Flat X-ray Detectors for Medical Imaging

Michael Overdick Philips Research Laboratories, Aachen, Germany

IWORID 2002, Amsterdam, 11 Sept. 2002

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 2

Outline

• Flat Detector Technology– Overview

– Key Components

• FD Performance

• Imaging Examples

• History & Future

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 3

Flat Detector Technology

Scintillator

AddressingReadout

ADC

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 4

Static detector: Digital Diagnost

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 5

Dynamic detector: Integris Allura

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 6

Scintillator: CsI:Tl needle crystals

• Thickness 550µm

good X-ray absorption

• Needles act as light-guides

sharp MTF

• CsI:Tl emits green light

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 7

Photodiode array: a-Si technology

• Same technology as used in active matrix LCDs (TFT displays)

• a-Si photodiodes: low dark currents,high sensitivityfor green light

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 8

Low noise readout electronics

• Driver and readout chips on flex modules

• Allowing very compact designs

20 cm

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 9

Main noise sources in Flat X-ray Detectors

Commonelectrode

CSA

TFTswitch

Photo-diode

Gateline

Columnread-out

line

Switching noise (kTC-noise) 750 e-

Amplifier noise 800 e-

X-ray quantum noise!

“Electronic noise” 1300 e-

other (TFT etc.) 600 e-

Shot noise from the photodiode (incl. refresh light)

300 e-

approx. ENC

Pixel circuit ofdynamic FD

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 10

Integris AlluraFlat Dynamic Detectorfor Cardio

Scintillator Photodiode array Refresh light

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 11

Technical Data

Digital Diagnost Integris Allura FD

Detection static static + dynamic(up to 30 frames/s)

Field of view 43 cm x 43 cm 18 cm x 18 cm

Number of pixels 3k x 3k 1k x 1k

Pixel size 143 µm x 143 µm 184 µm x 184 µm

DQE(0) 60 % 75 %

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 12

Detective Quantum Efficiency (DQE) for a Flat Dynamic Detector

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.5 1 1.5 2 2.5

frequency [mm-1]

DQ

E

1000nGy

100nGy

10nGy

5nGy

From: F. Busse et al., Proc. SPIE 4320 (2001) 287-298

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 13

DQE for a Static Flat Detector

0 1 2 3 4

lp/mm

0

0.2

0.4

0.6

0.8DQE

Screen-film (400)Fuji ST type VFlat-panel detector

Source: PMS Hamburg

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 14

Static FD performance vs. Screen/Film systems

• High DQE• Fully digital• Simple handling (no cassettes or films)• Large dynamic range

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 15

X-ray film: Dynamic range

Over-exposedUnder-exposed

8

mAs

0.5

mAs

2

mAs

4

mAs

16

mAs

32 mAs 63

mAs

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 16

Flat detector: Dynamic range

typical usage

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 17

Dynamic FD performancevs. Image intensifiers (II-TV systems)• Larger dynamic range• Size & weight• Undistorted images• Immune to magnetic fields• Strongly reduced:

Veiling glare and fixed pattern noise

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 18

Imaging examplesDynamic FD: Heart arteries with contrast agent

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 19

Imaging examplesStatic FD: Shoulder and finger

1 cm

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 20

History

1989 Start of FD Research Project at Philips1993 8”x8” FD Prototype1997 Joint venture with Thomson and Siemens (Trixell)2000 Introduction of static FDs by Trixell and GE Medical2001 Introduction of dynamic FDs for Cardio application (GE Medical and Trixell)

Apart from CsI:Tl based FDs also Selenium based FDs

are availble (e.g. from Anrad/Toshiba), mainly used for static applications (e.g. mammography).

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 21

Future

• Flat Detectors for further applications and with different sizes will enter the market.

• FDs will gradually replace II-TV systems.

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 22

And what about CMOS Pixels?

CMOS Pixel Electronics+ Amplifier in each pixel+ Additional functionality

But:• Cost and feasibility of large area CMOS coverage!• Please carefully check against FD performance

(as the new “gold standard”)

Counting vs. Integrating:• Nice topic for an extra talk!• Observe the high maximal count rates ( 109 counts/s

mm²)

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 23

Conclusions

• Flat Detectors are out now in the market(mainly static detectors and cardio detectors)

• FD technology offers various benefits as compared to conventional systems.

• Scintillator, a-Si technology and low noise electronics are the key FD ingredients.

• New X-ray detector developments should use FD performance as their new benchmark.

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 24

Acknowledgements

• Philips Medical Systems (Best, Hamburg and North America)

• Trixell (Moirans, France)

• Colleagues at Philips Research Aachen and Redhill

PFL-Aachen, M. Overdick, 11 Sept 2002, IWORID 2002 26

Residual signalsDependence on the Refresh Light Duration

10-1

100

101

10-4

10-3

10-2

Time in s after exposure

Nor

mal

ized

res

idua

l sig

nal @

30µ

Gy

RLD=0µs g= -0.92 RLD=10µs g= -1.06RLD=50µs g= -1.24

10-1

100

101

10-4

10-3

10-2

Nor

mal

ized

res

idua

l sig

nal @

21x

30µ

Gy

Time in s after last exposure

RLD=0µs g= -0.74 RLD=10µs g= -0.77 RLD=50µs g= -0.83 RLD=250µs g= -0.89

Stronger refresh light accelerates decay of Res. Signals

(M. Overdick et al., Proc. SPIE 4320 (2001) 47-58)