Chapter 6: Digital Radiographic Imaging

Digital Acquisition Methods 1. Digitize radiographs with a film digitizer

* Converts films to digital files

1. Digitize radiographs with a film digitizer Digital Acquisition Methods 1. Digitize radiographs with a film digitizer 2. Digitize the video signal with an ADC Advantages: Inexpensive Easy to install Disadvantages: Noisy cameras Poor signal-to-noise ratio (200:1, need 1000:1 for digital) Area beam Small matrix size

Digital Acquisition Methods 1. Scan radiographic films 2. Digitize the video signal 3. Scan projection radiography (SPR) * Greatly reduces the area of the beam, and scatter * Replaces the camera with detectors Fan shaped beam Depth of beam may be a cm, or smaller

Xenon Gas Detector From a CT Scanner Xenon gas chamber

Detectors 1. Xenon Gas 2. Scintillation

Detectors 1. Xenon Gas 2. Scintillation Photomultiplier (PM tube) Crystal

Ring of Xenon Detectors in a CT Scanner

SPR used for CT Scout Films X-ray tube Detectors

CT Scout Views Acquired by SPR, to produce a digital radiograph

Digital Acquisition Methods 1. Scan radiographic films 2. Digitize the video signal 3. Scan projection radiography (SPR) 4. Computed Radiography (CR) Photostimulable image plate (IP) technology Barium Fluorohalide doped with Europium

CR Advantages Uses existing radiographic hardware Relatively inexpensive to purchase Reduced number of repeats Increased latitude Filmless capture The CR IP looks like a conventional intensifying screen, and is housed in a conventional looking cassette.

CR Facts 300 RSV Only one speed (no detail or high speed) Standard film sizes Laser Film – wet or dry processing

Step 1. Make the exposure like any other radiographic Computed Radiography Step 1. Make the exposure like any other radiographic exposure, only use an IP instead of film. *Remnant photons strike plate *Photoelectric interaction causes barium fluorohalide to fluoresce as electron is ejected. *Electrons (that are of no more use in film radiography) are trapped in the energy traps created by the europium IP

Reading the IP Converting the stored energy to an electric current, point by point.

The CR IP Reader And Workstation

CR Workstations

Posterior bases obscured Problems Inherent to Conventional Chest Radiography Under- exposed Retrocardiac clear- space overexposed Posterior bases obscured by diaphragm on PA

Films

Latitude Logarithmic response of film Linear response of CR Maxed out Yet to respond Linear response of CR

Analog is continuous. Image is fixed in film Digital is discrete Analog is continuous. Image is fixed in film Digital is discrete. Image may be manipulated 1 15 15 1

CR Postprocessing & Characteristic Curves

Histogram

Processing Algorithms Poorly exposed image plates may be corrected by software to some extent

Patient Dose Calculated and displayed Fuji S number (200 ave) Low number = high exposure Kodak (1800-2200) Low number = low exposure

Fuji’s CR Cassette Readers * Four cassettes * 115 images per hour * Standard pixel density = 5 per mm * High pixel density =10 per mm * Single cassette * Built in ID terminal * Single cassette * Separate ID terminal

Cassetteless Readers Chest units. In table

Workstations Technologists consoles communicates with IP reader for: * QA of images * Examination status * Processing adjustments

Laser, Dry Image Hardcopy Devices

Now why didn’t they think of that sooner? Smart CR All in one unit Now why didn’t they think of that sooner?

Digital Acquisition Methods 1. Scan radiographic films 2. Digitize the video signal 3. Scan projection radiography (SPR) 4. Computed Radiography (CR) 5. Charged Couple Devices 6. Flat Panels Amorphous Silicon & Amorphous Selenium

Thin film transistors (TFT) in an Active Matrix Array (AMA), are incorporated in a “flat panel” detector that is used in place of a film cassette.

Thin Film Transistors (TFT) 139 microns (half a hair) Diodes connected to rows Current flows out columns

Liquid Crystal Displays 5. RGB colored filters 6. Second filter

Amorphous Silicon Cesium iodide (CsI) scintillator converts X-rays to light Light is converted to a charge by a photodiode at a TFT junction.

Amorphous Selenium Photon in Interaction creates electron-hole pairs (called Direct Radiography) Positive charge + + + + + + + + Electrode with a bias voltage Photoconductor material Photon in - - - - - - - - - - - Negative charge TFT Interaction creates electron-hole pairs Signal out

* Method by which the stored, latent electronic image is discharged Digital Radiography (DR) Receptor Reader* Energy Comment Transformations Video Target of camera Electron gun x-ray to light to Use limited by charged globules noise of camera to video signal SPR Xenon Interrogations of x-ray to ionized Dedicated cxr successive detectors electrons and CT scouts Scintillation Interrogations of x-rays to light successive detectors to current CR Photostimulable Helium-neon x-ray to light to Only portable phosphorIP laser to trapped electrons receptor to light to current CCD IC Point by point discharge x-ray to light to Potential next of photoelectric trapped electrons generation of IIs detectors (pixels) to current Amorphous TFT AMA point by point discharge x-ray to light Called direct silicon Flat panel of TFTs to current radiography Amorphous TFT AMA point by point discharge x-ray to current Called direct selenium Flat panel of TFTs radiography * Method by which the stored, latent electronic image is discharged