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Title:

Fluoroscopy

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May be recorded (barium swallow examinations) or unrecorded (catheter positioning) ... The photocathode (a thin layer of antimony and alkali metals, such as Sb2S3) ... – PowerPoint PPT presentation

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Title: Fluoroscopy


1
Fluoroscopy
2
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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4
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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10
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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14
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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17
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

18
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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22
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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24
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

25
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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27
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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29
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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