physics of flat‐panel detectorsbml.pusan.ac.kr/.../graduates/imagedetectors/6_fpd.pdf · 2020. 3....
TRANSCRIPT
Physics of Flat‐panel Detectors
Ho Kyung [email protected]
Pusan National University
Medical Imaging Detectors
Routine
• Load a film into a cassette (in the dark room)
• Carry it to the examination room
• Insert it into the x‐ray table
• Position the patient
• Make the x‐ray exposure
• Carry the cassette back to the processor to develop the film
• Check if the film is suitable for making a medical diagnosis
• Hang the film in a view box
Key issue in the development of digital diagnostic radiology
• Large‐area availability of a digital sensor enough to cover the patient body
• Flat‐panel active matrix array, originally developed for laptop‐computer displays
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Requirements
• Field of view (FOV)
• Dynamic range (DR)
‒ Range of x‐ray factors (attenuation) used to create images
• Pixel size
‒ Spatial resolution
• Noise level
‒ Lowest possible x‐ray exposure at which the system should be quantum‐noise limited
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General concepts
Digitization
• Sampling in space => pixels checkboard artifacts
• Quantization in intensity => bits contouring artifacts
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Dynamic range = the attenuation divided by precision
• Ratio between x‐ray attenuation of the most radiolucent & the most radio‐opaque paths
‒ 𝑒
• Precision of the x‐ray signal measured in the most radio‐opaque anatomy
‒ e.g., 1% precision in the signal attenuated by a factor of 50 results in the dynamic range of 5000
‒ Chest:
500, mammography:
4000, & fluoroscopy:
. 100
‒ Corresponding to 54 dB, 72 dB, & 40 dB or 10 bits, 12 bits, & 7 bits with the noise level of 1 LSB
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Dynamic range
Accounting for typical # x‐ray photons per pixel is ~5000, the design of 1 pF seems to be reasonable (5000 ph 18 keV / 18 eV for CsI 5 106 e– with 100% quantum efficiency)
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-5 V
Gate ~ 3,430 lines for 24 cm
1 M70 m
0.5 m
Data
𝐶 𝜀 𝜀𝐴𝑑
1 pF
𝜏 10𝑁 𝑅 𝐶 35 ms
𝑛𝐶 𝑉
𝑒3.2 10 𝑒
𝑛 𝜎 𝜎 𝜎𝐽 𝐴𝜏
𝑒𝜎
𝜎 100 𝑒 if 𝐽 10 pA mm
𝑛 𝜎 2 10 𝑒 (assumption)
DR 20 log 84 dB 14 bits (1 LSB is defined by the electronic noise level)
Intrinsic dynamic range
Implications
• Adjustment of the beam energy should be done to control the required dynamic range and dose w.r.t. imaging tasks. Optimization of spectrum is important (e.g., 25 keV results in 37 attenuation; hence the 12‐bit works)
• Without understanding x‐ray interaction physics, the proper design of detector cannot be achieved
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DR ≡ 𝑒 1207
whereas DR 2048 (11 bits);
enough then?
• ADC number around the full attenuation region would be "1" or "2" (2048/1027)
• Impossible to obtain any subtlety in image tone
What is the DR when considering
1% precision of attenuation measurements?
• DR.
120700, and
which requires 131,071 grayscale levels (or 17 bits)
8 cm70%
Fibroglandular
I0
I0/1,207
18 keV
2,048
Refer to E. D. Pisano, M. J. Yaffe, & C. M. Kuzmiak, Eds. | Digital Mammography | 2004
In this region, we have to discriminate lesion from neighboring background!
