Fluoroscopy

Real-time imaging Most general-purpose fluoroscopy systems use TV technology, operating at 30 frames/sec May be recorded (barium swallow examinations) or unrecorded (catheter positioning) Cinecardiography may operate at 120 fps using 35mm film Higher sensitivity than screen-film systems –1 to 5  R per frame versus 600  R for a 400-speed screen-film system to give OD = 1.0

Image intensifier Four principal components: –A vacuum bottle to keep air out –An input layer to convert the x-ray signal to electrons –Electronic lenses that focus the electrons –An output phosphor that converts the accelerated electrons to visible light

Input screen Input screen consists of four layers: –The vacuum window (thin Al window that is part of the vacuum bottle) –A support layer (also thin Al), curved for accurate electron focusing –The input phosphor (CsI in thin, needle-like crystals) –The photocathode (a thin layer of antimony and alkali metals, such as Sb2S3) that emits electrons when struck by visible light

Output phosphor Made from zinc cadmium sulfide doped with silver (ZnCdS:Ag), which emits green light Small phosphor particles (1 to 2  m) in a thin coating (4 to 8  m) to preserve high spatial resolution Anode is a very thin (~ 0.2  m) coating of aluminum on the vacuum side of the phosphor

Output phosphor (cont.) Much smaller image at the output phosphor than at the input phosphor (23- to 35-cm diameter input imaged focused onto a 2.5- cm diameter circle) Must deliver resolution >70 line pairs/mm to preserve a resolution of 5 line pairs/mm at the input plane

Quantum detection efficiency X-rays must pass through the vacuum window and the input screen substrate before reaching the phosphor This reduces the QDE of an image intensifier Maximal around 60 kVp –Dose to patient decreases at higher kVps, so optimal kVp for a given examination will generally be higher than 60 kVp

Modes of operation Continuous fluoroscopy –Basic form of fluoroscopy; continuously on x-ray beam High dose rate fluoroscopy –Specially activated mode allowing exposure rates of up to 20 R/min to the patient in the US Variable frame rate pulsed fluoroscopy –30, 15, and 7.5 frames/sec operation allows lower temporal resolution for parts of procedure Frame averaging

Fluoroscopy images generally noisy Sometimes beneficial to compromise temporal resolution for lower noise images Digitize fluoroscopic images and perform real-time averaging in computer memory for display

Automatic brightness control Purpose of ABC is to keep brightness of the image constant at the monitor Accomplished by regulating the x-ray exposure rate incident on the input phosphor of the II –As II pans from a thin to a thick region of the patient, thicker region attenuates more of the x-rays Video signal itself can be used to sense light output ABC can adjust both tube current and generator voltage

Image quality Spatial resolution of the II best described by modulation transfer function (MTF) The limiting resolution of an imaging system is where the MTF approaches zero Higher magnification modes (smaller fields of view) are capable of better resolution Video imaging system degrades the MTF substantially

Image quality (cont.) Contrast resolution of fluoroscopy is low compared with radiography because low exposure levels produce images with relatively low signal-to-noise ratio (SNR) Excellent temporal resolution of fluoroscopy is its strength and its reason for existence

Fluoroscopy suites Smaller facilities may use one fluoroscopic system for a wide variety of procedures Larger facilities have several suites dedicated to specific applications, such as: –Gastrointestinal suites –Remote fluoroscopy rooms –Peripheral angiography suites –Cardiology catheterization suites –Biplane angiography systems –Mobile fluoroscopy – C arms

Radiation dose Maximum entrance exposure rate for fluoroscopy to the patient is 10 R/min (see sc.gc.ca/hecs-sesc/ccrpb/publication/safety_code20a/toc.htm ) sc.gc.ca/hecs-sesc/ccrpb/publication/safety_code20a/toc.htm Low-dose techniques include heavy x-ray beam filtration, use of low frame rate pulsed fluoroscopy, and use of lower-dose ABC options Last-frame-hold features often reduce fluoroscopy time Using the largest field of view suitable to a given clinical study also helps reduce radiation dose to the patient

Dose to personnel Occupational exposure of physicians, nurses, technologists, and other personnel who routinely work in fluoroscopic suites can be high Lead aprons should be worn when the x-ray beam is on Portable lead glass shields should be available for additional protection to staff members observing or otherwise participating in the procedure Reducing total fluoroscopy time is beneficial to everyone