Digital Detectors for Industrial Applications-Nityanand Gopalika

Digital Radiography:a-Si Array Detectors for Industrial

Applications

Nityanand Gopalika, D. Mishra, V. Manoharan & Greg Mohr*

Industrial Imaging and Modeling LaboratoryJohn F Welch Technology Center

Bangalore

*GE Inspection TechnologiesOne Neumann Way MD K207

Cincinnati, OH 45215

Presentation Outline:

• Technology Development• Benefits of Digital Radiography• Quantifying Image Quality• Image Quality Metric for Digital Systems• Comparison of Imaging Devices: CCD, CMOS

and a-Si• Performance Study of Flat Panels

3 /Industrial Imaging and Modeling Laboratory

Evolution of Direct Digital X-ray Detectors

Technology Development

Film radiography

Image intensifiers

Computed radiography

CCD technology

Direct digital radiography

Productivity Resolution

NDT World

SizeCost

4 /Industrial Imaging and Modeling LaboratoryProductivity and cost benefits

Benefits of Digital RadiographyProductivity• Faster response• Elimination of chemical processing• Automated inspection• Elimination of retakes

Cost• Elimination film and consumables• ROI in two to three years

Quality• Image processing and analysis• Reduces operators fatigue• Consistency

Advanced Application• Volumetric CT for High Throughput


Measure of Image Quality

Quantifying Image Quality

Decreasing Noise

Incr

easi

ng C

ontr

ast

Contrast-to-Noise Ratio

Perceived Image Quality

What is a good Physical Measure of Image Quality?


MTF: Same Response to Signal and Noise

Image Quality Metric for Digital Systems


Higher MTF Does Not Mean Better Imaging System

Contrast Limiting Spatial Resolution (LSR), MTF measured at high contrast

• bar patterns » 100% input contrast MTF indicates fraction of signal that will be seen in image.

Noise MTF measured under noiseless conditions. MTF transfers noise in addition to signal. Image noise can interfere with object detectability.

MTF Limitations



High MTF; But Poor Performance

SNR = 1

Higher limiting resolution of smallerpixels may not provide betterdetectability in noisy images.

MTF: High Middle Low

MTFHighMiddleLow



MTF is One Metric; But Not Enough

Traditional gauge used for quantifying image quality cannot be used as a stand alone metric.

• Quantum and electronic noise are unavoidable in digital imaging chain.

• SNR can vary widely across systems

• High SNR is key to better inspection power

• To increase SNR often the only way is to increase radiation dose,

unacceptable trade-off

• Achieving high SNR at lower dose: better imaging system



DQE = SNR2 at detector outputSNR2 at detector input

SNR = signal-to-noise ratio

DQE: Detective Quantum Efficiency

Measure of SNR transmittance



Less Dose and Better Image

Input SNR2 proportional to radiation dose

DQEImage QualityInput Radiation Dose

• Traditional measures such as MTF, LSR are not sufficient to characterize detector performance

• Noise is a limiting factor for detectability, image processing, and advanced applications

Doubling DQE means:

• Same output SNR (“image quality”) at half the dose • 40% improvement in SNR at same dose


α


High DQE at Lower Dose

“Object”“Improved”DQE = 0.5

“Standard”DQE = 0.25

SNR = 5 SNR = 3.5 SNR = 2.5



High DQE Better Detectability

Film GE Detector



Detector Design Keeping DQE in Mind



DQE

One Image Quality

Measure

MTFHigh Resolution(for small object

detection)

Measurements and Requirements

Low Noise(for clear visualization)

Noise (NPS)

Efficient X-Ray Conversion (for minimum exposure)

Signal (S)

The Detector Properties

Pixel SizeSampling, Fill

Factor, Aliasing

Scintillator/CouplingCsI, Lanex, Se

lens/Direct

PhotodetectoraSi, CCD,

CMOS

ReadoutElectronic Noise

NPSMTFS

QDQE

221 ⋅=

Detector Design for High DQE

16 /Industrial Imaging and Modeling LaboratoryFlat Panel Technology Variation

Flat Panel Technology

Direct Conversion (Se)

Photons

Selenium

Digital Data

Electrons

Read Out Electronics

Photons

Cesium Iodide (CsI)

Light

Electrons

Read Out Electronics

Amorphous Silicon Panel

Digital Data

Indirect Conversion (CsI)


CsI vs. SeCesium Iodide

• Very high DQE; potential for high image quality at low dose

• Fluoro capable• Advanced application capable• Mature technology: 25-year history

with Image Intensifiers

Selenium

• Direct conversion of X-Ray into electrical signals

• Currently not capable of fluoro • Low X-Ray absorption• High sensitivity to temperature

Again Keep DQE in Mind!!


