The image intensifier is comprised of a large cylindrical, tapered tube with several internal structures in which an incident x-ray distribution is converted into a corresponding light image of non-limiting brightness. A picture of an image intensifier television (II-TV) system is shown below. X-ray to light amplification is achieved in several sequential steps. 第一个, x-rays incident on and absorbed by a cesium iodide (CsI) structured phosphor produce a large number of light photons resulting from the energy difference of x-rays (30-50 keV average) to light photons (1 -3 eV average). 吸收和转化效率分别为60%和10%. 一小部分光子与相邻的层在输入荧光粉背面的光电阴极相互作用, 释放成比例数量的电子(通常为5个光子/电子). 带负电荷, 电子通过大约25的电位差被加速,向位于真空管锥形侧的正阳极方向注入1000伏电压. Electro-magnetic focusing grids maintain focus and at the same time minify the electron distribution as it interacts at the output phosphor structure, producing a large increase in the light intensity compared to the amount of light originally produced at the input phosphor. Overall brightness gain of the II is achieved through the acceleration and kinetic energy increase of the electrons impacting on the output phosphor (known as electronic or flux gain) as well as the geometric area reduction of the electron density from the large area input phosphor to the small area output phosphor (known as minification gain, equal to the ratio of the input to output phosphor areas, or ratio of the square of the diameters). 电子增益和缩小增益的结合使亮度提高了5000X. Variable brightness gain occurs with a change in the input phosphor active area; as the field of view (FOV) is reduced, the minification gain is reduced, decreasing the overall brightness gain (and vice-versa). Optical coupling of the output phosphor to a 电视摄像机 or photospot, 电影, 或其它光检测器允许检测图像并进行后续显示.

II“速度”的控制是通过在共轭透镜系统中包含一个限光孔径来实现的. For situations requiring high dose (i.e.，数字减影血管造影)，孔径减小(大 f 号), while for low-dose fluoroscopic applications (i.e. upper GI fluoro), the aperture diameter is increased (small f 号).

图E illustrate a typical overtable II/TV system, 住房, carriage (allows vertical and horizontal positioning, 和工作台(x射线管安装在工作台下面，相对于II探测器有固定的几何形状). 图F shows an internal cross-section of the II, and important structures including the image intensifier envelope, 输入荧光粉由结构碘化铯(CsI)闪烁体材料组成, the photocathode comprised of a light-sensitive, electron emitting material (Sb2Cs3), the electron focusing electrodes, 阳极结构, the output phosphor comprised of zinc cadmium sulfide (ZnCdS:Ag), 具有限光孔和部分镀银镜的串联共轭透镜(部分反射光), and light photon detectors (film, 电视摄像机, CCD相机)捕获输出图像并转换成有用的图像供观看.

图E. 图像增强-电视系统的组成部分显示为传统的透视室.

图F. Cross section of the II-TV system illustrates the various components that are used to create the highly amplified output light image. A four stage process (x-rays to light; light to electrons; electrons to light; light to electronic signal) is shown.

图像增强器尺寸

图像增强器有一系列的输入视场(FOV)直径，用于诊断成像应用, from 6 inches (15 cm FOV) to 16 inches (40 cm FOV), and many dimensions inbetween, depending on the type of imaging procedure. II's have a spherical input phosphor structure, with a curvature designed to withstand the large force on the II enclosure, resulting from the internal vacuum required for operation. The output phosphor dimension is typically about 1 inch (2.54厘米)直径. 输入和输出之间的大小差异导致输出图像的缩小, whereby the electrons emerging from the photocathode of the input are focused and minified during acceleration through the evacuated tube. Brightness gain achieved by minification is equal to the area of the input phosphor to the area of the output phosphor, 由于在输出荧光粉处增加了电子密度和相应增加了光强. 在图G中, an illustration of two FOV examples demonstrate the electronic "magnification factor" that is available on most image intensifier systems.

图G. 图像增强器可以交互式地改变输入视场从大到小的区域. Effects on the output image are described in the figure inset information. 除了, an aperture collimator within the x-ray tube collimator assembly must limit the x-ray beam to the active area of the II.

Image intensifiers can electronically vary the size of the input radiation field of view whilst keeping the output field fixed, 等于2.54 cm (1 inch), corresponding to the size of most television cameras. If the input field of view is halved, 然后，被观察的病人的大小也减半，结果是图像放大了两倍. 这种类型的放大，被称为电子变焦，双倍的空间分辨率性能. 例如, 25厘米视场和传统电视摄像机(525线)的极限空间分辨率为~ 0.7 line pairs per mm; reducing the input field of view to half this value (i.e., 12.通过使用“电子变焦”将极限空间分辨率性能提高到~1.每毫米4线对.

图H. Image intensifer TV system with 4 FOV diameters: 37 cm, 30 cm, 22 cm, 17 cm, and corresponding intrinsic resolution capabilities (bar pattern is taped next to input phosphor of the image intensifier). 顶部行显示全视场图像，底部行显示条形模式放大视图.

If the input field of view is halved, then only one quarter of the input phosphor is being irradiated since the area is proportional to the square of the field of view. Halving the input field of view, while keeping all the other parameters constant, 在整个视野下，图像亮度会降低到原来亮度的四分之一吗. To compensate for this effect, 在图像增强器输入处入射的辐射量需要增加四倍 to compensate for reduction in exposed area. Automatic brightness control feedback circuits in the image intensifier / x-ray generator system accomplishes this with feedback signals to adjust the kVp (kV modulated with mA fixed), mA (mA modulated with kV fixed), 或两者(kV和mA都被调制)以保持输出荧光粉的亮度. 当使用“放大模式”时，对患者的后果是剂量增加, but also spatial resolution enhancement.

