets illustrating the value of tissue-only and bone-only image presentations:

On the left is the conventional single-energy image; in the middle is the bone-subtracted "soft-tissue only" image; on the right is the soft-tissue subtracted "bone only" image. 在上面的图像集中, 肺软组织病变在复合图像和单纯软组织图像上都清晰可见, 尽管在仅骨的图像中，心脏区域有其他病变，表明存在含钙病变. These might be due to calcium deposits in the vasculature. 在下面的图像集中, 没有明显的病变, but surgical clips are readily visualized on the bone only image.

演示灰度操作的图像示例. Of interest in these images is the presence of silicone in the breasts of this patient, and a soft-tissue lesion in the left upper quadrant of the lung.

用于平板双能量探测器系统, 当x射线系统在心脏快速收缩期间(收缩期结束)通电时，运动可能是一个问题。. Most motion artifacts appear in and around the cardiac anatomy, and often in the pulmonary architecture and diaphragm area.

在许多情况下，双能量x线摄影通过去除掩盖软组织病变的解剖阴影，通常可以提高投影x线摄影的诊断信息含量和灵敏度. 在下面的例子中, the composite image (left) does not show evidence of a pulmonary lesion, 哪一个被上面的肋骨信号所隐藏. 通过选择性地去除骨信号, a soft tissue lesion is clearly visible in the "tissue only" image (middle). 随后的CT穿刺活检显示病变的横截体积及其与双能图像的相关性. The value of cross-sectional imaging is nicely demonstrated, although with higher radiation dose and much higher costs.

总结:双能量成像

Dual energy imaging provides the capability of selectively imaging two clinically relevant materials, 即软组织和骨组织. 骨与软组织的能量依赖差异被用来消除一个组织或另一个组织, determined by energy spectra differences used for acquiring independent images. Elimination of structured anatomy (noise) is the major benefit of the technique.

两种主要的方法包括CR夹层(无源检测器)和检测器间滤波器(铜)，以实现低(前)和高(后)图像对. CR方法的属性是single-shot, 没有运动, 但能量分离差, resulting in noisy images for low dose typical of a chest x-ray examination. DR(使用快速平板读出检测器)获取不同kVp(通常为~60 kVp和~120 kVp)的图像，产生两种不同的图像，具有良好的能量分离，但时间响应时间较差, allowing motion artifacts to sometimes be a significant problem with image quality. DR双能量图像的特点是可能出现患者不自主的运动(尤其是心脏)。, but good energy separation and superior image noise properties for a given patient dose, 使图像具有优异的信噪比.