x‐ray detector physics - pusan national...

X‐ray Detector Physics

Ho Kyung [email protected]

Pusan National University

Detector Physics

Introduction

Review

Energy states in atoms are discrete (quantum mechanics)

Unstable nuclei (atoms) become stable by emitting radiations

Radiation can ionize atoms, resulting in ion pairs (ionizing radiation)

Fast electron interaction with target atoms produces bremsstrahlung & characteristic x rays

The number of x‐ray photons & their energy are controlled by the tube current & voltage, respectively

X‐ray photons interact with matter by the photoelectric absorption (𝜇 ) & Compton scattering (𝜇 ) processes

The interaction probability is characterized by the linear attenuation coefficient

• 𝜇 𝐸; 𝑍, 𝜌 𝜇 𝐸; 𝑍, 𝜌 𝜇 𝐸; 𝑍, 𝜌

As a result of interaction, the number & intensity of x‐ray photons are exponentiallyattenuated with material thickness (𝑡) or area density (𝜌𝑡)

• 𝑁 𝑡 𝑁 0 𝑒 𝑁 0 𝑒

Exposure describes x‐ & ‐ray fields in terms of their ability to ionize air, while the absorbed dose describes the energy imparted to matter by all kinds of ionization radiations

2

X‐ray imaging chain

generationinteraction

detection display

3

How to detect x‐ray photons?

Energy band diagram

SiSi Si

Si

Si

e-e-

e-

e-

e-

e-

e- e-

e-

e-e-

e-

e-

e-

e-e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-e-

e-

+14 e-

e-

e-

e-

Intrinsic semiconductor

• Elements from group IV of the periodic table

• 4 valence electrons in the outer (valence shell)

• Crystal structure by covalent bonds

• Intrinsic (carrier) concentration: number of free electrons

– 𝑛 1.5 10 electrons/cm3

4

e- e-

e- e-e- e- e- e-e-e-

e- e-e-

Conduction band

Energy

Valence band

Second band (shell 2)

First band (shell 1)

Energy gap

Energy gap

Energy gap

Energy band diagram

• Conduction band, valence band, forbidden gap (or energy gap)

• Electron conduction; recombination

e-

Energy

e- Free electron

Hole

Electron‐hole pair

5

• Hole conduction

e-Si

e-

e-

Si Si

e-

e-

SiSi

e-

e-

Si

e-

e-

Si

e-

e-

Si

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

6

Conductors, semiconductors, insulators

Valence band

Energy

Energy gap

Conduction band

Valence band

Energy gap

Conduction band

Energy

Valence band

Overlap

Conduction band

Energy

Insulators Semiconductors Conductors

7

Phosphors (scintillators)

Activator site

Valence band

Forbidden gap

Conduction band

EnergyEnergy

8

Semiconductors (photoconductors)

EnergyActivator site

Valence band

Forbidden gap

Conduction band

• Radiation signal (or energy) ∝ ∆𝑄 ∝ ∆𝐼 ∝ ∆𝑉 ∝ digital signal

9

W-value

Outline

Fundamental principles of digital x‐ray imaging detectors

Analog x‐ray imaging systems

• Film/screen system

• X‐ray image intensifier (XRII)

Digital x‐ray imaging detectors

• Operation principles

• Flat‐panel detectors

‒ Charge‐coupled device (CCD)

‒ Complementary metal‐oxide‐semiconductor (CMOS)

‒ Hydrogenated amorphous silicon thin‐film transistor (a‐Si:H TFT)

10

Fundamental principle of x‐ray detection

Detection material: x‐ray photon energy (light ) charge (electron‐ion or electron‐hole pairs)

Readout electronics: charge voltage (charge‐sensitive preamplifier)

Frame grabber: voltage digital bit (ADC + memory)

11

Detection material

𝐸

𝜇 𝐸; 𝑍, 𝜌

𝑥

Detection material

• Gas: 𝑒 𝑖𝑜𝑛 pairs

• Liquid (mostly scintillator, hence light photons)

• Solid: 𝑒 ℎ pairs

Detection (quantum) efficiency 1 𝑒 Good detection material

• Higher detection efficiency

‒ Higher signal & less patient dose

• Dominant photoelectric absorption events

𝐸 𝐸 𝐸

𝐸′ 𝐸

𝐸 𝐸′Photoelectric abs.

