Digital Radiography Used for and its Types

The term 'Digital Radiography' has come to imply a technique in which the traditional silver-halide film/screen radiation receptor has been replaced by a photo-electronic receptor controlled by a computer.

Types of Digital Radiography (DR) systems

Current DR systems can be divided into 2 broad categories:

1. Digital video-fluoroscopy systems (Digital fluoroscopy).
2. Scan projection radiographic (SPR) systems.

 

The most common type of digital radiography system at present is digital fluoroscopy and the procedure most commonly performed using digital fluoroscopy is digital subtraction angiography. Digital video-fluoroscopy utilizes a more or less conventional X-ray image intensifier in combination with a high-quality TV camera.   

In the scan projection digital radiography systems, collimated X-ray beams are used to scan the patient, either by moving the beam over the patient or by moving the patient through the beam. The detectors used in SPR systems are usually higher efficient. SPR is used currently as an adjunct to CT to produce the survey images that are very helpful for determining the appropriate tomographic plane.

Advantages of digital radiography over the film-based system

The advantages include:

1. Superior contrast detectability. Therefore, it is possible to image arterial structures by an I/V injection resulting in increased patient comfort and safety, decreased hospital stay.
2. Facility to manipulate image formation and presentation.

Disadvantages of the current digital radiographic system

These are:

1. A relatively small field of view.
2. Reduced spatial resolution as compared to traditional film images.

Components of a digital radiographic technique

In the DR system, an X-ray image intensifier replaces the film as the primary image receptor. X-rays interact with the film as the primary image receptor. X-rays interacting with the image intensifier input phosphor are first converted to light photons within a scintillation screen. The light is, in turn, converted to electrons by a photocathode.   

 The electrons are then accelerated and focused onto the image intensifier output phosphor. At the output phosphor, the increased energy of the electrons and the reduced size of the phosphor work together to produce an image with significantly enhanced brightness. The rather small image on the output phosphor (1-2 inches in diameter) is then viewed by a video camera (usually a lead oxide vidicon).   Up to this point, the system for digital radiography is identical to a conventional video-fluoroscopic system. In a conventional video-fluoroscopic system, however, the images are viewed on a video monitor or recorded on a videotape or video disk (analog storage device).

Summary

In a Digital radiographic system (DR) the video signals are converted to numbers and stored either in a computer memory or on a computer disc (digital storage devices). The part of the system which actually performs the task of converting the video signals to numbers is called an Analog-to-Digital Converter (ADC).   

 The numbers corresponding to each part of the image are stored at high speed in special image memories and then transfer to either the digital disk or to a video device/disk, tape, or monitor by way of a Digital-to-analog Converter (DAC).   In addition to the above basic components, numerous image manipulation techniques have been developed which can affect the perception of an image without changing its information content.