Imaging field (or field of view, FOV)
• Scanning
‒ Scanning with a 1D array detector
‒ Scanning with a multi‐line array (slot) detector
‒ Excellent scatter rejection
‒ Heavy load of x‐ray tube
• Optical coupling
‒ Lens, tapered fiber‐optic bundles
‒ Secondary quantum noise: secondary quantum sink (𝑁 𝑁 ) due to optical inefficiency
• Mosaic
‒ Fixed pattern noise, high dark current
‒ Stitching problems
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Flat‐panel detectors
Traditional x‐ray detection materials + large‐area active matrix readout structure
• X‐ray interactions with phosphors & photoconductors to generate a measurable response (detection)
• Storage of the response with a recording device
• The measurement of the stored response
Integrating the incoming signal over a finite period of time
• X‐ray fluence detector or charge‐integration detector
• Not a photon‐counting detector
Pixel = switch + sensor/storage
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Operation
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CCD transfers signal from pixel to neighboring pixel, whereas active‐matrix array transfers from the pixel directly to the readout amplifier
Energy band structures
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Resolution losses
Main intrinsic causes
• Geometrical blurring
‒ Oblique incidence of x‐ray beam
• Electron‐range blurring
‒ ~1 3 𝜇m at 10‐30 keV
‒ ~10 30 𝜇m at 50‐100 keV
• K‐fluorescence reabsorption
‒ ~50 200 𝜇m
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Independence of a‐Se thickness on MTF (due to high E‐field)
Mainly due to electron range
Mainly due to K fluorescence
Photoconductors
Typical properties
• 𝐸 ~ 2 eV; insulator with negligible free carriers at room temp.
• Sufficient lifetime of radiation‐induced carriers to reach the surface of the detection vol.
• High E‐field => fast traverse of charges => less time for lateral diffusion => high resolution
• 𝑊~3𝐸 to release an e‐h pair
Amorphous selenium
• 𝑊~ 50 eV at typ. 10 V/m due to (germinate & columnar) recombination of e‐h pairs
• Blocking contact (limited by the field strength)
• Low surface (transverse) conductivity (by introducing a high density of traps)
• a‐Se:0.5%As to prevent from crystallization; adversely, As causes large density of ℎ traps (short 𝜏 )
• a‐Se:0.5%As + 10‐20 ppm Cl (stabilized a‐Se) to reduce ℎ traps
‒ 𝜇 0.13 cm V s ; 𝜇 0.003 0.006 cm V s‒ 𝜏 50 500 𝜇s; 𝜏 100 1000 𝜇s‒ 𝑆 ~6.5 65 mm; 𝑆 ~0.3 3 mm @ 10 V/m (Schubweg, 𝑆 𝜇𝜏𝐸)
• 𝑍 = 34
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Other possible photoconductors
PbI2 & PbO have been used for nuclear radiation detectors & vidicon
TlBr has a high ionic conductivity, causing large dark current
Further candidates include CdZnTe, CdTe, & HgI2
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~𝟐. 𝟔𝑬𝒈 𝟎. 𝟕𝟓
~𝟐. 𝟐𝑬𝒈 𝟎. 𝟕𝟓
Phosphors
Conversion gain (or quantum amplification)
• Deexcitation of conduction‐band electrons through activators, emitting light (~2‐3 eV)
• In Gd2O2S:Tb, the intrinsic conversion efficiency of ~14.4% yields ~3,600 green‐light quanta (2.4 eV) for the absorption of x‐ray photon w/ 60 keV
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X‐ray screens
Highly scattering (or turbid) rather than transparent to light
• Phosphor powder (high refractive index) + binder (optically transparent)
• Trade‐off between the phosphor thickness (x‐ray interaction efficiency) & spatial resolution
‒ Thin phosphor for a high‐resolution imaging task
Factors affecting image quality
• Phosphor grain size, size distribution, bulk absorption, & surface reflectivity
• Back‐screen configuration improves the spatial resolution compared to the front‐screen one
• FPDs are configured in the front‐screen design because of the thickness (~0.7 mm) of glass substrate
Poor light‐collection eff. can limit the overall performance of the complete system
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A. Transparent w/ reflective backingB. Transparent w/ absorptive backingC. Turbid w/ reflective backingD. Turbid w/ 50% bulk absorptionE. Turbid w/ 50% absorptive backingF. Ideal
Swank's model Design C
Structured phosphors
CsI
• Needle‐like closely packed crystallites => pillar‐like (columnar) structure (~10 m, reduced density of ~80‐90% of single crystal)
• Fiber‐optic light guide (𝑛 1.78 𝑛 1): 83% internal reflection• Activator controls the emission spectrum
‒ Na: blue (~450 nm) well matched to the response of photocathodes of XRII
‒ Tl: green (~550 nm) well matched to the response of a‐Si:H layers
• Hygroscopic & mechanically weak
Fiber‐optic faceplates with scintillation impurities
• Intrinsic conversion efficiency = ~0.1 – 0.3%
Micro‐channel array filled with scintillators
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Higher light output
Noise in x‐ray converters
Mainly related to the x‐ray exposure
Further degraded by
• Lack of x‐ray absorption (quantum‐absorption inefficiency)
‒ 𝛼 𝐸 1 𝑒
• Fluctuations in the detector response to the x‐ray absorption
‒ Gain‐fluctuation noise or the Swank noise factor, 𝐼
• DQE 0 quantum absorption inefficiencygain fluctuation 𝛼 𝐼
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Wrong!
