Conventional Fluoroscopy

Objectives

• discuss development of fluoroscopy

• differentiate between fluoroscopic and radiographic imaging

• Describe the components of an image intensifier

• Explain the operation of an image intensifier

• Explain the operation of an image intensifier in magnification mode and its effect on image quality and patient exposure

Contect Spects

• exposure factors

• fluoroscopy time

• Automatic Brightness Control

• Receptor Positioning

• Magnification Mode

• Air Kerma Display

• Dose or Time Documentation

• Minimum Source-to-Skin Distance

History of Fluoroscopy

• invented bu Thomas C. Edison

• 1st fluoroscope available commercially in 1896

• Produced a faint image

  • Dark Adapt: operator would sit in a darkened room OR wear adaption googles with red lenses

• Dynamic radiographic examination

• views while standing in the path of the x-ray beam as it exits the patient

  • high dose to the operator

• 1950’s: Image intensifier

• Electronic vacuum tube

• brightened image

  • eliminated the need to dark-adapt

  • improved visible anatomic details

• indirect viewing

• low mA (.5- 5 mA)

• Round or Oval FOV

• Spatial resolution: 1-2 Lp/mm

Converts:

• remnant beam → light

• light → electrons

• electrons → light

Input Phosphor

• absorbs x-ray photons and emits light photons

• Cesium Iodide (CsI)

  • scintillating phosphor

  • emits light

• Closest to patient

• larger phosphor

  • 15 - 30 cm

Photocathode ( photo → light, cathode → source of electrons)

• emits electrons in response to light stimulus

  • photoemission

  • need many light photons to create1 electron

• Cesium and antimony

• Curved surface to ensure that all electrons emitted travel same distance to output phosphor

  • makes sure all electrons make it towards cathode

Electrostatic Focusing Lenses

• Negative charged plates

• repels electrons and focuses them toward the output phosphor

• electrostatic repulsion

Focal Point

• where electrons merge into a point

Accelerating Anode

• constant positive charge of 25 kV

• Attracts electrons toward the output phosphor

Output Phosphor

• absorbs electrons and emits light

  • 1 electron emits many light photons

• Silver-activated zinc cadmium sulfide

• increase light brightness compared to brightness at input phosphor

  • brightness gain

• smaller phosphor

  • 2,5 cm

Image Intensification

1. Input Phosphor absorbs the remnant beam and emits light in response

2. emitted light exposes the PHOTOCATHODE which emits electrons

Brightness Gain

• ability of the image intensifier to convert x-ray to light and increase the brightness of the image

• Brightness Gain = Flux Gain x Minification Gain

• Higher Brightness Gain = Greater Efficiency of the Image Intensifier

• Image Intensifiers have. brightness Gains of 50000 - 200000

• As the image intensifier ages it loses its efficient to convert light

  • more radiation is needed to produce the same level of brightness

Flux Gain

• conversion of efficiency of the I.I.

• Light photons ‘in’ vs light photons ‘out’

  • Flux gain = # of output light photons / # of input of x-ray photons

Minification Gain

• Expresses the degree an image is made smaller from input to output phosphor

• diameter of inout phosphor vs diameter of output phosphor

• brighter image output

• electrons concentrated into smaller surface area

• minification gain = ( d1/d2)²

Automatic Brightness Control

• ABC

• maintains the overall appearance of intensified image

  • contrast and brightness

• automatically adjusts the kVp and mA

• operator can select a desired brightness level at the control panel

• can be slow to respond to change sin patient tissue thickness and density as it moves

  • brightness lag

Magnification Mode

• “tri-focus” image intensifier

• adjustable inout phosphor size

• various sizes

  • most common: 25/17/12 cm

• increases patient dose

Camera Tube

• attached to image intensifier

• Converts light signal from output phosphor to an electronic signal

• Electron Gun: heated filament that supplies a constant electron current

• Electrostatic Grids: Accelerates and focuses the electron bea,

  • control grid

• Outside of tube:

  • electromagnet coils:

    • controls size and position of electron beam

• Target assembly: 3 layers

  • window: outside layer

  • signal plate: conducts video signal

  • target plate: photoconductive layer; conducts electrons when illuminated

Spot Film

• ‘Static’ imaging - one time frame

• receptor located between patient and image intensifier

• receptor i sparked out of beam path

  • stored in a lead lined shroud until needed

• until will shift from fluoroscopy mode to radiographic mode

Milliampaerage

• Fluoroscopic mode:

  • low mA

  • 2-5 mA

• Radiographic Mode:

  • High mA

  • 100-1200 mA

Photospot Camera

• allows radiologist to view image while it is being recorded

Photospot v Spot film

• photospot: image intensifier

• Spot film: image receptor

• Photospot: fluor FOV

• Spot Film: Image Receptor FOV

Cineradiography

• Dynamic recording

PinCushion Effect

• curved edges of image

• distortion caused by unequal magnification

Mottle

• insufficient information present to create image

• increase mA to compensate

Air Kerma

• kinetic energy released per unit mass in air

• should not normally exceed 5 cGy/min at the position wher ethe beam enters the patient

  • must not exceed 10 cGy/min

• air kerma from leakage radiation. measured at 1 m from the source, does not exceed 0.1 rad in any 1 hour

Dose Display

• updated every 6 sec

• irradiation time in minutes and tenths of minutes shall be continuously displayed

• flouro and radiation dose

Personnel Protection

• only persons whose presence is necessary shall be in the fluoroscopy room during exposures

Source-to Skin Distance

• SSD

• must not be less than 30 cm

  • should not be less than 38 cm (15 in.)

Bucky Slot Cover

• protective device

• automatically covers the bucky opening durng exposure

• at least 0.25 mm Pb equivalent

High Level Control

• requires positive action to start and maintain the high exposure rate

• continuous audible signal shall accompany the high rate