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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The fluoroscopic imaging chain
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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 (G1,
  G2, and G3 in the figure)
• An output phosphor that converts the accelerated
  electrons to visible light

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Internal structure of an image intensifier
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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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Input section of an image intensifier
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Scanning electron micrograph of a CsI input
phosphor
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Pincushion distortion arising because the input
surface of the II is curved and the output
surface is flat
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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 window of an image intensifier
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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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In normal operation, electrons emitted by the
photocathode over the entire surface of the input
window are focused onto the output phosphor.
In magnification mode, the voltages applied to
the electrodes are changed, such that only the
electrons released from the smaller diameter FOV
are properly focused onto the output phosphor.
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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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Comparison of QDE of an image intensifier with
that of a thin-film transistor digital detector
array
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The optical distributor
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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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The concept of frame averaging for noise reduction
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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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Two possible automatic brightness control curves
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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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Resolution of the components in the fluoroscopic
imaging chain
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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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Radiographic/fluoroscopic system

• Table may be tilted to different angles to
  facilitate the movement of contrast agents
  (typically barium-based) upward or downward in
  the patient.

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Angiography system

• Table does not tilt
• Table top floats left to right and top to bottom
  to allow easy adjustment of patients anatomy
  with respect to imaging chain
• Systems with two complete imaging chains permit
  simultaneous recording of anteroposterior and
  lateral views of a single contrast injection

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Original anterior and lateral fluroscopic images
Angiographic sequence showing movement of
contrast agent
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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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Entrance skin exposure as a function of kVp for
different thickness
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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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Scatter field incident upon the radiologist while
performing a fluoroscopic procedure
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Shoe store fluoroscopy

• A sales gimmick which showed room for the toes
• Over 10,000 units in the 40s and 50s
• Poorly regulated
• 7 to 14 R per dose (some as high as 116 R)
• Outlawed in the late 50s