Tube Current (mA)

x射线管电流决定x射线在x射线管中产生的速率.e., photons per second). 当x射线管旋转360º时，在1000个左右的投影中获得的x射线光子总数与mA和旋转时间(秒)的乘积成正比。, or the mAs. Since the x-ray tube rotation is normally fixed, 用于制作任何CT图像的光子数量与电子管电流(毫安)成正比。.

10 mA	20 mA	40 mA	80 mA

160 mA	320 mA	640 mA

Figure A. Cross sectional images of an anthropomorphic phantom. 除了x射线管电流从10毫安(左上)增加到640毫安(右下)的变化外，所有图像都是使用相同的技术获得和重建的。.

图A显示了在10 mA至640 mA的管电流范围内获得的一系列图像的示例, which is an increase of a factor of 64. 图B显示了从选定图像中名义上均匀区域的“放大”, and which clearly demonstrate how increasing the mA (i.e.(用于获取图像的光子)减少了生成的CT图像中的噪声(斑点). CT imaging is generally a quantum noise limited imaging system, where the only significant source of noise is quantum mottle, and other sources such as electronic noise are deemed to be negligible. For a quantum noise limited imaging system, 噪声与用于成像的光子数的平方根成反比. In other words, if the number of photons used is quadrupled, the noise in the resultant image should be halved.

10 mA	40 mA

160 mA	640 mA

Figure B. 从相同的幻影位置获取的小区域，但使用不同的mA值获得.

图C定量地说明了随着mA的增加图像噪声的减少, 并说明所选mA的四倍化大约使所测噪声值减半. 图C还显示，Hounsfield Unit值与mA值无关. Since the HU value is the amount of material contrast relative to water, this example also illustrates that changing mA does NOT affect contrast, but only noise. Image quality, and the ability to detect a lesion, is generally related to the contrast to noise ratio. 增加mA将通过降低噪声来改善图像质量，但不会影响对比度.

10 mA	20 mA	40 mA	80 mA

160 mA	320 mA	640 mA

Figure C. Analysis of a small region of interest showing that the mean attenuation (i.e., Hounsfield Unit) is independent of the mA used to acquire the image, but that the noise (standard deviation or Std. Dev.) in the region of interest is reduced as the mA increases.

X-Ray Tube Voltage (kV)

x射线管电压是所有基于x射线成像模式的重要参数, including CT. When the x-ray tube voltage increases, the number of x-rays produced increase. The increase is greater than the linear relationship that is observed when the mA is changed; doubling of the x-ray tube voltage would likely increase the total number of x-ray photons produced by about a factor of four i.e., x-ray tube output is taken to be approximately proportional to kV2.

Increasing the x-ray tube voltage will also increase the average photon energy. As the photon energy increases, x-ray attenuation is reduced. CT图像中的衰减系数用Hounsfield单位(HU)表示。, 哪个可以量化任何特定组织相对于水的衰减量. 一般情况下，x射线光子能量的变化也会引起组织HU值的变化. If a tissue (lesion) were surrounded by water, 组织HU量化病变相对于周围水的x射线衰减差异(i.e., contrast). 因此，kV的变化也会影响所得图像的对比度.

图D显示了使用四个x射线管电压从80到140千伏获得的四幅图像, but otherwise having identical techniques and acquisition geometry. The four x-ray tube voltages depicted in Figure D (i.e.(80,100,120和140kv)是临床CT成像中最常遇到的. In each image depicted in Figure D, 有一个感兴趣的区域(~20 cm2)，描述恒定组织等效材料中的平均HU值, 以及相应的被测HU值在均值附近的标准差. 将x射线管电压从80千伏增加到140千伏会改变两者的平均值, as well as the corresponding standard deviation:

80 kVp	100 kVp

120 kVp	140 kVp

Figure D.

Mean HU value: 80kv时平均HU值为115 HU, 140kv时平均HU值为141 HU. The increase in HU at 140 kV relative to that of 80 kV (i.e., 26 HU) means that relative to water, this tissue equivalent material is 2.x射线管电压越高，衰减越大6% [1 HU]，对应x射线衰减的变化, relative to that of water, of 0.1%]. 任何组织的HU的行为取决于三个因素:(a)光子能量, 增加光子能量通常会增加康普顿散射相互作用的可能性, and reduces the likelihood of photoelectric interactions; (b) tissue density, 在高光子能量下，组织类材料(i.e., those with atomic number similar to water [Z = 7.5]; (c) atomic number (Z), atomic numbers, where the higher the atomic number, the more likely the photon interactions are likely to be photoelectric. The variation of tissue HU with photon energy may be complex; for example, 在光电过程更为重要的低光子能量下，低Z材料可能比水的衰减更小, but more attenuating at high energies if the material density is high, 因为在高能量下，康普顿过程更为重要，而且这些过程与物理密度近似成正比. In general, tissue like materials (i.e., Z ~ 7.5) will show only modest variation in HU with photon energy in CT, of the order of a couple of percent or so, as depicted in Figure D. High Z materials, however, 如碘(Z = 53)和钡(Z = 56)的HU随光子能量(kV)的增加而迅速下降，因为这些材料的x射线相互作用主要由光电效应主导, and the PD effect is inversely proportional to E3 (i.e.例如，光子能量增加一倍将使光电效应减少约8倍。.

Standard deviation in HU:从图D的数据可以看出，提高x射线管电压可以降低图像噪声/斑点, as reflected by the observed reduction in the measured standard deviation. 观察到噪音/斑驳随千伏增加而减少的原因有两个:(a)随着千伏增加, (many) more photons are produced in the x-ray tube, and the number measured by the x-ray detectors will therefore be increased; (b) increasing the kV also increases the average photon energy, which increases the x-ray penetration, 所以入射到病人身上的x射线的透射率也增加了. For both of these reasons (i.e., more photons, and increased penetration), 增加x射线管电压将减少在生成的图像中观察到的斑驳的数量，如图D中的数据所示. 有趣的是，将x射线管电压从80千伏增加到140千伏，噪声从8千伏降低.9 HU to 4.3 HU; to achieve this twofold reduction in noise would have required a quadrupling of the mA, or scan time, at a constant x-ray tube voltage.