Biomedical Imaging II

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Biomedical Imaging II. Class 6 – Optical Tomography II: Instrumentation 03/13/06. Measurement 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 - PowerPoint PPT Presentation

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Slide 1Biomedical Imaging II
03/13/06
Measurement 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
BMI II SS06 – Class 6 “OT Instrum.” Slide *
DOT characteristics summary
Functional imaging method
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
BMI II SS06 – Class 6 “OT Instrum.” Slide *
Continuous 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 imaging
2-3 cm
Time-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
Prompt or ballistic Photons (t = d/c)
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Frequency-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)
Photon density waves
(* f = 200 MHz, μa = 0.1 cm-1, μs’ = 10 cm-1 n = 1.37
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Modulation
Principle components of a DOT system
Target
Light
source
Delivery
Source
scan
Detector
Detector
scan
DAQ
storage
Timing,
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Collection
Multi-detector implementation
Safes hardware components, cost
Feasible for “static imaging”
Used in TR, FD methods because of expensive detection hardware
BMI II SS06 – Class 6 “OT Instrum.” Slide *
Multi-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
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 range
(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 source
Solution: Detector gain switching
Semiconductors 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
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Semiconductors 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
Photodiodes (PD)
Diffusion of carriers potential across junction (n-type is left positively charged, p-type is left negatively charged)
Recombination at junction region of depletion of free carriers high resistance voltage drop
Carriers generated within diffusion length of the depletion region are separated by potential slope
Photoelectric current Ip produced by photodiode (proportional to irradiation intensity)
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Photodiode Operation
Photodiode (Transimpedance) Amplifier
Bandwidth:
PD characteristics
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)
BMI II SS06 – Class 6 “OT Instrum.” Slide *
PMT spectral sensitivity
Avalanche Photodiodes
Gain temperature sensitive -> requires cooling/regulation
Available in ready-to-use modules
Comparison PD vs. PMT
Light sources I
Power ~1-100 mW: Signal quality vs. exposure limit (~ mW/mm2)
Laser diodes (semiconductor lasers): Most widely used
Small
Semiconductor-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
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Types of laser diodes
Laser 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
Off shorting
LD require cooling and often temperature control to stabilize the output power Thermoelectric cooling (TEC) elements
BMI II SS06 – Class 6 “OT Instrum.” Slide *
Light sources II
Short pulses (< ps, time-resolved systems)
Good beam profile
Broad wavelength range
Light propagation in optical fibers I
Snells’ law:
n1 to n2 < n1:
Light 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)
BMI II SS06 – Class 6 “OT Instrum.” Slide *
Properties of some optical materials
Important interfaces
Fiber modes
Number of possible modes depending on core radius, refractive indices
Multimode (MM) fibers
Intensity profile for MM fibers
BMI II SS06 – Class 6 “OT Instrum.” Slide *
Fiber transmission losses
Coupling light into optical fibers
Focusing optics must provide:
Beam convergence angle acceptance angle
Mechanical alignment:
Beam perpendicular to front face (-)
Fiber face cut, polished
DYNOT system (best DOT imager around!!!)
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Fiber Optics
Source fiber bundle
Detector fiber bundle
Laser Diodes
Laser Diodes
780 nm
830 nm
Laser Controller
Thorlabs Inc. OEM laser driver
BMI II SS06 – Class 6 “OT Instrum.” Slide *
Fast Multi-Channel Optical Switch
fiber pigtails
incoupling unit
Commercial fiber-optic switch
Multi-Channel Detector
Gain switching
Gain (TTL)
The DYNOT (DYnamic Near-infrared OT) Instrument
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
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Helmet
Probes individually spring loaded
Measurement Geometries
Complete 56 arrangement
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Baby Helmet
Dual Breast Measurement Head
Patient in prone position
Fiber protrusion individually adjusted (manually; pneumatic possible)
Measuring cup positions individually adjusted
BMI II SS06 – Class 6 “OT Instrum.” Slide *
Highly Flexible 2×-Breast Setup
BMI II SS06 – Class 6 “OT Instrum.” Slide *
Adjusting Mechanism
Probe Placement
Animal Imaging Studies