Photographic film (not testable)

Before there was digital recording...there was film   

Ultrasound Transparency Film

This medium was the traditional “hard copy” method for recording ultrasound images.  Similar to X-ray film, the transparency film consisted of a large sheet of plastic film on which a series of images were recorded using a type of camera attached to the ultrasound system.  Similar to X-ray film, the film had to be chemically processed in order for the image to be seen.  Also similar to X-ray film, the film was viewed “backlit” from a viewing box.  The process worked like this: 

The film was loaded into a cassette and the cassette was placed inside a large camera attached to the ultrasound machine.  Inside the camera was a small high resolution cathode ray tube (CRT).  The image on the CRT in the camera was identical to the image on the display monitor viewed by the operator.  To record an image, the operator pushed the “print” button on the camera and the image from the CRT in the camera was recorded on the film. See atlas figure 3-23.

The transparency film was single-sided which meant it had a recording surface (emulsion) on only one side. The emulsion side of the film was placed in the film cassette to face the camera’s CRT screen. Once exposed, the cassette was removed from the camera and the film was processed using the same type of automatic wet processing used for X-ray film. 

Notable characteristics of ultrasound transparency film include:

• ∙ 8" x 10" or 14" x 17" size, depending on type of multi-format camera 

• ∙ single emulsion (image created on one side of film)

• ∙ wide latitude for good grayscale recording

• ∙ spectral sensitivity of film was matched to CRT phosphor (blue vs green etc)

Construction:

• ∙ the film’s base was a rigid plastic acetate on which the emulsion was layered

• ∙ the light-sensitive emulsion was a mixture of gelatin and light-sensitive silver halide crystals

• ∙ a supercoat layer covered the emulsion to protect it

Undeveloped film is dull on the emulsion side and shiny on backing.

Film Processing

The film was processed in an automatic chemical processor.  The usual processing time was 90 seconds per sheet of film. 

In the processor the film was automatically run through a series of chemical tanks to develop the image on the film.  The three primary tanks were the developer, the fixer, and the wash.

The “latent image” was the term for the invisible image on the film produced by the exposure of the film’s silver halide emulsion to the light from the image in the camera.

The developer chemical changed the latent image into the “manifest image”.  The manifest image is the visible image.  The fixer stopped the development process.  The wash removed any residual chemicals.  Finally, a heater, dried the film before it was expelled from the processor.

The temperature, concentration, and time in the developer could significantly affect the appearance of the film.  An increase in any or all of these factors tended to make the film darker.  Conversely, a decrease in these factors would make the image on the film lighter (See atlas figure 3-23).

In its time, the advantages of transparency film as a recording medium were:

• ∙ it used existing X-ray processing technology and fit well in a radiology department that used film  

• ∙ it was durable, making it a good medium for archiving

• ∙ it had excellent spatial and contrast resolution. Transparency film was a high-quality medium and was NOT a limiting factor to the spatial resolution of an ultrasound study

The disadvantages of transparency film were:

• ∙ it required chemical processing

• ∙ it required significant Quality Assurance.

• ∙ it was bulky and difficult to store

• ∙ the films could be lost

• ∙ film was expensive and time-consuming to duplicate