Compton scatter.

Escape





12

𝑒ℎ

𝑒ℎ

𝑡

𝑖 𝑡

𝑡

𝑣 𝑡 W‐value: the energy required to create a single 𝑒

𝑖𝑜𝑛 or 𝑒 ℎ pair

• Similarly used for scintillation photons

Driftmotion induces current at electrodes

• Schubweg 𝐿 𝜇𝜏𝐹 (mobility lifetime field strength)

Electric field

𝑣𝑄𝐶

𝑖 𝑡 d𝑡 𝑄

𝑡𝑖 𝑡





13

𝑡

𝑣 𝑡

Sample & hold

Voltage Bit

0 V

10 V

0

1023

Bit signal =

# 𝜂#

#𝐺

#𝐺

• Signal ∝ 𝐸





14

MUX

Pixel (picture element)

Operation modes

Energy (or charge)‐integrating detectors

• Measure the spectrum as the mean energy

• 200.1 + 400.35 + 600.41 + 800.1 + 1000.04 = 52.7 keV• Most x‐ray imaging detectors

Photon counting detectors

• Measure the number of incident photon as a function of energy (bins)

• (Ideal PCD) reproduces the incident spectrum

• All the ‐ray (imaging) detectors including recent x‐ray detectors for multi‐energy imaging

15

𝐸 (keV)

𝑁 𝐸

10 (0.04)25 (20.10)

85 (0.35)100 (0.41)

604020 80 100

16

Film radiography

Film

• Containing an emulsion with silver halide crystals (e.g., AgBr)

• Absorbed optical photons by the silver halide grains, and then metalized (dark)

• Precipitated metallic silver when developed

• Negative image

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• Graininess

‒ The image derived from the silver crystals is not continuous but grainy

‒ The larger the grains, the faster the film becomes dark (amount of photons needed to change a grain into metallic silver upon development is independent of the grain size)

• Speed

‒ Inversely proportional to the amount of light needed to produce a given amount of metallic silver on development

‒ Mainly determined by the silver halide grain size

‒ The larger grain size the higher the speed

‒ How many x‐ray photons are needed to produce a certain density on the film

‒ Speed in the screen‐film system: Reflector improves the speed

• Contrast

– Plot of the optical density 𝐷 vs. the logarithm of the exposure 𝐸 (called the sensitometric curve)

– 𝐷 log

– A larger slope implies a higher contrast at the cost of a smaller useful exposure range

– gamma: the maximal slope

• Resolution

– Depending on its grain size and the light scattering properties

18

A. A light photon removes the outermost electron from a bromide anion. The bromine atom (now uncharged) diffuses out the crystal. The liberated electron wanders through the crystal and is trapped at the sensitivity speck.

B. The speck is now negatively charged.

C. It draws an interstitial silver cation to itself.

D. The electron on the sensitivity speck neutralizes the charge of the silver ion, and the resulting silver atom is deposited there.

E. Another light photon causes the process to repeat. The deposition of 10 or so silver atoms at the sensitivity speck transforms it into a latent image center. A crystal with a latent image center will be transformed into a fleck of pure silver during the development process.

