Chapter 6: Digital Radiographic Imaging

Digital Acquisition Methods

1. Digitize radiographs with a film digitizer

* Converts films to digital files

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 Methods1. Digitize radiographs with a film digitizer

Digital Acquisition Methods

2. Digitize the video signal

* Greatly reduces the area of the beam, and scatter* Replaces the camera with detectors

3. Scan projection radiography (SPR)

Fan shapedbeam

Depth of beammay be a cm, orsmaller

1. Scan radiographic films

Xenon Gas Detector From a CT Scanner

Xenon gas chamber

Detectors

1. Xenon Gas

2. Scintillation

Detectors

1. Xenon Gas

2. Scintillation

CrystalPhotomultiplier

(PM tube)

Ring of Xenon Detectors in a CT Scanner

Detectors

SPR used for CT Scout Films

X-ray tube

Detectors

CT Scout Views

Acquired by SPR, to produce a digital radiograph

Digital Acquisition Methods

2. Digitize the video signal

3. Scan projection radiography (SPR)

4. Computed Radiography (CR)

Barium Fluorohalide doped with Europium

Photostimulable image plate (IP) technology

1. Scan radiographic films

CR AdvantagesUses existing radiographic hardware

Relatively inexpensive to purchase

Filmless capture

Reduced number of repeats

Increased latitude

The CR IP lookslike a conventionalintensifying screen,and is housed in aconventional lookingcassette.

300 RSVOnly one speed (no detail or high speed)

Laser Film – wet or dry processing

CR Facts

Standard film sizes

Computed Radiography

Step 1. Make the exposure like any other radiographic exposure, only use an IP instead of film.

IP

*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

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

The CR IP ReaderAnd Workstation

CR Workstations

Problems Inherent to Conventional Chest Radiography

Under-exposed

Posterior bases obscuredby diaphragm on PA

Retrocardiac clear-space overexposed

Films

Latitude

Logarithmic responseof film

Linear response of CR

Yet to respond

Maxed out

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* Separate ID terminal

* Single cassette* Built in 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

Smart CR

All in one unit

Now why didn’t they think of that sooner?

Digital Acquisition Methods

2. Digitize the video signal

3. Scan projection radiography (SPR)

4. Computed Radiography (CR)

6. Flat Panels Amorphous Silicon & Amorphous Selenium

5. Charged Couple Devices

1. Scan radiographic films

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 (halfa hair)

Diodes connectedto rows

Current flows out columns

Liquid Crystal Displays

5. RGB colored filters

6. Second filter

Amorphous Silicon

Cesium iodide (CsI) scintillatorconverts X-rays to light

Light is converted to a charge by a photodiode at a TFT junction.

Amorphous Selenium(called Direct Radiography)

Electrode with a bias voltage+ + + + + + + +

- - - - - - - - - - -

Photoconductor material

TFT

Photon in

Interaction createselectron-hole pairs

Positive charge

Negative charge

Signal out

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 signalSPR 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 currentCR Photostimulable Helium-neon x-ray to light to Only portable phosphorIP laser to trapped electrons receptor to light to currentCCD IC Point by point discharge x-ray to light to Potential next of photoelectric trapped electrons generation of IIs detectors (pixels) to currentAmorphous 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

Digital Radiography (DR)