Digital Radiographic Imaging 101

Digital radiography (DR), image receptors, film digitizer, noise, signal to noise ratio, area beam, xenon gas detectors, scintillation detectors, fan beams, pre and post patient collimation,slit radiography, translation, interrogation time, attenuation profile,extinction time, Computed radiography (CR), photostimulablephosphor image plate, barium fluorohalide, europium, IP reader,photomultiplier (PM) tube, photocathode, photoemission, dynode, CR workstation, linear energy response, duel image format,charged couple devices (CCD), amorphous silicon & selenium image receptors, thin film transistor (TFT), active matrix array (AMA),direct radiography.

Terms

Similar to a paper scanner,only for film

Film is read by a laser and the image file is sent to a designated secondary device

Digital Acquisition Methods

1. Digitize radiographs with a film digitizer

Advantages* Inexpensive* Easy to use* Small

Uses* Converts films to digital files* Teleradiology* Teaching files and presentations

Disadvantages* Conversion of analog to digital adds a step that degrades image quality* Impractical for converting large archives

2. Digitize the video signal with an ADC

Advantages: Inexpensive Easy to install

Disadvantages: Noisy cameras Poor signal-to-noise ratio (SNR) Area beam (Scatter) Small matrix size (525)

Digital Acquisition Methods

1. 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

Ring of Xenon Detectors in a CT Scanner

Detectors

SPR used for CT Scout Films

StationaryX-ray tube

Detectors

CT Scout Views

Acquired by SPR, to produce a digital radiograph

Scatter radiation is greatly reduced

Detectors

1. Xenon Gas

2. Scintillation

Detectors

1. Xenon Gas

2. Scintillation

CrystalPhotomultiplier

(PM tube)

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

So is 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 RSV

Only 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.

CR Workstations

Problems Inherent to Conventional Chest Radiography

Under-exposed

Posterior bases obscuredby diaphragm on PA

Retrocardiac clear-space overexposed

CR Film

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 displayedTotal energy absorbed by IP

Fuji S number (200 ave) Low number = high exposure

Kodak (1800-2200) Low number = low exposure

Cassetteless Readers

Chest units.

In table

Laser, Dry Image Hardcopy Devices

Digital Acquisition Methods

2. Digitize the video signal

3. Scan projection radiography (SPR)

4. Computed Radiography (CR)

5. Charge Coupled Devices (CCD)

1. Scan radiographic films

Charge-Coupled Device (CCD)

CCD’s havereplaced theVidicon tubein camcorders

Charge-Coupled Device (CCD)

Read-out row

Photoelectric detectorsembedded in layers ofsilicon

Each pixel is 6 to 25 microns in size, and can store 10,000to 50,000 electrons.

Charge-Coupled Device (CCD)

Incident light createsa charge in the pixels

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Charge-Coupled Device (CCD)

A shutter closes to stop furtherinteraction of lighton the pixels

The frame is readyfor reading

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Charge-Coupled Device (CCD)

Charges shift fromone pixel to anotheras they move tothe readout row

Charge-Coupled Device (CCD)

Charges shift fromone pixel to anotheras they move tothe readout row

Charge-Coupled Device (CCD)

Charges movealong the readout row, and exit thechip.

Charge-Coupled Device (CCD)

Charges movealong the readout row, and exit thechip.

Charge-Coupled Device (CCD)

Charges movealong the readout row, and exit thechip.

Question: When the chargesleave the CCD chip, wheredo they go?

Answer: If the CCD was functioning as a camera, they could be sent directlyto an analog monitor as the video signal, or...

or...they could be sent to an ADC, on to RAM, displayed on a digital monitor, and stored ina secondary memory device

ALU

CUPrimaryMemory

SecondaryMemory

(RAM)

ADCDAC

Charge-Coupled Device (CCD)

DACADC

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

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)