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Intensifying screen

Very inefficient photographic film for capturing x rays

• Only 2% of the incoming x rays contributes to the output image (quantum absorption efficiency)

• Would yield prohibitively large patient dose

• Typically, placed the film b/w two intensifying screens

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Screen

• Containing phosphors (Gd2O2S:Tb) with a high quantum absorption efficiency

• Absorbing most of the x‐ray photons

• 25% of QAE or QE of each screen instead of 2% for film

• Converting x rays into visible light (which is scattered in all directions, resulting in image blur)

• Fluorescence

‒ Prompt emission & stop of light and used in intensifying screens

‒ CaWO4, Gd2O2S:Tb, CsI:Tl

• Phosphorescence (or afterglow)

‒ Continuation of light emission (> 10‐8 s)

‒ Undesirable because it causes “ghost” images and image lag (and fogging in film)

21

22

no intensifying screen128 mAs, >12 lp/mm,

fine screen10 mAs, >7 lp/mm,

fast screen1.33 mAs, <5 lp/mm,

Image intensifier

Working principle

• Conversion of x rays into visible light by an input phosphor (or fluorescent) screen

• Emission of electrons from a photocathode hit by light

• Accelerated the ejected electrons by a potential difference b/w the cathode and the output

• Focused electron beam to the output phosphor screen by electrostatic or magnetic focusing

• Captured visible light from the phosphor screen by a camera

Capable of producing dynamic image sequences in real time at video rate (a process known as fluoroscopy)

Image degradation

• Less spatial resolution rather than that of a film‐screen system (because of the limited camera resolution)

• Increased noise due to the additional conversions (light electrons light)

• Geometric distortion, called pin‐cushion distortion, toward the borders of the image

23

24J. A. Seibert | Pediatr. Radiol. | 2006

Electronic focusing allows:

• Large FOV

‒ Large coverage

‒ Higher gain due to minification

‒ Pin‐cushion distortion in the periphery of the image (caused by mapping the spherical input phosphor electron image onto the planar output phosphor)

• Small FOV

‒ Magnification

‒ High spatial resolution

‒ Lower gain (or higher patient dose)

25J. A. Seibert | Pediatr. Radiol. | 2006

PACS

Picture archiving & communications systems

• System for the storage, transfer, & display of radiological images

• Able to include the teleradiology that transmits images for viewing at sites remote from where they are acquired

• Exchange information with:

‒ HIS (hospital information system)

‒ RIS (radiology information system)

‒ EMR (electronic medical record) system

DICOM (digital imaging & communications in medicine)

• Standards to facilitate the transfer of medical images & related information (patients, images, & studies)

26

Analog vs. Digital

27

Mrs. Roentgen, 22 Dec. 1895Taken from I. A. Cunningham’s Slides Me, 22 Sept. 2009

Digital images

Digitization = sampling (space) + quantization (intensity)

28

Space

Intensity

Pixel pitch

Sampling

• The conversion from a continuous function to a discrete function retaining only the values at the grid points

29

17921792 896896 448448 224224

141428285656112112

128 larger pixel

Quantization

• The conversion from analog samples to discrete‐value samples

30

8 bits 7 bits 6 bits 5 bits

1 bit2 bits3 bits4 bits

Digital images

• A set of possible (achromatic) gray levels or (chromatic) colors in a rectangular grid‐point (or pixel) array

• Sampling and quantization (integer)

• Dynamic range: the set of possible gray levels

• Contouring: an artificial looking height map

• How many gray values are needed to produce a continuous‐looking image?

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8 bits/pixel 4 bits/pixel

Why such a long time gap to digitalization?

The size does matter!

• Limited size of available imagers (e.g., CCD, CMOS photodiode arrays)

• Availability in large size wafers

• Marginable production yield in the wafer‐based fabrication process

Radiation hardness of silicon or other materials

17”

14” 17”

17”

CCD, LBNL Taken picture from M. J. Flynn’s Lecture Slides

32

Computed radiography

CR based on photostimulable phosphors, introduced in the early 1980s by the Fuji Photo Film Co., has been used until now (and still after)

33

P. Suetens | Fundamentals of Medical Imaging |Cambridge Univ. Press | 2009R. Schaetzing | RSNA Categorical Course | 2003

Storage phosphors

Storage phosphors

• Also called photostimulable phosphors

• Photo‐stimulated luminescence

‒ An extreme case of phosphorescence

‒ Released the temporarily stored energy in a form of light by stimulation (laser)

34

Computed radiography

• Use of the storage phosphor

• Trapped the excited electrons by electron traps (impurities in the scintillator)

