Digital Radiography

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Digital Radiographic Imaging 101
Terms
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), photostimulable
phosphor 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.
Digital Acquisition Methods
1. Digitize radiographs with a film digitizer
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
Uses
* Converts films to
digital files
* Teleradiology
* Teaching files and
presentations
Advantages
* Inexpensive
* Easy to use
* Small
Disadvantages
* Conversion of analog
to digital adds a step
that degrades image
quality
* Impractical for
converting large
archives
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 (SNR)
Area beam (Scatter)
Small matrix size (525)
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
Ring of Xenon Detectors in a CT Scanner
Detectors
SPR used for CT Scout Films
Stationary
X-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
Crystal
Photomultiplier
(PM tube)
Digital Acquisition Methods
1.
2.
3.
4.
Scan radiographic films
Digitize the video signal
Scan projection radiography (SPR)
Computed Radiography (CR)
Photostimulable image plate (IP) technology
Barium Fluorohalide doped with Europium
CR Advantages
Uses existing radiographic hardware
So is 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
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.
CR Workstations
Problems Inherent to Conventional Chest Radiography
Underexposed
Posterior bases obscured
by diaphragm on PA
Retrocardiac clearspace overexposed
CR
Film
Latitude
Maxed out
Logarithmic response
of film
Linear response
of CR
Yet to respond
Analog is continuous.
Image is fixed in film
Digital is discrete.
Image may be manipulated
15
1
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
Total 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
1.
2.
3.
4.
5.
Scan radiographic films
Digitize the video signal
Scan projection radiography (SPR)
Computed Radiography (CR)
Charge Coupled Devices (CCD)
Charge-Coupled Device (CCD)
CCD’s have
replaced the
Vidicon tube
in camcorders
Charge-Coupled Device (CCD)
Read-out row
Photoelectric detectors
embedded in layers of
silicon
Each pixel is 6 to 25
microns in size, and
can store 10,000
to 50,000 electrons.
Charge-Coupled Device (CCD)
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Incident light creates
a charge in the pixels
Charge-Coupled Device (CCD)
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A shutter closes
to stop further
interaction of light
on the pixels
The frame is ready
for reading
Charge-Coupled Device (CCD)
Charges shift from
one pixel to another
as they move to
the readout row
Charge-Coupled Device (CCD)
Charges shift from
one pixel to another
as they move to
the readout row
Charge-Coupled Device (CCD)
Charges move
along the readout
row, and exit the
chip.
Charge-Coupled Device (CCD)
Charges move
along the readout
row, and exit the
chip.
Charge-Coupled Device (CCD)
Question: When the charges
leave the CCD chip, where
do they go?
Charges move
along the readout
row, and exit the
chip.
Answer: If the CCD was
functioning as a camera,
they could be sent directly
to an analog monitor as
the video signal, or...
Charge-Coupled Device (CCD)
ALU
ADC
ADC
CU
Primary
Memory
or...they could be sent
to an ADC, on to RAM,
(RAM)
displayed on a digital
monitor, and stored in
a secondary memory Secondary
Memory
device
DAC
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
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
(called Direct Radiography)
Positive charge
++++++++
Electrode with a bias voltage
Photoconductor material
Photon in
----------Negative charge
TFT
Interaction creates
electron-hole pairs
Signal out
Digital Radiography (DR)
Receptor
Video
Target of camera
SPR
Xenon
Reader*
Energy
Transformations
CR
Photostimulable
phosphorIP
CCD
IC
Amorphous
silicon
TFT AMA
Flat panel
x-ray to light to
charged globules
to video signal
Interrogations of
x-ray to ionized
successive detectors
electrons
Interrogations of
x-rays to light
successive detectors
to current
Helium-neon
x-ray to light to
laser
to trapped electrons
to light to current
Point by point discharge x-ray to light to
of photoelectric
trapped electrons
detectors (pixels)
to current
point by point discharge x-ray to light
of TFTs
to current
Amorphous
selenium
TFT AMA
Flat panel
point by point discharge x-ray to current
of TFTs
Scintillation
Electron gun
* Method by which the stored, latent electronic image is discharged
Comment
Use limited by
noise of camera
Dedicated cxr
and CT scouts
Only portable
receptor
Potential next
generation of IIs
Called direct
radiography
Called direct
radiography
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