Upper limit of Swank noise considering only x‐ray interactions
Measurement of the Swank noise
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Depth‐dependent signal & noise transfer
• Known as the Lubberts effect
• Decreasing DQE as 𝑓 increases
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Fabrication
Amorphous Si by PECVD
• 𝜇 ~1 cm V s ; 𝜇 0.003 cm V s• Microcrystalline Si with an order of magnitude higher mobilities
• Polycrystalline Si with an order of magnitude higher again
• Crystalline Si: 𝜇 ~1300 cm V s ; 𝜇 ~500 cm V s
Difference in the pixel design compared to the conventional LCD process
• p+ deposition; thick i layer ( 1 μ𝑚)
Photolithography
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Indirect‐conversion pixel
Photodiode
• Light absorption coeff. of a‐Si:H (~10 to ~10 cm‐1) > that of c‐Si by an order of magnitude
‒ a‐Si:H 𝐸 ~1.7 eV whereas c‐Si 𝐸 ~1.1 eV
‒ ~0.5 μm‐thick i‐layer is sufficient to absorb most visible photons
‒ 1~2 μm is typical to reduce pixel capacitance
• Photoconductive gain mode with ohmic contacts
‒ Large dark current & low dynamic range
• Blocking contacts preventing the injection current
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Increased penetration of the i‐layer w/ increasing 𝜆
Increased absorption in the p‐layer w/ decreasing 𝜆
pin photodiode
• ITO (~50 nm + p+ (c‐Si:H alloy) (~10 20 nm + i (~1.5 μm + n+ (~10 50 nm‒ Lower dark current (~10 A cm )
Schottky photodiode
• No p+ layer
‒ Higher quantum efficiency; more compatible with the AMLCD process
‒ Higher dark current (~10 A cm )
MIS photodiode
• Insulator instead of the p+ layer; hence reverse n‐i‐p configuration
‒ Requiring the refresh cycle to remove hole build‐up (switching polarity)
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Charge capacity = 𝐶 𝑉 𝜀 𝜀 𝑉 8.85
10 Fcm 12.
5 V ~5 pC
Direct‐conversion pixel
Charge storage capacitor (insulator) + collection electrode (metal)
Require an electric field strength ~10 V μm• Additional designs protecting from the high voltage damage
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~1 pF (~10 V)
~5 pF cm‐2 => 0.002 pF for 200 m pixel (~4990 V)
Switch
Diodes
• Improvement in device yield because of the same as the sensor
• Smaller in area (~5%), increasing pixel fill factor
• Larger resistance at lower on bias voltage, causing image lag
• Large charge transient (i.e., pixel cross‐talk) during switching (due to higher capacitance)
TFT
• Structure
‒ Gate dielectric a‐Si3N4:H (~0.3 0.4 μm)/intrinsic a‐Si:H (~0.1 μm)/drain & source n+ a‐Si:H/passivation
‒ Additional metallic light shield
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• On (~+10 – 20 V)/off (~‐5 – 10 V)
‒ 𝐼 ~0.1 1 10 A per W in μm• 𝑊 ~8 128 μm• 𝐿 ~5 10 μm
‒ 𝑅 ~1 MΩ; independent of 𝑉 in contrast to the diode switch
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• Coupling capacitance due to geometrical overlaps
– Depending on the exact details of the array designs
• Gate/source‐drain; bias/gate/data lines
– Can be a significant contributor to the total data line capacitance (seen by the external electronics)
• Charge is injected into the sensor & onto the data line whenever the gate voltage is switched
– Small changes in the gate voltage along a data line can result in line‐correlated noise
– Fluctuations in the bias voltage are coupled into the data line
– Changes in the threshold voltage, 𝑉 affects the TFT resistance
Array design
Aperture: the dimension of active portion of each detector element
• Determine the spatial frequency response of the detector; MTF 𝑢 sinc 𝜋𝑎𝑢
Pitch: the sampling interval of the detector
• Nyquist frequency = 2𝑝• Aliasing
• Pre‐sampling blurring to reduce aliasing
Fill factor
• Geometrical fill factor: the fraction of the pixel area sensitive to the incoming signal
• Effective fill factor
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Build‐up potential on the insulator repels the electric lines, hence achieving the unity fill factor
Switch
• e.g. Larger switch has lower on resistance which improves the speed but lower off resistance which increases pixel leakage
Metal lines
• High resistive Cr (~12 μΩcm) vs. low resistive Al (~3 μΩcm)
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TFT
CSPAGround
Noise source intrinsic to array
kTC switching (thermal noise on capacitors)
• Switching = changing the value of a resistor (Johnson noise) = trapping charge on the capacitor
• 𝜎 2𝑘𝑇𝐶 /𝑒
• 𝜎 ~560 𝑒 if 𝐶 1 pF @ room temp.
