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BMI II SS06 – Class 6 “OT Instrum.” Slide 1

Biomedical Imaging IIBiomedical Imaging II

Class 6 – Optical Tomography II: Instrumentation

03/13/06


Measurement principleMeasurement principle

Measure optical intensity migrating from small irradiation spot (source, S) to detector (D) position

“Scan” object to obtain measurements for many S-D pairs

Light propagation is scatter-dominated Signals are obtained under any angle

Signal is strongest near source (backscattered)


DOT characteristics summaryDOT characteristics summary

Functional imaging method

Sensitive to hemoglobin oxygenation states (contrast mechanism)

Low spatial resolution (~mm - ~cm)

Excellent temporal resolution (~ms), capability of studying hemodynamics

DOT assesses tissue function rather than providing an accurate image of anatomical features.

Examples: breast cancer, functional brain imaging


Continuous Wave (C.W.) MeasurementsContinuous Wave (C.W.) Measurements

Simplest form of OT: lowest spatial resolution, “easy” implementation, greatest penetration

Measuring transmission of constant light intensity (DC)

Simple, least expensive technology most S-D pairs

High “frame rates” possible


Example: Optical brain imagingExample: Optical brain imaging

“Partial view” or back reflection geometry

Scalp

Bone

Cortex

CSF

2-3 cm

Source / Detector 1

Detector 2

Detector 3


Time-Resolved MeasurementsTime-Resolved Measurements

Measuring the arrival time/temporal spread of short pulses (<ns) due to scattering & absorption (narrowing the “banana”)

Expensive, delicate hardware (single-photon counters, fast lasers, optical reflections, delays…)

Long acquisition times (low frame rates)

Potentially better spatial resolution than DC measurements

t

I

t0 t

I

t0

Prompt or ballistic Photons (t = d/c)

d

“Snake” Photons

Diffuse Photons


Frequency-Domain MeasurementsFrequency-Domain Measurements

Propagation of photon density waves (PDW): PDW = 9 cm, cPDW = 0.06 c (*

Measure PDW modulation (or amplitude) and phase delay

RF equipment (100MHz-1GHz)

Wave strongly damped, challenging measurement

t

I

t0

t

I

t0

t

I

t0Photon density waves

t

I

t0

Phase

Modulation

(* f = 200 MHz, μa = 0.1 cm-1, μs’ = 10 cm-1 n = 1.37


Principle components of a DOT system Principle components of a DOT system

Target

Lightsource

Delivery

Sourcescan

Detector

Detectorscan

DAQstorage

Timing,control

Collection


Multi-detector implementationMulti-detector implementation

Scanning of single detector (only used in lab setups):

• Safes hardware components, cost

• Long acquisition times

Parallel multi-detector acquisition

• No “time skew”

• Stable setup

• Added hardware

Mixed approach (Scanning limited number of detectors)

• Feasible for “static imaging”

• Used in TR, FD methods because of expensive detection hardware


Multi-source position implementationMulti-source position implementation

Time-division multiplexing : One source position is illuminated at one time for the duration of the detection (~10-100 ms).

Time skew between sources

Switching mechanism necessary:

• Optical switch (challenges: isolation, stability, size)

• Electronic switching of multiple sources (multiple laser sources & drivers – cost, complexity)

Frequency encoding: All sources are on at the same time.

Intensity modulation at different frequencies allows electronic separation of the signals originating from different sources.

No time skew

Reduced dynamic range


Dynamic rangeDynamic range

Ratio of largest to smallest “useable” signal (saturation limit noise limit)

Typically 1:104 (80 dB) for detection electronics

Signal falls of rapidly (~ factor 10 per cm distance on surface)

Determines the maximum tissue volume that can be probed

With second sourceOne source


Solution: Detector gain switchingSolution: Detector gain switching

I1

12 10

II

13 100

II

14 1,000

II

15 10,000

II

S1

1

10

1001,000

10,000

1

10

1001,000

10,000

1

10

1001,000

10,000

1

10

1001,000

10,000

1

10

1001,000

10,000

0 1

0 1

0 1

0 1

0 1

S2

1

10

1001,000

10,000

1

10

1001,000

10,000

1

10

1001,000

10,000

1

10

1001,000

10,000

1

10

1001,000

10,000


Semiconductors ISemiconductors I

Energy levels in solids have band structure :

Thermal excitation creates intrinsic carriers (electron-hole pairs): ni = np 1.51010 cm-1 (Si at room temperature, kT = 0.025 eV)

