Roll Film Changers

Roll film changers are obsolete and are considered here only in a historical perspective. The operation of a rapid serial roll film changer can be compared with that of a movie projector. Roll film changers had four major parts: (1) changer and mounting stand; (2) supply magazine; (3) receiving magazine; and (4) program selector.

Cut Film Changers

Rapid serial cut film changers rapidly transported single sheets of film of a specific size. The 14- × 14-in (35.6- × 35.6-cm) and the 10- × 12-in (25- × 30-cm) sizes were most often used. These units were available as single-plane changers; however, two units on different mounting stands were often combined for biplane operation.

The cut film changer was similar to a roll film changer in that it also contained four major components: (1) changer and mounting stand assembly; (2) supply magazine; (3) receiving cassette; and (4) program selector.

An automatic cut film changer was also available for peripheral angiography, with an exposure frequency of two exposures per second. An interesting feature of this unit was the central radio transparent opening, through which fluoroscopy could be used to position the patient and place the catheter. Its operation was similar to that of the cut film changer previously described.

Basic Operational Considerations

All of the rapid serial changers discussed operated in a similar manner (Fig. 2-1). They moved the imaging material into place for the exposure and then removed it to a storage location. In other words, with all rapid serial changers there was a period of time during which the film or cassette was in motion (transport time) and a period of time during which no motion occurred and the film was in position for the exposure (stationary period). The radiograph had to be produced during the stationary period. The stationary period for each of the types of rapid serial changers was different; however, it was usually less than 50% of the total film cycle time (transport time + stationary period). Certain inherent electronic delays prevented the entire stationary period from being used to make the exposure. These were the zero time and phase-in time.

The actual amount of phase-in time varied with each exposure and could not be predetermined. However, the maximum exposure time that could be used for a particular system was calculated by deducting the greatest phase-in time delay from the stationary period. Table 2-1 illustrates the maximum phase-in time delays for single-phase and three-phase units.

Biplane Radiography

A discussion of image capture would not be complete without consideration of biplane operation. The term biplane as applied to rapid sequence radiography is defined as simultaneous radiography in two planes (Fig. 2-2).

When rapid sequence changers are used for biplane radiography, certain technical difficulties arise. The most important factor, scatter, necessitates the use of special crossed grids in the vertical changer. This grid should absorb the scatter radiation produced in a biplane setup that strikes the vertical changer. This causes an increase in the density of the film in the horizontal changer. These factors should be considered when choosing the technical parameters for biplane operation. If single-plane study is required on the same patient, an adjustment in technique will be necessary to compensate for this effect.

We can see, therefore, that during biplane studies, it is necessary to use different exposure factors in each of the two planes. This is possible only if the x-ray tubes are supplied with separate generators, thus providing flexibility of technical factors for each tube. If, however, the x-ray tubes are connected in parallel on one generator, the selection of these factors will be severely compromised, and technical adjustments for the increased scatter radiation to one plane will be limited.

Image Intensification

An image intensifier (II) is used to increase the brightness of the image. This is accomplished with the II tube (Fig. 2-3). The input side of the II tube is coated with a phosphor layer, usually cesium iodide, that produces the image as a direct result of the action of the remnant radiation. Photoemissive material is coated on the substrate opposite the phosphor layer; this material converts the visible light image into an electron image. The electrons making up the image are accelerated across the II tube by an electron lens system and focused to strike the output side of the tube.

The output side of the II tube is also coated with a phosphor, which converts the electron image to a smaller, corresponding light image. The image at the output side of the II tube is brighter. This results from the process of reducing the size of the image and the acceleration of the electrons.

A television camera, either high resolution charge-coupled device (CCD) video camera or pickup tube, is attached directly to the output side of the II tube via a special lens system or fiberoptic disc. The CCD is an integrated light-sensitive circuit (chip) that can store and display the data of an image. The image is produced as separate picture elements (pixels), which exhibit electrical charges of varying intensities. The intensities are related to a corresponding “color” of the visible spectrum. Charge-coupled devices are found in digital cameras. The advantage to this type of system is that images can be produced in low light situations without the loss of resolution. The images produced can be transmitted to a television monitor as well as other types of recording devices. The use of an image distribution device will also allow the attachment of a serial spot filming device, a cinefluorographic unit, or both, to the system.

Hard Copy Image Output

Hard copy images produced in the state of the art digital equipment results from the conversion of a digital image into its analog counterpart. This process is accomplished using a digital to analog conversion (DAC) system and usually a laser or dye sublimation printer.

