8/18/2011
Imaging Educational Course: Radiography IV
Digital Projection X-ray Detectors:
Review of Detector Technologies: J.Yorkston
Disclosure:
• John Yorkston works for Carestream Health that
manufactures and sells CR and DR systems for medical
applications
Review of Commercial Landscape: E.Gingold
John Yorkston, Carestream Health
Eric Gingold, Thomas Jefferson University Hospital
Digital Acquisition: Introduction
• “CR” technology introduced in 1980’s
– Originally referred to “storage” phosphor based systems
– Cassette based imaging with separate readout scanner
– Similar workflow as screen-film cassettes
• “DR” technology introduced in mid 1990’s
– Originally referred to a-Si:H (and CCD) based systems
– Integrated with x-ray system
– Fully “electronic” readout with no moving parts
• Since then a number of novel hybrid systems:
–“Avalanche” of new terms and nomenclature that confuse and confound
– Much of this originates in the “marketing” departments of companies
Confusing Acronyms/Nomenclature:
Particle in Binder Phosphor Powdered Phosphor Lens coupled system
Flat Panel Detector Direct Detector PSP: Photo Stimulable Phosphor
Fibre Optic coupled system
Amorphous Silicon Detector
Indirect Detector
AMFPI
FXD:
Flat Panel X-ray Detector
DDR: Direct Digital Radiography
Plasma DR Structured Phosphor
AMA: Active Matrix Array
SBDX: Scanned Beam Digital X-ray CR System
Needle Phosphor
Amorphous Selenium Detector
DR: Direct Radiography FD: Flat Detector Technology Needle IP
Photostimulable Luminescence
CCD based system Storage Phosphor
CR: Computed Radiography
DFP: Dynamic Flat Panel Laser Scanned System
CMOS based system
FPD: Flat Panel Detector
TFT Detector
FP: Focused Phosphor
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Educational Goals:
• Identify features that define the underlying technology
• Decipher confusing terminology present in marketplace
– Know what questions to ask
• Appreciate differences between competing approaches
– All systems have their pros and cons…
– Important to know which suits your application and budget
Digital Image Formation:
• X-rays modulated by object
Incoming X-rays
• X-Ray Converter/Absorber
X-ray “Converter”
• Secondary quanta response
• Secondary quanta detector
Outline:
• Discuss x-ray absorption material selection
• Review secondary quantum detection options
• Review current status of projection x-ray technology
Response
Detector
X-Ray Absorption Materials:
• Advantageous properties for an x-ray absorber include:
– Absorb as many x-rays as possible
– Provide accurate measure of how many x-rays interacted
– Maintain information on spatial location of point of interaction
– Manufacturable over suitably large physical areas
• Two different types of materials used:
– Phosphor materials that generate light
– Photoconductor materials that generate electrical charge
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X-Ray Phosphors:
X-Ray Phosphors:
• PIB configured from small phosphor grains in plastic
• Two categories of x-ray phosphor materials
– Prompt emission materials (e.g. Gd2O2S(Tb) also known as GOS)
» Emit light “instantaneously” on absorption of x-ray
» Formed basis for “modern” screen-film systems
– Photo-stimulated emission materials (e.g. BaFBr(I))
»
»
»
»
»
»
Fraction of x-ray energy stored in long lived “latent” sites
Require readout with stimulating radiation (typically laser)
Also known as “storage” phosphors
Formed basis for Computed Radiography (CR) systems
Actually emit ~50% of energy as “prompt” light
Require erasure step to remove remaining signal
• Traditionally created as particle-in-binder (PIB) layers
– Relatively “easy” to manufacture (after decades of development)
– Very physically robust
• Issue with creating thick layers to increase absorption
– Light scatters within material
– As thickness increases so does light spreading, reducing resolution
– Escape efficiency of light from screen varies through screen depth
» Results in increased noise (Swank noise)
phosphor
~100-150 mm
Air gaps
binder
» Also known as phosphor-in-binder or POWDERED Phosphor
X-Ray Phosphors:
X-Ray Photoconductors:
• More recently “structured” phosphors have been used
–
–
–
–
–
Prompt emission type: CsI(Tl)
Stimulated emission type: CsBr(Eu)
Reduces effect of thickness on spatial resolution: allows thicker layers
Improves light escape efficiency so reduces Swank noise
Allows higher “packing fraction” than PIB so higher effective absorption
~200 mm Mammo.
