Study Notes for Fluoroscopy - Chapter 25

OUTLINE OBJECTIVES

  • At the completion of this chapter, the student should be able to:

    1. Discuss the development of fluoroscopy.

    2. Explain visual physiology and its relationship to fluoroscopy.

    3. Describe the components of an image intensifier.

    4. Calculate brightness gain and identify its units.

    5. List the approximate kVp levels for common fluoroscopic examinations.

    6. Discuss the role of the television monitor and the television image in forming fluoroscopic images.

AN OVERVIEW OF FLUOROSCOPY

  • Historical Context:

    • The fluoroscope was invented by Thomas A. Edison in 1896.

    • It serves as a valuable tool for dynamic studies in medical imaging.

    • The fluoroscope allows real-time viewing of moving internal structures and fluids during x-ray examination.

  • Primary Function:

    • To provide dynamic viewing of anatomical structures through the use of fluoroscopy.

    • Often uses contrast media to enhance visibility of anatomy.

DEVELOPMENT OF FLUOROSCOPY

  • Fluoroscopic Systems:

    • Systems can have the x-ray tube situated over or under the patient couch, with various configurations possible.

    • Image-intensified fluoroscopy typically displays images on a television or flat panel monitor.

ILLUMINATION AND VISUAL PHYSIOLOGY

  • Illumination:

    • Critical for the interpretation of fluoroscopic images, with brightness dependent on the x-ray tube operation.

    • Measured in lux (lumen/square meter); typical illumination levels for fluoroscopy range from 100 to 1000 lux.

  • Visual Physiology:

    • The structures responsible for vision are rods and cones located in the retina.

    • Rods are sensitive to low light conditions (scotopic vision) and cones provide color perception and visual acuity under bright light (photopic vision).

    • The transition from rod to cone vision impacts image interpretation during fluoroscopy, particularly in dim conditions.

FLUOROSCOPIC TECHNIQUE

  • Image Quality Factors:

    • Maximum image detail requires high brightness.

    • Fluoroscopic tube operates at lower mA (< 5), but radiation exposure can be high due to prolonged exposure.

  • Kilovolt Peak (kVp):

    • Depends on the body section being examined; various examinations require different kVp settings for optimal image quality.

  • Table 25-1: Representative fluoroscopic kVp values for common examinations:

    • Gallbladder: 65–75 kVp

    • Nephrostogram: 70–80 kVp

    • Myelogram: 70–80 kVp

    • Barium enema (air contrast): 80–90 kVp

    • Upper gastrointestinal: 100–110 kVp

    • Small bowel: 110–120 kVp

    • Barium enema: 110–120 kVp

IMAGE INTENSIFICATION

  • Image-Intensifier Tube:

    • Converts the x-ray beam to a high-intensity visible light image.

    • Composed of several elements: input phosphor, photocathode, and output phosphor.

    • The input phosphor (cesium iodide) converts x-ray energy to visible light, while the output phosphor produces amplified visible light.

  • Brightness Gain:

    • An essential characteristic of the image intensifier:

    • Brightness Gain = Minification Gain x Flux Gain

      • Where Flux Gain = Number of output light photons / Number of input x-ray photons.

      • Minification Gain = (Diameter of input phosphor / Diameter of output phosphor)^2.

  • Bright Image Generation:

    • The image intensifier’s ability to amplify the incoming x-ray image is captured in the brightness gain ratio.

FLUOROSCOPIC IMAGE MONITORING

  • Television Monitoring:

    • Utilizes a camera tube (vidicon) to convert visible light images from the output phosphor into electrical signals.

    • Video signals are processed to adjust brightness and contrast for better image quality.

  • Image Recording:

    • Various methods used for recording, including spot films and photospot cameras, allowing for higher quality images with less interruption to the procedure.

FLUOROSCOPY QUALITY CONTROL

  • Quality Control Procedures:

    • Important to monitor entrance skin dose (ESD), which should not exceed certain limits for patient safety.

    • Routine QA procedures ensure that systems remain calibrated and capable of delivering consistent image quality without excessive patient dose.

  • Automatic Exposure Systems:

    • Ensure consistent image brightness during fluoroscopic operations, compensating for changes in patient anatomy and position.

CHALLENGE QUESTIONS

  1. Define or identify key concepts related to fluoroscopy:
    a. Photopic vision
    b. Automatic brightness control (ABC)
    c. Visual acuity
    d. Flux gain
    e. Angiography
    f. Vidicon
    g. Photoemission
    h. Bucky slot cover
    i. Spot-film camera sizes
    j. Modulation

  2. Summarize the relationships in fluoroscopy, including x-ray interactions and image formation.

  3. Review the importance of visual physiology in interpreting fluoroscopic images under varying illumination conditions.