• (it takes 8 h to decrease the stored energy by ~25%)

• Extraction of stored energy or latent image by pixelwise scanning with a laser beam

• Released visible light by the de‐excitation of electrons

• Captured light by an optic array and transmitted to a photomultiplier

• Converted analog electrical signal into a digital bit stream by an A/D converter

• Erased any residual image by a strong light source

35

Some tricks for large‐area imaging

Utilization of the conventional, small‐size photo‐imagers (e.g., CCD, CMOS)

• With various mechanical motions;

‒ May provide a better image quality due to the scatter rejections

‒ But, can we finish scanning within a single heart beat, and handle the heat load

• By coupling with optics;

• But, very special caution should be devoted when designing optics systems

• e.g. 𝜂 = 1.5% (𝜏 = 0.8, M = 0.5, & F = 1.2)

22

222

2

44)1(4M

F

M

MFM

M

M. Mahesh | RadioGraphics | 2004

36

M. J. Yaffe & J. A. Rowlands | PMB | 1997

Secondary quantum noise

Scanning radiography: panoramic radiography

37

Imaging Dynamic Co., Ltd., Canada

Lens‐coupled DR system

CCD

X-ray

LightLens

Mirror

38

• By butting small‐size imagers (mosaic method);

‒ But, should keep the butting‐gap be as small as a pixel pitch

‒ Needed additional image processing techniques for interpolation between gaps and different signal responses between the detector modules

Image courtesy of Dr. T Achterkirchen, Rad-icon Imag. Corp. Image courtesy of Vatech & E-Woo

A pixel

< 50 m

39

• By stitching small‐size imaging chips (or reticles) in wafer‐process level;

– Ideally, there are no gaps between reticles

– But, also needed an additional image processing technique for different characteristics between reticles due to the nonuniform fabrication process over large area

Single‐wafer (12”) detector

Image courtesy of Rayence (Vatech)SK Heo et al. | Proc. SPIE | 2011

28 kVp, 100 mAs

24.1 cm

17.1 cm

70 m

40

Breakthrough

Large‐area flat‐panel detectors (FPDs)

• Motivated by large‐area AMLCDs and initialized in the mid‐1980s

• Realization of 2D pixel arrays (TFT alone or a combination of TFT plus photodiode in a pixel) on large‐area glass substrate based on amorphous silicon process

‒ Lower fabrication cost compared to the crystalline counterpart

‒ Better radiation hardness

‒ But, worse electrical properties & a high density of charge traps, which may result in image lag & ghosting

Image Courtesy ofSamsung Electronics Co. & Vatech, Co., Ltd.

Scintillator to convertx‐ray into light

41

Amorphous materials

Availability in large area in a morphological form of short‐range order/long‐range disorder

Lower fabrication cost compared to crystalline counterparts

Better radiation hardness than crystalline counterparts

Worse electrical properties than crystalline counterparts

Charge trapping through dangling bonds

• Hydrogenated amorphous silicon, a‐Si:H

SiHydrogenated

Uncoupled

Void

42

X‐ray detection materials

Desirable properties

• Absorb as many x‐ray photons as possible

• Provide accurate measure of how many x‐ray photons interacted

• Maintain information on the spatial location of point of interaction

• Manufacturable over large physical areas

Two types of materials

• Phosphor materials that convert x‐ray into light

• Photoconductor materials that convert x‐ray into electrical charge

43

Particle‐in‐binder or powdered phosphors

• Gd2O2S:Tb

• Easy to manufacture

• Very physically robust

• Light scattering

• Increasing thickness: reducing resolution & increasing noise (Lubberts effect)

• Depth‐dependent escape efficiency: increasing noise (Swank noise)

Structured phosphors

• CsI:Tl

• Light guiding

• MG = ~ 200 𝜇m• DX = ~500–600 𝜇m

44

4 𝜇m

Phosphor

Binder Air gap

Photoconductors

• High voltage to allow charge collection

‒ Constraining lateral diffusion of released charges

‒ Near perfect spatial resolution (almost independent of thickness)

