Fluoroscopy

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Fluoroscopy
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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

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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

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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

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Output phosphor

• Made from zinc cadmium sulfide doped with silver
  (ZnCdSAg), 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

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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 gt70 line pairs/mm to
  preserve a resolution of 5 line pairs/mm at the
  input plane

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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

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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

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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

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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

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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

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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

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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

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Radiation dose

• Maximum entrance exposure rate for fluoroscopy to
  the patient is 10 R/min (see http//www.hc-sc.gc.c
  a/hecs-sesc/ccrpb/publication/safety_code20a/toc.h
  tm)
• 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

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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

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