TFT channel resistance (related to the bandwidth of the readout circuit)
• Thermal (Johnson) noise
• 1/f flicker noise
Dark current
• Shot noise
• 1/f noise
Distributed resistance/coupling capacitance of the metallic lines
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Radiation damage
Typical lifetime ~50 Gy
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Peripheral electronics
Circuitry that supplies the required voltages to the array elements, and amplifies & digitizes the signals from the pixels
Potential noise sources tending to reduce the quality of the final image
Preamplifier
• Charge‐integrating design (CSPA)
• Feedback capacitor determines the electronic gain (mV/pC)
• 𝜎 noise floor 𝑒 slope 𝐶 ~300 500 ~3 6 𝐶
‒ 𝐶 ~50 100 pF typical, so that 𝜎 ~500 2000 𝑒
Correlated‐double sampling
• To correct the kTC charge trapped on the CSPA feedback capacitor
Bias voltage/gate lines
• Line‐correlated noise to which human vision is extremely sensitive
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Example)
• Amplifier noise = 1000 𝑒• Coupling capacitance between TFT & gate/data lines = ~20 fF• 2000 pixels per a data line
• 5 μV variation in the bias voltage source during the pixel integration time (~10 50 μs)• Total coupling capacitance = ~40 pF
• Feedthrough charges coupled to the amplifier by un‐addressed 1999 pixels = ~
. ~1250 𝑒
Imply the careful design of the components & circuit boards supplying the bias voltages
Overlap capacitance b/w gate & data lines per intersection = ~0.01 0.1 pF• 10‐V gate operation gives rise to ~0.1 1 pC or ~6 10 6 10 𝑒• Fortunately, the on‐off cycle of each row of pixels sums to zero, but the amplifier should be
extremely linear!
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Image processing
To obtain a diagnostic‐quality radiograph, remove artifacts & adjust the appearance of the raw image information
Flat‐fielding correction
• Correction of variations in pixel sensitivity & offset (due to thickness & quality of layers)
• Dark‐field images for variations in pixel & electronic offsets
• Flood‐field images for variations in pixel sensitivity & electronic gain
Defects
• Thresholding to identify defects
• Difficult to set up the threshold limits for partially functional or nonlinear response pixels
• Median filter
‒ Difficult to correct the clusters of bad pixels/several adjacent bad lines
Image lag & ghosting
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Performance
Dynamic range
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𝑆100 1 mR𝑋 in mR
Imaging performance
DQE 𝑢
‒ 𝑊 𝑢 = normalized NPS
• A basis to determine the physical principles involved in the detectors
MTF 𝑢 MTF 𝑢 MTF 𝑢• MTF 𝑢 MTF 𝑢
‒ ~60% @ 𝑢
‒ Noise aliasing
‒ 𝛾 𝑢 𝑝 ; 𝑢 = the first zero freq.
