Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

higher. Cine radiography uses very short radiographic pulses, and therefore specialgenerators are needed. The opening of the cine camera shutter is synchronized withthe x-ray pulses. The x-ray tube loading is usually quite high, so high heat capacityx-ray tubes are used in the cardiac catheterization laboratory. An entrance exposureto the input phosphor of approximately 10 to 15 JlRlframe (23-cm-diameter II) isused for cine-radiography studies. Digital cine cameras are typically CCD-basedcameras that produce a rapid sequence of digital images instead of film sequence.Digital cine is increasingly used in the cardiology community.With the computerization of x-ray generators and imaging systems, a great deal offlexibility has been added to fluoroscopy systems compared to what was available inthe 1970s. Some systems come with enhanced features, and some of these alternatemodes of operation are described below.Continuous fluoroscopy is the basic form of fluoroscopy. It uses a continuously onx-ray beam using typically between 0.5 and 4 mA (depending on patient thickness).A video camera displays the image at 30 frames/sec, so that each fluoroscopic framerequires 33 milliseconds (1/30 second). Any motion that occurs within the 33-msecacquisition acts to blur the fluoroscopic image, but this is acceptable for most examinations.In the United States, the maximum entrance exposure to the patient is 10Rlmin.High Dose Rate FluoroscopySome fluoroscopy systems have high dose rate options, usually called specially activatedfluoroscopy. This mode of operation allows exposure rates of up to 20 Rlminto the patient in the U.S. The high dose rate mode is enabled by stepping on a differentpedal than the standard fluoroscopy pedal. An audible signal is required tosound when specially activated fluoroscopy is being used. Unless a practice sees anexceptionally high proportion of obese patients, the need for high dose rate fluoroscopyis questionable.In pulsed fluoroscopy, the x-ray generator produces a series of short x-ray pulses.Let's compare this with continuous fluoroscopy with 33-msec frame times, operatingcontinuously at 2 mA. The pulsed fluoroscopy system can generate 30 pulsesper second, but each pulse can be kept to ~1O msec and 6.6 mA. This would providethe same x-ray exposure rate to the patient (using 0.066 mAs per frame in thisexample), but with pulsed fluoroscopy the exposure time is shorter (10 msec insteadof33 msec), and this will reduce blurring from patient motion in the image. Therefore,in fluoroscopic procedures where object motion is high (e.g., positioningcatheters in highly pulsatile vessels), pulsed fluoroscopy offers better image qualityat the same dose rates.