Digital Radiography Receptors
Beth Schueler, Ph.D.
Mayo Clinic
Overview: Projection Radiography
• Computed Radiography (CR)
– Basic technology – Characteristics
– Recent developments
• Digital Radiography (DR)
– Types
3
Computed Radiography (CR)
• Based on use of a photostimulable storage
phosphor
• Can be configured as:
– Cassette-based – Cassette-less
• Manufacturers include:
– Fuji – Agfa
– Carestream (formerly Eastman Kodak) – Konica Minolta
CR: Cassette-based
• Photostimulable
storage phosphor
– Absorbs x-rays and stores a latent image
• Cassette
– Used like a film-screen cassette
• CR Reader
5
CR: Cassette-less
• No cassette
handling
Image Plate
Protective Laminate
Photostimulable Phosphor (PSP) Layer
Support
Light Shielding Layer
0.
7
CR Image Formation
• PSP material: BaFX:Eu
2+– Commonly contain barium and fluorine with bromine (Br), iodine (I) and/or
strontium (Sr) in a crystal structure
– Europium added trace amount as an activator
• With x-ray exposure
– About half of signal is prompt light emission
– Remaining is stored as a trapped electron
X-rays
Readout Process
• Exposed image plate
is scanned by a laser
– red-emitting diode: 670-690 nm
• Stored energy in
image plate is
released as a photon
– blue-violet: 400 nm
9
Readout Process
• Emitted light is
collected by a light
guide
– Light guide: curved acrylic plate
• Light is detected by a
photodetector (PMT)
and converted to a
voltage signal
PMT
11
Readout Process
Readout Process
• Residual signal is
removed by scanning
with a high intensity
erasure light
– Some low level residual signal remains
13
New Developments in CR
• Line scan readout
– single laser beam is replaced with a laser line source – single PMT is replaced by a linear CCD array
– readout time is 10X faster than point scan system
Dual-sided
Readout
• Image plate is
mounted on a clear
support/backing layer
• Emitted light is
detected from both
sides with 2 light
collection systems
• Decreases image
noise
Light Guide
15
Structured Phosphors
• Image plate has needle-like rods of phosphor
material
• Rods channel emitted light to improve spatial
resolution
Digital Radiography (DR)
Different types based on choice of x-ray absorber:
1. Indirect
• use a phosphor to produce light, then convert light into photoelectrons
• Gd2O2S – intensifying screen • CsI – image intensifiers
2. Direct
• Use a photoconductor to produce electron-hole pairs for direct signal capture
17
Digital Radiography (DR)
Different types based on choice of signal collection
method:
1. CCD
• Detectors are indirect with CCD collecting light
2. Thin-film-transistor (TFT) array
• Large area signal collection array
CCD-based DR
• Phosphors used
– Gd2O2S – CsI
• Configurations available
– Large area phosphor
– Linear array with slot scanning
• Coupling of phosphor to CCD through
– Lens
19
CCD-based DR
Manufacturer Model Size(cm) spacing Pixel (m)
X-ray
Absorber
Swissray ddR 35x43 167 CsI
Imaging
Dynamics Xplorer 1700 43x43 108 Gd2O2S
CCD-based DR: Issues
• For large area detectors, light collection
efficiency from the phosphor is very low
• Coupling system requires space – thick detector
4
3
cm
35 cm
21
TFT Arrays
• Made from amorphous silicon (a-Si:H)
– Layers deposited onto glass
– Etched to create pixel elements and connection lines for readout of signal
• Same technology that is used for flat panel
display systems
– Large market for TFT displays has allowed for
TFT: Pixel Elements
Switching element
Active area
• Indirect detector
– Active area is a
photodiode to collect light
• Direct detector
– Active area is a storage capacitor to collect
23
Indirect Type
X-rays
Cesium
Cesium
Iodide
Indirect DR Detectors
Manufacturer Model Size(cm) spacing Pixel (m)
X-ray
Absorber
Trixell
(Siemens, Philips)
Digital
Diagnost, Aristos
43x43 143 CsI
General
Electric Revolution, Definium 41x41 200 CsI
Varian PaxScan 30x40 194 CsI,
25
Direct Type
• Converts x-rays directly into electron-hole pairs
– High voltage draws + charge to electrode pixels
Direct DR Detectors
Manufacturer Model Size(cm) spacing Pixel (m)
X-ray
Absorber
Hologic (DRC – Direct
Radiography Corp)
Kodak
Directview, Fischer
VersaRad
35x43 139 a-Se
Anrad Toshiba 35x35 150 a-Se
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TFT: Pixel Elements
• Fill factor
– Dead space surrounds the active area
– Fraction of pixel area that is sensitive = fill factor
TFT: Array Operation
1. Initialized state
– Control voltage at -5 V for all control lines – All switching
elements are off
-5 V
-5 V -5 V
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TFT: Array Operation
2. X-ray exposure
is made
– Signal is stored in each active
area -5 V
TFT: Array Operation
3. Control voltage
set to +10 V for
one row
– Signal for this row of pixels is transferred out data lines
-5 V
+10 V
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TFT: Array Operation