• MTF 𝑢 MTF 𝑢‒ ~10% @ 𝑢
NPS 𝑢 const. NPS 𝑢 ~𝑢
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NPS shows a marked exposure dependence
DQE increases with exposure and a plateau is finally reached where further increase in exposure makes no difference to the DQE
DQE 0 DQE 0 ; DQE 𝑢 DQE 𝑢
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System performance modeling
Fill factor
• Sub‐unity 𝛾 reduces DQE(0) of the directdetector by the same factor, whereas no loss in the indirect detector due to sharing of signals from each x‐ray photon with many pixels
Aliasing
• Always present in Se detectors
• Reduction/removal by blurring prior to pixel sampling
‒ Inevitable in reduction of the high‐freq. components of signal
‒ Much more susceptible to external noise (e.g., electronic noise or secondary quantum statistics)
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DQE 𝑢 losses
• Lubberts effect
‒ Due to the depth‐dependent MTF characteristics in an x‐ray conversion layer (usually phosphors)
‒ Less or none effective in Se & fiber‐optic scintillation layer
‒ Vulnerable to aliasing
‒ Requiring anti‐aliasing blurring layer just before sampling
• K‐fluorescence reabsorption
‒ Partial energy absorption with K‐fluorescence escape or reabsorption at remote site give rise to substantial blurring & noise
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Predictive DQE
DQE 𝐮𝑞𝐺 MTF 𝐮
NPS 𝐮
𝑞 𝑎 𝛼𝛽𝜅𝜂 𝑇 𝐮 sinc 𝑎𝐮
𝑞𝑎 𝛼𝛽𝜅𝜂1𝛾 𝜅𝜂 ∑ 𝑇 𝐮 sinc 𝑎 𝐮
𝑇 𝐮 sinc 𝑎𝐮1
𝛼𝛽𝜅𝜂1𝛾 𝜅𝜂 ∑ 𝑇 𝐮 sinc 𝑎 𝐮
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DQE is dose‐independent only if the additive NNPS can be ignored
Additive noise is harmful to DQE at high frequencies where the number of secondary quanta lessens
H. K. Kim | JINST | 2011
Low‐resolution FPD
DQE 𝐮𝑇 𝐮
1𝛼𝛽𝜅𝜂
1𝛾 𝜅𝜂 𝑇 𝐮
𝛼𝐼𝑇 𝐮𝑇 𝐮
≡ 𝛼𝐼 𝐮
• Swank noise = the gap (or correction factor) between 𝛼 and DQE 0
High‐resolution FPD
DQE 𝐮sinc 𝑎𝐮
1𝛾𝛼𝛽𝜅𝜂 1 𝜅𝜂
𝛾𝛼𝐼sinc 𝑎𝐮
• Fill factor Swank noise = the gap between 𝛼 and DQE 0
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H. K. Kim | JINST | 2011
How to avoid secondary quantum sink?
𝜎𝛾𝑞𝑎 𝛼𝛽𝜅𝜂
→ 0
𝜎 ↓• Metal line coupling capacitance
• New metal line process
𝛾 ↑• Limited by the TFT design rule
• Critical to high‐resolution FPD (e.g. a‐Se)
‒ Electrostatic lens design
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H. K. Kim et al. | Int. J. Precis. Eng. Manuf. | 2008
H. K. Kim et al. | IEEE TNS | 2005
𝜎𝛾𝑞𝑎 𝛼𝛽𝜅𝜂
→ 0
𝑞𝑎 ↑ (quanta/active pixel)
• Wrong approach (∵ patient dose )
𝛼 ↑• High Z converters
• Thick converters ⇒ MTF 𝑢 ↓
𝛽 ↑• Converters having a lower W‐value
‒ e.g. CdZnTe, HgI2 10 a‐Se
𝜅𝜂 ↑• Block small leakages (optical and charge leakages)
• Optical mismatch, poor charge‐collection efficiency …
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H. K. Kim et al. | Int. J. Precis. Eng. Manuf. | 2008H. K. Kim et al. | Med. Phys. | 2012
DQE comparison of two different CsI‐based detectors
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(Taken from) I. A. Cunningham | SPIE Short Course | 2008
Good optical coupling
Less good optical coupling
Easy to maximize ?
