OD and Viewboxes

  • Differences in Mammography vs General Diagnostic Film

    • Film Characteristics:

    • Mammography films have a higher gradient and maximum optical density (OD).

    • General diagnostic films max out at 3 to 3.5 OD while mammography films can reach up to 4 to 5 OD.

    • Higher gradient increases contrast between breast tissues.

    • Higher maximum OD provides a wider dynamic range while maintaining high contrast.

  • Film Processing:

    • Chemistry: Specific processing chemistry is required for mammographic films.

    • Processing Time:

    • General diagnostic processors: 45 seconds cycle time.

    • Mammography processors: can take 90 seconds to 3 minutes.

    • Longer processing time increases contrast and reduces patient dose.

  • View Boxes:

    • Brightness: View boxes for mammography are brighter.

    • Luminance:

    • Mammography view boxes: about 3,000 candelas/m².

    • General radiographic systems: about 1,500 candelas/m².

    • Brightness compensates for the higher optical density of mammography films.

    • Shutter Mechanism:

    • Shutters eliminate excess light around films, allowing eyes to adjust to film brightness.

    • This process is called masking, which is crucial to avoid visualized contrast reduction due to external light.

  • Demonstration of Shutters:

    • Test Phantom Imaging:

    • Left side (no shutters): fewer aluminum squares visible.

    • Right side (with shutters): many more aluminum squares visible.

    • Brighter backlights may be required for visualization in regions with OD > 2 to 2.5.

    • Small hot lights (> 5,000 candelas/m²) facilitate visualization in excessively dark anatomical regions.

  • Cassettes in Mammography:

    • Material and Design:

    • Made from translucent, low-energy x-ray absorbing materials like plastic or carbon fiber.

    • No lead backing is used, which is essential in general radiography to minimize backscatter.

    • Phototimer Positioning:

    • In mammography, phototimers are located behind the cassette to avoid ratings shadows on film due to low energy radiations.

    • Conversely, general diagnostic systems use entrance phototimers situated in front of the cassette.

    • Calibration Challenges:

    • Phototimers in mammography are more challenging to calibrate since measuring occurs after radiation passes through the cassette.

  • Summary of Key Points:

    • Single Emulsion Film enhances spatial resolution by reducing parallax.

    • Higher Gradient Film improves contrast among varying breast tissue types.

    • Higher Optical Densities allow for greater latitude while maintaining a higher gradient.

    • Longer Processing Times are beneficial for improving contrast and reducing patient radiation dose.