4. Control voltage set to +10 V for next row 5. And so on until each
row is emptied
6. Entire array readout
takes 300-500 ms +10 V
TFT: Reinitialization
• Process of preparing the detector for the next
exposure
• If not done properly, there is residual signal left
over resulting in a ghost image
• Approaches vary depending on detector
manufacturer, but commonly include:
– Use of an applied bias voltage – Use of a light field
33
CR/DR Characteristics
• Image pre-processing
– initial image corrections
• Characteristic response
– dynamic range
• Detector x-ray absorption vs energy
– scatter sensitivity
• Exposure indicators
CR/DR Image Pre-Processing
• CR
– Laser beam and light guide variations
• DR
– Variations in phosphor or photoconductor thickness – Pixel-to-pixel variation in TFT array components
– Pixel and line malfunctions
• Image correction routines
– CR: Shading correction
35
CR Shading Correction
Uncorrected Image
DR Pixel Defect Correction
• Pixels that do not produce signal are identified
• Signal for that location is replaced with average
of surrounding pixels
• Manufacturers have specifications for number of
dead pixels, lines and pixel clusters that are
37
DR Offset Correction
• Requires acquisition of a dark image
– Image is readout without x-ray exposure
• Offset signal depends on
DR Offset Correction
• Requires acquisition of a dark image
– Image is readout without x-ray exposure
• Offset signal depends on
39
DR Gain Correction
• Requires uniform field images
– Multiple image are acquired and averaged to reduce quantum noise
• Gain depends on
– x-ray beam energy – SID
– exposure level
41
CR-DR Image Quality
0 Frequency (per mm)
D
Q
E
(
f)
43
Scatter Rejection for CR-DR
• Increased absorption in the scattered x-ray
energy range for BaFBr and CsI make scatter
rejection especially important
• Grids use is important for CR and indirect-type
detectors
– Need to be used in more situations (such as portable radiographs)
Grid Selection for CR-DR
• Stationary grids can result in aliasing patterns
caused by insufficient sampling by the image
receptor
– not a problem for moving or Bucky grids
• Stationary grids are used for:
– portable radiographic exams with CR or portable DR – free cassette (cross-table) views with CR or portable
DR
45
Grid Aliasing Patterns
• When the sampling rate exceeds 2 X grid
47
Grid Aliasing Patterns
• When the sampling rate is below 2 x grid
Grid Aliasing Patterns
49
Stationary Grid Specification for CR-DR
• For CR, position grid lines perpendicular to
reader scan line direction
• For both CR (when grid lines must be parallel to
scan lines) and DR, use high frequency grids
51
Exposure Indicators
• Underexposure/overexposure
– In film-screen, film density indicates under- and overexposure conditions
– In CR and DR, underexposure appears as a noisy image, overexposure is generally impossible to detect
• Exposure indicators provide a way to verify
proper radiographic exposure was used
Example Exposure Indicators
Manufacturer
Exposure Indicator
Name
Exposure Indicator Value
Under-exposed Target exposed
Over-AGFA lgM 1.9 2.2 2.5
Kodak Exposure
index, EI 1700 2000 2300
Fuji Sensitivity
number, S 400 200 100
General
53
CR-DR Implementation
• CR Configurations
– cassette
– cassette-less – operates much like DR
• DR Configurations
– table and wall stand
– portable – with wire connections and wireless (future) – U-arm
55
CR-DR Implementation
• Configuration should fit application
– Tables and wall stands:
• Cassette-less CR and DR are more efficient
– Portables:
• CR – flexible and inexpensive
• portable DR (wired or wireless) - expensive
– Free cassette views:
• CR – flexible, inexpensive, lightweight
• portable DR (wired or wireless) – heavier than CR, more sensitive to impact, wire difficult to handle in a sterile environment
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Vendor Model Detector Detector Size (cm)
GE Senographe DS,
2000D Indirect 19 x 23
GE Essential Indirect 24 x 31
Hologic Lorad Selenia, Siemens Mammomat
Novation
Direct 24 x 29
Fischer SenoScan CCD - line scan 21 x 29
Sectra MicroDose Photon counting
– line scan
24 x 26
Fuji ClearView CSm CR – Dual-
sided Readout 18 x 24 or 24 x 30
References
1. Rowlands JA. The physics of computed radiography. Phys Med Biol 2002; 47:R123–R166.
2. Schaetzing R. Computed Radiography Technology, in Advances in Digital Radiography Categorical Course in Diagnostic Radiology Physics, eds. Samei E, Flynn MJ, RSNA 2003, 7-22.
3. Seibert JA, et al., ‘‘Acceptance testing and quality control of
photostimulable phosphor imaging systems,’’ Report of the American Association of Physicists in Medicine #93, 2006.
4. Yorkston J. Digital Radiography Technology, in Advances in Digital Radiography Categorical Course in Diagnostic Radiology Physics, eds. Samei E, Flynn MJ, RSNA 2003, 23-36.
