Fluoroscopy provides real-time, interactive x-ray projection imaging. Fluoroscopic procedures are normally performed by using an image intensifier to detect the x-ray pattern emerging from the patient after removal of scattered radiation by the anti-scatter grid. The x-rays are captured by a CsI scintillator and are converted to light photons that are subsequently directed to the photocathode layered on the back side of the scintillator. A proportional number of electrons are generated and accelerated under the influence of a large voltage (~25,000 V) between the photocathode and the anode structure on the other side of the evacuated tube. Electromagnetic focusing coils maintain the focus of the electron trajectories from the input phosphor/photocathode assembly and reduce the large area electron distribution to the area of the output phosphor. The electrons impact on the output phosphor, and by virtue of the acceleration gain of the electrons and the geometric reduction of the electron distribution, the resultant light image is amplified by a factor of the order of 5000 or so in the image intensifier (i.e., flux [acceleration] gain of ~50 and minification gain of ~100). The light pattern can be detected by a television camera and displayed on a monitor In fluoroscopy, tube currents are normally low (a few mA), and typically a hundred times lower than those used for spot/photospot imaging (see below). In addition, 30 frames are normally acquired and displayed every second, and total fluoroscopy imaging times can be measured in minutes (or hours).
As a result, the dose per single frame is low and the image quality is very low because so few photons are used to make the image(i.e., high mottle limits visibility of low contrast lesions). Consequently, a single fluoroscopy frame (e.g., last image hold image) is therefore generally not considered to be of diagnostic quality. However, because so many images are acquired (1800 every minute of fluoroscopy time), the total radiation dose to the patient can be substantial, and much higher than doses associated with simple radiographic examinations (e.g., most chest x-ray examinations usually consists of only two images). Radiation doses in fluoroscopy are therefore substantially higher than in conventional radiography.
Instead of using continuous x-ray tube current, some systems create a short pulse of x-rays at the beginning of each frame, delivering the same dose per frame. For instance, if 3 mA is continuously on for 30 frames/second (frame/s) imaging, the effective mAs is 3 mA/30 frame/s = 0.1 mAs /frame. By increasing the tube current by a factor of 10 to 30 mA and decreasing the exposure time by a factor of 10 (1 frame is 0.033 s) to 0.0033 s/frame will result in the same effective mAs: 30 mA × 0.0033 s = 0.1 mAs / frame. An even higher value, such as 50 mA with 0.002 s exposure time delivers 0.1 mAs/frame. A higher tube current thus allows a shorter exposure time, which can assist in reducing intra-frame motion. In examinations where temporal resolution is not crucial, using digital frame buffers in conjunction with lower frame rates, such as 15 frame/s, 7.5 frame/s, and 3.75 frame/s (1/2, 1/4, 1/8 of 30 frame/s) can result in significant dose savings, where reduced temporal sampling is sacrificed for lower patient dose. Often, systems are configured to deliver more dose per frame with reduced frame rates to improve image quality through the capture of more x-rays/frame, but can still result in a lower overall dose. For instance, a 7.5 frame/s acquisition with two times the mAs / frame but one-fourth the number of frame/s, results in an overall reduction of dose by a factor of one-half for the same overall fluoroscopy time.
An x-ray tube + grid + image intensifier combination may be used to perform (dynamic) fluoroscopy, or to generate (static) diagnostic quality images. With regards to the latter category, there are three types of (static) diagnostic quality images – spot films, photospot films, and digital photospot films. A conventional radiograph, which does not make use of the image intensifier, is known as a spot film, and is acquired with the screen film cassette positioned in front of the image intensifier. A conventional photograph of the output of the image intensifier is known as a photospot and usually have a 100 mm single frame film or 105 mm roll film format. A partially-silvered mirror directs light in the optical coupling box to an objective lens that focuses the light image onto the film, which is processed using standard developer/fixer chemistry. A digital photspot image is acquired with a high-resolution TV camera (e.g., 1000 x 1000 or 2000 x 2000 matrix) producing an analog video signal that is subsequently digitized. It is important to note that spot films, photospot and digital photospot use high x-ray tube currents (hundreds of mA) as well as short exposure times to generate images that used for diagnostic purposes. Quantum mottle is much lower in a single fram spot/photosphot/digital photospot image compared to a single frame fluoroscopy sequence.