~500-600 mm
Gen. Rad.
• Somewhat different issues than phosphors:
Require applied voltage to “energize” layer and allow charge collection
Internal field constrains lateral drift of released charges
Near perfect spatial resolution almost independent of thickness
High collection efficiency so low Swank noise
Most mature material is amorphous selenium (Z=34)
» Low Z value limits x-ray absorption at diagnostic energies (>60kVp)
» Difficult to manufacture thick layers (~1000mm) over large area
» More suited to mammographic applications (<30kVp)
– Other materials include c-Si, CdTe, CdZnTe, HgI, PbI, PbO, Xenon.
–
–
–
–
–
~200 mm Mammo.
4 mm
~500-1000 mm
Gen.Rad.
E
- +
H.V.
~10V/mm
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Primary Photon Absorption:
Mammo. Photon Absorption vs. Thickness
Mammo
Gen.Rad.
Chest
CsI(Tl) ~84%
a-Se
Photon Absorption (%)
Attenuation Coefficient (mu)
(28kVp Mo/Mo 5cm PMMA )
CsI(Tl)
1.E+03
1.E+02
1.E+01
10
a-Se Photon Abs. (%)
1.E+00
CsI(Tl) Photon Abs. (%)
1
1.E-01
0
20
40
60
80
100
120
1
140
500mm CsI(Tl)
10 0
a-S e P h o to n A bs . (% )
100
1000
Clinical Image Comparisons: Lateral Chest (120kVp)
R Q A -9 Ph oton A b sorption vs. T h ic k ne ss
Cs I(T l) P h o to n A bs . (% )
10
Thickness (microns)
Photon Energy (keV)
P ho t o n A b so rp tio n (% )
a-Se ~97%
100
1.E+04
500mm a-Se
CsI(Tl) ~56%
a-Se ~27%
10
1
10
10 0
10 0 0
T h ick n es s (m icro n s )
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X-Ray Absorption Materials Summary:
• Can be divided into 3 main types:
– Prompt emitting phospors (Gd2O2S(Tb), CsI(Tl))
– Stimulated emission phosphors (BaFBr(I), CsBr(Eu))
– Photoconductors (a-Se)
• Phosphors can be sub-divided into:
– Powdered or Particle-in-binder layers
– Structured/Needle/Focused Phosphors
Secondary Quanta Detection:
• Issues are similar for phosphors and photoconductors
– Need accurate measure of generated light/charge over large areas
– Maintain image “quality” produced by x-ray absorption layer
• Possible approaches include
– Point by point scanning
– Line scanning
– Full area readout
• All have sufficiently good properties to be “useful”
• Which is “best” depends on specifics of application
Point by Point Scanning:
• Storage Phosphors lend themselves to this approach
–
–
–
–
Allow time to scan whole area with small laser spot
Spot ~100mm in size, 10mW power,
Dwell time ~few msecs/pixel
~30 or so seconds for full readout
Point by Point Scanning:
• Optics and mechanical motion require “large” system
• Issue with collection efficiency of stimulated light
– Secondary quantum sink at collection stage
• One solution: read out signal from both sides of phosphor
PMT
Light Guide
v
Laser Spot
PMT
Light Guide
Image courtesy R. Uzenoff Fuji Medical
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Line/Slot Scanning:
Line/Slot Scanning:
• To improve scan speed read out a line at a time.
