Unit 1: What is Digital Radiography?


Margaret Ewart I Margaret's Portfolio I ETAP 623 Fall 2012

DRS7.jpg Your first steps....

Target Objectives

At the conclusion of this unit, learners should be able to:

  • Provide a brief description of the history of digital radiography.
  • Identify key digital radiography modalities.
  • Define the major components of a digital imaging system.

The History of Digital Imaging

Remembering the Basics!

To learn new material we must first go back to basics. Since it has been quite some time since some of us were in X-ray school, I feel it is important to have a little refresher on how X-ray images are produced. Please watch the following video on the function of an X-ray tube before continuing on in this course.


The Evolution of Digital Imaging

During the past two and a half decades, digital radiography has replaced screen-film radiography in many radiology departments. Today, technological advances provide us with the tools we need to produce quality radiographs while utilizing state of the art equipment. Digital detectors allow implementation of a fully digital picture archiving and communication system, in which images are stored electronically and are available anytime and anywhere. Image distribution in hospitals can now be achieved electronically by means of web-based technology with no risk of losing images. Other advantages of digital radiography include improved quality of exams, increased dose efficiency, and the greater dynamic range of digital detectors with possible reduction of radiation exposure to the patient. The future of radiography will be digital, and it is of great benefit to the quality Radiographer to be familiar with the technical principles, image quality criteria, and radiation exposure issues associated with the various digital radiography systems that are currently available. The following table gives a timeline on the evolution of digital imaging in the Radiology world.


The Different Types of Systems

CR - Computed Radiography

In computed radiography (CR) imaging, a photostimulable phosphor (PSP) plate within the imaging cassette is exposed during the study. The latent image is captured in the plate as electrons in the phosphor are excited when exposed to radiation. With time, this latent image eventually degrades due to spontaneous phosphorescence.

After exposure, the cassette is placed in a reader to capture and analyze the image data. A laser extracts information from the plate as an analog electrical signal, and an analog-to-digital converter translates this signal to digital binary code. Once the analog image is converted to a digital image, the resulting digital image is sent to a review station and then to the PACS for storage.

Before the next imaging study, the latent image is erased from the PSP through exposure to fluorescent light in the reader and then put back into the cassette. This step is necessary because the process of image capture in CR is relatively inefficient, and approximately 50% of the electrons that were excited during the initial radiation exposure are not released during the laser reading process.

DR - Digital Radiography

Unlike CR imaging, DR systems use built-in image plates and no cassettes are required. This eliminates the manual steps required in manipulating the cassette and the time needed for PSP read-out and processing. A variety of image capture configurations are used in DR systems. These configurations can consist of either large-area, flat-panel detectors with integrated thin-film transistor (TFT) readout mechanisms, or integrated PSP plate scanning mechanisms. Alternatively, DR systems can house an optic lens that immediately translates the analog image to a digital image with a charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) image sensor.

Flat-panel DR systems can either be characterized as providing a direct or indirect image capture. Indirect systems accomplish image capture through a process in which a scintillator turns X rays into light during exposure. A silicon photodiode then converts this light into an analog electrical charge. The TFT accomplishes the storage, digital translation, and readout of this electrical charge. Direct systems, meanwhile, house a selenium-based X-ray photoconductor that turns X rays into an electrical charge, which can then be processed by the TFT. This eliminates the photodiode process and the step of converting X rays into light.

The Major Players in the System


Digital radiography is performed by a system consisting of the following functional components:

A digital image receptor- the imaging plate that intercepts the x-ray beam after it has passed through the patients body and produces an image in digital form.

A digital image processing unit- unit used to process the image after it has been recorded. This unit can be used to change the characteristics of the image and optimize image quality.

An image management system - this unit controls the movement of the images among the other components and associating other data and information with the images. This processing unit is better known at the PACS (Picture Archiving and Communications System) system.

Image and data storage devices - Device used for long term storage of images. Images are easily accessible for rapid retrieval.

Patient Information System - often referred to as a Health Information System (HIS) or Radiology Information System (RIS) this computer system interfaces to the digital equipment to transfer patient demographics properly, leaving little room for human error.

A communications network-network available for transferring digital images is made up of a variety of integrated systems and can enable accessing patient images from across town or across the world.

Display device - workstation by which the radiologist can view and edit image characteristics. The viewer can manipulate the image with tools such as zoom, pan, measurements, and window/level.

Homework and Quiz

Read Chapter 1- pages 3-14 of the Seeram text.

Use the follow link perform the module on Digital Radiography.


Test your knowledge - What is Digital Radiography? Quiz

Answer Key - DR quiz answer key

Course Home: Digital Radiology for the Imaging Professional

Next- Unit 2: The Digital Image Process