Pelvis Phantom (透视)

图我	图J	图K

图我 shows a single frame from a fluoroscopy run using an image intensifier diameter of 38 cm; 图J shows an magnified image (electronic zoom) with an image intensifier diameter of 25 cm and 图K shows further magnification achieved by reducing the image intensifier input diameter to 15 cm. Relative to the image in 图我, the magnification in 图J is x 1.5 (i.e. 38/25), and the magnification in 图K is x 2.5 (i.e., 38/15).

The choice of technique in 图我 was 75 kV and 2.4 mA, which resulted in an entrance skin air kerma of 35 mGy/minute. When the image was magnified by a factor of 1.5(图J)，系统将x射线管电压提高到85 kV，使用管电流为2.7 mA, which increased the entrance air kerma rate to 50 mGy/minute. When the image was magnified by a factor of 2.5(图K), 该系统进一步将x射线管电压提高到94千伏，并使用2千伏的管电流.8 mA, which increased the input air kerma to 61 mGy/minute.

Use of magnification modes in fluoroscopy is usually associated with an increase in the choice of x-ray tube voltage for two reasons:

(a) higher voltages will reduce the entrance skin air kerma which needs to be kept below 90 mGy/minute (10 R/min) for regulatory purposes. Adjusting the x-ray tube voltage with increasing magnification resulted in only relatively modest increases in the entrance air kerma rate (35 mGy/minute -> 50 mGy/minute -> 61 mGy/minute).

(b) the tube current needs to be kept below ~5 mA to minimize the power input into the x-ray tube anode permit continuous fluoroscopy operation without overheating the x-ray tube. The increased in power input to the anode (power is kV x mA watt) was also relatively modest (190 watt -> 230 W -> 260 W).

相比之下，保持x射线管电压恒定，放大倍率增加2倍.5将需要增加625%的入口皮肤空气kerma(和x射线管阳极的功率负荷)(i.e., 2.5^2)，因为输入荧光粉的暴露面积减少了六倍.

Pelvis Phantom (数字聚光灯 )

图1	图米	图N

图L显示了与图I (I)所示的透视框相对应的数字光斑图像.e., 38 cm diameter field of view). The choice of techniques used to acquire this image was 65 kV/9 mAs, and the corresponding entrance air kerma rate was 1.4 mGy. Increasing the magnification by a factor of 1.5 by reducing the field of view to 25 cm (图米) resulted in a digital photospot technique of 65 kV/18 mAs and an entrance air kerma of 2.8 mGy. A further increase in image magnification to x 2.5通过使用15厘米视场(图N)产生65千伏/33毫安的数字光斑技术, and an entrance air kerma of 4.8 mGy.

In digital photospot imaging, reducing the field of view does not normally require an increase in the x-ray tube voltage for the following reasons:

(a) Increased x-ray tube voltage will reduce the amount of image contrast and the corresponding contrast to noise ratio. 保持对比度噪声比是可取的，因为这提高了病变的可检测性.

(b)由于几个原因，对病人剂量的考虑比透视少得多. Diagnostic image quality is normally the paramount concern, 人们也不希望使用过少的辐射来损害诊断性能. 此外, there are no dose limits in radiography, 哪一种用于诊断, whereas there are dose limits in fluoroscopy (entrance air kerma must normally be < 90 mGy/minute). Even though the patient dose per frame is high compared to fluoroscopy, the total number of photospot images acquired is very low.

(c) There are no x-ray tube heating problems in digital photospot imaging. 沉积到阳极的总能量是功率(kV x mA)和总暴露时间(s) (i)的乘积.e.，能量(J) = kV x mAs). The total energy deposited in the three examples shown above ranges from 0.6 kJ (65 kV and 9 mAs) to 2.2kj (65kv, 33ma). The anode tube capacities of x-ray tubes in typical fluoroscopy/radiography imaging systems is typically hundreds of kJ and x-ray tube heating is generally not an important issue.

对比了x射线管电压在放大透视中的变化, 在数码光斑成像的放大模式下，x射线管电压通常保持近似恒定. 结果是, the entrance skin air kerma and energy deposition into the x-ray tube anode are approximately inversely proportional to the exposed area of the input phosphor of the image intensifier. 视野减半, which doubles the magnification and corresponding spatial resolution, 在数字光斑成像中，是否期望(大约)使入口空气的孔径增加四倍.

颅骨幻象(透视)

图阿	图P

图O显示了使用25厘米视场获得的头部幻影的透视运行的一帧. The radiographic techniques were 74 kV/2.2 mA, and the entrance air kerma rate was 26 mGy/minute. Reducing the field of view (图P) from 25 cm to 16 cm (i.e., increasing magnification by a factor of 25/16 or 1.6) resulted in the use of 92 kV/2.8 mA and an entrance air kerma rate of 49 mGy/minute.

在这个例子中, note that the 图阿 (25 cm field of view) includes a small region around the skull that consists of the image intensifier being directly irradiated by the x-ray beam. This is generally undesirable (see collimation section below), and will result the selection of technique factors that take into account the relatively large detected signal from these directly irradiated regions. Predicting changes in the selected radiographic techniques (kV/mA), 在这种情况下，病人的剂量和x射线管的负荷特别棘手.