FLUOROSCOPY & IMAGE INTENSIFICATION Sergeo Guilbaud, RT School of Radiologic Sciences

FLUOROSCOPY ABSTRACT: Since the invention of the fluoroscope by Thomas Edison, it has been a valuable tool in medicine more specifically diagnostic imaging. Since its invention, it has been modified to reduce the dosage received by the patient and operators. This lecture will detail the construction and operation of the fluoroscope and image intensifier. This lecture will also discuss briefly radiographic examinations performed with the use of the image intensifier and the proper operation of such equipment.

OBJECTIVES: By the end of this lecture, each attendee shall be able to: 1.Describe the purpose of a fluoroscope. 2.Describe the components of a fluoroscope. 3.Describe the purpose of the image intensifier. 4.Describe the benefits of an image intensifier versus a fluoroscope. 5.Describe the components of the image intensifier. 6.Describe the advantages and drawbacks of image magnification. 7.Describe the ancillary devices that may be attached to the II tube.

OBJECTIVES: OUTLINE: 1.Fluoroscope A.Purpose B.Construction C.Image viewing 2.Image Intensifier A.Purpose B.Construction C.Image enhancement 3.TV Camera A.Purpose B.Construction C.Image projection 4.Television monitors A.Purpose B.Construction C.Image viewing/display

FLUOROSCOPE Purpose: To perform dynamic studies. Visualize anatomical structures in real time or motion. View the motion and function of anatomic organs.

CONSTRUCTION Invented by Thomas A. Edison. In 1896 Construction: Zinc Cadmium Sulfide Screens backed by leaded glass. Screens emit light in the yellow-green region of the visible light spectrum. Close to the sensitivity of the human eye.

Construction Mirror-Optic System is used to view images. This was totally dependent of the fluoroscopist’s (rods) night vision. Before viewing, the fluoroscopist’s eyes had to become accustomed to dim light. (Dark adaptation = wearing of red goggles)

EYE ANATOMY Cornea: Transparent protective covering over lens. Iris: Regulates amt. of light that comes into the eye by its ability to constrict and dilate. Retina: Area on the internal surface of the eye where rods and cones are located.

EYE ANATOMY Cones are primarily located on an area called the Fovea Centralis (DIRECTLY) posterior to lens. Rods are located on the periphery of the cornea. Blind spot : The area where the optic nerve enters the eye.

EYE ANATOMY Fovea Centralis: Area where cones are concentrated. Cones: Less sensitive to light but, are capable of responding to intense light levels. They are much more capable of detecting differences in brightness levels than rods. (Contrast Perception)

EYE ANATOMY Rods: Very sensitive to light and are used during dim light situations. The ability to perceive small objects or fine detail is much worse than that of cones. Photopic Vision: Daylight vision. Scotopic Vision: Night vision. As a consequence, cones are primarily used for daylight vision whereas rods are used for night vision.

FLUOROSCOPY Brightness of the fluoroscopic image is dependent on anatomic structure being viewed. Since the patient’s anatomy cannot be controlled, the increase of kVp and decrease in mA is preferred as it is in radiographic imaging. Thus the need for an image intensifier. Modern radiographic and fluoroscopic (R&F) equipment are equipped w/ image brightness controls: ABC= Automatic Brightness Control. ABS= Automatic Brightness Stabilization. AEC= Automatic Exposure Control. AGC= Automatic Gain Control.

IMAGE INTENSIFIER What is an Image Intensifier tube? A complex electronic imaging device that receives the remnant beam and converts it to light and increases the intensity of the light. Tube is contained in a glass envelope in a vacuum and mounted in a metallic container which provides protection for the components.

IMAGE INTENSIFIER 1.Input Phosphor: Constructed of cesium iodide. Responsible for converting the incident photon’s energy to a burst of visible light photon. (similar to intensifying screens in cassettes) Standard size varies from cm. (normally used to identify the II tubes)

IMAGE INTENSIFIER 2.Photocathode: Thin metal layer bonded directly to the input phosphor. Usually made of Cesium and Antimony compounds that respond to light stimulation. Responsible for Photoemission. (electron emission after light stimulation) The number of electrons emitted is directly proportional to the intensity of light intensity of the incident x-ray photon.

IMAGE INTENSIFIER 3.Electrostatic Focussing Lenses: A series of lenses inside the II tube to maintain proper focus of the photoelectrons emitted from the photocathode. They contain a positive charge. They are located along the length of the II tube.

IMAGE INTENSIFIER The focussing lenses assist in maintaining the kinetic energy of the photoelectrons to the output phosphor. The photoelectrons contain the image in minified form. A 25kV potential is maintained within the Image Intensifying tube during fluoroscopy to assist in the acceleration of the photoelectrons.