‒ Low Swank noise (due to high collection efficiency)

• Amorphous selenium (Z = 34)

‒ Most suitable to mammographic applications

‒ MG = ~ 200 𝜇m‒ DX = ~500–1000 𝜇m

45

e h HV

46Courtesy J. Yorkston | Carestream Health

Lateral chest (120 kVp)


500 𝜇m CsI:Tl 500 𝜇m a‐Se

(Digital) X‐ray signal readout devices/methods

CCD (charge‐coupled device)

• Forms images from visible light

• Integrated circuit made of crystalline Si

‒ Limited size determined by the dimensions of x‐Si wafers

‒ Smaller pixel (< 20 𝜇m) to achieve charge transfer efficiency of 99.99% to keep additive electronic noise low

50

51

TFT (thin‐film transistor) passive pixel readout

• Based on amorphous Si process

• Operation

‒ Integration: storing electronic charges produced by light or x‐ray photons

‒ Readout: transferring the charges to the FB capacitor on the preamplifier

‒ Reset: closing the preamplifier switch

52

• High electronic noise due to large dataline resistance & capacitance

e.g., 𝜎 𝜎 𝛾 𝐶 250 𝑒 15 50 pF 1000 𝑒

‒ To minimize this noise, the input (pixel) & output (CSA) circuits should be separated by adding an additional amplifier in the pixel

53

CMOS (complementary metal‐oxide‐semiconductor) active pixel sensor

• Based on the x‐Si process

• RAM chips w/ built‐in photo‐sensitive detectors, storage capacitors, & active readout electronics

• Lower electronic noise than PPS but higher than CCD

• Operation

‒ Reset: 𝑇 = ON, 𝑇 = OFF; 𝑉 𝑉

‒ Integration: 𝑇 = OFF; 𝑉 𝑉

‒ Readout: 𝑇 = ON, 𝑇 = ON

54

H. K. Kim et al. | Int. J. Precis. Eng. Manuf. | 2008

Overlap cap.Crossover cap.

Weighing two methods

Indirect‐conversion FPDs Direct‐conversion FPDs

X‐ray converterScintillators

e.g. CsI:Tl, Gd2O2S:TbPhotoconductive semiconductorse.g. a‐Se, HgI2, PbI2, PbO, CdZnTe

Readout pixel array TFT + photodiode TFT + pixel electrode (storage cap.)

Bias voltage ‐5 ~ ‐10 V higher (e.g. 10 V/𝜇m @ a‐Se)

Fab. complication 12 ~ 14 masks 5 ~ 7 masks

Quantum efficiency Higher Lower (a‐Se)

Image blurring Additional light scattering Within intrinsic x‐ray interactions

Image sampling Lower aliasing Higher aliasing (white spectrum)

Amelioration

Higher intrinsic conversion eff.Less light scattering

Better optical couplingLess charge trapping

High Z materialsLower W‐value

Lower dark currentLarger 𝜇𝜏𝐸

55

H. K. Kim et al. | Int. J. Precis. Eng. Manuf. | 2008

Pre‐ & post processing

56

Raw image Corrected gain/offset Interpolated bad pixels/lines Post-processed

Taken from J. A. Rowlands' Slides

Can you tell which image was obtained from the scintillator or the photoconductor?

57

매일경제, 2007.12.13.

연합뉴스, 2007.11.22.

58

Fluoroscopy

59M. Overdick | Philips | IWoRID 2002

60

Photon counting detectors

Infinite dynamic range

No additional noise (low‐dose imaging!)

Extension to multi‐channel spectral imaging

T. Francke et al. | NIMA 471, 85 | 2001

Large DR

Small DR Only 0–10 photons/pixel

61

GF Knoll | Radiation Detection and Measurement | Wiley | 2010

M Campbell & V Rosso, IEEE NSS-MIC | Rome, Italy | 2004

~100 m

62

Large‐area PCD

63

4 5 Timepix’s 5.6 7 cm

10 10 Timepix’s 14 14 cm(Courtesy) WidePix | Czech

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