Photoelectric excitation possible for

g gh E hc E

Valence band

Conduction band

Ele

ctro

n en

erg

y

Bound electrons

Free electrons

Eg < 5 eV for insulatorEg 1 eV for semiconductorEg = 0 eV for metals

+

-

22 3

gE

kTi in p T e


Semiconductors IISemiconductors II

Doping with impurities increases number of free carriers (~ typ. by factor of 107) according to Ea,d 0.045 < kT

Donor: Pentavalent impurity (e.g., P) provides excess e- n-type semiconductor

Acceptor: Trivalent impurity (e.g., B) “captures” e- creates additional holes p -type semiconductor

Internal photoelectric effect:

Donor doped: n-type

Acceptor doped: n-type

, ,a d a dh E hc E

Ed

Ea


Diode junction of p-type and n-type semiconductors:

1. Diffusion of carriers potential across junction (n-type is left positively charged, p-type is left negatively charged)

2. Recombination at junction region of depletion of free carriers high resistance voltage drop

3. Carriers generated within diffusion length of the depletion region are separated by potential slope

4. Photoelectric current Ip produced by

photodiode (proportional to irradiation intensity)

Photodiodes (PD)Photodiodes (PD)

+ --

- - -

- --

- --

-++

+ ++

+ ++

++

+p n

Diffusion lengthNegative

net chargePositive

net charge

Anode CathodeIp


Photodiode OperationPhotodiode Operation


Photodiode (Transimpedance) AmplifierPhotodiode (Transimpedance) Amplifier

Converts photocurrent to voltage according to:

Bandwidth:

Highly linear

“Photovoltaic Mode”

out PD fV I R

3

1

2dbf

fRC

PD


PD characteristicsPD characteristics


Photomultiplier tubes (PMT)Photomultiplier tubes (PMT)

External photoelectric effect converts light intensity into current of free electron

Cascade of secondary electron emission / multiplication

Signal amplification G = N typ. ~106 (N: no. of dynodes, : gain per dynode ~4)


PMT spectral sensitivityPMT spectral sensitivity


Avalanche PhotodiodesAvalanche Photodiodes

Reverse biased with high voltage (~100V)

Internal 10-1000× amplification through avalanche effect

Gain temperature sensitive -> requires cooling/regulation

Available in ready-to-use modules

Pricey


Comparison PD vs. PMTComparison PD vs. PMT

Property PD PMT

Sensitivity 10-12 W 10-15 - 10-16 W

Active area ~ mm2 ~ cm2

Speed MHz - GHz (small area) ~ GHz

Dynamic range >109 <105

Size Small (mm) Medium to large (~cm)

Power supply Low voltage High voltage (~0.1-1 kV)

Ruggedness Very good Limited

Cost Cheap – moderate (~$10) Expensive (~$100)


Light sources ILight sources I

Near infrared range (600-900 nm)

Power ~1-100 mW: Signal quality vs. exposure limit (~ mW/mm2)

Laser diodes (semiconductor lasers): Most widely used

+ Small

+ Inexpensive (o.k…. ~$10 - $1000)

+ High efficiency, easy-to-operate

+ RF modulation possible

+ ps-pulsed systems available

(Poor beam quality)

Discrete wavelengths (760, 785, 800, 810, 830,.. nm)


Semiconductor-based light sourcesSemiconductor-based light sources

“Forward bias” causes reduction of potential wall diode in conducting mode

Electrons and holes recombine in depletion layer, carriers are replenished by current source

Emitting of recombination radiation light emitting diode

For special diode geometries and reflecting end faces, laser action can be achieved laser diode

+ --

- - -

- --

- --

-+ +

+ ++

+ ++

++

+p n

+

-- - -- ---

--

--

+ ++ +

++ ++ ++ + + + +

-

+ +

LED

LD


Types of laser diodesTypes of laser diodes

“Butterfly”

“HHL”

“TO3”

“C-mount”

“5-mm can /9-mm can:”Low / mid-power (mW-100 mW)

Hi-power (~W)

Hi-power (~W)

“Fiber pigtail”

Telco app.


Laser diode driversLaser diode drivers

Laser diodes require a controlled current source

LD are highly sensitive to ESD, short pulses, and all kinds of electromagnetic interference

Line filters

Power on ramping

Off shorting

LD require cooling and often temperature control to stabilize the output power Thermoelectric cooling (TEC) elements


Light sources IILight sources II

Solid state lasers: Optically active crystals (TiSa)

+ Short pulses (< ps, time-resolved systems)

+ Good beam profile

Bulky (requires pump laser)

Expensive

Difficult to operate

Non-laser sources: light emitting diodes (LED)

Broad wavelength range

Diffuse emitter

Power ~ 10mW


Snells’ law:

Total internal reflection for > c when going from n1 to n2 < n1:

Fiber components:

Core (n1)

Cladding (n2)Coating (mechanical stab.)

n1 > n2 “guided modes”