IMAGE INTENSIFIER 4.Output Phosphor: Usually constructed of zinc cadmium sulfide crystals. Serves to increase illumination of the images by converting photoelectrons to light photons. Upon interaction, the incident photoelectron is multiplied and converted to times as many light photons.

IMAGE INTENSIFIER Brightness Gain: The ability of the II tube to increase the illumination level of the image. =Minification gain X Flux gain Minification gain: The ratio of the square of the diameter of the input phosphor to the square of the diameter of the output phosphor. Flux gain: The ratio of number of light photons at the output phosphor to the number at the input phosphor.

Example: What is the brightness gain for a 17cm II tube having a flux gain of 120 & a 2.5 cm output phosphor? Brightness Gain = X =46 X 120 =5520

IMAGE INTENSIFIER Most image intensifiers have a brightness gain of 5,000 to 20,000. This number decreases as the tube ages. Because of the image intensifier, the information gathered may be stored in many ways. Spot film deviceCine camera TV monitorVideo recorder

MULTI-FIELD IMAGE INTENSIFIERS Image intensifiers allow great flexibility with the fluoroscopic image. Image intensifiers are manufactured in multi- fields (afford magnification of the viewed image). Most II tubes are tri-focus. (e.g... 25/17/12). The numbers refer to the dimensions of the input phosphor of the image intensifier.

IMAGE MAGNIFICATION In the larger II diameter, the entire input phosphor is used to produce the image. e.g cm As a smaller size is used, the voltage on the focussing lenses is increased thus causing the focal point to move further from the output phosphor. e.g cm The end result is a magnified image that is in direct proportion to the ratio of the diameters.

Example: A 25/17 II tube operated in the 17 cm mode will produce a magnified image 1.5 times larger than the image in the 25 cm mode. When in the magnification mode, the minification gain is reduced thereby producing a dimmer image. To compensate, the mA is automatically increased thus giving the patient a larger dose of radiation. This is so b/c more x-rays per unit area are used to make the image.

Vignetting A reduction in brightness at the periphery of the fluoroscopic image. This is due to magnification fluoroscopy. Only the central portion of the input phosphor is used, the periphery is inherently unfocussed.

TV MONITORS Because of the limitations of the mirror optic viewing system, a more practical and efficient viewing system was employed. TV monitors: 1.Afford viewing by multiple persons. 2.Monitors may be located in remote locations other than the radiographic room. 3.Image brightness and contrast can be manipulated. 4.Images may be stored on different medium for reviewing at a later time.

TV CAMERA TUBE When TV monitors are used to display the fluoroscopic image, TV Camera tubes are attached directly to the output phosphor of the image intensifier tube. The Plumbicon or Vidicon tubes are most often used.

TV CAMERA CONSTRUCTION: Cylindrical housing containing electromagnetic coils to focus the electron beam. Electrostatic grids to accelerate the electron beam. Electron gun which serves as a filament. Target assembly which receives the electron beam making it possible for viewing on the TV monitor.

TV CAMERA Target Assembly consists of: 1. Photoconductive layer made up of Antimony Trisulfite. Receives the electron beam. It is with this layer the electron beam interacts. 2. Signal Plate is a thin layer of metal or graphite. It conducts the video signal out of the tube into the external video circuit. It is thin enough to conduct light but thick enough to be an efficient electrical conductor.

TV CAMERA 3. Face Plate is the outside layer or window. It is the thin part of the glass envelope.

Coupling the TV Camera Two methods are used to couple the TV camera to the Image Intensifier. 1.Fiber optic bundle 2.Beam splitting mirror Fiber optic bundle is most common. TV camera tubes are manufactured to same diameter as the output phosphor of the image intensifier. (2.5 cm or 5 cm)

Beam splitting mirror

TV Monitor The video signal is amplified and transmitted to the TV monitor via a cable where it is transformed into a visible image. The monitor has only two controls; contrast and brightness. The video image is formed in Raster pattern.

Raster Pattern

Cinefluorography Normally used in Cardiac Catheterization or Angiography. The TV camera tube is replaced by a movie camera that records the images on film.

Spot Film Device Used to make permanent images during the radiographic examination. Film is positioned b/w the patient and the image intensifier. When the film is needed, the radiologist actuates the control that brings the cassette in position. This changes the tube from fluoroscopic mA to radiographic mA. During fluoroscopy, the tube is operated at less than 5 mA. There is also a Spot film camera that operates similar to a movie camera except that it exposes only one frame at a time.

Spot Film Device

References Bushberg et al, The Essentials of Physics and Medical Imaging, Williams & Wilkins Publisher. Bushong, S., Radiologic Science for Technologists, Physics, Biology and Protection, 8th Edition, C.V. Mosby Company. Carlton et al, Principles of Radiographic Imaging, An Art and Science, Delmar Publishing. Selman, J., The Fundamentals of X-Ray and Radium Physics, 8th Edition, Charles C. Thomas Publisher.