Competition between x‐ray interaction & light collection
One possible solution:
• Flatten 𝜅 𝑧 : uniform escape regardless of 𝑧‒ 𝑡 ↓ ⇒ 𝛼 ↓‒ 𝑅 at surface ⇒ MTF ↓
𝑧
X-ray interaction
Light escape
𝑧𝐸
𝑛 photons
𝑛 ( 𝑛 ) photons
𝐸
Uncertain! ⇒ 𝐼
𝐸
𝑛𝑛
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J. Tanguay et al. | Med. Phys. | 2010
R. K. Swank | JAP | 1973
Swank noise effect
0 1 2 3 4 5 6 70.0
0.2
0.4
0.6
0.8
1.0
u (mm-1)
MT
F
(a)0 1 2 3 4 5 6 7
10-7
10-6
10-5
10-4
u (mm-1)
NN
PS
(m
m2 )
(b)0 1 2 3 4 5 6 7
0.0
0.2
0.4
0.6
0.8
1.0
u (mm-1)
DQ
E
(Konstantinidis et al)
(c)
RadEye100
CsI (Monnin et al)
CsI (Rivetti et al)
a-Se (Monnin et al)
Dexela
J. C. Han et al. | JKPS | 2014
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Lubberts effect
DQE 𝐮 will be independent of 𝑢 if MTF 𝐮; 𝑧 is not dependent upon a depth 𝑧 because NPS 𝐮 ~MTF 𝐮
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G. Lubberts | JOSA | 1968 A. Badano et al. | Med. Phys. | 2004
PSF MTFX‐rays
Photodiode array
𝐿 𝑢MTF 𝑢
NPS 𝑢 /NPS 0where MTF 𝑢 d𝑧 𝑤 𝑧 MTF 𝑢; 𝑧
Direct x‐ray interaction
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S. Yun et al. | IEEE TNS | 2009H. K. Kim | APL | 2006
Optical gap
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Since noise due to direct x‐ray interactions is white in the spatial‐frequency domain, it is harmful to DQE at high‐spatial frequencies where the number of secondary quanta lessens
Therefore, optical gaps between the scintillator and the photodiode array further degradethe DQE performance by enhancing the direct interaction noise (for the less number of secondary quanta)
A. Koch (Thales Electron Devices) | Proc. SPIE | 2004
Substrate effect
Fluorescence x‐rays from glass substrates would cause a low‐frequency drop in MTF but their effect on NPS is modest; Ultimately, they cause a drop in DQE (a‐Se) of 10‐20%
K‐fluorescence backscatter from heavy elements in the glass substrates causes a low‐frequency drop in MTF of the Gd2O2S:Tb + a‐Si:H panel design
• Corning 7059F (~25% BaO), note: Ba, 𝐸 37 keV
• Corning 1737F (~10% BaO)
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M. J. Flynn et al. | Proc. SPIE | 1998J. Yorkston et al. | Proc. SPIE | 1998
However, there would be an increase (~22%) in zero‐frequency DQE as a result of the additional signal from the backscatter
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A. R. Rubinsky et al. | Proc. SPIE | 2006
Energies of 𝐾 fluorescence
• Gd: ~43 keV
• Cs: ~31 keV
• I: ~29 keV
Clinical applications
Chest radiography
• Difficult because of necessity of large dynamic range considering very radio‐lucent (lung fields) & very radio‐opaque (mediastinum) regions
• Possible solution with very highly penetrating x‐ray beams (130–150 kVp)
‒ Higher‐E x rays effectively reduce the contrast range of the image
• Dual‐energy imaging to isolate the bony details (i.e., the spine & ribs) from the soft tissues
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Fluoroscopy
• The most demanding potential application for FPDs
• Must be x‐ray quantum‐limited even at extremely low exposure levels
• XRII
‒ Bulky nature, veiling glare (x‐ray & light scatter w/i the XRII), geometric distortion
• FPDs
‒ Reduction of noise to permit quantum‐noise limited operation at the low end of fluoroscopic rates (i.e., 0.1 μR s )
‒ Reduction of the image carry over or lag
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Contrast ratio (veiling glare) =
Mammography
• X‐ray projection to visualize soft‐tissue contrast
• Breast compression to equalize the x‐ray path length to the point that the whole breast can be visualized
• Film followed by screen to ensure the highest possible image resolution (back‐screen design)
• FPDs with high dynamic range is compatible to visualize dense breasts
Tomography
• Blurring out the shadows of superimposed structures to allow better isolation of the structures of interest
• Digital tomosynthesis
• Volumetric CT
‒ Improved z‐direction resolution
‒ Problem with the increased level of scatter
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Portal imaging
• To confirm the correct positioning of the patient in the output portal of the therapy machine
• Not possible to see soft‐tissue contrast & difficult to visualize bones because of inadequate contrast/resolution
• Video‐based systems
‒ Secondary quantum sink where each x‐ray photon is represented by less than a single light photon
‒ Dominance of electronic noise
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Future prospects
Increased numbers of active elements per pixel, allowing an amplifier at every pixel
Integrated readout electronics to make an x‐ray imager on glass
Increased x‐ray to charge conversion gain (lower W or avalanche gain)
More sophisticated switching structures with reduced coupling capacitance, lower leakage current, smaller physical area, & more robust operating characteristics
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