Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Many fluoroscopists practice aggressive dose-saving measures, a commendablegoal. Variable frame rate pulsed fluoroscopy can be instrumental in this. Duringmuch of a fluoroscopic procedure, a rate of 30 frames per second is not required todo the job. For example, in a carotid angiography procedure, initially guiding thecatheter up from the femoral artery to the aortic arch does not require high temporalresolution, and perhaps 7.5 frames per second can be used-reducing theradiation dose for that portion of the study to 25% (7.5/30). Pulsed fluoroscopy atvariable frame rates (typically 30, 15, and 7.5 frames/see) allows the fluoroscopistto reduce temporal resolution when it is not needed, sparing dose in return. At lowframe rates, the fluoroscopic image would demonstrate intolerable flicker unlessmeasures were taken to compensate for this. Using a digital refresh memory, eachimage is digitized and shown continuously at 30 frames per second (or faster) onthe video monitors until it is refreshed with the next pulsed image. In this way, asometimes jerky temporal progression of images can be seen by the human observerat low frame rates, but the image does not suffer from on/off flicker.Fluoroscopy systems provide excellent temporal resolution, and it is this feature thatis the basis of their clinical utility. However, fluoroscopy images are also relativelynoisy, and under some circumstances it is beneficial to compromise temporal resolutionfor lower noise images. This can be done by averaging a series of images, asshown in Fig. 9-12. Frame averaging is performed by digitizing the fluoroscopicimages and performing real-time averaging in computer memory for display. Appreciableframe averaging can cause very noticeable image lag (reduced temporal resolution),but the noise in the image is reduced as well. The compromise between lagand image noise depends on the specific fluoroscopic application and the preferencesof the user. Aggressive use of frame averaging can allow lower dose imaging inmany circumstances. Portable fluoroscopy systems use x-ray tubes with limited outnoisier,less lagless noisy, more lag FIGURE 9-12. The concept of frameaveraging. The individual frames occurringas a function of time are weightedand averaged into a single displayimage, and typically "older" frames areweighted less than newer frames.