Physics of Imaging Systems Basic Principles of X-Ray Diagnostic II · 2008. 12. 9. · Prof. Dr....

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RUPRECHT-KARLS-UNIVERSITY HEIDELBERG

Computer Assisted Clinical MedicineProf. Dr. Lothar Schad

12/9/2008 | Page 1Master‘s Program in Medical Physics

Chair in Computer Assisted Clinical MedicineFaculty of Medicine Mannheim University of HeidelbergTheodor-Kutzer-Ufer 1-3D-68167 Mannheim, GermanyLothar.Schad@MedMa.Uni-Heidelberg.dewww.ma.uni-heidelberg.de/inst/cbtm/ckm/

Physics of Imaging Systems

Basic Principles of X-Ray Diagnostic II

Prof. Dr. Lothar Schad



12/9/2008 | Page 2

source: Morneburg. “Bildgebende Systeme für die medizinische Diagnostik” 1995

Anode MaterialFixed Anode Turning Anode Ele-

ment

Atomic

number Z

Max. Temperature Tmax

at 1,33×10-2 Pa

°C

Thermal

Conductivity λ

W/(cm K)

Q-Factor

Order

c: Specific Heat

Q-Factor

Order

Cu 29 1032 3,98 119113 8 3,68 110135 10

Mo 42 2167 1,38 125599 7 1,88 171106 8

Ag 47 832 4,18 163450 4 3,18 124350 9

Ta 73 2587 0,55 103868 9 1,13 213402 6

W 74 2757 1,3 265223 1 1,81 369273 1

Re 75 2557 0,71 136160 6 1,38 264650 4

Os 76 2280 0,87 150754 5 1,77 306706 3

Ir 77 2220 1,46 249572 3 2,06 352136 2

Pt 78 1742 0,71 96472 10 1,41 191585 7

Au 79 (1063) 3,14 263687 2 2,81 235975 5

U 92 (1132) 0,25 26036 11 0,75 78108 11

cTZ ⋅⋅⋅⋅ ρλmaxλ⋅⋅ maxTZ c⋅⋅ρλ

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12/9/2008 | Page 3

construction of turning anode

double angle anode

sliding-contact bearing(lubrication solvent: fluid metal In-Ga-Sn)

source: Dössel. “Bildgebende Verfahren in der Medizin” 2000

Anode Constructiontungsten-rhenium molybdenum graphite

- molybdenum: √λρc = 1.88 > 1.81 (tungsten)- graphite: cgr = 10 cW- material combination allows 106 Ws = 10 s at 100 kW !

high powerlow power



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t time for one rotation (= 1/f)A focus area (2δl)f rotating frequencyλ thermal conductivityρ densityc specific heat

Turning Anode: Geometry

focal spot

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12/9/2008 | Page 5

source: H. Morneburg. “Bildgebende Systeme für die medizinische Diagnostik”, 1995

Turning Anode: Temperature Increase

basic temperature

ring temperature

focus spot temperature

number of rotations



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Turning Anode: One Rotation Temperature

anode temperature ϑ

depth coordinate z

focal spot

maximum temperature ϑm (z)

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12/9/2008 | Page 7


High-Frequency Generator

- constant voltage over time (< 10 - 20%, for CT < 2%)- control of voltage by triodes (ideal: < 0.1% variation → expensive

band pass filterU1/U2 = - N2/N1

data transferto other

componentsof X-ray system

inverter

heating

turning anodestarter

X-ray tube

voltageregulation

datalink

mAsswitch

datacontrol

controlconsole

service exposureunit

electricitynetwork



12/9/2008 | Page 8


Generator Variation of Applied Dose: ΔD

10-20% for film/foil-combination3-7 % for Kino2-4 % for DSA

∫ ⋅=At

dttIQ0

)(

thyristor timing

imaging time tiexposure time te(IEC/DIN 0750)

single exposure time ts

“on” “off”shoot

chosenQref

Uref

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12/9/2008 | Page 9


focus spot 0.6 mm40 kW150 Hz

IA as a function of loading time tfor different UA

(for preheated anode with 300 W)

Turning Anode: Maximum Loading

- typical loading curve for turninganode

- max. mAs-product for given UA- switch-off at max. mAs to avoidoverheating !

anode current IA



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- “generator with fixed current” creates a constantcurrent and switch-off after reaching the requested dose or the loading limit(max. mAs-product)

