Image Receptor Systems, Computed Radiography, Digital Radiography and PACS

Thursday, July 26, 2018


Cassettes and Intensifying Screens

The cassettes serves as the film holder during the radiographic procedure. It provides a light-tight, rigid structure to protect the film and also houses the intensifying screens. Most cassettes contain two intensifying screens, one front and one at the back, and the film is places between them. Instensifying screens are plates coated with phosphors (fluorescent crystals) that give off light when exposed to xrays. Their purpose is to reduce the amount of exposure required to produce an image. Without intensifying screens, as much as 50 to 100 times more exposure would be needed to adequately expose film. Intensifying screens greatly reduce patient dose and also reduce the output capacity requirements of xray generators and xray tubes.

Intensifying Screens Phosphor

Most phosphor in common use today are salts from rare earth elements. When exposed to xrays, they give off green, blue, blue violet, or ultraviolet light, depending on the specific phosphor. The size of crystal and thickness of the phosphor layer determine the amount of exposure required. Larger crystals and/or thicker layers require less exposure. Screens with finer crystals and thinner layer produce sharper image detail. Most radiography departments that use screens have at least two types: fast screens with larger crystal for routine use, and detail or extremity screens that have smaller crystals and require more exposure. The detail screens are used only for relatively small parts, such as hand and feet, where fine detail is most important. They are used only on the tabletop, not with grids or buckys. The third type may be used in some departments for chest radiography, where the screen-film combination produces low contrast images (images with more shades of gray) to improve visualization of the lungs, airway, and vascular structures.

Marker side of the Cassette

Each cassette has a small area where there is no intensifying screen and where exposure is blocked from the film by lead foil. This area is reserved for the photographic imprint of the patient identification. It is indicated on the front of the cassette by the position of the identifying label.

Handling

It is important that you become familiar with the types of intensifying screens used in your department so that you can select cassettes correctly and use them with the appropriate exposure factors. Cassettes are marked according to the type of screens they contain, and the technique chart will state which screens are appropriate with a given set of exposures.

Intensifying screens are quite expensive and are easily damaged. Damaged areas, dirt, or stains on the screens prevent light from exposing the film and result in artifacts on the image. For these reasons, it is important to avoid touching the screen and to keep the film processing are free of dust and dirt.

Radiographic Film

Radiographic film is manufactured with a particular sensitivity to the light emitted by intensifying screens. Green sensitive film is used with screens that emit green light, blue sensitive film is matched with blue emitting screens, and ultraviolet light emitting screens are paired with film that is sensitive to ultraviolet light.

Double Emulsion Film

Film for routine radiography has emulsion coated on both sides of the base so that the film responds to the light from both intensifying screens. This system decreases the required exposure by half. Both side of the film are therefore identical; there is no right or wrong side to a sheet of double emulsion film.

Film and Cassettes Sizes

Film and cassettes (as well as other image receptors) come in standard sizes. You will work more effectively in the clinical area when you have learned to recognize them at a glance. The most common sizes are the following:
  • 8 X 10 in ( 20 x 25 cm)
  • 9 X 9 in (23 x 23 cm)
  • 10 X 12 in (25 X 30 cm)
  • 11 X 14 in (28 X 35 cm)
  • 7 X 17 in ( 18 X 43 cm)
  • 14 X 14 in ( 35 X 35 cm)
  • 14 X 17 in ( 35 X 43 cm)


Film Storage and Handling

Film must be stored correctly to avoid fog, a generalized exposure that reduces film contrast. A good storage area is clean, cool, and dry, and is protected from radiation and processing chemical fumes. Film boxes should stand on edge with the expiration date visible. This date is checked to be sure older film is used before its expiration date.

Proper Film Handling

To avoid artifacts from improper film handling, be sure your hand are clean and dry, and touch only the corners of the film when removing it from the cassette. Avoid bending and crimping the film by allowing it to hand vertically when holding it with only one hand. To place it horizontally, use both hands and hold the film on opposite corners.