Older systems used film to capture the image, and the cameras were usually attached to the image intensifiers (II). The image capture systems discussed next may be used in certain areas and warrant a brief discussion of the principles of their operation.

Photofluorographic Systems

This category includes the serial spot filming device and the cinefluorographic camera. It should be noted that the spot film devices are not necessary in the current “state of the art” equipment, digital systems. These components were attached to the II by an image distribution unit. This unit contains a beam-splitting mirror for diverting the optical image onto the lens of the photofluorographic device. The serial spot filming and cinefluorographic camera record the image from the output phosphor of the II in the same way; that is, the light image from the output phosphor creates the image on the photofluorographic film.

Serial Spot Film Cameras.

This type of unit is also called a large-format, spot film, or rapid sequence camera. Such cameras may be referred to by the size of film they use. The current film size formats were 70, 90, 100, and 105 mm; the most commonly used film formats were 100 and 105 mm. Spot film cameras are capable of recording from 1 to 12 pictures (frames) per second (fps) and can use either roll or cut film depending on the manufacturer and model.

The serial spot filming devices do not contain a shutter. The image is produced on the II by using a pulsed x-ray beam. When the beam is on, an image is recorded; when the beam is off, no image is produced on the output phosphor and therefore no image is recorded in the camera.

The major difference between the photofluorographic and serial spot film systems is the finished product. The serial spot filming units produce images that are separated by a specific time interval and demonstrate the changes in the anatomy and pathology as a sequence of static events. These are usually cut from the roll of film and are viewed in sequential order.

Cinefluorographic Systems.

The cine camera is similar in construction to a movie camera. The device contains (1) lens system; (2) supply spool; (3) motor-driven film transport mechanism; (4) take-up spool; and (5) rotating shutter. Film sizes for the cine camera are 16 and 35 mm. The larger format is more popular because it produces a better-quality image.

The operation of the cine camera is also similar to that of a home movie camera. The film is passed in front of an aperture (hole) in synchronization with the shutter system; therefore, the film is in motion when the shutter is closed and stationary when the shutter is open. The image on the output phosphor of the II tube is produced by a pulsed x-ray beam. The pulsed format reduces the radiation dose to the patient, decreases the heat loading of the x-ray tube, and helps reduce motion artifacts, allowing very short exposure times.

The motor used to drive the transport mechanism is capable of varying speed settings. The speed of the motor will govern the number of frames per second that are exposed. The range of frames per second can be from 8 to 200 depending on the film size used. Cinefluorography is usually accomplished at frame rates in excess of 16 fps to produce the effect of motion when the final images of the study are projected for viewing.

Cineradiographic film must be properly matched with the emission spectrum of the output phosphor of the II. The type of film used is either orthochromatic (yellow or green-blue sensitive) or panchromatic (orange, red, green, yellow, and violet sensitive). Film processing can be accomplished with a specialized processor that can handle film sizes ranging from 35 to 105 mm.

Cinefluorographic units are used during cardiac catheterization to record the sequences in the coronary vasculature. They can be used in a single-plane setup or, if a C-arm is used, as a biplane system.

Multiformat Cameras.

The systems discussed above all relied upon the photography of an image either on an intensifying screen, cathode ray tube (CRT), television, or oscilloscope. In all of the cases the image was captured, stored, and displayed or converted into a hard copy image. The quality of the image produced depended upon the components of the image recording system. This is a well-known principle in general radiography. The image is degraded by a variety of factors in the system. Among them were the CRT brightness, phosphor graininess, distortion, and artifacts as well as the optical systems themselves. The image output devices discussed earlier produced a single image on a single frame or film. The desire to place more images on a single film as in CT and ultrasound studies led to the development of the multiformat camera.

Initial systems used various methods to produce the images. These included motorized optics and multiple lenses to produce the images. By varying the distances of the lenses and the film, it became possible to change the size of the images produced. Many advances were introduced to improve on the quality of the output produced by these cameras. These included better optics, the use of flat screen monitors. and improved phosphors. Eastman Kodak produces several types of single-emulsion radiographic film for use with the multiformat camera. Figure 2-5 illustrates the sensitometric properties of three of Eastman Kodak’s multiformat films. Note that there is a suitable variety of film speeds and average gradients among the films. These are all sensitive to the higher wavelength green light and are compatible with most CRT monitors. These cameras are still employed in many institutions and find their greatest use in ultrasound and nuclear medicine.