- “generator with falling load”starts with large current whichdecreases during exposureleading to minimized exposuretime and wrong exposed films

Generator with “Falling Load”

generator with falling load

generator with fixed load

loading limit

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- components:

• X-ray tube• high-voltage generator• patient couch• dose measuring unit• scatter grid• film/foil cassette• operator console• film developing system• accumulator foilswhen using digital X-ray imagingwith laser readout unit

Complete X-Ray GeneratorX-ray tube

collimator system

radiation protection box

X-ray film cassette

bodyobject

used radiation cone

mirror forlight field

bulb forlight field



12/9/2008 | Page 12Examples

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12/9/2008 | Page 13

1 X-ray tube2 X-ray collimator3 lead shielding to protect

patient’s non exposed body4 patient couch with low Z5 monitor6 electronic image intensifier7 TV tube8 lead glasses with lateral

protection9 lead thyroid protection10 lead whole body protection11 lead-lamellae to protect

examiners

1

2

3

4

5

6

7

8

9

10

11

source: BfS 2003

Urological X-Ray Examination



12/9/2008 | Page 14

X-Ray Imaging

X-Ray Imaging

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12/9/2008 | Page 15

- double-sided X-ray film (cross section)

emulsion: suspension of silver/bromine crystals in gelatinesupporting layer: flexible polyester or cellulose acetate

source: Laubenberger and Laubenberger. „Technik der medizinischen Radiologie“, Deutscher Ärzte-Verlag 1999

X-Ray Film: Schema

polyester foil

cover

emulsion

undercoating

emulsion

cover

undercoating



12/9/2008 | Page 16X-Ray Film: Chemistry

film emulsion of AgBr:

- light:AgBr reduction to Ag and Br

- development:Ag atoms grow to small μm grains

- fixation:remove AgBr by appropriate solution

Br + hυ → Br + e-

Ag+ + e- → Ag

problem:- low absorption of emulsion !- emulsion on both sides of film → crossover: unsharp image on second side of film

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12/9/2008 | Page 17

source: Biehl and Zier. „Röntgenstrahlen - ihre Anwendung in Medizin und Technik“, Leipzig 1980

- optical dense S = grade of film darkening

- gradient of density curve= γ-valueγ large: large contrast at

low dose γ small: modest contrast but

large dose range

- at linearity:μ >> film bright (bones)μ << film dark (air)

X-Ray Film: Density Curve

optical dense

object coverage

dark

enin

g co

vera

ge

shoulder

solarization

fog linea

r par

t of c

urve

(opt

imal

exp

osur

e)

log (light intensity)or dose ~ μd



12/9/2008 | Page 18

only about 1% of the intrinsic X-ray photons are absorbed by the film emulsion !

problem

darkening at position of a developed film depends on the number of free silver atoms and therefore depends on the number of X-ray

photons which have been absorbed at this position

X-ray film alone is not practical for medical applications !

X-Ray Film: Problem

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12/9/2008 | Page 19


excitation and light emission in accumulator foils (energy level schema)

Accumulator Foils

- store X-ray information in foil- electrons are excited from valence to conduction band creating e-/hole-pairs- recombination of e-/hole-pairs by laser → stimulated luminescence- readout foil and directly convert digitally- foil handling like X-ray film (no new mechanical construction necessary)

light emission

valence band

conduction band

excitationX-ray

centers of light



12/9/2008 | Page 20

solid state material: energy band representation

Solid State Material I

- electrons can be in energy levels of an atomdiscrete: single atom; continuous: solid state material

- valence band: filled with electrons; conduction band: emptyoften: energy state between both bands forbidden → isolator

- impurities in the solid state material can produce energy levels in the forbidden area

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12/9/2008 | Page 21Solid State Material II

- by activation (energy larger than the difference between valence and conductingband) electrons can be moved from valence band to the conducting band

- when the electron is removed, a hole (positive) is appearing at electron´s position- holes behave like electrons but with positive charge- hole filled by neighboring electron, hole moves to that position- efficiency: 20 – 50%- 97% efficiency of absorption of generated light- reduction of radiation by factor of 10 – 20- shorter radiation time with less motion artifacts- smaller focus spot → higher resolution- material choice: high X-ray absorption, high conversion efficiency into visible light- total efficiency: product of absorption and conversion efficiency