Processing the Film

In a conventional processing system, the exposed cassette is taken to the darkroom where the film is removed and fed into the automatic processor in near darkness. Patient identification may be stamped on the film using a daylight system that identifies the film outside the darkroom while it is still in the cassette, or inside the darkroom after the film is removed from the cassette. After the film has entered the processor, the cassette is reloaded with fresh film from the film bin, a storage unit located under the counter.

Safelight Indicator on Processor

A safelight provides just enough illumination to see where things are located. A tone or a red light on the processor will indicate when it is safe to feed another film or to turn on the lights.

Passbox

Cassettes are often passed to and from the dark room without opening the door by using a passbox. This compartment is installed in the darkroom and one set in the outside wall. Because the inner and outer doors cannot be opened at the same time, cassettes can be transferred while the darkroom remains dark. The passbox has two compartments: one for exposed cassettes awaiting processing, and one for the unexposed cassettes that have been reloaded and are ready for use. Correct locations for cassettes are essential, because it is not possible to determine by looking at the cassettes whether the film is exposed. Only by following the established routings can radiographer be confident that a cassette is unexposed and ready for use.

Filmless Radiography

Most major imaging centers have converted to filmless systems for much of their radiographic imaging. Filmless systems are expensive, but when the savings in space, time and processing chemicals are added to the advantages of digital electronic images, the conversion is more than worthwhile. There are two basic types of filmless radiography: Computed Radiography (CR) and digital radiography (DR).

Image Receptor in Computed Radiography

The image receptor for computed radiography is an imaging plate that consists of photostimulable phosphors. It is exposed in a special cassette using conventional radiographic equipment. The radiographer inserts the exposed cassette into a special processor and selects the type of examination from a menu so that the image will be processed correctly. A small beam from a high intensity laser in the processor converts the latent image to a visible one that is captured by a photomultiplier tube similar to those used in fluoroscopic image intensifiers. The photomultiplier tube emits an electronic signal that is digitized and stored in a computer. The image can then be displayed on a high resolution monitor. Hard copies can be produced using a laser film printer.

Digital Radiography Filmless Imaging System

The other type of filmless imaging system, DR, does not use conventional equipment. Special radiographic tables and upright cabinets contain radiation receptors that react to the pattern of the remnant radiation and transmit a digital signal directly to the computer system, producing an image instantaneously on a monitor. No cassettes and no processing are involved.

Because both the CR and DR imaging systems automatically adjust the visual quality of the image, there is not tell-tale darkness or lightness of the image that indicates overexposure or underexposure as in conventional imaging systems. For this reason, these processing systems usually display an exposure indicator number on the monitor, also referred to as an exposure index number, S number, or other number, depending on the equipment. This number must be monitored by radiographer to ensure that exposure are not excessive.

While digital radiography has for some time been used for special applications such as fluoroscopy and angiography. Technical limitations and cost factors have prevented widespread adoption of digital systems for general radiography. As these technical limitations are conquered, digital radiography is becoming more and more feasible for general radiographic applications.

Storage, Archiving of Digital Images / PACS

Once stored in the compter system, digital images from either CR or DR systems are organized and catalogued and can be accessed on monitors from multiple locations connected to the system network. These digital images can be manipulated electronically to enhance visibility. Analog image (conventional radiographs) can be added to the system by scanning them with a laser device called a film digitizer.

Computer Hardware and Software in Radiography

The computer hardware and software technology used to manage digital images in hospital and large health care facilities is called a picture archiving and communication system (PACS). This system provide archives for the storage of images from all imaging modalities, connect images with patient database information, facilities laser printing of images or transfer them to CR-ROM media, and display both images and information at work stations throughout the network as needed. PACS may include transmission equipment for tele radiology, allowing images to be viewed in remote location such as a physician’s home, and receiving images from remote locations such as outlying clinics. PACS technology can transmit images directly over telephone lines and via the internet.

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