• With prompt emitting phosphors need to collimate x-rays
• With storage phosphor can readout lines after area exposure
• Numerous versions of line/slot scanned systems
– Incorporate line laser and solid state collector in compact single unit
– Significantly reduces space requirements for beam path optics
– Still requires mechanical motion
– Also possible with photoconductor ( e.g Thoravision, Fuji Aspire FFDM)
Laser Source +
Intensity Control
Beam Shaping
• Most use some form of linear CCD coupled to a phosphor as detector
– c-Si photon counting mammo system recently approved by FDA
– Gas wire chamber based systems have also been reported
• Good coupling between phosphor and CCD, good DQE
Light Collection Optics
• Excellent scatter rejection
Optical Filter
• Still require mechanical motion and collimation alignment
Photodetector
• Scan times of multiple seconds
• Commercial examples using CsI(Tl) coupled with linear CCD’s include:
Image Plate
Philips Thoravision a-Se Chest System
Line/Slot Scanning:
– Thorascan (Oldelft) chest system
– Senoscan (Fischer) mammo system
Line/Slot Scanning:
•Lodox Statscan
•Full body scan 13 secs
•Linear CCD with CsI(Tl)
•Biospace EOS
•Full body scan 20 secs
•Perpendicular wire/gas chambers
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Full Area “Electronic” Readout:
Line/Slot Scanning:
• Crystalline Si low x-ray absorption efficiency (Z=12)
–
–
–
–
• Earliest approaches used CCD detectors
Si chip fabrication uses thin layer of processed materials (~100’s mm)
Not thick enough for direct x-ray absorption
Increase effective thickness by rotating thin layer of c-Si
Used in commercial scanning “photon counting” mammo system
Sectra
43cm
3-4cm
Area Readout: Single CCD Configuration
X-rays
CsI(Tl)
• Lens or F.O. Demag. ~x10-12
• Photon Collection Efficiency ~0.1%
• 2nd quanta shot noise dominant
Area Readout: Multiple CCD Configuration
• SwissRay and Apelem
• reduces de-mag.
(Secondary quantum sink)
Mirror
CCD
High Quality Lens
or Fibre Optic Reducer
(Courtesy Imaging Dynamics Corp.)
(Source: SwissRay Corp.)
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Area Readout: Multiple CCD/CMOS Config.
• CaresBuilt and Naomi
• Tiling of image an issue
Area Readout: a-Si:H Flat Panel Readout
• Large area a-Si:H readout
• 14x17” or larger readily available with pixels down to <100mm
• Can use prompt emitting phosphor or photoconductor
• Directly coupled to x-ray absorption layer (high transfer effic.)
• “Electronic” readout can operated in static or fluoroscopic modes
Sharp Gen. 10 Glass Substrate 9x10’
Area Readout: a-Si:H Flat Panel Readout
Area Readout: Tiled CMOS/CCD Config.
• Recent development is “large” 3 side buttable CMOS tiles
• Very low “additive” readout noise
• High speed readout (30 f.p.s)
• Integrated electronics (e.g. ADC’s)
• Directly coupled to x-ray absorption layer (high transfer effic.)
43x43cm
2x3 Array of Tiles
Pixel Area
Large Area Tile
(~8x14cm)
Sensing/Storage
Element
(Image courtesy Dr. B. Polischuk)
Switching
Element
23x28 cm FFDM CMOS detector Dalsa Corp.
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Performance Summary:
Point/Line
Scanning
Stimulated Emission
Phosphor
Speed of
Readout
BaFBr(I)
CsBr(Eu)
--
-
Conclusions:
Full Area Readout
Prompt Emission Phosphor
Photoconductor
a-Si:H
+GOS
a-Si:H
+CsI(Tl)
CCD/CMOS
lens/fib.opt.
Tiled
CMOS
200mm a-Se
(mammo)
500mm a-Se
(gen.rad.)
++
++
+++
+ + + (?)
++
++
(+ if integ.)
(+ if integ.)
Image
Quality
+
++
++
+++
+
+ + + (?)
+++
++
Robustness
++
-
+++
-
+++
+ + (?)
-
--
Size
--
-
+
+
--
+
+
+
Cost
+++
+
-
-
+++
- (?)
-
-
Adv. Apps.
(tomo& DE)
-
-
+
++
+
+ + + (?)
++
+
• Confusing nomenclature and acronyms abound in digital radiography
• An understanding of fundamental detector components can help to:
–
–
–
–
Decipher details of a specific commercial offering
Understand the advantages and limitations of a specific system
Help in discussions with vendors and generation of RFP’s
Help to ensure final detector choice will satisfy intended application
• The introduction of new detectors and new applications will only make
things more confusing:
– Dual energy, contrast enhanced dual energy, photon counting,
tomosynthesis, phase contrast imaging, cone beam CT, multispectral
imaging,…..
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