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X-ray film cassette with accumulator foil

conversion of X-ray radiation into visible light and subsequent detection using film → luminescence

luminescent substances: calcium / tungsten (CaWO4)lanthanum / bromide endowed with terbium (LaOBr:Tb)gadolinium / sulfur endowed with terbium (Gd2O2S:Tb)


Accumulator Foils: Material

cassette front

front foilfilmback foilfoamcassette backwith lead

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12/9/2008 | Page 23

• luminescence layer can have a much larger X-ray attenuation factor than the emulsion layer; luminescence layer can contain elements with large atomic number Z and large density ρ

• luminescence layer can be much thicker than emulsion layer because luminescence layer will not be developed or fixed

• luminescence layer can create from one X-ray quantum many visible photons

example: 100 keV-quantum is absorbed by 100 % and converted by 100 % intovisible light at 550 nm (2.26 eV) → 44.000 visible photons !

Accumulator Foils: Advantages



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- quality criteria for accumulator foils:- high absorption for X-rays- high light efficiency and quantum yield- good adaptation of light emission spectrum

to film sensibility for dose reduction

- result: low radiation exposure, short exposure times, minimization ofmovement artifacts, small focus spot of the tube, large image contrast

Accumulator Foils: Quality

(for same film darkening)

- amplifying factor V of accumulator foil:

V =

typical values: V = 10 - 20

dose with accumulator foildose without accumulator foil

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12/9/2008 | Page 25

accumulator foils X-ray absorption in a 100 µm foil at:

40 keV 60 keV 80 keV

efficiency of light emission

% % % %

CaWO4 33 13 27 4

LaOBr:Tb 73 33 17 13

Gd2O2S:Tb 37 51 28 19


important for dose !

Accumulator Foils: Absorption Conversion



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- mass absorption coefficient μ/ρ of different light emission materials as a function of X-ray energy

- Gd2O2S:Tb best material withcharacteristic K-line at 50 keV fits wellto the spectral distribution of diagnostic X-ray radiation


Mass Absorption Coefficient

energy of X-ray quants

relative spectraldistributionof X-ray radiation

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12/9/2008 | Page 27


Spectral Light Emission

wavelength

intensity of light emission

X-ray film blue-sensitive

X-ray filmgreen-sensitive



12/9/2008 | Page 28


- scattered light:radiation of fluorescence lightin all directions

- “cross over” effect:fluorescence light penetratesfilm base layer and developsthe foil at backside emulsionof the film

Image Blurring and Foil Thickness

thin foil

filmbaselayer

emulsion

foil

thick foil

foil

emulsion

reflection effect

“cross over” effect

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12/9/2008 | Page 29

X-ray imaging using accumulator foils(semi-conductor foil using “heavy metal halogenide phosphor” compound)

→ was the first step to “digital radiology”

conversion from light centers to ground state is “optically forbidden”

light centers stay excited


Accumulator / Storage Foils: Readout

accumulator foil is transported in a light-proofed cassette from the X-ray unitto the readout system

1. exposure

4. delete and regenerate

3. readout

2. storage

conducting band

valence band

light centers



12/9/2008 | Page 30

scanning of the stored image by a laser with very small focus

wavelength of the laser is adapted to the stimulation of the light centers

detection of emitted light using a photomultiplier

photomultiplier signal is digitized line-by-line and stored in a digital image

→ digital luminescence


Accumulator / Storage Foils: Laser Scanner

accumulator foil

photomultiplier

color filter

deflection mirrorlaser

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Digital Luminescence Radiography (DLR)

cassette

image data

foil depot

semi-conductor

foil

image readout delete station

light sources



12/9/2008 | Page 32

linear characteristic curve over 5 to 8 order of magnitudes !


Signal / Dose Relation

filmdarkening

luminescent material

film / accumulator foil

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12/9/2008 | Page 33

• digital image, i.e. data processing with computer possible

• not sensitive to wrong film development because of the large dynamic range

• linear characteristic line

DLR Advantage



12/9/2008 | Page 34

X-ray beam

readout ICsline driver ICs

detector area:43 cm x 43 cm

pixel size:143 µm

resolution:3.5 LP/mm

Direct Digital Solid State Detector: CsI, aSiX-ray radiation

photocathode

- future: CCD (charge coupled devices)

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Physics of Imaging Systems Basic Principles of X-Ray Diagnostic II · 2008. 12. 9. · Prof. Dr....

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