Light propagation in optical fibers ILight propagation in optical fibers I

01

0 1

sin

sin

n

n

0

1

2

1

sin c

na

n


Light propagation in optical fibers IILight propagation in optical fibers II

Acceptance angle a: Maximum incoupling angle ai resulting in guided transmission:

“Numerical Aperture” NA = sin a

Divergence angle: Maximum exiting angle ad (ai ad aa)

2 21 2

0

1sin a n n

n


Properties of some optical materialsProperties of some optical materials

Important interfaces

Various fiber materials


Fiber modesFiber modes

Different modes of optical propagation = different spatial intensity patterns

Number of possible modes depending on core radius, refractive indices

Multimode (MM) fibers

large core > 50 m

Higher efficiency

Higher power

(Cheaper)

Single-mode (SM) fibers

small core < 10 m

Better beam quality

No pulse shape

distortion (Telecom apps)


Intensity profile for MM fibersIntensity profile for MM fibers


Fiber transmission lossesFiber transmission losses

Absorption losses

Bending losses


Coupling light into optical fibersCoupling light into optical fibers

Focusing optics must provide:

Focus spot size s core diameter

Beam convergence angle acceptance angle

Mechanical alignment:

Focus on fiber core front face (x-y-z)

Beam perpendicular to front face (-)

Fiber face cut, polished


DYNOT system (best DOT imager around!!!)DYNOT system (best DOT imager around!!!)

referencesignals

Target

LD 1

Incouplingoptics

MDU

Source fiber bundles

Collecting fiberbundles

Laser current

DPS 1

Laserdiodes

Fiberpigtails

Motor controller

Beamsplitter

LD 2

Laser controller

Mirror

DPS 2

PCI busData acquisition

board

Measuringhead

OTDM

LDD + TECD

LDD + TECD

f 1 f 1

f 2 f 2

Personalcomputer

Bifurcatedfibers

Referencesignals

1

2

3

4

5

6

7

8

910

11

12

13


Fiber OpticsFiber Optics

Deliver light to/from tissue

Bifurcated design ( co-located S/D pairs)

Source fiber bundle

Detector fiber bundle

Probing end

Reinforced jacketing

Soft jacket

Bifurcation


Laser DiodesLaser Diodes

Laser Diodes

780 nm

830 nm

“Fiber pigtails” = opt. output

Electrical connectors


Laser ControllerLaser Controller

Commercial Newport 8000 Laser diode and temperature controller

Thorlabs Inc. OEM laser driver


Fast Multi-Channel Optical SwitchFast Multi-Channel Optical Switch

Multi-wavelength

32 fibers

~70 Hz switch speed = 2Hz frame rate @ 30 sources

fiber pigtails

incoupling unit

circular fiber array

DC servomotor

beam-splitter cube

focusing optics

rotating mirror

source fiber bundles


Commercial fiber-optic switchCommercial fiber-optic switch


Multi-Channel Detector Multi-Channel Detector

Gain switching

32 Parallel detection channels

Electronic wavelength separation

Gain Setting of detector determines its sensitivity

Gain (TTL) S/H

Out @

Out @

Ref. f2

PTIA

PTIA

PGA

PGA

Lock-in@ f2

Lock-in@ f2

S/HS/HLock-in@ f1

Lock-in @ f1

SiPD

1 1000

S/HS/H

1 1000

Ref. f1


The DYNOT (DYnamic Near-infrared OT) InstrumentThe DYNOT (DYnamic Near-infrared OT) Instrument

7

3

4

5

1

2

9

8

1

2

3

4

5

5

6

6

1 – power supply, 2 – motor controller, 3 – detector, 4 – laser controller, 5 – host PC w/ monitor, 6 – fiber optics, 7 – optical switch, 8 – optics shielding cover, 9 – laser diodes


HelmetHelmetHelmet kit can be configured depending on application

Probes individually spring loaded


Measurement GeometriesMeasurement Geometries

1. Unilateral temporal arrangement (motor cortex)

2. Distributed Arrangement (frontal, temporal, parietal)

3. Installing / adjusting the optical probes

4. Complete 56 arrangement 30 sources 30 detectors = 900 data channels

1

2

3 4


Baby HelmetBaby Helmet


Dual Breast Measurement HeadDual Breast Measurement Head

Patient in prone position

Simultaneous dynamic bilateral breast imaging

Fiber protrusion individually adjusted (manually; pneumatic possible)

Measuring cup positions individually adjusted


Highly Flexible 2×-Breast SetupHighly Flexible 2×-Breast Setup


Adjusting MechanismAdjusting Mechanism


Probe PlacementProbe Placement


Optical Fibers

Animal Imaging StudiesAnimal Imaging Studies