透视提供了解剖结构的动态视图, 并保持病人的总剂量低, the image intensifier is operated at a very high gain at low air kerma entrance rates in order to achieve the brightness necessary for the TV camera to produce a reasonably bright image on the in-room TV monitor. A real-time sequence of images at 30 frames/second presented to the viewer is perceived with much less mottle than a static last frame hold image because of the internal lag characteristics of the human eye-brain response, 大约是200毫秒. The consequence is that a noisy image sequence viewed in real-time will appear as an average of approximately 3-5 frames of image information content, thus presenting with less quantum mottle than the last frame hold image. Digital photospot images are acquired through the image intensifier / TV system using a much higher entrance air kerma (and of course much higher patient dose) but much better image quality in terms of resolution (typically a higher resolution TV camera is used in conjunction with a larger digital image matrix with correspondingly smaller pixel dimension) and lower quantum mottle are achieved, 如下所示. Exposure differences are on the order of 50 - 100 times greater for the digital photospot compared to the single frame fluoroscopy image. 这一点必须考虑到, 然而, in the context of a much larger number of images acquired during fluoroscopy compared to a relatively few digital photospot images acquired for the purposes of documentation of the anatomical findings with much better detail and image quality.

骨盆的幻影

图一个. 一个 frame 骨盆幻象的透视图像	图B.骨盆幻像的数字光斑图像.

图一个 is a single frame of a fluoroscopy run of a pelvic phantom with a nominal image intensifier diameter of 38 cm. 用于获得该透视运行的技术为75kv和2kv.4 mA，这是典型的透视. 在幻体入口处的辐射空气速率(i.e.，入口皮肤空气热率)为35mgy /min (i.e.， ~ 4r /分钟). 透视是通过每秒生成30帧来完成的, so that in one minute there will be a total of 60 seconds x 30 frames/second image, 或者1800张图片. 图一个 shows just one of these 1800 images generated every minute and the patient entrance dose is thus ~(35 mGy entrance air kerma)/(1800 acquired images), or ~0.每帧019mGy的入口空气密度.

图B shows an example of a digital photospot image (radiograph) that is obtained by increasing the x-ray tube current to a high value, about hundred or so times higher than the low tube used during fluoroscopy (i.e.. 2.4 mA). 在这个例子中, x射线管电压降至65kv, and the digital photospot image was acquired using an x-ray beam intensity of 9 mAs. 与图B相关的皮肤入口气孔为~1.4mgy，或74 (i)的因数.e., 1.4/0.019)高于图A所示的单透视框架.

The number of photons used to generate a radiographic image determines the amount of mottle (AKA noise) in the image. 图一个 makes use of very few photons and has a much higher level of mottle than figure B. 透视图像的质量通常很低, and are used to identify the location of a catheter rather than for diagnostic interpretation. Digital photospot images are acquired using radiation intensities that use ~100 or so times more photons, 并且被认为是诊断性的.

头幻影

图C. 一帧颅骨幻影透视图(AP投影)	图D. 一个 digital photospot image of the same skull phantom shown in 图C.

图C shows one single frame of a fluoroscopy run of a skull phantom, 标称图像增强器直径为25厘米. The techniques used (selected by the II system) to acquire this image were a x-ray tube voltage of 74 kV and tube current of 2.2 mA. The choice of x-ray techniques for the skull phantom are slightly lower than those used to image the pelvis phantom (图一个) because this projection of the skull phantom is less attenuating the pelvis AP projection. The radiation air kerma at the entrance of the phantom was 26 mGy/minute (~2.9 R/minute), so the single frame shown in 图C required an entrance skin air kerma of 0.014年mGy.

图D shows the corresponding digital photospot image (radiograph) taken at the completion of the fluoroscopy run. 生成图D所示的x光片, the imaging chain used an x-ray tube voltage of 68 kV and a total tube current exposure time product of 6 mAs (high current/short exposure time). The entrance skin air kerma required to generate the image in 图D was 0.94毫戈瑞，或高出67倍(i.e., 0.94/0.014)比图C所示的单透视镜框更清晰. 正如预期的那样，图像质量(i.e., mottle) of the image in 图D is low, and the image is acceptable for diagnostic purposes. 相比之下, 图C图像中的斑点非常高, and the single frame shown in 图C would not be deemed to be of diagnostic quality for most clinical applications.

In comparing a single frame from a fluoroscopy run (图一个 and 图C) with a digital photospot (图B and 图D), it is important to note that the imaging chain is identical except for the amount of radiation used to acquire the image. The amount of mottle (noise) in an image is inversely proportional to the square root of the number of photons used to acquire the image. 换句话说, a single frames from a fluoroscopy series that uses ~100 times less radiation than a digital photospot image will have approximately ten times more mottle. This increased mottle will limit the ability to detect low contrast lesions; a fluoroscopy frame is expected to have about a tenfold reduction in lesion detection (contrast or lesion effective thickness) than the corresponding digital photospot image.