Point Spread Function for Different Detector Types

Electrons

Image Intensifiers CSI Flat panels Se Flat panels

MTF is One Part of the Story, DQE is the Other BIG Part


Photons

Scintillator

LightFiber Optic Taper

Electrons

CCD

Amorphous Silicon• Potentially high image quality at low

dose (high DQE)• Active Research on New Applications • Designed for X-Ray from the start• Compact packaging• Very high development cost

CCD• CCDs are easily available• Low development costs• “Transition” technology to

flat panel • High CCD cost• Tiling and design complexity

CCD Technology


All will convert visible energies into an electronic charge

Imager Dimensions• CCDs: 10 – 60-mm on a side• CMOS: 50-mm on a side• a-Si: 200 – 410-mm on a side

Size governed by silicon process• CCDs and CMOS – 6” wafers• Multiple chips/wafer – yield• a-Si – Large area deposition/glass

Pixel Dimensions• CCDs: 9 – 25 microns• CMOS: 40 – 50 microns• a-Si: 100 – 400 microns

• Pixel size governed by architecture

Silicon Imaging Devices, CCD, CMOS & a-Si


Silicon Device Coupling deviceor Method

Phosphor orPhotoconductor

CCD or CMOS

Fiber optic coupler

Requires 10X more Exposure

Fiber optic scintillator and/or phosphor

ShieldingGlass

LensCooled CCD Camera

Requires 100X more Exposure

Silicon Imaging Devices, CCD, CMOS & a-Si


3 Types of GE Digital X-Ray Panels All feature high efficiency & fast 14-bit readoutHighest resolution (DXR-500)

• 7” x 9” (19 cm x 23 cm) @ 100 micron

• Over 20% MTF at 5 lp/mmHighest efficiency (DXR-250)

• 16” x 16” (41 cm x 41 cm) @ 200 micron

Fastest Imaging (DXR-250RT)• Up to 30 Hz• 8” x 8” (20 cm x 20 cm) @ 200

micron

GE Digital X-Ray Detectors

Panels Optimized for Different Applications


Detector Characteristics Suitable for Industrial Applications

Productivity:• High Speed (mR vs. R)• Minimized Rework• High Latitude Coverage

Advantages• Less radiation field• Micro focus (Faster response

enables high definition radiography)

Useful exposure rangeMin and Max exp ratio 2

Comparative study

Typical Exposure ratio requirements

Material LattitudeX-ray tube potential

Subject contrast

Ti 3 to 20 mm 160 kV 12.7Steel 1 to 10 mm 160 kV 7.28

Characteristics DXR-500 Digital Detector Industrial X-ray film ( Medium speed)

Speed In the order of few mR to get signal level of 12000

1.3 R to get Optical density of 2

Dynamic range Useful minimum to maximum exposure ratio 12

Useful minimum to maximum exposure ratio 2

DXR-500 DR system

02000400060008000

100001200014000

0 5 10 15 20 25 30 35

Relative exposure

Sign

al(A

DC)

Performance Study: Radiation Exposure


Dynamic range

• High latitude coverage• ~ 10 times > image-

intensifier• No blooming or

saturation• Window leveling • No Lead masking

Wide dynamic range enables- high latitude imaging

2& 3 window level adjusted

Ti step wedge image 2 –20 mm

1

2

3


Artifacts

Image Intensifier- distortionFlat panel- no distortion

> No distortion > No blooming> Uniform sensitivity

over entire area> Brightness uniformity Brightness uniformity

Source:GE Health care Source:GE Health care

Source:GE Health care

Flat panelXII


Performance study-Spatial resolution

Observation:• System can be designed to match with film MTF.• Digital detector with mini/micro focal tube

outperforms film radiography with large focal spots.• DQE of DXR-500 is comparatively good.

System Design for Meeting Requirements

FS = 1.8mmFS = 0.4 mm

FS = 1.8 mmFS = 0.4 mm


Detective quantum efficiencySource: GE healthcare

Better defect detectability in useful spatial resolution range

DQE comparison- XII vs. DR


Performance study-Noise response• Poisson distributed noise

• Noise Quantum limited

• Averaging of frames reduces noise

Effect of no. of X-ray photons

Effect of no. of frames

Quantum Limited Noise Performance

1 mAs

5 mAs

5 frames

10 frames

20 frames


Performance study-IQI sensitivity

Mat. – TiThick. – 25mmKVp – 120mAs – 1.0FS – 0.4 mm

Mat. – TiThick. – 10mm KVp – 120mAs – 1.0FS – 0.4 mm

2-1T sensitivity over range of thickness

Mat. – SSThick. – 10mm KVp– 140mAs – 1.0FS – 0.4 mm

Mat. – AlThick. – 40 mm KVp – 120mAs – 1.0FS – 0.4 mm


Performance study-Imaging

KVp – 125

mAs – 1.0

FS – 0.4 mm

Mat. – CSI

Porosity2-2T

Sensitivity

Lack of penetration

KVp – 125

mAs – 1.0

FS – 0.4 mm

Mat. – CS

Range of applications with 2-1T sensitivity


Object – IC

KVp – 70

mAs – .5

FS – 10 microns

Mag. – 50X

Object – IC

KVp – 70

mAs – .5

FS – 10 microns

Mag. – 50X

Object –ceramics

KVp – 70

mAs – .5

FS – 10 microns

Mag. – 50X

Enable high definition radiography

Performance study-Imaging


Welding defects-Lack of penetration and spatter

Material: CS

Plate, 12 mm thk

SW SI – Offset

FS-0.4 mm

SDD-700 mm

KVp: 130, 2 mAs

Filter: Cu –0.4 mm


• System can be designed to meet image quality requirements

• Quantum limited noise performance• Faster response and wide dynamic range • Real time (fluoro)

• Range of applications with 2-1T sensitivity

• Enables high definition radiography • Advanced image processing for image

optimization

Summary

Digital Detectors for Industrial Applications-Nityanand Gopalika

Technology

digital imaging

high image quality

image noise

output snr image quality

years quality image

digital systems dqe

high mtf

noise mtf